API Standard 2T

Recommended Practice for Planning, Designing, and Constructing Tension Leg Platforms

API RECOMMENDED PRACTICE 2TSECOND EDITION, AUGUST 1997

http://www.cssinfo.com

One of the most signiÞcant long-term trends affecting the future vitality of the petroleumindustry is the publicÕs concerns about the environment. Recognizing this trend, API mem-ber companies have developed a positive, forward looking strategy called STEP: Strategiesfor TodayÕs Environmental Partnership. This program aims to address public concerns byimproving our industryÕs environmental, health and safety performance; documenting per-formance improvements; and communicating them to the public. The foundation of STEP isthe API Environmental Mission and Guiding Environmental Principles.

API ENVIRONMENTAL MISSION AND GUIDING ENVIRONMENTAL PRINCIPLES

The members of the American Petroleum Institute are dedicated to continuous efforts toimprove the compatibility of our operations with the environment while economicallydeveloping energy resources and supplying high quality products and services to consum-ers. The members recognize the importance of efÞciently meeting societyÕs needs and ourresponsibility to work with the public, the government, and others to develop and to use nat-ural resources in an environmentally sound manner while protecting the health and safety ofour employees and the public. To meet these responsibilities, API members pledge to man-age our businesses according to these principles:

¥ To recognize and to respond to community concerns about our raw materials, productsand operations.

¥ To operate our plants and facilities, and to handle our raw materials and products in amanner that protects the environment, and the safety and health of our employees andthe public.

¥ To make safety, health and environmental considerations a priority in our planning,and our development of new products and processes.

¥ To advise promptly, appropriate ofÞcials, employees, customers and the public ofinformation on signiÞcant industry-related safety, health and environmental hazards,and to recommend protective measures.

¥ To counsel customers, transporters and others in the safe use, transportation, and dis-posal of our raw materials, products, and waste materials.

¥ To economically develop and produce natural resources and to conserve thoseresources by using energy efÞciently.

¥ To extend knowledge by conducting or supporting research on the safety, health, andenvironmental effects of our raw materials, products, processes, and waste materials.

¥ To commit to reduce overall emission and waste generation.

¥ To work with others to resolve problems created by handling and disposal of hazard-ous substances from our operations.

¥ To participate with government and others in creating responsible laws, regulations,and standards to safeguard the community, workplace, and environment.

¥ To promote these principles and practices by sharing experiences and offering assis-tance to others who produce, handle, use, transport, or dispose of similar raw materi-als, petroleum products and wastes.

Recommended Practice for Planning, Designing, and Constructing Tension Leg Platforms

Exploration and Production Department

API RECOMMENDED PRACTICE 2TSECOND EDITION, AUGUST 1997

SPECIAL NOTES

API publications necessarily address problems of a general nature. With respect to partic-ular circumstances, local, state, and federal laws and regulations should be reviewed.

API is not undertaking to meet the duties of employers, manufacturers, or suppliers towarn and properly train and equip their employees, and others exposed, concerning healthand safety risks and precautions, nor undertaking their obligations under local, state, orfederal laws.

Information concerning safety and health risks and proper precautions with respect to par-ticular materials and conditions should be obtained from the employer, the manufacturer orsupplier of that material, or the material safety data sheet.

Nothing contained in any API publication is to be construed as granting any right, byimplication or otherwise, for the manufacture, sale, or use of any method, apparatus, or prod-uct covered by letters patent. Neither should anything contained in the publication be con-strued as insuring anyone against liability for infringement of letters patent.

Generally, API standards are reviewed and revised, reafÞrmed, or withdrawn at least everyÞve years. Sometimes a one-time extension of up to two years will be added to this reviewcycle. This publication will no longer be in effect Þve years after its publication date as anoperative API standard or, where an extension has been granted, upon republication. Statusof the publication can be ascertained from the API Authoring Department [telephone (202)682-8000]. A catalog of API publications and materials is published annually and updatedquarterly by API, 1220 L Street, N.W., Washington, D.C. 20005.

This document was produced under API standardization procedures that ensure appropri-ate notiÞcation and participation in the developmental process and is designated as an APIstandard. Questions concerning the interpretation of the content of this standard or com-ments and questions concerning the procedures under which this standard was developedshould be directed in writing to the director of the Authoring Department (shown on the titlepage of this document), American Petroleum Institute, 1220 L Street, N.W., Washington,D.C. 20005. Requests for permission to reproduce or translate all or any part of the materialpublished herein should also be addressed to the director.

API standards are published to facilitate the broad availability of proven, sound engineer-ing and operating practices. These standards are not intended to obviate the need for apply-ing sound engineering judgment regarding when and where these standards should beutilized. The formulation and publication of API standards is not intended in any way toinhibit anyone from using any other practices.

Any manufacturer marking equipment or materials in conformance with the markingrequirements of an API standard is solely responsible for complying with all the applicablerequirements of that standard. API does not represent, warrant, or guarantee that such prod-ucts do in fact conform to the applicable API standard.

All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise,

without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C. 20005.

Copyright © 1997 American Petroleum Institute

iii

FOREWORD

This Recommended Practice for Planning, Designing, and Constructing Tension Leg Plat-forms incorporates the many engineering disciplines that are involved with offshore installa-tions, either ßoating or Þxed. DeÞned herein are guidelines adapted from successfulpractices employed for related structural systems in the offshore and marine industries.

A Tension Leg Platform (TLP) is a vertically moored, buoyant, compliant structural sys-tem wherein the excess buoyancy of the platform maintains tension in the mooring system. ATLP may be designed to serve a number of functional roles associated with offshore oil andgas exploitation. It is considered particularly suitable for deep water applications.

A TLP system consists of many components, each of which has a precedent in the off-shore or marine industry. The uniqueness of a TLP is in the systematic inßuence of one com-ponent on another. Consequently the design is a highly interactive process which mustaccount for functional requirements, component size and proportion, equipment layout andspace allocation, hydrodynamic reaction, structural detail, weight and centers of gravity, etc.All disciplines involved in the design process should anticipate several iterations to achieveproper balance of the design factors.

This document summarizes available information and guidance for the design, fabricationand installation of a TLP system. The recommendations are based on published literatureand the work of many companies who are actively engaged in TLP design. As TLP technol-ogy develops, this document will be updated to reßect the latest accepted design and analysismethods.

This Recommended Practice has three parts: the main body, the commentary, and theglossary. The main body contains basic engineering design principles which are applicableto the design, construction, and operation. Equations for analyses are included where appro-priate. In many cases these equations represent condensations of more complete analysisprocedures, but they can be used for making reasonable and conservative predictions ofmotions, forces or component strength. More detailed discussions of these engineering prin-ciples, describing the logic basis and advanced analytical concepts from which they weredeveloped, are given in the commentary. The designer and operator are encouraged to usethe most current analysis and testing methods available, and bring forth to the Institute anynewfound principles or procedures for review and consideration.

API publications may be used by anyone desiring to do so. Every effort has been made bythe Institute to assure the accuracy and reliability of the data contained in them; however, theInstitute makes no representation, warranty, or guarantee in connection with this publicationand hereby expressly disclaims any liability or responsibility for loss or damage resultingfrom its use or for the violation of any federal, state, or municipal regulation with which thispublication may conßict.

Suggested revisions are invited and should be submitted to the director of the Explorationand Production Department, American Petroleum Institute, 1220 L Street, N.W., Washing-ton, D.C. 20005.

v

CONTENTS

Page

1 SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3 DEFINITIONS AND TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1 DeÞnitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 PLANNING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.2 The Design Spiral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.3 Operational Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.4 Environmental Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.5 Seaßoor Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.6 Systems Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.7 Fabrication and Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.8 Materials and Welding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.9 Safety and Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.10 Codes, Standards, and Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.11 Operating and In-Service Manuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 DESIGN CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.2 Operational Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.3 Stability Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.4 Environmental Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135.5 Design Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 ENVIRONMENTAL FORCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.2 Wind Forces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.3 Current Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186.4 Wave Forces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196.5 Ice Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.6 Wave Impact Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.7 Earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.8 Accidental Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.9 Fire and Blast Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

7 GLOBAL DESIGN AND ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.2 Extreme Responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247.3 Responses For Fatigue Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297.4 Hydrodynamic Loads For Hull Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307.5 Static and Mean Response Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307.6 Equations of Motion and Solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327.7 Random Process Statistics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.8 Hydrodynamic Model Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.9 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

vi

Page

8 PLATFORM STRUCTURAL DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398.2 General Structural Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398.3 Design Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408.4 Structural Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408.5 Structural Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428.6 Fabrication Tolerances. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

9 TENDON SYSTEM DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449.2 General Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469.3 Design Loading Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499.4 Load Analysis Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509.5 Structural Analysis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529.6 Structural Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529.7 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549.8 Installation Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549.9 Operational Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549.10 Corrosion Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559.11 Inspection and Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

10 FOUNDATION ANALYSIS AND DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5510.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5510.2 Foundation Requirements and Site Investigations . . . . . . . . . . . . . . . . . . . . . . . 5610.3 Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5710.4 Analysis Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5810.5 Design of Piled-Template Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5910.6 Design of Piles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5910.7 Design of Shallow Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6010.8 Material Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6110.9 Fabrication, Installation, and Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

11 RISER SYSTEMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6111.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6111.2 Riser Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6311.3 Riser Analysis Methodology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6711.4 Component SpeciÞcation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6711.5 Operating Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7111.6 Special Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

12 FACILITIES DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7112.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7112.2 Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7212.3 Drilling Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7412.4 Production Systems Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7512.5 Hull System Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7612.6 Personnel Safety Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7912.7 Fire Protection Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8012.8 Interacting Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

vi

Page

13 FABRICATION, INSTALLATION, AND INSPECTION. . . . . . . . . . . . . . . . . . . . . 8213.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8213.2 Structural Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8213.3 Tendon System Fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8513.4 Platform Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8713.5 Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8813.6 Installation Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8913.7 Inspection and Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

14 STRUCTURAL MATERIALS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9614.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9614.2 Steel ClassiÞcation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9614.3 Manufactured Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9714.4 Special Applications for Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9814.5 Fracture and Fatigue Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10014.6 Structural Welding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10114.7 Corrosion Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10214.8 Cement Grout and Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10214.9 Elastomeric Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

APPENDIX A COMMENTARIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105APPENDIX B REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Figures1 Tension Leg Platform Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Design Spiral For a Tension Leg Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Inertial CoefÞcient Found By Equating the MacCamy-Fuchs Solution

to the WFE Inertia Term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 Wave Force Calculation Method and Guideline. . . . . . . . . . . . . . . . . . . . . . . . . 225 TLP Motion Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 Design Analysis Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Surge Motion Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 Maximum Tendon Tension Up Wave Leg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Minimum Tendon Tension Down Wage Leg . . . . . . . . . . . . . . . . . . . . . . . . . . . 2810 Restoring Force With Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3211 Simple Model For TLP Response Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3312 Typical Tendon Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4513 Tendon Design Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4814 Net Section and Local Bending Loads On a Cylindrical Section . . . . . . . . . . . 5315 Stress Distribution Across Section A-A For Axisymmetric Cross Section . . . . 5316 Combined Net Section and Local Bending Stress Linear Interaction Curve. . . 5417 Components of an Integrated Template Foundation System . . . . . . . . . . . . . . . 5518 Components of an Independent Template Foundation System . . . . . . . . . . . . . 5519 Components of a Shallow Foundation System. . . . . . . . . . . . . . . . . . . . . . . . . . 5520 Deck-Level Completion Production Riser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6221 Subsea Completion Riser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6322 Riser Design Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6523 Misalignment of Butt Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

vii

vi

Page

24 Misalignment of Cruciform Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8425 Beam Column Deßections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8426 Truss Deßections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8427 Girder Deßections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8528 Stiffened Plate Deßections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8529 Maximum Permissible Deviation From Circular Form,

e

, For Cylinders . . . . . 8630 Arc Length For Determining Deviation From Circular Form . . . . . . . . . . . . . . 8631 Major Activities and Options For Installation Operations . . . . . . . . . . . . . . . . . 9032 Options For Tendon Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92A-33 TLP Global Design Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106A-34 Residual Pile Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115A-35 Riser Governing Differential Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Tables1 Design Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 Typical Data List For Analysis of Risers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643 Allowable Stress Limits For Riser System Design and Operation . . . . . . . . . . 664 Structural Steel Plates and Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975 Structural Steel Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986 Impact Testing Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

viii

1

Recommended Practice for Planning, Designing, andConstructing Tension Leg Platforms

1 Scope

This Recommended Practice is a guide to the designer inorganizing an efÞcient approach to the design of a TensionLeg Platform (TLP). Emphasis is placed on participation of allengineering disciplines during each stage of planning, devel-opment, design, construction, and installation. Iteration ofdesign through the design spiral, Figure 2, is recommended.

2 References

References for this Recommended Practice are listed bysection in Appendix B.

3 Definitions and Terminology

3.1 DEFINITIONS

For the purposes of this Recommended Practice the fol-lowing deÞnitions apply:

3.1.1 added mass:

Effective addition to the system masswhich is proportional to the displaced mass of water.

3.1.2 bluff body:

An opaque object located in a fluid flowstream and developing a high drag force because it lacksstreamlining.

3.1.3 braces:

Structural members that serve to stiffen thehull structure and provide deck support.

3.1.4 bulkhead:

Stiffened vertical or horizontal loadbearing diaphragm.

3.1.5 buoyancy equipment:

Devices added to tendonor riser joints to reduce their weight in water, thereby reduc-ing top tension requirements. The devices normally used forrisers take the form of syntactic foam modules or open-bot-tom air chambers.

3.1.6 connectors:

1. Riser devices used to latch andunlatch risers and lower marine riser packages to subseaequipment. 2. Tendon devices used to latch and unlatch ten-dons to the foundation system and to connect the tendon tothe platform.

3.1.7 deck beams:

Secondary structural elements span-ning between intermediate girders and/or main girders.

3.1.8 deck plate:

Flat plate or grating spanning betweendeck beams.

3.1.9 design life:

Maximum anticipated operational yearsof service for the platform, i.e., the period of time from com-mencement of construction until removal of the structure.

3.1.10 elastomer:

Any of the class of materials, includ-ing natural and synthetic rubbers, which return to their origi-nal shape after being subjected to large deformations.

3.1.11 extreme offset:

An estimated maximum offset ofthe platform corresponding to given environmental condi-tions.

3.1.12 flat:

Horizontal stiffened bulkhead.

3.1.13 flex element:

Any of a variety of devices that per-mit relative angular movement of the riser or tendon in orderto reduce bending stresses caused by vessel motions andenvironmental forces.

3.1.14 guidance equipment:

Guidance equipment isused to direct and orient risers or tools to the seafloor tem-plate. Guidelines, tendons, submersibles, etc., can be used forthis purpose.

3.1.15 heave:

Platform motion in the vertical direction.

3.1.16 hydrodynamic damping:

Component of hydro-dynamic force proportional to the velocity of the body and180 degrees out of phase with the velocity.

3.1.17 intermediate columns:

Vertical, cylindrical, ormultifaceted buoyancy members of the hull structure whichprimarily assist in deck and/or pontoon support.

3.1.18 intermediate decks:

Deck levels between lowerdeck and upper deck consisting of girder, beam and plateelements.

3.1.19 intermediate girders:

Primary deck elementsspanning between main girders.

3.1.20 jumper hoses/fluid transfer system:

Systemfor transmitting fluid flow between the top of the risers to theplatform mounted manifold. Jumper hoses or an articulatedsystem of hard piping may be used to accommodate the rela-tive motion between these points.

3.1.21 load:

Any action causing stress or strain in thestructure.

3.1.22 lock-in:

Synchronization of vortex-shedding fre-quency and structural vibration frequency producing resonantflow induced vibration.

3.1.23 low frequency motion:

Motion response at fre-quencies below wave frequencies typically with periods rang-ing from 30 to 300 seconds.

3.1.24 lower deck:

Lowest primary deck level consistingof girders, beams and plate elements.

2 API R

ECOMMENDED

P

RACTICE

2T

3.1.25 main columns:

Vertical, Cylindrical or multifac-eted buoyancy members of the hull structure which provideplatform stability and deck support. Tendons are supportedby these columns.

3.1.26 main girders:

Deck elements spanning betweenthe primary load carrying subsystem.

3.1.27 mating joints:

Intersection of deck and hull struc-tures on a non-integrally constructed platform.

3.1.28 mean offset:

The average offset, corresponding tothe average horizontal forces on the TLP in the given envi-ronmental conditions.

3.1.29 moment controlling device:

Devices such asball joints or elastomeric joints used to reduce bendingstresses induced by relative angular movements at the ends ofthe riser. When curvature control is necessary, tapered jointsmay also be used.

3.1.30 negative buoyancy:

If a body weighs more thanthe weight of sea water that it displaces, then it is consideredto be negatively buoyant.

3.1.31 offset:

Horizontal distance of the platform at anyinstant from its static, stillwater, still air, equilibrium position.

3.1.32 pitch:

Platform rotation about the plant east-westhorizontal axis.

3.1.33 pontoons:

Horizontal, cylindrical or rectangularbuoyancy members of the hull structure which interconnectwith columns to form a frame below the waterline.

3.1.34 preload:

Load purposely induced in a componentto improve its in-service strength, fatigue life, or sealingcapabilities.

3.1.35 pretension:

Tension applied to a tendon in itsstatic, zero offset equilibrium position.

3.1.36 primary load carrying subsystem:

Structuretying column tops together and supporting deck levels. Thisstructure may consist of either trusses, box girders, plate gird-ers or a combination thereof.

3.1.37 ringing:

High frequency vertical vibration of theTLP spring-mass system excited by impulsive loading.

3.1.38 riser joint:

A riser joint consists of a section ofpipe, with couplings on each end. It may have provision forsupporting integral and non-integral auxiliary lines (flow-lines, choke and kill lines, control bundles, etc.) and buoy-ancy devices.

3.1.39 riser running/handling equipment:

Usuallyconsists of a riser handling sub and a riser spider. The risersub latches on to the end of the riser joint permitting it to beconnected to the surface lifting device. The riser spider is

used to support the riser string, during deployment/retrieval,as a joint is being made or broken.

3.1.40 riser spacer frame:

A purpose designed frame tomaintain lateral separation among risers.

3.1.41 riser spider:

A device used to support the riserstring as a joint is being made or broken during riser deploy-ment/retrieval operations.

3.1.42 riser spoilers:

Used in areas where high velocitycurrents are encountered to preclude vortex-induced riservibration. Various types of spoilers have been effective inreducing these vibrations; however, they frequently result inan increase in drag forces.

3.1.43 riser sub:

A device which latches on to the end ofthe riser joint permitting it to be connected to the surface lift-ing device.

3.1.44 roll:

Platform rotation about the plant north-southaxis.

3.1.45 setdown:

The increase in TLP platform draft withoffset due to tendon system restraint.

3.1.46 springing:

The high frequency vertical vibrationof the TLP spring-mass system excited by cyclic loading at ornear the TLP pitch or heave resonant periods.

3.1.47 subsea diverter:

A piping manifold positioned atthe top of the drilling riser to divert formation gas and liquidto an acceptable discharge point, preventing flow to workingareas.

3.1.48 subsea manifold:

The subsea well template mayincorporate a subsea manifold when wells are completed withsubsea trees. Here, production fluid is conveyed from thetrees via pipes on the template to a subsea manifold at thebase of a production riser. Production fluid may be commin-gled at the manifold if the number of subsea wells exceedsthe number of production risers available.

3.1.49 subsea well template:

A structural frame whichprovides location and anchor points for the subsea wellheads,riser systems, and guidance systems.

3.1.50 surface trees:

A combination of valves whichmay be placed on the top of production risers to control pres-sure and divert flow.

3.1.51 surge:

Horizontal motion of the platform in theplant north-south direction.

3.1.52 sway:

Horizontal motion of the platform in theplant east-west direction.

3.1.53 telescopic joint:

Riser joint designed to permit achange in length of the riser to accommodate platform move-ments. Sometimes called a slip joint.

R

ECOMMENDED

P

RACTICE

FOR

P

LANNING

, D

ESIGNING

,

AND

C

ONSTRUCTING

T

ENSION

L

EG

P

LATFORMS

3

3.1.54 tendon:

A system of components which form alink between the TLP platform and the subsea foundation forthe purpose of mooring the TLP.

3.1.55 tendon access tube:

A conduit within a platformcolumn between the bottom of the column and the tendon topconnector through which a tendon passes.

3.1.56 tendon connector:

A device used to connect atendon to the platform hull (top connector) or to the founda-tion template (bottom connector).

3.1.57 tendon coupling:

A device which connects onetendon element to another or to a specialty component.

3.1.58 tendon element:

Each of the similar or identicalbut discrete structural components which, when assembledwith the flex elements, top and bottom connectors, and anyother special components, comprise a complete tendon.

3.1.59 tension leg:

The collective group of tendons asso-ciated with one column of the platform.

3.1.60 tensioner:

A device, usually pneumatically orhydraulically powered, used to apply tension to tendons orriser.

3.1.61 tensioner systems:

Tensioner units are used tomaintain risers in tension as the platform moves in responseto wind, waves, and current.

Horizontal motions, heave, and setdown of the platformnecessitate changes in length of the risers. Tensioners accom-modate these movements, as well as relative angular motionbetween the platform and riser, while maintaining a nearlyconstant tension on the risers.

3.1.62 upper deck:

Upper or roof deck level consistingof girder, beam and plate elements.

3.1.63 yaw:

Platform rotation about the vertical axis.

3.2 TERMINOLOGY

The following terms used in this Recommended Practiceare depicted in Figure 1:

3.2.1 hull:

Consists of buoyant columns, pontoons andintermediate structure bracing.

3.2.2 deck structure:

A multilevel facility consisting oftrusses, deep girders and deck beams for supporting opera-tional loads.

3.2.3 platform:

Consists of hull and deck structure.

��

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

��

ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ

��

yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

Risers(drillingproductionpipeline)M

oorin

g sy

stem

Temporarymooringsystem

Well template

Foundation template

Piles

Tendons

Ten

sion

leg

plat

form

Hull

Pla

tform

Deck

Drilling deck

Skid base

Substructure

Upper deck

Lower deck

Pontoon

Col

umn

Figure 1—Tension Leg Platform Terminology

4 API R

ECOMMENDED

P

RACTICE

2T

3.2.4 foundation:

Consists of templates and piles, or agravity system.

3.2.5 mooring system:

Consists of tendons and foun-dation.

3.2.6 risers:

Include drilling, production, and pipelinerisers.

3.2.7 well systems:

Include risers, riser tensioners, well-head, and subsea well templates.

3.2.8 tension leg platform:

Includes all of the aboveplus all deck equipment and hull marine systems.

4 Planning

4.1 GENERAL

4.1.1 Configuration Selection

4.1.1.1

Most TLP designs have certain basic features thatare common. SpeciÞcally, the distribution of buoyancybetween the vertical columns and the submerged pontoon orbuoyancy volume is selected to minimize the net verticaloscillating wave force on the hull by taking advantage of thehydrodynamic cancellation effects, thus reducing oscillatingloads on the tendons.

4.1.1.2

Another common feature is the relationshipbetween the pretension in the tendons and the displacementof the hull, wherein the pretension can be selected to result ina predetermined static offset due to steady forces. Generally,the minimum pretension should be chosen to result in positivetension loads at the foundation tendon connections for allloading conditions. This should result in a long natural periodin surge, with beneÞcial attenuation of surge motion.

4.1.1.3

It is the designerÕs responsibility to select the mostsuitable buoyancy distribution and pretension that will com-plement the functional requirements and the operatorÕs pref-erence for deck and hull conÞguration. The latter isinßuenced by the selected well system. Experience indicatesthat a very close designer/operator relationship is requiredduring the entire design process in order to produce a satisfac-tory design.

4.1.2 Planning Considerations

4.1.2.1 Recognition of the need for several iterations of thedesign process and operational requirements is important inplanning and scheduling the design. Time is needed to evalu-ate the effects of parameter variations before rational designdecisions can be made. Hydrodynamic model testing shouldbe included in the design process to verify the analyticalresults.

4.1.2.2 A weight and center of gravity control procedurefor the entire system should be incorporated into the designprocess at the very earliest stage. The control procedureshould be one that can be used throughout the design, con-struction and operational life.

4.2 THE DESIGN SPIRAL

4.2.1 General

4.2.1.1 An understanding of the entire design sequence andits relationship to external constraints such as Þnancial,scheduling and manpower requirements is essential. Thedesign steps involved are illustrated in the Design Spiral(Evans, 1959) as an iterative process working from functionalrequirements to a detail design, Figure 2.

4.2.1.2 In planning the design process it is important torecognize the operatorÕs contracting strategy for constructionand to identify the stage at which plans and speciÞcations areto be released for construction contracting. Traditional ship-building practice refers to this stage as the ÒcontractÓ designin which the main structural members are speciÞed but sec-ondary structure and appurtenances are loosely deÞned. TheÞnal deÞnition of these items is assigned to the builder withassistance from the designer. In this manner the design is tai-lored to best utilize the methods and facilities provided by thebuilder and is still subject to the approval of the designer.

4.2.1.3 The industry practice for offshore platforms is gen-erally to separate the design and construction activities bycompleting the detail design before committing for construc-tion. This procedure allows the operator to become directlyinvolved in the development of the fabrication methods. Bothdesign/construction strategies have their advantages and dis-advantages; however, the operator should recognize the dif-ferences and decide which method to utilize for the TLP.

4.2.2 Conceptual Design

4.2.2.1 Conceptual design translates the functional require-ments into naval architectural and engineering characteristicsduring the initial iteration around the design spiral. It embod-ies technical feasibility studies to determine such fundamen-tal elements as length, width, depth, draft, hull shape,mooring system, well and riser systems, all of which satisfythe environmental and functional criteria. The conceptualdesign includes initial lightship weight estimates and moor-ing pretension.

4.2.2.2 Alternative designs are generally considered inparametric studies during this phase to determine the mostpractical design solution. The selected concept then is used asa basis for obtaining approximate construction and installa-tion costs and schedule, which often determine whether or notto initiate a preliminary design.

RECOMMENDED PRACTICE FOR PLANNING, DESIGNING, AND CONSTRUCTING TENSION LEG PLATFORMS 5

4.2.3 Preliminary Design

Preliminary design further reÞnes the characteristics affect-ing cost and performance. Certain controlling factors such asplatform geometry, number and type of wells, mooring pre-tension and payload should not change after completion ofthis phase. Its completion provides a precise deÞnition thatwill provide the basis for development of contract plans andspeciÞcations.

4.2.4 Final Design

4.2.4.1 The Þnal design stage yields a set of plans andspeciÞcations which form an integral part of the fabricationcontract document. It encompasses one or more loops aroundthe design spiral. This stage delineates precisely such featuresas hull shape, dynamic response, structural details, use of dif-ferent types of steel, spacing and type of frames and stringers.Paramount among the Þnal design features is a weight andcenter of gravity estimate. The Þnal general arrangement isalso developed during this stage. This Þxes the overall vol-umes and areas for consumables, machinery, living and utilityspaces, and handling equipment.

4.2.4.2 The accompanying speciÞcations delineate qualitystandards of hull and outÞtting and the anticipated perfor-mance for each item of machinery and equipment. Theydescribe the tests and trials that shall be performed success-fully to have the TLP considered acceptable.

4.2.5 Detail Design

The last stage of design is the development of detailed fab-rication drawings and construction speciÞcations. These arethe installation and construction instructions to yard trades-men and are subject to the approval of the designer.

4.3 OPERATIONAL REQUIREMENTS

4.3.1 Function

A TLP can perform a variety of missions such as drilling,producing, storage, materials handling, living quarters, orsome combination of these. The platform conÞgurationshould be determined by studying equipment layouts ondecks, as well as addressing those aspects of system designwhich assure hydrodynamic and aerodynamic performance,stability, weight considerations, and constructability. The rel-atively broad column spacing required for stability andhydrodynamic performance will probably permit convenientequipment arrangements on the available deck area.

4.3.2 Site Considerations

4.3.2.1 Location

Environmental conditions depend on geographic location,and within a given geographic area, the foundation conditionswill vary as will such parameters as design wave height andperiod, currents, tides, and wind speeds.

Configuration proportions

Arrangements

Hydrostatics subdivision

Hydrodynamics (including model tests) Structural design and analysis

Mooring and foundation design

Weight estimates

Costs contracting plans

Functional requirements

Figure 2—Design Spiral For a Tension Leg Platform

6 API RECOMMENDED PRACTICE 2T

4.3.2.2 Water Depth

Accurate data on water depth and tidal variations areneeded to fabricate tendon components so that the TLP oper-ates at its design draft.

4.3.2.3 Orientation

The orientation of a platform refers to its position refer-enced to true north. Orientation will be controlled by thedirections of prevailing and extreme design waves, winds,and currents and by operational requirements.

4.3.3 Arrangements

4.3.3.1 Equipment and Consumables

Layout and weight of equipment for mooring, drilling and/or production, consumables, and other payload items shouldbe carefully accounted for in the design and operation.Weight and weight distribution affect both the steady anddynamic tensions in the tendons. Consideration should begiven to future operations such as gas and/or water injection.

4.3.3.2 Personnel and Material Handling

Plans for handling personnel and materials should bedeveloped at the start of the platform design. The type andsize of supply vessels and the mooring system required tohold them in position can affect the platform. The number,size, and location of boat landings, if required, should bedetermined. The type, capacity, number, and location of thedeck cranes should also be determined. If equipment or mate-rials are to be placed on a lower deck, adequate hatchesshould be provided on the upper decks. The use of helicoptersshould be established and adequate facilities provided.

4.3.3.3 Access and Auxiliary Systems

The location and number of stairways, access routes, andboat landings should be controlled by both safety and opera-tional requirements.

4.3.3.4 Fire Protection

Fire protection systems, including Þre walls and pressur-ized spaces, should be provided for the safety of personneland equipment. The systems selected should be suitable forthe anticipated hazards (e.g., electrical or hydrocarbon Þre)and should conform to all applicable regulations.

4.3.3.5 Emergency Evacuation

Emergency equipment such as launchable lifeboats or sur-vival capsules should be provided for personnel evacuation.The types of equipment and evacuation methods should meetall applicable regulations.

4.3.3.6 Spillage and Contamination

Provision should be made for handling spills and potentialcontaminants. A deck and process vessel drainage systemwhich collects and stores liquids for subsequent handlingshould be provided. The drainage and collection systemshould meet applicable regulations.

4.3.3.7 Hull Systems

The platform should be provided with systems for transfer-ring ballast water to or from hull compartments (ballast sys-tem), for monitoring tank contents, and for permitting safeaccess to tanks and void spaces. Compartmentalization of thehull will be required to limit the effects of damage, leakage orother unintended water ingress. Such compartments may beuseful for temporary ballast to control draft and stabilitybefore and during installation. Access for inspection shouldbe provided in the design.

4.4 ENVIRONMENTAL CONSIDERATIONS

4.4.1 General

Winds, currents, waves, and tides cause steady and oscilla-tory lateral movements, variations in tendon loads, and/or dis-tributed loadings on the structure and its elements. Theresulting TLP response requires the use of dynamic analysismethods in the design. Environmental data consistent withthe analysis technique should be utilized.

4.4.2 Design Considerations

The design of all systems and components should antici-pate extreme and normal environmental conditions which canbe experienced at the site.

Environmental loading and platform response are impor-tant design considerations for several subsystems includingfoundations, tendons, risers, hull and deck equipment.

4.4.2.1 Extreme Environmental Conditions

Extreme environmental conditions are those which pro-duce the extreme response that has a low probability of beingexceeded in the lifetime of the structure. A minimum returnperiod of 100 years for the design event should be used unlessrisk analysis can justify a shorter recurrence interval fordesign criteria. The design of the structure and its key sub-systems shall be such that they will be capable of withstand-ing the extreme environmental event in a safe condition.

4.4.2.2 Normal Environmental Conditions

Normal environmental conditions are those which areexpected to occur frequently during the construction and ser-vice life. Since different environmental parameters and com-binations affect various responses and limit operations


differently (e.g., installation, crane usage, etc.), the designershould consider the appropriate environmental conditions forthe design situation.

4.4.3 Environmental Data

4.4.3.1 Responsibility

Selection of the environmental data required is the respon-sibility of the operator. The dynamic nature of the TLPrequires that the platform designer work closely with a mete-orological-oceanographic specialist to develop data and inter-pretations in the form needed for the particular design/analysis methods to be used.

4.4.3.2 Statistical Models

Recognized statistical methods and models should beapplied in the assessment of extreme and normal environmen-tal conditions. All data used should be carefully documented.The estimated reliability and the source of all data should berecorded, and the methods used in developing available datainto models should be described. Sensitivity of design topoorly established parameters/distributions in statistical mod-els should be recognized.

4.4.3.3 Specific Environmental Conditions

Selection of speciÞc environmental conditions for designshould be based on factors related to risk. Section 5.4 con-tains speciÞc guidance on the choice of environmental param-eters for design. API Recommended Practice 2A, Chapters 1and 2, give general discussions of most of these parametersand their speciÞc use in design analysis for Þxed platforms.

4.5 SEAFLOOR CHARACTERISTICS

4.5.1 Seafloor Surveys

The primary purpose of a seaßoor site survey is to providedata for a geologic assessment of foundation soils and thesurrounding areas. A secondary purpose is to identify opera-tional hazards such as seaßoor irregularities, shallow gaspockets and man-made objects. Geophysical equipment suchas side-scan sonar devices, sub-bottom proÞlers, boomers andsparkers are available for deÞning the physical features of thesurface of the seaßoor to sub-bottom depths of several hun-dred feet.

For more detailed description of commonly used sea-bot-tom survey systems, refer to Sieck and Self, 1977.

4.5.2 Site Investigations

On-site soil investigations should be performed to deÞnethe various soil strata and their corresponding physical andengineering properties. If practical, the soil sampling and test-ing program should be deÞned after reviewing the seaßoor

survey. The foundation investigation for pile supported struc-tures should yield at least the soil test data necessary to pre-dict axial capacity of piles in tension and compression, axialand lateral pile load deßection characteristics, and mudmatpenetration vs. resistance.

4.5.3 Seafloor Instability

Large movement of the seaßoor may be caused by waves,earthquakes and soil loads. Such soil movement can imposesigniÞcant lateral and vertical forces against foundations.

The scope of site investigations in areas of seaßoor insta-bility should be sufÞcient to develop design criteria for theeffects of soil movement.

4.5.4 Scour

Scour is removal of seaßoor soils caused by currents andwaves, and can result in removing vertical and lateral supportfor foundations. Where scour is a possibility, it should beaccounted for in design to avoid settlement of the foundationand overstressing of the foundation elements.

4.6 SYSTEMS DESIGN

4.6.1 Platform

4.6.1.1 Types

There are several variations of platforms which can be dis-tinguished either by platform use (i.e., production-only ordrilling/production), or by drilling arrangement. Some exam-ple variations are:

a. Production well platform without drilling capability. Thistype should be considerably smaller and lighter than a drill-ing/production platform. Production risers generally areattached to the deck structure.b. Drilling/production platform with drilling at deck levelthrough a well bay.c. Drilling/production platform with drilling at deck levelthrough the columns and tendons. The tendons are designedto act as conductors and drilling equipment is designed tomove from column to column.

4.6.1.2 Functional Requirements

Many functional requirements of a platform require specialattention during the planning stages of design. In all cases,personnel and material requirements must be considered inrelationship to the safety and efÞciency of the platform. Thefollowing critical requirements will signiÞcantly impact thedesign and layout of the platform:

a. Drilling facilitiesÑThe number, type and location of drill-ing rigs should be ascertained prior to commencement ofdesign.


b. Production facilitiesÑThe weight, area, and center ofgravity of the production facilities should be determined inso-far as possible prior to commencement of design of the plat-form. Because platform design is sensitive to the values ofweight, area, and center of gravity, these values should not bepermitted to deviate beyond speciÞed tolerances, otherwiseredesign may be required.c. Drilling/production risersÑSufÞcient clearance must beprovided between risers and adjacent structural members toavoid interference during severe environmental conditions.d. Well SystemsÑThe number of platform wells, completionand workover method, minimum well spacing, and well baylocation have a direct inßuence on the size and layout of thedeck structure and the hull. These features should be deter-mined prior to commencement of preliminary platformdesign.e. Hull compartmentationÑHull damage from falling objects,boat collision, or other means should be considered during thedesign. The subdivision of the hull should allow for accidentalßooding of at least one watertight compartment. Damage con-trol procedures should be developed during the design phaseand included in the operating manual.f. AirgapÑThe minimum clearance between the lowest deckor any underdeck temporary maintenance equipment and awave crest is an important parameter in the design of the TLP.The airgap has an effect on the center of gravity and in turnthe maximum and minimum tendon tensions. The designerhas two general options: provide a minimum deck clearanceor allow for wave impact in the design of the platform.

4.6.2 Tendon System

The tendon system consists of the tendons, ancillary com-ponents needed for operation, including load measurementsystems and inspection or monitoring apparatus.

The tendon system restrains motion of the platform inresponse to wind, waves, current, and tide to within speciÞedlimits. Legs of the system, composed of an array of tendons,connect points on the platform to corresponding points on aseaßoor foundation (see Figure 12). By restraining the plat-form at a draft deeper than that required to displace itsweight, the tendons are ideally under a continuous tensileload that provides a horizontal restoring force when the plat-form is displaced laterally from its still water position. Gener-ally very stiff in the axial direction, the tendon system limitsheave, pitch, and roll response of the platform to small ampli-tudes while its softer transverse compliance restrains surge,sway, and yaw response to within operationally acceptablelimits.

The number of legs, as well as the number of tendons ineach leg, is determined by the platform conÞguration, loadingconditions, and design philosophy, including intended servicerequirements and redundancy considerations speciÞed by theoperator for a particular installation. The designer should

allow for the possibility of material deterioration during theservice life of the platform and provide a means of detectingand repairing such defects.

4.6.2.1 Tendon Types

The tendons may take one of several forms, for example:

a. Tubular members with connectorsÑThe members may bedesigned to be either buoyant or fully ßooded. They may befabricated as one piece or constructed by welding the connec-tors to a tubular. The members may be made of metal or com-posite Þber reinforced resins (e.g., carbon Þber/epoxycomposites), with either integral or metallic connectors.b. Tubular or solid rod members with Þeld welded connec-tionsÑThe tubulars are fabricated from seamless or rolledand welded steel and are designed to be welded together,prior to or during offshore installation, to form a continuoustendon element.c. Tendon strandÑThese tendons are fabricated from smalldiameter high tensile strength wire or Þber strands and areformed into bundles. These tendons are designed to beinstalled offshore using a continuous one-piece spoolingoperation without the need for intermediate connectors.

4.6.2.2 State of Technology

Investigating items such as coupled tendon/platformmotions, vortex-induced vibrations, and the fatigue life ofcomplex mechanical connections requires a high level oftechnical sophistication. Technology in this area is relativelynew and rapidly advancing. The designer is encouraged tomake use of modern but proven equipment and analyticalmethods.

4.6.2.3 Engineering

Certain tendon components, because of their complexity,may warrant extensive engineering development and prototypetesting to determine the fatigue, fracture, and corrosion charac-teristics and the mechanical capabilities of the components.

4.6.2.4 Tendon Fabrication

The time required to fabricate the tendons may exceed theduration required to construct the hull and deck structure.Consideration should be given to the fabrication lead timerequirement of the tendons to avoid unnecessary delays ininstallation.

4.6.2.5 Tendon Installation

Installation of the tendons may require the use of largecapacity lifting and handling equipment. Installation proce-dures and their implication to the design should be consideredearly in the planning stages. Onboard storage area, if required


for the tendons during installation, can affect the layouts ofthe deck and hull and warrants early attention during design.

4.6.3 Foundations

4.6.3.1 Foundation Types

There are several types of foundations that may be utilizedfor a TLP. For example:

a. A foundation consisting of a foundation templateanchored to the ocean ßoor by piles which carry both lateraland tensile loads. The loads are transmitted to the piles by thetendons which may be directly connected to the piles orattached to the template.

b. Shallow foundations such as non-piled gravity foundationsor suction pile foundations to which the tendons are directlyattached.

c. Combination of a and b with a template for each leg or onetemplate common to all legs.

d. Auxiliary foundations consisting of anchor piles, dead-weight clumps, drag anchors, or other types of anchors towhich a catenary mooring system is attached for temporaryuse during installation.

4.6.3.2 Pile Types

Three commonly used types are the driven pile, the drilled-and-grouted pile, and the combination driven-drilled-and-grouted pile. The type most appropriate for a particular foun-dation will depend on the soil conditions at the site and thepile performance, as well as on the installation equipmentavailable. Further discussion on these pile types can be foundin API Recommended Practice 2A.

4.6.4 Well Systems

The design of a well system should achieve cost effectivesafety and reliability in the containment, control, and trans-mission of produced ßuids from the oil or gas reservoir to theprocessing system. While risers are an integral part of the wellsystem, they can also be used for other functions, such as forpipeline connections. Systems will commonly be capable ofbeing run and retrieved by vertical deployment from the deck.

4.6.4.1 General

Integration of the design of the well systems into the designof the TLP should be an early priority. The selection of wellriser tension levels, the platform motion effects, the effect ofthermal loads when wells and tendons are congruent, andriser/hull clearances are examples of items requiring closecoordination. The weight and size of the well system equip-ment will have a signiÞcant impact on hull size and cost.

4.6.4.2 Well System Selection

Different types of risers between the platform and seaßoormay be utilized, including integral and nonintegral risers, andrisers integral to the tendons. Drilling BOPs and well comple-tion systems may be located either at the platform deck levelor subsea. Anticipated workover frequency and wellheadmaintenance will inßuence the decision as to surface or sub-sea completions. Anticipated changes in future operation(e.g., gas lift or water injection) might require the need forßexibility within components selected.

4.6.4.3 Well System Reliability

Well component design and selection should be primarilyon the basis of reliability and safety of the system. Fieldproven technology and equipment should be used where pos-sible. Design reliability should include redundancy, back upprocedures, and fail-safe designs whenever practical. Compo-nent and well system reliability studies could be useful indetermining the consequences of failure, and identifyingthose components needing a higher degree of reliability. Iden-tiÞcation of those components that cannot be retrieved to thesurface, the consequences of such components being dam-aged, and how to mitigate the consequences should be con-sidered. In all cases, consideration should be given to anacceptable means of stopping the well ßow near the seaßoorin the event of an accident.

4.6.5 Facilities

4.6.5.1 General

The planning and selection of facilities involve many prob-lems which are unique to compliant structures. The selectionand design of the facilities should consider the platformmotions. Facilities will have interfaces between individualsystems and the overall structure, including dynamic loadinput from drilling rigs, sharing of utilities between drilling/production systems and hull systems, and escape means forvarious damage states. Such loads and interfaces should beidentiÞed and considered.

4.6.5.2 Facilities Design

TLP facilities design must recognize the highly interactivenature of the design process, and the importance of propercoordination and integration of drilling rig, production, hullsystems, and structural needs. SpeciÞc deÞnition of all facili-ties criteria and requirements early in the design processshould prevent changes in the platform resulting fromchanges in facilities. There should be close coordinationbetween the facilities and structural designers throughout thedesign project to ensure that routine interactions, changes,and interfaces are properly addressed.


4.6.5.3 Facilities Layout

Facilities layout should be considered in the initial stagesof design when the development of the overall conÞgurationis being made.

Layouts should initially be guided by the overall functionof the platform and should include the inßuences of welllocation(s), production systems needs, accommodationrequirements, and area classiÞcation considerations. Facilitiesconstruction, whether fully integrated, semi-integrated, ormodular, will affect the layout and weight as well. Damagecontrol, personnel safety and evacuation, and spillage/con-tainment requirements also inßuence the facilities layout. Itmay be beneÞcial to examine a variety of facilities layouts.

4.6.5.4 Facilities Weight, Center of Gravity, and Space Management

4.6.5.4.1 Weight, CG, and space requirements should bemanaged to develop a facility efÞcient in cost and operation.

4.6.5.4.2 The design process should consider the use ofÒgrowth allowancesÓ in the form of weight and space factors,which can help in two respects. First, platform facilities havea tendency to grow during the design process with potentiallydetrimental implications. Thus, realistic allowances forweight and space growth during the design process shouldhelp to prevent major design recycling at late stages. Second,experience has shown that the originally intended operationalparameters for offshore facilities frequently are no longeradequate once the facility has been in operation for severalyears. Accordingly, it is appropriate to utilize space andweight growth allowances as a means of allowing ßexibilityin future operations. Operational growth scenarios shouldalso include examination of the weight or space ßexibilitythat may be gained by the removal of certain facilities at laterstages in the operation.

4.6.5.4.3 BeneÞts may result from keeping the designgrowth and operational growth allowances separate duringdesign. Operational growth allowances can easily be pre-empted by unexpected design problems, but the implicationsto future facility operation should be considered. Both designgrowth allowances and operational growth allowances shouldrecognize the impact of weight and space on ßoating facilities.

4.7 FABRICATION AND INSTALLATION

4.7.1 Fabrication Methods

The method of platform fabrication should be consideredprior to completion of the preliminary design since themethod selected will signiÞcantly affect not only structuraldesign but also the feasibility of fabrication at a chosen site.There are two basic methods of platform fabrication:

a. Deck ßoatoverÑBy this method the deck is constructed inone piece separately from the hull, ßoated (usually by barge)over the hull and lowered and mated to it using controlledballast and jacking procedures. OutÞtting of the deck is usu-ally completed prior to deck mating.b. Integral deck and hullÑBy this method the deck is con-structed integrally with the hull. A sufÞciently deep dry dockor a convenient, sheltered deepwater site is a prerequisite forthis type of construction. OutÞtting of the deck may be com-pleted together with the construction of the deck subassem-blies (as in modular construction) or may take placesubsequent to deck and hull construction.

4.7.2 Fabrication Site Selection and Preparation

The proper selection and preparation of the fabrication siteis instrumental to the successful construction. Important con-siderations are:

a. Coastal siteÑThe fabrication yard should have a deepwa-ter dry dock or means for transferring the hull into the water.It may be skidded onto a submersible barge or launcheddirectly into the water. If the dry dock does not have sufÞcientdepth, the use of auxiliary buoyancy to support the hull dur-ing construction may be acceptable.b. Sheltered offshore construction areaÑDeepwater con-struction facilities may be located offshore, away from thefabrication yard, and in sufÞciently deep and sheltered watersto allow convenient access for either ßoatover deck mating orintegral deck construction.c. Deepwater channelÑA deepwater channel must be avail-able to permit towing the completed structure to sea. Theminimum channel depth must be sufÞcient to allow the plat-form to be towed at a draft commensurate with speciÞed sta-bility criteria.

4.7.3 Towout

Precautions should be taken during towout to sea to avoiddamage to the structure. Escort tugboats to provide protectionagainst damage should be considered.

Stability criteria for towout should be selected as appropri-ate for the time, duration and location of the tow as well as forthe degree of damage protection and control afforded. Spe-ciÞc towing requirements will depend on whether or not thetow is manned.

4.7.4 Installation Equipment

The function, type, and size of the major equipmentselected for installation can affect the design and should beconsidered during the planning stages of design. For example,the response of the platform will change considerably duringthe transition from freely ßoating to vertically restrained;therefore, the temporary restraining equipment should besized accordingly.


4.7.5 Installation

In planning the installation of the sub-sea well and moor-ing template(s), due consideration should be given to avoid-ing interference with seaßoor returns of well cuttings andgrout. These factors should also be considered in design ofthe connection equipment and methods to be used for the ris-ers and tendons. The Þnal design of the production tem-plate(s) and well system, the temporary mooring system, thefoundation templates, and the piles will depend on the instal-lation methods and equipment selected.

4.8 MATERIALS AND WELDING

4.8.1 Materials

Selection of the strength and quality levels for steel,cement grout, concrete and other materials for the platform,foundation or other components will generally follow the cri-teria commonly used for offshore structures. This Recom-mended Practice edition emphasizes steel as the primarystructural material but speciÞcally does not preclude the con-sideration of other materials. Future revisions of this Recom-mended Practice will cover these other materials asappropriate. Steel for the tendons may be higher strength thannormal structural steel and will affect the method of tendonfabrication and inspection as well as tendon type and service.The tendons will operate under high cycle fatigue stressessuperimposed on the mean stress tensile load in a seawaterenvironment. The material should have acceptable propertiesin the Þnal condition to meet the requirements of strength,toughness and resistance to corrosion and corrosion fatigue.The material should possess adequate fracture toughness soas to withstand the largest possible ßaw (undetected) atdesign maximum loads and minimum exposure temperatures.Resistance to stress corrosion cracking under operating con-ditions is critical since detection of such cracks is difÞcultduring service. In-service inspection requirements, intervals,and methods of determining allowable defect size should beconsidered.

4.8.2 Welding and Inspection

4.8.2.1 Selection, qualiÞcation, and application of weldingand weld inspection procedures will generally follow criteriaused for offshore platform fabrication where applicable (e.g.,platform, foundation templates, etc.).

4.8.2.2 Where welding is allowed in the fabrication of ten-dons, the resulting weldments should have properties com-mensurate with the considerations given above for the tendonparent material. Fabrication procedures should be followedwhich assure the required properties in the installed tendon.These properties may be more difÞcult to obtain in a weld-ment than in the parent steel, especially as the strength level

increases. Consideration may be given to fabricating the ten-dons without any weld; however, the effect on cost, availabil-ity and fabrication lead time should be accounted for.

4.8.2.3 The inspection method should be sufÞcient todetect and locate all potentially damaging ßaws. This requiresconsideration of the local geometry as well as the toughnessof the material and the applied stress. Inspection methodsshould be designed and tested to demonstrate an adequateability to detect, resolve and size defects. Frequency of moni-toring should be determined to ensure that an unacceptabledefect does not occur during service.

4.9 SAFETY AND RELIABILITY

4.9.1 The design should maximize the safety of personneland the protection of property within a framework of effi-cient, cost effective design. Safety and reliability depend onthe ability of a facility to survive the loads anticipated overthe operational life. The designer should examine not only theintact facility and structure, but also examine the structureunder damaged conditions and ensure that the remainingstrength, fire resistance, and escape means are adequate.

4.9.2 Qualitative reliability analyses of certain systemssuch as the tendon system are possible. Such analyses canhelp to understand the differing degrees of reliability amongdesigns utilizing different numbers of tendons, different typesof connectors, and/or end terminations, etc. Such analysescan help assess reliability versus system cost, and pinpointcritical elements deserving special attention.

4.9.3 Hull damage state scenarios should be developedwith the implications of compartment flooding. Facilitiesdesign should consider damaged state scenarios and possibleimplications upon the deck structural system. Personnelescape routes should be designated for damaged states, andalternate routes provided. Damage control systems, includingfirefighting means, ballast redistribution capabilities, andbackup power supplies should all be selected considering theneed for reliable operation during periods of severe service.Redundant means of monitoring major platform functions,such as trim, ballast condition, etc., should be considered.

4.10 CODES, STANDARDS, AND REGULATIONS

A determination of the applicable codes, standards and reg-ulations should be made at the commencement of a project.Differences between such requirements or standards shouldbe identiÞed immediately, and a project decision or agree-ment with the responsible regulatory organization expedited.Other sections in this document and the API RecommendedPractice 2A discuss applicable rules and regulations pertain-ing to a TLP.


4.11 OPERATING AND IN-SERVICE MANUALS

4.11.1 The designer should provide manuals which com-municate to the operator the correct practices to be used forsafe and efficient operation. These manuals describe the prac-tices and procedures necessary for normal operation, mainte-nance, in-service inspection and emergency procedures fordamage state conditions and other emergency situations.Operating personnel should be required to review and under-stand the Operating and In-Service manuals.

4.11.2 In addition to the topics common to most offshoreoperations, the operating manuals should address:

a. In-place performance monitoring systems.b. Tendon handling systems.c. Riser handling systems.d. Ballast, weight, and center of gravity control systems.e. In-place inspectionÑhull and tendons.f. Environmental considerationsÑRelevant informationabout extreme environmental conditions and the predictedresponse effects.g. Corrosion monitoring and maintenance.h. Platform installation and removal.i. Emergency procedures:

1. FireÞghting.2. Evacuation.3. Personnel protection.4. Emergency and routine drills.5. Damage control.6. Well blow out.

5 Design Criteria5.1 GENERAL

5.1.1 This section defines the criteria commonly neededfor the design of a TLP. The format of the design criteria isconsistent with Section 6, Environmental Forces, and thosesections that deal with the design of the various subsystems.

5.1.2 Design and analysis of the TLP and the associatedsubsystems require that a series of design cases be specified.This requires that each phase of construction, transportation,installation, and operation be coupled with design environ-mental events and associated allowable stresses and/or safetyfactors. Statistical procedures involving probabilistic predic-tions of environmental parameters and platform responsesshould be established in order to select design cases. Knowl-edge of the system response characteristics must precede thedetermination of design environmental conditions. Specifica-tion of such conditions requires establishing maximum valuesof wind, waves, current and tidal variation together with therange of weight and center of gravity variations of the plat-form. Other environmental conditions, including long-termdata for fatigue analyses, etc., are also needed.

5.2 OPERATIONAL REQUIREMENTS

Design criteria dictated by operational requirements shouldbe reviewed during each iteration of the design spiral. Thecost and weight consequences of these requirements shouldbe fully established for the operator before a Þnal design deci-sion is made. Examples of such requirements usually involve:

a. Simultaneous drilling and production.b. Consumables resupply procedure and frequency.c. Maintenance procedures and frequency.d. Manning schedule and rotation.

5.3 STABILITY REQUIREMENTS

5.3.1 General

Stability should be established for relevant operating andpre-operating conditions, for both intact and damaged statesof the structure.

5.3.2 Free-Floating Condition

The intact and damaged stability while aßoat during con-struction, tow-out and installation stages should, in general,satisfy requirements applicable to column-stabilized mobileoffshore drilling units, as promulgated by the U.S. CoastGuard, American Bureau of Shipping, I.M.O., or NationalAuthorities.

5.3.3 Inplace Condition

The tension in the tendons should be sufÞcient to ensureintegrity of the platform and tendons. The sufÞciency of thetension should be demonstrated by appropriate analysis and/or testing. Alternately, a margin against tendon slackingshould be selected which depends on the state-of-knowledgeof design and operating conditions, and on the consequencesto the tendon system and components in case of slacking.

5.3.3.1 Intact Condition

The intact condition should include the full range of possi-ble center of gravity variations permitted by acceptable oper-ating procedures during extreme conditions. A minimumtension margin may be expressed in terms of some relevantcomponent of the tension, such as initial pretension, maxi-mum ßuctuating tension component, or tension measurementaccuracy.

5.3.3.2 Damaged Condition

5.3.3.2.1 Accidental ßooding of a buoyant compartmentwill result in added weight on the platform and changes intendon tensions. The hull should be subdivided to surviveaccidental damage of any one compartment below the water-line, or of a tendon compartment. Subdivision in way of the


waterline should be consistent with column-stabilized mobileoffshore drilling unit standards.

5.3.3.2.2 At the time of ßooding, the environmental condi-tions should be assumed to be the ÒnormalÓ environment (see5.5.4.2). Adequate tendon tension should be demonstratedthrough analysis.

5.3.3.2.3 Under any of the above-assumed damage condi-tions the unit should be capable of being restored to adequatetendon tension for the reduced extreme environment (see5.5.4.3) within three hours.

5.3.4 Weight and Center-of-Gravity Determination

An inclining test should be conducted, when constructionis as near to completion as practical, to accurately determinethe platform weight and the position of the center-of-gravity.Changes of on-board load conditions after the inclining testand during service should be carefully accounted for.

5.4 ENVIRONMENTAL CRITERIA

5.4.1 General

5.4.1.1 Environmental criteria should be associated with arecurrence interval of the response of the structure. For exam-ple, the 100 year design event should be that which producesthe worst platform response in 100 years. There may be dif-ferent design events which give the worst response for differ-ent parts of the structure. It is also noted that the largestresponses of TLPs are not necessarily produced by the high-est wave conditions.

5.4.1.2 The amount of detail required in the environmentaldescription is dependent on the level of analysis being under-taken and on the safety factors being used in design. Becauseof its compliant and dynamic nature, criteria for design of aTLP should have more detail for the same level of analysisthan for statically designed structures. Selection of the actualdata needed should be made only after consultation with boththe platform designer and meteorological/oceanographicspecialists.

5.4.1.3 Available statistical data and/or realistic statisticaland mathematical models should be utilized to develop thedescription of normal and extreme environmental conditions.Considerations include:

a. Normal environmental conditions are important both dur-ing the construction and the service life of a platform.b. Extreme conditions are important in formulating platformdesign loadings.

5.4.1.4 All data used should be documented. The qualityand the source of all data should be recorded, and the meth-ods employed in developing data into the desired environ-mental values should be deÞned.

5.4.1.5 The following sections brießy describe the environ-mental criteria which are required for use in design. For guid-ance on actual values to use in design, the designer shouldrefer to data collected at the intended site, to appropriateoceanographic numerical models, and to API RecommendedPractice 2A.

5.4.2 Wind

Wind is signiÞcant in TLP design and analysis. Bothsteady wind and a wind spectrum which represents ßuctuat-ing wind components should normally be used. SeeA.Comm.5.4.2.

5.4.3 Waves

5.4.3.1 Wind-driven waves are a major source of environ-mental forces on offshore platforms. Such waves are irregularin shape, can vary in height and length, and may approach aplatform from one or more directions simultaneously.

5.4.3.2 The development of wave criteria should generallybe done in accordance with API Recommended Practice 2A,Section 1.3.3.

5.4.3.3 Because of the random nature of the sea surface,the seastate is usually described in terms of a few statisticalwave parameters such as signiÞcant wave height, spectralpeak period, spectral shape and directionality. Other parame-ters of interest can be derived from these. See A.Comm.5.4.3.

5.4.4 Current

Current data collected at the site should be included in thedesign criteria if available. Currents should include winddriven, tidal, and background circulation components. Indeep water the currents might produce large system loads.Near boundary currents (e.g., the Gulf Stream), meanders andeddies should be considered.

5.4.5 Tide and Water Level

Tidal components for design include astronomical, wind,and pressure differential tides. A high design water level(HDWL) and low design water level (LDWL) should beestablished for each design event. The tidal range will affectthe required tendon pretension.

5.4.6 Joint Statistics

Environmental data such as wind, tide, wave, and currentscan have speciÞc relationships regarding their interaction andjoint occurrences. The commonly used assumption of takingthe combined maximum of each parameter might not alwaysproduce the worst design condition. When collecting data orperforming analytical work, the various relationships shouldbe included if possible. Of particular importance are wind/


wave, wave height/wave period, wave/current, wind/current,and wave/tide relationships.

5.4.7 Physical Properties

Various seawater physical properties, such as temperature,salinity, and oxygen content, may be important for steelrequirements, corrosion, and buoyancy calculations. Furtherguidance can be found in API Recommended Practice 2A.

5.4.8 Ice

In Northern climates, ßoating ice or atmospheric icing canaffect the loading on the platform (see API RecommendedPractice 2N).

5.4.9 Earthquakes

Seismic accelerations should be considered for areas thatare determined to be seismically active. Both vertical and hor-izontal accelerations are important for TLP design. Thedegree of risk for seismic activity in the United States isshown in API Recommended Practice 2A. Local regulationsand conditions should be considered and may result in morestringent requirements.

