0015 4 Concrete

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TECHNICAL POLICY BOARD

www.gl-nobledenton.com

GUIDELINES FOR CONCRETE GRAVITY STRUCTURECONSTRUCTION & INSTALLATION

0015/ND

Once downloaded this document becomes UNCONTROLLED.

Please check the website below for the current version.

22 Jun 13 4 RLJ Technical Policy Board

6 Dec 10 3 RLJ Technical Policy Board

31 Mar 10 2 RLJ Technical Policy Board

16 Dec 08 1 RLJ Technical Policy Board05 Oct 87 0 RLJ Technical Policy Board

Date Revision Prepared by Authorised by

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GUIDELINES FOR CONCRETE GRAVITY STRUCTURE CONSTRUCTION & INSTALLATION

0015/ND Rev 4 Page 2 of 28

PREFACE

This document has been drawn with care to address what are considered to be the primary issues in relation to thecontents based on the experience of the GL Noble Denton Group of Companies (“the Group”). This should not,however, be taken to mean that this document deals comprehensively with all of the issues which will need to beaddressed or even, where a particular matter is addressed, that this document sets out a definitive view for allsituations. In using this document, it should be treated as giving guidelines for sound and prudent practice, butguidelines must be reviewed in each particular case by the responsible organisation in each project to ensure thatthe particular circumstances of that project are addressed in a way which is adequate and appropriate to ensure thatthe overall guidance given is sound and comprehensive.

Reasonable precaution has been taken in the preparation of this document to seek to ensure that the content iscorrect and error free. However, no company in the Group

shall be liable for any loss or damage incurred resulting from the use of the information contained herein or

shall voluntarily assume a responsibility in tort to any party or shall owe a duty of care to any party other than to its contracting customer entity (subject always to the terms

of contract between such Group company and subcontracting customer entity).

This document must be read in its entirety and is subject to any assumptions and qualifications expressed therein aswell as in any other relevant communications by the Group in connection with it. Elements of this document containdetailed technical data which is intended for analysis only by persons possessing requisite expertise in its subjectmatter.

© 2013 Noble Denton Group Limited. The content of this document is the copyright of Noble Denton Group Limited. All rights reserved. Any reproduction in other material must have written permission. Extracts may be reproduced

provided that their origin is clearly referenced.

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CONTENTSSECTION PAGE NO.

1 SUMMARY 5 2 INTRODUCTION 6 3 DEFINITIONS 8 4 THE APPROVAL PROCESS 10

4.1 GL Noble Denton Approval 10 4.2 Scope of Work Leading to an Approval 10 4.3 Technical Studies 11 4.4 Surveys 11 4.5 Limitation of Approval 12

5 PROJECT SAFETY AND CONTROL 13 5.1 Health, Safety and Environment 13 5.2 Organisation, Communication and Documentation 13 5.3

Weather / Metocean Criteria and Forecasts 13

5.4 Weight Control 13

6 STRUCTURAL STRENGTH 14 6.1 Loadcases 14 6.2 Structural Concrete & Compressed Air 14 6.3 Structural Steel 14 6.4 Penetrations 15

7 MOTION RESPONSES 16 7.1 Purpose 16 7.2 Motion Response Determination 16 7.3 Human Limit 16

8

STABILITY AND FREEBOARD 17

8.1 General 17 8.2 Inclining Tests 18 8.3 Intact Stability and Freeboard Requirements 18 8.4 Damage Stability and Freeboard Requirements 19 8.5 Instrumentation 20 8.6 Air Cushion 20

9 CONSTRUCTION AND OUTFITTING AFLOAT 21 9.1 Introduction 21 9.2 Structural and Stability Limitations 21 9.3 Construction Spread 22

10 OTHER SYSTEMS & PHASES 23 10.1 Building /Construction Basin 23 10.2 Tow Out from Dry-dock / Building Basin 23 10.3 Temporary Ballasting and Compressed Air Systems 23 10.4 Moorings 23 10.5 Deck Mating 23 10.6 Lifting 23 10.7 Towages 23

11 INSTALLATION 24 11.1 General 24 11.2 Site Location 24 11.3 Seabed Preparation 25 11.4 Installation Method Principles 25 11.5 Positioning Systems 25 11.6 Docking Piles 26 11.7 Skirt Penetration 26

