Ocean Engineering & Oceanography

Volume 4

Series editors

Manhar R. Dhanak, Florida Atlantic University SeaTech, Dania Beach, USANikolas I. Xiros, New Orleans, USA

More information about this series at http://www.springer.com/series/10524

Zafarullah Nizamani

1 3

Environmental Load Factors and System Strength Evaluation of Offshore Jacket Platforms

Zafarullah NizamaniDepartment of Environmental Engineering Jalan University KamparMalaysia

Springer Cham Heidelberg New York Dordrecht London© Springer International Publishing Switzerland 2015This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made.

Printed on acid-free paper

Springer International Publishing AG Switzerland is part of Springer Science+Business Media (www.springer.com)

ISSN 2194-6396 ISSN 2194-640X (electronic)Ocean Engineering & OceanographyISBN 978-3-319-15050-5 ISBN 978-3-319-15051-2 (eBook)DOI 10.1007/978-3-319-15051-2

Library of Congress Control Number: 2014959854

For my family Mother, Father, Jamila and Shahzad

vii

Preface

API (WSD) and ISO 19902 (LRFD) codes are being used nowadays for design of Jacket platforms all over the world. ISO code is a probabilistic code which takes into account the uncertainties of material and loads and thus enables economised designs. This advantage is not available for API code. The sustainable develop-ment of physical structures depend not only on reliability of structures but also on cost saving. ISO load factors are calibrated using Gulf of Mexico and North Sea environmental data. In this book, three offshore regions of Malaysia have been taken separately. The probabilistic uncertainty models for resistance and loads for local conditions are determined. Resistance uncertainty is evaluated using data collected from fabrication yard in Malaysia. Geometrical and mate-rial variations are statistically analysed from this data using probability distribu-tions. Uncertainty model for nine component stresses and eleven joint stresses is analysed using MATLAB and statistical distributions. Environmental load uncer-tainty model included wave, wind and current parameters. The platform-specific and regional data is used for the analysis. The extreme distributions, i.e. Weibull and Gumbel are fitted for the analysis and their parameters are evaluated. SACS software is used to find the component stresses. Morrison Equation is used for application of wave load and BOMEL and Heideman’s Equations are used to find the response from the stresses. 100-year loads are used to find the reliabil-ity. Seven code Equations are used to find the component reliability. The member selection for reliability analysis is based on diameter, thickness and slenderness ratios. The component reliability is found through FORM method of reliability using MATLAB code. For the target reliability API WSD code is used. Thus the environmental load factor which gives higher reliability than the target is selected. Codes define three types of Joints, K, T/Y and X in Jacket platforms. The environ-mental load factor is proposed using local geographic conditions. Though codes use component and joint-based environmental load factors only it is found nec-essary to include and check the system-based approach for the load factor also. ISO requires that to assess the strength of structure for extension of life, change in load or resistance of Jacket, 10,000-year load should be applied and Jacket strength evaluated. API and ISO code require that they should be checked against

Prefaceviii

probability of failure of 10−4. In this text the probability of failure is determined and it is updated by applying the Bayesian updating technique.

This research is made at Universiti Teknologi PETRONAS (UTP) through its graduate assistantship scheme. Therefore my deep appreciation goes to the univer-sity without which it would not have been possible for me to do this research. I am thankful to Mr. Mohd Sapihie Ayob, Principal Engineer (Structural Mechanics) of PETRONAS Group Technical Solution, Technology & Engineering Division (PGTS), Malaysia for his support for a project related to this work. Many faculty staff and col-leagues at Universiti Teknologi PETRONAS shared their knowledge and experience to complete this task. Without their help and sharing knowledge this work would not be as is shown in this final shape, such as Dr. Paul Frieze for useful discussions at initial stage of research. I am thankful for guidance on environmental load parameters from Dr. R.V. Ahilan and on structural reliability and FORM from Dr. Kaisheng Chen from Noble Denton. I am thankful to Mr. Bishwa Mohan Jha and Mr. Che Wahab from KENCANA HL Sdn Bhd who provided help to obtain onsite statistics of tubular members. I am also thankful to Dr. Hamid M.F.A., Scientige Sdn Bhd (on leave from Universiti Teknologi Malaysia). I am also thankful to my supervisors Dr. Narayanan S.P., V.J. Kurian and M.S. Liew for their guidance. I am especially thankful to my col-leagues at UTP Mr. Cossa N.J. I would like to thank my other colleagues who were encouraging and supportive during this period of trial. I am also thankful for an open source code named as FERUM available through University of California, Berkeley.