5.4.10 Marine Growth

The type and accumulation rate of marine growth at thedesign site may be necessary for determining design allow-ances for weight, hydrodynamic diameters and drag coefÞ-cients. Refer to API Recommended Practice 2A forappropriate guidance.

5.5 DESIGN CASES

5.5.1 General

5.5.1.1 DeÞning a design case requires selection of the fol-lowing parameters:

a. Project phase.b. System condition.c. Environment.d. Safety criteria.

5.5.1.2 All appropriate load types must be quantiÞed andincluded for each design case. Load types and design parame-ters are discussed below. Table 1 shows how the parametersmay be combined to deÞne design cases. This table isintended only to provide an example and is not necessarilycomplete. Other environmental criteria may be used if prop-erly justiÞed.

5.5.2 Project Phase

This describes the phase of the platform or component,such as hull construction, deck transport, or platform in place.

The construction, ßoatout, loadout, and transportation phaseshave various loading conditions that should be examined.These conditions are described by the stage of construction(percent complete), draft during tow, or stage of loadout.

5.5.2.1 Fabrication

Loading conditions imposed on the hull and deck duringfabrication could control the structural design of some com-ponents. Intact and damaged stability of the freely ßoatinghull must be insured at all phases of the fabrication.

5.5.2.2 Transportation

The freely ßoating platform behaves like a semisubmers-ible during transportation to the installation site. Since it isfree to pitch and roll, lateral deck accelerations can be largerthan after installation. Since it is free to heave, and heave res-onant periods tend to be short, adequate deck clearance withthe waves should be veriÞed.

5.5.2.3 Installation

The stiffness and mass properties change as the platform istransformed from a freely ßoating vessel to a verticallymoored platform, thus changing the dynamic response char-acteristics. The tendons and their handling equipment shouldbe designed for loading conditions representative of the vari-ous installation phases.

5.5.3 System Condition

This describes the condition of the platform or component.

Table 1—Design Cases

DesignCase

ProjectPhase

SystemCondition Environment

SafetyCriteria

1 Construction Various stages Ñ A

2 Loadout Intact Calm A

3 Hull/deck mating

Intact Mating B

4 Tow/transport Intact/damaged

Route B

5 Installation Intact Installation A

6 In place Intact Normal A

7 In place Intact Extreme B

8 In place Damaged Reduced extreme

B

9 In place Tendon removed

Normal A

10 In place Tendon removed

Reduced extreme

B

11 In place Intact Seismic B

12 In place Intact Fatigue C


5.5.3.1 Intact

The platform or component is as designed.

5.5.3.2 Damaged

The transport barge or ßoating component has one com-partment ßooded.

5.5.3.3 Tendon Removed

If the platform is designed on the basis that tendons can beremoved for inspection, maintenance, or replacement, itshould be designed for combinations of static, environmental,motion induced and construction loads with one or more ten-dons removed.

5.5.4 Environment

The environment should be deÞned quantitatively in termsof wind, wave, current, and tide. Environmental events shouldbe determined by the operator.

5.5.4.1 Extreme Environment

Extreme environmental conditions are those which pro-duce TLP responses having a low probability of beingexceeded in the lifetime of the structure. A minimum returnperiod of 100 years for the design response should be usedunless risk analysis can justify a shorter recurrence interval.The design of the structure and its key subsystems shall besuch that they will be capable of withstanding extreme envi-ronmental conditions in a safe operable condition.

5.5.4.2 Normal Environment

Normal environmental conditions are those which areexpected to occur frequently during the construction and ser-vice life. Since different environmental parameters and com-binations affect various responses and limit operationsdifferently (e.g., installation, crane usage, etc.), the designershould consider the appropriate environmental conditions forthe design situation.

5.5.4.3 Reduced Extreme Environment

Reduced extreme environmental conditions are thosewhich have a low probability of being exceeded when the hullis damaged or a tendon is removed. Joint statistics may beused to determine a return period which, combined with theprobability of damage, produces a risk level equal to that ofthe extreme environment.

5.5.4.4 Calm Environment

Some operations are performed only during calm condi-tions. Where such a choice is available, the design case is per-mitted to use calm conditions.

5.5.4.5 Route Environment

Transportation cases should use appropriate conditions forthe transportation route. The return period selected shouldconsider the length of exposure and an appropriate risk level.

5.5.4.6 Seismic

The TLP should be designed with strength and stiffness toensure no signiÞcant structural damage occurs for the level ofearthquake shaking which has a reasonable likelihood of notbeing exceeded during the life of the structure.

5.5.5 Safety Criteria

Safety criteria are classiÞed as categories A or B. Thesecorrespond to the intent of API Recommended Practice 2A,where safety factors are related to the probability of loadingoccurrence. Others may also be considered, such as criteriacorresponding to ultimate survival or damaged redundancydesign cases. SpeciÞc recommendations for safety factors foreach system component are given in their respective sections.

5.5.5.1 Category A

Category A safety criteria are intended for those conditionswhich exist on a day-to-day basis.

5.5.5.2 Category B

Category B safety criteria are intended for rarely occurringdesign conditions.

5.5.5.3 Category C

Category C safety criteria are intended for the design of thestructure against fatigue failure.

5.5.6 Load Types

Loading type categories are as follows:

a. Dead loadsÑDead static weight of the platform structureand any permanent equipment which does not change duringthe life of the structure.b. Live loadsÑVariable static loads, which can be changed,moved or removed during the life of the structure. Maximumand minimum payloads should be considered.c. Environmental loadsÑLoads on the structure due to theaction of wind, wave, current, tide, earthquake, or ice.d. Inertial loadsÑMotion induced loads that are conse-quences of the environmental loads.e. Construction loadsÑLoads built into the structure duringthe fabrication and installation phases.f. Hydrostatic loadsÑBuoyancy of, or submerged pressureon, submerged members.


The combination and severity of loads should be consistentwith the likelihood of their simultaneous occurrence.

6 Environmental Forces6.1 GENERAL

The purpose of this section is to describe methods for cal-culating forces which act on a TLP due to environmentaleffects, such as waves, winds, currents, ice, earthquakes, etc.Forces due to platform motion responses are also signiÞcantand are discussed herein. Environmental parameters neededfor these calculations are deÞned in Section 5. Methods forestimating platform motions and mooring system loadscaused by these environmental forces are given in Section 7.

Environmental forces must be calculated at four distinctfrequency bands to evaluate their effects on the system. Thefour frequency bands are:

a. Steady forces such as wind, current, and wave drift are con-stant in magnitude and direction for the duration of interest.b. Low frequency cyclic loads can excite the platform at itsnatural periods in surge, sway, and yaw: typical natural peri-ods range from 1 to 3 minutes.c. Wave frequency cyclic loads are large in magnitude andare the major contributor to platform member forces andmooring system forces. Typical wave periods range from Þveto twenty seconds.d. High frequency cyclic loads can excite the platform at itsnatural periods in heave, pitch, and roll: typical natural peri-ods range from one to Þve seconds.

6.2 WIND FORCES

6.2.1 General

The wind conditions used in a design should be determinedwith appropriate means from wind data collected in accor-dance with Section 5 and should be consistent, in terms ofjoint probabilities of occurrence, with other environmentalparameters assumed to occur simultaneously. A TLP has longnatural periods in surge, sway, and yaw which may be excitedby energy in the wind spectrum. The effects of the completewind spectrum, including sustained and ßuctuating winds,should be considered in determining the wind induced plat-form loads and responses. Such analyses may require knowl-edge of the wind turbulence intensity, spectra, and spatialcoherence. These items are addressed below.

6.2.2 Wind Properties

Wind speed and direction vary in space and time. Onlength scales typical of even large offshore structures, statisti-cal wind properties (e.g., mean and standard deviation ofvelocity) taken over durations of the order of an hour do notvary horizontally, but do change with elevation (proÞle fac-tor). Within long durations, there will be shorter durations

with higher mean speeds (gust factor). Therefore, a windspeed value is only meaningful if qualiÞed by its elevationand duration. A reference value VH is the one hour meanspeed at the reference elevation, H, of 33 feet (10 meters).

Note: A duration of one hour is assumed unless otherwise noted.

Variations of speed with elevation and duration, as well aswind turbulence intensity and spectral shape, have not beenÞrmly established. The available data show signiÞcant scatter,and deÞnitive relationships cannot be prescribed. The rela-tionships given below provide reasonable values for windparameters to be used in design. Alternative relationships areavailable in the public domain literature, or may be developedfrom careful study of measurements.

6.2.2.1 Mean Profile

The mean proÞle for the wind speed averaged over onehour at elevation z can be approximated by:

Vz = VH(z/H)0.125 (1)

6.2.2.2 Gust Factor

The gust factor G(t,z) can be deÞned as:

(2)

Where:I(z) = the turbulence intensity described below.

t = the gust duration with units of seconds.

The factor g(t) can be calculated from:

g(t) = 3.0 + 1n[(3/t)0.6] for t £ 60 sec. (3)

6.2.2.3 Turbulence Intensity

Turbulence intensity is the standard deviation of windspeed normalized by the mean wind speed over one hour.Turbulence intensity can be approximated by:

Iz º s (z)/Vz = (4)

Where:zs = 66 feet (20 meters) is the thickness of the surface

layer.

6.2.2.4 Wind Spectra

As with waves, the frequency distribution of wind speedßuctuations can be described by a spectrum. Due to the largevariability in measured wind spectra, there is no universally

G t z,( ) V t z,( ) V¤ z 1 g t( )I z( )+=º

0.15(z zs )¤ Ð0.125 for z zs£

0.15(z zs )¤ Ð0.275 for z zs>


accepted spectral shape. In the absence of data indicating oth-erwise, the simple shape given by the following equation isrecommended:

(5)

Where: Suu(f) = the spectral energy density at elevation z.

f = the frequency in hertz.s(z) = the standard deviation of wind speed, i.e.,

s(z) = I(z)/Vz.

Measured wind spectra show a wide variation in fp about anaverage value given by:

fpz/Vz = 0.025 (6)

Due to the large range of fp in measured spectra, analysis ofplatform sensitivity to fp in the range:

0.01 £ fpz/Vz £ 0.10 (7)

is warranted. Tension Leg Platform response, particularlymaximum offset, may be very sensitive to this parameter. Itshould be noted that fp is not at the peak of the dimensionalwind energy spectrum, since Equation 5 gives the reducedspectrum.

6.2.2.5 Spatial Coherence

Wind gusts have three dimensional spatial scales related totheir durations. For example, 3 second gusts are coherent overshorter distances and therefore affect smaller elements of aplatform superstructure than 15 second gusts. The wind in a 3second gust is appropriate for determining the maximumstatic wind load on individual members; 5 second gusts areappropriate for maximum total loads on structures whosemaximum horizontal dimension is less than 164 feet (50meters); and 15 second gusts are appropriate for the maxi-mum total static wind load on larger structures. The oneminute sustained wind is appropriate for total static super-structure wind loads associated with maximum wave forces.In frequency domain analyses of dynamic wind loading, itcan be conservatively assumed that all scales of turbulenceare fully coherent over the entire superstructure.

The variable nature of the wind Þeld can alternatively bedescribed by two components: a sustained component (Vz)and a gust component (u'). The total wind speed is then:

u = Vz + u¢ (8)

Where:u = the instantaneous speed and direction.

u¢ = the instantaneous speed and direction variationfrom the sustained wind.

6.2.3 Wind Force Relationship

6.2.3.1 The instantaneous wind force on a TLP can be cal-culated by summing the instantaneous force on each memberabove the water line. This should be calculated by an appro-priate equation such as:

(9)

Where:

F = wind force (pounds).ra = mass density of air (slugs/cubic foot).Cs = shape coefÞcient (may also account for

shielding).A = projected area of the object/feet2.

= the instantaneous velocity of the structural mem-ber, feet/second.

z = elevation of the centroid of the member, feet.

6.2.3.2 For all angles of wind approach to the structure,forces on ßat surfaces should be assumed to act normal to thesurface and forces on vertical cylindrical objects should beassumed to act in the direction of the wind. Forces on cylin-drical objects which are not in a vertical attitude should becalculated using appropriate formulas that take into accountthe direction of the wind in relation to the attitude of theobject. Forces on sides of buildings and other ßat surfacesthat are not perpendicular to the direction of the wind shallalso be calculated using appropriate formulas that account forthe skewness between the direction of the wind and the planeof the surface.

6.2.3.3 The total wind force on the structure may also becalculated using the total exposed area of the structure withappropriate coefÞcients determined by model tests or someother appropriate method.

6.2.3.4 When using the wind spectrum, it is common tolinearize the force for spectral and frequency domain calcula-tions (see Simiu and Leigh, 1983; Kareem, 1980),

(10)

where the Þrst term is the constant or steady force, and thesecond term is linear in the ßuctuating velocity. The higherorder term which is neglected in this approximation is gener-ally small. It does contribute a small amount to the steadyforce.

Suu f( )s z( )2

-----------------f f¤ p

[1 1.5 f f¤ p]5/3+----------------------------------------=

F12---raCs A|¢V z u¢ xú| V z u¢ Ð xú+( )Ð+=

xú

F12--- raCsAVz

2 ra+ Cs AV z u¢=


6.2.4 Steady Wind Force

The Þrst term of Equation 10 is the steady wind force. Vz

should correspond to the mean wind speed used in generatingthe wind spectrum.

6.2.5 Fluctuating Wind Force

The ßuctuating wind force may be calculated in the time orfrequency domains. In the time domain, the total wind forceis calculated from a time series of the instantaneous totalwind velocity using Equation 9. In frequency domain calcula-tions. Equation 10 may be used with the wind spectrum toderive the wind force spectrum as follows:

Sff(f) = c2 (f) Suu (f) (ra Cs AVz)2 (11)

Where:Sff = wind force spectrum.

c = aerodynamic admittance function (see 6.2.7).Suu = wind gust spectrum (see Equation 5).

6.2.6 Shape and Shielding Coefficients

The following shape coefÞcients are recommended for per-pendicular wind approach angles:

Shielding coefÞcients may be used when the proximity of asecond object relative to the Þrst is such that it does not expe-rience the full effect of the wind. (See Simiu and Scanlan,1978; API Recommended Practice 2P; Hoerner, 1965; andMeyers, Holm, and McAllister, 1969.)

6.2.7 Aerodynamic Admittance

6.2.7.1 Wind gusts measured at two locations becomeuncorrelated as the distance between the two locationsincrease. When the lateral dimensions of a structure are large,the reduction in gust forces can be accounted for by an aero-dynamic admittance factor. Simiu and Scanlan (1978) presentdata on transverse gust correlations which can be used todetermine admittance factors. The aerodynamic admittancecoefÞcient c modiÞes the force equation as follows:

(12)

6.2.7.2 The admittance coefÞcient is frequency dependent,is smaller for higher frequencies, and varies between 0 and 1.A value of c = 1.0 is conservative and appropriate for low fre-quency wind oscillation, and should be used for high frequen-cies when data is not available to establish a lower value.

6.2.8 Wind Tunnel Data

Wind pressures and resulting forces may be determinedwith properly executed wind tunnel tests on representativemodels. An example of tests for a semi-submersible is givenin Macha and Reid (1984). Such tests may be suitable for asimilarly shaped TLP.

6.3 CURRENT FORCES

6.3.1 General

The current velocity used in design should be determinedby the means described in 5.4 and should be consistent (as toreturn period) with other design parameters such as waveheight and wind velocity. The joint statistics of current andother environmental events should be considered.

6.3.2 Current Drag

In the absence of wave induced water motions, the dragforce exerted on a bluff cylindrical member by a current isproportional to the square of the current velocity. The dragforce acts in the direction of the component of current that isnormal to the member axis. The drag coefÞcient should bebased on the best empirical data available. Drag force can bedetermined using the following formula:

(13)

Where:FD = drag force (per unit length) normal to the

member, lbsrW = mass density of water, slugs/ft3

CD = drag coefÞcientA = projected area per unit length, ft2

V = current velocity normal to the member axis, ft/sec

6.3.3 Vortex-Induced Vibration

6.3.3.1 In a ßow which is steady, or nearly steady, vorticeswill be shed from a bluff body. As they shed, the vorticesinduce a lift force, i.e., a force normal to the ßow direction.Usually this lift force is of an alternating nature and is ran-domly distributed over the body. The net force on the body isof little or no consequence. If, however, the body is capable ofvibrating and the vortices are being shed at or near one of thebodyÕs natural frequencies, the body may be excited to vibra-tions of signiÞcant amplitude. Currents acting with nearly

ShapeCoefÞcients Object

1.5 Beams1.5 Sides of rectangular sections0.5 Cylindrical sections1.0 Overall projected area of platform

(Should be conÞrmed by model testing)

F12---raCsAVz

2 ra+ cCsVzu¢=

FD12---= rWCD AV 2


uniform speed over a substantial portion of the body are morelikely than waves to excite such vibration.

6.3.3.2 The vortex shedding frequency can be predicted bythe formula:

(14)

Where:S = Strouhal number.f = oscillation frequency (Hertz).

D = member diameter, feet (meters).V = ßow speed normal to member, feet/second

(meters/second).

The Strouhal number varies with Reynolds number, but isusually taken to be 0.2 in the range of Reynolds numbers rel-evant for TLP columns, tendons and risers.

6.3.3.3 The responses of a TLP which are sensitive to vor-tex shedding are strumming of the tendons and risers, andyaw of the hull caused by vortex shedding of the columns.Towers and cylinders on the topsides may also be susceptibleto wind vortex shedding.

6.3.3.4 Vortex-induced vibration is a hydroelastic phenom-enon whose amplitude is not amenable to prediction by con-ventional methods of forced oscillation analysis. Thevibration amplitude has, however, been correlated with a non-dimensional parameter called the reduced damping (GrifÞn,1981). Further information on vortex shedding may be foundin Blevins (1977).

6.4 WAVE FORCES

6.4.1 General

6.4.1.1 The wave spectrum and/or deterministic waveheight and period used in the design should be determined asdiscussed in 5.4. When applied with other environmentalparameters the wave parameters should be consistent withrespect to return period.

6.4.1.2 Two approximate methods for calculating waveforces are commonly used. These are the Diffraction Theoryand the Wave Force Equation (WFE). (Morison, et al., 1950.)Recommendations for wave force theory selection are givenin 6.4.4.

6.4.1.3 The Þrst order calculation methods described in6.4.2 and 6.4.3 provide wave forces at the wave frequency.Higher order wave forces are discussed in 6.4.5.

6.4.2 Diffraction Theory

6.4.2.1 General

6.4.2.1.1 Wave forces are calculated in diffraction theoryby the integration of the total water pressure Þeld acting on abody. The method is appropriate when the body is large rela-tive to the water motion amplitude so that viscous forces arerelatively unimportant, and that the body is sufÞciently largerelative to the wavelength to modify the wave Þeld throughdiffraction and radiation.

6.4.2.1.2 In diffraction theory, the ßuid is described by apotential ßow function which satisÞes the Laplace equationwithin the ßuid domain and satisÞes boundary conditions atthe bodyÕs surface, at the free surface, at the ocean bottom,and at inÞnity. For response calculations, linear steady statesolutions to the potential function provide adequate estimatesfor Þrst order wave forces. In general, the potential isexpressed as the superposition of three different wave sys-tems:

a. The incident wave system.b. The diffracted (or scattered) wave system as if the body isÞxed.c. The radiated wave system generated by the moving bodyin calm water.

The Þrst two wave systems provide the wave excitationforces while the last wave system gives rise to added massand wave damping forces.

6.4.2.1.3 The solution for the potential function is usuallysought on the submerged body surface, and integrals of pres-sure over the body surface give the appropriate forces andmoments acting on the body. Analytical solutions for thepotential function for an arbitrary body are not available, sonumerical techniques are generally used. Discussion of someof the techniques used to solve the complete body diffractionproblem may be found in Faltinsen and Michelson, 1974; andYue, et al., 1978.

6.4.2.1.4 One solution available for large vertical columnsis the potential function for a solitary vertical cylinder extend-ing to the seaßoor (see MacCamy and Fuchs, 1954). Anapproximation to the diffraction forces on a TLP can then bemade by summing the forces on each column.

f SVD----=


6.4.2.1.5 The solution for the wave force per unit length(F) on a cylinder where the wave is deÞned by h = H/2 cos(kx Ð wt) is given by:

F = (2rW gH/k)[cosh k(d+z)/cosh kd] (15)A (kr) cos (kxÐwt+f)

with A(kr) = [J¢12 (kr) + Y¢1

2 (kr)]Ð1/2

and f = tanÐ1 [J¢1 (kr)/Y¢1 (kr)]

Where:H = wave height (feet).k = wave number (=2p/l)(feet-1).r = radius of cylindrical member (feet).d = water depth (feet).g = gravitational acceleration (feet/second2).z = elevation above mean water surface (feet) (sub-

merged members have negative values).x = the distance along the direction of wave propaga-

tion from the coordinate origin to the center lineof the member (feet).

J'1 & Y'1 = derivatives with respect to kr of Bessel functionsof the Þrst and second kinds, respectively.

l = wavelength (feet).w = wave frequency (rad/sec).

rW = mass density of water.

The equation is a linearized solution which is based onwater waves of small steepness incident on a circular cylinderof inÞnite extent. It can be applied to vertical columns withÞnite draft, providing that the member has sufÞcient draft toextend below most of the wave action.

6.4.2.1.6 Pontoons can be treated by means of slenderbody or strip theory. The usual inner expansions of theLaplace equation reduce the problem to 2-D ßow involving apontoon cross section. The 2-D boundary value problem canbe solved without much computational effort compared to the3-D problem. One of the most commonly used techniques isgiven by Frank (1967). The total force acting on a pontooncan be obtained by integrating sectional forces along the lon-gitudinal axis.

6.4.2.1.7 The approximations based on MacCamy andFuchs and Frank are valid as long as the interaction betweenmembers of the body are small. For closely spaced columns,or where spacings are multiples of half wavelengths, interac-tion effects become important. These should be checked witha full 3-D analysis or model testing.

6.4.2.2 Limitations of Diffraction Analysis

6.4.2.2.1 Linear wave diffraction theory is based on theassumption that wave heights and platform excursion ampli-

tudes are small. However, experiments (Chakrabarti, 1971)have shown the method to be valid for vertical cylinders inmoderate-to-steep waves.

6.4.2.2.2 The diffraction solutions do not include viscousforces. For hull shapes composed of large members (greaterthan ten feet diameter) the viscous forces are usually insignif-icant at wave frequencies. At high frequencies viscous forces,along with wave radiation effects, provide damping. At lowfrequencies viscous forces provide both drift forces anddamping. Linearized viscous damping may be added to thediffraction solution.

6.4.3 The Wave Force Equation (WFE)

6.4.3.1 General

6.4.3.1.1 When body members are relatively slender orhave sharp edges, viscous effects may be important and thewave force may be expressed as the sum of a drag force andan inertial force. The wave force equation (WFE) is an empir-ical formula for calculating forces on a member for givenwater velocity and acceleration conditions (Morison, 1950;Ippen, 1966; Newman, 1977, Sarpkaya and Isaacson, 1981).It is based on the assumption that the presence of the memberdoes not appreciably alter the wave form. The wave forceequation given below has been modiÞed to account for thevelocity and acceleration of the structure.

F = Fd + Fi (16)

Where:F = the force vector per unit length acting normal to

the member axis (pounds) and:Fd = drag force vector:

F = (17)

Fi = inertia force vector:

F = (18)

F =

D = diameter of member (feet)CD = drag coefÞcientCA = added mass coefÞcient for body accelerations.

With this formulation, added mass should not beincluded in the mass matrix.

CM = virtual mass coefÞcient (for ßuid accelerations)CM = CA + 1

12---rW CDD u xúÐ u xúÐ( ) lbs( )

p4---rWCAD2 uú xúúÐ( ) p

4---+ rWD2uú lbs( )

p4---rWD2 CMuú CAxúúÐ( ) lbs( )


u = water velocity normal to cylinder axis (feet/second).

= water acceleration normal to cylinder axis (feet/second2).

= velocity of member normal to its axis (feet/second).

= acceleration of member normal to its axis (feet/second2).

6.4.3.1.2 The water velocities and accelerations can be cal-culated from several wave theories.

6.4.3.1.3 The drag and inertia force components are vectorquantities which act in the directions of the normal compo-nents of velocity and acceleration vectors, respectively. Thedrag and mass coefÞcients are empirical coefÞcients whichare generally coupled with a wave kinematics theory. Theyshould be used with the same theory that was used in theirderivation.

6.4.3.1.4 In the situation where current and waves occursimultaneously, prediction of the kinematics can be complex.A simple way is to vectorially combine the water particlevelocities from the contributing wave and current systems.However, if the current is not uniform, then this superpositionis not correct. Vectorial combination is conservative, and isgenerally used as the best available method.

6.4.3.2 Drag Coefficients

The drag coefÞcient is a function of Reynolds number,Keulegan-Carpenter number, roughness, and other factors.

Model tests do not normally cover the appropriate parameterranges. Field tests have been conducted on Þxed offshoreplatforms, but member sizes are not indicative of TLP hullmembers. CoefÞcient determination will require carefulextrapolation of test results. Commonly accepted values arebetween 0.6 and 1.2 (see Sarpkaya and Isaacson, 1981). Val-ues well below 0.6 have been shown to occur for low Keule-gan-Carpenter numbers (Verley and Moe, 1980).

6.4.3.3 Mass Coefficients

6.4.3.3.1 The mass coefÞcients are frequency dependent.Model tests are often the most appropriate way to producesufÞciently accurate estimates of CM. (See 7.8.) Mass coefÞ-cients can be found theoretically by Þrst performing a diffrac-tion analysis and then equating the resulting force to theinertial term of the WFE. This has been done for large verticalcylinders (Meyers, Holm, and McAllister, 1969) using thediffraction solution by MacCamy and Fuchs. When equatingthe two force terms, the inertia coefÞcient yields:

CM = [4l2A(kr)/(p3D2)] cos (wt-a)/cos wt (19)

6.4.3.3.2 The coordinate system used sets the wave zeroupcrossing at t = 0.

6.4.3.3.3 Figure 3 gives values of a and CM as functions ofD/l.

6.4.3.3.4 This formula assumes that there is no interactionbetween columns. For short waves where the vessel dimen-

uú

xú

xúú

0

1.0

2.0

0¡

10¡

20¡

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Member diameter/wave length, D/l

Iner

tia c

oeffi

cien

t, C

M

CM

a

a

Figure 3—Inertia Coefficient Found By Equating the MacCamy-Fuchs Solution to the WFE Inertia Term


sions or column spacings are on the order of half a wave-length, interaction effects may become important. This maybe checked with a 3-D diffraction calculation.

6.4.4 Force Calculation Method Guideline

The basic assumptions associated with each of the theoriesshould be considered when selecting the force calculationmethod. Figure 4 (taken from Pearcey, 1979) provides a gen-eral guideline for applicability of the diffraction and WFEmethods based on the ratio of diameter to wave height.

6.4.5 Other Hydrodynamic Forces

6.4.5.1 Subharmonic and Superharmonic Wave Forces

When second order terms in the potential theory, or whenÞnite wave height kinematics are used with the WFE, then itcan be shown that several different phenomena occur:

a. In regular waves a steady wave drift force is generated inthe horizontal plane. In the case of diffraction theory thissteady force results from free surface integrals and the evalua-tion of the full Bernoulli equation over the body boundary.

From WFE, the steady drift force results from the free surfaceintegral and viscous effects.b. In regular waves, both potential theory and WFE predict asteady vertical force. Additionally, potential theory predicts adouble frequency force in the vertical direction.c. In irregular waves, the potential theory and WFE predict asteady and a slowly varying horizontal force called the wavedrift force. In potential theory this occurs at the difference fre-quencies of the wave energy. A current adds to the drift forcethrough an interaction with the viscous forces resulting fromwave kinematics.d. In irregular waves, both potential theory and WFE predictsum frequency forces, which, when they occur at a pitch-roll-heave resonant frequency, excite springing response. In regu-lar waves, potential theory predicts double frequency forces.

These theories are still in a state of development. Theforces, however, have been shown to be very important, sincesubharmonic frequencies relate to rigid body resonance insurge, sway, and yaw, while superharmonic frequencies relateto the heave, pitch and roll resonance modes. These topics areaddressed in a number of references (Ogilvie, 1964; Eatock-Taylor and Rajagopalan, 1981; Pinkster, 1979; Newman,1974; Faltinsen and Michelson, 1976; Faltinsen and Loken,1979; Pinkster, 1980; Standing, 1982).

0.5 1.0 3 6 10 20 40 100 200

1008060

40

30

20

1086

4

3

2

1.0

90%

Dra

g

10%

Dra

g

Iner

tia

Diffrac

tion

Drag regime

Wav

e he

ight

(fee

t)

Diameter (feet)

Tendonsand risers

Columnsand pontoons

Subcritical

105

Post criticalRe

106

107 108

Figure 4—Wave Force Calculation Method and Guideline


6.4.5.2 Damping

6.4.5.2.1 Damping in resonant modes is important in thecalculation of responses.

6.4.5.2.2 Verley and Moe (1980) have published workinvestigating the drag for very small oscillations of large cyl-inders. Model tests can be used to determine the damping, butthe viscous effects are ReynoldÕs number dependent. Careshould be exercised when estimating full-scale damping val-ues from model test results.

6.4.5.2.3 Special attention should be given to the highReynolds number, low Keulegan-Carpenter number depen-dence of damping for vertical mode resonances.

6.4.5.2.4 The presence of current, waves, or both generallyincreases damping. (cf., Wichers and Huisman, 1984; Simiuand Leigh, 1983.) For severe condition responses, the damp-ing should be estimated for appropriate conditions. Modeltests or time domain calculations are appropriate for thisdetermination.

6.4.5.2.5 The damping in the vertical modes (heave, pitch,and roll) includes contributions from structural and soildamping, as well as from hydrodynamics. These should beestimated with consideration for the mode shapes and thestrains in the various parts of the system.

6.5 ICE LOADS

Superstructure icing can affect tendon tension and increaselocal wind loads due to increased frontal area. Wave inducedmotions of ßoating ice can impose local impact forces whichshould be considered in the design of the structure.

6.6 WAVE IMPACT FORCES

Wave slamming must be evaluated for its local effect onstructural or ßotation members and, if warranted, be includedin the overall solution of the equation of motion.

6.7 EARTHQUAKES

For TLP sites where earthquakes are a concern, appropriateground acceleration time histories should be obtained. ForTLP tendon tension responses, the vertical ground motion ismuch more critical than horizontal ground motion. Additionalguidelines for earthquake ground motion are referred to inAPI Recommended Practice 2A.

6.8 ACCIDENTAL LOADS

The potential for accidental loads arising from variouskinds of collision, dropped or swung objects, or other eventsshould be considered in the design of the structure. Consider-ation should be given to employing active and passive mea-sures in the design to resist or absorb such loads. These

measures could include, but not be limited to, thickening deckplate in areas where material handling is performed, shieldingrisers in the wave zone, or determining the energy absorptioncapacity of the structure and/or mooring system. For the latterconsideration, such absorption capacity should be consistentwith the size and actual speed of vessels working close to theplatform.

6.9 FIRE AND BLAST LOADING

Offshore facilities treating hydrocarbons have a potential,however small, for either Þre or explosion or both. The resultof an explosion from a structural sense is an overpressure thattends to dissipate with distance. Any object in the vicinity ofthis explosion interacts with the overpressure. Fire causes athermal loading on nearby objects which in turn causes bothdeformation and stress. Prolonged thermal loading also canresult in changes in material modulus and yield point. Thedesign of an offshore structure should include a systematictreatment of these potentially adverse loadings.

7 Global Design and Analysis7.1 GENERAL

7.1.1 The purpose of this section is to describe methods forperforming the response analysis of Tension Leg Platforms.Environmental information needed for the analysis is pre-sented in Section 5, the derivation of environmental forcesand moments on the platform is given in Section 6. The cal-culation of platform motions, mooring system loads, andloading on the structure are included in this section. Theresponse analysis described herein is directed, when possible,at a working stress design (WSD) approach. Extremeresponses are calculated to a design return period, and cansubsequently be used in a WSD or load resistance factordesign (LRFD) approach.

7.1.2 The design of a tension leg platform requires theapplication of analysis methods to estimate a number andvariety of responses which are not commonly considered inthe design of conventional fixed offshore structures. Environ-mental forces result in steady and dynamic platform displace-ments and loading on the overall TLP system, see figure 5.

Analysis methods of varying degrees of complexity havebeen adapted from practices developed for design and analy-sis of ships, semisubmersibles, large volume gravity baseplatforms and steel space-frame structures. Solutions may becharacterized as linear or nonlinear, frequency domain ortime domain, and deterministic or probabilistic: these will bediscussed below in the context of the several relevant modesof response. Emphasis is placed on the use of multi-parameterforcing function inputs for a number of the design responses.The designer must work with a relatively extensive environ-mental data set in order to identify events and combinationsof events giving design responses.


7.2 EXTREME RESPONSES

Extreme responses are considered to be those responseswhich govern the design of the TLP. They generally includeoffset, yaw, minimum and maximum tendon tension, anddeck clearance. The design events are those environmentalevents which produce the extreme responses. The environ-mental events are generally associated with various load casesas described in Section 5 to provide complete design cases.

7.2.1 Statistical Analysis

The prediction of extremes for a number of major systemresponses requires the selection of appropriate design eventsand the use of analytical tools discussed in 7.7. The methodsdescribed are based on a deterministic analysis whenever pos-sible and are intended to be used as part of a working stressdesign. A probabilistic analysis combined with reliabilitybased design methods (see Moses, 1983 and Leverette, 1982)may provide a more versatile method for developing an opti-mum design, but requires extensive analysis. The main difÞ-culty in using a deterministic approach is identifyingappropriate design events. The speciÞcation of these events isdependent on the system design, and therefore should bedone as part of the preliminary design. Because each responseis somewhat different, this section includes separate com-ments on each major system response.

The different approaches to calculating design responsesare given schematically in Figure 6. The alternate paths are

similar in that all involve the two components of 1) responseanalysis of the system in the environment, and 2) probabilisticanalysis of the extremes. The differences lie in the order inwhich these operations are performed, and in the amount ofcomputation involved.

7.2.1.1 Deterministic Response Analysis

See path ÒCÓ in Figure 6. Deterministic design analysisinvolves performing statistical analysis on the environment asthe Þrst step. A suitable extreme event is identiÞed which isexpected to produce the most severe response. The secondstep is the actual calculation of the response to this event.This method minimizes the amount of work involved in cal-culating system responses. The combination of maximumwave, maximum wind, maximum storm current, and maxi-mum tide does not necessarily produce the worst design casefor all parameters of interest and the same event does not pro-vide the extremes for all of the major design responses.

7.2.1.2 Probabilistic Analysis

See path ÒAÓ in Figure 6. A full probabilistic analysisinvolves calculating responses to the entire suite of possibleenvironmental conditions. Statistical analysis of theseresponses is then performed in order to predict a suitableextreme for each response. This requires a much more exten-sive set of environmental data.

7.2.1.3 Semi-Probabilistic Analysis

7.2.1.3.1 See path ÒBÓ in Figure 6. The deterministic pro-cedure is often inadequate in the proper speciÞcation ofdesign cases. A full probabilistic analysis requires more datathan is commonly available and requires more analysis casesthan are generally feasible. An intermediate approach is asemi-probabilistic method. This path involves some reductionin the environmental data to identify the generally more

Surge(platformnorth)

Roll

Incident waves

Heave

Yaw

Pitch

Sway

C.G.

b

Figure 5—TLP Motion Nomenclature

Ensemble oflifetime responses

ExtremeResponse

Extreme stormresponses

Fieldenvironment

Extremestorm

Extremewave

Statistical analysis

Res

pons

e an

alys

is

A B C

Figure 6—Design Analysis Paths


severe conditions and combinations of events. The systemresponses to these are calculated and these are furtherreduced to predict the extremes.

There are a number of advantages to this compromise:

a. The number of analysis cases are kept within reason.b. The requirements are within the bounds of typical dataavailability.c. The designer assists in the process of determining combi-nations of events producing extreme responses.

7.2.1.3.2 Through the course of a design, various pathsthrough Figure 6 will be followed. In conceptual design, adeterministic analysis is usually used for initial sizing. Duringthe Þnal design, a semi-probabilistic analysis may be used topredict the maximum global responses. Once the extremeevents are identiÞed, a deterministic analysis may be used forlocal structure design and scantling sizing. Following com-pletion of the design, a probabilistic analysis may be used forrisk analysis and design veriÞcation.

7.2.2 Environmental Parameters

7.2.2.1 Environmental parameters important to responseare:

a. Wind:1. Mean wind speed.2. Mean wind direction.3. Wind power spectral density function.

b. Wave:1. SigniÞcant wave height.2. Mean wave period.3. Wave amplitude spectral density function.4. Mean wave direction.5. Wave directional spreading function.

c. Current:1. Surface current speed.2. Surface current direction.3. Current proÞle (speed and direction).

d. Tide:1. Astronomical tide.2. Storm surge.

7.2.2.2 The number of parameters involved cause the pri-mary shortcoming of deterministic analysis. While more rigidstructures are typically dominated by wave forces, responsesof a TLP are also strongly affected by wind, tide/storm surge,higher order wave drift forces, currents, and wind dynamics.Simultaneous occurrence of independent extremes for each ofthe above parameters produces results which range fromoverly conservative for some responses to marginally non-conservative for others. In order to produce a reasonable andsafe design, the designer must evaluate and specify the designevent or condition for each response.

7.2.3 Maximum Offset

The prediction of maximum horizontal excursion is impor-tant for analysis of riser and tendon systems and for speciÞca-tion of riser and tendon hardware. The maximum offset alsopartially governs the deck height requirement because ofplatform setdown with offset. Analysis of the horizontalmotions (surge, sway, and yaw) is often termed station-keep-ing analysis.

The designer may consider directional distributions ofenvironmental conditions. The assumption of colinear wind,wave, and current is appropriate in the absence of data. Ifenough information is available, use of appropriate non-colin-ear environments may be used and can result in a more accu-rate estimate of maximum offset.

7.2.3.1 Environmental Forces Contributing to Offset

The environmental forces of interest can be classiÞed bytheir frequency content as 1) steady or mean forces, 2) wavefrequency forces, and 3) low frequency forces.

The steady or mean forces result in a mean offset of theplatform. The low frequency forces excite motion which isbelow wave frequencies and predominately at frequenciesnear the surge, sway, or yaw natural frequencies of the plat-form. The wave frequency forces result in wave frequencysurge, and sway motions.

7.2.3.1.1 Mean or Steady Forces

The following steady forces should be considered:

a. Mean wind forcesÑSee 6.2. The wind speed used shouldbe an average wind speed for an extended period (on theorder of one hour) based on the appropriate environmentalconditions (return period).b. Current forceÑSee 6.3.2. The drag contribution due totendons and risers should be included in estimating the totalforce.c. Steady wave driftÑSteady wave drift forces result fromwave diffraction and from second order viscous effects. Theinteraction between waves and current can also result insteady forces which must be considered. See 6.4.3 for a dis-cussion of these forces.

7.2.3.1.2 Wave Frequency Forces

The wave frequency forces are dominated by Þrst orderdrag and inertial wave loads on the platform. These forces arediscussed in 6.4 (Wave Forces).

7.2.3.1.3 Low Frequency Forces

The following forces contribute signiÞcantly to low fre-quency motion and should be considered:


a. Wind forcesÑThe wind gust spectrum (see 5.4 and 6.2.3)should be used to model the wind speed variation from theaverage wind speed.

The mean wind speed which is used as a parameter in mostgust spectrum models should be the same as the wind speedused to estimate the mean wind force. Simiu and Leigh(1983) discuss the calculation of time varying forces on theplatform from the wind gust spectrum.b. Wave/current forcesÑA force spectrum for slowly vary-ing (low frequency) wave drift forces should be estimated forthe appropriate wave conditions.

Pinkster (1980) gives a method to estimate the force spec-trum from second order potential wave drift given the steadywave drift forces vs. wave frequency and the appropriatewave spectrum. It may also be appropriate to consider contri-butions from viscous wave drift and wave/current interac-tions. Burns (1983) gives an approximate method to estimatethe effects of wave/current interactions.

7.2.3.2 Offset Components

7.2.3.2.1 The TLP horizontal motions can be convenientlydivided into three contributions corresponding to the classiÞ-cation of forces discussed above:

a. Mean offsetÑThe mean vessel offset can be estimated bydeveloping a restoring force vs. offset function and applyingthe estimated mean force to give an estimate of mean offset.The restoring force function depends on setdown, tendonstretch, catenary effects, and the effect of time varying tendontension.b. Wave frequency motionsÑThe wave frequency horizontalmotions can be modeled by using either the time or frequencydomain techniques discussed in 7.6.3 and 7.6.4.c. Low frequency motionsÑThe low frequency motions canalso be modeled using either time or frequency domain tech-niques.

7.2.3.2.2 For frequency domain modeling, the force spec-trum for wave drift (or total low frequency wave forces) canbe combined with the wind force spectrum to give a total lowfrequency force spectrum (see 6.2.3 and 6.4.5). Assumingthat the low frequency wind and wave forces are independent,the total low frequency force spectrum is given by the sum ofthe low frequency wave force spectrum and the low fre-quency wind force spectrum.

7.2.3.2.3 This total force spectrum can then be multipliedby the appropriate motion transfer function to obtain thedesired horizontal motion. Time domain simulation can alsobe used to obtain the low frequency motion. However,because the low frequency motions are dominated by thesurge, sway and yaw resonances with natural periods on theorder of 60 to 150 seconds, a long time simulation is requiredto include a signiÞcant number of cycles. Modeling of thetime series should be done carefully. The use of discrete fre-

quencies is discussed by Tucker et al., 1984. For time simula-tion a time history of low frequency forces is generated fromwind and wave force spectra or directly from wind velocityand wave proÞle time histories.

7.2.3.2.4 Because low frequency motion is dominated by aresonant response, the estimation of damping is important.Further discussion of damping is provided in 6.4.5.2, 7.6.3.5,and 7.6.4.5.

7.2.3.3 Estimating Extreme Offset for Design

7.2.3.3.1 Due to the contribution of the low frequency res-onance, the resulting motion (combined low and wave fre-quency motion) is broad banded. Figure 7 shows an examplemotion spectrum for a TLP in an extreme storm.

7.2.3.3.2 The estimation of the extreme value of offsetshould consider the wide band nature of the process and bebased on wide band statistical methods such as thosedescribed in A.Comm.7.7.

7.2.3.3.3 It is possible to estimate extreme offset by per-forming full length time domain simulations. However, inmany cases the simulation times and number of simulationsrequired will be prohibitive. A frequency domain approach oran approach based on estimating spectral parameters from ashort time domain simulation is more tractable than fulllength time simulations.

7.2.4 Maximum Yaw

The prediction of maximum yaw is important for predict-ing the maximum rotation of riser and tendon top termina-tions and for the yaw contribution to horizontal excursions.The prediction of maximum yaw is similar to predicting max-imum offsets. The methods of 7.2.3 apply to yaw predictionexcept that moments on the vessel must be modeled insteadof forces. This section will discuss only considerations spe-ciÞc to yaw which are not identiÞed in 7.2.3.

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

00 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Sur

ge e

nerg

y de

nsity

(th

ousa

nds)

Frequency (hertz)

Figure 7—Surge Motion Spectrum


7.2.4.1 Environmental Forces Contributing to Yaw

7.2.4.1.1 In modeling the environmental forces contribut-ing to yaw, the designer must consider the likely conditionsand environmental directions which would result in extremeyaw. For example, the wind directions resulting in the great-est steady and oscillating moments on the vessel should beconsidered. Modeling of wind induced moments is particu-larly difÞcult. Consideration should be given to momentsinduced by spatial variability.

7.2.4.1.2 Non-central center of gravity locations will tendto increase yaw responses to waves. Any likely loading condi-tions such as non-symmetric placement of drilling rigs ornon-symmetrical riser installations should also be considered.Checking of various wave headings may be required becausesymmetric wave loadings will minimize the yaw response towaves. Multi-directional seas tend to excite yaw more thanuni-directional seas.

7.2.4.1.3 Another source of yaw excitation can be vortexshedding from the columns in a steady current. Resonant cou-pling between the vortex shedding and yaw motion has beenshown to occur in model tests, and should be checked for inhigh current areas. (See 6.3.3.)

7.2.4.2 Estimating Yaw Response

The yaw response can be estimated using the same tech-niques outlined for offset in 7.2.3.3. The modeling of damp-ing is also important in estimating yaw response.

7.2.5 Maximum Tendon Tension

7.2.5.1 Tendon loads arise from pretension and environ-mental effects. The nominal pretension is selected in order tocontrol minimum tension, or to limit maximum offset. Thefollowing components should be considered for maximumtendon tension determination:

a. Quasi-static:1. ToÑDesign pretension at mean water level.2. TtÑTide/storm surge water level variation loads.3. TlÑLoad and ballast condition/weight variations/design margin.4. TmÑTension due to overturning moment from windand current forces.5. TsÑTension caused by setdown due to static andslowly varying offset (wind, wave drift, and current).

b. Wave induced tension:1. TwÑTension variation from wave forces and waveinduced vessel motion about the mean offset (includingany coupled tendon responses).2. TfÑLoads from foundation mispositioning and theinstantaneous offset.

3. TrÑLoading due to heave, pitch, and roll oscillations attheir natural frequency (ringing and springing, includingpossible underdeck slamming loads).

c. Individual tendon effects:1. TiÑIndividual tendon load sharing differential (one ofa group of tendons generally carries a greater share of theload because of anchor template rotational tolerances, ves-sel yaw, initial setting tolerances, etc.).2. TvÑTension induced by vortex shedding responses ofan individual tendon.

7.2.5.2 Maximum tendon tension may be estimated using alinear superposition incorporating the effects discussedabove:

Tmax = To + Tt + Ti + Tm + Ts + Tw + Tf + Tr + Ti + Tv (20)

7.2.5.3 Figure 8 illustrates the superposition of these ten-sion contributions. This approach is adequate if a maximumdesign condition can be determined which accounts for thestatistical characteristics of the random sea condition. Thevalue of Tmax should correspond to the maximum expectedtension over the design return period. This should be deter-mined from a statistical evaluation of the components ofEquation 20, including their joint probabilities of occurrence.

7.2.5.4 When calculating the maximum stresses in the ten-dons, there are other stresses and loads which sometimeshave to be included. For example, if the tendons contain ßuidat a temperature other than seawater temperature, then ther-mal stresses should be considered.

7.2.5.5 If all of the terms in Equation 20 are taken to beextreme or maximum values, the resulting estimate of tensionwill be very conservative, with a much lower probability ofoccurrence than the return period of any one of the terms. Inorder to calculate a reasonable design value for maximum

Foundation mispositioning

Wave

WindOffset

Tide/surgePretension

Ten

don

tens

ion

Maximum Tendon TensionUp Wave Leg

Figure 8—Maximum Tendon Tension Up Wave Leg


tension, the joint statistics of the driving functions should beconsidered. The joint statistics of wind, wave, current, andtide/surge can be used with the vessel response functions toestimate a tension corresponding to the design return period.See Leverette, 1982.

7.2.5.6 An alternate method to the superposition formulagiven in Equation 20 is to perform a full dynamic analysis ofthe vessel to wave, wave drift, and wind dynamic forces andperform a statistical analysis of the tension responses. Thiscan be done in time or frequency domains. Care must betaken to properly model the correlation of the various inputparameters. This procedure is a more rigorous way to predictthe tension response and includes the joint statistics as well asthe system response to joint events. This method is recom-mended if the data and analysis capabilities are available.

7.2.6 Minimum Tendon Tension

7.2.6.1 Like maximum tension, minimum tension is deter-mined by linear superposition of pretension and environmen-tal effects:

Tmin = To Ð [Tt + Tl + Tm Ð Ts + Tw + Tf + Tr + Ti] (21)

7.2.6.2 The minimum tension calculation is illustrated inFigure 9. Although the form of this equation is similar toEquation 20, the values of the terms may be different. In par-ticular, the loading condition and conÞguration correspond-ing to the most probable minimum tension conditions aredifferent than those corresponding to the maximum tensionconditions. For example, the positive tide and surge term willbe different from the negative term, and the minimum tensionwill generally come from the minimum rather than maximumslowly varying offset which is associated with an extremewave event.

7.2.6.3 For negatively buoyant tendons, the minimum ten-sion occurs near the lower tendon connection, hence the ten-don weight (in water) needs to be subtracted from the

minimum tension at the upper ßex element. Otherwise, thetension components may be evaluated by a method similar tothose described above for maximum tensions. Note that theterm representing vortex shedding due to wave and currentaction need not be considered for minimum tension calcula-tions.

7.2.7 Tendon Angle

7.2.7.1 Maximum tendon angle at the upper and lower ßexassemblies is closely tied to maximum surge and yaw, withthe addition of any tendon motion effects. Accuracy of fabri-cation and of foundation installation should also be consid-ered. The maximum value is used for design of the ßexassembly, for hull and foundation clearance allowance, andfor calculating bending stresses in the tendon.

7.2.7.2 The maximum angles may be calculated using themaximum surge and yaw calculation methods together to pre-dict a maximum excursion for the upper ßex joint. This canbe used as input to an analysis of the tendon motion response(see Section 9) which provides the angle responses. Due con-sideration of the consequences of exceeding the predictedextreme should be included when calculating the response(see discussion of risk parameter in 7.7).

7.2.7.3 In addition to maximum angle, the ßex jointdesigners generally need the envelope of load range and anglefor completing the ßex joint design. Minimum load at highangles is likely to be of as much concern as maximum load.Joint statistics of wind, wave, and current are needed to prop-erly estimate the load/angle envelope. Frequency domainmethods can be used if phase information between offset andtension is retained. Time domain and model test methods arealso suitable.

7.2.8 Deck Clearance

7.2.8.1 The minimum clearance between the deck and awave crest is an important parameter in the design. The deckclearance has an effect on the vertical position of the center ofgravity and in turn on the maximum and minimum tendontensions. The deck elevation also affects the wind load andwind overturning moments. In general, a higher deck hasadverse effects on tendon tension responses. However, largetendon tension variations may result if the deck is too low andwaves strike the lower deck.

7.2.8.2 In determining the deck clearance, one should con-sider:

a. Wave crest elevation.b. Storm and astronomical tides.c. Platform setdown due to platform offset and yaw.d. Local wave modiÞcations due to the presence of the hull.e. Dynamic vertical motion of platform.

Foundationmispositioning

Wave

Wind

Offset

Tide/surge

Pretension

Ten

don

tens

ion

Minimum Tendon TensionDown Wave Leg

Figure 9—Minimum Tendon Tension Down Wave Leg


f. Air gap.g. Phase angle between wave crest and platform position.

7.2.8.3 The designer has two general options: Provide aminimum deck clearance, or allow for wave impact on por-tions of the deck, hull, or lower appurtenances.

7.2.8.4 In the second option, the designer must have conÞ-dence in the accuracy of the wave crest elevation and the con-tact should only be localized. The deck bottom should bedesigned for the anticipated local and global wave slammingforces. The effect of wave slamming on tendon tensions andthe hull heave and pitch accelerations should be considered.

7.2.8.5 Members subjected to wave run up should bedesigned for the associated forces.

7.2.9 Lateral Accelerations

The lateral accelerations of the platform are used in thedesign of the structure and in the design of equipment andequipment supports. In addition to the in-service conditionother conditions during fabrication, transportation, and instal-lation phases of the platform life should be considered. Theresponse of interest is generally the extreme horizontal accel-eration, although operating condition accelerations may beimportant in the design of process equipment and deck drain-age systems.

7.2.9.1 Contributing Factors

Global lateral accelerations are governed by the dynamichorizontal offset forces acting on the platform. These aredominated by Þrst order wave forces, but may also includewind and wave drift components. Local lateral accelerationsare also produced by the rotational responses of pitch, rolland yaw.

7.2.9.2 Estimating Lateral Accelerations

The wave frequency motions and accelerations are esti-mated by frequency or time domain solutions to the equationsof motion or by model tests. Deterministic or short term prob-abilistic methods can be used to predict the maximum wavefrequency response.

7.3 RESPONSES FOR FATIGUE ANALYSIS

7.3.1 General

This section provides guidance in the prediction of theresponse histories for fatigue calculations. The prediction offatigue life or damage is by nature a statistical procedure. Inthe past, deterministic analysis with high safety factors havebeen the norm. However, numerous investigators are develop-ing more accurate probabilistic approaches (see Wirsching,1983). These methods inherently furnish the designer with

more information on the fatigue properties, but the probabi-listic approach requires more input data than is often avail-able. The three methods following are in order of increasingcomplexity and data requirements.

7.3.1.1 Method 1—Discrete Wave Height and Period

The wave height/period joint statistics are operated on byappropriate transfer functions which yield force and motionstatistics that are period dependent. Because of the sensitivityto wave period, traditional methods using a mean period foreach wave height are not applicable. Wave drift, wind, andcurrent response statistics can be estimated using similarmethods.

7.3.1.2 Method 2—Frequency Domain

The wave spectrum statistics are operated on by appropri-ate response amplitude operators (RAOs) which yield theforce and motion spectra statistics.

The response statistics to wave drift force, current, wind,and platform loadings are estimated using Method 1. TheRAOs can be generated with a regular sea or random sea andwith frequency, or time domain simulations. Regular sea orrandom sea model tests can also be used.

7.3.1.3 Method 3—Random Time Domain

A simulation of the response to the environment is per-formed for an extended period of time. The response statisticsare generated from this time history.

7.3.2 Tendon and Foundation Loads

The tendon and foundation loads can be calculated usingone of the above methods; however, the results should bemodiÞed to include the following effects, if present:

a. Ringing and springing (see 6.4.5 and 7.2.5).b. Wave slamming force on the deck, if this will be permitted(see 7.4.5).c. Effect on the tension RAO of the setdown caused by surge,sway and yaw, the tides and the platform loading.

7.3.3 Surge Offset/Tendon Flexjoint

The offset/tendon angle can be calculated using one of theabove methods. The effect of the tide, storm surge, and plat-form loading on the offset should be considered (see 7.2.3).

7.3.4 Hull Forces

The hull forces (see 7.4) can be calculated using one of theabove methods. Forces generated by tendon ringing and waveslamming should be considered.


7.4 HYDRODYNAMIC LOADS FOR HULL DESIGN

7.4.1 General

7.4.1.1 Local loadings are deÞned as those loads which arelocally applied to the platform for the detailed design of indi-vidual members and nodes. The loads produced for this pur-pose are used as input to the procedures deÞned in Section 8.Local loading can be derived either from time domain analy-sis at various phases within the steady state wave cycle orfrom frequency domain analysis by obtaining maximumloads at a prescribed wave frequency. Loads may be resolvedat discrete nodes or may be left as pressures acting over sec-tions of structural members.

7.4.1.2 Environmental conditions for deriving local load-ings are given in Section 5. Not all of these conditions need tobe investigated when determining loads for structural design.Conditions which are important are related to hydrostaticpressures on submerged members, squeezing, prying andracking of the hull frame, wave slamming, or extreme dragsuch as high current or towing conditions.

7.4.1.3 Local loads relating to equipment arrangement,deck mating, and internal member pressures are given else-where in this recommended practice.

7.4.2 Hydrostatic Head

7.4.2.1 Hydrostatic head on each submerged structuralmember is determined from a combination of submergenceand tide. Loads are applied statically at various elevationsalong each memberÕs length. Pressures are determined by theequation:

p = rW g(z + t) (22)

Where:p = local pressure.g = acceleration of gravity.

rW = unit mass of seawater.z = vertical distance from mean water level (MWL)

to point of applied pressure (positive down).t = height of tide above MWL.

7.4.2.2 Additional local pressure is applied to submergedmembers in the presence of waves and current. This pressureshould be consistent with the force applied to the platform bywaves and current and will vary with the sophistication of theforce model.

7.4.3 Squeeze-Pry Racking Loads

7.4.3.1 Major joints and braces of the hull are affected bysqueezing and prying action of the hull. A squeeze conditionis deÞned as one where lateral loads from a wave are maxi-mum inward toward the center of the platform. A prying con-

dition occurs when lateral loads act outward away from theplatform center.

7.4.3.2 The maximum squeezing and prying loads gener-ally occur in waves that are twice the length of the platform.A deÞnition of maximum wave height as a function of wavelength or period permits a rapid scan to identify the designcondition for this response. This wave will not generally bethe design case for all aspects of the structure. Load condi-tions with head, quartering, and beam seas should bechecked.

7.4.4 Maximum Wave Loads

In addition to local loads which affect the global frame ofthe platform, individual member designs may be governed byloads resulting from extreme wave heights. Pressures on indi-vidual members should be calculated for the extreme wavesas determined by the design wave spectra.

7.4.5 Wave Slamming Loads

Allowance should be made for members subject to waveslam. These members include columns, vertical braces, risers,and possibly members forming the underside of the deck. Ifappropriate, waveborne ice impacts should be considered.

7.4.6 Towing Loads

During tow to the installation site, the platform will be sub-jected to towing loads. Depending upon the location andlength of the tow, certain members may experience loadswhich exceed the inplace design loads.

7.4.7 Construction and Installation Loads

Loading conditions during construction and installationshould be considered.

7.5 STATIC AND MEAN RESPONSE ANALYSIS

7.5.1 General

Static and mean response analysis consists of determiningthe static equilibrium with no wind, wave, or current present,and then determining a mean position due to steady environ-mental loads acting on the platform. The determination of amean or equilibrium position is necessary to proceed with afrequency domain dynamic analysis. Similarly, an initial con-dition is needed for a time domain dynamic analysis.

7.5.2 Determination

7.5.2.1 Static Equilibrium in Still Water Condition

7.5.2.1.1 The determination of the static equilibrium (orÒweight balanceÓ) with the Òstill waterÓ condition, is funda-mental to sizing of the TLP and is the starting point for fur-


ther analysis. A static equilibrium analysis should beperformed for each loading condition to be analyzed (see8.3.3).

7.5.2.1.2 Determination of the static equilibrium shouldinclude the following:

a. The total platform weight associated with each loadingcondition to be analyzed.b. The total platform displacement (the total platform buoy-ancy) for each draft to be analyzed.c. All riser and tendon tensions acting on the platform ineach loading condition to be analyzed.d. All hook loads which are signiÞcant for the loading casesto be analyzed.

7.5.2.1.3 The platform weight should include the weight ofall structural elements, permanent appurtenances, and allequipment permanently mounted on the platform. In additionthe platform weight should include all weights which areappropriate to the loading condition to be analyzed. Thesetemporary loads should include the weight of equipment, con-sumable supplies, ballast, marine growth or ice on the struc-ture, and any other temporary weights which are appropriatefor the loading case being analyzed. Note that the variousloading cases (identiÞed in 5.5.2, 8.3.3, and 9.3.2) mayinvolve signiÞcant variations in temporary or removableweights and loads to be included in static equilibrium analysis.

7.5.2.1.4 A general representation of the vertical force bal-ance of the TLP in static equilibrium is given by:

B = WDS + WHS + WDP + WHP +WB + PR + PT +WM (23)

Where:B = platform buoyancy (total buoyancy of the plat-

form for a given draft).WDS = weight of deck structure.WHS = weight of hull structure.WDP = weight of all equipment in or on deck including

production equipment, drilling equipment, utili-ties equipment, marine equipment, consumables,stored liquids, quarters, lifesaving equipment,and future growth.