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11.8 Anti-Scour Precautions 26 REFERENCES 27 APPENDIX A - INITIAL INFORMATION REQUIRED FOR APPROVAL 28

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1 SUMMARY1.1 These guidelines have been developed by GL Noble Denton for the marine aspects of construction,

towages and installation of offshore concrete Gravity Base Structures (GBS) including those used by

the oil and gas and renewable sectors. They are intended to be applicable to structures that are towedon their own buoyancy, both deeper draught structures, where much of the construction work is carriedout afloat, as well as shallower draught structures where the construction of the Gravity Base Structure(GBS) can be essentially completed in dry dock.

1.2 These guidelines do not apply to low freeboard structures such as tunnel segments, dock gates,breakwater sections, etc. which should meet the requirements of GL Noble Denton 0030/ND“Guidelines for Marine Transportations”, Ref. [5].

1.3 These Guidelines are intended to lead to an approval by GL Noble Denton.

1.4 Certification of the structure in its installed state is not covered as it is not normally part of a marineinsurance warranty. However it has been assumed that the structural strength of the structure hasbeen adequately checked by others and the results of specified loadcases agreed with GL Noble

Denton.1.5 This Revision 4 supersedes Revision 3 dated 6 December 2010 and the main changes are described

in Section 2.10.

1.6 There are Sections on:

Definitions

A description of the approval process

Health Safety and Environment

Weather criteria

Weight control

Structural strength

Motion responses

Stability and freeboard

Ballasting and compressed air systems

Tow out from dry dock

Construction and outfitting afloat

Installation

1.7 The following phases are covered in separate guidelines as follows:

Criteria and recommendations that are common to many types of operation in 0001/ND,“General Guidelines for Marine Projects, Ref. [1].

Towages of the GBS, completed platform, components or integrated decks in 0030/ND,“Guidelines for Marine Transportations” Ref. [5].

Inshore or offshore deck mating in 0031/ND, “Guidelines for Float-Over Installations /Removals” Ref. [6]

Moorings in 0032/ND, “Guidelines for Moorings” Ref. [7].

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2 INTRODUCTION2.1 This document summarises the general guidelines developed by GL Noble Denton for application to

the marine aspects of construction, towages and installation of offshore concrete Gravity Base

Structures including those used by the oil and gas and renewable sectors. The guidance is intended tobe applicable to structures that are towed on their own buoyancy, both deeper draught structures,where much of the construction work is carried out afloat, as well as shallower draught structureswhere the construction of the Gravity Base Structure (GBS) can be essentially completed in dry dock.

2.2 These guidelines do not apply to low freeboard structures such as tunnel segments, dock gates,breakwater sections, etc. which should meet the requirements of GL Noble Denton 0030/ND“Guidelines for Marine Transportations”, Ref. [5].

2.3 This document does not specifically cover platform removal, but it should be assumed that the designmust allow technically and economically feasible removal operations, in accordance with internationalenvironmental legislation.

2.4 As each platform will differ in design, building location and destination, detailed recommendations will

apply to individual cases. However, this document is submitted for general guidance during projectdevelopment and it is emphasised that discussions with GL Noble Denton at an early stage would bedesirable.

2.5 These Guidelines are intended to lead to an approval by GL Noble Denton. Such approval does notimply that compliance with national codes and legislation, or the requirements of other regulatorybodies, harbour authorities and/or any other parties would be given.

2.6 This document covers specific aspects of the towages of the GBS ’s and completed platforms that areadditional to the requirements of 0030/ND “Guidelines for Marine Transportations” , Ref. [5]. Thetowage of components and integrated decks is also covered by 0030/ND.

2.7 Similarly deck-mating is covered in 0031/ND “Guidelines for Float-Over Installations /Removals”,Ref. [6] and moorings in 0032/ND “Guidelines for Moorings, Ref. [7].

2.8 This document refers to, and should be read in conjunction with other GL Noble Denton Guidelinedocuments, particularly:

a. 0001/ND “General Guidelines for Marine Projects”, Ref. [1]

b. 0013/ND “Guidelines for Load-Outs”, Ref. [2]

c. 0021/ND “Guidelines for the Approval of Towing Vessels”, Ref. [3]

d. 0027/ND “Guidelines for Marine Lifting Operations”, Ref. [4]

e. 0030/ND “Guidelines for Marine Transportations”, Ref. [5]

f. 0031/ND “Guidelines for Float-Over Installations /Removals” , Ref. [6]

g. 0032/ND “Guidelines for Moorings”, Ref. [7].