November 2014 Zafarullah Nizamani

ix

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Design Codes of Practice for Jacket Platforms . . . . . . . . . . . . . . 11.2 Geographic Region of Offshore Malaysia . . . . . . . . . . . . . . . . . . 31.3 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1 Uncertainty of Resistance . . . . . . . . . . . . . . . . . . . . . . . 41.4 Uncertainty of Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.5 Structural Safety and Reliability . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.5.1 Environmental Load Factor for Component and Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.5.2 System Reliability and Environmental Load Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.6 Bayesian Updating of Probability of Failure . . . . . . . . . . . . . . . . 81.7 Outline of the Chapters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Past Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.1 Design Codes of Practice for Jacket Platforms . . . . . . . . . . . . . . 13

2.1.1 API RP2A-WSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.1.2 API RP2A-LRFD/ISO 19902 . . . . . . . . . . . . . . . . . . . . 152.1.3 Benefits of Limit State Design Code . . . . . . . . . . . . . . 152.1.4 Safety Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Geographic Region of Offshore Malaysia . . . . . . . . . . . . . . . . . . 172.2.1 History of Offshore Oil Production . . . . . . . . . . . . . . . 172.2.2 Jacket Platform Design in Malaysia . . . . . . . . . . . . . . . 18

2.3 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.1 Uncertainty of Loads and Resistance . . . . . . . . . . . . . . 192.3.2 Basic Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.3.3 Sources of Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.4 Parameters of Uncertainty . . . . . . . . . . . . . . . . . . . . . . 212.3.5 Types of Resistance Uncertainty . . . . . . . . . . . . . . . . . 22

Contents

Contentsx

2.4 Resistance Uncertainty-Background Study . . . . . . . . . . . . . . . . . 242.4.1 Material Uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4.2 Characteristic Resistance . . . . . . . . . . . . . . . . . . . . . . . 262.4.3 Geometric Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . 262.4.4 Resistance Model Uncertainty . . . . . . . . . . . . . . . . . . . 262.4.5 Critical Review of Resistance Uncertainty . . . . . . . . . . 27

2.5 Load Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.5.1 Load Uncertainty Parameters . . . . . . . . . . . . . . . . . . . . 282.5.2 Statistical Data Uncertainty for

Environmental Load . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.5.3 Critical Analysis of Load Uncertainty . . . . . . . . . . . . . 33

2.6 Environmental Load Modelling of Jacket Response . . . . . . . . . . 332.6.1 Environmental Load Uncertainty Model . . . . . . . . . . . 332.6.2 Dead Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.6.3 Live Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.7 Structural Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.7.1 Reliability Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.7.2 Parameters of Structural Reliability . . . . . . . . . . . . . . . 352.7.3 Review of Structural Reliability Methods . . . . . . . . . . 39

2.8 Component Reliability and Previous Work . . . . . . . . . . . . . . . . . 412.8.1 Component Reliability Index-critical Review . . . . . . . 42

2.9 Resistance Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422.10 Joint Reliability and Previous Work . . . . . . . . . . . . . . . . . . . . . . . 42

2.10.1 Joint Reliability Index-critical Review . . . . . . . . . . . . . 432.11 Reliability and Environmental Load Factor . . . . . . . . . . . . . . . . . 43

2.11.1 Code Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.12 Nonlinear Collapse Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.13 System Reliability and Reserve Strength Ratio (RSR) . . . . . . . . 45

2.13.1 Previous Work on System Reliability and Load Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.13.2 System-based Environmental Load Factor-Critical Review . . . . . . . . . . . . . . . . . . . . 46

2.14 Assessment of Jacket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.14.1 Bayesian Updating and Probability of Failure . . . . . . . 472.14.2 Damaged Structural Members . . . . . . . . . . . . . . . . . . . 482.14.3 Critical Review of Updating of Probability

of Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482.15 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Contents xi

3 Research Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.2 Resistance Uncertainty for Jacket Platforms in Malaysia . . . . . . 56