WHP = weight of all equipment and stored liquids in thehull.

WB = weight of ballast in platform.PR = riser pretension (at the top of riser, where

attached to platform).PT = tendon pretension (at the top of the tendon. where

attached to the platform).WM = any other weight appropriate for the loading case

considered including, if appropriate, ice loading,marine growth, and any signiÞcant hook loads.

7.5.2.1.5 A similar balance should be performed for otherdegrees of freedom of the platform. Because of the weightsensitivity of the tension leg platform, it is important that thevarious weight components be estimated as accurately as pos-sible. It is normal to include a weight margin which is consis-tent with the conÞdence bounds of the estimate. Since futurepayload cannot be increased without modifying the hull dis-placement, this helps minimize the number of design itera-tions which will be required. See 4.6 for further comments onweight estimating.

7.5.2.2 Tidal Effects

7.5.2.2.1 Changes in buoyancy due to tidal effects can sig-niÞcantly affect mean tendon tensions. Therefore, the choiceof a tide condition for static equilibrium analysis is important.Changes in tide conditions should be considered in evaluatingthe various maximum responses of interest:

a. A high mean water level tends to increase maximum ten-don tensions, hydrostatic loading on the hull, and currentloading on the hull, and tends to decrease deck clearance.b. A low mean water level tends to decrease minimum ten-don tensions and to decrease the horizontal restoring forcesfor a given horizontal offset.

7.5.2.2.2 These effects of tide may be taken into accountby performing a static balance at the various appropriate tidelevels to provide a starting point for further analysis or bymaking allowances for the appropriate tide level in calculatingextreme responses. For example, the effect of the highest tidelevel consistent with the probability of simultaneous occur-rence of other extreme environmental conditions should betaken into account in estimating maximum tendon tensions.

7.5.3 Mean Response Analysis

7.5.3.1 The analysis of the TLP response to mean or steadyenvironmental forces may be used to determine the initialcondition for time domain analysis of the platform dynamicresponse or for frequency domain analysis of the platformdynamics. The dynamic analysis of risers or tendons oftenrequires an estimate of mean platform position as input forfurther analysis.

7.5.3.2 The estimate of mean response should begin withthe still water condition discussed in the previous section.Then the following effects should be added:

a. Tendon effects including pretension, tendon weight inwater (catenary effect), foundation mispositioning, and plat-form ÒsetdownÓ effects. Platform setdown increases tendontension as the platformÕs horizontal displacement increases.In some cases tendon stretch is important and should beincluded in modeling tendon effects.b. The mean forces and moments acting on the platform dueto wind.


c. The mean forces and moments acting on the platform dueto wave drift and current forces.d. The effect of current forces on the risers and tendons.

7.5.3.3 Note that an analysis of mean response will berequired for various loading cases (identiÞed in 8.3.3 and9.3.2). Weight variations and tidal variations might be impor-tant in addition to changes in wind, wave, and current forces,and should be included in the loading cases used for analysisof mean response.

7.5.3.4 The steady offset and setdown are important out-puts from analysis of mean response. Figure 10 illustrates thenonlinear behavior of the horizontal restoring forces versushorizontal offset for a typical TLP.

7.5.3.5 If the wind, wave, and current directions areassumed colinear, and incident on an axis of symmetry of thevessel, the mean response analysis can be done consideringresponses only in the direction of the environment. In suchcases, three degrees of freedom are eliminated from the plat-form response and only platform heave, surge, and pitch in thedirection of the environment need be considered. This simpli-Þcation is often used as a preliminary design calculation.

7.6 EQUATIONS OF MOTION AND SOLUTIONS

7.6.1 General

7.6.1.1 This section describes the equations of motion gov-erning the dynamic response and discusses the frequencydomain and time domain techniques used to solve these equa-tions.

7.6.1.2 Frequency domain analysis is the closed form solu-tion of ordinary or partial differential equations by means ofLaplace or Fourier transform techniques. Frequency domainanalysis has been applied extensively to problems of ßoatingvessel dynamics. Frequency domain techniques can estimateresponse to random waves including platform motions and

accelerations, tendon loads and angles, and hydrodynamicloads.

7.6.1.3 The most signiÞcant limitation of frequencydomain techniques is that all nonlinearities in the equations ofmotion must be ignored or replaced by linear approximations.In cases where both time and frequency domain techniquesare applicable, the frequency domain often has the advantageof simplifying the computations, so is used for preliminarydesign. In addition, the frequency domain input and output isoften more convenient and useful for the designer.

7.6.1.4 Time domain analysis is the direct numerical inte-gration of the equations of motion allowing the inclusion ofall system nonlinearities. For example, ßuid drag forces pro-portional to velocity squared can be incorporated withoutprior linearization. This advantage over the frequency domaintechnique is gained at the expense of increased computingtime and increased complexity in calculation results.

7.6.1.5 The equations of motion are the same no matterwhich solution technique is adopted, but their formulationmust reßect the limitations and strengths of the selectedtechnique.

7.6.2 Equations of Motion

7.6.2.1 The forces acting on the system are generally afunction of both time and vessel position. In order to try toseparate the forces into separate terms to allow simple solu-tion, a number of assumptions and linearizations are usuallymade. The expression generally used to describe the motionis given by:

(24)

Where:M(t) = inertial mass matrix.N(t) = damping matrix.K(t) = stiffness matrix.

x = system displacement vector.= system velocity vector.= system acceleration vector.

7.6.2.2 A common assumption is to limit the system modelto a rigid platform and exclude riser and tendon displace-ments. The system then has the 6 degrees of freedomdescribed by Figure 11. The Þxed coordinates are coincidentwith the principal directions of the platform when the plat-form is at rest. The assumption of no interaction between ten-don and riser dynamic response and the platform dynamicresponse leads to the label of Òuncoupled analysisÓ for thissimple case.

7.6.2.3 A convenient coordinate system is a Þxed righthanded coordinate system with origin at the mean position ofthe center of gravity of the platform. For the simple model

Res

torin

g fo

rce

Offset

Figure 10—Restoring Force With Offset

M t( )xúú N t( )xú K t( )x F(x,t )=+ +

xúxúú


described in Figure 11, the position vector coordinate isgiven by:

7.6.2.4 Models with more than 6 degrees of freedom can bedeveloped by including displacement degrees of freedom onthe risers and tendons. Such models couple the analysis ofplatform and tendons/risers. These models are useful for deepwater or when tendon/riser masses are a signiÞcant portion ofthe total system mass. These are referred to as Òcoupled analy-ses.Ó Some analysts have also relaxed the assumption of a rigidplatform and added degrees of freedom to the platform model.

7.6.2.5 The mass matrix include the mass of the platformsteel, equipment and variable loads, and the Òadded massÓ ofthe surrounding water. Inertia forces introduced by the ten-dons and risers should be accounted for in the mass matrix inan approximate fashion for 6 degrees of freedom (DOF) mod-els, or more exactly in coupled tendon/riser/platform analyseswith a large number of DOFs.

7.6.2.6 The damping matrix is important in limiting plat-form resonant responses and has signiÞcant contributionsfrom platform wave radiation and drag on the hull, tendons,and risers.

7.6.2.7 The stiffness matrix contains hydrostatic terms forthe platform, geometric terms due to tendon/riser tensioncombined with platform offset to produce forces restoringoffset, and elastic terms introduced by tendon and foundationßexibility.

7.6.2.8 The time-dependent force vector includes the manyexternal forces discussed in Section 6. These forces are oftencategorized by their frequency relative to the resonant fre-quencies of the platform/tendon/riser system:

a. Nearly steady forces that can be considered static becausethey vary at frequencies much lower than any platform reso-nant frequencies.

b. Slowly varying forces near the surge, sway and yaw natu-ral frequencies. These responses typically have periods in therange of 1 to 4 minutes.

c. Forces at wave frequencies.

d. Forces at frequencies near the heave, pitch and roll naturalfrequencies. These resonances typically have periods in therange of 1 to 5 seconds.

7.6.3 Frequency Domain Modeling and Solutions.

7.6.3.1 General

7.6.3.1.1 Frequency domain analysis refers to the solutionof the equations of motion by methods of harmonic analysisor methods of Laplace and Fourier transforms. The result of afrequency domain analysis is a description of the variables ofinterest (platform motions, platform forces, tendon forces,etc.) as functions of frequency. The method is naturally suitedto the analysis of systems exposed to random environmentsbecause it provides a clear and direct relationship between thespectrum of the environmental loads (see 5.4.3) and the spec-trum of the system response. The system response spectrumcan then be used to estimate the short term statistics of thevariable of interest.

7.6.3.1.2 In cases where both time and frequency domaintechniques can be considered, the frequency domain often hasthe advantage of fewer and simpler computations. For largeßoating structures where wave scattering and radiation isimportant, the inviscid hydrodynamic properties are mostconveniently calculated in the frequency domain. Frequencydomain analysis has been applied extensively to problems ofßoating vessel dynamics, including analysis of both motionsand structural forces (see Price and Bishop, 1974, and Salve-sen et al., 1970). It has been applied to TLP analysis as well(see Botelho et al., 1984).

X1 Surge(platformnorth)

X4 Roll

Incident waves

X3 Heave

X6 Yaw

X5 Pitch

X2 Sway

C.G.

b

Figure 11—Simple Model For TLP Response Analysis

x

x1 surge

x2 sway

x3 heave

x4 roll

x5 pitch

x6 yawîïïïïíïïïïì

=


7.6.3.1.3 The most developed and widely used frequencydomain solution techniques require linear equations ofmotion. The linear assumption is also predominant in the ran-dom process theory used to interpret the solution. This isinconvenient when modeling velocity squared drag loads,time varying geometry, horizontal restoring forces and vari-able water surface elevation, since these effects are nonlinear.In most cases, these nonlinearities can be satisfactorily linear-ized. The linearization may take the form of linearizing aboutsome operating point or an equivalent linearization technique.

7.6.3.1.4 This section includes the analysis of a generallinear system in the frequency domain. Discussions of themodeling of the hydrodynamic loads, tendons, and low fre-quency loads are also presented.

7.6.3.2 Linear System Solution

7.6.3.2.1 The motions of a TLP can be modeled by thesix degree of freedom set of differential equations given inEquation 24. Using the technique of harmonic analysis, theload vector is assumed to be a sinusoidal function with fre-quency w:

F = F1 cos (wt) + F2 sin (wt) (25)

7.6.3.2.2 For simplicity it is assumed that there is only onesource of the external load. This is not a restriction, since for alinear system the solutions for each type of load can be calcu-lated separately and then superimposed. The response vectoris a sinusoidal function with frequency w:

X = H1 cos (wt) + H2 sin (wt) (26)

7.6.3.2.3 This requires that the coefÞcients of X (Equation24) be constant for a given frequency. By substituting Equa-tion 25 and Equation 26 into Equation 24, two matrix equa-tions for the vectors H1 and H2 are obtained:

[ÐMw2 + K]H1 + [Nw]H2 = F1 (27)[ÐNw]H1 + [ÐMw2 + K]H2 = F2

7.6.3.2.4 Where the load vector F is calculated for a unitamplitude sinusoidal wave, the vectors H1 and H2 representthe response of the structure to a unit amplitude input. This isusually called the transfer function. The transfer function isoften presented as a magnitude or response amplitude opera-tor (RAO):

(28)

and a phase,

(29)

The subscript n denotes the nth element of a vector. Thusfor some arbitrary input amplitude, A, the response would be:

Xn = A(RAO)n cos(wt + fn) (30)

The transfer function is often written in complex form:

H = H1 + iH2 (31)

Following this notation the response X can be written as:

X = A REAL [HeÐiwt] (32)

7.6.3.3 Random Excitation

7.6.3.3.1 The spectrum of a variable can be interpreted asthe average mean square of the variable between two frequen-cies divided by the difference between these frequencies. Alinear system which transforms the amplitude of the input tothe amplitude of the output by multiplication by RAO, trans-forms the mean square of the input to the mean square of theoutput by multiplication by the square of the RAO. Thus,

SXX = RAO2 SAA (33)

Once the system RAO and the input spectrum are known,obtaining the output spectrum follows from a simple multipli-cation.

7.6.3.3.2 Gaussian (Normal) Random ProcessÑA lineartransformation of a Gaussian process is also a Gaussian pro-cess. The power spectrum of the excitation and the responseamplitude operator deÞne the probability density function ofthe response. The probability density function for a Gaussianrandom process is:

P(x) = (2ps2)Ðò exp [Ðò (x/s)2] (34)

assuming x has a zero mean. The standard deviation of theresponse, s, is obtained from the spectrum of the response;

(35)

The use of a Gaussian distribution to predict extremes of aprocess is described in C7.7.

7.6.3.4 Stiffness Modeling

7.6.3.4.1 For a general problem, the tendon stiffness willcontribute terms for each element of K. A simple tendonmodel which can be used to model vertical motions (heave,pitch, and roll) and tendon tension variations is an elasticspring (no mass, no ßuid effects) modeling each tendon orgroup of tendons. If this simple elastic spring model is used

RAOn2 H1n

2 H2n2+=

fn tanÐ1H2n

H1n

--------è øæ ö=

s2 Sxxdw0

¥

ò=


for modeling tendon attachment point forces on the vessel,the forces can be linearized to generate the stiffness terms ofK. Even with this simple spring model for the tendons, geo-metric non-linearities are present which will cause the termsof K to change depending on the offset point chosen for lin-earization.

7.6.3.4.2 In deepwater, the tendon mass may be an impor-tant contribution to vertical and horizontal mode natural fre-quencies. One simple approach is to augment the mass oradded mass matrices to reßect the contribution of tendons.Uncoupled time or frequency domain tendon analysis may beused to generate terms added to M, K, and N to model tendondynamic effects as a function of frequency. It is also possibleto augment the six degrees of freedom, Equation 24, by add-ing degrees of freedom for the tendons. (The tendon dynamicequations are adjoined to the vessel dynamics of Equation24.) In addition to the tendons, the risers contribute to the sys-tem stiffness.

7.6.3.5 Modeling Hydrodynamic Added Mass, Damping, and Exciting Forces

7.6.3.5.1 There are various assumptions and degrees ofapproximation which can be used in modeling hull hydrody-namics for frequency domain analysis. One useful approxi-mation is to assume that members are cylindrical in shapewith cross-sectional dimensions that are small in comparisonwith both the cylinder length and wave lengths of interest. Ifhydrodynamic interaction between members is neglectedthen the exciting force, added mass, and damping contribu-tion of each individual member can be computed at the mem-berÕs mean location in the ßow Þeld ignoring the effect ofother members. End effects where columns join pontoonsmay be important. For analysis using these assumptions, freesurface effects are usually ignored and added mass and dragare computed as though the members are deeply submerged.Using these assumptions only the viscous drag contributes todamping. The viscous drag is usually assumed to be a qua-dratic function of velocity and is linearized for frequencydomain analysis. Paulling and Horton, 1971, discussed theseassumptions and their application to response prediction.

7.6.3.5.2 If hull members have large cross-sectionaldimensions compared with the wave length, then free surfaceeffects become important and should be modeled. Also asmembers become larger compared with the wave length andmember to member spacing then hydrodynamic interactionbetween the members becomes important. The complete, lin-earized potential ßow hydrodynamics can be solved by using3-dimensional integral equation (sink-source) techniques orßuid Þnite element techniques (see 6.4). These techniquesmodel completely the linear (Þrst order) free surface effectsand hydrodynamic interactions. However, the viscous dragcontribution is not modeled and, if signiÞcant, must be

accounted for by linearized drag terms added to the dampingmatrix.

7.6.3.6 Modeling Wind, Wave Drift, and Current Forces for Frequency Domain

7.6.3.6.1 In frequency domain analysis the steady andoscillatory forces due to wind, wave drift, and current can betaken into account using a number of different assumptionsand approximations. In some cases it is appropriate toaccount for steady and low frequency forces and moments byadding constant forces and/or moments to arrive at a quasis-tatic equilibrium point. The TLP dynamics including tendonstiffnesses may be linearized for motions about this quasis-tatic equilibrium point. In other cases the steady and low fre-quency response may not appreciably affect the dynamics forwave frequency response so that the equilibrium point or lin-earization point may be taken as a convenient point such asthe zero-offset still water condition.

7.6.3.6.2 For some analyses, such as the analysis of offset,it is important to account for the low frequency motions asaccurately as possible. In such cases, force or moment spectraincluding wind and wave drift low frequency excitation maybe applied as input to a frequency domain model. Note thatsuch an approach is useful where the response to forces ormoments is linear. It does not depend on a linear relationshipbetween the environment (wind speed, wave amplitude, etc.)and the forces or moments. If a force spectrum can be esti-mated it can be added to the other contributions to F in Equa-tion 24. This technique is particularly useful where thestatistics of low frequency response (such as offset, tendonangles, tendon stresses, etc.) are needed.

7.6.3.6.3 The response to wind forces including responsesat all frequencies of interest can be modeled by estimatingwind force and moment spectra. For response at wave fre-quencies or above, the aerodynamic admittance may reducethe effective force of the wind because of wind spatial corre-lation effects. In such cases the aerodynamic admittanceshould be modeled carefully (Bearman, 1971).

7.6.4 Time Domain Modeling and Solutions

7.6.4.1 General

Time domain solution methods are generally used for Þnaldetailed design stages and for checks on frequency domainsolutions. Their primary advantage is in allowing changingboundary conditions and nonlinear forcing and stiffness func-tions. Time domain methods are usually used for extremecondition analysis, but are not used for fatigue analysis oranalysis of more moderate conditions where linearized analy-sis works much more efÞciently. The main drawback of timedomain methods is the computation times involved. Periodicanalysis must be carried far enough to achieve steady state.


Irregular analysis must be carried far enough to achieve sta-tionary statistics. The time domain analysis procedure con-sists of a numerical solution of the rigid-body equations ofmotion for the platform subject to external forces which mayoriginate in the ßuid motion due to waves, current, and plat-form motion, the platform positioning system, and other dis-turbing effects such as wind. Time domain analysis methodsfor ßoating bodies have been proposed by a number ofauthors (Cummings, 1962; Van Oortmerssen, 1976; Paulling,1977). Since a direct numerical integration of the equations ofmotion is performed, effects may be included which involvenonlinear functions of the relevant wave and motion vari-ables. Typical effects are drag forces which are nonlinearfunctions of the ßuid velocity, Þnite motion and Þnite waveamplitude effects, and nonlinear positioning or anchoringsystems. In comparison to the commonly used linear fre-quency domain technique, the direct numerical solution per-mits the user to investigate nonlinear, Þnite amplitudephenomena which the former method is incapable of treating,but this advantage is gained at the expense of increased com-puting time.

7.6.4.2 Regular vs. Irregular Wave Analysis

7.6.4.2.1 Regular wave time domain analysis is determin-istic. A discrete maximum design regular wave with one orseveral selected periods is used to predict the worst systemresponse to this event. Regular wave analysis requires lessnumerical effort in solving equations of motion in the timedomain (Paulling, 1977; Salvesen, 1982). All the added massand damping terms may be frequency dependent value whichcan be obtained from the frequency domain analysis (see7.6.3.5).

7.6.4.2.2 A wave spectrum is used to generate randomtime series when simulating irregular wave kinematics. TheÞrst order wave exciting forces and second order slowly vary-ing wave drift forces are both represented in the form of ran-dom time histories.

7.6.4.3 Solution Techniques

7.6.4.3.1 There are many numerical methods that havebeen developed for solving the equations of motion in thetime domain using direct step-by-step integration techniques.

7.6.4.3.2 The Newmark-Wilson method and the Runge-Kutta method are commonly used to solve a second order dif-ferential equation. If the analysis is carried out for a singleregular wave, then the frequency dependent added mass anddamping coefÞcient for the speciÞc wave period can bedirectly used in solving the above equation.

7.6.4.3.3 When the analysis is performed in a random sea,then consideration should be given to the frequency depen-dency of the added mass and damping coefÞcients. There are

a number of ways that have been proposed to include the fre-quency dependency in time domain calculations (Van Oort-merssen, 1976).

7.6.4.4 Stiffness Modeling

This can be handled in the same manner as in frequencydomain analysis (see 7.6.3.4).

7.6.4.5 Modeling Hydrodynamic Added Mass and Damping

7.6.4.5.1 The treatment of added mass and damping fortime domain calculations is based on the same principles andprocedures as discussed for frequency domain calculations.Besides a frequency dependent damping (so-called radiationdamping) derived from diffraction theory and which mainlylies within the normal wave frequency region, there are otherdamping mechanisms involved in the entire dynamic system.

7.6.4.5.2 For instance, in high frequency resonant motions(pitch, roll, and heave), damping comes from foundation/soilinteraction, hull and tendon structure damping, and fromlocal hydrodynamic drag effects around small members andsharp corners. A strong nonlinear coupling between heave,pitch and roll modes is almost always ensured, and the totalsystem damping rather than the damping in each modeshould be considered.

7.6.4.5.3 The low frequency damping (surge and sway)includes both radiation and drag effects. It also depends onthe wind and current Þelds, and contributions come from riserand tendon hydrodynamic drag.

7.6.4.6 Modeling Wind, Wave, and Current Forces for Time Domain

A time series of wave forces may be generated from awave spectrum.

7.6.4.6.1 First order wave forces:

(36)

Where:= 1st order time dependent wave forces.= frequency dependent Þrst order wave exciting.

forces per unit wave amplitude.ei = phase angle of ith wave component.ai = amplitude of ith wave component = .

S(w) = spectral density function of wave spectrum.

7.6.4.6.2 Second order slowly varying drift forces:

Fwv(1) t( ) Fwv

(1)

i 1=

N

å wi( ) cos wit ei+[ ]ai=

Fwv(1) t( )

Fwv(1) w( )

2S wi( )dw


(37)

Where:Dij = drift force per unit wave amplitude squared in

bichromatic waves.

Newman (1974) has given an approximation for thismatrix using only the diagonal terms. This has been shown tobe valid for ships by Faltinsen and Loken (1979), but is notconsidered valid for semisubmersible like structures.

7.6.4.6.3 Dynamic wind forces:

(38)

Where:ra = mass density of air.A = projected area.

CS = shape coefÞcient.ca = aerodynamic admittance.

= instantaneous platform velocity.VWD = instantaneous wind speed.

7.6.4.6.4 Viscous drag forces:

(39)

Where:rw = mass density of water.A = projected area.

CD = drag coefÞcient. = instantaneous platform velocity.

VC = water particle velocity (includes current).

7.6.4.7 Output of Time Domain Analysis

The output of time domain analysis is a time series ofresponses:

a. Regular wave simulations can be used to predict transferfunctions by taking the ratio of the response amplitude to theinput wave amplitude.b. The spectrum of the response can be calculated from thetime series, providing similar information to the frequencydomain analysis. This can be used for predicting extremes asdescribed in 7.2.1.c. The extreme response can be estimated directly from thepeaks of the responses during a simulation.

7.7 RANDOM PROCESS STATISTICS

For a system operating in an ocean environment, the forc-ing from the environment is a random process. The calcula-

tion of responses to this forcing involves random processanalysis. For most economically feasible systems there arepossible conditions which will cause the structure to fail itsperformance goals. Random process theory as an element inprobabilistic design provides rational methods for estimatingthe probability of occurrence of such events and thus fordesigning these probabilities. In addition, certain aspects ofstructural design can only be treated with random processtheory. These include estimating dynamic response of a reso-nant system and estimating fatigue damage of any system.These aspects are analyzed as random processes even forÞxed platforms. See A.Comm.7.7.

7.8 HYDRODYNAMIC MODEL TESTS

7.8.1 Purpose

7.8.1.1 Physical model experiments are another means toobtain design estimates of the response. Model tests may beused either as a calibration of analytical predictions or todetermine those responses not directly calculable. The pri-mary objectives of model tests may be broadly categorized asthe following:

a. Tests to determine the responses of a particular design. Inthis case, the model experiment is treated as an analog com-puter which is capable of predicting the full scale responses.b. VeriÞcation of methods for analytical or numerical predic-tion of system responses. In this case, less emphasis is placedon the details of the physical model since the dimensions andparameters of the system can usually be modiÞed more easilyin the numerical model than in the physical model even at asmall scale. In fact, it is recommended to keep the physicalmodel simple, if possible, to reduce cost and avoid complica-tions which might obscure the most important results. It ismore efÞcient to make minor parametric variations to the sys-tem through a numerical model if it can be shown throughphysical experiments that the numerical model is accurate. Itis important in this type of model testing to place enoughemphasis on the measurement of the incident wave Þeld. Thedetails of the wave form must be known to the same degree ofaccuracy as the vessel responses.

7.8.1.2 The numerical predictions and model experimentresults are complementary to each other. Through carefulinterpretation, each of these results can be used to partiallycircumvent the limitations of the other. One of the greatestvalues of model tests is that the results are obtained withoutrequiring any a priori assumptions about the nature of theresponses. This is almost never true of numerical models.

7.8.2 Sources of Error

When comparing the results of model experiments withanalytical predictions the following potential sources of dis-crepancies should be considered:

Fwv(2) t( ) ai

j 1=

N

åi 1=

N

å ajDij cos wi w jÐ( )t ei e jÐ( )+[ ]=

FWD t( ) òra A Csca VWD xúÐ VWD xúÐ( )=

xú

FDRAG òrw ACD Vc xúÐ Vc xúÐ( )=

xú


a. Possible errors due to scale effects. Improper scaling ofReynolds Number is inevitable since Froude scaling is almostalways used. This will affect the viscous component of ßuiddrag and the location of the boundary layer separation. Thelatter will affect the form drag and the nature of vortex shed-ding.b. Possible errors resulting from Þnite tank dimensions.Since most offshore engineering model tests are done withlow/zero forward speed, wave reßections from side walls,wave, etc., may have signiÞcant inßuence on test results.c. Possible errors resulting from limitations on the accuracyof modeling physical parameters and dimensions. In somecases, the instrumentation itself might affect the responses.This should be minimized wherever possible.d. Limitations on accuracy of the experimental results result-ing from Þnite record lengths, Þnite sample rates and numeri-cal accuracy of the data analysis procedures.e. Discrepancies arising from assumptions made in thedevelopment of the numerical model which might not accu-rately depict the physical model. An example is the assump-tion of linearity of the responses with respect to wave heightwhich is almost always made in the frequency domain analy-sis. This might cause signiÞcant discrepancies between thenumerical and experimental results for very steep waves or insituations where viscous forces play an important role.

7.8.3 Modeling Parameters

Typically, the following parameters should be modeledwith care in the physical model or otherwise properlyaccounted for in the interpretation of the results:

a. The physical dimensions of the platform. Some relativelyminor dimensional features such as the radius of corners onrectangular elements might signiÞcantly affect the results. Inother cases, it might be unnecessary to model the completedetail. The effect of any such simpliÞcations to the modelshould be considered before the model construction.b. The mass properties of the platform including the center ofgravity and the radii of gyration.c. The restoring force characteristics of the tendon system.This usually requires modeling both the axial stiffness and thelength of the tendons.d. The principal physical characteristics of the tendonsincluding the outer dimensions, mass and immersed weight.This might be of only minor importance if the water depth issmall. In large water depths, the inertial and drag forces act-ing on the tendons might be a signiÞcant component of thetotal force acting on the system. The effects of the responsesof mechanical damping in the tendons might also be impor-tant.e. The effect on stability of internal free surface. For exam-ple, during the installation phase there might be a free surfacein the ballast compartments which will affect the static stabil-

ity. The effect on dynamic stability will probably be minimalbut should be examined.f. Structural stiffnesses of any components which mightaffect the responses of the system. Such components includethe bottom foundation, connections at the upper and lowerends of the tendons and the platform itself. Due to the difÞ-culty and expense of scaling down material properties mostsmall scale platform models are considerably stiffer than theprototype. However, this is not always the case, particularlywhen instrumentation is placed in series with the structuralcomponent to measure loads. For example, tension measuringdevices attached to the model tendons might signiÞcantlyreduce their effective axial stiffness causing erroneous results.

7.8.4 Types of Tests

7.8.4.1 Types of tests which are commonly conducted andwhich might be useful include:

a. Tests of the full system consisting of hull, deck, tendonsand risers in the drilling and operating conÞgurations in regu-lar or irregular waves. Current and wind are more difÞcult tomodel but can also provide useful information.b. Free oscillation tests to determine the natural period anddamping of the system in various modes of motion.c. Towing tests to measure seakeeping characteristics of theplatform during the transportation phase.d. Measurements of motions and interface loads during mat-ing of deck structures and hull.e. Measurements of system responses under simulated dam-age conditions or in the partially installed state.

7.8.4.2 The responses which can be tested commonlyinclude motions, loads, deck clearance, riser and tendonresponses, and installation procedures.

7.8.4.3 In order to maximize the usefulness of the testinformation, a test program should be developed in advancewhich deÞnes the test objectives, the needed products, therequired instrumentation and data analysis procedures. It isdesirable to have data analysis and display capabilities on lineduring the testing so that ßaws in the instrumentation, modelor data acquisition system which might affect the Þnal resultscan be discovered and corrected.

7.9 SYMBOLS

The following symbols are typical in equations used fordesign and analysis:

= structural acceleration.F(x, t)= load vector.

w = radian frequency.H1 = real part of system response.H2 = imaginary part of system response.F1 = real part of force.F2 = imaginary part of force.

xúú


f = phase of system response.A = input amplitude of energy.ai = amplitude of ith wave component.

Sxx = response spectrum.Saa = input energy spectrum.wo = natural frequency.

ei = phase angle of ith wave component.P = probability density function.s = standard deviation of response.t = time.

M = frequency independent added mass matrix.R = retardation function.

Am = frequency dependent added mass coefÞcient.= Þrst order wave force.

Sa = wave spectral density.S(w) = wave spectral density at frequency w.

= second order slowly varying drift force.D = drift force per wave amplitude squared.

FWD = dynamic wind force.A = projected area.

CS = shape coefÞcient.VWD = instantaneous wind speed.

FDRAG = viscous drag forces.rw = mass density of water.CD = drag coefÞcient.VC = water particle velocity.

8 Platform Structural Design8.1 GENERAL

This section addresses the structural design and analysis ofthe hull and deck. Discussions of fabrication, materials,inspection, monitoring and maintenance are also included.

8.2 GENERAL STRUCTURAL CONSIDERATIONS

8.2.1 Project Phases

The hull and deck structures should be designed for load-ings which occur during all project phases including construc-tion, transportation, installation and in place phases (see 5.5).

8.2.2 Damage Conditions

The structural design should consider the possibility of acci-dental events (see 5.3.3.2 and 8.5.7). Accidental events includecollisions, dropped objects, Þre, explosion, or ßooding.

8.2.3 Redundancy

The capability of the deck and hull to redistribute loadsshould be considered when selecting the structural conÞgu-ration.

8.2.4 Reserve Strength

The design of the structure should include details that pro-vide reserve strength beyond the allowable design load.

8.2.5 Interfaces with Other Systems

The structural design of the deck and hull should considercritical interfaces with other systems, such as tendon and riseranchor points, tendon cross load bearings, tendon and riserinstallation equipment, moonpool requirements, drilling andproduction equipment, hull systems, and foundations. (SeeSections 9, 10, 11, and 12.)

8.2.6 Safety

The arrangement of the main structural deck elementsshould be coordinated with topside facilities equipment andoperational requirements. The inßuence of the structure onproper ventilation of hazardous areas, access for Þre Þghting,Þre protection and escape routes should be considered. (SeeSection 12.)

8.2.7 Deck Clearance

The lower deck elevation should be established based on7.2.8. If wave impact on the underside of the lower deck isanticipated, local strengthening of the deck structure is rec-ommended.

8.2.8 Weight Engineering

Because of their effect on the platform buoyancy and ten-don tension requirements, all weights and centers of gravityshould be accurately and continuously monitored through-out the design, construction, and in place project phases.Design practices should minimize structural weight wher-ever possible.

8.2.9 Corrosion Allowances

A corrosion allowance appropriate for the environment anddesign life of the platform should be provided on all membersat the waterline. The corrosion allowance should be based onthe provisions of NACE Standard RP-01-76. Unprotectedsteel (plates with stiffeners and girders) in ballast and drillwa-ter tanks should also be provided with a corrosion allowance.

8.2.10 Vibrations

The effect of machinery vibrations should be included inthe design. Reinforcing of the structure may be needed toreduce the level of local stresses. Structural details in areas ofhigh vibration should be designed to reduce the effect of reso-nance and local member fatigue.

Fwv1

Fwv2


8.3 DESIGN CASES

8.3.1 General

A design case is a combination of loads due to the projectphase, system condition, and environment with the appropri-ate safety criteria as described in 5.5. Some design casesgiven in 5.5 may not be required for a speciÞc concept, whileothers not listed may be required. The designer should reviewthe proposed concept to be sure that all appropriate designcases are considered.

8.3.2 Safety Criteria

The safety criteria for categories A, B and C are deÞned ingeneral in 5.5.5. SpeciÞc recommendations for safety factorsfor platform design are given in 8.5.2 for categories A and B,and in 8.4.4.1 for category C.

8.3.3 Design Loading Conditions

8.3.3.1 For each design case, the platform should bedesigned for the loading conditions that will produce the mostsevere effects on the structure. Environmental loads, with theexception of earthquake loads, should be combined in a man-ner consistent with the probability of their simultaneousoccurrence during the design case being considered. Earth-quake loads should be imposed on the platform as recom-mended in 5.5.

8.3.3.2 For drilling and production platforms, loads fromsimultaneous drilling and production operations should beconsidered.

8.3.3.3 Variations in consumables and the locations ofmovable equipment such as a drilling substructure should beconsidered in order to determine the maximum design stressin the platform members.

8.4 STRUCTURAL ANALYSIS

8.4.1 Analysis Methods

8.4.1.1 The platform may be analyzed for the applied load-ings using a variety of computational methods. A linear, elas-tic space frame computer analysis is recommended. DetailedÞnite element analyses may be necessary to more accuratelydetermine the local stress distribution in complex structures.Supplementary manual calculations for members subjected tolocal loads may be adequate in some cases.

8.4.1.2 Environmental loads acting on the platform shouldbe calculated according to Section 7. These loads should betransferred to the structural model, and the effects of motioninduced loads should be accounted for.

8.4.2 Modeling

8.4.2.1 Space Frame Model

A space frame model generally consists of beam elements,plus other elements needed to model speciÞc structural char-acteristics. All primary structural elements should be modeledin the space frame analysis. The effects of secondary mem-bers (if not included in the space frame model) should beaccounted for in detailed local analysis. The effect of jointeccentricities and joint ßexibility should be accounted for inthe model, as should the in-plane stiffness of the deck plating.

8.4.2.2 Finite Element Models

Finite element analysis is recommended for complex jointsand other complicated substructures to determine local stressdistributions more accurately and to verify the stiffness of thespace frame model. The loads applied to Þnite element mod-els should come from the global space frame analysis andfrom local loads acting on the structure. Columns and pon-toons with complex stiffeners, ßats, or bulkheads will requireÞnite element analysis unless manual calculations are sufÞ-cient to accurately determine stress distributions.

8.4.2.3 Manual Calculations

Manual calculations may be performed where a detailedÞnite element analysis is not needed, and may use empiricalformulas or basic engineering principles. The loads used forthese calculations should come from the global space frameanalysis and from local loads acting on the structure.

8.4.2.4 Stress Concentration Factors

Stress concentration factors should be determined bydetailed Þnite element analysis, by physical models, by otherrational methods of analysis, or by published formulas.

8.4.2.5 Stability Analysis

Formulas for the calculation of the buckling strength ofstructural elements are presented in API Recommended Prac-tice 2A, API Bulletin 2U, Stability Design of CylindricalShells, and API Bulletin 2V, Design of Flat Plate Structures.As an alternative, buckling and post-buckling analyses ormodel tests of speciÞc shell or plate structures may be per-formed to determine buckling and ultimate strength loads.

8.4.3 Stress Analysis

8.4.3.1 Structural analyses may be performed quasistati-cally when the structure does not respond dynamically. Thenatural periods referred to in this section are those of the elas-tic vibration of the platform (hull and deck) and do not referto the rigid body periods of the platform and tendon system.The natural periods are expected to be small enough in com-


parison with periods with signiÞcant wave energy that struc-tural dynamics need not be considered. This assumptionshould be veriÞed for each speciÞc platform design. If theÞrst natural period of the platform structure is greater than 3.0seconds, dynamic structural analysis of the platform shouldbe performed.

8.4.3.2 Environmental loads applied to the platform aretime varying and can be calculated using two different meth-ods, frequency domain or time domain, as described in7.3.1.2 and 7.3.1.3, respectively.

8.4.4 Fatigue Analysis

8.4.4.1 Fatigue Life Requirement

The allowable fatigue life is a function of inspectability,repair ability, redundancy, the ability to predict fatigue dam-age, and the consequences of failure of a structural element.In general, it is recommended that the design fatigue life ofeach structural element of the platform be at least three timesthe intended service life of the platform. For critical elementswhose failure could be catastrophic and for elements not eas-ily accessible for inspection or repair, use of an additionalmargin of safety should be considered. This corresponds tosafety criteria category C as deÞned in 5.5.5. Fatigue shouldbe checked not only at joints, but also at any details with highstress concentrations, such as doubler plate welds, thicknesstransitions, etc. Structural details should follow good designpractice as described in 8.5.6.

8.4.4.2 Fatigue Loading

8.4.4.2.1 Fatigue can be caused by cyclic environmentalloads and machinery vibrations.

8.4.4.2.2 The main cause of platform fatigue is cyclicwave loading. The wave climate should be derived on the bestavailable basis. The wave environment description can beestablished from recorded data and/or hindcasts. The waveclimate is the aggregate of all sea states expected over thelong term, including the heading of the sea state, and may becondensed into discrete sea state blocks. Each sea state blockmay be characterized by a spectral description.

8.4.4.2.3 Stress ranges can be associated with the sea statedescription. Histories of cyclic stresses due to other types ofloads should be calculated and included with the waveinduced stresses for the fatigue analysis.

8.4.4.2.4 Space frame analysis should be performed toobtain stresses for each wave frequency applied to the struc-ture. An inertial load set corresponding to platform motionsshould always be included. Dynamic effects should be takeninto account when it is believed that they make a signiÞcantcontribution to the response of the structure. The short-term

stress response and number of wave cycles can be developedfor each sea state.

8.4.4.2.5 Fatigue is a localized problem; therefore detailedstructural models of complex joints and other complicatedstructures may be needed to develop local stress distributions.

8.4.4.3 Fatigue Analysis

8.4.4.3.1 Fatigue life estimates are made by comparing thelong-term cyclic loading in a structural detail with the resis-tance of that detail to fatigue damage.

8.4.4.3.2 Two different approaches have been developedfor determining fatigue damage:

a. S-N approach.b. Fracture mechanics approach.

8.4.4.3.3 The S-N approach uses an S-N curve which givesthe number of cycles to failure for a speciÞc structural detailor material as a function of constant stress range, based on theresults of experiments.

The long-term stress distribution is used to calculate thecumulative fatigue damage ratio, D:

(40)

Where:ni = number of cycles within stress range interval i.Ni = number of cycles to failure at stress range i as

given by the appropriate S-N curve.

D should not exceed unity for the design fatigue life.

8.4.4.3.4 The fracture mechanics approach can be used topredict the growth rate of a fatigue crack and the crack lengthat which failure will occur, thus giving the fatigue life. Thefatigue strength of a particular model being analyzed can becalculated using the Paris law expression:

(41)

Where:da/dN = crack growth rate.

DK = range of stress intensity factor occurring at thecrack tip.

C and m are constants for a particular material and loadingcondition.

These material constants depend on material, structuraland environmental conditions. By integration and proper cal-culation for DK, a relationship can be established between thecyclic stress and the number of cycles to failure taking intoaccount initial defect sizes and material toughness.

Dni

Ni

-----å=

dadN------- C KD( )m=


This method can also be used to help establish inspectionintervals by predicting the time necessary for a crack to growfrom an undetectable size to failure. Fracture mechanics anal-ysis can be used to determine required material toughness,maximum allowable initial ßaw size (for in place inspection),and inspection intervals.

8.4.4.3.5 Any fatigue analysis should account for materialproperties in sea water and cathodic protection effects. Referto A.Comm.8.4.4.3 and API Recommended Practice 2A for amore detailed discussion of fatigue analysis techniques.

8.5 STRUCTURAL DESIGN

8.5.1 Design Basis

8.5.1.1 The design basis adopted in this document is theworking stress design method, whereby stresses in all com-ponents of the structure are not allowed to exceed speciÞedvalues.

8.5.1.2 The structural components of the deck and hullshould be designed in accordance with the applicable provi-sions of API Recommended Practice 2A, AISC and API Bul-letins 2U and 2V. In general, cylindrical shell elements shouldbe designed in accordance with API Bulletin 2U, ßat plateelements in accordance with API Bulletin 2V, and all otherstructural elements in accordance with API RecommendedPractice 2A or AISC as applicable.

8.5.1.3 In cases where the structureÕs conÞguration orloading condition are not speciÞcally addressed in API Rec-ommended Practice 2A, AISC and API Bulletins 2U and 2V,other accepted codes of practice can be used as a design basis.Where alternative codes are followed, the designer mustensure that the safety levels and design philosophy implied inAPI Recommended Practice 2T are adequately met.

8.5.1.4 In API Recommended Practice 2A and AISC,allowable stress values are expressed in most cases as a frac-tion of the yield stress or the buckling stress.

In API Bulletin 2U, allowable stress values are expressedin terms of critical buckling stresses.

In API Bulletin 2V, the allowable stresses are classiÞed interms of limit states. Two basic limit states are considered:ultimate limit states and serviceability limit states. Ultimatelimit states are associated with the failure of the structure.Serviceability limit states, such as material yield, local buck-ling, excessive deformations, etc., are associated with the ade-quacy of the design to meet its functional requirements.While an ultimate limit state, if reached, leads to structuralfailure, reaching the serviceability limit state implies that thestructureÕs ability to serve its intended purpose has beenimpaired, but the structure is still capable of carrying addi-tional loads before reaching an ultimate limit state.

8.5.1.5 The loss of equilibrium of a part or the whole struc-ture must be avoided by careful selection of the overall struc-tureÕs conÞguration, the layout of its members, the degree ofredundancy, the availability of alternate load paths, and theadequate design of foundation and support conditions.Design judgment must be used to ensure that the possibilityof this type of failure is excluded.

8.5.2 Allowable Stresses

8.5.2.1 For structural elements designed in accordancewith API Recommended Practice 2A or AISC, the safety fac-tors recommended in API Recommended Practice 2A andAISC should be used for normal design conditions associatedwith safety criteria A. For extreme design conditions associ-ated with safety criteria B, the allowable stresses may beincreased by one-third.

8.5.2.2 For shell structures designed in accordance withAPI Bulletin 2U, a factor of safety equal to 1.67 y is recom-mended for all buckling modes for safety criteria A. Forsafety criteria B, the corresponding factor of safety is equal to1.25 y. The parameter y varies with the buckling stress and isdeÞned in API Bulletin 2U. It is equal to 1.2 for elastic buck-ling stresses below the proportional limit and reduces linearlyfor inelastic buckling from 1.2 at the proportional limit to 1.0when the buckling stress is equal to the yield stress.

8.5.2.3 For ßat plate structures designed in accordancewith API Bulletin 2V, the allowable stress depends on thelimit state under consideration (ultimate or serviceability).For each limit state, the allowable stress is obtained by divid-ing the limit state stress by an appropriate factor of safety.Factors of safety for different service conditions and limitstates are as follows:

If both limit states are checked, the lower of the two allow-able stress values should be used. In the case of serviceabilitylimit states associated with deformation, the allowable defor-mation is obtained by dividing the limit state deformation bythe applicable factor of safety given in the table above.

8.5.3 Design of Deck Structure

8.5.3.1 The design of the deck structure should conform tothe provisions of API Recommended Practice 2A, AISC andAPI Bulletin 2V.

Factor of Safety

Safety CriteriaServiceability

Limit StateUltimate

Limit State

A 1.67 2.0

B 1.25 1.5


8.5.3.2 The design of stiffened ßat panels, grillages, anddeep plate girders should comply with the provisions in APIBulletin 2V.

8.5.3.3 API Recommended Practice 2A and AISC shouldbe used for the design of truss elements, rolled beams, shal-low plate girders and built up members such as open box typebeams.

8.5.3.4 Alternate rational methods may be used where nec-essary.

8.5.4 Design of Hull Structure

8.5.4.1 The design of the hull structure components shouldcomply with the applicable provisions of API RecommendedPractice 2A, AISC and API Bulletins 2U and 2V.

8.5.4.2 The design of circular cylindrical columns andpontoons should comply with the provisions in API Bulletin2U.

8.5.4.3 The design of stiffened and unstiffened tubularbraces against local buckling should comply with the provi-sions of API Bulletin 2U. For other design considerations, theapplicable provisions of API Recommended Practice 2A andAISC should be used.

8.5.4.4 The design of stiffened ßat plate hull componentsshould comply with the provisions in API Bulletin 2V.

8.5.4.5 Alternate rational methods may be used where nec-essary.

8.5.5 Design of Nodes and Connections

8.5.5.1 Tubular Joints

8.5.5.1.1 The design of small, unstiffened tubular jointsshould comply with the provisions of API RecommendedPractice 2A.

8.5.5.1.2 Stiffened tubular joints should be designedaccording to 8.5.5.2.

8.5.5.2 Pontoon to Column and Deck to Column Joints

Joint designs should be checked by using a Þnite elementanalysis to determine the load path through the joint. Jointsshould be designed to provide a continuous transfer of loadsfrom the pontoons and decks through the columns. Primarystresses in the joint shell and internal stiffening may be com-pared to the limit state strength formulas for curved and ßatstructural elements. Tensile limit states should be checked toguard against fracture of node material or welds. Model tests

may be useful to determine the stress distribution in complexjoint geometries.

Cast insert pieces may be used to reduce stress concentra-tions at these joints.

8.5.5.3 Transition Joints and Stiffened Plate Intersections

The same general principles described in 8.5.5.2 apply inthe case of transition joints and stiffened plate intersections.Whenever the complexity of the geometry justiÞes, a Þniteelement analysis should be performed and may be comple-mented by model testing.

8.5.6 Design of Structural Details

8.5.6.1 The design of structural details is important forproducing a complete structure that will be free of localcracks, buckles, and severe localized corrosion during the lifeof the platform.

8.5.6.2 Details and penetrations in main structural mem-bers should be checked for compliance with 8.5.2. The detailsincorporated in the Þnal construction drawings should bereviewed by the platform designer to ensure that the designhas not been compromised.

8.5.6.3 Guidance for sizing beam brackets and spacing ofpanel stiffeners can be found in the rules of the major classiÞ-cation societies.

8.5.6.4 Sections 9, 10, and 11 should be consulted by thedesigner. These references provide some history of serviceperformance of structural details used on oceangoing ships.

8.5.7 Design for Accidental Loads

8.5.7.1 General

Structural design should consider the possibility of acci-dental events. The term Òaccidental eventÓ is a collective termfor exceptional conditions, such as collisions, droppedobjects, Þre, explosion, or ßooding.

8.5.7.2 Design Philosophy

Satisfactory protection against accidental damage can beobtained by a combination of two means:

a. Low damage probability.b. Acceptable damage consequences.

8.5.7.3 Energy Absorption Capability

The structure should behave in a ductile manner to absorbenergy caused by impact loads. Measures to obtain adequateductility are:


a. Make the strength of connections greater than the strengthof the members.b. Provide redundancy in the structure, so that alternate loadredistribution paths may be developed.c. Avoid dependence on energy absorption in slender strutswith a limited degree of postbuckling reserve strength.d. Avoid pronounced weak sections and abrupt changes instrength or stiffness.e. Use materials which are ductile in the operating tempera-ture range.

8.5.7.4 Collisions and Dropped Objects

8.5.7.4.1 The direct loads and consequential damage dueto collisions and dropped objects should not cause completestructural collapse or loss of platform stability.

8.5.7.4.2 It may not be possible to design the platform tosurvive a collision with a very large object such as a tanker oran iceberg.

8.5.7.5 Damage Tolerance

A damaged platform should resist functional and reducedextreme environmental loads (see 5.5, design case 8). Thisimplies that the platform should maintain its structural integ-rity and be stable with no immediate need for conductingrepairs. The residual strength of a damaged member may beincluded provided its magnitude can be assessed by rationalanalysis or tests. If such residual member strength is notproven, damaged members should not be considered effective.

8.6 FABRICATION TOLERANCES

Guidance on fabrication tolerances is given in 13.2.3 and inAPI Bulletin 2U. Any change in these tolerances as a conse-quence of speciÞc fabrication methods should be consideredin the design.

9 Tendon System Design9.1 GENERAL

9.1.1 Purpose and Scope

9.1.1.1 The purpose of this section is to discuss majorparameters such as loading conditions, analysis methods,design criteria, and operational considerations which shouldbe taken into account during the design of the tendons. Asused here, the term ÒtendonsÓ includes all components associ-ated with the vertical mooring system between (and includ-ing) the top connection to the platform and the bottomconnection to the foundation.

9.1.1.2 Because the tendons link the platform to the oceanßoor, there is interaction among the tendons, the platform,and the foundation, all of which the designer must take intoaccount in selecting the tendon system. This section of the

recommended practice addresses those design considerationswhich apply speciÞcally to tendons; design considerationsrelating to the interaction with other structural systems arealso referenced as appropriate.

9.1.1.3 Determination of tendon tensile loads induced byplatform motions is addressed in Section 7 with referencesand further description in this section. Determination of ten-don bending loads induced by platform motions and by directhydrodynamic forces is addressed in this section.

9.1.2 Description of Tendon

9.1.2.1 Tendon Components

9.1.2.1.1 An individual tendon is composed of three majorparts: an interface at the platform, an interface at the seaßoor,and a link between the two (see Figure 12). Each part maycontain several components, with each component taking avariety of forms depending on the speciÞc design.

SpeciÞc designs may provide individual pieces of equip-ment for each function or combine two or more functions intoone unit. The functions of the three major parts are describedbelow:

a. Platform interfaceÑComponents at the platform interfacemust perform the following three functions:

1. Apply and adjust a prescribed level of tension to thetop of a tendon.2. Connect a tensioned tendon to the platform.3. React side loads and control the bending stresses of atensioned tendon.

b. Seaßoor interfaceÑComponents at the seaßoor interfacemust perform the following functions:

1. Provide a structural connection between the tendonand foundation.2. React side loads and control the bending stresses of atensioned tendon.

c. Link between platform and subsea interfaceÑThe mainbody of a tendon may take a variety of forms including tubu-lars, solid rods or bar shapes, stranded construction such asparallel or helical wire rope, or any other conÞguration thatmeets the tendon service requirements.

9.1.2.1.2 In the case of tubular tendons, the bore may beconsidered for use in routing umbilicals between the seaßoorand surface, or as a ßuid ßow conduit.

9.1.2.1.3 To date, steel has been the material of preferencefor tendons. Nonmetallic materials and composites, such asaramid or graphite Þbers in a composite matrix, have beenproposed, however, and may be considered as a tendonmaterial.

9.1.2.1.4 Regardless of material or conÞguration, all ten-dons will have, as a minimum, terminations top and bottomand in most cases, intermediate connections or couplings


Top connectorand/or tensioner

Top flexelement

Tendonelement(TYP)

Coupling (TYP)

Foundation template

Bottom flex element

Well productionrisers

Bottom connector

Tendon (TYP)

Hull column

Tendon accesstubes (TYP)

Leg(includes all tendons

at a corner)

Figure 12—Typical Tendon Components


along their length. Tendon connections may take the form ofmechanical couplings (threads, clamps, bolted ßange, etc.),welded joints, or any other type of structural connectionmeeting the service requirements.

9.1.2.2 Specialized Components

The tendon design might incorporate specialized compo-nents, such as the following:

a. Corrosion protection system components, which mayinclude coatings, sacriÞcial anodes, elements of an impressedcurrent system, or combinations of any of these.b. Buoyancy devices such as air cans or foam modules to off-set a portion of the tendonÕs submerged weight.c. Devices intended to suppress vortex induced vibrationsand/or reduce hydrodynamic drag.d. Sensors, tattletales, or other forms of instrumentationintended to provide information about the performance orcondition of the tendons.e. Auxiliary lines, umbilicals, or similar conduits, for tendonservice requirements, or for some function not related to thetendons.f. Provisions for the tendons to be used as guidance struc-tures for running other tendons, or equipment packages to beused in support of other operations.g. Elastomeric elements, as described in 14.9.

9.2 GENERAL DESIGN

9.2.1 Tendon Removal

Tendons may be designed to be permanent or to be remov-able for maintenance and/or inspection. If tendons can beremoved for maintenance and/or inspection, the allowablestresses recommended in 9.6.2 should not be exceeded whenone tendon is missing in an appropriate environmental condi-tion. This condition should be selected taking into account theexpected frequency of tendon removal and the length of timefor which one tendon is likely to be out of service.

9.2.2 Service Life

The tendon system should have a speciÞed service life;typically, the nominal tendon life might match the design lifeof the platform. There might be cost or risk incentives, how-ever, that could extend or shorten the tendon design servicelife criteria. The initial tendon cost, the cost of tendonreplacement, tendon in-place inspectability, and the risksassociated with tendon retrieval and reinstallation are amongthe factors that may affect the tendon design service life.

9.2.3 Material Considerations

9.2.3.1 See Section 14, and speciÞcally, 14.4.3.

9.2.3.2 The tendon cross-sectional area may be set byfatigue life, axial stiffness or maximum allowable stressrequirement. If the fatigue or stiffness requirement domi-nates, the material yield strength should be speciÞed low,consistent with maximum stress prediction, so as to facilitateachievement of other important properties such as toughness,weldability, etc.

9.2.3.3 Maximum wall thickness of the tendon or tendoncouplings might be limited by the material properties achiev-able in thick sections and by the fabrication process envi-sioned (e.g., welded or unwelded construction).

9.2.3.4 Material fatigue performance should be consid-ered. Tendons will be subjected to long-term exposure to sea-water, possibly with certain components located in the splashzone. Relatively high mean tensile stress with a high numberof cycles at relatively low stress ranges will typically domi-nate the load history over its lifetime.

9.2.3.5 Tendon material fracture toughness and maximuminitial defect size should be established using fracture mechan-ics methods to reßect the effects of mean and alternatingstresses. Selection of toughness and defect size should bemade in conjunction with the choices of tendon cross-sectionalarea and tendon inspection/ replacement strategy (see 9.6.5).

9.2.4 Design Procedure

9.2.4.1 This section discusses some of the major steps inthe design procedure necessary to develop a tendon systemwhich satisÞes operational, installation, material, inspection,stress, and fatigue requirements. These steps are shown in theform of a ßow chart in Figure 13. This ßow chart is shownonly as an example and will be referred to for purposes of thisdiscussion.

9.2.4.2 In general, the sequence of major activities in thedesign process is as follows:

a. Platform sizingÑDetermine overall TLP conÞguration.b. Preliminary tendon designÑEstimate pretension and otherinput required for platform sizing.c. Response analysisÑDevelop vessel motions and maxi-mum and minimum tendon loads.d. Tendon horizontal responseÑCalculate tendon bendingloads and horizontal motions.e. Minimum tensionÑEstablish minimum allowable tendontension.f. Preliminary stress analysisÑCheck preliminary maxi-mum stress level, fatigue life, and hydrostatic collapse.g. Operational limits checkÑCheck for acceptable vesseloffsets and tendon motions and displacements.h. Fatigue lifeÑCalculate fatigue life under combined axialand bending loading.i. Final design checkÑCheck maximum stress, minimumtension, fatigue life, fracture mechanics and inspection/


*May be combined asa coupled analysis.

Set Tendon PropertiesArea, D/t, yield strength

pretension, stress concentrationfactor, toughness, etc.

Platformresponse analysis*

Tendon axialstress responses

Tendon toplateral motions

Tendon maximumand minimum

tensions

Minimum allowabletension analysis

Increasepretension

Min.allowabletension

Tendon bendingresponse analysis*

da/dn Data,detectable crack

size

Combinationof stresses

Tendon bendingstress responses

Increaseyield strength

Fracturemechanics

analysis

Lifetimestresses

Max.allowabletension

Not OK

OK

Not OK

OK

Not OK

OK

Not OK

OK

Fatigueanalysis

S – NCurves

Increasearea

Requiredfatigue

life

Checkhydrostatic

collapse, max.angles,

etc.

Inspectioninterval

Increase areaand/or decrease

diameter

Tendondimensions,material, etc.

Prototypetesting

Figure 13—Tendon Design Flow Chart


replacement strategy, hydrostatic collapse, and vortex inducedvibrations.j. Coupled analysis checkÑDetermine whether a coupledresponse analysis is necessary.k. Model test (optional)ÑVerify tendon motions and loads.

9.2.4.3 The procedures, special equipment and tolerableenvironmental conditions for tendon installation must bedeveloped. This step will generally follow selection of Þnalplatform and tendon design parameters. Depending on thetype of tendon and tendon top attachment, the installationrequirements could inßuence the platform and tendon design.It may, therefore, be advisable to develop the tendon installa-tion design, including some tendon analysis, in parallel withthe platform/tendon design procedure.

9.2.4.4 The ßow chart, Figure 13, illustrates the iterativeinteraction between the platform sizing and response analysis(Section 7) and the tendon design analysis. Tendon loads andmaximum angles also inßuence the design of the foundation.

9.2.5 Design Data Requirements

9.2.5.1 Environmental Data

The tendon loads depend on platform response to wind,waves, current, and tide, and possibly to marine fouling, seis-mic activity, ßoating ice, and platform snow or ice accumula-tion. The extreme environmental conditions as well as otherreturn period conditions might be pertinent a) for damageconditions, b) for less than a full complement of tendons, c)during installation, or d) for other speciÞed operationalevents. Wind and wave spectra and directionality data arevaluable for assessing tendon fatigue. Current proÞles, sea-state probabilities, and weather persistence data help establishthe design of equipment and the techniques needed for tendoninstallation and retrieval. Water temperature, salinity, andoxygen content can be used to establish cathodic protectionrequirements. For a more complete discussion of environ-mental data requirements (see 5.4).

9.2.5.2 Platform, Tendon, and Foundation Characteristics

9.2.5.2.1 The properties of the tendons affect the platformdisplacement and response, and vice versa.

9.2.5.2.2 Tendon pretension (tension in still water) is estab-lished to satisfy the maximum platform offset limit and to con-trol minimum tendon tension. Tendon pretension has a directeffect on platform displacement and tendon maximum tension.

9.2.5.2.3 Tendon cross-sectional area establishes tendonaxial stiffness which is a major factor in the platform naturalperiods of heave, pitch and roll vibration. These periods mustbe kept low enough to limit fatigue damage due to waveexcitation.

9.2.5.2.4 The diameter-to-thickness ratio for air-Þlled ten-dons is also important because it establishes the tendonweight in water which, in turn, establishes the differencebetween the top and bottom tensions in the tendons. Thesetensions inßuence platform displacement and offset.

9.2.5.2.5 The foundation system design is also affected bythe number of tendons, their pattern and their loading. If sep-arate foundations are used, the tendon load is affected by theplacement accuracy of the foundation system.