2.9 Revision 3 superseded Revision 2 dated 31 March 2010. Principal changes included: the addition of sections on definitions (Section 3), the approval process (Section 4), health,

safety and environment (Section 5), organisation, planning and documentation (Section 6),weight control (Section 8) and installation (Section 17).

Mooring, deck-mating and towing were then covered in separate guidelines, Ref. [7], Ref. [6]and Ref. [5] respectively.

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2.10 This Revision 4 supersedes Revision 3 dated 6 Dec 2010. Principal changes include:

Moving to 0001/ND “General Guidelines for Marine Projects”, Ref . [1], the bulk of Sections 4(The Approval Process), 5 (Health Safety & Environment), 6 (Organisation, Planning &Documentation), 7 (Weather Criteria), 8 (Weight Control), part of 9 (Structural Strength), 12

(Ballasting and Compressed Air Systems, 13 (Building Basin) and 14 (Tow-out from Dry-dock).

Additional information on seabed preparation in Section 11.3

Additional recommendations for installation starting at Section 11.4

Additional information on ant-scour precautions in Section 11.8

The information required for approval in summarised in Appendix A.

2.11 All GL Noble Denton Guidelines can be downloaded from:

http://www.gl-nobledenton.com/en/rules_guidelines.php.

2.12 Please contact the Technical Policy Board Secretary at [email protected] with any queries or

feedback.

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http://www.gl-nobledenton.com/en/rules_guidelines.php


mailto:[email protected]





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3 DEFINITIONS3.1 Referenced definitions are underlined.

Term or Acronym Definition

Approval The act, by the designated GL Noble Denton representative, of issuing a Certificate of Approval.

Barge A non-propelled vessel commonly used to carry cargo or equipment.(For the purposes of this document, the term Barge can beconsidered to include vessel or ship where appropriate.)

Certificate of Approval A formal document issued by GL Noble Denton stating that, in its judgement and opinion, all reasonable checks, preparations andprecautions have been taken to keep risks within acceptable limits,and an operation may proceed.

Client The company to which GL Noble Denton is contracted to performmarine warranty or consultancy activities.

Deck mating The act of installing integrated topsides over a substructure,generally by float-over and ballasting. Deck mating may take placeinshore or offshore, onto a floating or a previously installedsubstructure.

DP Dynamic Positioning or Dynamically Positioned

FMEA or FMECA

Failure Modes and Effects Analysis or Failure Modes, Effects and Criticality Analysis

Freeboard The distance from the waterline to the upper watertight deck level.

In commercial vessels, it is measured relative to the ship's load line.

On a GBS, under construction or under tow, it will normally be theheight above sea level of the lowest down-flooding point.

GBS Gravity Base Structure.

GL Noble Denton The legal entity trading under the GL Noble Denton name which iscontracted to carry out the scope of work and issues a Certificate of Approval, or provides advice, recommendations or designs as aconsultancy service.

GM Initial metacentric height.

HAZID Hazard Identification StudyHAZOP Hazard and Operability Study

HIRA Hazard Identification Risk Assessment

Insurance Warranty A clause in the insurance policy for a particular venture, requiring theapproval of a marine operation by a specified independent surveyhouse.

JSA Job Safety Analysis

Load-out The transfer of a major assembly or a module onto a barge, e.g. byhorizontal movement or by lifting.

MBL The minimum allowable value of breaking load for a particular sling,grommet, wire or chain etc.

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http://en.wikipedia.org/wiki/Waterline

http://en.wikipedia.org/wiki/Deck_(ship)

http://en.wikipedia.org/wiki/Ship%27s_load_line#Load_line

http://en.wikipedia.org/wiki/Ship%27s_load_line#Load_line

http://en.wikipedia.org/wiki/Deck_(ship)

http://en.wikipedia.org/wiki/Waterline

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Term or Acronym Definition

Operation Duration The planned duration of the operation excluding a contingencyperiod from the Point of No Return to a condition when theoperations /structures can safely withstand a seasonal design storm

(also termed “safe to safe” duration).