3.2.1 Collection of Data for Resistance Parameters . . . . . . . 563.2.2 Statistical Analysis of Geometric

and Material Variables . . . . . . . . . . . . . . . . . . . . . . . . . 583.2.3 Component and Joint Stress Model Uncertainty . . . . . 59

3.3 Load Uncertainty for Offshore Jacket Platforms in Malaysia . . . 623.3.1 ISO and Metocean Criteria . . . . . . . . . . . . . . . . . . . . . . 623.3.2 Environmental Load Uncertainty Parameters. . . . . . . . 633.3.3 Geographical Data for Environmental Load

Parameters for Offshore Malaysia . . . . . . . . . . . . . . . . 663.3.4 Statistical Analysis of Environmental Load

Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.3.5 Weibull Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.3.6 Gumbel Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.3.7 Environmental Load for SACS . . . . . . . . . . . . . . . . . . . 71

3.4 Structural Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.4.1 Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733.4.2 Monte Carlo Simulations for Determination

of Probability of Failure . . . . . . . . . . . . . . . . . . . . . . . . 753.4.3 Selection of Jacket Platforms for Reliability

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.4.4 SACS Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773.4.5 Load Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.4.6 Soil Conditions Effect on Component and Joint . . . . . 80

3.5 Component Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813.5.1 Single Stresses Case Study: Axial Tension . . . . . . . . . 82

3.6 Joint Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843.6.1 Target Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

3.7 Environmental Load Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.8 Resistance Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.9 System Reliability-Based Environmental Loads . . . . . . . . . . . . . 85

3.9.1 SACS Collapse Module . . . . . . . . . . . . . . . . . . . . . . . . 863.9.2 Collapse Analysis of Jacket . . . . . . . . . . . . . . . . . . . . . 873.9.3 SACS Load Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883.9.4 SACS Jacket Model for Pushover Analysis . . . . . . . . . 883.9.5 Wave and Current Loads in Malaysia. . . . . . . . . . . . . . 893.9.6 Curve Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.9.7 Safety Factor for Jacket System: API WSD

and ISO 19902 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.9.8 Limit State Function for System

Environmental Loading . . . . . . . . . . . . . . . . . . . . . . . . 923.9.9 Target System Probability of Failure . . . . . . . . . . . . . . 92

Contentsxii

3.10 System-Based Environmental Load Factor . . . . . . . . . . . . . . . . . 933.11 Assessment of Jacket Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

3.11.1 Uncertainty Model for Resistance and Load . . . . . . . . 943.11.2 Bayesian Updating of Probability

of Failure-Intact Structure . . . . . . . . . . . . . . . . . . . . . . 943.11.3 Bayesian Updating of Probability

of Failure-Damaged Structure . . . . . . . . . . . . . . . . . . . 963.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

4 Uncertainty Modelling of Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.2 Resistance Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.3 Statistical Properties of Fundamental Variable for Resistance . . . 102

4.3.1 Geometric Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 1034.3.2 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4.4 Probabilistic Model Stresses Used in ISO Code 19902 . . . . . . . . 1084.4.1 Component Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.4.2 Joint Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

5 Uncertainty Modelling of Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255.2 Load Factor and Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255.3 Load Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1265.4 Wave and Current Directionality for Offshore Malaysia . . . . . . . 126

5.4.1 South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1275.4.2 Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285.4.3 Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.5 Wave Load Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1325.5.1 PMO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1325.5.2 SBO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345.5.3 SKO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365.5.4 Gulf of Mexico (GOM) and North Sea (NS) . . . . . . . . 137

5.6 Wind Load Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395.6.1 PMO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395.6.2 SBO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415.6.3 SKO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1435.6.4 Gulf of Mexico (GOM) and North Sea (NS) . . . . . . . . 143

5.7 Current Load Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.7.1 PMO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.7.2 SBO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.7.3 SKO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.7.4 Gulf of Mexico (GOM) and North Sea (NS) . . . . . . . . 150