9.2.5.2.6 The size of the platformÕs columns determinesthe space available for handling and installing tendons. Thenumber of tendons per corner might be determined in part bythis available space. The column structural arrangementshould accommodate the localized tendon loads and the pos-sible presence of tendon access tubes.

9.2.5.3 Operating Limits and Other Design Considerations

The limits imposed on or by the tendon design mayinclude:

a. Flexure limitsÑThe ßex element must accommodate themaximum tendon angles at top and bottom due to platformsurge.b. InterferenceÑInterference between tendons and platformor foundation template structure or the maximum distance ofthe upper ßex element above the bottom of the tendon accesstube and the lower one inside the foundation is governed bythe tendon tube diameter and ßex element angle.c. Tendon centerline spacingÑMinimum spacing is set byequipment space requirements, by possible interferenceamong tendons during deployment, and by column fabrica-tion space requirements. Large spacing may cause differencesin loading among tendons at a corner due to pitch/roll plat-form motions.

9.3 DESIGN LOADING CONDITIONS

9.3.1 Load Types

9.3.1.1 Tendon loads include axial, bending, shear, torque,radial and hoop loads. Axial loads may be determined bysuperposition of the load components described in 7.2.5(Maximum Tension) and 7.2.6 (Minimum Tension). Bendingand shear loads may arise from:

a. Dynamic response to platform motions.b. Bending induced by ßex element stiffness.c. Hydrodynamic drag and inertial forces.d. Vortex induced vibrations.e. Unusual loading during installation.f. Manufacturing alignment errors.g. Gravity when platform is offset (catenary effect).


9.3.1.2 Hoop loads result from a difference in hydrostaticpressure on the outside and inside. Torque may be induced byplatform yaw motion.

9.3.1.3 While tendon design is characteristically domi-nated by axial tension loads, the other load types should beevaluated as appropriate to ensure adequate design margins.Buoyant tendon designs in deep water could be controlled byhydrostatic collapse, and large diameter tendon design couldbe dominated by bending.

9.3.2 Loading Conditions

9.3.2.1 General

9.3.2.1.1 Tendon structural analysis should consider, as aminimum, load cases associated with the following:

a. Maximum tension.b. Minimum tension.c. Largest ßex element angle.d. Lifetime fatigue conditions.e. Installation.f. Hydrostatic collapse.g. Maximum loading on speciÞc components.

9.3.2.1.2 Maximum and minimum tension are discussed in7.2.5 and 7.2.6 respectively, and maximum angles in 7.2.7.

9.3.2.2 Extreme Events

9.3.2.2.1 Selection of environmental conditions and a TLPconÞguration for each load case should account for the likeli-hood of joint events occurring which could lead to an extremeload occurrence.

9.3.2.2.2 For any selected survival load case, speciÞcationof the design wave should include a spectral representation ora range of wave heights and frequencies. The design shouldnot be based exclusively on a single Òmost probable maxi-mumÓ wave height and an associated period since this singlewave representation might not correspond to the maximumloading condition.

9.3.2.2.3 The consequence of minimum tension should beconsidered. Loss of tendon tension could result in tendonbuckling and/or damage to ßex elements. If tension loss ispermitted, tendon dynamic analysis should be conducted toevaluate its effect. (See Brekke and Gardner, 1986.) See 7.2.6for a more complete discussion of minimum tendon loads.

9.3.2.3 Normal Conditions

9.3.2.3.1 Lifetime operating load conditions for the ten-dons should consider a range of combinations of wind, wave,and current conditions which will commonly occur. Life timeoperating loads are particularly important in the evaluation oftendon fatigue life and inspection interval.

9.3.2.3.2 Cyclic loads in the tendon can lead to fatiguecrack initiation and growth, thus these loads should be con-sidered. The combined effect of primary and secondary waveeffects, including high frequency axial responses (e.g.,springing and ringing), plus vortex shedding should be evalu-ated. Derivation of life cycle tendon loads should be based onlong-term climatic and oceanographic data which includesdata on the joint occurrence of waves and factors which resultin static platform offset (wind and current).

9.3.2.4 Installation

9.3.2.4.1 Loads during tendon installation can result fromany of the environmental conditions discussed. Tendondynamics prior to and during latch-up should be consideredto minimize the risk of tendon damage due to free hangingphenomena or due to resonance response with part of the ten-dons installed.

9.3.2.4.2 As in the case of operational loads, spectral seastate criteria for installation should be based on site speciÞcspectral measurements or hindcast estimates.

9.3.2.5 Damage

Over the structure life, tendons might become damaged.Hence, tendons may be designed to be removable andreplaceable. Damage to the platform such as a ßooded com-partment could also affect tendon loads and responses todynamic forces. DeÞnition of these load cases should con-sider the following factors:

a. Missing tendonsÑThe absence of a tendon can increasethe static load shared by the remaining tendons as well as thedynamic loads resulting from platform response to environ-mental conditions. In determining tendon loads under thiscondition, the designer should consider the frequency ofplanned tendon replacement (if any), and the likelihood forunplanned tendon removal (for example, following discoveryof serious damage). These considerations should determinethe appropriate operational environments applicable for amissing tendon case.b. Damaged platform compartmentsÑA ßooded platformcompartment could lead to a reduction in tendon tension andthe possibility of loss of tension in the tendon under certainconditions. Adequate pretension should be provided to insurethat the tendon load does not drop below levels which couldcause excessive tendon stress or damage tendon componentsunder the worst environmental conditions expected with oneor more ßooded compartments as speciÞed in 5.3.c. Damaged tendonÑStructural damage to a tendon may bedetected either by in-service inspection (e.g., NDT), tendonleaking, or loss of load carrying capability (e.g., from dam-aged bottom connector or failure of foundation). The operatormay elect to leave such a tendon in place for a period of timeprior to replacement. The designer should consider appropri-


ate load cases with a damaged tendon in place to determinethe effects of such conditions as reduced pretension on a dam-aged tendon, or complete or partial ßooding of an air-Þlledtendon.

9.3.2.6 Seismic

9.3.2.6.1 Seismic loads on the tendons should be based onsite speciÞc geotechnical conditions, and the design spectrashould include the vertical and two orthogonal horizontalcomponents of ground acceleration.

9.3.2.6.2 Either the response spectrum method usingresponse spectra presented in API Recommended Practice 2Aor the time history method may be used. If the response spec-trum method is used, suitable adjustments should be made tothe response spectra to account for the best estimate of damp-ing. Also, the response spectra should be extended to coverperiods corresponding to lateral (bending) mode vibrations ofthe tendon in the surge and sway directions. If the time his-tory method is used, ground motion time histories should alsoinclude energy contributions in this frequency range (typi-cally up to 12 second periods).

9.4 LOAD ANALYSIS METHODS

9.4.1 General Considerations

9.4.1.1 Tendon loads consist of both static and dynamiccomponents. Static loads arise from tendon pretension, tide,platform offset due to steady environmental forces and foun-dation installation position errors. These loads may be deter-mined from the equilibrium conditions of the platform,tendons, and risers as discussed in 7.2.5 and 7.2.6.

9.4.1.2 Dynamic tendon loads arise from platform andseismic motions, wind gusts and direct hydrodynamic forces.Calculation of these loads and forces is described in the fol-lowing paragraphs.

9.4.2 Dynamic Analysis Considerations

9.4.2.1 General

9.4.2.1.1 Dynamic axial and bending tendon loads ariseprimarily from platform motions. Platform response analysisshould include both primary wave effects (loads at the pri-mary wave frequency) and secondary wave effects (loads atharmonic or subharmonic frequencies to primary wave fre-quencies, or at sum/difference frequencies in response tospectral wave input). Dynamic analysis of TLP tendon loadsshould take into consideration the possibility of platformpitch/roll resonant excitation due to primary and secondaryeffects. Linearized dynamic analysis does not include some ofthe secondary wave effects and it may not model accuratelyextreme wave responses. A check of linear analysis resultsusing non-linear methods may be necessary and a suitably

scaled model test may be used to conÞrm analytical results.Care should be exercised in interpreting model test results forresonant responses, particularly for loads due to platformpitch or heave, since damping may not be accurately modeled.

9.4.2.1.2 Tendon dynamic analysis may be coupled non-linear, coupled linear or uncoupled. In coupled analysis, thetransverse (bending) response of the tendons is calculatedsimultaneously with the platform response. Dynamic hori-zontal forces interact between the tendons and platform at thepoints of attachment. If dynamic tendon tension is alsoallowed in the tendon bending response analysis, then theprocedure is non-linear as well as coupled.

9.4.2.1.3 Platform response analysis with tendons mod-eled as springs having no transverse inertia, is uncoupled.Tendon bending response analysis can be performed sepa-rately using the results of uncoupled platform analysis. Fordiscussions of non-linear and coupled analyses, see de Boom,1983; Denise, 1979; and Halkyard, 1983.

9.4.2.1.4 One approach for uncoupled dynamic analysis isas follows:

a. Compute equilibrium (zero offset) conditions based onwater level and tendon pretension (see 7.6.2).b. Compute steady loads and offsets due to environmentalforces (see 7.6.3).c. Compute a linearized spring constant for the riser and ten-don reactions acting on the platform. This spring constantmay be a function of the steady offset and drawdown.d. Apply an equivalent vertical mass (typically one-third ofthe total tendon mass) to the platform tendon attachmentpoints to account for tendon mass. No mass should be addedfor accelerations in the horizontal directions.e. Compute platform response including tendon top tensions,to unsteady environmental forces (see 7.7)f. Perform a tendon lateral (bending) analysis using a risertype program as discussed in API Recommended Practice2Q. The platform horizontal motions at the tendon attach-ments, calculated in e should be imposed on the tendon top.The tendon tension may be constant (linear analysis) or varyaccording to the results of e, if the analysis is nonlinear. Also,rotational stiffness of the ßexible termination joints should beincluded in the analysis model.

9.4.2.1.5 This approach can be carried out using eithertime domain or frequency domain (linear only) techniques,although each has its advantages and disadvantages as dis-cussed below.

9.4.2.2 Frequency Domain Analysis

9.4.2.2.1 Frequency domain solutions for tendon responsecan be obtained by numerical solution of the linearized Þniteelement or Þnite difference equations for the tendons. Thisapproach requires modeling the hydrodynamic drag as a lin-


ear damping term and assumes small excursions from themean position. The solution will be in the form of a transferfunction relating tendon response (bending, displacement,etc.) as a function of frequency. The forcing function will befrom platform motions and direct hydrodynamic forces.Therefore a linear relationship, including phase relationships,between these forces and primary wave height and frequencyneeds to be determined from the motion analysis as input tothe tendon load analysis.

9.4.2.2.2 Frequency domain analysis is well suited forfatigue and operational analysis, since the transfer functionsmay be applied directly together with sea spectra to arrive atexceedance statistics for tendon loads and stresses. In order todo this, it is best to combine the results for tension, bending,and shear loads to arrive at a response amplitude operator fortotal combined stress prior to application of spectral tech-niques. Otherwise, the phase relationships for the combinedload cases are not taken into account.

9.4.2.2.3 The linearizations required for frequency domainanalysis might lead to inaccurate results for extreme loads. Inthis case, time domain analysis should be carried out andcompared.

9.4.2.3 Time Domain Analysis

9.4.2.3.1 Time domain solutions offer the advantages ofallowing the direct inclusion of non-linear effects and thedetermination of the phase relationship for combined load-ings (tension, bending, and shear) under a maximum designcondition. Several riser type programs are available for thisanalysis.

9.4.2.3.2 Uncoupled time domain analysis of tendon loadsrequires as input the time history of top tendon loads and plat-form offsets, hence a time domain platform motion isrequired. A simulated time history of platform motions andtop tensions can be constructed from frequency domain plat-form motion solutions if appropriate phase relationships aremaintained between the incident wave proÞle, motions, andtension and shear components.

9.4.2.3.3 To determine the extreme values for key parame-ters (e.g., tension or angle), cases should be run for severalwave frequencies at amplitudes consistent with expectedmaximum wave heights. Non-linear wave forms should beinvestigated to examine the possibility of tendon responses atresonant frequencies other than the primary wave frequency.Ideally, a random time series representing the wave form inthe design storm should be used to develop a histogram ofpeak loads. A sufÞcient number of complete cycles should becomputed in order to obtain a distribution of peaks.

9.4.2.4 Instabilities and Resonances

The tendon system may possess certain dynamic charac-teristics which should be given special consideration. Anexample of these are the axial forces caused by platformheave and pitch/roll oscillations at resonance. Other effectswhich should be considered include vortex induced vibra-tions (see 6.4.3.2) and seismic loads. In considering thesedynamic responses, the design should include an approxi-mate damping model accounting for both mechanical andhydrodynamic effects.

9.4.2.4.1 Axial Vibrations

Natural heave and pitch/roll frequencies may be determinedfrom the above analyses or from simple harmonic oscillatortheory. Natural periods typically will be in the range of 2 to 5seconds. While these periods are lower than the predominantwave energy, resonant ampliÞcation can lead to signiÞcanttendon tension oscillations at the natural period of heave orpitch. Two forcing mechanisms should be considered:

a. High frequency primary wave platform heave and pitch/roll response.b. Non-linear heave and pitch/roll motions at two and threetimes the primary wave frequency.

9.4.2.4.2 Transverse Vibrations

Transverse vibrational modes for the tendons can have nat-ural periods in the range of primary wave periods. Riser typeanalyses such as those discussed above and in API Recom-mended Practice 2Q should include a sufÞcient number ofnodes and time step intervals (for time domain analysis) toinclude modes with natural frequencies in the range of pri-mary waves.

9.4.3 Hydrodynamic Loads

9.4.3.1 Wave and Current Loads

The in-line force acting on tendons can be described by themodiÞed MorisonÕs equation (see 6.4.3).

9.4.3.2 Vortex-Induced Loads

Vortex-induced vibrations are described in 6.3.3.

9.4.4 Seismic Loads Analysis

Seismic response can be estimated using a dynamic anal-ysis method which includes the coupled responses of the plat-form, tendons, and foundations. This analysis can be madeusing either time history or response spectrum methods.


9.4.5 Representation of Multiple Tendons

Multiple tendons at each leg may be modeled as a singletendon provided the cross section, mass, hydrodynamic drag,and added mass properties are chosen appropriately. In cou-pled analysis, particular care must be taken in calculating thebending stress. The axial and ßexural stiffness of elastomericßex elements in the tendon system should also be modeled.

9.5 STRUCTURAL ANALYSIS METHODS


The design of components for the TLP tendon systemrequires a detailed knowledge of the stresses in each compo-nent. In some components, such as tubulars, this informationcan be obtained by simple calculations. However, for mostcomponents (couplings, top and bottom connections, ßex ele-ments, etc.), the determination of stress distributions shouldbe obtained from appropriate Þnite element analysis. Recom-mendations for ÒDetermination of Stresses by AnalysisÓ canbe found in API Recommended Practice 2R.

9.5.2 Stress Distribution Verification Tests

9.5.2.1 After completion of the design studies, it is recom-mended that the initial prototypes of the tendon componentsbe tested to verify the stress analysis. The testing has two pri-mary objectives: to verify any assumptions made in the analy-sis, and to substantiate the analytical stress predictions. Thiscan be accomplished by strain gauge testing or photoelasticmethods. Normal design qualiÞcation tests may be performedin conjunction with this testing. Components having beenproven by prior service or qualiÞcation may not require addi-tional testing.

9.5.2.2 The testing should serve to verify the pattern ofstrain in regions surrounding the critical points and to ensurethat no areas of stress concentration were overlooked in theanalysis.

9.5.3 Fatigue Analysis

9.5.3.1 In the design of tendon components, considerationshould be given to the fatigue damage that will result fromcyclic stresses. A detailed fatigue analysis should be per-formed using a PalmgrenMiner, S-N curve approach or frac-ture mechanics. The fatigue life of the tendon is deÞned as thetotal life to tendon failure, i.e., the life to which the tendonparts (critical failure).

9.5.3.2 The combined axial and bending stress historyshould be determined by dynamic analysis (see 9.4.2) andmay consider variations around the tendon circumference.Care should be taken to ensure that the S-N curve or crackgrowth parameters are derived under conditions representingactual materials, environments, cyclic stress range and fre-

quency, mean stress, level of cathodic protection, sectionthickness, shape and stress gradients.

9.5.4 Inspection Interval Selection

9.5.4.1 Periodic inspection of the tendons for fatiguecracks is desirable. A nominal inspection interval should beestablished by a fatigue analysis based on MinerÕs cumulativedamage theory and appropriate crack growth data asdescribed in 9.5.3.

9.5.4.2 The inspection interval may be determined usingthe fracture mechanics approach described in A.Comm.9.5.3.In this case, however, the initial ßaw size should be taken as,the maximum ßaw size likely to be missed, using the methodof inspection to be employed. For example, if a leak beforebreak criterion is used in tendon design, then the initial ßawsize would be the tendon wall thickness. The residual lifeshould be a multiple of the inspection interval. An example ofthis method is given in Halkyard (1986).

9.6 STRUCTURAL DESIGN CRITERIA

9.6.1 Design for Maximum Loading

9.6.1.1 The design of tendon system components for maxi-mum loading requires that they support their design loadwhile keeping the maximum stresses within the allowablelimits in this section. These limits are intended to preventstructural deformation that could lead to failure and ductilefracture with a factor of safety. Local peak stresses are notconsidered for maximum loading, but can be of primary con-cern for evaluating fatigue life as discussed in 9.6.4.

9.6.1.2 The allowable limits for maximum loading relateto the linear stress distribution at each of the critical crosssections within a component. The linear stress distribution ata cross section represents the combination of two categoriesof stress: net section stress and local bending stress. Net sec-tion stress at a cross section results from axial load, pressureand general bending moments, while local bending stressresults from local bending moments created by an eccentricload path.

9.6.1.3 The location of critical sections and the stress dis-tribution at these cross sections are obtained from an appro-priate analysis as described in 9.5.1; the stress distributionwill usually vary within the cross section, the area of maxi-mum stress being that of interest.

9.6.1.4 The critical cross sections within a component aregenerally cylindrical cross sections, or portions of cylindricalcross sections, obtained by passing a plane perpendicular tothe axis of the tendon.

9.6.1.5 The component internal loads acting on such across section are illustrated in Figure 14 for a hypotheticalcoupling design with an axisymmetric cross section, assumed


here to simplify the illustration. The component internal loadsshown result in the linear stress distribution used to evaluatethe component for maximum loading.

9.6.1.6 Referring to the axisymmetric cross section shownin Figure 14, the net section stress (sp) at the cross section isseen to result from the combined axial tension and generalbending moment, while the local bending stress (ss) is seen toresult from an eccentric load path. For combined axial tensionand general bending, both the net section stress and localbending stress vary around the circumference. For axial ten-sion only, the net section tensile stress and local bendingstress are constant around the circumference.

9.6.1.7 The stress output provided by typical programsgives the distribution of total stress, where the total stress rep-resents the combination of net section stress, local bendingstress and peak stress categories, as illustrated in Figure 15 foran axisymmetrical cross section. The net section stress is cal-culated as the constant stress distribution which results in thesame multiaxial loads on the cross section as that developedby the total stress distribution. The local bending stress is cal-culated as the linear bending stress distribution which resultsin the same bending moment on the cross section as thatdeveloped by the total stress distribution. The local peak stressand the resulting total stress, while not used for static loading,are more important for fatigue life and fracture mechanics cal-culations as discussed in 9.6.4 and 9.6.5. For each of thesestress categories, the Von Mises equivalent stresses are calcu-lated after formulation of the component stresses.

9.6.1.8 Although not all tendon system components havecylindrical cross sections, the loads and stress distributionswill be similar for noncylindrical cross sections (such as col-lets, segments, dogs, etc.). Therefore, the same principles dis-cussed are applicable.

9.6.1.9 The net section stress and the local bending stressat a cross section have different degrees of signiÞcance. Forthis reason they are given different allowable limits, theallowable limit for local bending stress being higher in accor-dance with the Limit Theory for combined bending and ten-sion. See ASME Boiler and Pressure Vessel Code, SectionVIII, Division 2.

9.6.2 Allowable Stresses

9.6.2.1 The allowable stresses for tendon components,safety criterion A in 5.5, are as follows:

a. Net section stress sp £ 0.6 Fy or 0.5 Fu, whichever is less.b. Local bending stress ss £ 0.9 Fy or 0.7 Fu, whichever is less.

9.6.2.2 The allowable stresses for tendon components,safety criterion B in 5.5 are as follows:

a. Net section stress, sp £ 0.8 Fy or 0.6 Fu, whichever is less.b. Local bending stress, ss £ 1.2 Fy or 0.9 Fu, whichever is less.

Where:Fy = minimum yield strength, ksi (MPa).

��

��

�

��

��

�Sectionthroughtendon

Generalbendingmoment

Tensile load

Sectionthroughcoupling

Localbendingmoment

Net sectionload due totension

Net sectionload due totension

Localbendingmoment

Net sectionload due tobending

Net sectionload due tobending

Note:�Local bending moment varies around circumference when general bending moment is considered; for design load condition of tension only, local bending does not vary around circumference.

Figure 14—Net Section and Local Bending Loads On a Cylindrical Section

Total stressdistribution

Localpeakstress

Localbendingstress

Netsectionstress Thickness

Von Misesstress

Equivalentlineardistribution

Localbendingmoment

Tensile load

Verticalplane throughaxisymmetriccoupling

A A

CL

Figure 15—Stress Distribution Across Section A-AFor Axisymmetric Cross Section


Fu = minimum ultimate strength, ksi (MPa).

9.6.2.3 In addition to complying with the above allowablestresses, the tendon static loading design should reßect theinßuence of the series and parallel nature of the tendon con-Þguration. The effect of multiple tendon elements on ultimatestrength reliability is discussed in Stahl and Geyer, 1984.

9.6.2.4 The net section stress and local bending stress arecombined using linear interaction as shown in Figure 16.Both categories of stress should be expressed in the form ofthe Von Mises equivalent stress.

9.6.2.5 The allowable stresses given in this section apply tosteel components but do not apply to chains or wire ropes.Alternate analysis methods and allowable stresses reßectingchain and wire rope technology should be used for thesetypes of tendon systems.

9.6.3 Hydrostatic Collapse

Hydrostatic collapse of the tendons should be preventedby using the appropriate analysis methods and safety factorsfrom the latest edition of API Recommended Practice 2A.

9.6.4 Fatigue Life

9.6.4.1 A nominal tendon component fatigue life (crackinitiation plus propagation) of 10 times the tendon design ser-vice life is recommended. This factor includes allowances foruncertainties in lifetime load prediction, S-N curve data scat-ter, approximations in linear damage theory and the effect ofmany tendon components being connected in series. This fac-tor should be used in conjunction with the component S-N

curve corresponding to a lower bound usually deÞned as thelower bound of a two-sided, 95-percent prediction interval.

9.6.4.2 A lower design fatigue life factor may be usedwhen a proven, reliable inspection/crack detection method(see 9.6.5) and expedient replacement (repair) plan are to beemployed. The factor might also be reduced by reduction inthe uncertainties, scatter and approximations mentionedabove (see Wirsching, 1986).

9.6.4.3 In no event, however, should the fatigue life be lessthan three times the design tendon service life.

9.6.5 Inspection/Replacement Interval

The inspection/replacement interval should not be morethan one-Þfth (1/5) of the time necessary for a reliably detect-able crack to grow to tendon failure. Besides in-place NDT,the inspection method could be leak-before-break or removalfor dry inspection.

9.7 FABRICATION

Procedures for the fabrication of tendon components arediscussed in 13.3.

9.8 INSTALLATION PROCEDURES

Procedures for the installation of tendon systems are dis-cussed in 13.6.4.

9.9 OPERATIONAL PROCEDURES

9.9.1 Load Monitoring

9.9.1.1 The tendon system should be suitably instrumentedand monitored to aid in operations and to ensure that the sys-tem is performing within design limitations.

9.9.1.2 Provision should be made to monitor tendon toptension. In addition, it may be desirable to monitor platformmean offset position and tendon upper and/or lower ßexjoint angles.

9.9.2 Tendon Retrieval and Replacement

The need to retrieve a tendon could arise: (1) as part of ascheduled plan for inspection or replacement, (2) in the eventof damage or suspected damage, or (3) in removing the TLPfrom site. Regardless of the reason for retrieval, the equip-ment, operations, and procedures involved should be carefullypreplanned and personnel trained to carry out the procedures.

9.10 CORROSION PROTECTION

Provisions for corrosion protection of tendons should beprovided as discussed in 14.7.

1.2 s Yield

0.8 s Yield

s Yield

Allowable stress

= Materialyield stress

*Net section stress (sp)

*Loc

al b

endi

ng s

tres

s (s

s)

*As derived from Von Mises�Equivalent Stress

Figure 16—Combined Net Section and LocalBending Stress Linear Interaction Curve


9.11 INSPECTION AND MAINTENANCE

Procedures for the inspection and maintenance of tendonsare discussed in 13.7.

10 Foundation Analysis and Design10.1 GENERAL


This section addresses the analysis and design of TLPfoundations. Discussions of fabrication, transportation, instal-lation, materials, monitoring, inspection, and maintenance asrelated to the foundation are also included.

10.1.2 Description of Foundation Systems

The term foundation system refers to the foundations usedto anchor the tendon legs to the seaßoor. A foundation systemcan consist of structures such as independent leg templatesand well templates or an integrated single piece foundationsupported or anchored by piles, gravity, mudmats or combi-nations of each.

10.1.2.1 Piled-Template Foundations

Foundations comprised of piles and template structures(integrated or independent) are addressed. Well templates arealso addressed since the well template may be integrated withthe leg templates. Figure 17 illustrates an integrated founda-tion and Figure 18 independent templates.

10.1.2.2 Shallow Foundations

Shallow foundations principally address gravity foundationsystems but also include the piled template during installationprior to pile placement. Figure 19 is an example of a shallowfoundation system. Mudmat design and analysis is covered inthe shallow foundation subsection.

10.2 FOUNDATION REQUIREMENTS AND SITE INVESTIGATIONS

10.2.1 Foundation Requirements

10.2.1.1 The primary function of the foundation system isto anchor the tendons.

10.2.1.2 Load transfer to the soil can be accomplished in anumber of ways. For example, through tendons directlyattached to piles, through templates which distribute tendonforces to the soil via piles, or through a gravity base.

10.2.1.3 The use of a template structure requires consider-ation of several factors including: template conÞguration,structural strength, installation feasibility, required positionaland alignment tolerances, connections with the tendons and

Figure 17—Components of an IntegratedTemplate Foundation System

Figure 18—Components of an IndependentTemplate Foundation System

Figure 19—Components of a ShallowFoundation System


risers, and if applicable, connections between the templateand piles.

10.2.1.4 The design of the foundation structure shouldensure that permissible limits of stress, displacement, andfatigue are not exceeded during and after installation. Particu-lar attention should be given to loading eccentricity arisingfrom tendon/riser force variations within a group, tendon/riserinstallation sequences, and possible tendon/riser retrieval andredeployment during the platformÕs operational life. The per-missible soil stress and displacement should be establishedconsidering variations in soil properties resulting from cyclictensile and lateral loadings, and in the case of pile supportedfoundations, potential creep due to sustained axial tensionloadings. Consideration should also be given to the loss offoundation capacity due to scour or soil instability (e.g., mud-slides and liquefaction).

10.2.1.5 The foundation system design should include pro-vision for inspection and maintenance. The extent of inspec-tion and maintenance should be commensurate with theredundancy relative to overall safety and performance.

10.2.1.6 As the primary objective of an analysis is toobtain realistic predictions of foundation response to loading,soil properties that best represent the parameters of interestshould be used. Design, on the other hand, seeks to assurethat particular levels of safety are possessed by the foundationto resist loads predicted by analysis. Therefore in design, soilparameters should be modiÞed by factors to reßect uncer-tainty and risk.

10.2.2 Site Investigations

10.2.2.1 Requirements for site investigations should beguided primarily by the type and function of the platform tobe installed, the availability and quality of data from prior sitesurveys, and the consequence which would result from a par-tial or complete foundation failure. Special problems includedeepwater sites and unusual loading conditions.

10.2.2.2 The measured properties of soil samplesretrieved from deep waters may be different from in-situ val-ues. Without special precautions, the relief of hydrostaticpore pressure and its resulting effect on any dissolved gasescan yield soil properties signiÞcantly different from in-situconditions. Because of these effects, in-situ or special labo-ratory testing to determine soil properties is warranted. Sinceinstallation sites may be remote from areas for which exten-sive site data are available, regional and local site studies toadequately establish soil characteristics may be required.Previous site investigation and experience may permit a lessextensive site investigation.

10.2.2.3 The upward static and dynamic loadings are dif-ferent from those typically experienced by a jacket-typestructure. Piled TLP foundations are subjected to constant

and cyclic tensile load components which can result in ten-sile creep of the foundation. Tests to ascertain the long-termsoil-pile response when subjected to these loadings shouldbe performed.

10.2.2.4 A site investigation program should be accom-plished for each platform location. The program should, as aminimum and preferably in the order listed, consist of thefollowing:

a. Geological surveyÑBackground geological data shouldbe obtained to provide information of a regional characterwhich may affect the analysis, design and siting of the foun-dation. Such data should be used in planning the subsurfaceinvestigation, and to ensure that the Þndings of the subsurfaceinvestigation are consistent with known geological conditions.b. Seaßoor and sub-bottom surveyÑGeophysical informa-tion should be obtained relating to the conditions existing atand near the surface of the seaßoor and include:

1. Soundings and contours of the seaßoor and shallowstratigraphy.2. Position of bottom shapes which might affect scour.3. The presence of boulders, obstructions, and smallcraters.4. Gas seeps.5. Shallow faults.6. Slump blocks.7. Scarps.8. Cuttings for pre-installed templates.9. Previous usage of seaßoor.

The background geological data should be incorporatedinto the results of the geophysical survey to provide an overallgeological evaluation of the area and to determine the exist-ence of any geologic hazards.c. Geotechnical investigationÑThe subsurface investigationshould obtain reliable geotechnical data concerning thestratigraphy and the lateral variability of the soil. The sam-pling and in-situ testing intervals should ensure that a reason-ably continuous proÞle is obtained.

The design soil parameters in various soil strata should bedetermined from a Þeld program that tests the soil in as nearlyan undisturbed state as feasible. Because the quality of soilsamples can be expected to decrease with increasing waterdepth, the use of in-situ testing techniques are recommendedfor deepwater sites. In addition, soil samples will be requiredto characterize the soil types and provide other basic engi-neering property data.

The number and depth of soil borings required will dependon the foundation geometry and the natural variability of thesite geology.

For pile foundations, the minimum penetration of at leastone boring should exceed the anticipated design penetration.For piled or non-piled gravity foundations, the minimum pen-etration of each boring should be related to the expected zone


of inßuence of the loads imposed by the base. Appropriate in-situ tests should be carried out, where possible, to a penetra-tion that will include the soil layers inßuenced by the founda-tion components.

Additional shallow sampling and testing should be per-formed to allow accurate predictions of near surface soil-foundation interaction, and to assess the variation of soilstratigraphy across the site.

Recovered samples which are to be sent to an onshore lab-oratory should be carefully packaged to minimize distur-bance, changes in moisture content, and temperaturevariations. Samples should be labeled and the results of initialinspection of the samples recorded, including soil fabric andsample disturbance.d. Soil testing programÑThe soil testing program shouldconsist of in-situ and laboratory tests to establish classiÞca-tion properties for all soil and rock strata and to obtain classi-Þcation properties and initial estimates of the soilÕs strengthand deformation properties. When applicable, testing shouldbe performed in accordance with ASTM or other applicablestandards. Additional testing should be performed to deÞnethe dynamic and cyclic behavior of the soil to allow predic-tion of soil structure interaction due to sustained and cyclicloading. Consideration should be given to the performance ofpermeability and consolidation tests.e. Preinstallation seaßoor surveyÑA seaßoor survey shouldbe made immediately prior to the installation of the founda-tion system to determine the most recent changes to the bot-tom topography.f. Additional studiesÑAs applicable, analytical studies orscaled tests should be performed to assess the effects listedbelow:

1. Scouring potential.2. Hydraulic instability and occurrence of sand waves.3. Ground response studies or analysis.4. Seaßoor instabilities in the area where the foundationsystem is to be placed.

10.3 LOADING

The basic deÞnition of load types and conditions are foundin 5.5. These deÞnitions are ampliÞed here as they apply tothe foundation design.

10.3.1 Load Types

The load types deÞned in 5.5.1.2 need to be considered aseither static or dynamic loads when applied to the foundationdesign.

10.3.1.1 Static

A static load is an externally applied force of constantmagnitude. Some loads that vary over relatively long timedurations can also be considered constant. For example, the

forces from movable drilling equipment or the forces due toastronomical and wind-driven tides and currents are consid-ered static.

10.3.1.2 Dynamic

A dynamic load is due to an externally applied force ordisplacement whose time rate of change can produce addi-tional external and/or inertial forces. Dynamic loads can beclassed as:

a. CyclicÑrecurring, varying forces induced by waves, windor earthquake motions.b. ImpactÑnon-cyclic forces induced by dropped objects,boat collision or drill rig hook loads.c. TemporaryÑdynamic forces induced by an event of shortduration such as those due to launching, lifting, placement orpile driving.

10.3.2 Loading Conditions

Forces at the connections between the foundation and thetendons, risers and pipelines should be applied as loads.Loads due to the effective weight of the templates, appurte-nances, piles and conductors should be applied as appropri-ate. The following loading conditions represent the minimumrequirements for the analysis and design of the foundationsystem (see Table 1):

10.3.2.1 Extreme

Drilling and/or producing equipment loads appropriate forcombining with tendon and riser forces and foundation sys-tem loads resulting from extreme environmental events.

10.3.2.2 Normal

Drilling and/or producing loads appropriate for combiningwith tendon and riser forces and template loads resultingfrom normal condition environmental events (conditionswhich occur frequently during the life of the platform).

10.3.2.3 Fatigue Loading

Consider cyclic tendon and riser forces and towing loadson the template. Also consider forces induced by pile driving.

10.3.2.4 Damage

Tendon and riser forces imparted to the foundation systemin which it should be assumed that one compartment of thehull has been damaged (ßooded). Environmental conditionsshould be the reduced extreme event.

10.3.2.5 Tendon Removed

A recurrence interval based upon the time required toreplace a tendon should be determined. Tendon and riser


forces imparted to the foundation system loads should bebased upon the environmental conditions established for thiscase.

10.3.2.6 Transportation and Installation

10.3.2.6.1 Static and/or dynamic loads are imposed on thecomponents of the foundation system during the operations ofmoving them on and off a barge, during tow and placement.

10.3.2.6.2 Environmental effects on transport vessel orself-ßoating motions appropriate to the tow route and installa-tion site should be considered in determining the following:

a. Forces produced during lifting, launching, self-ßoating orlowering of the foundation system components.

b. Forces that result from on-bottom placement and levelingof the structure such as those due to mudmat reactions.

c. Impact forces due to latching and/or stabbing piles, ten-dons and risers. The anticipated running rate and the effect oftensioning devices should be considered.

d. For driven piles, the static weight and impact load causedby the hammer should be considered. The effects of oceancurrents on the freestanding portion of the pile and hammerassembly should be included.

The penetration of the pile should be calculated accountingfor pile/hammer assembly weight, restraint offered by thetemplate, and soil friction and end bearing. The length of thefreestanding portion of the pile can then be determined.

e. For foundation systems utilizing drilled and grouted piles,the effective weight of piles supported by the structure shouldbe included. After grouting to the soil, a pile should not beassumed capable of carrying its own weight until after a suit-able set-up time. Pile buoyancy should be considered becauseof differences in internal and external ßuid densities.

f. Pipeline pull-in forces on the foundation.

10.3.2.7 Seismic

Reference 9.3.2.6 and 9.4.4.

10.4 ANALYSIS PROCEDURES

10.4.1 General

This section presents guidelines for analysis procedures forthe response of TLP foundations under the loading conditionsdetailed in 10.3.2. Analysis is differentiated from design bythe range of behavior (response) of the foundation and theneed to provide an understanding of the primary factors con-trolling the interaction between structure, piling and the sup-porting soil for given geometry, soil characteristics, andloading conditions. Analysis provides the internal memberforces and displacements for use in design.

10.4.2 Analysis of Piled-Template Structures

Because piled foundation templates are similar to Þxedplatform foundations, the recommendations given in APIRecommended Practice 2A should be considered whereappropriate.

10.4.2.1 Template Modeling

The foundation template should be analyzed using a modelwhich represents the geometric, stiffness, and damping char-acteristics of the structure. Normally, a three-dimensionalspace frame model of the template should be used to predictload distribution and stress levels in members of the template.Simpler two-dimensional models may be sufÞcient if thegeometry and loading characteristics allow such simpliÞca-tion. Three-dimensional analyses may be required to furthercheck the validity and adequacy of any two-dimensional anal-yses. The interaction between tendon connectors, template,and piles should be considered.

10.4.2.2 Soil Modeling

10.4.2.2.1 The manner in which the foundation soil ismodeled and the selection of the values (and range) of engi-neering properties of the soil is important. The soil modelshould reßect the characteristics and interactive response ofthe affected soil zones and be consistent with modeling tech-niques and level of sophistication used for the rest of thestructure. For example, the soil may be modeled as a contin-uum or a set of discrete springs.

10.4.2.2.2 Selection of engineering properties of the soil,such as undrained shear strength, effective friction angle,PoissonÕs ratio, elastic and shear moduli should be consistentwith the soil model, loading condition, and type of analysis.In addition, the designer/analyst should estimate possiblevariations and ranges of the parameters and assess the effectsof these variations on the response of individual componentsand the overall system. Effects of possible liquefaction shouldbe considered.

10.4.2.3 Pile Soil Interaction

Where appropriate the pile may be modeled by discreteelements which account for the stiffness and damping charac-teristics of soil-pile interaction. For foundations with closelyspaced piles, the effects of group interaction on response andcapacity should be evaluated. The effects of lateral load onaxial behavior should be considered because of the conse-quences of large upward foundation movements.

10.4.2.4 Conductor Modeling

For a template with drilling conductor slots (whether sepa-rate or integral), the effect of the conductors on the behaviorand strength of the foundation system should be assessed.


The same analysis, design and installation considerationsused for the pile would apply to a conductor, but with theaddition of fracture gradient considerations.

10.4.3 Analysis of Shallow Foundations

10.4.3.1 Mudmat Modeling

Mudmats, in general, are similar to those for jacket-typestructures for analysis purposes. The designer/analyst shouldgenerally follow the recommendations in API RecommendedPractice 2A, accounting for the expected loading conditionsand duration of service.

10.4.3.2 Gravity Template Modeling

Modeling of a gravity type foundation subjected to higheccentric uplift loads should consider the problems of poten-tial suction under the base and lateral stability under theeccentric uplift loads, together with the interaction betweenfoundation, soil and skirts. Linear or nonlinear analysis meth-ods may be used. Analyses should model the cyclic nature ofthe loads and pore pressure generation and dissipation undercyclic loads.

10.5 DESIGN OF PILED-TEMPLATE STRUCTURES

10.5.1 General

10.5.1.1 Internal member forces derived by analysis (see10.4) should be used to determine the required member sizes.Local details and secondary members such as pile guidecones, tendon stabbing cones, padeyes, grouting systemattachments and other appurtenances not included in the anal-ysis may be designed by detailed local analysis.

10.5.1.2 For the design of a steel template structure refer-ence should be made to API Recommended Practice 2A forallowable design stresses and fatigue estimation procedures.Reference should be made to Section 8 for the minimumdesign fatigue life of the steel foundation components.

10.5.2 Tendon Connection

The tendon attachment should be detailed to ensure all loadcomponents are safely transmitted into the main templatestructure or directly into the piles. Detailed analyses may berequired to determine the stress distribution in the regionaround the connection. The template should be detailed toprovide adequate clearance between the tendons and the tem-plate structure during the maximum platform offset, consider-ing the effects of marine growth or debris.

10.5.3 Pile-Template Connection

Piles may be connected to the template by grouted pilesleeves, mechanical connectors or other means. The use of

shear keys on the pile and pile sleeve can signiÞcantlyincrease grout bond strength. Reference can be made to APIRecommended Practice 2A for the design of grouted pile-template connections. Detailed analyses may be required todetermine the stress distribution in the region of the pile-tem-plate connection.

10.5.4 Installation Aids

Details of the template structure should consider therequirements for installing piles, tendons and conductors.Appurtenances should be designed to withstand the effect ofimpact, driving, or other loads during pile installation.

10.5.5 Corrosion Protection

Reference can be made to API Recommended Practice 2Afor corrosion protection considerations.

10.6 DESIGN OF PILES

Pile design should conform to the practice given in APIRecommended Practice 2A except as noted below for axialpullout loads and factors of safety.

10.6.1 Axial Capacity

10.6.1.1 Ultimate pullout capacity should be calculatedusing API Recommended Practice 2A.

Q = fAs (42)

Where:

Q = ultimate pullout capacity

f = unit skin friction capacity

As = side surface area of pile

10.6.1.2 The design pile penetration, including the appro-priate safety factors, must be sufÞcient to develop adequatecapacity to resist axial loads unique to TLP foundations.Allowance should be made for maximum computed axial ten-sion load, cyclic degradation about a sustained tension load,axial ßexibility of the pile, the effects of sustained tensionloading, group effects, and the potential of near surface axialcapacity reduction from gapping caused by lateral loading,scouring, or liquefaction.

10.6.1.3 The following factors of safety are recommendedfor use in conjunction with Equation 42 for each pile in agroup and to the group taken as a whole:

Where B is a bias factor modifying API RecommendedPractice 2A recommended jacket compression pile factors ofsafety for tension pile applications. B may vary between indi-vidual piles and pile group.


10.6.1.4 Five speciÞc aspects of pile foundation design areto be considered in the determination of B, as follows:

a. Uncertainties in understanding soil-pile behavior undertensile loadings.

b. Lack of residual strength of the soil-pile system.

c. Load redistribution capabilities of the foundation system.

d. Relative difÞculty of foundation installation, and relativeintegrity of soil samples obtained from deep water.

e. Refer to the commentary for a detailed discussion of thesefactors.

10.6.1.5 The recommended range for the minimum valueof B is 1.0 through 1.5. As an example, a highly redundantfoundation system capable of effectively redistributing theload from a failing pile to the other piles in the group wouldhave a lower B value. A higher B value would apply to a sin-gle pile or ßexible template foundation system incapable ofeffective redistribution.

10.6.2 Laterally Loaded Piles

The ability of the piles to resist lateral loads and momentsshould be checked using the criteria given in API Recom-mended Practice 2A. Also the calculated lateral displacementshould be consistent with the rest of the foundation design.

10.6.3 Installation Method

10.6.3.1 Driven Piles

Refer to API Recommended Practice 2A.

10.6.3.2 Drilled and Grouted Piles

Refer to API Recommended Practice 2A. A deep waterenvironment requires additional installation considerations,such as formation (hydraulic) fracturing, inspection and qual-ity control.

10.6.3.3 Belled Piles

Refer to API Recommended Practice 2A. A deep waterenvironment requires special consideration of construction,inspection, quality control and formation fracture potential.

10.6.3.4 Jacked-in Piles

In contrast to driven piles, the capacity of jacked-in pilesduring and immediately after installation may be higher thanthe long-term capacity. This short-term penetration resistanceand long-term capacity should be evaluated.

10.7 DESIGN OF SHALLOW FOUNDATIONS

10.7.1 Soil Characteristics

The ability of the soil to resist loads from shallow founda-tions should be evaluated by considering the stability againstoverturning, bearing, sliding, uplifting or a combinationthereof. The foundation load-deformation behavior is gener-ally characterized using the stiffness and damping of thefoundation. Soil properties needed for such an evaluationinclude, but are not limited to, the soil shear strength, moduli,compression index and unit weight. An understanding of thepresent and past state of stress (stress history) of the soildeposit is also necessary.

10.7.2 Design of Gravity Template

A gravity template should be designed in accordance withthe Shallow Foundation design practice given by API Recom-mended Practice 2A with the following additions.

10.7.2.1 Uplift Capacity

The resistance of a gravity template against an uplift forcecan be obtained from the following equations:

10.7.2.1.1 For undrained total stress (f = 0¡) analyses

Q = Wb + a cAs (43)

Where:a = empirical adhesion factor, typically less than 0.5.f = soil friction angle.c = undrained shear strength.

Q = maximum vertical uplift load at failure.Wb = submerged weight of the gravity template.As = embedded side surface area of the gravity tem-

plate and/or skirt.

10.7.2.1.2 When a gravity template is subject to a dynamicuplift force, a temporary suction caused by negative porepressure and adhesion may develop at the base of the tem-plate and the uplift capacity will be greater than that com-puted from Equation 43. This additional capacity depends on,among other factors, the permeability of soil, drainage paths,duration of applied load, and geometry of the foundation. Asits effect is temporary, it shall not be accounted for in thedesign unless substantiated by appropriate analysis or experi-mentation.

10.7.2.1.3 For drained condition:

Load Condition Safety Factor

Extreme environment 1.5 ´ B

Normal environment 2.0 ´ B

Damage (w/reduced extreme env.) 1.5 ´ B

One tendon removed (w/reduced extreme env.)

1.5 ´ B


Q = Wb + (c¢ + K Po¢ tan d) As (44)

Where:c¢ = effective cohesion intercept of Mohr-Coulomb

envelope.d = effective friction angle along slip surface.K = coefÞcient of lateral earth pressure at rest.Po = effective overburden pressure.

10.7.2.1.4 Safety FactorsÑAPI Recommended Practice2A (Safety Factors) should be modiÞed to include UpliftFailure.

10.7.2.1.5 The design based on Equations 43 and 44should satisfy one of the following safety factors:

a. When considering only the submerged weight, Wb in theabove equations a factor of safety of 1.25 is acceptable. b. When considering both terms, a factor of safety of 2.0 isrequired.

10.7.2.2 Sliding Capacity

API Recommended Practice 2A (Sliding Stability) is mod-iÞed as follows:

10.7.2.2.1 Undrained analysis:

H = cA (45)

Where:H = horizontal load at failure.c = undrained shear strength.A = effective bearing area of foundation.

10.7.2.2.2 Drained analysis:

H = c¢ A + V tan d (46)

Where:c¢ = effective cohesion intercept of Mohr envelope.V = maximum vertical load at failure.

10.7.2.2.3 Skirts may increase the sliding capacity of thefoundation. Guidance for design/analysis is currently underdevelopment.

10.7.2.2.4 Safety factorsÑThe design based on Equations45 and 46 should satisfy a 1.5 factor of safety for all loadingconditions.

10.7.3 Design of Mudmats

Mudmats are used to temporarily support the foundationtemplate during installation and should be designed to con-sider short-term bearing failure, sliding stability and shortterm deformation in accordance with API RecommendedPractice 2A (Shallow Foundations). In the event mudmats aremade a permanent part of the foundation template, their inßu-

ence on the performance of the foundation should be consid-ered. However, mudmats should not be relied upon to providelong term sliding or uplift resistance.

10.7.4 Site Preparation

Obstructions on the seaßoor should be removed prior totemplate installation. The foundation surface should be pre-pared to avoid high localized contact pressures. Voidsbetween the gravity template structure and the seaßoorshould be considered and may be Þlled with grout to achieveeffective contact during installation. The grout should bedesigned so that its strength properties are compatible withadjacent surface soil.

10.8 MATERIAL REQUIREMENTS

Section 14 describes general material requirements. Itemsof special interest are: grout for a drilled and grouted-pilefoundation, and the material connecting piles to templatesand templates to tendons.

10.9 FABRICATION, INSTALLATION, AND SURVEYS

For design considerations arising from fabrication andinstallation requirements refer to Section 13. Monitoring, sur-veys, and maintenance requirements for the foundation arealso provided in Section 13.

11 Riser Systems11.1 GENERAL

11.1.1 Scope

This section discusses structural analysis procedures,design guidelines, component selection criteria, and typicaldesigns for riser systems. Guidance is also given for develop-ing load information for the equipment attached to the ends ofthe riser.

This section primarily addresses rigid steel risers (i.e., notßexible pipe), where risers are not used as/or part of/the ten-dons. When the functions of risers and tendons are combined,structural design should also consider the recommendationsoutlined in Section 9.

11.1.2 Functions of Risers

A TLP requires risers to provide conduits between the sea-ßoor equipment and the surface platform. Risers may per-form the following functions:

a. Export, import, or circulate ßuids.b. Guide drilling or workover tools to the wells.c. Support auxiliary lines.d. Serve as, or be incorporated in, the tendon.


11.1.3 Description of Risers

Drilling and production riser conÞgurations depend onwhether the drilling BOP and well completions are located atthe surface or subsea. In general, risers have a tensioningdevice at the top and a moment controlling device and a con-nector at the bottom. The tensioning device may allow rela-tive vertical motion between the riser and the platform, whilethe subsea connector provides a seal between the removableriser and the subsea well or pipeline.

Risers, regardless of function, may be classiÞed in twobroad conÞguration categories: integral and nonintegral. EachconÞguration is further subdivided into internal or externaltube types.

11.1.3.1 Integral Risers

11.1.3.1.1 The component ßowlines of an integral risermay not be retrieved separately. An integral external riserconsists of a central structural pipe which may carry ßuids orprovide structural support. Smaller diameter ßowlines areattached to the central pipe with brackets. Each end of thecentral pipe is Þtted with half of a quick make-up coupling. Asection of the central pipe, the external ßowlines and the cou-pling are together called a Òjoint.Ó When two joints are matedtogether by connecting their coupling halves, the ßowlinesare also connected with full design pressure sealing integrity.

11.1.3.1.2 The integral internal riser contains smallerdiameter ßowlines within the bore of a large pipe. Thesetubulars are supported by plates attached to the internal shoul-ders of the large pipe at the junction of the coupling half andthe pipe. Centralizers at intermediate points within a jointprevent the tubulars from buckling. An example of an integralinternal riser is a workover riser with multiple bores provid-ing direct access to the internal bores of a subsea tree andcontinuity to the surface.

11.1.3.2 Non-Integral Risers

11.1.3.2.1 The component ßowlines of a non-integral risermay be retrieved separately. The non-integral riser canassume several conÞgurations. The concept consists of a ten-sioned structural pipe and provision for lateral support ofinternal or external tubulars. This pipe is installed and ten-sioned Þrst to establish the structural support. Then the tubu-lars are installed through external guides or through the boreof the structural pipe.

11.1.3.2.2 Non-integral external riser concepts have beenused as production risers from semisubmersible ßoating pro-duction systems. They have been proposed for use with TLPproduction systems. The weight of the tubing may be carriedat the mudline or at deck level.

11.1.3.3 Riser Examples

11.1.3.3.1 One type of non-integral internal productionriser, Figure 20, which has been built (Goldsmith, 1980), is anextension of the well bore and permits a deck-level comple-tion similar to that used on a conventional Þxed platform.

11.1.3.3.2 Alternatively, as shown in Figure 21, a subseacompletion system may be used. Several possible riser con-Þgurations are illustrated in Figure 21.

11.1.3.3.3 The type of riser system selected impacts plat-form design (Nikkel, et al., 1982). For example, when oneriser is required for each well, as in Figure 20, substantialdeck area, and payload capacity, may be needed. Similarly,well spacing and deck area can be impacted by the size of thesubsea equipment deployed through the deck openings.

11.2 RISER DESIGN


11.2.1.1 Riser design requires that the riser response to theplatform motions and the environmental loads be obtained.Local forces and moments derived from the response analysisare then used for the design of the individual riser compo-nents. This section describes input parameters for responseanalysis. A.Comm.11.3 describes recognized analytical meth-ods, and 11.4 outlines the use of riser response results in com-ponent design.

��

�

��

��

��

Productiontree

Platform

Fluidtransfersystem

Tensioner

Riser joints

Couplings

Guidanceequipment

Wellhead

Mudline

Taperedjoint

Connector

Template

Figure 20—Deck-Level CompletionProduction Riser


11.2.1.2 A riserÕs ability to resist environmental loading isderived largely from the applied top tension. Riser design andthe selected top tension should be based on their response toenvironmental loads and their functional performancerequirements. The riser loads include hydrodynamic forces ofcurrent and waves, the motions of the platform, and the loadsimposed by contained ßuids and tubulars. Some of the func-tional constraints are top and bottom angles, steady and alter-nating stresses, and resistance to column buckling andhydrostatic collapse.

11.2.1.3 The design of riser components should accountfor operational procedures. For example, a drilling riserÕs ßexjoint angle should be maintained within rather low limitswhen drilling. However, during extreme conditions, drillingmay be suspended and the limitation on riser angles relaxed.Limiting criteria for riser design may then become maximumcombined stresses. During such a change of operation theriser top tension may be changed. Such decisions are a part ofthe riser design process and require analysis of the riser ineach potential operational mode. Refer to API RecommendedPractice 2Q.

11.2.1.4 Because TLPs may have multiple risers, mechani-cal interference, hydrodynamic interaction and hydroelasticvibrations may require special attention.

11.2.1.5 The following paragraphs describe the requireddata and the recommended procedure for analyzing riserresponse.

11.2.2 Riser Design Analysis Procedure

11.2.2.1 The following items describe a step-by-step pro-cedure for establishing riser component design loads, select-ing the appropriate top tensions for the range ofenvironmental conditions, and specifying the limiting condi-tions for each operational mode.

a. Drilling and production requirementsÑThe risersÕ conÞg-uration and properties as well as the internal ßuid densitiesare determined by the drilling program and productionrequirements.b. Site environmental dataÑWater depth and statistical dataon wind, wave, current, ice, tides, and seismicity for the siteshould be acquired in accordance with 5.4.

Figure 21—Subsea Completion Riser

Non-integralexternal

Integralexternal

Integralinternal

Production Riser Joint ArrangementsA – A

Subseatrees

Template

Productionriser

Tensioner

A A


c. Platform motionsÑAnalytical and/or model test data onplatform response should be acquired in accordance with 7.2.Platform response data may be used for translating wavemotion into riser top motions. Long-period excursions andsetdown should be included.

d. Design limitsÑParametric limits such as top and bottomriser angles, riser stresses, internal ßuid density, etc., deter-mine the required top tension and component design require-ments. Stress-range limits should be established based onfatigue considerations.

e. Parametric riser analysisÑRiser analyses should be con-ducted for a range of environmental and operational parame-ters to determine the required top tensions and limitation onoperational modes. Riser design should start with a proposeddesign and then iterate to identify the riser parameters, suchas wall thickness and material strength, etc., which satisfy thedesign objectives. Table 2 shows a typical data list for itera-tive riser analysis. Necessary analysis should be conductedfor installation/retrieval situations and installed riser cases.Computation of clearances between risers and the platformmay be necessary.

f. Riser installation and operational proceduresÑRiser oper-ating procedures should be prepared for all phases of riseroperations.

11.2.2.2 Figure 22 presents an example ßowchart for riserdesign showing major activities. Many other alternative pro-cedures are suitable. This ßowchart shows the interaction ofthe response analysis and component design phases and theirrelationship with other platform design activities.

11.2.3 Site Data

Site-speciÞc data required for the design and operation ofrisers should be gathered as deÞned in 5.4. The followingparagraphs present a general summary of the information thatshould be considered.

11.2.3.1 Waves

Waves produce motion of the platform and exert oscillatingforces on the risers. Wave data needed for riser analysisshould be developed from the data base describing the site, asdiscussed in 5.4.3.

11.2.3.2 Currents

11.2.3.2.1 Currents exert lateral forces on the risers. Thetype of current data which should be considered includes:

a. Current speed and direction proÞles for each month orseason.b. Components of total current such as wind driven or tidalportions.

11.2.3.2.2 For riser design proper combination of wavesand current should be considered. In some areas, phenomenasuch as internal waves and current eddies may affect riserdesign.

11.2.3.3 Earthquakes

Seismic motions should be considered in riser design forseismically active areas. Vertical and horizontal groundmotion data should be acquired.

11.2.4 Platform Data

11.2.4.1 Platform Configuration Data

The location of risers relative to each other and to the plat-formÕs structural members are needed for installation and in-place riser interference analyses.

11.2.4.2 Platform Response Data

11.2.4.2.1 The platform response to environmental load-ing is transmitted directly to the top end of the riser. Thesemotions constitute a source of dynamic loading on the riser.The various platform motion data that should be determinedin accordance with 7.3, are:

a. Static Offset (steady wave drift, wind, current forces).b. Wave Frequency Motions (1st Order).c. Low Frequency Wind and Wave Motions (2nd Order).d. Setdown.

11.2.4.2.2 First order motions are characterized, for theuse of riser analysis, by transfer functions which are linear inwave height and depend on wave period and direction.

11.2.4.2.3 The phasing described in the transfer functionsis normally relative to the platform center of gravity. Thisphasing should be corrected to account for the offset of risers.The exact phase shift depends on riser location, wave direc-tion, and wave length.

Table 2—Typical Data List For Analysis of Risers

Water Depth

Boundary Conditions (top tension, restraints, etc.)

Arrangement of Risers (relative to platform and other risers)

Riser ConÞguration (auxiliary lines, elevation of terminations, ßex joints and lateral constraints, etc.)

Riser Joint Properties (area, weight, material, etc.)

Buoyancy Device Properties (mass, lift in water, etc.)

Hydrodynamic Parameters (diameter, force coefÞcients, etc.)

Site Environmental Data

Platform Motions

Internal Contents (weight, pressure, etc.)


Figure 22—Riser Design Activities

OK

OK

OK

OK

*Include vortex-inducedoscillation analysis

Start

Environmentaldata

Operationalrequirements

Platformmotion data

Riser designlimits

Riseranalysis

load cases

Preliminaryresponseanalysis

Preliminaryriser design

Adjust:• Design• Platform• OperationalrequirementsCompare

Finalresponseanalysis*

Compare

Genericfatigue

analysis

• SCF Range• Fatigue data

Fatigue life?

Componentdesign loads

Componentstress analysis

Compare

Finalfatigue analysis

Fatigue life?

Risk analysis Acceptable? Riser designPlatform loadsEquipment loadsTop tensions

Componentpreliminary

designs

Adjustdesign


11.2.4.2.4 The low frequency wind and wave motions maybe considered as static in the riser response analysis. The plat-form undergoes a vertical setdown with horizontal offset thatshould be considered in the riser analysis.

11.2.5 Functional and Operational Requirements

Internal diameters and pressure ratings, bend radii, corro-sion protection, etc., should be based on the riserÕs functionaland operational requirements.

11.2.6 Limits for Design and Operation

Five types of limits for drilling and production riser systemdesign and operation are typically considered; maximumstress, maximum deßection, fatigue damage, stability, andfracture:

11.2.6.1 Maximum Stress

Dynamic riser simulation provides estimates of the loaddistribution on a riser system needed for design of riser com-ponents. These load distributions are typically converted tolocal stresses in riser components, incorporating the effects ofinternal and external pressures and thermal gradients. APIRecommended Practice 2R and 9.6.1 and 9.6.2 deÞne com-ponents of stress acting on a cross section. The net sectionstress and local bending stress are combined using linearinteraction as shown in Figure 15. These stresses, combinedwith other stress components on the basis of von Mises,should not exceed the allowable stresses recommended inTable 3.

11.2.6.2 Maximum Deflection

11.2.6.2.1 Maximum deßection limits should be based onthree factors:

a. The need to limit riser curvature to permit passage ofdownhole tools, and limit wear (Fowler and Gardner, 1980;API Recommended Practice 2Q).b. The maximum angles permitted by such riser componentsas ßex joints (API, Recommended Practice 2Q).c. The need to avoid collision between adjacent risers (Nikkelet al., 1982), and between risers and platform members.

11.2.6.2.2 API Recommended Practice 2Q provides limitsfor lower ball joint angles. The operator may select purpose-built drilling riser equipment for a TLP with limits differentfrom those in API Recommended Practice 2Q. Thus, riser-angle limits should be selected on the basis of the equipmentto be used.

11.2.6.3 Fatigue Damage

A detailed cumulative damage and/or crack growth analy-sis, accounting for all load cycling, should be performed forcritical regions in the riser system (Wybro and Davies, 1981).Total stresses, including local peak stresses as deÞned in9.6.2, should be used for fatigue analysis. S-N fatigue orcrack growth curves appropriate for the material and the envi-ronment should be used. Data should be characteristic of (1)the corrosive environment and corrosion protection, (2) themean stress, stress ratio (minimum stress/maximum stress),and frequency, and (3) the fabrication method and surface Þn-ish. The design fatigue life should be based on the intendedservice life and the inspection program schedule. A designfatigue life of a minimum of three times the intended servicelife should be used for each component. Consideration shouldalso be given to insuring adequate riser system life.

11.2.6.4 Stability

Risers should be designed to avoid collapse caused byhydrostatic pressure and column-buckling caused by insufÞ-cient tension. API Recommended Practice 2Q providesguidelines for calculating limiting collapse pressures forhydrostatic loading and combined axial and hydrostatic load-ing, and for evaluating effective tension.

11.2.6.5 Fracture

Riser materials and manufacturing procedures should beselected to avoid unstable crack propagation or fracture. Frac-ture mechanics methods should be used to evaluate the frac-ture toughness required for the expected service conditionsand to establish the limits for tolerable defect sizes.

11.3 RISER ANALYSIS METHODOLOGY

An analytical approach is used for determining the riserresponse. The mathematical models and the solution tech-niques have been widely treated in the literature. A bibliogra-phy is provided for the reader desiring detailed information,and a general discussion of the pertinent aspects of riser anal-ysis is contained in the commentary.

11.4 COMPONENT SPECIFICATION

11.4.1 Introduction

11.4.1.1 This section describes the various componentsthat make up the riser system and their relevant design con-

Table 3—Allowable Stress Limits For Riser System Design and Operation

Design Case

Use Lower Value Installation Normal Extreme

Net Section Stress% yield strength: 67 67 80% ultimate strength: 50 50 60

Local Bending Stress% yield strength: 87 87 120% ultimate strength: 67 67 90


siderations. General requirements common to all componentsare outlined in 11.4.2, and individual components are dis-cussed in subsequent paragraphs according to the followinggeneral format:

a. FunctionÑthe basic function of components within theriser system is described.

b. Selection/acceptance criteriaÑgeneral performance andqualiÞcation requirements are outlined.

c. Typical designsÑTypical existing designs are described.

11.4.1.2 Proven technology and hardware should be usedwhere practical. Concepts or designs derived from applicableßoating drilling or production equipment and current TLPconcepts are described.


The design engineer should have two primary objectives indeveloping speciÞcations for riser components:

a. The speciÞcations should be sufÞciently comprehensive toassure the required performance of each component and thetotal riser system.

b. The speciÞcations should include qualiÞcations criteria todemonstrate that the component design complies with thespeciÞcations.

The following list of general requirements for all compo-nents of a riser system should be considered in preparing thespeciÞcations.

11.4.2.1 Structural Integrity

The design load for each component should be based inpart on the riser response analysis described in 11.2. Suchloadings as tension, bending, torsion, pressure and thermalgradients should be included in the component speciÞcations.The resistance of the component to yielding, collapse, andfatigue should be demonstrated by analysis and test. Suchtechniques as Þnite element analysis and strain gage testingmay be speciÞed.

11.4.2.2 Sealing

Almost all riser components have some provision for seal-ing to segregate internal ßuid from the seawater, to isolatehydraulic or electric actuation systems, or to protect againstdirt contamination. The sealing devices are most critical whenthe pressure differentials are high and when there are movingsurfaces. The speciÞcations should call out required pressuredifferentials, types of ßuids which may come in contact withthe seals, and the cyclic life if there are moving surfaces.

Except for Þeld proven designs, qualiÞcation testing to thedesign environment should generally be required.

11.4.2.3 Handling and Storage

For many components handling during installation,retrieval and storage may impose the most severe loadingsand may warrant special consideration. Reference to APIRecommended Practice 2K, which is speciÞcally for drillingrisers, could give general guidance for the other riser systems.

11.4.2.4 Prototype Testing

Riser component designs may change from those currentlyused in offshore service. Riser components may require proto-type testing to verify the designÕs compliance with the designrequirements. The requirements for such testing should beincluded in the component design speciÞcation. QualiÞcationtesting of existing equipment may also be necessary.

11.4.2.5 Failure Propagation

A failure analysis (Woodyard, 1980) may be useful to iden-tify possible component malfunctions or failures which canpropagate, causing the failure of other system components.Frequently, component speciÞcations can be developedwhich will reduce or preclude major system failures resultingfrom individual component failure. The failure analysis canidentify where a redundancy or fail-safe design philosophy isappropriate.

11.4.2.6 Inspection

11.4.2.6.1 Inspection and testing should be performedperiodically during the operational life of the risers.

11.4.2.6.2 Riser systems may comprise a number of com-ponents made by various manufacturers. Proper speciÞcationof the components should reßect the requirements of the inte-grated system. Interface speciÞcations ensure dimensional,electrical, hydraulic, structural, and welding compatibilitybetween the components. The inspection program shouldassure that these speciÞcations are followed.

11.4.2.7 Maintenance

Replacement of seals, lubrication, inspection for structuralcracks, painting, etc., should generally be performed on ascheduled basis for drilling risers. The equipment should bedesigned to facilitate maintenance operations. ComponentspeciÞcations should include required maintenance opera-tions. Appropriate procedures should be developed for otherriser systems (API Recommended Practice 2K).

11.4.2.8 Corrosion

Protection of a riser system against corrosion involves con-sideration not only of individual components, but the risersystem and the equipment to which it is attached. Dissimilarmetals in the same area and electrical circuits set up by elec-


trically powered equipment are two examples of inßuenceswhich can accelerate corrosion. Another hazard is stress cor-rosion in high strength metals which are under continuousloading. These and other corrosion related effects, such asH2S and CO2 exposure, should be considered in preparing thecomponent speciÞcations. Additional corrosion consider-ations are presented in Section 14.

11.4.3 Tensioner System

11.4.3.1 Function

Tensioner units are used to maintain risers in tension as theplatform moves in response to wind, waves, and current andare commonly motion compensated, although motion com-pensation may not be necessary for deep water designs.

11.4.3.2 Selection Criteria

Riser tensioners with motion compensation should reliablymaintain top tension within a speciÞed range over a speciÞedstroke for the design life of the platform. A redundant strategymay be used to enhance tensioner reliabilities.

11.4.3.3 Typical Designs

One current production riser tensioner example consists offour hydraulic cylinders attached directly to the riser. Anyopposing pair of cylinders will support the riser. The cylin-ders are contained in a removable module which Þts into arecess in the deck. During normal operations, this tensionerrequires no outside power.

Tensioner support systems are discussed in 12.5.3.

11.4.4 Drilling Riser Telescopic Joints

11.4.4.1 Function

A telescopic joint may be needed, based on the strokerequirements of the tensioner, to allow for elongation due toplatform motions and environmental load. The telescopic jointhas a sliding seal and transmits the ßow of ßuids. It may alsoprovide attachment points for jumper hoses and tensioners.

11.4.4.2 Selection/Acceptance Criteria

The selection of a telescopic joint should include consider-ation and evaluation of the following basic items:

a. StrengthÑThe telescopic joint should be designed tosupport the imposed load and to resist the design internalpressures.b. Stroke lengthÑThe maximum stroke should accommo-date platform and riser relative motions.c. ServiceabilityÑThe packing should be easily serviced andmaintained.


Telescopic joints have an outer barrel connected to the riserand an inner barrel connected to the platform. A hydraulicallyor pneumatically actuated seal is used between the barrels.

11.4.5 Riser Joint

11.4.5.1 Function

A riser joint forms the basic element of a riser system. Ariser joint may support ßowlines in various conÞgurations asdescribed in 11.1.3.


Riser joint couplings should have strength equal to orgreater than that of the riser pipe. The riser joint designshould be evaluated to determine its ability to carry expectedtension, bending and internal pressures, and unexpected situa-tions such as internal pressure resulting from a tubing failure.Particular attention should be given to fatigue considerations(API Recommended Practice 2R).


Riser joints can take many forms, but a riser joint normallyconsists of a length of pipe with couplings on each end. Manytypes of couplings have been used including: threaded,ßanged, dog-type, clamped, breachlock, and welded.

11.4.6 Moment Controlling Device

11.4.6.1 Function

A moment controlling device is used at the bottom andsometimes at the top of the riser string to minimize bendingmoments or control curvature. Devices such as ball joints orelastomeric ßex joints reduce bending stresses induced by rel-ative angular movements at the ends of the riser. Taperedjoints are also used to control curvature and stresses.


The following should be considered when specifying ordesigning a moment controlling device:

a. The required bending ßexibility, angular limits and tensioncapacity should be determined by the riser analysis asdescribed in 11.3.b. The effect of temperature and internal and external pres-sure should be considered in the design.c. The fatigue life requirements should be established andevaluated.d. Corrosion and elastomeric degradation due to containedßuids may be an important consideration in an evaluation ofthe ßexible joint life. Protection against wear from the run-


ning and rotation of the drill pipe and the necessity of piggingsome risers may also be important.


The types of moment controlling device which have beenconsidered include: pressure balanced ball joints, universaljoints, elastomeric ßex joints, and tapered joints.

11.4.7 Connector

11.4.7.1 Function

A connector is used to latch risers to the subsea terminationand provide mechanical and pressure continuity. The connec-tor is used to connect the riser to the seaßoor equipment.Effective sealing is a primary consideration.


Mechanical strength of the connector should be sufÞcientto safely resist internal pressure and externally applied (riser)loads (API Recommended Practice 2R). Fatigue resistance isimportant, especially for use with production risers. The con-nectorÕs ability to engage and latch and to disengage in thepresence of angular, rotational and translational misalignmentshould be considered in the design. The ability of the connec-tor to maintain long term pressure seal integrity should beconsidered.


Hydraulic connectors use a large annular piston or multiplehydraulic pistons to activate the locking segments within theconnector. These segments mechanically engage the matingparts to effect a mechanical and pressure-tight connection.Other designs use mechanical latches with a hydraulic run-ning tool in which no permanent hydraulics remain subsea.

11.4.8 Spacer Frame

11.4.8.1 Function

Some production riser systems consist of a primary struc-tural tubular surrounded by an array of smaller, externallymounted ßowlines or an array of similar production risers.During installation, retrieval or operation, spacer frames canbe used to guide and position the tubing relative to the pri-mary structural tubular or to similar tubing. Spacer framesmay also be designed to provide structural support to reduceor eliminate hydroelastic vibrations.


Spacer frames should be designed to accommodate staticand dynamic loads between the tubulars. Their effect shouldbe included in the riser response analysis.


A number of conÞgurations are possible and the typeselected is determined by the riser concept used.

11.4.9 Production Tubing

11.4.9.1 Function

Production riser systems may incorporate tubing in addi-tion to a primary structural riser tubular. The tubing maytransmit produced ßuids to the platform or permit access tothe well annulus.


11.4.9.2.1 Production tubing is subjected to, and should bedesigned for, various types of operational loads and theirfatigue consequences, including:

a. Internal pressure.b. Thermal expansion/contraction.c. Hydroelastic excitation (when external).d. Tension.e. Bending.f. Buckling.

11.4.9.2.2 Other design guidelines, such as ANSI B31.4 orAPI Recommended Practice 14E, should also be used for tub-ing design.

11.4.9.2.3 Typical tubular arrangements are described in11.1.3.

11.4.10 Buoyancy Equipment

11.4.10.1 Function

Buoyancy, in the form of foam modules or air cans, isadded to risers to reduce the top tension required, to reducetension in the pipe wall and occasionally to reduce the heatloss from production lines.


11.4.10.2.1 The buoyancy system design should reßectservice conditions, handling loads, compatibility with risertype, maintenance requirements, and reliability (Agbezugeand Noerager, 1978; Watkins, 1978).

a. Foam ModulesÑFoam modules are passive, their volumeand density sized to provide the desired amount of buoyancy.The density and type of the foam depends on the servicewater depth and the required submerged lift.b. Air CansÑThe air can system requires a supply of com-pressed air. Selection of the air can volume should accountfor the weight of the pressurized air, which may be signiÞcantin deep water. Air cans may be fabricated from a number ofmaterials.


11.4.10.2.2 The air can normally has a much larger stiff-ness than the riser and may add signiÞcant stress to the cou-pling. These stresses should be considered in the design.


11.4.10.3.1 Foam Modules

These are cast sections of microspheres and hollow balls,in a high strength plastic matrix, designed to Þt the contour ofthe riser.

11.4.10.3.2 Air Cans

These are open-ended cans that encase a typical riser joint.The cans get their buoyancy by displacing the water withinthem with air. The buoyancy is a function of displaced air col-umn which can be controlled by adjusting the position of theair shut-off valve within the individual riser.

11.4.11 Instrumentation

11.4.11.1 Function

Riser instrumentation (Evans et al., 1981) may be desirableas an operations aid or in gathering data for conÞrmation ofdesign analysis methods. The instrumentation may includemeasurement of top and bottom angles, stresses, motions andexternal pressures. Other information, including environmen-tal and vessel motion data, may be desirable.


The choice of riser instrumentation should be based onaccuracy, reliability, measurement range, maintainability andability to resist service loading.

11.4.11.3 Acceptance Criteria

The riser instrumentation should be qualiÞed to functionunder extended exposure to the ocean environment for therequired service life. Particular attention should be given toelectrical insulation, instrumentation attachments, andmechanical wiring terminations. An Instrumented Riser Joint(IRJ) should be qualiÞed to the same strength and fatiguerequirements as other riser components.


The usual design for an IRJ is a special Òpup jointÓ whichreplaces the principal riser pipe. The pup joint carries instru-ments to measure stresses, pressures, angles, etc. ÒHard Wir-ingÓ is normally used to transmit the data to surface.

11.4.12 Vortex Induced Vibration Reducers

11.4.12.1 Function

This equipment suppresses vortex-induced vibrations andin some cases is used to reduce ßuid drag from ocean currentson risers.


In certain cases risers, and external ßowlines attached torisers, may be subject to vortex induced vibrations (Albersand DeSilva, 1977, Shanks, 1983). Susceptibility to suchvibrations depends on the coincidence of a structural naturalfrequency and a vortex shedding frequency acting over a sub-stantial extent of the riser. (See 6.3.3.)

As described in 9.4.3.2, appropriate analyses should beperformed to assess the likelihood of the occurrence of thisphenomenon.

11.4.12.3 Typical Design

Helical strakes of various types have been successfullyemployed to suppress vortex induced vibrations. Vanes orfairings also suppress vibrations and reduce drag.

11.4.13 Fluid Transfer System

11.4.13.1 Function

The purpose of the ßuid transfer system is to provide aßuid conduit which accommodates the relative motionsbetween the platform and the top of the riser. Some designsmay require such a system at the bottom of the riser.


Selection of a suitable ßuid transfer system should includethe following criteria:

a. Relative motion.b. Pressure and temperature.c. Resistance to corrosion/erosion/chemical attack.d. Gas diffusion.e. Conduit size.f. Reliability and maintainability.g. Routing and minimum bend radius required.


Telescopic joints and angular articulations (i.e., bell nip-ples or ball joints) typically accommodate the relativemotions for the bore of the drilling riser. For drilling and pro-duction risers, high pressure ßuids are normally transferredthrough ßexible or articulated pipe.


11.4.14 Guidance Equipment

11.4.14.1 Function

Guidance equipment is used to direct and orient risers ortools to the seaßoor template.


Design of guidance equipment should consider maximumenvironmental conditions selected for deployment andretrieval operations; size, weight, and drag characteristics ofequipment to be run; and proximity of operations to installedrisers, tendons, or platform structure.

11.4.14.3 Typical Design

A wide variety of approaches have been proposed. Thedesign selected should be compatible with the productionriser design and installation requirements. Guidelines, ten-dons, submersibles, etc., can serve as guidance equipment.Guidance may be a particular problem because positioningthe platform to facilitate stabbing is difÞcult.

11.5 OPERATING PROCEDURES

11.5.1 The operator should develop procedures for safeand efficient riser operations which are suitable to the particu-lar application. Such procedures should be developed in con-sultation with riser designers to ensure that limits establishedare consistent with the original riser specifications.

11.5.2 Operating procedures should cover all aspects ofriser operation including, but not limited to:

a. Riser handling and storage (on deck).b. Riser installation.c. Installed riser operation.d. Riser retrieval.e. Riser inspection and maintenance.

11.6 SPECIAL PROBLEMS

11.6.1 Loads on Sea Floor Equipment

11.6.1.1 Risers often impose signiÞcant forces andmoments on wellheads or other connecting subsea equip-ment, which should be designed to withstand them (Bednar etal., 1976). Estimation of these design forces should beobtained as a part of the riser response analysis.

11.6.1.2 The vertical design force is approximately equalto the riser pipe wall tension. The horizontal force is the hor-izontal component of the effective tension; i.e., the tensioncalculated by subtracting the wet weight of the entire riserand its contents from the top tension. This horizontal forcewill induce a bending moment in equipment below the riser.An additional bending moment may also be transmitted by

the riser, depending on the type of moment controllingdevice used.

11.6.1.3 In addition to implications for subsea equipmentdesign, riser loads may be important for the design of subseatemplates or conductor pipe to which they may be attached.Consideration should thus be given to riser load implicationsfor Section 10, Foundation Analysis and Design.

11.6.2 Interface with Platform Structure

Information on riser installation, retrieval, operation, clear-ance, hydraulic or pneumatic supply and structural loads isimportant for the design of the platform. This informationwill thus be needed for the design aspects treated in Sections8 and 12, Platform Design and Facilities Design, respectively.

12 Facilities Design

12.1 GENERAL

12.1.1 This section contains guidelines for planning,designing and arranging facilities for a TLP in U.S. waters,while recognizing the requirement for safe, environmentallyacceptable, efficient production of oil and gas. This sectionincludes hull systems, and addresses drilling and productionconsiderations unique to a TLP.

12.1.2 These guidelines:

a. Identify interfaces unique to a TLP with regard to:1. Structures.2. Production systems.3. Drilling systems.4. Hull systems (systems required for life support andfunctional operations excluding drilling and production).

b. Emphasize and provide recommendations on the need tolimit and control weight (dry and operating) and center ofgravity (CG) when selecting equipment and determiningequipment arrangements.c. Identify unique static or dynamic loads which could affectequipment selection.d. Identify industry codes, standards or guides which mightbe applicable to TLP design and which have acceptance byindustry and governmental bodies.e. Identify federal regulatory agenciesÕ established require-ments and indicate how they might inßuence design andarrangement of equipment.f. Provide guidelines for the selection of hull system equip-ment such as bilge, ballast, machinery, etc.

12.1.3 Detailed specific design guidelines for sizing, rat-ings, safety factors, etc., for production or drilling equipmentare not provided. The designer should refer to other API pub-lications for this information.


12.2 CONSIDERATIONS

12.2.1 Structural

12.2.1.1 The type of deck structure adopted will affectfacilities design. Plate girder members and bulkheads willimpose constraints on type and quantities of penetrationswhich may result in less than optimum routing of services.Structural design utilizing plate girders and bulkheads requireearly service routing agreement to assure that penetrationrequirements are identiÞed in time to allow structural designto proceed.

12.2.1.2 Deck height limitations desired to maintain CG aslow as possible can present facilities design with unique con-cerns:

a. Restricted height for installation of gravity systems suchas vents, drains, separation trains, etc.b. Mezzanine level having height restrictions more severethan for Þxed structure.c. May dictate that some equipment be installed on maindeck when optimal locations may be on lower decks.

12.2.1.3 Weight restrictions can dictate selection of mini-mum deck area loading criteria (PSF) and maximum totalload for a given area. Requirements for higher deck area load-ing for maintenance (egress, setdown and handling of equip-ment/machinery) need to be established early in design.

12.2.2 Arrangements

The principles of good practice, as applied to any offshorestructure, should be observed in the arrangement of equip-ment. For planning equipment arrangements, the guidelinesprovided in API Recommended Practice 2G, ProductionFacilities on Offshore Structures, should be consulted. Therelatively broad column spacing required for stability willprobably permit convenient equipment arrangements on theavailable deck area. In addition to the many equipment spac-ing considerations provided in API Recommended Practice2G, the following items should be considered:

a. A TLP is sensitive to the effects of weight and CG and theeffects of equipment arrangement/selection must be givenspecial considerations during the planning stages.b. Area classiÞcations and the separation of hazardous andnonhazardous areas need early resolution to minimize theneed for additional bulkheads, long vent ducting or additionalstructure.c. Adequate escape routes and equipment maintenanceaccess should be given attention since imposed space limita-tion will require compromises probably greater than thosefound on Þxed offshore structures.d. Installation of production or drilling equipment in hullspaces will provide greater ßexibility in CG location, but will

require special venting and bilge requirements and may haveother design penalties.

e. The hull structure may provide available space for con-sumable storage. Location of storage tanks upon or under thedeck presents additional operating weight, load and stabilityconcerns.

f. Inßuent and discharge caissons must be minimized. Con-sideration should be given to using sea-chests for both seawa-ter intake and efßuent disposal.

g. Enclosing areas should be minimized. These enclosuresnot only add structural weight, but also increase costs for util-ities, electrical, HVC, and safety systems.

12.2.3 Weight and Center of Gravity

12.2.3.1 Control of weight and CG location are essentialduring design and should be an initial design consideration.Weight and weight distribution will affect both the steady anddynamic tensions in the tendons.

12.2.3.2 Preliminary weight estimates should be as accu-rate as possible and adequate margins should be allowed forestimating inaccuracies, design growth, fabrication devia-tions, and platform operating requirements such that, whencompleted, the platform will remain within the establisheddesign parameters. Equipment, machinery, and tanks whichpresent heavy concentrated loads require early coordinationwith response analysis disciplines.

12.2.3.3 A strict policy of weight control should be imple-mented throughout design and fabrication and weight mar-gins reduced as veriÞed weight information becomesavailable.

12.2.4 Dynamic/Static Loads

In addition to the static and dynamic loads encountered onÞxed platforms, a TLP is subject to horizontal accelerationsthroughout its operational life. The designer should becomeaware of the methods available to determine the effects ofthese loads on facilities equipment. Section 7 of this recom-mended practice provides data on what acceleration forcesmay be expected. In addition, special loading conditionsshould be anticipated for construction, tow-out and temporarymooring phases.

12.2.5 Construction, Transportation and Installation

Methods of construction, transportation, and installationhave a basic inßuence on facility design. The designer shouldbecome aware of the methods planned for construction, trans-portation, and installation to assure that design criteriaselected supports the total project.


12.2.6 Environmental and Geographical Considerations

The degree of environmental protection required is a func-tion of the Þnal geographic location. The personnel andequipment protection options, such as wind walls, enclosedstructures, heating, ventilation, and air conditioning require-ments and their impact on equipment arrangement, stability,weight, and CG should be considered.

12.2.7 Resupply

Platform location and prevailing weather conditions shouldbe evaluated to establish an acceptable cycle for consumableresupply to minimize required consumable storage. The loca-tion and the maximum weight of supplies should be consid-ered in the weight estimate. Regulatory requirements couldhave an impact on consumable storage requirements andshould be investigated. Ballast adjustments during resupplyshould be considered.

12.2.8 Simultaneous Drilling and Production

Weight and CG penalties are associated with simultaneousdrilling and production. Drilling of wells prior to installationof production equipment could result in a signiÞcant reduc-tion of topside loads. However, weight savings achieved maybe at the expense of future drilling program requirements.

12.2.9 Industry Codes and Standards

Various organizations have developed numerous standards,codes, speciÞcations, and recommended practices which havesubstantial acceptance by industry and governmental bodies.These documents could be useful references and helpful indesigning TLP drilling and production facilities. Some of themore commonly accepted documents are listed herein. Thislisting is intended to be general in nature and is not all-inclu-sive. More speciÞc references may be contained within thelisted documents.

a. American Petroleum Institute:

1. API SpeciÞcation 2C, SpeciÞcation for OffshoreCranes.

2. API Recommended Practice 2G, Recommended Prac-tice for Production Facilities on Offshore Structures.

3. API Recommended Practice 2L, Recommended Prac-tice for Planning, Designing, and Constructing Heliportsfor Fixed Offshore Platforms.

4. API Recommended Practice 2Q, Recommended Prac-tice for Design Operation of Marine Drilling Riser Sys-tems.

5. API Recommended Practice 14C, RecommendedPractice for Analysis, Design, Installation and Testing of

Basic Surface Safety Systems on Offshore ProductionPlatforms.6. API Recommended Practice 14E, Recommended Prac-tice for Offshore Production Platform Piping Systems.7. API Recommended Practice 14F, Recommended Prac-tice for Design and Installation of Electrical Systems forOffshore Production Platforms.8. API Recommended Practice 14G, RecommendedPractice for Fire Prevention and Control on Open TypeOffshore Production Platforms.9. API Recommended Practice 500B, RecommendedPractice for ClassiÞcation of Areas for Electrical Installa-tion of Production Facilities.10. API Recommended Practice 520, RecommendedPractice for the Design and Installation of Pressure-Relieving Systems in ReÞneries, Parts I and II.11. API Recommended Practice 521, Guide for PressureRelief and Depressuring Systems.

b. American National Standards Institute:1. ANSI B 31.1, Power Piping.2. ANSI B 31.3, Chemical Plant and Petroleum ReÞneryPiping.3. ANSI B 31.4, Liquid Petroleum Transportation PipingSystems.4. ANSI B 31.8, Gas Transmission and Distribution Pip-ing Systems.

c. American Society for Testing and Material Annual Bookof ASTM Standards.d. American Society of Mechanical Engineers ASME Boilerand Pressure Vessel Code.e. National Fire Protection Association:

1. National Fire Codes.2. Fire Protection Handbook.3. National Electric Code.

f. The Offshore OperatorÕs Committee Manual of Safe Prac-tices in Offshore Operations.g. FAA Advisory Circular 100/5390-1B, ÒHeliport DesignGuide.Ó

12.2.10 Regulations

Regulatory organizations have established rules whichmight inßuence the design. Rules developed for mobile off-shore drilling units, offshore installations, marine and electri-cal engineering are applicable in part. The TLP is apermanent OCS facility for drilling and/or production, andhas been deÞned as a Òßoating OCS facilityÓ in 33 Code ofFederal Regulations, 140.10 and 33 Code of Federal Regula-tions, 143.120 which describes the speciÞc application of theregulations. These regulations establish certain requirementswith respect to safety equipment and the promotion of safetyof life and property at sea.


12.2.10.1 United States Coast Guard Jurisdiction

Applicable regulations include:

a. 33 Code of Federal Regulations, 140-147 (Subchapter N),ÒOuter Continental Shelf Activities.Ó These regulations stipu-late requirements for identiÞcation marks for platforms,means of escape, guard rails, Þre extinguishers, life preserv-ers, ring buoys, Þrst aid kits, etc.

b. 46 Code of Federal Regulations, 107-109 (Subchapter I-A), ÒMobile Offshore Drilling Units.Ó These regulations gov-ern the inspection and certiÞcation design and equipment andoperation.

c. 46 Code of Federal Regulations, 50-64 (Subchapter F),ÒMarine Engineering.Ó These regulations prescribe therequirements for materials construction, installation, inspec-tion and maintenance of boilers, unÞred pressure vessels, pip-ing and welding.

d. 46 Code of Federal Regulations, 110-113 (Subchapter J),ÒElectrical Engineering.Ó These regulations prescribe in detailthe electrical engineering requirements for vessels.

e. 33 Code of Federal Regulations, 67, ÒAids to Navigationon ArtiÞcial Islands and Fixed Structures.Ó These regulationsprescribe in detail the requirements for installation of lightsand foghorns on offshore structures in various zones.

12.2.10.2 Minerals Management Jurisdiction

30 Code of Federal Regulations, 250. Oil and gas and sul-fur operations in the Outer Continental Shelf and Outer Con-tinental Shelf orders for U.S. Waters. These orders govern themarking, installation, operation and removal of offshorestructures and facilities.

12.2.10.3 International

Applicable international regulations include:

a. International Maritime Organization (IMO), Code for theConstruction and Equipment of Mobile Offshore DrillingUnits [Resolution A.414 (XI)].

b. International Regulations for Preventing Collisions atSea, 1972, Rule 24a, ÒTowing and Pushing.Ó Designer isadvised to consult the above organization to determine cur-rent rules and regulations.

12.2.11 Classification Societies

A classiÞcation society should be consulted if the TLP is tobe certiÞed or classed as an offshore installation. Applicableportions of appropriate classiÞcation society rules should beinvestigated if the TLP is to be classed.

12.3 DRILLING CONSIDERATIONS

12.3.1 General

When establishing a drilling program, attention should begiven to the potential effects of this program on weight andcenter of gravity. Depth and drift angles along with casing,mud, and completion programs can determine equipment,storage, utility, and consumable requirements. These require-ments may have weight and space impact. Early coordinationof the drilling program with other design disciplines can min-imize storage and equipment size. Design criteria which spec-iÞes simultaneous drilling live loads for two rigs presentweight and CG penalties.

Shared utilities (such as electrical power, utility air, fuel,etc.) between production, hull systems and drilling should beconsidered to minimize equipment/machinery requirements.

12.3.2 Modular Package

A modular package approach is often used when designingdrilling rigs. Alternative approaches such as palletizing equip-ment and integrated deck design should be consideredbecause of the TLP sensitivity to weight. Minimum equip-ment size which meets realistic drilling program require-ments should be planned.

12.3.3 Pollution Containment

12.3.3.1 Drilling system discharges (cement slurry, oilywater, clean water, solids, bit cuttings, or chemical dis-charges) should be integrated into the overall pollution con-tainment and drainage system. Care with the arrangement ofcement discharges is suggested to avoid coating or blockageof drain lines resulting in additional weight/CG consider-ations. The close coordination between drilling rig and facil-ity design is recommended to assure efÞcient interfaces.

12.3.3.2 The location of discharge casings for cementslurry, bit cuttings and other solids should consider the effectof settlement or cement fouling of subsea equipment and suc-tion sea chests.

12.3.4 Tank Sizing and Arrangement

Mud (wet and dry), cement, drill water, and fuel storagerequirements should be identiÞed after the drilling programhas been established. These consumables require signiÞcantspace and present concentrated loads on the platform. Theirlocation and effects on CG should be carefully consideredsince their relocation after Þnal design is in progress couldresult in extensive redesign or use of ballast adjustmentallowances. Horizontal accelerations during operation couldrequire tank bafßing. CG or weight sensitivity may requireintegrating tanks into the deck structure and/or hull.


12.3.5 Area Classification

When determining the extent and boundaries of hazardousareas for drilling equipment, the following regulations, stan-dards and codes should be considered since they can inßu-ence design and arrangement of equipment:

a. National Electrical Code, Article 500. Industry standardcode providing deÞnitions and electrical standards.b. API Recommended Practice 500B, Recommended Prac-tice for ClassiÞcation of Areas for Electrical Installation ofProduction Facilities.c. 46 Code of Federal Regulations, 111.105-33 (SubchapterJ), ÒElectrical Engineering.Ó USCG requirements for areaclassiÞcations.

12.4 PRODUCTION SYSTEMS CONSIDERATIONS

12.4.1 General

12.4.1.1 Initial facility design should emphasize the earlyestablishment of Þrm design premises, equipment sizing, andlayouts in order to establish accuracy in weight estimates.

12.4.1.2 Detailed speciÞc guidelines for process designcan be found in the referenced industry codes (12.2.9). Wherethe process system design and equipment selection and loca-tion should stress weight reduction and low center of gravity,some additional items to be considered are:

a. Minimizing surge and storage capacities.b. Use of horizontal vs. vertical separators (to minimizeretention time).c. Use of centrifugal or vane type pumps and compressorsvs. reciprocating units.d. Use of high strength materials and higher equipment oper-ating speeds.e. Use of gas turbine vs. reciprocating drivers.f. Use of central electric power plants vs. individual enginedrivers.g. Shared use of production and TLP utility systems.h. Liquid storage integrated into hull and deck structures.i. Use of plate frame heat exchangers vs. tubular heatexchangers.j. Lightweight valves and Þttings.

12.4.1.3 TLPs are subject to vertical, horizontal, and rota-tional movements which vary in magnitude in both themoored and tow modes. Such movements and the associatedforces generated should be considered in the design of sup-port structures for piping, vessels, and other facility equip-ment as well as in the design of riser connections. Installationof bafßes in storage tanks and process vessels should also beconsidered to restrict liquid movement and stabilize processlevels.

12.4.1.4 Cooler wellhead temperatures and possiblehydrate formation due to extreme water depths may requireadditional equipment considerations.

12.4.1.5 The recommended practices of API Recom-mended Practice 14C for design and analysis for surfacesafety systems should be consulted. API RecommendedPractice 14E and ANSI B31.3 should be consulted for pipingsystems.

12.4.2 Packaging

Integrated deck construction should be considered in lieuof skid packages. This design approach provides potentialweight and space savings but requires close facility/structuredesign coordination. Design data provided to outside vendorsshould identify speciÞc packaging requirements.

12.4.3 Drain System

12.4.3.1 Two liquid spillage handling systems should beprovided, a bilge system and a drainage system. Liquid spill-age within the conÞnes of the decks should be handled by thedrain system. Bilge system recommendations are found in12.5.1.

12.4.3.2 The drain system guidelines provided in API Rec-ommended Practice 2G and API Recommended Practice 14Gshould be consulted. Unique considerations for the TLP are:

a. Liquid accumulations in drain system should be minimum.b. Draft limitations of hull may not provide structural supportof outside discharge casings.c. Low deck heights could limit gravity drain system capac-ity within deck spaces.d. Vertical segregation of water intakes and facilities outfallsmay be limited by keel draft. Large hoses have been usedfrom the sea chest to achieve the desired segregation.e. Acceleration forces may affect ßow of liquids in gravitydrain systems.


12.4.4.1 When determining the extent and boundaries ofhazardous areas on production decks, the following should beconsidered since they will inßuence design, arrangement, andtype of equipment:

a. National Electrical Code, Article 500. Industry standardcode providing deÞnitions and electrical standards.b. API Recommended Practice 500B, Recommended Prac-tice for ClassiÞcation of Areas for Electrical Installation ofProduction Facilities.c. API Recommended Practice 14F, Design and Installationof Electrical Systems for Offshore Production Platforms.

12.4.4.2 The extent and boundaries of hazardous areas onproduction decks is not speciÞcally covered in the Code of


Federal Regulations (CFR). However, the designer shouldexpect the U.S. Regulatory Agency to examine the plans todetermine the overall effect on general safety.

12.4.5 Utility Systems

12.4.5.1 Utility systems account for a large portion of thetotal topside equipment. Sharing of these utilities betweendrilling, production and hull systems will provide opportuni-ties for weight savings.

12.4.5.2 Utility demands should be controlled withdemand margins identiÞed and modiÞed as the informationbecomes available.

12.4.5.3 Applicable regulations and codes that apply toindividual drilling, production and hull systems could beimposed on the overall shared utilities.

12.4.6 Riser Connection

The production and pipeline risers present special condi-tions due to the relative motion between the deck and risers.In selecting a ßexible connection to accommodate this rela-tive movement, the following should be considered:

a. Vertical pig launcher/receivers may provide a more desir-able solution than horizontal, since a vertical launcher can bemounted on the riser structure.b. Well shutdown sequences should consider subjecting ßex-ible connection to well shut-in pressure.c. Amount of wellhead spacing required because of spacerequired for ßexible connection.

12.4.7 Vent/Flare System

Vent system design should be studied early in the design.Since these structures will have signiÞcant effects on weight,wind loading and center of gravity, it is important to establishrealistic relief rates and system sizing criteria in the initialdesign phase. API Recommended Practice 520, Parts I and II,and 521 provide guidelines for pressure relief systems.Dynamic loads from platform accelerations should be consid-ered during the design of ßares, vent stacks, and booms.

12.5 HULL SYSTEM CONSIDERATIONS

12.5.1 Bilge

12.5.1.1 With the exception of ballast compartments, allcompartments, passageways, and machinery spaces in thehull should be serviced by a bilge liquid removal system. Pro-visions for removal of bilge liquid should be made for instal-lation free-ßoating and fabrication phases. Watertight hullcompartments and hazardous and non-hazardous spacesshould be provided with separate drainage or pumpingarrangements.

12.5.1.2 All valves in machinery spaces controlling thebilge suctions from the various columns or hull compart-ments should be the stop check type; where Þtted at the openends of pipe, the valves should be of the nonreturn type.

12.5.1.3 Bilge pumps should be of the self or automaticpriming type and capable of continuous operation in theabsence of liquid ßow. Bilge pumping capacity should beadequate to remove the maximum liquid input from nonfail-ure operations (e.g., service water washdown, Þrewater fromdeluge, or hose reels). For machinery spaces containingequipment essential to safety, independently powered pumpsshould be considered with one supplied from an emergencysource. Any hull compartment containing equipment essentialfor the operation and safety of the platform should be capableof being pumped out when in the extreme inclined damagedcondition (that is maximum incline or list angle).

12.5.1.4 If bilge piping is tied into topside treatment facil-ity, back ßow into the bilge system should be prevented. A sys-tem which ensures for gravity free fall should be considered.

12.5.1.5 These regulations should be considered since theycan inßuence the design. However, the designer should deter-mine speciÞc applicability to the TLP.

a. 46 Code of Federal Regulations, 56.50 (Subchapter F),ÒMarine Engineering.Ób. 46 Code of Federal Regulations, 111.101 (Subchapter J),ÒElectrical Engineering.Ó

12.5.2 Ballast

12.5.2.1 The ballast system design serves numerous func-tions, including:

a. Adjustment of platform center of gravity during fabrica-tion, towing, installation, and operation.b. TLP draft changes during fabrication, hull/deck mating,ßoating-out, sea trial, tow, and installation.c. Tendon tensioning or tension adjustment.d. Hull compartment dewatering for inspection, or mainte-nance, after installation.e. Damage stability, correction of center of gravity.

12.5.2.2 Redundancy and reliability of utilities, controland monitoring instruments and equipment during all phasesof TLP operation should be given design emphasis and prior-ity. A single point failure on any piece of equipment, or ßood-ing of any single watertight compartment, should not disablethe damage control capability of the ballast system. Where itis apparent that the free-ßoating inclined damaged conditiontrim impairs the operability of the ballast system, additionalmeans are to be provided for this phase of the operation.

12.5.2.3 Ballast pump and controls should be designed fornumerous differential head conditions without damage due toexcessive velocity or cavitation. Dewatering of ballast com-


partments may require a separate stripping system to lowerthe water below the level set by main ballast pump Net Posi-tive Suction Head (NPSH) requirements. The stripping sys-tem may also serve as a partial rate backup to the ballastsystem. Provisions should be made to dewater ßoodedmachinery spaces with consideration given to the inclineddamage (maximum list of TLP) angle and available NPSH toremaining pumps. Integrating seawater supply and ballastingfunctions into a common system should be considered, butthe reliability of the ballast system for in-place operationsshould not be impaired.

12.5.2.4 Control systems should be provided to preventaccidental opening of ßooding valves for all modes of opera-tion. Blinding off of systems not in use should be considered.The ballast system design should prevent uncontrolled ßowof ßuids passing into one compartment from another whetherfrom the sea, water ballast or consumable storage. Ballasttank valves should be designed to remain closed except whenballasting.

12.5.2.5 Remote controlled valves should fail closed andshould be provided with open and closed position indicationat the ballast control station. Position indication power shouldbe independent of control.

12.5.2.6 Provision for in-situ isolation of the sea chest andintake system or any discharge below the waterline levelshould be provided.

12.5.2.7 The potential for hazardous contamination of theballast system and tanks should be considered in the designand appropriate access should be provided for maintenance.Selection of tank vents and overßow locations should con-sider damage stability effects and the location of the Þnal cal-culated immersion line in the assumed damage condition.Tank vents and overßows should be located so that they willnot cause progressive ßooding unless such ßooding has beentaken into account in the damage stability review. Arrange-ment and design of the vent systems should provide forthrough tank ventilation, when required and should preventliquid accumulation in the vent pipes. All watertight tanksshould be provided with leak detection and level measure-ment capability.

12.5.2.8 Regulations that are applicable to a TLP shouldbe determined by the designer. One regulation to be consid-ered is 46 Code of Federal Regulations, 56.50 (Subchapter F),ÒMarine Engineering.Ó

12.5.3 Riser Tensioner Support System

12.5.3.1 Many tensioning systems utilize high pressuregases to minimize riser tension ßuctuation in response to plat-form motions. Such tensioners act as air springs maintainingnearly constant tension on drilling, production, and pipelinerisers, while the platform moves with the wind, waves, and

current. Similar tensioners may be used during tendon instal-lation to apply tendon pretension while also acting as motioncompensators. Tension device guidance can be found in APIRecommended Practice 2Q.

12.5.3.2 As a minimum, the high pressure gas supply sys-tem should provide a dew point below the temperature real-ized with expansion cooling from design pressure andminimum design temperature to atmospheric pressure. Pres-surization of any tensioner should be possible withoutrecharge of storage. Design of storage volume, standby sup-ply, compression capacity, and redundancy should considerthe potential effects and allowable time of response to partialor complete depressurization of any single tensioning device.

12.5.3.3 Necessary utilities for supply of high pressure gasshould be available during the installation phase.

12.5.3.4 The potential for ignition or explosion of hydrau-lic ßuid/high pressure gases should be considered in thedesign of tensioning equipment and selection of ßuids.

12.5.4 Utility System

Hull system utilities are those required for life support andfunctional operation of the platform excluding drilling andproduction. The TLP presents no signiÞcant unique utilityrequirements from those found on Þxed structure platforms.However, as in process design, weight reduction and loweringof CG should be stressed. The considerations of 12.4.1 shouldbe investigated for application to utilities.

12.5.5 Electrical

API Recommended Practice 14F provides recommendedpractices for electrical design which should be consulted.

The designer is advised to investigate all applicable regula-tions during initial design because they will inßuence designof the hull electrical system. The USCG Electrical Engineer-ing Regulations (46 Code of Federal Regulations, SubchapterJ) establish requirements with respect to safe electrical instal-lations and repair aboard vessels and mobile offshore drillingunits. Because of the similarities between the TLP and aMobile Offshore Drilling Unit (MODU), the designer shouldanticipate that MODU rules may be applied to electricalinstallations.

12.5.6 Machinery Area

In the arrangement of machinery, provisions for the safetyof personnel responsible for the repair and maintenance of theequipment should be considered. Provisions should be madeto ensure that all equipment installed and operated belowwater level can be readily serviced or replaced. The equip-ment should be placed and protected to minimize the proba-bility of mechanical injury or damage by leaks or fallingobjects. Clear working space should be provided around the


equipment to enable personnel full access for inspection orrepair of the equipment as required as well as its handling andoperation. Electrical equipment liable to arc should be venti-lated or placed in ventilated spaces in which dangerous gasesand oil vapors cannot accumulate. Additionally, the layout ofmachinery should incorporate the following provisions:

a. Complete access to the areas as required for manual ÞreÞghting where necessary.b. Personnel safe escape routes in an emergency.c. Facilities for the remote shutdown of any equipment dur-ing an undesirable event.d. Equipment handling facilities.


When establishing classiÞcation of hazardous areas forhull system equipment, the following will inßuence design,selection and arrangement of equipment:

a. National Electrical Code, Article 500.b. API Recommended Practice 500B, Recommended Prac-tice for ClassiÞcation of Areas for Electrical Installation ofProduction Facilities.c. 46 Code of Federal Regulations, 111.105 (Subchapter J),ÒElectrical Engineering.Ó

12.5.8 Subsea Inspection Support

Plans for subsea inspection and maintenance should bemade early in the design process. Space and weight allow-ances and any utility requirement for diving support/inspec-tion facilities need to be made prior to Þnalizing equipmentarrangements.

12.5.9 Navigation Aids

12.5.9.1 The platform must have obstruction lights and fogsignals installed. Applicable guidance for these requirementsis found in 33 Code of Federal Regulations, 67, ÒAids to Nav-igation on ArtiÞcial Islands and Fixed Structures.Ó

12.5.9.2 Depending upon the installation site as it relatesto the Outer Continental Shelf line of demarcation, the struc-ture will be classed A, B, or C. Obstruction light and fog sig-nal requirements for these classes are detailed in Subparts67.20, 67.25, and 67.30, respectively.

12.5.9.3 While being towed to the installation site, theplatform is required to exhibit appropriate navigational lightsas prescribed by the International Regulations for PreventingCollisions at Sea, 1972 (72 COLREGS). Rule 24a, ÒTowingand Pushing,Ó is the pertinent section, and it requires a plat-form being towed to exhibit sidelights, a sternlight, and whenthe length of the tow exceeds 200 meters, a diamond shapewhere it can best be seen.

12.5.10 Hull Compartment Penetration, Access and Inspection

12.5.10.1 Penetration plans and locations must beaddressed and Þxed early in the design process since struc-tural analyses and facilities routing plans cannot proceedwithout mutually agreed penetrations.

12.5.10.2 The designer should consider the following:

a. Provisions should be made for thermal expansion of pip-ing between watertight bulkheads.b. Access/egress, independent of all support utilities, shouldbe provided from all hull access and inspection spaces.c. Inspection access plans for ballast compartments mustconsider damage stability as well as ventilation to removepotential accumulation of hazardous vapors.d. Ventilation and access requirements during inspection ofhull compartments might create potential for multiple com-partment ßooding.e. In addition to ventilation provisions, sampling connectionsshould be provided on all hull compartments for test veriÞca-tion of a safe atmosphere prior to entry.

12.5.11 Accommodation Area

12.5.11.1 When establishing accommodation needs, theUSCG regulations will inßuence the design. Of interest areUSCG 46 Code of Federal Regulations, 107Ð109, SubchapterI-A, ÒMobile Offshore Drilling Units,Ó and in particular Sub-part B, ÒConstruction and ArrangementÓ (Structural Fire Pro-tection Accommodation Spaces).

12.5.11.2 The accommodation area will probably havesigniÞcant effects on wind loading and center of gravity. Itssize and location should be Þnalized early in design. Installa-tion of the accommodation requirements wholly or partlywithin the deck structure should be considered.

12.5.12 Helicopter Facilities

12.5.12.1 Facility design for helicopter operation shouldreview the requirements of 46 Code of Federal Regulations,108.231Ð108.241 and 108.486Ð108.489, for USCG consid-erations. In addition, the following are of use for designguidelines:

a. API Recommended Practice 2L, Planning, Designing andConstructing Heliports for Fixed Offshore Platformsb. FAA Advisory Circular 150/5390-1B, ÒHeliport DesignGuide.Ó This booklet sets forth requirements for markingtowers, poles, and similar obstructions. Platforms with der-ricks, antennas, etc., are governed by the rules set forth in thisbooklet.

12.5.12.2 Refueling requirements for helicopters shouldbe considered when required. Special attention to Þre Þghting


and area classiÞcation requirements of regulatory agencies isadvised.

12.5.13 Cranes

The methods for establishing rated loads for cranes can befound in API SpeciÞcation 2C. 46 Code of Federal Regula-tions, 107.258-260 provides additional requirements forcrane certiÞcation, inspection and testing. In addition theeffect of TLP motions on all crane operations should be con-sidered.

12.6 PERSONNEL SAFETY CONSIDERATIONS

12.6.1 General

Regulatory agencies have established certain requirementsfor personnel safety which will affect the design. Thedesigner is advised to consult the regulations identiÞed in thissection during initial project planning.

12.6.2 Means of Escape

12.6.2.1 The space limitations imposed will require earlyplanning for means of escape for personnel. Each space thatis (1) an accommodation space over 300 square feet, (2) con-tinuously manned, or (3) used on a regular working basisneeds two means of escape. Escape means should be plannedto allow personnel to move from the uppermost level of theTLP to successively lower levels to lifeboats and, if possible,the water level. Whenever possible two separate isolatedescape routes from any working or accommodation areashould be provided.

12.6.2.2 When planning/arranging escape routes, the fol-lowing should be considered since they can inßuence thedesign:

a. Outer Continental Shelf ActivitiesÑ33 Code of FederalRegulations, 142.b. Requirements for Mobile Offshore Drilling UnitsÑ46Code of Federal Regulations, 108.151Ð108.167.

12.6.3 Life Saving

12.6.3.1 The following regulatory requirements will inßu-ence the design criteria for life saving:

a. Outer Continental Shelf ActivitiesÑ33 Code of FederalRegulations, 144.b. Requirements for Mobile Offshore Drilling UnitsÑ46Code of Federal Regulations, 108.501Ð108.527.

12.6.3.2 The above regulations stipulate requirements forlifeboats, life rafts, ring buoys, preservers, communications,distress signals, and methods of embarkation and should beconsulted.

12.6.4 Alarms

12.6.4.1 A general alarm system is required. The systemshould be capable of being activated by manually operatedalarm boxes and by an automatic Þre detection system, ifprovided.

12.6.4.2 The alarm system should be continuously pow-ered with an automatic change over to standby power in caseof loss of normal power supply. The alarm system should bedesigned to handle simultaneous alarms with the acceptanceof any alarm not inhibiting another alarm.

12.6.4.3 The alarm system is required to be audible in allparts of the platform. In high ambient noise level workingareas, a visible means of alarm should be provided.

12.6.4.4 The following regulatory requirements will inßu-ence the design criteria for alarms:

a. Electrical Engineering RegulationsÑ46 Code of FederalRegulations, 113.25.

b. Outer Continental Shelf ActivitiesÑ33 Code of FederalRegulations, 146.

The regulations stipulate speciÞc requirements for alarmsystems.

12.7 FIRE PROTECTION CONSIDERATIONS

12.7.1 General

12.7.1.1 API Recommended Practice 2G, API Recom-mended Practice 14C, API Recommended Practice 14G pro-vide guidelines on design of Þre protection systems andshould be consulted.

12.7.1.2 Regulatory agencies have established certainrequirements for Þre protection which will affect the design.

12.7.1.3 The designer is advised to consult the regulationsspeciÞed herein during initial design.

12.7.1.4 Due consideration should be given to 46 Code ofFederal Regulations, 108 requirements which provide guide-lines for MODUs. Further guidance can be found in:

a. USCG Navigation and Vessel Inspection Circular NVICNo. 6-72, ÒGuide to Fixed Fire Fighting EquipmentÓ includ-ing Change 1.

b. USCG Navigation and Vessel Inspection Circular No. 6-80, ÒGuide to Structural Fire Protection Aboard MerchantVessels.Ó

12.7.1.5 These circulars provide a USCG interpretationof regulatory requirements for equipment structural Þre pro-tection.


12.7.2 Structural Fire Protection

12.7.2.1 The USCG Circular NVIC 6-80 provides guide-lines for regulatory rules. This circular provides an accurateinterpretation by the USCG of the regulatory requirements.

12.7.2.2 The merits of an active or passive system for pro-tection of the structural steel should be determined. Consider-ations are:

a. Active systems can increase water system capacityrequirements and demand provisions for drainage for Þrewater run off.b. Passive system materials such as intumescent coating pro-vide protection but may not represent a minimum weightsolution.c. Requirements for structural inspection under passive sys-tem coating.d. Testing requirements for active system.

12.7.3 Ventilation

46 Code of Federal Regulations, 108.181Ð108.187 andABS Rules for Building and Classing Offshore Installationsprovide regulatory requirements for ventilation systems forenclosed spaces and equipment. The following are consider-ations:

a. Ventilation requirements for hull inspection and its impacton multiple compartment ßooding.b. Increase duct runs for fresh source air and discharge.c. Increase need for powered air circulation to column/pon-toon area.

12.7.4 Detection Systems

Gas and Þre detection systems utilized on Þxed structuresare applicable. ABS Rules for Building and Classing Off-shore Installations and 33 Code of Federal Regulations, 144provide guidelines along with USCG 46 Code of FederalRegulations, 108.404Ð413. Provisions for portable detectiondevices should be planned for hull inspection. Remote airsampling should be planned for closed compartments requir-ing inspection access.

12.7.5 Fire Extinguishing

46 Code of Federal Regulations, Part 108 provides regula-tory agency established requirements. The USCG CircularNVIC 6-72 and 6-72 Change 1 provide accurate interpreta-tion of the regulatory rules. The following are considerations:

a. Use of sea chest versus outside casing for Þre water liftpump.b. Location alternatives for diesel powered Þre pumps versus

1. use of electric power pumps with separate powersource or2. diesel hydraulic units.

c. Use of lightweight piping for Þre water where permittedby regulations.d. Maximum use of portable dry chemical and gaseous extin-guishers to minimize weight.e. Hull system protection should consider systems other thandeluge.

12.8 INTERACTING CHECKLIST

An optimized design will require close coordinationbetween the facilities designer and other design disciplines.The following drilling and production interacting checklistwill assist in identifying coordination points.

12.8.1 Drilling Rigs

A good working design is often seriously weakened by alack of deÞnition between the various design disciplines onhow the drilling rigs interface with the TLP. Drilling rig inter-face points with TLP are:

12.8.1.1 Structure Layout

Considerations include:

a. Position and strength of beams and trusses on platformdeck(s).b. Allowable loadings for deck areas between major supportmembers.c. Deck layout plans for open areas.d. Design loads due to wind on drilling packages.e. Dynamic loads from drilling packages resulting from hori-zontal accelerations of platform.f. Loading conditions from drilling packages due to con-struction, tow out and temporary mooring phases.g. CG and weight impact due to rig layouts, drilling liveloads, and rig packaging/module design.h. Rig skid beam spacing, position, and strength.i. Jacking systems that can be accommodated.j. Elevation of well heads.k. Strength and layout of local beams around wellhead area.l. Strength points available for pulling heavy loads; e.g.,hanging BOP units.m. Position of ÒratÓ and Òmouse holes.Ó

12.8.1.2 Utilities


a. Sharing with production systems.b. Lightweight machinery valving and piping.c. Area classiÞcation requirements resulting from drilling rigmodules.d. Liquid storage in hull vs. decks and associated weight andCG impact.e. Piping for fuel, water, etc., and transfer to and along theplatform:


1. Location distribution, size and number.2. Access points.

f. Electrical power and communication/instrumentation:

1. Number, distribution and location2. Remote BOP control points (including prerun hydrau-lic lines)3. Drilling instrumentation

12.8.1.3 Rig Services


a. Drilling drains, sumps, and solids handling to avoid coat-ing or plugging lines, sumps, or other discharge points.

1. Downcomers for overboard disposal of cuttings fromshale shakers, desilters, desanders, and sandtraps.2. Drilling mud from active and reserve mud pits.3. Cement and cooling water.

b. Impact of solid discharges on under sea equipment orinstallations.c. Supply barge handling and consumable loading/unloading.d. Handling and operations of drilling risers, blowout preven-ters and deep well tools.

12.8.1.4 Safety


a. Two escape routes from working areaÑfrom rig modulesonto platform then to lifeboats.b. Coordinated communications.

12.8.1.5 Regulations

Review for impact on design resulting from drilling rig.

12.8.2 Production Systems

Production equipment and piping can be placed in a varietyof positions. Early coordination between platform designerand facility designer will result in a more workable optimumdesign. The designers should develop good working relation-ships to avoid lack of interfacing deÞnitions. The following isprovided to assist the facility designer in identifying uniqueinteraction with facilities and hull systems which may notexist on a Þxed platform.

12.8.2.1 Structural and Layouts


a. Deck height limitations.b. Weight and CG limitations.c. Plate girder bulkhead design and routing of services.d. Height restrictions on mezzanine levels.

e. Hazardous area impacts due to equipment location selec-tions (bulkheads, Þrewalls).f. Dynamic loads from and on production equipment result-ing from horizontal accelerations of platform.g. Loading conditions from and on production systems dueto construction, tow out, and temporary mooring phases ofproject.h. Integrated or palletized construction.i. Watertight bulkhead penetrations.j. Deck PSF loading required for maintenance and equip-ment access/egress.k. Bulk storage in hull/columns.l. Ballast tank and compartment arrangements and effect onventilation and access for inspection.

12.8.2.2 Production System


a. Limitation on gravity systems caused by inadequate deckheight.b. Equipment height limitations.c. Maximum use of lightweight equipment.d. Shared utilities with drilling and hull systems.e. Loads generated by horizontal, vertical, and rotationalmovements of TLP during fabrication, tow out, mooring, andoperations.f. Damping liquid movement and stabilize process levels.g. Drain system inßuence on buoyancy and CG.h. TLP hydrodynamics impact from facilities installations.i. Riser connections and support.j. Drain/bilge system interfaces.k. Regulatory requirement impact on design.l. Cool ßowing wellhead temperatures and possible hydrateformation due to extreme water depth.

12.8.3 Hull Systems

The following is provided to assist the facility designer inidentifying requirements which may not exist or are differenton a Þxed platform:

a. Bilge system requirements.b. Vents and overßow locations.c. Multi-compartment ßooding during inspection.d. Tension device redundancy.e. Shared utilities with production and drilling.f. Subsea inspection requirements.g. Ventilation and access for inspection of compartments andtanks.h. Accommodation needs.i. Handling of solids discharges and oily water from bilgesand drains.


13 Fabrication, Installation, and Inspection

13.1 GENERAL

13.1.1 Scope and Objectives

This section deals with fabricating, installing, and inspec-tion of a Tension Leg Platform. It speciÞcally addresses fabri-cation, assembly, outÞtting and inspection for the platform,mooring system, foundation, and well templates. Addition-ally, installation is addressed, including such items as launch-ing, transporting, stability, positioning and inspection as theyapply to the platform and its foundations and well templates,respectively. Further, tendon running, connecting, pretension-ing, and riser running are discussed.

13.1.2 State of Technology

Techniques and procedures to be used in the fabricationand assembly of the various components of a Tension LegPlatform combine the experience of Þxed platforms and ßoat-ing drilling units, as well as introducing new challenges dueto size, geometry and service requirements. The designer andbuilder are encouraged to take full advantage of availabletechnology as well as ongoing research and development, off-shore contractor experience, and innovation by equipmentmanufacturers.