Contents xiii

5.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

6 Tubular Strength Comparison of Offshore Jacket Structures Under API RP 2A and ISO 19902 . . . . . . . . . . . . . . . . . . . . 1556.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1556.2 Design Codes for Jackets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1566.3 Numerical Analysis Background . . . . . . . . . . . . . . . . . . . . . . . . . 1596.4 Comparison of Tubular Strength Equations

in Different Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1596.4.1 Axial Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1606.4.2 Axial Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1616.4.3 Bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1666.4.4 Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1676.4.5 Hydrostatic Pressure (Hoop Buckling). . . . . . . . . . . . . 1676.4.6 Combined Stresses Without Hydrostatic Pressure . . . . 1696.4.7 Combined Stresses with Hydrostatic Pressure . . . . . . . 170

6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

7 Component Reliability and Environmental Load Factor . . . . . . . . . . 1757.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1757.2 Selection of Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1757.3 Component Target Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . 1777.4 Component Reliability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 178

7.4.1 Code Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1787.4.2 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 1827.4.3 Effect of Variation of Environmental Load Factor . . . . 1837.4.4 Effect of Column Slenderness Ratio . . . . . . . . . . . . . . 1837.4.5 Calibration Points for Jackets . . . . . . . . . . . . . . . . . . . . 1867.4.6 Selection of Environmental Load Factor . . . . . . . . . . . 1867.4.7 PMO Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877.4.8 SBO Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1897.4.9 SKO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

7.5 All Regions and All Components Combined Result . . . . . . . . . . 1947.6 Resistance Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

7.6.1 Axial Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1957.6.2 Axial Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

7.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

8 Joint Reliability Analysis and Environmental Load Factor . . . . . . . . 1998.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1998.2 Selection of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Contentsxiv

8.2.1 K-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1998.2.2 T/Y-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2028.2.3 X-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

8.3 Beta Factor (β) Effects (d/D) on Reliability Index . . . . . . . . . . . . 2068.3.1 K-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2068.3.2 T/Y-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2068.3.3 X-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

8.4 Gamma Factor (γ) Effects (D/2T) . . . . . . . . . . . . . . . . . . . . . . . . 2098.4.1 K-Joints: Tension/Compression . . . . . . . . . . . . . . . . . . 2108.4.2 T/Y-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2118.4.3 X-Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

8.5 Variation of Environmental Load Factor . . . . . . . . . . . . . . . . . . . 2148.6 Calibration of API (WSD) and ISO (LRFD)

Reliability Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2158.7 Environmental Load Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

8.7.1 PMO Region Platform . . . . . . . . . . . . . . . . . . . . . . . . . 2168.7.2 SBO Region Platform . . . . . . . . . . . . . . . . . . . . . . . . . . 2178.7.3 SKO Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

8.8 All Regions and All Joints Combined Result . . . . . . . . . . . . . . . . 2228.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

9 System Reliability-Based Environmental Loading . . . . . . . . . . . . . . . 2259.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2259.2 System Strength Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

9.2.1 Wave and Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2269.2.2 Curve Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2269.2.3 Selection of RSR for Jackets in Malaysia . . . . . . . . . . 230

9.3 System Environmental Load Factor . . . . . . . . . . . . . . . . . . . . . . . 2329.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239

10 Extension of Life of Jacket Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . 24110.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24110.2 Collapse Analysis of Jacket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

10.2.1 Wave Effect on Collapse Load . . . . . . . . . . . . . . . . . . . 24210.2.2 Directional Base Shear . . . . . . . . . . . . . . . . . . . . . . . . . 24310.2.3 Wave Directional Effects on Collapse Base Shear . . . . 24310.2.4 System Redundancy . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

10.3 Updating the Probability of Failure . . . . . . . . . . . . . . . . . . . . . . . 24810.3.1 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 24810.3.2 Bayesian Updating the Probability of Failure . . . . . . . 25210.3.3 Bayesian Updating Probability of Failure

with Damaged Members . . . . . . . . . . . . . . . . . . . . . . . 25710.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

Contents xv

11 Conclusions and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . 26311.1 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

11.1.1 (a) Resistance Uncertainty . . . . . . . . . . . . . . . . . . . . . . 26311.1.2 (b) Environmental Load Uncertainty . . . . . . . . . . . . . . 264

11.2 Load Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26411.2.1 Component Reliability and Environmental

Load Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26411.2.2 Joint Reliability and Joint-based Environmental

Load Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26411.2.3 System-based Environmental Load Factor . . . . . . . . . . 265

11.3 Bayesian Updating of Probability of Failure for Reassessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

11.4 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26511.5 Time Variant Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26611.6 Accidental Limit State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26611.7 Operational Condition Reliability . . . . . . . . . . . . . . . . . . . . . . . . 26611.8 Structural Reliability of Floaters . . . . . . . . . . . . . . . . . . . . . . . . . 26611.9 Environmental Load Parameter Modelling . . . . . . . . . . . . . . . . . 26611.10 Reassessment of Jacket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26711.11 Bayesian Updating Due to Change of Conditions . . . . . . . . . . . . 26711.12 Reliability of Offshore Mooring Foundations . . . . . . . . . . . . . . . 267References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

Appendix A: Tubular Member API WSD and ISO 19902 Code Provisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

Appendix B: Tubular Joints API WSD and ISO 19902 Code Provisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

Appendix C: MATLAB Programing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

Appendix D: Load Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

Appendix E: Wave Load Against Corresponding Base Shear in 8 Directions at SBO, SKO1, SKO2 and SKO2a Jacket Platforms . . . . . 291

Appendix F: Evaluation of RSR of 1.0 and System Redundancy . . . . . . . . 303

Glossary of Useful Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

xvii

A Cross-sectional area, activity factor for JacketAi Variable area of tubular member, load uncertainty modelAn Nominal area of tubular membera1, a2, a3 Load coefficientBi Resistance model uncertaintyC Critical elastic buckling coefficientCm,y, Cm,z Moment reduction factors corresponding to the member y and z axesCx Elastic critical buckling coefficientd Water depth, Brace outside diameter, dead load ratioD Outside diameter of member, random dead loadDl Dead load, gravity loadE Young’s Modulus of elasticityEl Environmental loadfa Absolute value of acting axial stressfb Absolute value of acting resultant bending stressfc Representative axial compressive strengthfe Smaller of Euler buckling strength in Y-Z directionsFh Absolute value of hoop compression stressFhc Critical hoop stressfhe Representative elastic critical hoop buckling strengthfxe Representative elastic local buckling strengthFS Factor of SafetyFt Allowable tensile stressft Representative axial tensile strength,fti Variable tensile strengthftn Nominal tensile strengthFy Yield strengthFyi Random yield strengthFyn Nominal yield strengthfe,y, fe,z Euler buckling strengths corresponding to member y and z axesfxe Representative elastic buckling coefficient

Symbols

Symbolsxviii

fyb Brace yield strengthfyc Representative local buckling strengthfyc Yield strength of chord member or (0.8 of tensile strength)fx(X) Probability density function (PDF) of variable X at a value of xG(x) Performance/Limit state function. G(x) = 0, Limit state surface with

respect to design value of xg Acceleration due to gravity, gap between bracesH Maximum wave heightHd Design wave heightHmax Maximum wave heightHmax(des) Design wave height (100 year)HR Wave height which gives RSR of 1.0Hvar Random wave heightI Moment of Inertia of cross-sectionICSF Implicit code safety factorK Effective length factorKy, Kz Effective length factor for y and z directionsL Wave length or Component span, random live loadLl Live Loadl Unbraced length, live load ratioLy, Lz Un braced lengths in y and z directionsM Bending moment due to factored actionsMa Allowable capacity for brace bending momentMc Bending force in chord memberMF Material factorMp Plastic moment strength of chordMuj Joint Bending MomentMy Elastic yield momentN Total number of simulationNf Number of failuresn Platform life in yearsnp Average number of people on JacketP Return period probabilityp Annual probability that the event will not occurPa Allowable capacity for brace axial loadPc Axial force in chord memberPd Gravity load proportionPf Probability of failurePfn Target probability of failurePs Probability of survivalPw Environmental load proportionPuf Updated probability of failurePuj Joint Axial StrengthPy Yield strength of chord, Axial strength due to yielding, Py = A× fy