13.1.3 Practices and Procedures

Fabrication, assembly, and installation procedures for theplatform and seaßoor foundations should be developed dur-ing the design process so as to meet work requirements. Closecoordination between the designer, fabricator, installer, andoperator is essential in developing these procedures. The fol-lowing serves as guidance for acceptable practices:

a. Fabrication should be in accordance with the ÒSpeciÞca-tions for the Design, Fabrication and Erection of StructuralSteel for BuildingsÓ AISC, latest edition, and applicable sec-tions of API Recommended Practice 2A, unless otherwisespeciÞed herein. For welding guidance, see 14.6.b. All work should be carefully executed with proper qualityand testing procedures to assure that the work product meetsdesign speciÞcations and drawings. All faults and/or deÞcien-cies should be corrected before the material is painted, coatedor otherwise made inaccessible.c. Prior to commencement of work, speciÞcation for the fab-rication and quality control procedures covering criticalaspects pertinent to the productÕs quality should be developedand agreed upon by the designer, operator, fabricator, andapplicable regulatory agencies. Further, a program for nonde-structive testing (NDT) should be developed and agreedupon. This program should contain information and docu-ments for planning, controlling, reporting, standards, etc.

d. Personnel safety during all phases of fabrication andinstallation must be maintained.e. Consideration should be given to providing temporaryaccess, lighting, ventilation, Þre Þghting, etc., during allphases of fabrication, assembly, and installation.

13.2 STRUCTURAL FABRICATION

13.2.1 General

This section addresses fabrication of the hull, deck struc-tures, foundation system, and well template.

API Recommended Practice 2A, Section 4, should be con-sulted for guidance on splices, welded tubular connections,and plate girder fabrication.

13.2.2 Welded Stiffened Plates and Shells

Fabrication of large diameter shells stiffened by either ringframes or a combination of ring frames and longitudinal stiff-eners requires consideration of local fabrication tolerances,global fabrication tolerances, weld details, connection details,and fabrication sequence.

If localized heating is proposed for straightening or repairof out of tolerance, its effects on the material propertiesshould be assessed by the designer. Assembly of substructurescan result in built-in stresses. A construction sequence shouldbe developed for all structures to minimize this condition.

13.2.2.1 Fabrication Sequence

13.2.2.1.1 A fabrication sequence should be established soas to minimize the extent of residual stresses. Some recom-mended guidelines for establishing this sequence are:

a. Welds should progress in a direction from higher to lowerrestraint.b. Joints with expected signiÞcant shrinkage should bewelded before those of lesser expected shrinkage.c. Joints should be welded with as little restraint as possible.d. Splices in component parts of built-in members should bemade before welding to other component parts.

13.2.2.1.2 The overall fabrication sequence should be suchthat defects and deÞciencies can be found and corrected priorto completion of the affected part being made inaccessible.

13.2.2.2 Weld Details

In fabricating cylindrical, and non-cylindrical sectionsincluding ßat plate structures, the following recommendationsshould be considered in addition to those discussed in 14.6:

a. Weld ProximityÑAll parallel welds (butt or Þllet) shouldbe separated considering the thickness of the material and theheat affected zone. However, in no case should this toe to toeseparation be less than 4 times the plate thickness (4t). The


most common case is the seam weld in a plate or cylinderparallel to a stiffener.

Longitudinal butt welds should be offset a sufÞcient dis-tance to avoid having all welds intersecting at the same corner.

b. Weld IntersectionsÑWhere the intersection and overlap ofbutt welds by butt or Þllet welds is unavoidable, the inter-sected (Þrst) weld should be ground ßush for a distance of 2inches (50 millimeters) where possible on either side of theabutting plate. Additional NDT may be required.

c. Weld ContinuityÑIntermittent welds are a stress concen-tration source which could lead to fatigue cracks if oriented inthe direction of varying tensile stresses. At points of intersect-ing stiffeners, stiffeners with larger varying stress rangeshould be welded continuously with interruptions in weldingon the secondary stiffeners. Cutouts (mouseholes) or clear-ances at these intersections should be of adequate size to pro-vide access for complete end welds. The radius of thesecutouts or clearances should be at least 2 inches (50 millime-ters). Intermittent welds should not be used where corrosionis likely.

d. Seal WeldsÑFillet welds in tank spaces should be detailedso as to limit the possibility of ßuids encroaching behind thewelds. This can be accomplished by use of seal welds.

13.2.2.3 Fabrication Details

When detailing the Þnal construction drawings, consider-ation should be given to:

a. DrainageÑAdequate drainage should be provided for thewebs of deep ring frames.

b. Cut-outsÑThe cut-outs where stiffeners pass throughring-frames should be designed to provide support for thestiffener, retain adequate ring-frame shear area, and limit theextent of stress concentration.

c. Plate orientationÑThe direction of rolling of platesshould, where practical, line up with the direction of the pri-mary load path to minimize fatigue effects in critical areas,such as pontoon or brace connections to columns.

d. TapersÑWhen joining items of different thicknesses andwidths, tapers of 4 to 1 are generally recommended.

13.2.3 Tolerances

Deviations from planeness of unstiffened plating, straight-ness of plate stiffeners, or roundness of circular membersshould not exceed the design assumptions regarding bucklingstrength; therefore, each member should be fabricated andpositioned accurately to the Þnal tolerances stated herein. Tol-erances not stated herein or referenced to other existing codesshould be in accordance with AISC ÒSpeciÞcations for theDesign, Fabrication, and Erection of Structural Steel forBuildings,Ó latest edition, and AWS D1.1, latest edition.

Special consideration should be given to alignmentbetween welded structural members. Allowable misalignmentdepends on stress level and type of loading, as well as typeand importance of the joint. Unless otherwise speciÞed in thefabrication speciÞcations, the maximum misalignmentsshould not exceed the values given in 13.2.3.2.

13.2.3.1 Tubular Members

Unless otherwise speciÞed, fabricated tubular membersshould conform to the tolerances as given in API SpeciÞca-tion 2B, Fabricated Structural Steel Pipe, latest edition.

13.2.3.2 Welded Plates

13.2.3.2.1 Misalignment of plate edges in butt welds (seeÞgure 23) should not exceed the following values with amaximum of 1/8 inch (3 millimeters):

a. Primary structural elements: 0.15 ´ t1.b. Secondary structural elements: 0.30 ´ t1.c. t1 = thickness of the thinner plate.

13.2.3.2.2 Primary Structural elements refers to primaryload carrying members of a structure where the occurrence ofa fracture could induce a major structural failure, i.e., externalshell plating of a column or pontoon.

13.2.3.2.3 Secondary structural elements refers to less crit-ical members due to a combination of lower stress and favor-able geometry, or where an incidence of fracture is not likelyto induce a major structural failure, i.e., internal memberswhich do not provide continuity at major structural intersec-tions.

13.2.3.2.4 Misalignment, m, (see Þgure 24) of the noncon-tinuous plates in cruciform joints should not exceed the fol-lowing values:

a. Primary structural elements: 0.30 ´ t1.b. Secondary structural elements: 0.50 ´ t1.c. t1 = thickness of the thinnest of the two noncontinuousplates.

��

��

��

��

��

��

��

�

��

��

��

��

��

��

t2t1a

Figure 23—Misalignment of Butt Joints

a = min{0.15 t1 primary elements0.30 t1 secondary elements1/8 inch (3 millimeters)


13.2.3.3 Beam Columns

The lateral deßection, e1, measured at the midspan of thebeam column should not exceed 0.0015 times the columnlength, or the compression ßange length between lateral sup-ports. The inclination of columns, e2, should not exceed0.0015 times the column length. See Figure 25. For columnsshorter than 84 inches, use a lower limit of 1/8 inch (3 millime-ters) for e1 and e2.

13.2.3.4 Trusses

The inclination of a truss, e4, should not exceed 0.008times the truss depth, h. The lateral deßection, e1, of a chordmember of a truss should not exceed 0.0015 times its length,L. Similarly, the lateral deßection, e2, of a truss bracing mem-ber should not exceed 0.0015 times the depth of the truss, h.Unless speciÞcally incorporated in the design approach, theeccentricity of framing members at a truss member intersec-tion, e3, should not exceed 0.0015 times the truss depth, h.

See Figure 26. For trusses shorter than 84 inches, use a lowerlimit of 1/8 inch (3 millimeters) for e1 and e2.

13.2.3.5 Girders

The lateral offset, e4, of the top ßange with respect to thebottom ßange of a built-up girder should not exceed 0.008times the girder depth, h. The deviation of the centerline ofthe web from the centerline of the ßange, e5, should notexceed 0.015 times the width of the ßange. The slope of theßange from its true position (e6/b) should not exceed 0.025.The lateral deßection of the web, e7, should not exceed 0.01times its depth. See Figure 27.

13.2.3.6 Stiffened Plates

The lateral deßection of a plate between longitudinal stiff-eners, Wp, should not exceed 0.01 times the width betweenthe stiffeners, b. The lateral and in-plane deßection of com-pressed longitudinal and transverse stiffeners, Ws, should notexceed 0.0015 times the length of the stiffener. See Figure 28.For stiffeners with lengths less than 84 inches, use a lowerlimit of 1/8 inch (3 millimeters) for Ws.

��

��

��

��

��

t1

t2

t3m

Figure 24—Misalignment of Cruciform Joints

e1 e2

L L

Figure 25—Beam Column Deflections

e1 £ 0.0015 LL = member length

or

L = unsupported flange lengthe2 £ 0.0015 L

��

��

AA–A

A e3

e3

e4

L

h

CL

Figure 26—Truss Deflections

e1 £ 0.0015 Le2 £ 0.0015 he3 £ 0.0015 he4 £ 0.008 h

��

��

��

e5

e6

e7

e4

b

b

h

Figure 27—Girder Deflections

e4 £ 0.008 he5 £ 0.015 be6 £ 0.012e7 £ 0.01 h


13.2.3.7 Cylindrical Shells

13.2.3.7.1 The local deviation of a straight line generatoron a cylindrical shell, eg, should not exceed 1/100 times thegauge length, Lg; where Lg is deÞned as 4(rt)1/2, but not toexceed the maximum ring stiffener spacing. The stiffener tol-erances should satisfy 13.2.3.6.

13.2.3.7.2 The out-of-roundness, i.e., the deviation of theactual radius (ra) of a cylinder from the true circle radius (r)should not exceed 0.005 r. For very large cylinders, thedesigner may specify a smaller out-of-roundness. Local devi-ation from a true circle should not exceed the permissibledeviation, e, obtained from Figures 29 and 30. The baylength, Lr, is the distance between bulkheads or stiffeningrings. For additional reference, see American Society ofMechanical Engineers (ASME), ÒBoiler and Pressure VesselCode,Ó Section VIII, Division 2 and Section III, SubsectionNE, latest addition.

13.2.3.8 Deck and Cap Beams, Piles, Grating Handrails, and Fences

Refer to Section 4, Fabrication, of API RecommendedPractice 2A, latest edition, for recommended tolerances.

13.2.4 Corrosion Protection

Generally, corrosion can be prevented by multicoat paint-ing systems. Additional protection can be provided bycathodic protection systems.

13.2.4.1 Coatings

Guidelines for the installation and inspection of coatingsystems can be obtained from the National Association ofCorrosion Engineers (NACE) publication RecommendedPractice-01-76. Unless speciÞed otherwise by the designer,the applications of coatings should conform to the above ref-erenced publication.

13.2.4.2 Cathodic Protection

The cathodic protection system components should be inaccordance with drawings and/or speciÞcations and theguidelines established by NACE RP-01-76 for the platform,foundation system, and well template and NACE RP-06-75for the tendons and production risers.

13.3 TENDON SYSTEM FABRICATION

13.3.1 General

Tendons of various structural conÞgurations such as steel,wire rope, or nonmetallic components like synthetic Þber arecandidates for use.

13.3.2 Manufacturing and Assembly

The designer should require the use of existing industrycodes, where practical, in the manufacture and assembly ofcomponents. The manufacturer should develop and use acomprehensive manufacturing and assembly system, includ-ing a complete quality control procedure.

13.4 PLATFORM ASSEMBLY

13.4.1 General

13.4.1.1 Assembly of subcomponents depends on thestructure design and proposed fabrication technique.

13.4.1.2 Several different methods for fabrication andassembly may be considered. Some methods include:

a. Hull assembled on land and launched into the water. Deckassembled at a different site, and loaded onto a barge. Bothhull and deck transported to a preselected mating site. Deckadded by a hull/deck mating operation or by individual mod-ules added to a framework.b. Hull assembled in dry dock and ßoated out; deck added asin (a) above.c. Hull and deck assembled on land and launched into water;deck modules may or may not be added at a later ßoatingstage.d. Hull and deck assembled in dry dock and ßoated out.e. Hull fabricated in sections and joined in free ßoating con-dition or on barges; deck added as in (a) above.f. Hull assembled on shore, loaded onto barge and launchedoff barge at mating site. Deck assembled as in (a) above.

WP

WS

WS

L

b

WS

Figure 28—Stiffened Plate Deflections

Wp £ 0.01 bWs £ 0.0015 L


1000900800700600500

400

300

200

150

1009080706050

40

30

250.05 0.10 0.2 0.50.40.3 0.6 0.8 1.0 2 3 4 5 6 7 8 9 10

Bay length Ö outside diameter, Lr/Do

Out

side

dia

met

er Ö

thic

knes

s, D

o/t

e = 1.0 te = 0.8 t

e = 0.6 te = 0.5 te = 0.4 t

e = 0.3 te = 0.25 te = 0.20 t

0.01 0.02 0.04 0.06 0.10 0.2 0.4 0.6 1.0 2 3 4 5 6 8 10 20

Bay length Ö outside diameter, Lr/Do

10

20

30

40506080

100

200

300

400500600

8001000

2000

Out

side

dia

met

er Ö

thic

knes

s, D

o/t

Arc = 0.06 D

o

Arc = 0.

07 D

o

Arc

= 0

.08

Do

Arc

= 0.

11 D

o

Arc

= 0.

13 D

o

Arc

= 0.

15 D

o

Arc

= 0.

17 D

o

Arc

= 0.

20 D

o

Arc

= 0.

25 D

o

Arc

= 0.

30 D

o

Arc

= 0.

35 D

o

Arc

= 0.

40 D

o

Arc

= 0.

50 D

o

Arc

= 0.

60 D

oAr

c =

0.78

Do

Arc

= 0.

09 D

o

Figure 30—Arc Length For Determining Deviation From Circular Form

Figure 29—Maximum Permissible Deviation From Circular Form, e, For Cylinders


13.4.1.3 In the selection of a fabricating and assemblymethod, some factors to be considered include:

a. Platform size, weight, and draft.b. Number of pontoons and columns.c. Type of deck construction, e.g., open truss or plated.d. Size and type of deck modules.e. Fabricators facilities (inclusive of size of dry docks, over-head cranes, skidways, etc.).f. Depth of water at launch site, and connecting waterwaysor channels to open sea or mating site.

13.4.1.4 Additional temporary buoyancy may be consid-ered to reduce draft.

13.4.2 Erection Sequence

The fabricator should develop a detailed erection sequenceshowing a step-by-step plan for assembling subcomponentsto make up the hull and/or deck. Careful planning is requiredto assure that subcomponents Þt-up within speciÞed toler-ances and that overall global dimensions are met.

13.4.3 Dimensional Control

13.4.3.1 Careful attention to dimensional control is requiredto assure that proper Þt of fabricated modules and structures isachieved during the assembly, mating and installation phases,so that time delay is minimized and the Òas-builtÓ modules andstructures meet the Òas-designedÓ requirements.

13.4.3.2 The Òas-builtÓ global geometry of the structure isnot to deviate from the calculation model in a way whichmay cause a signiÞcant change in load path. In the absence ofsupportive calculations, the recommended practices of Sec-tion 4 of API Recommended Practice 2A, latest edition, maybe used.

13.4.3.3 Dimensional control operations should beplanned to suit the design, fabrication and erection plans, andprovide early warning for necessary corrective and/or reanal-ysis requirements.

13.4.3.4 The following steps should be considered:

a. IdentiÞcation of control parameters in accordance withdesign, fabrication and erection plans.b. Ongoing survey of fabricated structures.c. Evaluation of results.d. Corrective and/or reanalysis requirements.

13.4.3.5 The corrective and/or reanalysis requirementsshould be determined based on the survey results and the cal-culated dimensions of the modules and structures for theactual support and loading conditions during the particularphase of construction.

13.4.3.6 For guidance on dimensional tolerances, refer to13.2.3.

13.4.4 Weight Control

13.4.4.1 The actual weights of the fabricated modules andstructures often vary from the calculated weight. An effectiveweight control program should ensure the Òas-builtÓ weightmeets the design requirements.

13.4.4.2 Selected subcomponents should be weighed toverify the calculated weights and centers of gravity. Timelycorrective or reanalysis action could minimize possible con-struction delays or major rework.

13.4.5 Heavy Lifts

Refer to API Recommended Practice 2A, Section 2, forÒLifting Forces.Ó

13.4.6 Hull/Deck Mating Operations

The designer should prepare a detailed plan for this opera-tion. Complete procedures for the intended method should beincluded.

13.4.6.1 Site Selection

If the fabrication sequence of the platform involves sepa-rate fabrication of the hull and deck, the mating site selectionshould provide, as a minimum, the following:

a. Adequate water depth to allow for the mating consideringballasting operations and changes in hull draft and transporta-tion vessels.

b. SufÞcient shelter from prevailing environmental condi-tions to assure that hull/deck motions are acceptable, and thatthe structures are not overstressed during critical interfacingoperations.

13.4.6.2 Mating Hardware

Hardware involved with the mating operation may includeguidance cones, pins or slots, rubber shock pads or absorbers,and hydraulic jacks.

13.4.6.3 Load Analysis

Detailed deßection and load analysis should be performedto determine applied forces on structural connections andmating hardware due to environmental loads, changes in bal-lasting arrangements, or load transfer during mating opera-tions. SufÞcient clearance between the transportation bargeand the deck after load transfer may be achieved throughdeballasting of the hull and/or ballasting of the barge. It mayalso be achieved through mechanical means (e.g., pull outblocks or hinged drop arms). In this manner, dynamic impactloads between the hull and the barge may be lessened.


13.4.6.4 Equipment Testing

All mating equipment (such as ballasting arrangements,temporary generators, jacks and control systems, etc.) shouldbe thoroughly tested before ßoat out or commencement ofoperations.

13.4.6.5 Stability

Stability calculations should be performed on both the plat-form and transportation barge (or vessel) to assure positivestability during all phases of the ballasting operations andload transfer to the hull.

13.5 TRANSPORTATION

13.5.1 General

Transportation operations should be planned concurrentlywith the structural design in order that loading conditions dur-ing loadout, towing, and launching are clearly deÞned. Appli-cable regulations and/or codes such as those of the U.S. CoastGuard should be considered.

13.5.2 Platform

13.5.2.1 Launching

Calculations should be made to conÞrm that the platformcan be safely skidded into the water or on a barge or, if builtin a dry dock, that it can transfer from the on-bottom condi-tion to the ßoating mode without instability or overstressing.

13.5.2.2 Ballast Systems

The systems for ballasting and deballasting should bedesigned with sufÞcient numbers of valves and pumps to pro-vide a Òfail safeÓ operation during mating, launching, towing,and installation.

13.5.2.3 Stability

Sea-keeping characteristics along with heeling and rightingmoment curves of the platform should be produced for thevarious towing conditions anticipated. The mathematicalanalyses may be correlated with a series of tank tests on anaccurately scaled model. The designer should perform calcu-lations to show that adequate buoyancy and stability will existif damage occurs during transit. Refer to 5.3.3.2 and 5.3.4 foradditional guidance.

13.5.2.4 Forces

The platform should be designed to resist all hydrostatic,environmental and towing forces imposed on it during transit.Towing attachments to the platform should be stronger thanthe breaking strength of the largest towing wire and should be

accessible during transit. Fatigue at towing attachmentsshould also be considered.

13.5.2.5 Towing Vessels

The proper number of seagoing tugs should be providedwith sufÞcient power and size to operate safely in any seaenvironment that may develop for each particular transitroute, transportation time, and time of year.

13.5.3 Foundations and Well Templates

Foundations and well templates may be transported andhandled by a variety of methods. Possible operations caninclude load-out or launching at the fabrication site, transpor-tation by self-ßoating or as deck cargo, and either lift-off orlaunching at the installation site.

For additional guidance in transportation to an installationsite, reference is made to the provisions of API Recom-mended Practice 2A, Section 5.2, latest revision.

13.5.3.1 Launch

A method of launch should be considered which utilizesthe procedures and criteria established for the launching ofjacket-type offshore platforms. Structural strength, launchtrajectory, and structure equilibrium after launch should beassessed. A hypothetical damage condition should be consid-ered when making these calculations. Prior to launching, allwatertight closures and valves should be secured.

13.5.3.2 Barge Transport

Foundations and well templates transported as deck cargoare subject to loads generated by the vesselÕs dynamicmotions. As required, motion analysis should be undertakento determine the dynamic load components, which should beincorporated in the structural and tiedown analyses.

13.5.3.3 Free-Floating Transport

Foundations and well templates transported by ßotationwill be subject to drag forces, stillwater bending, waveinduced bending, and hydrostatic forces. Environmental loadsshould be predicted on worst case tow criteria and analyzed inaccordance with procedures in 10.5. Points of attachment fortowing hawsers should be designed to limit stress concentra-tions and crack initiation at these locations. Stability calcula-tions, including a one-compartment damage condition,should be performed.

13.5.3.4 Offshore Lifts

Structures lifted from transport barges offshore should bedesigned following the guidance of API Recommended Prac-tice 2A. Prior to lifting, all watertight closures and valvesshould be secured.


13.5.4 Tendons

The transportation and handling of tendons between thefabrication site and the platform requires careful evaluation toensure that the tendons, including the connectors, coatings,anodes and other appurtenances, are not damaged.

13.6 INSTALLATION OPERATIONS

13.6.1 General

Installation can be considered as Þve major activities:installation of the foundation system; deployment of the tem-porary mooring system; installation of the tendons; attach-ment of the tendons to both the foundation and platform,which includes establishing the desired level of pretension inthe tendons; and installation of risers. The sequence and spe-ciÞc tasks that will occur during each of these activities mayvary depending on a number of factors including:

a. Geometry of the overall system design and individualcomponents.

b. Soil, environmental and geographic parameters.

c. Equipment capability and availability.

d. Long-term Þeld development plans and other economicconsiderations.

Many of the options that may be viable for a speciÞc Ten-sion Leg Platform installation are illustrated in the networkdiagram Figure 31.

13.6.1.1 Site Survey

Prior to the initiation of installation operations, a survey ofthe proposed site should be carried out. This should include abottom survey to ensure that no recent changes to the installa-tion area such as debris and cuttings have occurred that wouldprevent installation of the seaßoor structures. After templateinstallation, an accurate depth survey should be made to deter-mine the Òas installedÓ position and depth of each template.

13.6.1.2 Environmental Envelopes

Based on the environmental conditions anticipated at theproposed installation site, calculations should be performed(and conÞrmed by wave tank testing, if necessary) to predictmotion response of the platform. In particular, the designershould determine the acceptable range of motions from acombination of wind, wave, current, and swell that the plat-form can experience while being maintained in position andinstalled. These calculations should establish the acceptablelimits of motion response, i.e., heave, pitch, surge, sway, yaw,and roll. These responses may be critical when the platform istransferring from a Òfree ßoatingÓ to a Òtension mooredÓ con-dition.

13.6.1.3 Installation Plan

Installation procedural and training plans should be devel-oped to ensure that the installation is accomplished in a satis-factory manner. In addition to deÞning the sequence andinterface for completing the tasks of each major phase of theinstallation, it should deÞne conditions for weather, equip-ment status and logistic support under which installationoperations should be: (a) initiated, (b) suspended, (c) termi-nated, or (d) reversed for each major phase of the procedure.

13.6.1.4 Contingency Plans

Comprehensive contingency plans covering all phases ofthe installation should be included in the installation plan andprocedures. Each phase of installation operations should bereversible if a malfunction occurs.

13.6.1.5 Mooring and Station Keeping

During installation, a temporary mooring system or othermeans of station keeping may be required. This systemshould be capable of maintaining the platform in positionduring the entire installation period including an anticipatedstorm condition. Calculations and/or wave tank testing maybe conducted to ensure that this mooring system will handlethe forces that result from platform motions.

13.6.2 Foundations and Well Templates

The means adopted for installing foundations and welltemplates may be inßuenced by the following:

a. Type of foundation:1. Multi-template.2. Combined base template.3. Piled structure.4. Shallow foundation.5. Hybrid structure.

b. Type of installation vessel:1. Moored or dynamically positioned.2. Semisubmersible, shipshape or ßat-bottom barges.3. Tension Leg Platform (self-installed).4. Other.

c. Water Depth:1. Within diver capability or not.

d. Environmental conditions:1. Mild or hostile area.2. Length of installation periods.

e. Installation equipment:1. Underwater hammer.2. Drilling/grouting tools.

Proposed installation procedures should be consideredearly in the design process, as they might impact Þnaldesigns.

90A

PI R

EC

OM

ME

ND

ED P

RA

CT

ICE 2T

Foundationsystem

installation

Gravity

Tensionpile

Hybrid

Multiplepiece

Singlepiece

Installedat same

time

Stagedinstallation

Installed priorto platform

arrival

Installed withplatform

at location

Installationassisted or

accomplishedby platform

Installed byequipmentother thanplatform

A

AMooringsystem

deployment

Accomplishedfrom or

assisted byplatform

Installed byequipmentother thanplatform

Legs installedsequentially

Legs installedinstantaneously

Individualtendonsinstalled

simultaneously

Individualtendonsinstalled

sequentially

B

BInterfacing

andpretensioning

Legsinterfaced

sequentially

Legsinterfaced

simultaneously

Individualtendons

interfacedsimultaneously

Individualtendons

interfacedsequentially

Pretensionby ballast

Pretensionby pull-down

Pretensionby pull-downand ballast

Pretensionall legs

simultaneously

Pretensionindividualtendons

simultaneously

Pretensionindividualtendons

sequentially

Finalinstalledcondition

Figure 31—Major Activities and Options For Installation Operations


13.6.2.1 Handling and Lowering

When installation equipment and methods are selected,calculations should be performed to quantify dynamic loadsand stresses during the lowering and placement of the struc-tures on the seabed. Factors that might impact handling ofseaßoor foundation structures at the surface (e.g., transferfrom ßoating condition to lowering condition) and also low-ering will include:

a. Size of structure; large structures might require buoyancyassistance during surface handling and lowering.b. Amount of lowering capacity available on installationequipment.c. Stability of structure during surface transfer operation.Structures although stable in a ßoating mode on the surfacemight become unstable when negatively buoyant.d. Type of lowering equipment on installation vessel (e.g.,whether lowering from a crane, a special winch module, ordrill string).e. Rotational control and stability of the structure may createproblems during lowering.f. Dynamic loads due to two-body interaction. This may beminimized through the use of motion compensators or elasticsynthetic lowering lines.

13.6.2.2 Positioning

13.6.2.2.1 Positioning encompasses the means to monitorand hold accurate location, azimuth, and levelness. Position-ing may be relative to a seaßoor grid (e.g., in the case of com-bined foundations and well templates) or relative toindependently installed seaßoor foundations (e.g., anchortemplates relative to foundation/well template). Although themeans to monitor positioning tolerances are readily availablewith conventional electronic underwater acoustic devices, themeans to achieve the necessary tolerances requires carefulevaluation.

13.6.2.2.2 Possible positioning means available include:

a. Maneuvering installation vessel on conventional (cate-nary) mooring system.b. Maneuvering installation vessel with dynamic positioningsystem.c. Maneuvering lowered seaßoor structure with localizedthrusters.d. Use of a bottom founded positioning structure loweredseparately to the templates. This would act as a guide aroundthe well template (it may or may not be removed).e. Use of pin piles (positioning piles).

13.6.2.3 Leveling

The means for obtaining leveling tolerances of seaßoorfoundation(s) depends on: nature of soil conditions, size ofbearing mats, mechanism to operate bearing mats, and other

factors. Leveling methods may include hydraulic jacks, rackand pinion mechanisms, pile elevators, ballasting, and grout-ing. Generally the leveling adjustments with multi-templatesshould be small (this can be achieved through accurate bot-tom surveys and structure design considerations). Levelingadjustments of large combined or hybrid structures are likelyto be difÞcult to achieve due to their overall mass.

13.6.2.4 Pile Installation

13.6.2.4.1 Template piles may be installed using underwa-ter driving techniques or by drilling and grouting. With under-water driving, calculations will have been performed toestablish penetration rates and blow count based on soil data,pile size and hammer energies. If considerable deviations arenoted from the design criteria, these could mean incorrect soildata or inadequate transfer of energy of the hammer to the pile.

13.6.2.4.2 Drilled and grouted piles depend on the abilityto drill the pilot hole because of possible cave-in problemsand the efÞciency of the grouting operation. Grouting tech-niques, although well established for deep water oilwell drill-ing operations, are still somewhat of an unknown technologywith large diameter tension piles. Effective means to monitorthe integrity of the grout in place are available through spe-cialized monitoring tools lowered from the surface vessel ordeployed from underwater remotely operated vehicles.

13.6.2.4.3 For both pile design and installation techniques,a comprehensive soils survey, engineering analysis and testprogram are recommended.

13.6.2.5 Template to Pile Connection

Present technology for the interface of piles to seaßoorfoundations includes grouting and mechanical connections.Grouting may be performed through drill strings or hoses.The grout may be monitored with special instrumentation(e.g., nuclear densimeters) either mounted on the template ordeployed with ROVs. Mechanical connections include meansof deforming the shape of the pile inside the template sleeveso that a connection is made between the two. This may beaccomplished with high pressure hydraulics or with explo-sives. It is necessary to ensure that the pile has deformed tothe design speciÞcations.

13.6.2.6 Shallow Foundations

13.6.2.6.1 Shallow foundations may be fabricated fromconcrete or steel and may include deadweight or hybrid pile/deadweight structures.

13.6.2.6.2 It is anticipated that these structures mayinclude additional buoyancy for lowering operations, and theitems mentioned in 13.5.2 would apply. After installation, thefoundations should be inspected and monitored to ensure thatinstallation has been achieved in accordance with plans.


13.6.3 Platform

The platform may be installed over previously installedseaßoor foundations and/or tendons. In this event, careful con-sideration should be paid to the ballasting system, temporarymoorings, etc., when moving the platform over the location.

13.6.4 Tendons

Developing a concept and a procedure for installation of atendon system is intimately linked with the tendon designprocess. (See Section 9, Tendon System Design.) It is recom-mended that the installer consult and work closely with thedesigner in developing an installation plan.

In view of the variety of conÞgurations and peculiar designfeatures that may be involved in a speciÞc tendon systemsdesign, it is not reasonable to attempt to outline detailedinstallation procedures. The ßow chart of Figure 32 deÞnessome of the designerÕs options that relate the design of bothan individual tendon and the entire tendon system to installa-tion considerations. Regardless of the speciÞcs of the designof an individual tendon, or the tendon system, the installationoperations will involve the sequential completion of the tasksdiscussed below.

13.6.4.1 Preparing to Install Tendons

The following is a partial list of tasks the designer andinstaller should consider in developing procedures to be fol-lowed in preparation for tendon installation:

a. Inventory, inspect and document the conditions of all ten-don components.b. Functionally test all equipment and systems to be usedduring installation.c. Develop contingency plans and alternate procedures to befollowed in the event of damage to tendon components orinstallation equipment malfunction or failure.

13.6.4.2 Tendon Installation

13.6.4.2.1 Tendon installation operations will generallyinclude handling and running operations similar to drill pipeor casing running procedures. As illustrated in Figure 32,there are a number of options for equipment utilization,sequencing and procedure that may be employed dependingon the speciÞcs of the individual tendon and overall mooringsystem design.

13.6.4.2.2 The following is a partial list of tasks thatshould be considered in developing procedures to be followedduring tendon installation, including landing and connectionoperations:

a. DeÞne conditions for weather, equipment status and logis-tic support under which installation operations will be:

1. Initiated.2. Suspended.3. Terminated or reversed.

b. Develop contingency plans and alternate procedures to befollowed in the event of:

1. Damage to tendon components.2. Installation equipment malfunction or failure.3. Unexpected deterioration of weather.4. Interruption of logistic support.

c. Evaluate the need for inspection procedures to be con-ducted during installation operations.

13.6.4.2.3 A comprehensive plan that includes appropriateinspection and record keeping procedures to ensure that indi-vidual tendons are deployed in a manner consistent with theirdesign and service requirements should be developed andform the basis for Þeld operations. In many designs it may beprudent to conduct deployment operations with surfaceequipment offset from the location of previously installedseaßoor equipment to minimize the damage potential fromdropped objects. Regardless of the speciÞcs of the installationprocedure, the need and potential beneÞts of comprehensiveoperator training and equipment familiarization includinginstallation system trials cannot be overemphasized.

13.6.5 Risers

13.6.5.1 Procedures for running risers should be developedconsidering the following factors:

a. Water depth.b. Type of riser system, e.g., integrated or nonintegrated.

Individual tendoninstallation

Tendon runas a singlecontinuous

piece

Spooled pipe:Parallel strand

helical wound orW.R. constructionNo intermediate

connections

Tendons runas several

long segments

Few

intermediate

connections

Tendons runas many

short segments

Many

intermediate

connections

Mech. intermediateconnections

Threads, clamps,bolted flange, dogs,

collets, etc.

Fused intermediateconnections

Welded, explosive-formed, etc.

Figure 32—Options For Tendon Installation


c. Surface or subsurface completion.d. Type of connections and latching devices.e. Whether buoyancy is included (either internal or externalair cans or foam).f. Whether guidelines are to be used or not.

13.6.5.2 General recommendations on the running of ris-ers can be found in API Recommended Practice 2K Care andUse of Marine Drilling Risers, and API Recommended Prac-tice 2Q Design and Operation of Marine Drilling Riser Sys-tems. Previously installed multiple production risers mayintroduce clearance and contact problems during running ofriser joints. The stab-in of riser joints into a seaßoor templatefrom a TLP offers somewhat different problems than a drill-ing vessel since the station keeping components do notreadily provide lateral platform positioning adjustments as inthe case of drilling vessels. The use of a temporary catenarymooring system, thrusters, tugs, or other positioning mecha-nisms may be considered for the stab-in operations.

13.6.6 Special Operations

13.6.6.1 Positioning Systems

The accuracy required to position and align the seaßoorcomponents is limited by installation equipment but must beconsistent with design tolerances. Consideration should begiven to special alignment appurtenances and monitoring sys-tems such as acoustic positioning equipment, TV, ROVs, etc.,in performing these tasks.

13.6.6.2 Environmental Monitoring

If installation is to be performed during a time of year and/or in an area where weather windows are relatively short orunpredictable, consideration should be given to specialweather monitoring means to ensure that operations may becompleted within the constraints of the design environmentalenvelopes.

13.6.6.3 Standby Equipment

Adequate numbers of tugs, anchor handling vessels, etc.,should be on location at all times during the installationshould it become necessary to secure operations and/or aban-don location.

13.7 INSPECTION AND TESTING

13.7.1 General

Inspection and testing throughout the life of the TLP isnecessary to assure the continued performance of the designgoals. The platform, being a buoyant structure, is weight sen-sitive and, accordingly, inspection during fabrication is neces-sary to assure weight control in addition to the usual concernsof fabrication quality and dimensional tolerances. Tendon

systems are sensitive to fatigue and, accordingly, fabricationinspection to detect ßaws which could reduce fatigue life isessential. Upon installation, the platform will likely remainonsite throughout the life of the Þeld. The monitoring of theplatformÕs performance and continued inspection of struc-tural components to detect deterioration or damage should bedone to avoid removing the structure for major repairs.

13.7.2 Fabrication

13.7.2.1 Platform/Seafloor Structures

13.7.2.1.1 Inspection and testing undertaken during fabri-cation of the platform and seaßoor structures componentsshould cover at least the following:

a. Material quality.b. Material traceability.c. Welder performance qualiÞcations.d. Weld procedure qualiÞcations.e. Thickness tolerances.f. Fit-up and edge preparation.g. Weld inspection, visual and NDT.h. Weld size.i. Dimensional and alignment checks.j. Coating and anode application.k. Support for handling, shipment and storage.l. Documentation of all inspection/testing

13.7.2.1.2 The techniques for this inspection and testingare speciÞcally deÞned in other sections, and in recognizedcodes currently being utilized in marine fabrication facilities.

13.7.2.2 Tendon System

13.7.2.2.1 Fabrication inspection should be performed toensure adherence to the drawings, speciÞcations, and proce-dures which contain the detailed instructions necessary toobtain the desired quality and service in the Þnished product.Inspection should be performed during both the fabricationand assembly phases to ensure compliance with the require-ments. The most effective quality control system is one whichprevents the introduction of defects into a component ratherthan Þnding the defects when they occur. The plans and spec-iÞcations for a component should clearly indicate whichmaterials and items are to be inspected and by what method.To the fullest extent practical, inspection should be performedas the fabrication progresses.

13.7.2.2.2 The manufacturer should use a system to main-tain the traceability of each tendon assembly and its compo-nents. This system should assure an adequate level oftraceability for both metallic and nonmetallic components.Prior to manufacture, the manufacturer should determine thelevel of traceability required jointly with the owner, regula-tory authority, and designer. The manufacturer should then


maintain the necessary detailed records consistent with thelevel of traceability agreed by the parties.

13.7.2.2.3 The manufacturer, designer, and/or owner maywish to demonstrate the acceptability of the product for itsintended application. Such testing methods should be realisticand consistent with the factors of safety designed into thecomponent. The testing of components such as ßex jointsshould avoid damage to the component prior to initiation intoservice.

13.7.3 Assembly

13.7.3.1 The assembly of the structural components of theplatform should maintain the quality control and alignmentcontrol established during the component fabrication as per13.2. Particular care should be taken when mating cylindricalsections of the major columns.

13.7.3.2 When two-phase construction is undertaken, aprocedural plan should be prepared which clearly outlines allsteps of the mating. The information to be included in thisplan includes environmental constraints, alignment checks,weld details and procedures, inspection points and criteria,platform monitoring equipment, etc. This operation should beanalyzed by the designer as a load condition including allow-able limits of misalignment, platform bending due to load andenvironmental conditions, and possible misapplications ofload. It should be recognized that, for mating, the platformmay be submerged beyond its design operating draft, andthus, special damage control precautions may be necessary.Instrumentation to measure the motions of the deck and hullcomponents, the alignments, and the stresses at the matingsurfaces throughout the mating process may be considered.

13.7.3.3 Module lift procedures should be developed fol-lowing current offshore practice with the additional consider-ation of the platform as a buoyant structure. This will requirethat the relative motions of the platform and lift vessel beincluded as an aspect of the lift analysis.

13.7.4 Preparation for Tow-Out

13.7.4.1 An inclining experiment should be carried out todetermine the platformÕs center of gravity. The condition ofthe platform at the time of inclining should be clearly docu-mented so that an accurate assessment of the effects of addedloads can be made.

13.7.4.2 A Þnal testing program should be undertakenprior to the transportation of the platform. This programshould verify the operation of the ballast system, ÞreÞghtingsystem, normal and emergency utility systems, communica-tion systems, alarms (as practical), life saving appliances, etc.In addition watertight closures and automatic closing doorsshould be tested. Functional testing of tendon running toolsand trials of tendon running operations is also recommended.

13.7.4.3 The buoyancy compartments and the ßoodingcontrol system of the subsea structures should be tested priorto tow-out.

13.7.5 Installation

13.7.5.1 Upon arrival at the installation site, the platform,seaßoor foundations, and tendons should be inspected fordamage during transportation, the positioning devices shouldbe tested for accuracy, and all necessary equipment be avail-able and in good working order.

13.7.5.2 The template installation should be completedwithin tolerance and positively connected to the pile founda-tion. Measuring devices for determining level and positionshould be tested for accuracy prior to deployment of the tem-plate. ÒAs builtÓ centerline to centerline dimensions of accesstunnels or hawse pipes at the platformÕs vertical columnswhere tendons terminate as well as similar dimensions of theseaßoor foundations should be available to further assist inthe overall installation. Final template position should bemeasured and recorded in the platformÕs operating booklet.

13.7.5.3 A functional test of the equipment used to couplethe template to the pile should be undertaken prior to deploy-ment. If grout connections are to be used, grout samplesshould be taken for strength tests, grout volume should bemonitored during installation and compared with calculatedvolumes, and instrumentation such as nuclear densimetersdeployed to monitor grout quality. Mechanical connectionsshould be inspected by remote techniques to assure that theconnection is satisfactory.

13.7.5.4 The platform installation requires running andstabbing the tendons within the anticipated weather window.During this procedure, inspections should monitor:

a. Tendon damage during handling.b. Tendon element makeup sequence (identiÞcation).c. Coating scars.d. Tendon motions during deployment (monitored by divers,ROVs, acoustical devices, etc.).e. Connector makeup.f. Tensioning.

13.7.6 In-Service

13.7.6.1 Maintaining the platform on station will require acontinuing inspection program to limit the possibility ofrequiring major repairs. The design of the platform, seaßoorstructures, tendons and risers should take inspection intoaccount.

13.7.6.2 All platform compartments should be accessible,vented, and either temporary or permanent means of lighting,staging, and cleaning provided to allow complete internal


inspection. Sea-chests should be provided with means ofclosing off so that they may be dewatered while at sea.

13.7.6.3 Seaßoor structures should be designed such thatunderwater inspections of critical members utilizing ROVs orsimilar devices can be undertaken.

13.7.6.4 Tendon and riser system components should beinspected to detect deterioration and allow corrective action,if required, to be taken in a timely manner. Inspection inter-vals and methods should take into account design methodsand assumptions, particularly those which concern fatiguelife and corrosion. Innovative inspection methods may beconsidered by the designer and operator to meet uniquerequirements. The operator may elect to pull tendons and ris-ers, or inspect them in place. A slightly less thorough exami-nation, in place, may provide better information than acomplete examination of a few pulled tendons and risers.

13.7.6.5 General inspection of all components is recom-mended to check for corrosion and damage. Detailed inspec-tion for cracking or other deterioration of metallic componentsis recommended. Areas of geometric discontinuity whichcause stress concentrations should be examined with particu-lar care. The condition of anodes, coatings, and other compo-nents of the corrosion protection system should be carefullyinspected. Components having elastomeric or other nonmetal-lic parts should be inspected for failure or deterioration.

13.7.6.6 A complete examination can be performed on atendon or riser pulled from the water after marine fouling hasbeen removed. This examination typically would include athorough visual inspection, appropriate nondestructive test-ing, and measurements of material loss due to corrosion orerosion. The entire tendon or riser should be inspected withattention to areas of stress concentration and accelerated cor-rosion. Protective coatings may be removed if required toconduct a thorough examination. A sufÞcient number of ten-dons should remain in place to withstand environmental con-ditions likely to occur during the inspection period. Selectionof tendons or risers to be inspected should take into accountload history and any indication of damage. Thorough exami-nation of one or more used as an indication of the conditionof all tendons and risers having an equal operational life.

13.7.6.7 In place or in-situ examination of tendons and ris-ers requires advanced techniques having a high probability ofdetecting cracks and other deterioration. Such inspectionsmay be substituted for pulling tendons. Any inspection pro-gram, whether involving pulled tendons or in situ checks,should include an in place visual examination to check thecondition of corrosion prevention components.

13.7.6.8 Immediately prior to initial installation of ten-dons, a thorough examination of each tendon componentshould be made and the exact location of any ßaws or damagenoted and appropriate corrective action taken. As soon aspractical after installation, an in place visual inspection of the

tendons should be performed to check for damage whichmight have occurred during installation. Tendon loading his-tory and information on the size and location of cracks andßaws may be used to modify the inspection frequency overthe life of the structure.

13.7.6.9 In addition to the regularly scheduled inspectionsdiscussed above, inspections should be considered under thefollowing circumstances:

a. An appropriate number of additional tendons may need tobe inspected after a failure or damage to a tendon if a similarproblem could have occurred on other tendons.b. Tendons should be inspected when they are pulled for anyreason.c. A tendon should be inspected when the maximum allow-able stress has been exceeded.d. All tendons should be inspected if the TLP is relocated toa new producing or drilling site.

13.7.6.10 After inspection, coatings should be repaired orreapplied in accordance with the manufacturerÕs recommen-dations.

13.7.6.11 An inspected tendon should be reinstalled inaccordance with the manufacturerÕs and designerÕs speciÞca-tions or stored in a protected location if future use is planned.

13.7.6.12 Detailed records should be maintained by theowner of the inspection history of each component includingthe exact location and size of cracks, ßaws, and damage.

14 Structural Materials14.1 GENERAL


The purpose of this section is to deÞne materials appropri-ate for use in design and construction of a TLP. Steels arecovered in some depth; cement grout, concrete, and elas-tomers are discussed in less detail.

Fracture and fatigue considerations are indicated for allcritical components with a yield strength greater than 70 ksi(480 MPa); for steels with yield strengths below 70 ksi refer-ence is made to API Recommended Practice 2A.

14.1.2 Specifications

Steel should conform to a deÞnite speciÞcation and to theminimum strength level, group, and class in accordance withthe design. In situations where an appropriate ASTM, API, orABS speciÞcation does not exist, a materials speciÞcationshould be developed, subject to preproduction qualiÞcation(See API SpeciÞcation 2Z) and used as appropriate for eachsituation.

CertiÞed mill test reports or certiÞed reports of tests madeby the fabricator or a testing laboratory in accordance with


ASTM A6, A450 or equivalent constitutes evidence of con-formity with the speciÞcation.

14.2 STEEL CLASSIFICATION

14.2.1 Steel Groups

Steel may be grouped according to strength level and weld-ing characteristics as follows:

14.2.1.1 Group 1

Designates mild steels with speciÞed minimum yieldstrengths of 40 ksi (280 MPa) or less. Carbon equivalent (CE)is generally 0.40 percent or less, where CE is deÞned:

CE = C + (47)

These steels may be welded by an appropriate weldingprocess as described in AWS D1.1.

14.2.1.2 Group II

Designates intermediate strength steels with speciÞed min-imum yield strengths of over 40 ksi (280 MPa) through 52 ksi(360 MPa). Carbon equivalent ranges up to 0.45 percent andhigher, and these steels should be welded utilizing low hydro-gen welding processes.

14.2.1.3 Group III

Designates high strength steels with speciÞed minimumyield strengths of 52 ksi (360 MPa) to 70 ksi (480 MPa). Suchsteels may be used, provided that each application is investi-gated with regard to:

a. Weldability and special welding procedures which may berequired.b. Fatigue problems which may result from the use of higherworking stresses.c. Toughness in relation to other elements of fracture control,such as service stress, service temperature and environment.

Additional guidance is provided in 14.5.

14.2.1.4 Group IV

Designates high strength steels, often quenched and tem-pered, with speciÞed minimum yield strengths in excess of 70ksi (480 MPa), and are primarily for use as tendons. Such

steels may be used provided that investigation into applica-tion includes the following areas in addition to that for theGroup III materials:

a. Heat treatment procedures necessary to meet mechanicalproperty requirements, including fracture toughness require-ments.

b. Welding procedures which might require preheat andpostweld heat treatment.

c. Susceptibility to hydrogen embrittlement and/or stresscorrosion cracking in seawater and other possible environ-ments which may be present.

Additional guidance is provided in 14.5.

14.2.2 Steel Classes

14.2.2.1 Steels should be selected with toughness charac-teristics suitable for the conditions of service. For this pur-pose steels in Groups I and II may be classiÞed as follows:

a. Class A steels are suitable for use at sub-freezing tempera-tures and for critical applications involving adverse combina-tions of the factors as cited in Class B. Critical applicationsmight warrant Charpy testing at 35¡ to 54¡F (20¡ to 30¡C)below the lowest anticipated service temperature. Steels enu-merated herein as Class A can generally meet the Charpyrequirements stated for Class B steels at temperatures rangingfrom Ð4¡ to Ð40¡F (Ð20¡ to Ð40¡C).

b. Class B steels are suitable for use where thickness, coldwork, restraint, stress concentration, impact loading, and/orlack of redundancy indicate the need for improved notchtoughness. Where impact tests are speciÞed, Class B steelsshould exhibit Charpy V-notch energy of 15 ft-lbs (20 J) forGroup I, and 25 ft-lbs (34 J) for Group II, at the lowest antici-pated service temperature. Steels enumerated herein as ClassB can generally meet these Charpy requirements at tempera-tures ranging from 50¡ to 32¡F (10¡ to 0¡C).

c. Class C steels are those which have a history of successfulapplication in welded structures at service temperaturesabove freezing, but for which impact tests are not speciÞed.Such steels are applicable to primary structural membersinvolving limited thickness, moderate forming, low restraint,modest stress concentration, quasi-static loading (rise time 1second or longer) and structural redundancy such that an iso-lated fracture would not be catastrophic. Examples of suchapplications are frame members, and deck beams and trusses.

Mn6

--------Ni Cu+

15-------------------

Cr M0 V+ +5

-----------------------------+ +


14.2.2.2 High levels of Charpy energy might be required

for high strength steels, in Groups III and IV. Additional guid-ance on selection of toughness requirements is given in 14.5.

14.3 MANUFACTURED STEEL

14.3.1 Structural Shape and Plate Specifications

Unless otherwise speciÞed by the designer, structuralshapes and plates should conform to one of the speciÞcationslisted in Table 4.Steels above the thickness limits stated maybe used, provided applicable provisions in 14.2.1 are consid-ered by the designer. In situations where an appropriate ASTMor API speciÞcation does not exist, a materials speciÞcationshould be developed, qualiÞed and used for each situation.

14.3.2 Structural Steel Pipe

14.3.2.1 Specifications

Unless otherwise speciÞed, seamless or welded pipeshould conform to one of the speciÞcations listed in Table5.Fabrication

Structural pipe should be fabricated in accordance withAPI SpeciÞcation 2B, ASTM A139, ASTM A381, or ASTMA671 using grades of structural plate listed in Table 4 exceptthat hydrostatic testing may be omitted.

Table 4—Structural Steel Plates and Shapes

Yield Strength Tensile Strength

Group Class SpeciÞcation & Grade ksi MPa ksi MPa

I C ASTM A36 (to 2² thick)ASTM A131 Grade A (to 1/2² thick) (ABS Grade F)ASTM AA285 Grade C (to 3/4² thick)

363430

250235205

58-8058-7155-75

400-550400-490380-515

I B ASTM A131 Grades B, D (ABS Grades E, D)ASTM A516 Grade 65ASTM A573 Grade 65ASTM A709 Grade 36T2

34353536

235240240250

58-7165-8565-7758-80

400-490450-585450-530400-550

I A ASTM A131 Grades CS, E (ABS Grades CS, E) 34 235 58-71 400-490

II C ASTM A441 (strength varies w/thickness)ASTM A572 Grade 42 (to 2² thick)ASTM A572 Grade 50 (to 1/2² thicka)ASTM A588 (to 2² thick)

42-50425050

290-345290345345

63-70 min.60 min.65 min.70 min.

435-485415 min.450 min.485 min.

II B ASTM A709 Grades 50T2, 50T3ASTM A131 Grade AH32 (ABS Grade AH32)ASTM A131 Grade AH36

5045.551

345315350

65 min.68-8571-90

450 min.470-585490-620

II A API SpeciÞcation 2H Grade 42Grade 50

API SpeciÞcation 2W Grade 42Grade 50

API SpeciÞcation 2Y Grade 42Grade 50

4250

42-6250-7042-6250-70

290345

289-427345-482289-427345-482

62-8070-90

62 min.65 min.62 min.65 min.

430-550485-620427 min.448 min.427 min.448 min.

ASTM A131 Grades DH32, EH32(ABS Grades DH32, EH32)Grades DH36, EH36(ABS Grades DH32, EH32)

ASTM A537 Class I (to 21/2² thick)ASTM A633 Grades A, B

Grades C, DASTM A678 Grade AASTM A737 Grade B

45.5

51

5042505050

315

350

345290345345345

68-85

71-90

70-9063-8370-9070-9070-90

470-585

490-620

485-620435-570485-620485-620485-620

III A ASTM A537 Class IIASTM A633 Grade EASTM A678 Grade B

606060

415415415

80-10080-10080-100

550-690550-690550-690

API SpeciÞcation 2W Grade 60API SpeciÞcation 2Y Grade 60

60-8060-80

415-550415-550

75 min.75 min.

517 min.517 min.

aTo 2² Thick for Type 1, Killed, Fine Grain Practice.


14.3.2.2 Selection for Service

Consideration should be given to the selection of steelswith toughness characteristics suitable for the conditions ofservice. For tubes cold formed to D/t less than 30, and notsubsequently heat-treated, due allowance should be made forpossible degradation of notch toughness, e.g., by specifying ahigher class of steel or by specifying notch toughness testsrun at reduced temperature.

14.3.2.3 Steel Forgings and Castings

In situations where an appropriate ASTM speciÞcationdoes not exist, a detailed speciÞcation should be developedand qualiÞed for the speciÞc application. CertiÞed mill testreports and mechanical property tests should be in conform-ance with the appropriate ASTM A6 or A370 speciÞcations.

14.4 SPECIAL APPLICATIONS FOR STEEL

14.4.1 Tubular Nodes

Welded tubular joint intersections are subject to local stressconcentrations which may lead to local yielding and plasticstrains at the design load. During the service life, cyclic load-ing may initiate fatigue cracks, making additional demandson the ductility of the steel, particularly under dynamic loads.These demands are particularly severe in heavywall joint-cans designed for high brace loads.

14.4.1.1 Underwater Joints

14.4.1.1.1 Group I and II steels for underwater joints, suchas a tubular joint, cans, or through members in overlapping

joints should meet the following notch toughness criteria atthe temperatures given in Table 6. Charpy V-notch energy

should be 15 ft-lbs. (20 Joules) for Group I steels and 25 ft-lbs. (34 Joules) for Group II steels (Transverse test). Guid-ance on toughness selection for Group III and IV steels isgiven in 14.5.

14.4.1.1.2 For water temperatures of 40¡F (4¡C) or higher,these requirements may normally be met by using the Class Asteels listed in Table 11.3.1.

14.4.1.2 Above Water Joints

For above water joints exposed to lower temperatures andpossible impact from boats or dropped objects and for criticalconnections at any location in which brittle fractures are to beprevented, the tougher Class A steels should be considered,e.g., API SpeciÞcation 2H. Special attention should be givenin developing welding procedures for higher strength steels toensure that the required mechanical and toughness propertiesare maintained throughout the welded joint. Additional guid-ance for Group III and IV steels is given in 14.5.

Table 5—Structural Steel Pipe

Yield Strength Tensile Strength

Group Class SpeciÞcation & Grade ksi MPa ksi MPa

I C API 5L Grade Ba

ASTM A53 Grade BASTM A135 Grade BASTM A139 Grade BASTM A381 Grade Y35ASTM A500 Grade AASTM A501

3535353535

33-3936

240240240240240

230-270250

60 min.60 min.60 min.60 min.60 min.45 min.58 min.

415 min.415 min.415 min.415 min.415 min.310 min.400 min.

I B ASTM A106 Grade B (normalized)ASTM A524 Grade I (through 3/8Ó w.t.)ASTM A524 Grade II (over 3/8Ó w.t.)

353530

240240205

60 min.60 min.55-80

415 min.415 min.380-550

I A ASTM A333 Grade 6 35 240 60 min. 415 min.ASTM A334 Grade 6 35 240 60 min. 415 min.

II C API 5L Grade X42 2% max. cold expansion 42 290 60 min. 415 min.API 5L Grade X52 2% max. cold expansion 52 360 66 min. 455 min.ASTM A500 Grade B 42-46 290-320 58 min. 400 min.ASTM A618 50 345 70 min. 485 min.

II B API 5L Grade X52 with SR5, SR6 or SR8 52 360 66 min. 455 min.II A See 14.3.2.2

aSeamless or with longitudinal seam welds

Table 6—Impact Testing Conditions

D/tTest

TemperaturesTest

ConditionOver 30 36¡F (20¡C) below LASTa Flat plate20-30 54¡F (30¡C) below LASTa Flat plateunder 20 18¡F (10¡C) below LASTa As fabricated

aLAST = Lowest Anticipated Service Temperature


14.4.1.3 Brace Ends

Although the brace ends at tubular connections are alsosubject to stress concentration, the conditions of service arenot quite as severe as for joint-cans. For critical braces, forwhich brittle fracture would be catastrophic, considerationshould be given to the use of stub-ends in the braces havingthe same class as the joint-can, or one class lower. Similarconsiderations may apply where stress risers are encounteredalong the member between joints.

14.4.2 Critical Joints and Plate Intersections

Joints formed by the intersection of stiffened plates mayform areas of high restraint, through-thickness shrinkagestrains, high residual stresses and may be subjected tothrough-thickness tensile loads in service. Special care shouldbe given to the design of stiffened intersections or termina-tions to minimize or avoid the formation of stress intensiÞers,notches, etc. For these and other highly restrained criticaljoints, consideration should be given to the use of castings,forgings, or steel having improved through-thickness (Z-direction) properties.

14.4.3 Tubular Tendon Materials

This section only considers materials for steel tubular ten-dons, although other structural conÞgurations are candidatesfor use with TLPs. Tendon metallurgy is of utmost impor-tance since this component is the key element in the satisfac-tory performance of the Tension Leg Platform. The tendonmaterial should therefore possess optimum mechanical prop-erties including tensile and yield strength, fatigue strength,toughness, and ductility. Additional guidance is provided in14.5. These properties should be uniform through the thick-ness of the tendon. The material selection process shouldtherefore consider the dimensions and shape of the connec-tors as well as the tendon body to determine required harden-ability of the alloy. The material or tendon joint conÞgurationshould also be inspectable. If a connector is welded to the ten-don body the material should have satisfactory weldability.Corrosion control procedures should also be a considerationin the material selection process. Non-metallic as well as sac-riÞcial metallic coatings have been used to control corrosionin the splash zone and below the water as well as during stor-age prior to installation. The method(s) of corrosion protec-tion may have a direct bearing on materials selection. Highhydrogen overpotential from the cathodic protection system,for example, may aggravate stress cracking of high strengthsteels.

A quality assurance program is also an important consider-ation in tendon material selection. Tendon alloy production,fabrication and testing requirements should be commensuratewith the material quality and properties.

The paragraphs below discuss the materials selection con-siderations for various tendon conÞgurations.

14.4.3.1 Connector Integral With Tendon Body

This conÞguration consists of a single length of tendonbody, probably a tubular, with the end connections machinedinto the ends. The connection may be either a thread or Òdog-typeÓ engagement. The tendon is produced from a single bil-let or forging. The material should have adequate hardenabil-ity to assure uniform properties through the cross section. Theends of the tendon may be required to be thicker than thebody to provide adequate load carrying capacity, and there-fore heat treatment and alloy composition should be balancedto assure uniformity. A quality assurance program should beimplemented to conÞrm that microstructure and mechanicalproperties in the upset and tendon body are uniform. Testingof a prototype might be desirable to assure conformance tothe tendon speciÞcations. Testing of selected productionpieces for hardness, microstructure, or other required proper-ties by removal of a test ring should be considered.

14.4.3.2 Multiple Piece Tendon Joints

This conÞguration may consist of a tendon body which is atubular onto which are welded end connectors. The connectorbody might be a forging of heavier size to accommodate athread or other mechanical fastening device. In this design,the weldment between the body and the connector should begiven critical attention. The materials must have adequateweldability. The weld should be thoroughly inspected toassure that it meets the speciÞcation requirements, which mayinclude volumetric and surface examination using radiogra-phy, ultrasonic or other techniques.