Symbols xix

Pult Ultimate loadQ LoadQ̄ Mean loadQe 100 year design loadQ1 Large value of QQi Nominal loadQf Chord force factor, base shear (damaged state)Qu Strength factorR Resistance effectR̄ Mean resistanceR1 Low value of resistance RRn Nominal resistanceRult Ultimate resistance of Jacket

r Radius of Gyration r =√

I/A

SR System redundancyT Chord wall thicknesst Wall thickness of member, brace wall thicknesstl Design life of JacketS Strength given by code EquationTapp Apparent wave periodTp Peak periodTz Mean zero-crossing period and is assumed to equal to Tp/1.4u Current speedVc Current speedvr COV of resistanceVpf COV of failure probabilityvq COV of loadVar (x) Variance of xv Poisson’s ratio = 0.3W Load effects, random environmental loadw Environmental load ratioWe Environmental load ratioWf Warning factor for sudden failurex* Design pointX Random variableXm Resistance model uncertaintyXw Load model uncertaintyz Safety marginZ Theoretical value of plastic section modulus of componentZe Elastic section modulus, Ze = π

64[D4

− (D− 2t)4]/(D/2)

ZP Plastic section modulus, ZP =16

[

D3− (D− 2t)3

]

γD Dead load factorγd Gravity load factorγL Live load factor

Symbolsxx

γR Resistance factorγW Environmental load factor∅ Material strength safety factorγi Load factorγR,t Partial resistance factor for axial tensile strength (γR,t = 1.05)μ Meanµg Mean of limit state functionµQ Mean loadµR Mean resistanceµs Social criteria factorσ Standard deviationσb Bending stress due to forces from factored actions; when M > My, σb

is to be considered as an equivalent elastic bending stress σb = M/Ze

σb,y, σb,z σb,y = Bending stress about member y-axis or z-axis (in plane) due to forces from factored actions

σc Axial compressive stress due to forces from factored actionsσg Standard deviation of limit state functionσh Hoop stress due to forces from factored hydrostatic pressureσQ Stand deviation of loadσR Stand deviation of resistanceσt Axial tensile stress due to forces from factored actionsσti Variable tensile stressσtn Nominal tensile stressγc Compression resistance factorγd Gravity load factorγw Environmental load factorγR,b Partial resistance factor for bending strength, γR,b = 1.05

γR,c Partial resistance factor for axial compressive strength, γR,c = 1.18

γRq Yield strength factor (1.05)� Column slenderness parameter�ult Factor which increases until collapseθ Angle between brace and chordβ Reliability index� Cumulative distribution function for the standard normal variables

xxi

AASHTO American Association of State Highway and Transportation Officials

ACI American Concrete InstituteAISC American Institute of Steel ConstructionAPI RP2A (WSD) American Petroleum Institute-Recommended Practice

(Working Stress Design)API RP2A (LRFD) American Petroleum Institute-Recommended Practice

(Load and Resistance Factor Design)BOMEL Company nameCDF Cumulative Distribution FunctionCOV Coefficient of VariationDNV Det Norske Veritas (Norwegian Certifying Authority)DSF Damaged Strength FactorFERUM Compiler for FORM Method of ReliabilityFORM First Order Reliability MethodGOM Gulf of MexicoHmax Maximum wave heightIPB In-Plane BendingISO 19902 International Standard Organization code for (Petroleum

and natural gas industries—Fixed steel offshore structures)ISO LRFD (MS) Environmental load factor from this research (Malaysian

study)LRFD Load and Resistance Factor DesignLSD Limit State DesignMC Mean CoefficientMCS Monte Carlo SimulationMS Malaysian Study (This Research)M–S March–SeptemberNE North EastN–M November–MarchNPD Norwegian Petroleum Directorate

Abbrevations

Abbrevationsxxii

NS North SeaNW North WestOPB Out Plane BendingPAFA Company namePDF Probability Density FunctionPF Probability of FailurePMO Peninsular Malaysia OperationPTS PETRONAS Technical StandardRSR Reserve Strength RatioS SouthSACS Structural Analysis Computer SoftwareSBO Sabah OperationSF Safety FactorSKO1 Sarawak Operation (Platform No. 1)SKO2 Sarawak Operation (Platform No. 2-Fixed at Mud line)SKO2a Sarawak Operation (Platform No. 2 with Pile Soil Foundation)SNS-NNS Southern North Sea, Northern North SeaSP Statistical ParametersSW South WestUPF Updated Probability of FailureVC Variation CoefficientWe/G Environmental Load to Gravity Load RatioWSD Working Stress Design