14.4.3.3 Seam Welded Tendons

This conÞguration involves a welded cylinder, such aswelded line pipe with upsets or joints attached to the ends.Quality requirements should be speciÞed on the seam welds.Critical attention should be paid during the fatigue analysis tothe intersections between the longitudinal seam weld and theend connection girth weldment.

14.4.3.4 Tendon End Connections

The tendons may be connected together with a threadedintegral connection or may be joined with dog-latch typedesign. Thickness and strength requirements should be con-sidered in the materials selection process. A fatigue analysisof the connectors should be performed to assure that thedesign is adequate for the intended purpose. Also, thedesigner should be aware of possible galling of connectionsin the event the tendons are pulled for inspection.


14.5 FRACTURE AND FATIGUE CONSIDERATIONS

14.5.1 Group I and II Steels

These steels are as deÞned in Tables 4 and 5 and the rec-ommendations in API Recommended Practice 2A should befollowed.

14.5.2 Group III and IV Steels

14.5.2.1 Experience with these steels in offshore applica-tions is limited and little data exists in the public domain.

14.5.2.2 The operator should select materials and fabrica-tion processes that lead to adequate levels of toughness andfatigue properties under service conditions. Fatigue propertiesof tendon materials should be deÞned with appropriate meantensile stress.

14.5.3 Toughness Testing

14.5.3.1 The materials and fabrication methods usedshould have sufÞcient toughness to avoid brittle failure. ForGroup I and II materials, this can be achieved by speciÞcationof Charpy V-Notch toughness requirements at prescribedtemperatures. Steel with increased thickness or increasedstrength may require alternate means for assessing toughnessrequirements.

14.5.3.2 CTOD (Crack Tip Opening Displacement) Testingtogether with rational target toughness values may be used.This approach involves testing of specimens of full thicknessin the parent plate, heat affected zone (HAZ) and weld metal.The target value of CTOD should ensure that readily detect-able (and rejectable) fabrication ßaws will not propagateunstably under service conditions. Effects of stress (or strain)concentration, residual stress, and loading rate should beincorporated into the selection of target values. For example,50 ksi materials, where material thickness exceeds 11/2 inch,the following target values (average of three) have been speci-Þed and achieved at typical seawater service temperatures:

a. 0.012 inch (0.3 millimeter) as welded.b. 0.008 inch (0.2 millimeter) post-weld heat treated.

14.5.3.3 Application speciÞc threshold thicknesses forrequiring CTOD properties and the target levels should beselected by the designer.

14.5.3.4 Charpy V-Notch impact values achieved on pro-cedures passing the CTOD requirements can be used as aquality control indicator in production plate testing and rou-tine qualiÞcations.

14.5.4 Fatigue Resistance

14.5.4.1 This practice recommends fatigue analysis basedeither on the Palmgren-Miner S-N approach or the fracture

mechanics approach. API Recommended Practice 2A pro-vides recommended fatigue S-N curves that may be appliedwhen utilizing materials speciÞed therein. However, whenmoving to higher strength material it is recommended thatsound justiÞcation be established before accepting an S-Ncurve for design purposes. Such curves should be based ontests carried out on specimens of the appropriate materialhaving micro-structures and weld proÞles or notch effects(where appropriate) that model the general characteristics tobe found in prototype components. Testing should be carriedout under conditions consistent with the actual prototypeoperating environment with respect to loading frequency, areaof application (in air, splash zone, submerged), level ofcathodic protection, temperature, bio-organic environment,and stress level. Curves should cover the range of variableswhere they have signiÞcance to design.

14.5.4.2 The fracture mechanics approach to fatigue analy-sis should have reliable and pertinent fatigue crack growthdata. As with S-N curves, the data should be gathered fromtests on specimens having similar

a. Material chemistry and microstructure.b. Environment.c. Loading frequency.d. Cathodic protection.e. Temperature.f. Mean stress.

appropriate to the design situation. In particular, data shouldbe collected at cyclic stress intensity levels pertinent todesign. Testing should be carried out on specimens withknown KI calibrations. Standard compact and three-pointbend specimens should be considered.

14.6 STRUCTURAL WELDING

14.6.1 Specifications

Welding should be done in accordance with applicable pro-visions of the AWS Structural Welding Code AWS D1.1 andother applicable AWS documents as follows:

a. Sections 1 through 6 are applicable and constitute a bodyof rules for the construction of any welded steel structuregoverned by the AWS D1.1 Code. Part C of Section 6, cover-ing ÒUltrasonic Testing of Groove WeldsÓ should not apply totubular nodes. API Recommended Practice 2X provides guid-ance on ultrasonic testing techniques, procedures, reports andqualiÞcations of technicians for tubular nodes.b. Section 8 applies for general structural welding of platesand structural shapes, e.g., portions of deck sections.c. Section 10, ÒTubular StructuresÓ may apply to the variousTLP components.d. AWS D1.1 alone may not be adequate for Group III andIV steels.


14.6.2 Welding Procedures

Written welding procedures should be required for allwork, including repairs, even where prequaliÞed. The essen-tial variables speciÞed in AWS D1.1, Section 5 should beshown in the welding procedure and adhered to in productionwelding.

14.6.3 Welders

Welders should be qualiÞed for the type of work assigned,and be issued certiÞcates of qualiÞcation stating such limita-tions as required by AWS D1.1, Section 5 and Appendix E.

14.6.4 Qualification

Welding procedures, welders, and welding operatorsshould be qualiÞed in accordance with AWS D1.1 as furtherqualiÞed herein.

a. Impact RequirementsÑImpact requirements should beincluded in the fabrication or purchase speciÞcation. Whenthe purchase speciÞcation does not specify impact require-ments for Group II, Class A and B, Group III, and Group IVthe as-deposited weld metal and heat affected zone in theprocedure qualiÞcations should meet the minimum tough-ness requirements speciÞed in 14.2.2. Additional guidancefor the selection of toughness requirements is given in 14.5.Charpy V-notch tests should be performed in accordance withASTM A370. The longitudinal axis of the specimen shouldbe at a minimum depth of T/2 for T = 3/4 inch or less and T/4for T > 3/4 inch, from a weld surface.

b. HardnessÑHardness requirements should be by agree-ment of the manufacturer and the purchaser.

c. Gas Metal Arc WeldingÑThe short arc process should notbe used without prior approval of the purchaser.

d. Large Diameter PipeÑThe procedure for submerged arcmetal welding of girth joints on large diameter pipe should bequaliÞed on the smallest diameter for which the procedurewill be used during production.

14.6.5 Performance Qualification Tests

QualiÞcation tests should be performed by a competenttesting laboratory.

14.6.6 Prior Qualifications

New qualiÞcations may be waived if prior qualiÞcationsand experience are deemed acceptable.

14.6.7 Weld Size

Welding should conform to sizes of welds and notes on thedrawings.

14.6.8 Inspection

The degree of inspection required should be speciÞed. As aminimum a visual inspection of welded joints should con-form to the appropriate requirements of the AWS D1.1 code.

14.6.9 Unspecified Welds

Intersecting and abutting parts should be joined by com-plete penetration groove welds, unless otherwise speciÞed.This includes hidden intersections, such as may occur inoverlapped braces and pass-through stiffeners.

14.6.10 Groove Welds Made From One Side

At intersecting tubular members, where access to the rootside of the weld is prevented, complete penetration groovewelds conforming to AWS Figure 10.13.1A may be used.When tubular members are large enough to allow welderaccess to the inside of the member, consideration may begiven to cutout windows to allow access for welding the backside of the joint. The cutout should be replaced and reweldedusing a properly Þt back-up bar with all splices in the back-upbar full penetration welded.

14.6.11 Seal Welds

Unless speciÞed otherwise, all faying surfaces should besealed against corrosion by continuous Þllet welds. Sealwelds need not exceed 1/8 inch (3.2 millimeters), regardless ofbase metal thickness, provided low hydrogen welding is used.

14.6.12 Post Weld Heat Treatment (PWHT)

Stress relief by PWHT of cylindrical members fabricatedfrom carbon steels is generally not required where the weldjoint thickness is 2 inches or less. Where post-weld heat treat-ment is to be used in production it should be included in thewelding procedure qualiÞcation tests. A detailed PWHT pro-cedure should be written for each heat treatment. In specify-ing PWHT other factors such as high weld joint restraintshould be considered in addition to thickness.

14.6.13 Weld Toughness

Where minimum toughness requirements are speciÞedthey should be applicable to the entire weldment (base mate-rial, weld deposit and heat affected zone).

14.6.14 Alternate Specifications

At the option of the operator, hull and deck fabrication mayfollow the ABS ÒRules for Building and Classing MobileDrilling Units.Ó Riser and tendon fabrication may follow theASME Boiler and Pressure Vessel Code.


14.7 CORROSION PROTECTION

14.7.1 General

The steel materials should be protected from the effects ofcorrosion by the use of a corrosion protection system that isin accordance with NACE Standard RP-01-76. The corrosionprotection systems include coatings, cathodic protection, cor-rosion allowance and corrosion monitoring. Overprotectionwhich may cause hydrogen embrittlement should be avoided.

14.7.2 Antifouling

In areas where marine fouling is signiÞcant, organisms areactive and the use of antifouling coatings may be consideredto reduce the effects of marine growth.

14.8 CEMENT GROUT AND CONCRETE

14.8.1 Cement Grout

14.8.1.1 The designer should consider the recommenda-tion on cement grout set forth in API Recommended Practice2A. Non shrinking grout should be used when the integrity ofthe connection depends solely on the bond strength betweenthe grout and the surface.

14.8.1.2 Non-shrinking grouts are generally classiÞed intofour types: gas liberating, metal oxidizing, gypsum-forming,and expansive cements which derive their non-shrink proper-ties from the expansive nature of the cementitious system.Some types are designed to maintain their as-cast volumewhile other types are designed for a continual but pro-grammed expansion with time. The level of expansion andthe force it exerts on adjacent structures and formations canbe controlled and designed accordingly.

14.8.1.3 Mixing and placing should be in conformancewith the material manufacturerÕs instructions, if available. Inmost instances, the grout should not be mixed for more than 3minutes after the water is added. Prolonged mixing, whileincreasing strength, tends to reduce expansions.

14.8.1.4 Batches should be of a size to allow continuousplacement of the freshly mixed grout. Grout not used within30 minutes after mixing should be discarded. Placing shouldbe continuous, Þlling the volume to be grouted from the bot-tom to the top. Grout should not be retempered. Placementunder pressure using a pump is acceptable.

14.8.1.5 When the material is a prepackaged productrequiring only the addition of water, no additional ingredientsshould be added without evaluating the effect of these addi-tions on the non-shrink (expansion) behavior of the grout.When the grout is formed by the addition of a volume con-trolling ingredient to a cementitious binder, the ingredientmay be dispensed in solid or liquid form. If in a solid form, itshould be thoroughly dry-mixed before adding water.

14.8.1.6 Water content should be the minimum that willprovide a ßowable mixture and completely Þll the space to begrouted without segregation, bleeding, or reduction instrength. The water-cement ratio (by weight) should notexceed 0.5. Unless recommended by the material manufac-turer, the Þnal proportions should be based on the results fromsample mixtures of the grout.

14.8.2 Concrete

The designer should consider the recommendation on con-crete material set forth in API Recommended Practice 2A.

14.9 ELASTOMERIC MATERIALS

14.9.1 Function

Elastomer compounds may be used in the articulating ele-ment of a TLP Mooring System. Selection of a material ishighly dependent on the user speciÞcations and the design ofthe ßexible joint. See A.Comm.14.9.

14.9.2 Typical Materials Used in Flexjoint Design

Selection of elastomers for use in TLP ßexjoints is depen-dent upon the laminated structure design approach employedby the ßexjoint manufacturer. Many of the speciÞc com-pounds used are proprietary to the manufacturer. Althoughliterally thousands of elastomeric compositions exist, thesefall within general types including synthetic and natural rub-bers. Synthetics include the nitriles, neoprenes, SBRs, butylsand polysulÞdes. Some blends of two or more polymers arealso used.

14.9.3 Selection Criteria

14.9.3.1 The material selection, design, manufacture, andtest of the TLP mooring ßexjoint are normally functions ofthe ßexjoint contractor. The selected material should havesufÞcient successful use history to demonstrate to the userÕssatisfaction its adequacy for its intended purpose.

14.9.3.2 The designer should be provided with speciÞca-tion deÞnition in enough detail to ensure the best design/material combination is selected. This includes the following:

a. Tension loads as a function of local leg deßection angleincluding normal operating and design maximums and mini-mums.b. Maximum angular deßection at design load.c. Design life.d. Site long term stress distribution spectrum in terms of ßuc-tuating tension loads and angular excursions.e. Internal conduit pressures if applicable.f. External environment consideration, i.e., sea water tem-perature and depths.g. Diameter and weight constraints.


h. Fatigue resistance.i. Maximum angular stiffness spring rate at operating condi-tions.j. Minimum axial stiffness.k. Corrosion protection requirements. Any special require-ments, for instance, compatibility with internal ßuids,unusual site speciÞc conditions.l. Proof loads.

14.9.4 Contractor Specifications

14.9.4.1 The ßexjoint contractor should provide speciÞ-cations for user approval which include the followingrequirements:

a. The type of elastomer to be used.b. Material mechanical property testing requirements such as:

1. Tension Testing of Rubber ASTMÑD-4122. Adhesion Test ASTMÑD-4293. Tear Resistant Requirement ASTMÑD-6244. Hardness of Rubber ASTMÑD-2240

c. As applicable, the shear modulus requirement at a particu-lar set of strain conditions.d. Bonding agents and surface preparation methods.

14.9.4.2 The ßexjoint contractor should demonstrateappropriate controls of materials, processing, and testing toensure uncured green rubber is delivered to the molding oper-ations. Age and storage controls must be documented.

14.9.4.3 The ßexjoint contractor should have suitablemolding process speciÞcations for molding techniques, mold

design, cure time, temperatures, pressures and should providethese for review.

14.9.4.4 The ßexjoint manufacturer should provide in-depth analysis of design, material selection including docu-mented and demonstrated history of satisfactory operation insimilar design, operational, and environmental situations.

14.9.5 Acceptance Criteria

14.9.5.1 Acceptance criteria for the molded elastomershould include the following:

a. Normal dimensional requirements.b. Non-destructive examination.c. Proof loads and angles.d. Inspection techniques to ensure proper location of rein-forcements and adequate rubber coverage.e. SpeciÞc criteria for voids and surface blemishes.f. Local interfacing with corrosion protection coatings.g. Repair methods and limitations.

14.9.5.2 Acceptance criteria for reclamation of metal com-ponents of rejected moldings should be deÞned. ReclamationprocessesÑchemical, thermal or cryogenicÑshould have nodeleterious effect on the metal.

14.9.5.3 Acceptance criteria for ßexjoint assembly testingshould include the following:

a. Hydro testing (if applicable), axial and spring rate testing.b. Proof loads (optional).

105

APPENDIX A—COMMENTARIES

Appendix A includes commentaries on certain sections of Recommended Practice 2T. The paragraph numbers correspond tonumbered paragraphs in the referenced section. References are included in Appendix B.

A.COMM.4 Planning

A.COMM.4.2 THE DESIGN SPIRAL

The design of a TLP system is an interactive processinvolving both design and analysis. The TLP system is inter-dependent among its parts and requires trade-offs and balanc-ing from conceptual design all the way through Þnal design.The design spiral discussed in 4.2 describes the iterationwhich is necessary in performing the design of a TLP. Thesystem design of a TLP is a small subset of this spiral whichincludes some of the most important interactions and trade-offs. The parameters which are considered primary in thisÞrst level of design are the size and number of columns, thedraft and sizing of pontoons, the tendon pretension, the deckclearance, maximum tendon loads, number and tension of ris-ers, and the payload. The design process balances operationalrequirements of the TLP with constraints of environment,regulation, and economics. The design of TLPs shouldinclude appraisal of probabilistic risk, including subjectiveuncertainties as well as statistical uncertainties, and the use ofspectral and statistical methods of predicting designresponses. The following provides an overview of the systemdesign process, while subsequent sections provide details onthe steps involved in the design.

A.COMM.4.2.2 Conceptual Design

The conceptual design and sizing of a TLP centers aroundthe pretension in the tendon system. The weight of the sys-tem, including riser tension and payload, must be balancedagainst the pretension requirements for the tendon system asdictated by the environmental loads. The Þnal weight of theplatform is ultimately determined by the end product ofdesign, while the pretension requirement is determined byanalysis of the Þnal conÞguration. The relationship betweenweight and pretension is a Þne balance which involves mostmajor aspects of design. The proper balance between weightand pretension is approached in an iterative manner. The pre-tension is provided by excess displacement of the hull.Increased displacement generally increases both weight andthe dynamic response of tension in the tendons to environ-mental forces. Therefore, in addition to providing additionalpretension, increased displacement also increases the preten-sion requirements. The estimate of weight at each level ofdesign is critical, with excessive errors in either direction hav-ing a detrimental impact on the design. Weight marginsshould be considered carefully. Too large an estimate resultsin a signiÞcantly larger and more expensive structure, and too

small an estimate results in insufÞcient weight carryingcapacity which cannot be increased without a complete re-sizing of the hull.

A.COMM.4.2.3 Preliminary Design

Because of the nature of the response calculations for theTLP design, it is important to have a complete design basisavailable at the end of the preliminary design cycle, and torecognize that later changes in the design requirements canhave a very large impact on the global design. The importantfactors in a design basis include performance requirements,the site parameters, and design factors and margins. These arediscussed elsewhere in Sections 4 and 5, but are listed herefor completeness:

a. Performance requirements.b. Function and payload (space and weight).c. Riser number, spacing, and tension.d. Maximum offsets and riser angles.e. Motion limits.f. Design life.g. Site parameters.h. Location.i. Water depth.j. Environmental criteria (operation and extreme).k. Wind.l. Wave.m. Current.n. Tide/surge.o. Directionality.p. Joint statistics of events.q. Geotechnical criteria.r. Soil strength/stiffness.s. Bottom survey/contours.t. Design factors and margins.u. Regulatory requirements.v. Safety factors.w. Tendon pretension margins.x. Weight estimating margins.y. Construction and fabrication tolerances.z. Stress/load/resistance factors.aa.Deck clearance margin.

The design process is shown schematically in Figure A-33.The design loop within which the system is conÞgured andsized includes three steps. The Þrst is the conÞguring of theplatform and tendon system and a weight estimate. The sec-ond is the analysis of the system extreme responses to envi-ronmental forcing, and the third is checking the system


responses against the requirements established in the DesignBasis. Any unacceptable performance results in a return to theconÞguration and sizing step for modiÞcation of the systemand subsequent analysis and performance checks. During theinitial passes through this loop, the designer will be con-cerned with meeting the fundamental performance require-ments and surviving extreme environmental conditions. Theweight estimates will be based on preliminary structuralsketches and/or typical weight densities for similar structures.In subsequent passes through the design loop, the designerwill include more detailed requirements such as fatigue life of

the tendons, and perform a much more complete analysis tobe certain of covering environmental conditions and combi-nations which will give extreme responses. By the Þnal passthrough the loop, it is important that the weight estimate bevery close to the Þnal weight. Appropriate margins for errorand growth should be included. The result of this phase of thedesign should be the system conÞguration with a Þxed hulldisplacement and the maximum loads and responses for thedetailed design of the platform tendons, foundation, risers,and platform systems.

Performancerequirements

Siteparameters

Designfactors and

margins

Tendonconfiguration

Platformconfiguration

and weight est.

Towoutstabilitycheck

Globalresponseanalysis

Modeltesting

Performancechecks

Vesselconfiguration

and responses

Riser/wellsystemdesign

Detailedfoundation

design

Detailedtendondesign

Detailedstructural

design

Figure A-33—TLP Global Design Process


A.COMM.4.2.4 Final Design

The Þnal design of the various subsystems of the TLP pro-ceed with the results of the preliminary design from thedesign loop described above. There continues to be someinterface, primarily at the physical intersection points such asthe tendon attachment to the hull or to the foundation. Itremains critical to monitor the weight, center of gravity,moments of inertia, and any other factors which may affectthe system responses. It may at some time be necessary to re-enter the primary design loop if major changes are required.The primary resources available to prevent such iterations areweight margins and weight shedding exercises.

A.COMM.5 Design Criteria

A.COMM.5.4.2 Wind

Steady wind speeds are deÞned in this RecommendedPractice as the average speed occurring for a period of onehour duration. The steady speeds are considered to be themean speed measured at a reference height, typically 30 feet(10 meters) above the mean still water level. The directional-ity of the wind may be important in some applications and ifso should be investigated. Wind speeds may be extrapolatedto heights other than the reference height and to other averagetime intervals as shown in 6.2.2.

Wind spectra are used to deÞne variable wind speedswhich cause dynamic loadings on the platform. Measuredwind data may be needed to develop an appropriate windspectrum for a speciÞc site. The spectrum so deÞned maythen be used to calculate ßuctuating wind forces (see 6.2.5).

There is considerable variability in wind gust characteris-tics as revealed by measurements. A number of generalempirical formulations for wind spectra have been proposed(Kareem, 1982; Simiu and Scanlan, 1978). A simple windspectra is given in 6.2.2.

A.COMM.5.4.3 Waves

The signiÞcant wave height (HS) is often used as the mainparameter to deÞne a seastate. Statistically, the signiÞcantwave height is four times the standard deviation of the seasurface elevation about its mean level, or is approximately theaverage height of the one-third highest waves.

The distribution of wave heights, including an expectedmaximum, can be derived from the signiÞcant wave height(see A.COMM.7.7 or Recommended Practice 2A).

Wave PeriodsÑIn addition to the signiÞcant wave height, acharacteristic wave period must be given to deÞne a seastate.This characteristic can be either the spectral peak period, Tp,or the average zero-crossing period, Tz (the average timebetween consecutive up or down crossings of the mean sealevel). Tz is more commonly used, but suffers from havingmany differing deÞnitions in use. Tz should be deÞned in

terms of the zero and second statistical moments of the waveheight spectrum, and should use an upper cutoff frequency inits deÞnition. If a cutoff frequency is not used, the spectrallydeÞned zero-crossing periods will be biased to values smallerthan observed by other methods. The criteria for selecting thecutoff frequency may be based on preserving a percentage ofthe zeroth moment of the spectra, or be a multiple of the spec-tral peak frequency.

(A.COMM-48)

Where:Tz = Average zero-crossing period.

(A.COMM-49)

Where:S(f) = height spectral density, ft2/Hz or m2/Hz.

f = Frequency, Hz.fc = Cutoff frequency, Hz.

Mn = n-th spectral moment.

Wave SpectraÑThe wave spectrum describes irregular seaconditions. Measured wave spectra are highly variable innature, and smoother parametric spectral models are com-monly used for analysis.

Many empirical formulations for wave spectra have beenproposed (Bretschneider, 1959; Hasselmann et al., 1973;Pierson and Moskowitz, 1964). Various spectral forms havebeen developed for different condition such as fully arisenseas (long duration and fetch or causing winds), short-fetchseas, combined seas, directionally spread seas, etc.

Among the empirical spectra available, most have the fol-lowing form:

S(f) = A fÐm exp (ÐBfÐn) (A.COMM-50)

Where:

(A.COMM-51)

Where:s2 = variance of the sea surface elevation about its

mean level.s2 = (HS/4)2.

A, B = dimensional constants related to the signiÞcantwave height and period.

m, n = integers.

Wave KinematicsÑWave induced water particle velocityand acceleration are functions of wave height, wave period,water depth, distance above bottom and time. These functions

Tz Mo/M2=

Mn fc S f( ) fdo

fcò=

S f( ) fd0

¥

ò s2=


may be determined by any defensible method for deepwaterwaves, e.g., linear wave theory, StokesÕ Þfth order wave the-ory, stream function wave theory, Chappelear wave theory, orextended velocity potential wave theory.

Linear theory is discussed in detail by Wiegel, 1964, and isespecially useful when performing spectral analysis. Sincethis is a Þrst order approximation to actual wave kinematics,values are calculated only to the mean water level. To obtainvalues up to the free surface, the kinematics from the meanwater level must be either stretched or extrapolated. If astretched approach is used the kinematics at the free surfacewill be identical to those originally calculated for the meanwater level. Extrapolating kinematics to the free surface con-sists of applying the velocity potential to the actual free sur-face. This method could lead to extremely conservativeresults for random seas. For this case the stretched methodmay be used.

Equations for StokesÕ Þfth order wave theory can be foundin Skjelbreia and Hendrickson, 1961. Equations for the Chap-pelear wave theory can be found in Chappelear, 1961. Thestream function wave theory equations can be found in Dean,1974. Lambrakos and Brannon, 1974, contains the equationsfor the extended velocity potential wave theory.

A.COMM.7 Global Design and Analysis

A.COMM.7.7 RANDOM PROCESS STATISTICS

For short term Gaussian processes, there are simple formu-lae for estimating extremes. Wave and wave response statis-tics are generally Gaussian in nature. The peaks of Gaussianprocesses are Rayleigh distributed. The most probable maxi-mum value, gn of a zero-mean narrow-band Gaussian randomprocess may be obtained by the following formula, for a largenumber of observations, n:

gn = (2M01n(n))1/2 (A.COMM-52)

The parameter M0 is the zeroth spectral moment of the pro-cess, either calculated or measured.

If the process is non-narrow band, the solution can still besimply estimated for bandwidth parameter, e, less than 0.90.That is:

(A.COMM-53)

Where:

(A.COMM-54)

M2 and M1 are the second and fourth spectral moments ofthe process.

The most probable extreme value can also be expressed interms of time instead of the number of observations. The timefor a given number of observations can be derived from theaverage number of zero crossings per unit time. Thus, theabove formula can be written as:

(A.COMM-55)

Where:T = total time (in hours) of exposure to the sea state in

question.

This relationship holds for any value of e and is not depen-dent on the bandwidth.

Although Equation A.COMM-55 is the same irrespectiveof the bandwidth parameter of the spectrum, the number ofpeaks for a broad-banded spectrum is larger than that for anarrow-banded spectrum for the same period of time.

gn is the most likely extreme to occur in n events. The larg-est response in these n events has a 63 percent chance ofexceeding gn. In order to obtain an extreme response in whichthe probability of being exceeded is small, a risk parameter,a, may be incorporated in the design extreme response pre-diction. a is between 0 and 1, and represents a fractionaloccurrence in n events. Equation A.COMM-53 becomes:

(A.COMM-56)

For a narrow-band assumption, e = 0, Equation A.COMM-52 becomes:

(A.COMM-57)

The preceding formulae are for Gaussian stationary pro-cesses, and are for short-term probabilistic analyses. For pro-cesses with e greater than 0.9, the Gumbel distribution maybe used. For longer term analyses, a number of other tech-niques can be used. These are generally referred to as extremedistributions, and include Weibull, Gumbel, and Ochi distri-butions. Long-term wave statistics are often Þtted to aWeibull distribution. Further information can be found inWeibull, 1951, Gumbel, 1954, and Ochi, 1973.

Response analysis often requires the consideration of mul-tivariate processes. In this case, the joint distributions of thedifferent parameters become important. To predict theresponse distributions in a fully probabilistic analysis, thejoint distributions of all relevant environmental events mustbe speciÞed. For deterministic analysis, an assumption of fullcorrelation between different parameters will usually provide

g n 2M0 1n

2n 1 e2Ð

1 1 e2Ð+--------------------------

è øç ÷æ ö

1 2¤

=

e 1M2

2

M02 M4

2-----------------Ðè ø

æ ö1 2¤

=

g n 2 M0 1n 3600 T2p

-----------------M2

M0

-------è øç ÷æ ö

1 2¤

=

g n 2 M0 1n2 1 e2Ð

1 1 e2Ð+--------------------------

na---

è øç ÷æ ö

è øç ÷æ ö

1 2¤

=

g n 2 M0 1nna---è ø

æ öè øæ ö

1 2¤

=


a conservative estimate of the extreme combination, but thisis often excessively costly to the design. Further informationon the application of joint statistics to TLP design may befound in Leverette, 1982.

A.COMM.8 Platform Structural Design

A.COMM.8.4.4.1 Fatigue Life Requirement

API Recommended Practice 2A recommends a designfatigue life for components of Þxed offshore platforms equalto twice the intended service life of the structure. For criticalelements whose sole failure could be catastrophic, use of anadditional margin is recommended. Tension leg platforms area new structural concept. There is no body of historical dataor extensive analytical studies for TLPs that can be used tocalibrate a fatigue life requirement to assure reliabilities forTLP structural components like those implicit in API Recom-mended Practice 2A for Þxed offshore platforms.

The recommendation in this TLP design practice of afatigue life requirement for the hull and deck of at least threetimes the intended service life of the platform reßects thisimmaturity of TLP technology. This recommendation maychange as more information on TLPs becomes available.

A.COMM.8.4.4.2 Fatigue Loading

The wave climate may be described, for fatigue analysispurposes, by approaches such as described in API Recom-mended Practice 2A.

A.COMM.8.4.4.3 Fatigue Analysis

There are two main categories of methods for generatingstress range distributions for fatigue life assessment: deter-ministic and stochastic. Four approaches for generatingstresses are presented. The Þrst three are deterministic and thefourth is stochastic:

a. A simple fatigue analysis is based on the assumption thatthe stresses in the structure have a probability distributionsimilar to the probability distribution of the wave height mea-sure (e.g., visual wave height, signiÞcant wave height, etc.).The parameters of the stress distribution are derived from alimited number of response analyses of the structure sub-jected to regular waves. The wave period assigned to each ofthese regular waves may be a deterministically deÞned quan-tity corresponding to a given wave height. Alternatively, thisperiod may be of stochastic character related to a given waveheight through scatter diagrams that describe the joint proba-bility of wave heights and periods. Dynamic effects should betaken into account in determining the probability distributionof stresses if their contribution is expected to be signiÞcant.The number of cycles is obtained by the return period conceptfrom the probability distribution of wave height.b. Instead of assuming the shape of the probability distribu-tion for stresses, the following approach called discrete

fatigue analysis may be used. From the probability distribu-tion of the wave height measure, discrete ÒblocksÓ of severalregular waves of different height are formed. The number ofcycles per ÒblockÓ is obtained by the return period conceptfrom the probability distribution of wave height. Theresponse of the structure is obtained for a series of singlewaves each being representative of one Òblock.Óc. Maddox (1974, 1975) presented a deterministic approachfor fatigue analysis. The wave climate is composed of manysea states. Each short-term sea state is represented by a waverecord (usually generated from a wave spectrum). The struc-ture response and stresses are obtained for each wave record.Stress range is deÞned from the record. The number of cyclesoccurring for a given stress range is obtained Þrst from thestress record; then it is scaled according to the percent ofoccurrence per year for the given sea state and the percent ofoccurrence for the corresponding Þnite length record.d. In a spectral fatigue analysis (see Maddox and Wildenstein1975, Vugts and Kinra, 1976; Kinrad and Marshall 1979), thewave climate is composed of many sea states, each describedby a wave spectrum. Transfer functions for response quanti-ties can be developed by either time domain or frequencydomain analysis. The objective is to generate stress responsespectra for critical locations on the platform. Short-term sta-tistics are generated by evaluation of moments of the stressspectra. The stress response spectral may be used to estimatethe stress range distribution by assuming:

1. Rayleigh distribution in the case of narrow bandedstress response spectra.2. Rice distribution in the case of broad banded stressresponse spectra.3. Time series simulation and cycle counting via rainßow,range pair, or some other algorithm.

Long-term statistics are evaluated by considering the prob-ability distribution of the parameters characterizing the wavespectrum. The stress range distribution is then used to calcu-late the cumulative fatigue damage ratio, D, according toEquation 40.

The choice of proper S-N curves is important. The S-Ncurves used should be related to the material, constructiondetail, and environment. The fatigue of TLPs is a high cycle,low stress phenomenon. In the high cycle range, S-N curvesare characterized by a degree of uncertainty.

A 95 percent conÞdence level S-N curve should be used.For welded connections the S-N curve used should reßectlocal weld proÞle, and should be used with a stress concentra-tion factor that reßects overall joint geometry. The effect ofmean stress should be considered.

For S-N curves to be used for tubular connections, see APIRecommended Practice 2A. Proper S-N curves should be usedfor connections different from tubular, such as tubular to rect-angular connections (column to pontoon), stiffened plates, etc.

Fatigue ScreeningÑA simple method to determine themaximum stress amplitude for fatigue screening during pre-


liminary stages of design is outlined here. The fatigue life ofthe Þnal design must be conÞrmed by a detailed fatigue anal-ysis as outlined in 8.4.4.3.

This simple method is based on the assumption of aWeibull probability density function for the stress range. Thetheory behind the method and the derivation of the equationfor fatigue damage is given in Marshall and Luyties, 1982.The lifetime fatigue damage, D, is:

(A.COMM-58)

Where:

D = cumulative damage ratio.m, A = empirical coefÞcients for the S-N curve.Srmax = maximum cyclic stress range.

NT = total number of cycles.x = Weibull shape parameter.

G() = the complete gamma function.

This equation can be solved for the maximum cyclic stressamplitude Smax (equal to Srmax divided by 2.0), in terms of thedamage D:

(A.COMM-59)

The designer should decide on the appropriate level ofdamage for this preliminary screening procedure.

As an example of the calculation of maximum screeninglevel stress, assume that the damage allowed over a one hun-dred year period is 1.0, and that the API X fatigue damagecurve is applied with no endurance limit. For this example thevariables in Equation A.COMM-59 are as follows:

NT = 5.0 E+8 number of cycles over 100 years

D = 1.0 damage allowed over 100 years

x = 1.0 Weibull shape parameter.

Substituting these variables into Equation A.COMM-59yields a maximum fatigue screening level hot spot cyclicstress amplitude of 18.7 ksi in the 100 year event.

A value of 0.5 to 0.8 is often used for x for Þxed platforms.A value closer to 1.0 may be more appropriate for TLPs. Thevalue of x is a function of the long-term environment and theplatform response. The designer should develop a basis for xbased on more detailed fatigue analyses.

A.COMM.8.5 STRUCTURAL DESIGN

Deterministic DesignÑIn deterministic design, discretevalues are assigned to each design variable rather than rangesassociated with probability of occurrence. To deÞne loads,extreme values that have a small chance of being exceededare usually adopted. Uncertainty is only indirectly included inthe design process through factors of safety, usually based onexperience or commonly accepted practice.

Probabilistic DesignÑIn probabilistic design all quantitiesthat enter into design calculations are associated with proba-bility of occurrence. Thus, probabilistic design attempts toaccount for all random and systematic uncertainties at thecode development stage. Probabilistic design attempts tomodel the mechanics of the structure, loading and materialrather than use a catchall factor of safety. This designapproach is expected to result in a more uniform level ofsafety for all structures.

Ultimate Strength DesignÑLimit state design checks thesafety of the structure against criteria for performance anduse. A structure, or part of a structure, is considered unsafewhen its performance or use is seriously impaired. TheseÒlimit statesÓ can be placed in two categories:

a. Ultimate limit states, which correspond to the maximumload carrying capacity.b. Serviceability limit states, which relate to the criteria gov-erning normal use and durability.

A.COMM.8.5.2 Allowable Stresses

Tension Leg Platforms are a new structural concept. Thereis no body of historical data or extensive analytical studies forTLPs that could be used to calibrate the allowable stressesused in this working stress design based practice to assurereliabilities for TLP structural components like those implicitin API Recommended Practice 2A for Þxed offshore plat-forms. The implicit approach to safety embodied in this prac-tice, like any working stress design based practice, willprovide structures that have a great range of component andsystem reliabilities.

The allowable stresses used here have been chosen to belike those used in API Recommended Practice 2A. The one-third increase in allowables has been used here for theÒExtremeÓ environmental condition, corresponding to theincreased allowables in API Recommended Practice 2A, Sec-tion 2.5.1b, ÒIncreased Allowable Stresses.Ó This allowableincrease was adapted from the AISC provision for wind andearthquake loading, Section 1.5.6 of the AISC code. Thedesigner should bear in mind that the behavior of TLPs underapplied loads may not be like that of other types of structures,and should use appropriate caution when using the allowablestresses recommended in 8.5.2.

DG m/x 1+( )

A--------------------------- NT

Srmax( )m

1nNT( )m/x------------------------=

Smax1nNT( )1/x

2-----------------------

D AG m/x 1+( ) NT

-----------------------------------

1 m¤

=

m 4.38=

A 2.44 E 11+= þýü

From the API X curve


A.COMM.9 Tendon System Design

A.COMM.9.1.2 Description of Tendon System

For some applications it can be useful to consider tendonsystem designs where there are substantial differences in theform, geometry, and intended service conditions of individualtendons either within a leg or from leg to leg. Examples ofthis might include a case where a solid rod or stranded cabletendon is to be installed inside a previously installed tubulartendon. It is not mandatory that an individual tendon have anidentical appearance throughout its length.

A.COMM.9.2 GENERAL DESIGN

The general design philosophy of this section is that thestructural failure of a tendon, i.e., parting of a tendon, isdeÞned as a critical failure. This philosophy has been adoptedin light of the high uncertainty in predicting the consequencesof a parted tendon. The goal of this design procedure is toavoid the parting of a tendon.

A.COMM.9.3 DESIGN LOADING CONDITIONS

Since tendon loadings can be sensitive to the occurrence ofwave energy at a resonant frequency, or to the number of ten-dons in service, tendon survival analysis should considerloadings which might arise from conditions other than themaximum design storm. For example, an annual or 10 yearstorm occurring while one or more tendons are removed forinspection or service, or with a ßooded compartment on theplatform, could result in higher tendon loads than a maximumdesign storm with all tendons in place. Selection of the maxi-mum design load should therefore consider the range of pos-sible storms which might occur during the various operationalconÞgurations of the platform. The selection of the maximumdesign load should be based on a consideration of the relativeprobabilities of each combination occurring.

A.COMM.9.3.2 Loading Conditions

The loading conditions listed in 9.3.2.1 deÞne representa-tive conditions for which tendons need to be designed to pre-clude overload, buckling and fatigue modes of failure.

A.COMM.9.3.2.2 Extreme Environment

In designing for minimum tension conditions, the designershould consider the possibility of extreme tidal variationsoccurring in conjunction with storms or improper ballasting.Uneven ballasting could result from a ßooded compartmentor from equipment failure. Note that the minimum tensioncondition is likely to occur on a leg other than the one experi-encing the maximum tension, hence the loads on all legsshould be considered. Also, some error or uncertainty in the

platform weight and ballasting should be considered in deter-mining minimum tension conditions.

A.COMM.9.3.2.3 Normal Conditions

Lifetime operating load conditions deÞne the expectedmagnitude and frequency of occurrence of tendon loads on anannual or service life basis. These can be used to determinecyclic stress range data needed for fatigue analysis.

A.COMM.9.3.2.6 Seismic

In considering seismic loads, if time history analysis is per-formed, the procedures for selecting ground motion describedin API Recommended Practice 2A should be followed.Where possible, three historic records from sites similar tothe TLP location should be selected, Þltered, and truncated.Each component on each record should be scaled on theamplitude and/or time axis so that its damped response spec-trum matches the appropriate recommended API Recom-mended Practice 2A spectrum, with an appropriate extensionto longer periods as discussed above, at the oscillating periodcorresponding to the structural response.

A.COMM.9.4.2 Dynamic Analysis Considerations

The importance of non-linear and coupling effects on ten-don tensions has been the subject of research in recent years.Generally any analytical method contains uncertainties whichmust be accounted for in design procedures and safety fac-tors.

Uncoupled linear analysis is the easiest method to employ.It can give adequate results for primary wave fatigue loadsand maximum loads provided a spectral wave input is usedwith appropriate statistics applied. A single deterministicÒdesign waveÓ could give inaccurate results for extreme loadsif the peak of the tension Response Amplitude Operator(RAO) does not happen to fall at the period of the designwave.

The importance of coupled tendon analysis may increaseas the mass of tendons approaches a signiÞcant percentage ofplatform mass. This would be the case for very deep TLPapplications and/or tendon designs based on very large diam-eter tendons or large tendon clusters. An important consider-ation is the amount of hydrodynamic added mass associatedwith tendon transverse accelerations.

Although an uncoupled tendon load analysis can be per-formed using a riser-type program. the designer must use carein using such a program because of the following two impor-tant differences between risers and tendons:

a. Risers normally have constant tension at the top with theirlength changing in accordance with vessel heave motions,with only a small variation in tension due to tensionerresponse. Tendons will be rigidly attached (in the axial direc-


tion) to the platform columns and will experience large ten-sion variations due to smaller heave motions.b. Heave and pitch motions of the platform may occur at res-onant frequencies close to the primary wave frequencies, or inthe range of second order wave force frequencies. The analy-sis needs to consider the effective amplitude of theseresponses since they could contribute a high number offatigue cycles.

A.COMM.9.5.3 Fatigue Analysis

While well understood in the offshore industry, the S-Napproach assumes the availability of lower bound S-N curvesfor the components being analyzed. Further, these curves areintended to be representative of the material, environment,cyclic load range and frequency, mean load and level ofcathodic protection. As of 1986, few tendon componentdesigns have been tested (Salama, et al.), so particular S-Ncurves cannot be given.

Fatigue life estimates can also be made via fracturemechanics, for which some general guidance can be found inDnV, 1977 (Appendix C). The life is thereby a function of therange of stress intensity, initial and Þnal ßaw sizes, and mate-rial crack growth constants (e.g., Burnside, et al., 1984) fromthe Paris equation. However, there are numerous pitfalls inachieving accurate fatigue life predictions using Òconven-tionalÓ fracture mechanics, even if the Þnal ßaw size andcrack growth constants are carefully established. For exam-ple, there is difÞculty in establishing accurate stress intensityexpressions for three-dimensional geometries, particularly inregions of high stress gradient and instances where the peakstresses are displacement-induced. Also, many componentsof interest have non-welded details, and crack initiation peri-ods, during which fracture mechanics is not applicable, canrepresent a substantial portion of the overall fatigue life.

Methods for determining stress intensities for ßaws innotches are discussed in Shah, 1976 (thumb nail cracks) andBuchalet, et al., 1976 (circumferential cracks). Hammouda, etal., 1979, discuss the general problem of fatigue crack initia-tion and propagation from notches, indicating the necessityfor test data for both phases of fatigue life. Given all theuncertainties with respect to procedures and input values ofkey parameters, calibration of analysis techniques to the con-servative side of experimental results for each component iswarranted. However, such calibration may fall short of pro-viding a sufÞcient amount of data to establish a lower boundS-N curve.

A.COMM.9.6.2 Allowable Stresses

The maximum allowable stress levels of 0.8 Fy for net sec-tion and 1.2 Fy for local bending are intended to preclude ten-don failure due to gross overload and yielding. Theseallowable stress levels were selected with the recognition thatpeak or Òhot spotÓ stresses, such as those found in the roots of

threads in threaded connectors and other locations of stressconcentration are likely to be much higher. These peak orÒhot spotÓ stresses are principal determinants of tendonfatigue life and crack growth rates. They should be deter-mined as accurately as possible due to the sensitivity offatigue life to variations in cyclic stress range predictions.

A.COMM.9.6.3 Hydrostatic Collapse

Interaction between axial and hoop stresses should be con-sidered in the collapse check. Both storm conditions andoperating conditions should be considered since safety factorsassociated with the operating condition are 33 percent higherthan those of the storm condition.

A.COMM.9.6.4 Fatigue Life

Since tendons typically consist of many components con-nected in series, fatigue design requirements should reßectthe fact that the predicted fatigue life of the whole tendon willbe less than the predicted fatigue life of any component.

Several approaches have been suggested for predicting thetendon fatigue life from individual component lives (e.g., seeWirsching, 1984). The approaches vary in the sense ofwhether both stress and resistance or only resistance is con-sidered and, if stress is included, the degree of correlationassumed among stresses of various components. Most of theapproaches assume that the predicted fatigue lives of individ-ual components are equal, although fatigue analyses revealsubstantial life variation among points along the length whenbending is signiÞcant. Furthermore, the number of compo-nents to assume is questionable, particularly when girthwelds, connectors, and even the main body of the tendon havecomparable fatigue lives. The number increases with waterdepth.

All of the approaches to tendon fatigue life make use ofreliability concepts and characteristic values of statistics.While research into appropriate statistics is ongoing, highuncertainties exist. High uncertainties are apt to remain untilthere is extensive, in-service, TLP experience.

The component fatigue life factor of ten is considered areasonable blanket requirement. The factor could be substan-tially less than ten if the inspection were proven reliable andcontinuous or very frequent, and there were an expedientrepair/replacement plan.

A.COMM.9.6.5 Inspection/Replacement Interval

Because of their criticality, the tendons warrant regularinspection. The uncertainty in detecting small cracks in sucha large volume of material suggests the small ratio of inspec-tion interval to time-to-failure. This ratio could be increasedbased on the use of a demonstrably reliable in-place inspec-tion system. The inspection interval must allow sufÞcient


time for replacement of a damaged tendon including provi-sion for weather windows, bringing out a new tendon, etc.

A.COMM.9.9.1 Load Monitoring

The load monitoring described in 9.9.1 is intended to aid inoperations and ensure that tendon system design and operat-ing limits are not exceeded. Additionally, multichannel simul-taneous recording of selected parameters can be used tocorrelate theoretical predictions of platform response and ten-don tension variations. Data logging and processing of tendontop tensions can be used to estimate tendon stress histogramsin service. Such information can, in turn, be used to verifystress-cycle assumptions employed in design fatigue analysesand crack growth predictions. For validation of analytical pre-dictions, it may also be useful to monitor stress and deßectionalong a tendonÕs length.

Selection of a particular instrument to monitor a speciÞcparameter should be based on the instrumentÕs required ser-vice life, accuracy, reliability, maintainability, and its poten-tial detrimental effects on a tendon (e.g., coating damage,galvanic cell creation, reduced strength, etc.)

Initial qualiÞcation and calibration of each instrumentationand monitoring system should be performed by the manufac-turer. Operating and maintenance manuals should also besupplied by the manufacturer. Recalibration procedures andschedules, or methods to identify the need for recalibration,should also be developed and provided to on-board operatingpersonnel. Operating personnel should be properly trained onthe use and care of the systems and should understand theiroperation and limitations.

The following paragraphs give general descriptions ofinstrumentation functions and design considerations:

a. Tendon tension instrumentationÑTendon axial loadsshould be monitored during installation to set tendon preten-sion and during operation to ensure that tendon loads aremaintained within design limitations.

Electro-hydraulic load cells, strain gauges, or other loadsensing instrumentation may be used to monitor tendon ten-sions.

Strain gauges can be located on the tension members justbelow the upper connector to measure top tension. Accessi-bility for periodic replacement should be provided.b. Position reference systemÑA platform position referencesystem can be provided to monitor horizontal offset and plat-form oscillatory motions. Example systems which can beused are an acoustical system, a taut-line system, and a radioposition reference system.

Platform heave, roll, pitch, and yaw can be measured usingan array of accelerometers and vertical axis gyroscopes.These motions are expected to be small and might not need tobe monitored for operational purposes but intermittent datacollection can be used to compare actual motions with analyt-ical predictions.

c. Center of gravity monitoringÑMonitoring of platformcenter of gravity location can be done to ensure that a rela-tively uniform distribution of tendon pretension is maintainedas weights aboard the platform vary with operations. Esti-mated weight distribution should be periodically comparedwith tendon tensions and discrepancies investigated.

This monitoring can be achieved by maintaining accuraterecords of ballast volume changes and signiÞcant platformweight and weight location changes. The latter considerationrequires a system for controlling and recording drilling der-rick location and load, drilling and auxiliary consumableinventories and other signiÞcant variable or movable weights.Estimated platform weight distributions should be comparedwith tendon tension distributions during calm conditions (nooffset) and signiÞcant discrepancies investigated. Shifts in theplatform center of gravity can then be calculated and counter-acted by re-ballasting. In this way the center of gravity can bemaintained within pre-determined safe limits. These limits orenvelopes can vary with the severity of environmental condi-tions, more tendon load distribution non-uniformity being tol-erable in mild conditions than in severe storm conditions.

d. Meteorological and oceanographic instrumentationÑAvast array of meteorological and oceanographic instrumenta-tion is commercially available. Meteorological instrumentsnormally include a barometer, anemometers, and a wet/drythermometer. Oceanographic instruments include wave staffs,wave rider buoys, and current meters.

e. Additional tendon monitoring instrumentationÑMonitor-ing lower and upper ßex joint angles and tendon stress anddeßection might be desired to verify in-service performancewith analytical predictions.

Tendon vertical angle and azimuth can be monitored usingßuid-damped pendulum potentiometers or other systemsdeveloped to monitor drilling riser angles. Such units can bestrapped to tendons adjacent to the upper and lower ßex jointsor deployed down the tendon bore on a wire-line. Data can betransmitted to the platform either by hard wire or by remotetelemetry.

Strain gauges can be installed at points of predicted highbending stress, such as adjacent to the upper and lower ßexjoints.

A.COMM.9.9.2 Tendon Retrieval and Replacement

Contingency planning for retrieval in the case of known orsuspected damage should address the feasibility of retrievalfor a variety of damage scenarios; for example, couplingdamage, lower/upper ßex joint damage, template or connec-tor damage, etc. Retrieval equipment and tendon replacementparts can be maintained onboard the platform or at an onshorelocation. When the latter option is selected, considerationshould be given to the time and procedures required to mobi-lize and install the equipment onboard accounting for


expected environmental conditions and platform motions atthe site.

Planning of retrieval operations should also consider:

a. The need to install and pretension a replacement tendoninto a spare template receptacle prior to removing the tendonto be retrieved.b. The selection of guideline or guidelineless retrieval andthe limiting platform offset, oscillation motions, and environ-mental conditions associated with the selected method.c. Crack growth predictions of time to failure for variouscrack sizes and locations and the ability to retrieve the tendonwithin the available time.d. Time required to install a replacement tendon or reinstallthe retrieved tendon.

A.COMM.10 Foundation Analysis and Design

A.COMM.10.6.1 Discussion on Safety Factors to be Applied to the Axial Capacity of Piled Foundations

a. Background.Pile design is dependent on past successful practice, that is,

empiricism. Lacking experience with TLP foundations, thedesign approach adopted utilizes the jacket type platform piledesign as the baseline for safety consideration.

Factors which could inßuence the safety of a TLP pilefoundation have been identiÞed and compared to the designinßuence each factor has for a conventional jacket pile.

Listed below are eight factors which were consideredimportant for TLP pile design. A qualitative comparison orbias is discussed relating the TLP and jacket pile application.The last three factors were not included in the body of APIRecommended Practice 2T because their inßuence on designwas deemed the same for the TLP and jacket foundations.b. Factors considered as having possible inßuence on TLPpiled foundations in comparison to compression piles injacket type structures:

1. Uncertainties in understanding soil-pile behavior undertensile loadings. Considerations include:

a. Potential reduction of near surface soilÕs effectiveness.

b. Cyclic degradation.c. Axial ßexibility of the pile-soil system.d. Effects of sustained tension.e. Suction.

¥ Item (a)ÑRelative to a jacket structure and driven pil-ing, there appears to be no reason to apply any explicitpenalty for this consideration.

¥ Items (b), (c), and (d) relate to considerations that werefelt to be difÞcult to quantify given the present state ofknowledge. It was also felt that it would not be appro-

priate to suggest testing of calculation methodologiesto help quantify these effects. Instead, these consider-ations should be explicitly mentioned as needing thor-ough investigation. Recommended safety factorsshould then be applied to the pileÕs ultimate axialcapacity after it is suitably modiÞed to account forthese items.

¥ Item (b) considers the degradation of pile capacity dueto the combination of sustained and cyclic loads. Sev-eral proprietary Þeld studies are underway to quantifyclay-pile behavior under sustained loading. Theseinclude a pile study by the Norwegian GeotechnicalInstitute at Haga and small diameter model pile seg-ment tests by the Earth Technology Corporation atShellÕs Beta pile test site, ConocoÕs Gulf of MexicoTLP pile test site, and ChevronÕs pile test site atEmpire. In addition, J. L. Briaud at Texas A&M Uni-versity is studying cyclic axial behavior under APIÕssponsorship. While none of the study results have beenpublished, a generally conservative interpretation ofsome of these data indicate pile pullout does not beginuntil the sum of the sustained load plus the cyclic com-ponent reaches about 80 percent of the static ultimateload.

¥ Item (c) relates to the development of progressive pilefailure due to the axial ßexibility of the pile-soil systemunder cyclic loading conditions. Two-way (i.e., tension-compression) load tests conducted in the above men-tioned studies show an immediate degradation in pilecapacity. Results from the full scale Beta pile test(Doyle & Pelletier, 1985) showed temporary reductionsof 61 percent to 85 percent of the pre-cyclic pile capac-ity within 16 fully reversed load cycles. However, othertesting at Haga by NGI suggests that one-way cyclicloading of the pile relative to the soil causes signiÞ-cantly less degradation than two-way loading at dis-crete pile elevations. For long ßexible piles, fullyreversed cycling may occur even though the pile top isunder a sustained bias loading. Thus, pile load capacitymay be reduced due to cyclic loading for ßexible piles.

¥ Item (d) considers the effect of sustained tension load-ing on soil behavior. Soils under sustained shear stressmay deform in time. This effect should be consideredin determining the long-term capacity of piles undersustained loading.

¥ Item (e) represents a consideration which can beexpected to aid the foundationÕs load carrying capacity.Since here again it is not possible to reliably quantifythe effects of suction, it was decided to ignore this con-sideration.

2. Lack of residual strength of the soil-pile system.A consideration included under this factor is the rela-

tive lack of residual strength for a pile loaded in tensioncompared to one loaded in compression. Schematically,


the relative residual strength after pile overload in tension(T), and compression (C) may be viewed as shown in Fig-ure 34.

The potential for dramatic decreases in strength in theevent of pile overload warrants an increased safety factorwhen compared to a compression pile.

The assignment of speciÞc numerical values to accountfor a tension pileÕs relative lack of residual strength isuncertain.

3. Load redistribution capacities of the foundation.

Redundancy in the form of clustered or grouped pilescan mitigate concern over the lack of residual strength,since all piles would not reach their maximum capacitysimultaneously. The bias factor should reward redundancy.

4. Relative difÞculty of foundation installation.

The relative ability to install satisfactory driven piles isdeemed comparable to the skirt piles of a jacket typestructure.

For drilled and grouted piles and for connectionsbetween a pile and a foundation template, it is believedthat these should be attainable in a manner comparable toa jacket platform. However, it is believed necessary toexplicitly mention that special means should be providedto verify that grouted piles and pile to template connec-tions are installed in a manner conforming with the design.(Provided explicit mention was made of the need to pro-vide means to verify the adequacy of grout and pile totemplate connection installation.)

5. Relative integrity of soil samples obtained from deepwater.

In this regard it is believed that adequate precautionsare already mentioned in the text on Òsoils investigation.ÓIt is also felt that the geotechnical consultant in conjunc-tion with the operatorÕs staff, could properly interpretengineering soil properties resulting from deep water soilsamples.6. Relative character and reliability of load determination.

The determination of loading is assumed to be compa-rable to that obtained for a jacket structure.7. Relative lack of ability to inspect and repair TLP foun-dation piles.

The ability to inspect and repair TLP piles was deemedto be very similar to that of jacket type structure.8. Consequences of a foundation failure to the integrity ofthe overall structural system.

Relative to a jacket type structure the consequence of afoundation failure was deemed to be comparable to that ofa TLP.

A.COMM.11 Riser Systems

A.COMM.11.3 RISER ANALYSIS METHODOLOGY

A.COMM.11.3.1 Structural Model

For the purposes of response analysis, the riser is a simplestructure although in some cases it may comprise multipletubulars with the potential for structural interaction. Basically,it may be represented as one or more tensioned beams whichrarely develop an angle greater than 10 degrees from the ver-tical. This qualiÞes it for analysis using the fundamental Ber-noulli-Euler beam theory. When large angles are possible, theanalysis should be modiÞed accordingly.

The beam equation for the riser is developed by Þrst exam-ining a differential element and the forces which act upon it.

Figure A-35 shows the hydrostatic pressures of sea waterand internal ßuid, the tension in the pipe wall, and the weight.The internal ßuids may be drilling mud, gas, oil, water, etc.The Þgure also shows the deformation of the riser pipe overthe elemental length. Finally, the horizontal hydrodynamicforces are indicated. Imposition of the equations of equilib-rium and simple beam theory leads to the equation of motion.The terms forming the coefÞcient y equal the tension calcu-lated by considering only the wet weight of the riser and theinternal ßuid. If the equation is rewritten with that term repre-sented by To, called the Òeffective tension,Ó it then takes theform of the classical beam-column equation. This concept hasbeen thoroughly discussed (Young et al., 1978; McIver, 1983;Lubinski, 1977; Chakrabarti, 1982). Real riser problems aresolved by converting it to a system of simultaneous equationsfor a discretized or lumped parameter representation of theriser. This allows for variations in riser properties and for theintroduction of such other non-uniformities as joints and soilrestraints.

Compression pile

Tension pile

Displacement Z

Pile

forc

e

T

Figure A-34—Residual Pile Strength


y« + y«« Æx

y«

y

fp

fy

di

sp

db

Px

sp + rp Æp g Æx

rw g (hw Ð x)

x + Æ x

Æ x

x

Undeformedriser

position

Governing Differential Equation

Where:

and:

s yúú EI y '' ''ddx------ T o T ' x+( ) y '[ ]Ð+ f y=

s r p App4--- ri di

2 Cm 1Ð( ) rw dh2+[ ]+=

Tp4--- rw g hw xÐ( ) db

2 Px di2Ð[ ] T p+=

Px Pt ri g hi xÐ( )+=

f ypg4

------ ri di2 rwÐ db

2( ) r p Ap g+ y 'Ð=

Terminology in Equations

Ap = cross sectional area of the riser pipe.

Cm = hydrodynamic mass coefÞcient.

db, di, dh = bouyancy internal, diameter of riser and hydrodynamic.

EI = ßexural rigidity.

fy = distributed hydrodynamic force acting in y direction.

g = gravitational acceleration.

hi, hw = total depth of internal ßuid, sea water.

Px = internal ßuid pressure at elevation x.

Pt = internal ßuid pressure at top of riser.

To = effective tension at bottom of riser.

Tp = actual tension in pipe wall.

T = effective tension.

x = vertical coordinate measured from bottom of riser.

y = horizontal riser translation at station x.

r1, rp, rw = density of internal ßuid, pipe, and water.

s = distributed mass.

( ¥ ) =

( )« =

ddt----- ( )

ddx------ ( )

Figure A-35—Riser Governing Differential Equation


A.COMM.11.3.2 Hydrodynamic Model

Probably the greatest uncertainty in riser response analysiscomes in formulating the hydrodynamic model (Hansen etal., 1979). This is especially true for the production riser sys-tem in an external multiple-tube conÞguration (Bennet andWilheilm, 1979). There are three different aspects to beresolved:

a. Sea surface.

b. Wave kinematics.

c. Force algorithm.

All three are primarily dependent on empirical evidence.Although extensive data have been gathered in each area(Sarpkaya and Isaacson, 1981), there is as yet no Þnal resolu-tion as to the most accurate general model. Active researchcontinues and promises further advances.

Sea SurfaceÑAlthough the surface of the ocean is a ran-dom, multi-directional process, most design analyses for off-shore structures are based on a unidirectional wave model.The principal reasons for this are: Þrst, the unidirectionalwave is much simpler to deal with and, second, it is generallybelieved to give more conservative loading, especially forfatigue calculation.

Another consideration that may warrant attention is thedistortion of the wave pattern by the platform (Connolly andWybro, 1984). 6.4 provides guidance for predicting the waveÞeld as altered by the TLP.

It is common practice to use a unidirectional, random wavemodel (Hudspeth, 1975; Sexton and Agbezuge, 1976), eitherin a linearized frequency domain method, or in the moreaccurate time domain solution. In the latter approach, a timehistory wave proÞle is synthesized based on the design wavespectrum, e.g., JONSWAP or Pierson-Moskowitz. A time his-tory solution is carried out long enough to assure that the riserresponse has statistical signiÞcance.

Wave KinematicsÑA number of models have been pro-posed for the ßuid velocity and acceleration proÞle beneaththe wave surface. Several nonlinear models have been devel-oped to satisfy boundary conditions. Each of these is rathercomplicated and for practical reasons, is generally used onlywith the single, periodic wave model. Recently gathered envi-ronmental data indicate that the simple, Airy linear function isquite adequate for modeling a random surface condition,especially with the Òstretched kinematicsÓ feature. The effectof the platform on wave kinematics may need consideration,since the forces acting on the riser are dependent on waveparticle kinematics.

Hydrodynamic Force AlgorithmÑAlso the subject of exten-sive study based on empirical results the widely used Morisonequation, described in 6.2.3, has survived intact. There is,however, extensive debate as to the selection of the drag andmass coefÞcients, especially in high wave conditions.

While available experimental and theoretical hydrody-namic data provide guidelines for the riser design, Þnaldesign veriÞcation may require design-speciÞc testing (Roweet al., 1978). This is most important for complex riser geome-tries involving multiple tube arrays.

The following are examples of the complicating factorsthat inßuence the hydrodynamic loading on risers:

a. Relative motion between riser and ßuid.

b. Drag variation due to surface roughness, marine growth,and variation in Reynolds Number and Keulegan-CarpenterNumber.

c. Flow disturbance due to nearby bodies.

d. Wake encounter in oscillating ßow.

e. Fluctuating in-line and transverse forces due to vortexshedding.

f. Drag ampliÞcation due to vortex-induced vibrations.

A.COMM.11.3.3 Lumped Parameter Model

The partial differential equation which governs riserbehavior is not directly usable for analyzing general problems(Morgan and Peret, 1976). It is therefore, usually converted toa system of discrete coordinates.

The two methods presently used are Þnite-element (Gard-ner and Kotch, 1976) and Þnite difference (Tucker, 1972)both involve; division of the riser into a series of Þnite lengthregions. The behavior of the riser can then be described interms of the nodes at which these elements are joined. Thesolution involves Þnding the translations and rotations, bend-ing moments, etc., at each node of the riser. While each ofthese idealized elements has uniform properties, the non-uni-formities of the riser are accounted for by the variation ofproperties from element to element. The signiÞcant conse-quence of this discretization of the riser is that it leads to aseries of simultaneous equations which are conveniently andrapidly solved on a digital computer.

A critical consideration is the number of elements intowhich the riser is divided. The nodes should be closelyspaced in the areas where high bending moments tend tooccur. These are always in the wave zone and near the bottomof the riser where the tension is the lowest. An importantsource of riser excitation is the platform motions. Theyshould be modeled accurately with emphasis on the phaserelationship between platform motion and the wave.

The upper and lower boundary conditions should reßectactual support conditions including tensioner forces (Kozikand Noerager, 1976), ßex-joint stiffness, etc.

Multiline risers may be modeled with a single beam repre-sentation having equivalent properties. Depending on theriser design, it may be necessary to use a multiple beammodel in order to determine detailed structural loads onmulti-tube risers in which spacer frames are included.


A.COMM.11.3.4 Solutions of the Simultaneous Equations

The discussion in the previous section dealt with the math-ematical techniques for converting the spatial derivatives intodiscrete translation coordinates for solution as simultaneousequations. This section deals with solution of the time deriva-tive portion of the equations.

It is the wave action and associated platform motion thatprovide the dynamic excitation. The waves impose time-vary-ing hydrodynamic forces on the riser. The platform drives theriser back and forth, producing additional contributions to thetime-varying forces. The applied riser tension may have time-varying components due to the non-ideal characteristics ofthe tensioner system.

Time Domain or Frequency DomainÑThere are two dif-ferent approaches for solving the riser dynamic equations, theapplication of time or frequency domain methods, as appliedto the platform response problem. The direct integration ortime domain solution permits the inclusion of all non-lineari-ties such as the non-linear hydrodynamic force, varying freesurface, and non-linear ßex joint behavior (Hachemi Safai,1983).

The alternative is the frequency domain method in whichnon-linear functions are linearized about a quasi-steady (Kro-likowski and Gay, 1980; McIver and Lunn, 1983) or meanvalue. An iterative procedure is used whereby the equivalentlinear drag is varied in successive solutions until it gives aminimum error solution. This method has been shown toyield reliable results for drilling riser analyses at substantiallylower cost than the time domain solution.

The frequency domain approach leads to a linear transferfunction for the riser which is a mathematical expression ofits dynamic characteristics. This transfer function (sometimescalled the complex frequency response) can be used directlywith the power spectra (wave energy vs. frequency) deÞni-tion of seastate. The result is riser response amplitude,including bending stresses, for example vs. frequency. This isprecisely the kind of information which is often used forfatigue analysis.

Modal AnalysisÑThe riserÕs natural frequencies andmodes of vibration may be determined from the equation ofmotion (McIver and Lunn, 1983). This information supple-ments the dynamic analysis and may be used to evaluate theriserÕs susceptibility to vortex-induced vibrations.

Regardless of the means of solution, riser response analysisshould result in an accurate description of mean and alternat-ing stresses, deßections, and angles over the length of theriser.

Reference is made to API Bulletin 2J, Comparison ofMarine Drilling Riser Analysis.

A.COMM.12 Facilities Design

A.COMM.12.2.1 Structural

The type of structure adopted for the deck is of paramountimportance to the facilities designer. The exclusive use ofplate girder bulkheads in the deck may offer certain advan-tages to the structural designer but can create signiÞcant prob-lems in the routing of services. This type of structural designwill impose constraints on the size, number and location ofpenetrations through the stressed plate members to avoidlocal overstressing particularly with respect to fatigue. Thismay preclude the optimized routing of services and accessroutes and will place demands on the facilities designer toagree to Þnal penetration requirements at an early stage in thedesign. For this reason the facility designer should recom-mend the use of truss girders or a combination of truss andplate girders for the main deck members.

The proposed method of deck construction can create thenecessity for temporary supports for services. For example,with a palletized deck construction, individual pallets cannotreceive the beneÞts of overhead supports from the palletabove until both pallets are installed into the main deck struc-ture. Additional temporary service supports for each discretepallet increases the scope of design and fabrication work andcan cause access difÞculties during fabrication.

To maintain the center of gravity as low as possible, limita-tions may be placed on the depth of the deck structure. Thelatter can have a signiÞcant impact on facilities design, andearly coordination between structural and facilities designersis necessary. InsufÞcient deck depth may cause problems inthe following areas:

a. There may be inadequate height for the installation ofgravity systems within the deck space (vent and drain sys-tems, separation trains, etc.).b. To achieve adequate vertical separation, some componentsof a deck system may have to be installed in the hull. Thismay introduce a hydrocarbon hazard into areas of the hullwhich would otherwise be free from hydrocarbons.c. The use of a mezzanine ßoor in the deck may be restricted.Some equipment may have to be installed on the exposedweather deck (e.g., HVAC plant/ducting and turbine wasteheat recovery systems) which may increase service runsthereby adding to weight and wind loads and raising the cen-ter of gravity.

Deck height above sea level can impose limitations on thelocation of ventilation intakes (which are commonly placedbelow deck) due to the ingestion of aerosol salt spray oreven physical contact with the crests of waves under stormconditions.

The desire of the structural designer to reduce overallweight will result in the use of minimal PSF deck loadingcriteria.


Maintenance routes for major equipment must be estab-lished at an early stage to assure adequate strength for theseareas.

A.COMM.12.2.3 Weight and Center of Gravity

Until equipment arrangements are established, the effect ofequipment weight and location on locating the center of gravity(CG) at the platform desired coordinations should be consid-ered a facilities design function. Thereafter, further CG adjust-ments probably will need to be made by ballast adjustments oraccommodated through allowances in the vessel/mooringdesign. These ballast adjustments caused by misplacement ofequipment could result in substantial weight penalty.

The amount of rework resulting from relocating majorequipment can be extensive once piping, electrical and instru-ment detailed design efforts get underway. Once a shift ismade, there is no way to know that subsequent informationwill not necessitate rearranging the equipment.

A.COMM.12.2.10 Regulations

The TLP has been deÞned as a Òßoating OCS facilityÓ in33 Code of Federal Regulations, 140 Subchapter NÑÓOuterContinental Shelf Activities.Ó The agency assigned to providethe regulations (CFR) is the Coast Guard. 33 Code of FederalRegulations, 143.120 deÞnes the applicable regulations for aßoating OCS facility and should be consulted at initiation ofproject.

The regulations establish certain requirements with respectto safety equipment and promotion of safety of life and prop-erty at sea. However, where unique TLP design or equipmentrequirements make regulation compliance impractical, alter-nate proposals that provide an equivalent level of safety maybe acceptable to the USCG. The designer is advised tobecome familiar with Òequivalent requirementsÓ of theagency involved.

A.COMM.12.4.2 Packaging

Construction methods should provide capability for anintegrated deck/facilities approach providing opportunitiesfor considerable weight savings.

A common approach to the design of production facilitiesfor Þxed offshore platforms has been that of prepackagingequipment items on individual structural skids. Subsequently,these skids are set on the platform deck structure and pipedtogether. This approach might result in faster fabricationtimes by allowing concurrent facilities equipment packagingand platform structural fabrication. It might not, however, beeconomically attractive for TLP facilities due to the weightaddition resulting from duplication of structural support. Asan alternative, the facilities designer might consider an inte-grated deck/facilities approach wherein all equipment sup-ports and drain pans are fabricated onto pallets which then

become an integral part of the platform deck structure. Eachpallet should be Þtted-out with services (piping, cable, etc.) asfar as is practical. In particular, fabrication tolerances must bemaintained since services run between structures and pallets,misalignment could cause remedial work when bulkheadpenetrations are involved.

A.COMM.12.4.6 Riser Connection

The relative motion of the deck to the launcher receiverwill be predominantly in the vertical direction. Verticallaunchers may present advantages over horizontal. Theweight of a vertical launcher may be supported by the uppersupport of the riser since this will probably be a device main-taining riser tension. In liquid service a vertical launcherrequires additional height for drainage.

Horizontal launchers would require an additional supportunder the launcher since the pipe would not support the canti-levered weight of the launcher. If a ßexible connection wasplaced downstream of the horizontal launcher, it could beattached to the deck and loaded in the conventional way. Ifthe ßexible connection is upstream of the launcher, thelauncher support may need to maintain support while allow-ing the launcher to ßoat with the riser.

A.COMM.12.5.5 Electrical

The designer should review all applicable electricalrequirements of the U.S. Coast Guard. The Electrical Engi-neering Regulations (46 Code of Federal Regulations, Sub-chapter J) establish requirements with respect to safeelectrical installations and repair aboard vessels and mobileoffshore drilling units.

Due to the similarities between the TLP facility and amobile offshore drilling unit compliance with Subchapter Jwill probably be found to be applicable for hull design. 46Code of Federal Regulations, 111.107 should be noted forspeciÞc requirements applicable to industrial systems. Theelectrical installation for production systems is not speciÞ-cally covered in Coast Guard regulations. Appropriate indus-try standards should be considered.

A.COMM.14 Structural Materials

A.COMM.14.9 ELASTOMERIC MATERIALS

The material selection guidelines for the elastomeric struc-ture of TLP mooring ßexjoints must consider the following:

a. Elastomers are both elastic and viscous materials.b. Various generic elastomers are satisfactory but mechanicalproperty requirements are dependent on the laminated struc-ture design. Different design approaches for the same appli-cation may dictate different mechanical characteristics of theelastomer(s) selected. More than one elastomer compoundmay be utilized in a single composite molding.


In addition to the above two technical differences over nor-mal structural materials, evaluation of elastomer(s) for theintended application is made more difÞcult because: (1) labo-ratory data is of limited use to the design engineer, especiallyin highly loaded, laminated structures, and (2) although manyrubber compositions exist, for the most part these are propri-etary to the manufacturer and limited data is available forcomparative or other technical purposes.

Since a laminated structure is utilized, the ÒmaterialÓ andother properties of the composite along with compatibility,bondability, manufacturability of this structure must also beconsidered.

Elastic Nature of RubberÑThe word ÒelastomerÓ clearlynotes the one unique property of rubber-like materialsÑtheyare highly elasticÑi.e., they can be repeatedly grosslydeformed in tension, shear, or compression at low stressesand return to their original condition. These are the propertiesthat permit bearings such as used in the TLP to ßex to largeangles, without imposing high bending loads on the mooringsystem. However, these same highly desirable ßexing proper-ties also present major limitations when used in structuraldesign. As a result, rubber is seldom used in tension loading.All tension leg ßexjoints and similar bearings use a laminatedstructure design approach such that the mooring systemÕs ten-sion and rotational loads result in compressive and shearstresses in the rubber. Through the use of high strength lami-natesÑe.g., steel, the low compressive (YoungÕs) modulus ofelastomers can be signiÞcantly increased. With a sufÞcientnumber of laminates, i.e., increasing the bonded areas to thenonbonded, edge areaÑthe compressive modulus of thenearly incompressible elastomer approaches its bulk-modu-lus, which is thousands of times greater than its YoungÕs mod-ulus. The shear modulus, however, is not affected. It is theunique combination of these characteristics, the high effectivecompression modulus and low shear modulus, that permitsthe ßexible elastomeric bearing to work.

The extremely high compressive loads imposed on thelaminated bearings must be taken up by the elastomer and thereinforcements (often called ÒshimsÓ). Although the mathe-matics of the bearings are fairly complex, the compressiveloads principally result in elastomer shear stresses. The rota-tional motion of the spherical bearing segments also results inshear stresses. These shear stresses must be containedthrough bonding, by the reinforcements. It follows then thatthe higher the tension leg induced compression loads thehigher the hoop tensile stresses in the reinforcements. It alsofollows that a low shear modulus rubber (which results in lowbending stresses on the tendon) creates high tensile stresses inreinforcements from tendon tension (rubber compression).

The laminate system designer then must match the overalldesign approach with characteristics of both the elastomersand the reinforcement materials. The choice of materials aTLP ßexjoint designer has to work with often becomesextremely narrow considering the high performance, rigorous

environments, and long fatigue life requirements imposedalong with state-of-the-art manufacturing capabilities of boththe rubber and metal forming industries.

Viscoelastic ConsiderationsÑSince elastomers are vis-coelastic materials, the dynamic response and mechanicalbehavior are dependent upon their viscous components. Thestress or strain history, strain amplitude, strain frequency,rates of loading, and elastomer temperature affect the behav-ior of the material. This viscous component also determinesthe internal energy loss, or hysteresis, which is converted intoheat. Although elastomers are poor heat conductors, even thehigh hysteresis rubber compounds do not have heat build-upproblems in TLP bearings because of the low motion energyspectrum inputs of these systems.

It is important that the mooring system designer as well asthe ßexjoint designer appreciate the signiÞcance of the vis-cous element of the elastomer. Some examples of viscoelasticbehavior that must be considered in the TLP system are:

a. Inßuence of temperature on rotational spring constant(increased bending stress with lower temperatures).b. Non-linearity of shear modulus (higher spring constants atlow and high angular deßection).c. High degrees of stress relaxation (reduction in rotationalspring constant with time).d. Rate dependence (higher rotational spring constant athigher angle rate changes).

Certain peculiarities of carbon black Þlled elastomersshould also be notedÑe.g., the ÒMullinsÓ effect in which thestress/strain relationship is affected by the immediately preced-ing stress/strain history. Also certain crystallizing elastomerssuch as natural rubber and Neoprene compounds may have aunique low temperature aging effect on mechanical properties.

Chemical ChangesÑElastomers can be greatly inßuencedby externally or internally produced chemical changes. It isassumed that the elastomers selected for the TLP ßexjointwill have been proven capable of long-term survivability inthe surrounding environments (ßuids, temperatures), underhighly loaded, dynamic conditions. It is also assumed that theelastomer has been properly compounded and that the adhe-sive and reinforcement system are such that no internal chem-ical actions take place that are deleterious to the operation andreliability of the ßexjoint.

Elastomeric Bearing DesignÑThere are various propri-etary and non-proprietary design methods used by the elasto-meric bearing industry to determine the structural, fatigue,and performance adequacy of the bearings. These variousmethods can be broken down into three categories:

a. The Òshape factorÓ approach.b. Closed form mathematical solution approach.c. Finite element approach where the analysis method iscapable of handling elastomeric materials with PoissonÕsratio very nearly 0.5.


The Òshape factorÓ approach has been the engineeringstandard for simple elastomeric bearings such as bridge padsbut is limited to applications in which the strains are verysmall, the conÞguration is very simple and the Òshape factorÓremains low. It is not considered useable in the design of ten-sion leg platform bearings.

The closed form analysis methods are based on linear the-ory of elasticity. The derivation of the equations normallyused by the industry are based on the works of Timoshenko1

and Sokolhikoff2. Because of the complex nature of the ßex-joints, i.e., many rubber laminates and reinforcements all of adifferent radius and all of a different area, it is normally nec-essary to go to computerized solutions. Several such comput-erized solutions are used by the industry, but for the most partthese are proprietary. Although expensive, computerizedÞnite element analysis techniques are seeing more and moreuse in highly loaded elastomeric bearings, particularly inthose applications where high rubber and reinforcementstresses are indicated.

DiscussionÑEither the closed form or the Þnite elementmethods are acceptable design analysis techniques providingthe designer can support the speciÞc analysis method(s) withactual product use and test data. This is necessary because:(a) it has not been possible to directly measure the actual rub-ber or elastomer shear stresses; (b) little correlation existsbetween methods; (c) differences in elastomers from manu-facturer to manufacturer, and (d) poor correlation betweenlaboratory tests and actual product performance. Thus, corre-lation of both the structural and fatigue analysis with actualproduct use and product test data is mandatory. SufÞcientdata must be presented to demonstrate that the various analyt-ically derived stress levels have performed satisfactorily inother designs which used both the same analysis techniqueand the speciÞc elastomer selected. Stresses in the reinforce-ments must be determined by analysis and correlated withdata available from actual tests of earlier designs (equivalentloadings and/or strain gauge data).

The designer must demonstrate that catastrophic form offailure in the way of column buckling instability will notoccur. This demonstration can be through analysis and labo-ratory experimental efforts (e.g., sub-scale tests) or in-serviceuse of data of similar conÞgurations and materials. In thoseareas where insufÞcient experience may be available, say atextremely high angles and high loads, for a particular designapproach, subscale data may be developed to demonstratedesign adequacy.

The service or fatigue life must be determined from thestresses as determined from the previously discussed analysesand the load and angular motion requirements speciÞed bythe TLP operator. Material property data used in the fatigueanalysis for both the reinforcements and the elastomer must

be based on actual use data in the particular environmentsinvolved.

The terms Òstructural failureÓ and Òfatigue failureÓ must bedeÞned in the TLP userÕs speciÞcation. For instance, therequired fatigue life may be speciÞed by a ÒMinerÕsÓ number.The number should be consistent with the remainder of themooring system, but the deÞnition of ÒfailureÓ in the elas-tomer must consider the unique characteristics of the lami-nated rubber systems, i.e., it is Òfail-safeÓ or highly damagetolerant in most modes.

Elastomer SelectionÑThe material selection, design, man-ufacture, and test of the bearing normally is the prime respon-sibility of the ßexjoint contractor. This document is notintended to place any limitations on the selection of a speciÞcmaterial or class of materials other than noting that the partic-ular material(s) selected must have sufÞcient successful usehistory to demonstrate its adequacy for the intended purpose.The material selection process will be a function of the detailsof the bearing design used, compatibility with the design andoperational environment, manufacturability, and generalexperience. This applies to both the elastomer and the rein-forcement materials. The selection of the manufacturingmethods for both the reinforcements and the elastomer mold-ing/bonding process and metal surface preparation and adhe-sive system is nearly inseparable from the materials selectedby a given manufacturer. Here again, demonstrated successfulexperience is the key to methods selection.

The speciÞc mechanical characteristics of the materials willbe dictated by the design approach. Because of ever-increasingknowledge and advancements in the state-of-the-art of elas-tomers, no speciÞc restrictions should be placed on the choiceof a particular elastomer. Obviously, one would not want toutilize an elastomer compound that was brittle, incompatiblewith the surrounding ßuids, or would not meet the age life orother critical requirements. However, there is a wide range ofelastomeric compounds when used in compatible designs thatwould be suitable for most TLP applications. There are com-petitive designs and material combinations some of which willbe superior to others. There are combinations which will pro-vide improvements in one area at the expense of the other.Here it is important that the user provide the designers withadequate speciÞcation deÞnition to ensure that the best design/material choice combination is selected. Selection of the rein-forcement material may be restricted by strength requirementsand manufacturing as well as heat treating limitations. How-ever, there are few restrictions in this area; low carbon, lowalloy, or stainless steels may be used providing they have theability to meet the structural, fatigue, and processing require-ments. Aluminum materials for shim stock are not permitted,but other materials such as titanium may be utilized. In allcases, demonstrated bondability of the elastomer must havebeen proven through long-term service or test.

123

APPENDIX B—REFERENCES

References for Section 51. Bretschneider, C. L. (1959), Wave Variability and WaveSpectra for Wind Generated Waves, Beach Erosion Board, T.M. No. 118.2. Chappelear, J. E., ÒDirect Numerical Calculation ofWave Properties,Ó Journal of Geophysical Research, Vol. 66,1961.3. Dean, R. G., Evaluation and Development of Water WaveTheories for Engineering Application, Vol. I & II, U. S. ArmyCorps of Engineers, Coastal Research Center, 1974.4. Hasselmann, K., Barnett, T. P., Bouws, E., Carlson, H.,Cartwright, D. E., Enke, K., Ewing, J. A., Gienapp, H.,Kruseman, P., Meerburg, A., Muller, P., Olbers, D. J., Richter,K., Sell, W., and Walden, H., ÒMeasurements of Wind-WaveGrowth and Swell Decay During the Joint North Sea WaveProject,Ó Deutche HydrograÞca Zeitung, Supplement A, 8,No. 12, 1973.5. Kareem, ÒDynamic Effects of Wind on Tension Leg Plat-forms,Ó OTC Paper No. 4229, 1982.6. Lambrakos, K. F., and Brannon, H. R., Wave Force Cal-culations for Stokes and Non-Stokes Waves, OTC PreprintsNo. 2039, 1974.7. Pierson, W. J., Jr., and Moskowitz, L. (1964), ÒA Pro-posed Spectral Form for Fully Developed Wind Seas Basedon The Similarity Theory of S. A. Kitaigorodskii,Ó Journal ofGeophysical Research, Vol. 69.8. Saunders, Hydrodynamics in Ship Design, Society ofNaval Architects & Marine Engineers, 1965.9. Simiu, E. and Scanlan, F. H., Wind Effects on Structures,John Wiley & Sons, New York, 1978.10. Skjelbreia, Lars, and Hendrickson, James, ÒFifth OrderGravity Wave Theory,Ó Proceedings of Seventh Conferenceon Coastal Engineering, Vol. 1, Chap. 10, 1961.11. Wiegel, Robert L., Oceanographical Engineering, Pren-tice-Hall, 1964.12. Planning, Designing, and Constructing Fixed OffshoreStructures in Ice Environments, API Bulletin 2N.13. Planning, Designing and Constructing Fixed OffshorePlatforms, API Recommended Practice 2A, 16th Edition.14. Rules for Building and Classing Mobile Offshore Drill-ing Units, American Bureau of Shipping, 1980.15. Rules for the Construction and ClassiÞcation of MobileOffshore Units, Det norske Veritas.16. Rules for The Design, Construction, and Inspection ofOffshore Structures, Det norske Veritas, 1977.

References for Section 61. Blevins, R. D., Flow-Induced Vibration, Van NostrandReinhold Co., New York, 1977.2. Brebbia, C. A. and Walker, S., ÒBoundary Element Tech-niques in Engineering, Newnes-Butterworths London, 1980.

3. Chakrabarti, S. K., ÒMoored Floating Structures andHydrodynamic CoefÞcients,Ó Ocean Structural DynamicsSymposium, Corvallis, Ore., Sept. 1984.4. Daily, J. W. and Harleman, D. R. F., Fluid Dynamics,Addison-Wesley Publishing Co., Reading, Massachusetts,1966.5. DeBoom, W. C., Pinkster, J. A., and Tan, S. G., ÒMotionand Tether Force Prediction for a Deepwater Tension LegPlatform,Ó OTC Paper 4437, OTC, Houston, 1983.6. Det norske Veritas (DnV), Rules for the Design, Con-struction, and Inspection of Offshore Structures, Appendix B,Loads, 1977.7. Eatock-Taylor, R. and Rajagopalan, A., ÒSuperharmonicResonance Effects in Drag Dominated Structures,Ó Integrityof Offshore Structures, Faulker, Cowling and Frieze, editor,Glasgow 1981, p. 85, Applied Science Publishers, London.8. Faltinsen, O. M. and A. E. Loken, ÒSlow drift oscilla-tions of a ship in irregular waves,Ó Appl. Ocean Res., Vol. 1,1979.9. Faltinsen, O. M., and Michelsen, F., ÒMotions of LargeStructures in Waves and Zero Froude Number,Ó Proc. Gov-ernment Symposium Dynamics of Marine Vehicles and Struc-tures in Waves, London, 1974.10. Frank, W., ÒOscillation of Cylinders in or Below the FreeSurface of Deep Fluids,Ó NSRDC Report #2375, October,1967.11. GrifÞn, O., ÒOTEC Cold Water Pipe Design for Prob-lems Caused by Vortex-Induced Oscillations,Ó Ocean Engi-neering, 1981, Vol. 8, No. 2, pp. 129-209.12. Hoerner, S. F., Fluid Dynamic Drag, published by theauthor, 1965.13. Ippen, A. T., Editor, Estuary and Coastline Hydrodynam-ics, McGraw-Hill, Inc., New York, 1966.14. Kareem, A., ÒDynamic Effects of Wind on OffshoreStructures,Ó Proceedings of the Offshore Technology Confer-ence, Houston, Texas, May 1980.15. MacCamy, R. C. and Fuchs, R. A., Wave Forces on Piles:A Diffraction Theory, TM 69, Corps of Engineers, Beach Ero-sion Board, 1954.16. Macha and Reid, ÒSemisubmersible Wind Loads andWind Effects,Ó Paper presented at the Annual Meeting of theSociety of Naval Architects and Marine Engineers, New York,N.Y., November 7-10, 1984.17. Mei, C. C., The Applied Dynamics of Ocean SurfaceWaves, John Wiley and Sons, New York, 1983.18. Meyers, Holm, and McAllister, Handbook of Ocean andUnderwater Engineering, McGraw-Hill, 1969.19. Molin, B., ÒComputation of Drift Forces,Ó Paper No.3627, OTC, Houston, 1979.20. Morison, J. R., et al., ÒThe Forces Exerted by SurfaceWaves on Piles,Ó Journal of Petroleum Technology, 1950.


21. Newman, J. H., ÒSecond-order, slowly-varying forces onvessels in irregular waves,Ó Proc. Int. Symp. on the Dynamicsof Marine Vehicles and Structures in Waves, University Col-lege, London, 1974.22. Newman, J. N., Marine Hydrodynamics, MIT Press,Cambridge, Massachusetts, 1977.23. Ogilvie, T. F., ÒFirst and Second Order Forces on a Cyl-inder Submerged under a Free Surface,Ó Journal of FluidMechanics, Vol. 16, Part 3, pp. 451-472, 1963.24. Pearcey, H. H., ÒSome Observations on FundamentalFeatures of Wave-Induced Viscous Flows Past Cylinders,ÓProceedings, IAHR Symposium on Mechanics of Wave-Induced Forces on Cylinders, ed. by T. L. Shaw, September1978.25. Pinkster, J. A., ÒMean and Low Frequency Wave DriftForces on Floating Structures,Ó Ocean Energy, Vol. 6, pp.593-615, 1979.26. Pinkster, J. A., ÒLow Frequency Second Order WaveExciting Forces on Floating Structures,Ó NSMB, Wageningen,1980, Publ. No. 650.27. Sarpkaya, T. and Isaacson, M., Mechanics of WaveForces on Offshore Structures, Van Nostrand Rienhold Co.,New York, 1981.28. Simiu, E. and Scanlon, R. H., Wind Effects on Structures,John Wiley and Sons, New York, 1978.29. Simiu, E. and Leigh, S. D., ÒTurbulent Wind Effects onTension Leg Platform Surge,Ó NBS Building Science Series151, National Bureau of Standards, U.S. Government Print-ing OfÞce, Washington, March 1983.30. Standing, R. G. and N. M. C. Dacunha, ÒSlowly-Varyingand Mean Second-Order Wave Forces on Ships and OffshoreStructures,Ó Proceedings, 14th ONR Symposium on NavalHydrodynamics, Ann Arbor, Michigan, 1982.31. Verley, R. L. P. and Moe, G., ÒHydrodynamic Dampingof Offshore Structures in Waves and Currents,Ó Proceedingsof the Offshore Technology Conference, No. 3798, Houston,May 1980.32. Wickers, J. E. W. and Huijsmans, R. M. H., ÒOn the LowFrequency Hydrodynamic Damping Forces Acting on Off-shore Moored Vessels,Ó OTC Paper 4813, OTC, Houston,1984.33. Young, R. W. and Bai, K. J., Numerical Solution to FreeSurface Flow Problems, 10th Symposium ONR, 1974.34. Yue, D. K., H. S. Chen, and C. C. Mei, ÒA Hybrid Ele-ment Method for Diffraction of Water Waves by Three-Dimensional Bodies,Ó International Journal for NumericalMethods in Engineering, Vol. 12, pp. 245-266, 1978.35. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.36. Chakrabarti, S. K. and Cotter, D. C., ÒFirst and SecondOrder Interaction of Waves with Large Offshore Structures,Ó2nd International Symposium on Offshore Mechanics andArctic Engineering. ASME, Houston, Feb. 1983.

References for Section 71. Bearman, P. W., ÒWind Loads on Structures in TurbulentFlow,Ó The Modern Design of Wind-Sensitive Structures,Construction Industry Research and Information Association,London, U.K., 1971, pp. 42-48.2. Bedrosian, E. and Rice, S. O. (1971), ÒThe Output Prop-erties of Volterra Systems (Nonlinear Systems with Memory)Driven by Harmonic and Gaussian Inputs,Ó Proc. of the IEEE,Vol. 59, No. 12, December.3. Bendat, J. S., and Piersol, A. G. (1971), Random Data:Analysis and Measurement Procedures, Wiley-Interscience,New York.4. Beynet, P. A., Berman, M. Y., and von Aschwege, J. T.,ÒMotion, Fatigue, and the Reliability Characteristics of a Ver-tically Moored Platform,Ó 1978 OTC 3304.5. Botelho, D. L. R., Finnigan, T. D. and Petrauskas, C.(1984), ÒModel Test Evaluation of a Frequency Domain Pro-cedure for Extreme Surge Response Prediction of TensionLeg Platforms,Ó Offshore Technology Conference, OTC4658.6. Burns, G. E., ÒCalculating Viscous Drift of a TensionLeg Platform,Ó Proc. Second International Offshore Mechan-ics and Arctic Engineering Symposium, ASME, New York,1983.7. Caughey, T. K. (1963), ÒEquivalent Linearization Tech-niques,Ó The Journal of the Acoustical Society of America,Vol. 1, No. 35, November 11, 1963.8. Crandall, S. J., and Mark, W. D. (1963), Random Vibra-tion in Mechanical Systems, Academic Press, New York.9. Crandall, S. H., Khabbaz, G. R. and Manning, J. E.(1964), ÒRandom Vibration of an Oscillator with Non-linearDamping,Ó The J. of the Acoustical Society of America, Vol.36, No. 7, July.10. Cummins, W. E., ÒThe Impulse Response Function andShip Motions,Ó D.T.M.B. Report 1661, Washington, D.C.,1962.11. Doob, J. S. (1953), Stochastic Processes, John Wiley andSons, New York.12. Faltinsen, O. I., Van Hoof, R. W., Fylling, I. J., andTeigen, P. S., ÒTheoretical and Experimental Investigations ofTension Leg Platform Behavior,Ó BOSS Proceedings, ThirdInternational Conference, Massachusetts Institute of Technol-ogy, 1982.13. Faltinsen, O. M., ÒStructures at Zero Froude Number,ÓProceedings of International Symposium on the Dynamics ofMarine Vehicles and Structures in Waves, University College,London, 1974.14. Faltinsen, O. M., and Loken, A. E., ÒSlow Drift Oscilla-tions of a Ship in Irregular Seas,Ó Applied Ocean Research,Vol. 1, No. 1, 1979.15. Gelb, A., and Vander Velde, W. E., Multiple-InputDescribing Functions and Non-linear System Design,McGraw-Hill, N.Y., 1968.


16. Gumbel, E. J., ÒStatistical Theory of Extreme Values andSome Practical Applications,Ó National Bureau of StandardsApplied Mathematics Series 33, February 1951.17. Iwan, W. D. (1973), ÒThe Generalization of the Conceptof Equivalent Linearizations,Ó Int. J. of Non-Linear Mechan-ics, Vol. 8.18. Jefferys, E. R., and Patel, M. H. (1982), ÒDynamic Anal-ysis Models of Tension Leg Platforms,Ó Journal of EnergyResources Technology, Vol. 104, September, 1982.19. Kan, D. K., and Petrauskas, C., ÒHybrid Time FrequencyDomain Fatigue Analysis for Deepwater Platforms,Ó OTC3965, 1981.20. Kareem, A., ÒDynamic Effect of Wind on OffshoreStructures,Ó Offshore Technology Conference, No. OTC 3764,May 1980.21. Leverette, S. J., Bradley, M. S. and Bliault, A., ÒAn Inte-grated Approach to Setting Environmental Design Criteria forFloating Production Facilities,Ó Behavior of Offshore Struc-tures (BOSS) Conference, Cambridge, Mass., August 1982.22. MacCamy, R. C., and Fuchs, R. A., ÒWave Forces onPiles: A Diffraction Theory,Ó U.S. Army Corps of Engineers,Beach Erosion Board, Technical Memo No. 69, Washington,1954.23. Moses, F., ÒUtilizing a Reliability-Based API Recom-mended Practice 2A FormatÓ API PRAC project 82-22, FinalReport November, 1983.24. Newman, J. N. (1977), Marine Hydrodynamics, TheMIT Press, Cambridge, Mass.25. Newman, J. N., ÒSecond Order Slowly Varying Forceson Vessels in Irregular Waves,Ó International Symposium onthe Dynamics of Marine Vehicles and Structures in Waves,University College of London, April 1974.26. Ochi, M. K., ÒOn Prediction of Extreme Values,Ó Journalof Ship Research, March 1973.27. Ogilvie, T. F., and Tuck, E. O., ÒA Rational Strip Theoryof Ship Motions,Ó Part I, University of Michigan, Departmentof Naval Architecture and Marine Engineering, Report No.013, March 1969.28. Papoulis, A. (1965), Probability, Random Variables andStochastic Processes, McGraw Hill, New York.29. Paulling, J. R., ÒTime Domain Simulation of Semi-Sub-mersible Platform Motion with Application to the TensionLeg Platform,Ó SNAME Spring Meeting, May 25-27, 1977.30. Paulling, J. R., and Horton, E. E., ÒAnalysis of the Ten-sion Leg Platform,Ó Society of Petroleum Engineers Journal,Sept., 1971, pp. 285-294.31. Pinkster, J. A., ÒLow Frequency Second Order WaveExciting Forces on Floating Structures,Ó Dissertation, DelftUniversity, October 1980.32. Price, W. G., and Bishop, R. E. D., Probabilistic Theoryof Ship Dynamics, John Wiley and Sons, N.Y., 1974.33. Salvesen, N., Tuck, E. O. and Faltinsen, O., ÒShipMotions and Sea Loads,Ó Transactions SNAME, Vol. 78,1970, pp. 250-287.

34. Salvesen, N., vonKerczek, C. H., Yue, D. K. and Stern, J.(1982), ÒComputation of Nonlinear Surge Motions of Ten-sion Leg Platforms,Ó 14th Offshore Technology Conference,No. OTC 4394.35. Simiu, E., and Leigh, S. D., ÒTurbulent Wind Effects onTension Leg Platform Surge,Ó NBS Building Science Series151, National Bureau of Standards, U.S. Government Print-ing OfÞce, Washington, March 1983.36. Spanos, P-T. D., and Iwan, W. D. (1978), ÒOn the Exist-ence and Uniqueness of Solutions Generated by EquivalentLinearization,Ó Int. J. of Non-Linear Mechanics, Vol. 13.37. Thomson, W. T. (1965), Vibration Theory and Applica-tions, Prentice-Hall, Englewood Cliffs, New Jersey.38. Tickell, R. G. (1977), ÒContinuous Random Wave Load-ing on Structural Members,Ó The Structural Engineer, Vol.55, No. 5.39. Tucker, M. J., Challenor, P. G., and Carter, P. S. T.,ÒNumerical Simulation of A Random Sea: A Common Errorand Its Effect on Wave Group Statistics,Ó Applied OceanResearch, Vol. 6, No. 2, 1984.40. Van Oortmerssen, G., ÒThe Motions of a Moored Ship inWaves,Ó N.S.M.B. Publication No. 510, 1976.41. Weibull, Waloddi, ÒA Statistical Distribution Function ofWide Applicability,Ó Journal of Applied Mechanics, Septem-ber 1951.42. Wirsching, P. H., ÒProbability Based Fatigue Design Cri-teria for Offshore Structures,Ó API Ñ PRAC Project #81-15,January 1983.43. Yosida, K., Ozaki, M., and Oka, N. (1984), ÒStructuralResponse Analysis of Tension Leg Platforms,Ó Journal ofEnergy Resources Technology, Vol. 106, March.44. Yue, D. K., Chen, H. S., and Mei, C. C., ÒThree Dimen-sional Calculations of Wave Forces by a Hybrid ElementMethod,Ó Proceedings of 11th Symposium of Naval Hydrody-namics, OfÞce of Naval Research, 1976.45. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.

References for Section 81. AISC, SpeciÞcation for the Design, Fabrication andErection of Structural Steel for Buildings, American Instituteof Steel Construction, Latest edition.2. Recommended Practice for Planning, Designing, andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, Latest edition.3. Bulletin on Stability Design of Cylindrical Shells, APIBulletin 2U, Latest edition.4. Bulletin on Design of Flat Plate Structures, API Bulletin2V, Latest edition.5. NACE Standard RP-01-76, 1983 Revision, ÒCorrosionControl of Steel, Fixed Offshore Platforms Associated with


Petroleum Production,Ó National Association of CorrosionEngineers, Houston, Texas, 1983.6. SSC 266, ÒReview of Ship Structural Details,Ó ShipStructure Committee, 1977.7. SSC 272, ÒIn-Service Performance of StructuralDetails,Ó Ship Structure Committee, 1978.8. SSC 294, ÒFurther Survey of In-Service Performance ofStructural Details,Ó Ship Structure Committee, 1980.9. Kinra, R. K., and Marshall, P. W.; ÒFatigue Analysis ofthe Cognac Platform;Ó Proceedings of the 11th OffshoreTechnology Conference; 1979; Paper No. OTC 3378, pp. 169-187.10. Maddox, N. R.; ÒFatigue Analysis for Deepwater Fixed-Bottom Platforms;Ó Proceedings of the 6th Offshore Technol-ogy Conference; 1974; Paper No. OTC 2051, pp. 191-203.11. Maddox, N. R.; ÒA Deterministic Fatigue Analysis forOffshore Platforms;Ó Journal of Petroleum Technology; July,1975; pp. 901-912.12. Maddox, N. R., and Wildenstein, A. W.; ÒA SpectralFatigue Analysis for Offshore Structures;Ó Proceedings of theOffshore Technology Conference; 1975; Paper No. OTC2261, pp. 185-198.13. Marshall, P. W.; ÒDynamic and Fatigue Analysis UsingDirectional Spectra;Ó Proceedings of the 8th Offshore Tech-nology Conference; 1976; Paper No. OTC 2537, pp. 143-157.14. Marshall, P. W., and Luyties, W. H.; ÒAllowable Stressesfor Fatigue Design;Ó Proceedings of the Third InternationalBOSS Conference; Boston; 1982.15. Vugts, J. H., and Kinra, R. K.; ÒProbabilistic FatigueAnalysis of Fixed Offshore Structures;Ó Proceedings of the8th Offshore Technology Conference; 1976; Paper No. OTC2608, pp. 889-906.

References for Section 91. Brekke, J. N., and Gardner, T. N., ÒAnalysis of Brief Ten-sion Loss in TLP Tethers,Ó Presented at Offshore Mechanicsand Arctic Engineering Conference, Tokyo, Japan, April,1986.2. Buchalet, C. C., and Bamford, W. H., ÒStress IntensityFactor Solutions for Continuous Surface Flaws in ReactorPressure Vessels,Ó Mechanics of Crack Growth, ASTM STP590, 1976, pp. 385-402.3. Burnside, O. H., Hudak, S. J., Oelkers, E., Chan, K., andDexter, R. J., ÒLong Term Corrosion Fatigue of WeldedMarine Steels,Ó Southwest Research Institute, March, 1984.4. de Boom, W. C., Pinkster, J. A., and Tan, S. G., ÒMotionand Tether Force Prediction for a Deepwater Tension LegPlatform,Ó Paper 4487, Offshore Technology Conference,1983.5. Denise, J. P. F., and Heaf, N. J., ÒA Comparison BetweenLinear and Non-Linear Response of a Proposed Tension LegProduction Platform,Ó Proceedings, Offshore TechnologyConference, 1979.

6. GrifÞn, O. M., ÒOTEC Cold Water Pipe Design for Prob-lems Caused by Vortex-Excited Oscillations,Ó Ocean Engi-neering, Vol. 8, 1981, pp. 119-209.7. Halkyard, J. E., and Liu, Shin-Lin, ÒCoupled vs. Uncou-pled Analysis for the Determination of TLP Tendon Loads,ÓProceedings, Oceans Õ83, Marine Technology Society AnnualMeeting, San Francisco, California, 1983.8. Halkyard, J. E., ÒDamage Tolerance and Inspection Cri-teria for Tension Leg Platform Tendons,Ó Energy-SourcesTechnology Conference & Exhibition, ASME, New Orleans,February, 1986.9. Hammouda, M. M., Smith, R. A. and Miller, K. J., ÒElas-tic-Plastic Fracture Mechanics for Initiation and Propagationof Notch Fatigue Cracks,Ó Fatigue of Engineering Materialsand Structures, Vol. 2, pp. 139-154, 1979.10. Isaacson, M., and Maull, D. J., ÒTransverse Forces onVertical Cylinders in Waves,Ó Journal of Waterways, Harborsand Coastal Engineering, ASCE, Vol. 102, No. ww1, Febru-ary 1976, pp. 49-60.11. King, R., ÒA Review of Vortex Shedding Research andIts Application,Ó Ocean Engineering, Vol. 4, 1977, pp. 141-171.12. Larsen, C. M., White, C. N., Fylling, I. J., and Nordsve,N. T., ÒNon-linear Static and Dynamic Behavior of TensionLeg Platform Tethers,Ó Proceedings of the Third InternationalOffshore Mechanics and Arctic Engineering Symposium, Vol.I, pp. 41-50, 1984.13. Salama, M. M., and Tetlow, J. H., ÒSelection and Evalua-tion of High Strength Steel for Hutton TLP Tension Leg Ele-ments,Ó OTC 4449, 1983.14. Sarpkaya, T., ÒVortex-Induced Oscillations, A SelectiveReview,Ó Trans. ASME, J. Applied Mechanics, Vol. 46, 1979,pp. 241-258.15. Shah, R. C., ÒStress Intensity Factors for Through andPart Through Cracks Originating at Fastener Holes,Ó Mechan-ics of Crack Growth, ASTM STP 590, ASTM 1976, pp. 429-459.16. Stahl, B., and Geyer, J. F., ÒFatigue Reliability of ParallelMember Systems,Ó Journal of Structural Engineering, Vol.110, No. 10, October 1984.17. Stahl, B., and Geyer, J. F., ÒUltimate Strength Reliabilityof Tension Leg Platform Systems,Ó 17th Annual OTC, 1984.18. Wirsching, P. H., ÒA SimpliÞed Approach to Establishinga Target Damage Level for Fatigue in a Tendon Component,ÓTechnical Report No. 11 to American Bureau of Shipping,February 1984.19. Wirsching, P. H., and Chen, Y. N., ÒFatigue Design Crite-ria for TLP Tendons,Ó Submitted for publication in Journal ofStructural Engineering, ASCE, August, 1986.20. ÒCriteria of the ASME Boiler and Pressure Vessel Codefor Design by Analysis,Ó Sections VIII, Division 2.21. Det Norske Veritas (DnV), Rules for the Design, Con-struction and Inspection of Offshore Structures, 1977(Reprint 1981), Appendix C, ÒSteel Structures.Ó


22. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.23. Design, Rating, and Testing of Marine Riser Couplings,API Recommended Practice 2R, May 1984.

References for Section 101. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.2. Doyle, E. H. and Pelletier, J. H. (1985), ÒBehavior of aLarge Scale Pile in Silty Clay,Ó XI International Conferenceon Soil Mechanics and Foundation Engineering, San Fran-cisco, CA.

References for Section 111. Agbezuge, L. K., and Noerager, J. A., ÒDeepwater RiserFlotation Systems,Ó An article in Ocean Resources Engineer-ing, February 1978.2. Albers, J., and DeSilva, M. L., ÒInnovative Risers, SpeedDrilling, and High Current Areas off Brazil,Ó Oil & Gas Jour-nal, April 18, 1977.3. Bednar, J. M., Dickson, W. P., and Dumay, W. H., ÒEffectof Blow-Out Preventor End Connections on the PressureIntegrity of a Subsea B.O.P. Stack Under Riser Loads,Ó OTCPaper 2649, May 1976.4. Bennett, B. E., and Wilhelm, G. P., ÒAnalysis of Produc-tion Riser Systems,Ó OTC Paper 3536, May 1979.5. Chakrabarti, S. K., and Frampton, R. W., ÒReview ofRiser Analysis Techniques,Ó Applied Ocean Res., Vol. 4, #2,1982.6. Childers, M. A., and Martin, E. B., ÒField Operations ofDrilling Marine Risers,Ó J. Petroleum Tech., March 1980.7. Connolly, J. T., and Wybro, P. G., ÒRiser Analysis Meth-ods Comparison with Measured Field Data,Ó OTC Paper4735, May 1984.8. Evans, J. L., Ganser, D. A., and Williams, S. J., ÒDevel-opment of an Instrumented Marine Riser Joint,Ó Paper pre-sented at the Energy Technology Conference and Exhibition,ASME, January 18-22, 1981.9. Fowler, J. R., and Gardner, T. N., ÒCriterion for Allow-able Lower Ball-Joint Angle in Floating Drilling,Ó Trans,ASME, J. of Energy Resources Tech., Vol. 102, December1980.10. Gardner, T. N., and Kotch, M. A., ÒDynamic Analysis ofRisers and Caissons by the Finite Element Method,Ó OTCPaper 2651, 1976.11. Goldsmith, R. G., ÒTLP Well Design,Ó Paper presented atthe 55th Fall Technical Conference and Exhibition of SPE,September 21-24, 1980.12. Hachemi Safai, V., ÒNon-Linear Dynamic Analysis ofDeepwater Risers,Ó App. Ocean Res., Vol. 5, #4, 1983.13. Hudspeth, R. T., ÒWave Force Predictions from Nonlin-ear Random Sea Simulations,Ó OTC Paper 2193, 1975.

14. Jones, M. R., ÒProblems Affecting the Design of DrillingRisers,Ó SPE Paper 5268, SPE London, April 1975.15. Kozik, T. J., and Noerager, J., ÒRiser Tensioner ForceVariations,Ó OTC Paper 2648, May 1976.16. Krolikowski, L. P., and Gay, T. A., ÒAn Improved Linear-ization Technique for Frequency Domain Riser Analysis,ÓOTC Paper 3777, 1980.17. Loken, A. E., Torset, O. P., Mathiassen, S., and Arnesen,T., ÒAspects of Hydrodynamic Loading and Design of Pro-duction Risers,Ó OTC Paper 3538, May 1979.18. Lubinski, A., ÒNecessary Tension in Marine Risers,ÓRevue de lÕInstitute Francais du Petrole, March 1977.19. McIver, D. B., ÒValidation of Marine Riser ComputerPrograms,Ó ASME Energy Technology Conference and Exhi-bition, 1983.20. McIver, D. B., and Lunn, T. S., ÒImprovements to Fre-quency Domain Riser Programs,Ó OTC Paper 4559, May1983.21. Morgan, G. W., and Peret, J. W., ÒApplied Mechanics ofMarine Risers,Ó Petroleum Engineering Publishing Company,Dallas, Texas, 1976.22. Nikkel, K. G., Cowan, R., and Labbe, J. R., ÒClearanceProblems of Individual Well Risers and Tension Leg Plat-forms,Ó ASME Energy Technology Conference and Exhibi-tion, February 1982.23. Ottesen Hansen, N-E, Jacobson, V., and Lundgren, H.ÒHydrodynamic Forces on Composite Risers and IndividualCylinders,Ó OTC Paper 3541, May 1979.24. Rowe, S. J., Fletcher, R. H., and Headley, C., ÒTheModel Testing of a Tethered Buoyant Platform and Its RiserSystem,Ó SUT Symposium on Models and Their Use asDesign Aids in Offshore Operation, May 1978.25. Sarpkaya, T., and Isaacson, M. de St. Q., ÒMechanics ofWave Forces on Offshore Structures,Ó Van Nostrand, NewYork, 1981.26. Sexton, R. M., and Agbezuge, L. K., ÒRandom Wave andVessel Motion Effects on Drilling Riser Dynamics,Ó OTCPaper 2650, May 1976.27. Shanks, F. E., ÒDrilling High Current Locations RequiresSpecial Preparation,Ó World, Oil, August 1983.28. Tucker, T. C., ÒA Mathematical Model for the Nondeter-ministic Analysis of a Marine Riser,Ó Ph.D. Dissertation, Uni-versity of Illinois, 1972.29. Watkins, L. W., ÒOperating Experience with MarineRiser Buoyancy,Ó ASME Paper 78-PET-56.30. Woodyard, A. H., ÒRisk Analysis of Well CompletionSystems,Ó Paper presented at the 55th Fall Technical Confer-ence and Exhibition of SPE, September 21-24, 1980.31. Wybro, P. G., and Davies, K. B., ÒThe Dorada Field Pro-duction Risers,Ó OTC Paper 4042, May 1981.32. Young, R. D., Fowler, J. R., Fisher, E. A., and Luke, R.R., ÒDynamic Analysis an Aid to the Design of Marine Ris-ers,Ó Trans. ASME, J. of Pressure Vessel Tech., Vol. 100, May1978.


33. Recommended Practice for Care and Use of MarineDrilling Risers, API Recommended Practice 2K, January1982.34. Recommended Practice for Design and Operation ofMarine Drilling Riser Systems, API Recommended Practice2Q, April 15, 1984.

Supplementary References for Section 1135. Azar, J. J., and Soltveit, R. E., ÒA Comprehensive Studyof Marine Drilling Risers,Ó ASME Paper No. 78-PET-61.36. Bennett, B. E., and Metcalf, M. F., ÒNonlinear DynamicAnalysis of Coupled Axial and Lateral Motions of MarineRisers,Ó OTC Paper 2776, May 1977.37. Bernitsas, M., and Kokkinis, T., ÒBuckling of RisersUnder Tension Due to Internal Pressure: NonmoveableBoundaries,Ó Trans. ASME, J. of Energy Resources Tech.,Vol. 105, September 1983.38. Botke, J. C., ÒAn Analysis of the Dynamics of MarineRisers,Ó Delco Electronics Report N. R75-67A, September1975.39. Brouwers, J. J. G., ÒResponse and Near Resonance ofNon-Linearly Elastic Systems Subject to Random Excitationwith Applications to Marine Risers,Ó Ocean Engineering, Vol.9, #3, 1982.40. Burke, B. G., and Tighe, J. T., ÒA Time Series Model forDynamic Behavior of Offshore Structures,Ó OTC Paper 1403,1971.41. Burke, B. O., ÒAnalysis of Marine Risers for Deepwa-ter,Ó J. Pet Tech., April 1974.42. Caldwell, J. B., and Gammage, W. E., ÒA Method forAnalysis of a Prototype Articulated Multi-Line Marine Pro-duction Riser System,Ó ASME Petroleum Mechanical Engi-neering Conference, Paper No. 76-PET-46, Mexico City,Mexico, September 1976.43. Castela, A., et al., ÒDeepwater Drilling,Ó Ocean ResourceEngineering, Part IÑApril 1978, Part IIÑJune 1978, andPart IIIÑAugust 1978.44. Chung, J. S., ÒMotion Analysis of a Riser Joint,Ó ASMEPaper No. 77-PET-40.45. Dareing, D. W., ÒNatural Frequencies of Marine DrillingRisers,Ó SPE Paper 5620, Dallas, Texas, September 1975.46. ÒDynamic Stress Analysis of the Mohole Riser System,ÓNESCO (National Engineering Science Company) Report183-2A, (NSF Report PB175258) January 1965.47. Egeland, O., and Solly, L. P., ÒSome Approaches to theComparison of Riser Analysis Methods Against Full-ScaleData,Ó OTC Paper 3778, May 1980.48. Etok, E. U., and Kirk, C. L., ÒRandom DynamicResponse for the Tethered Buoyant Platform ProductionRiser,Ó Applied Ocean Res., Vol. 3, #2, 1981.49. Fischer, W., and Ludwig, M., ÒDesign of Floating VesselDrilling Risers,Ó J. Pet. Tech., March 1966.

50. Gardner, T. N., ÒWhatÕs New in Drilling Riser Technol-ogy,Ó Ocean Industry, June 1978.51. Gnone, E., Signorelli, P., and Guiliano, V., ÒThree-Dimensional Static and Dynamic Analysis of Deepwater SeaLines and Risers,Ó OTC Paper 2326, 1975.52. Gosse, C. G., ÒThe Marine RiserÑA Procedure forAnalysis,Ó OTC Paper 1080, May 1969.53. Graham, R. D., Frost, M. A., and Wilhoit, J. C., ÒAnaly-sis of the Motion of Deepwater Drill Strings,Ó Parts 1 and 3, J.of Engineering for Industry, May 1965.54. Harris, L. M., and Ilfrey, W. T., ÒDrilling 1300' ofWaterÑSanta Barbara Channel, California,Ó OTC Paper1018, May 1969.55. Hartnup, G. G., Patel, M. H., and Sarohia, S., ÒHydrody-namic Tests on Marine Risers,Ó OTC Paper 4319, May 1982.56. Huang, T., and Dareing, D. W., ÒMarine Riser VibrationResponse Determined by Modal Analysis,Ó Petroleum Engi-neering International, May 1980.57. Huang, T., Dareing, D. W., and Beran, W. T., ÒBendingof Tubular Bundles Attached to Marine Risers,Ó Trans.ASME. J. of Energy Resources Tech., Vol. 102, March 1980.58. Kirk, C. L., Etok, E. U., and Cooper, M. T., ÒDynamicand Static Analysis of a Marine Riser,Ó App. Ocean Res., Vol.1, #3, 1979.59. Kozik, T. J., ÒAn Analysis of a Riser Joint TensioningSystem,Ó OTC Paper 2329, May 1975.60. Kwan, C. T., Marion, T. L., and Gardner, T. N., ÒStormDisconnector of Deepwater Drilling Risers,Ó published byASME in Deepwater Mooring and Drilling, OED-Vol. 7,1979.61. Maison, J. R., and Lea, J. F., ÒSensitivity Analysis ofParameters Affecting Riser Performance,Ó OTC Paper 2918,1977.62. Morgan, G. W., ÒDynamic Analysis of Deepwater Risersin Three Dimensions,Ó ASME Paper No. 75-PET-20.63. National Engineering and Science Company, ÒStructuralDynamic Analysis of the Riser and Drill String for ProjectMoholeÓ, NESCO Report 5234, Parts 1 and 2, January 1966.64. Natvig, B. J., ÒA Large Angle-Large Displacement Anal-ysis Method for Marine Risers,Ó Paper presented at theInteMaritec, September, 1980.65. Nordgren, R. P., ÒOn Computation and Motion of ElasticRods,Ó J. Applied Mech., Vol. 41, #31, September, 1974.66. Ortloff, J. E., and Teers, M. L., ÒThe Development of aMulti-Line Universal Joint for Subsea Applications,Ó OTCPaper 2328, May 1975.67. Rajabi, F., Zedan, M., and Mangiavacchi, A., ÒVortexShedding Induced Dynamic Responses of Marine Risers,ÓTrans. ASME Vol. 106, June 1984.68. Spanos, P., and Chen, T. W., ÒVibrations of Marine RiserSystems,Ó Trans. ASME Journal of Energy Resources Tech-nology, Vol. 102, December 1980.


69. Sparks, C. P., ÒMechanical Behavior of Marine RisersMode of Inßuencing Principal Parameters,Ó Trans. ASME, J.Energy Resources Tech., Vol. 102, December 1980.70. Sparks, C. P., ÒThe Inßuence of Tension, Pressure andWeight on Pipe and Riser Deformations and Stresses,Ó Trans.ASME, J. of Energy Resources Tech., Vol. 106, March 1984.71. Sparks, C. P., Cabilic, G. P., and Schawann, J. C., ÒLon-gitudinal Resonant Behavior of Very Deepwater Risers,ÓTrans. ASME, J. Energy Resources Tech., Vol. 105, Septem-ber 1983.72. St. Denis, M., and Armijo, L., ÒOn the Dynamic Analysisof the Mohole Riser,Ó Proceedings Ocean Science, OceanEngineering Conference, Marine Technological SocietyASIO, 2:1240, 1965.73. Tidwell, D. R., and Ilfrey, W. T., ÒDevelopments inMarine Drilling Riser Technology,Ó ASME Paper No. 69-PET-14.

74. Wardlaw, H. W. R., ÒFundamentals of Marine RiserDesign,Ó Prepared for presentation at Joint PetroleumMechanical Engineering and Pressure Vessels and PipingConference, ASME Paper No. 76-PET-35, Mexico City,Mexico, September 1976.75. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.

References for Section 131. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.

References for Section 141. Recommended Practice for Planning, Designing andConstructing Fixed Offshore Platforms, API RecommendedPractice 2A, 16th Edition.

PC-01200—8/97—8C ( )

Additional copies available from API Publications and Distribution:(202) 682-8375

Information about API Publications, Programs and Services is available on the World Wide Web at: http://www.api.org

Order No.: G02T02

Documents

API Standard 2T