Operational referenceperiod

The Operation Duration, including a contingency period

Platform The completed platform after deck mating, consisting of the GBSand Topsides.

PNR /Point of No Return

The last point in time, or a geographical point along a route, at whichan operation could be aborted and returned to a safe condition.

QRA Quantified Risk Analysis

Reduced exposure towage

/transportation

A weather-unrestricted operation which has an exposure sufficiently

short (typically under 30 days) for the design extremes to bereduced. See Section 7.2.2.3 of 0001/ND, Ref. [1], and Section 6.4of 0030/ND, Ref. [5].

ROV Remotely Operated Vehicle.

Structure The object to be transported and installed, or a sub-assembly,component or module.

Survey Attendance and inspection by a GL Noble Denton representative.

Other surveys which may be required for a marine operation,including suitability, dimensional, structural, navigational and Classsurveys.

Surveyor The GL Noble Denton representative carrying out a Survey’

An employee of a contractor or Classification Society performing, for instance, a suitability, dimensional, structural, navigational or Classsurvey.

Vessel A marine craft designed for the purpose of transportation by sea or construction activities offshore. See Barge

Weather-restrictedoperation

A marine operation which can be completed within the limits of anoperational reference period with a favourable weather forecast(generally less than 72 hours), taking contingencies into account.

The design environmental condition need not reflect the statisticalextremes for the area and season.

A suitable factor should be applied between the operational weather limits and the design weather conditions (see Section 7.3.3 of 0001/ND, Ref. [1]).

Weather-unrestrictedoperation

An operation with an operational reference period greater than thereliable limits of a favourable weather forecast (generally less than72 hours).

The design weather is typically a 10 year seasonal storm, butsubject to Section 7.2.2 of 0001/ND, Ref. [1].

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h. Lifting and installation of equipment and sub-assemblies.

4.2.4 INSTALLATION

a. Details and survey reports of installation location

b. Installation method

c. Loads and stresses during installation

d. Stability

e. GBS ballasting system and limitations

f. Mooring, positioning and handling systems

g. Installation procedures.

4.3 TECHNICAL STUDIES

4.3.1 Technical studies leading to the issue of a Certificate of Approval may consist of:

a. Reviews of specifications, procedures and calculations submitted by the client or hiscontractors, or

b. Independent analyses carried out by GL Noble Denton to verify the feasibility of the proposals,or

c. A combination of third party reviews and independent analyses.

4.4 SURVEYS

4.4.1 Surveys and attendances carried out as part of GL Noble Denton's scope of work typically include, asmay be relevant to the particular operation:

a. Pre-flooding survey of the dry-dock, including handling arrangements

b. Tug suitability surveys (see note)

c. Suitability surveys of other major marine equipment used in the marine operations, such asbarges, crane vessels, mooring laying vessels (see note)

d. Attendance prior to and during lift-off in dry-dock

e. Attendance during tow out from dock, to inshore moorings or hand-over location

f. Attendance during connection to inshore moorings

g. Periodic attendances during construction afloat, particularly during any critical ballastingoperations, changes of mooring configuration, critical lifts, etc.

h. Attendance during deep submergence tests

i. Attendance during preparations for, and during, inshore deck-mating

j. Attendance during preparations for towage offshore

k. Attendance during towage, either on the structure if manned, or on the lead tug

l. Attendance during preparations for installation, and during installation operation

m. Attendance during offshore construction activities, including deck mating or module lifting.

Note: For tugs registered in the GL Noble Denton Towing Vessel Approvability Scheme,Ref. [3], with a valid bollard pull certificate, tug suitability surveys may be carried out immediately prior to sailaway. Similarly, for other equipment known and generally acceptable to GL Noble Dentonsuitability surveys may be carried out immediately prior to the operation, although a review of documentation may be made in advance. When a vessel is unknown to GL Noble Denton it issuggested that a preliminary survey be undertaken before committing to the vessel use or charter toensure that it complies with the requirements of 0030/ND, Ref [5].

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4.5 LIMITATION OF APPROVAL

4.5.1 The following limitations are in addition to those in Section 4.5 of General Guidelines 0001/ND,Ref. [1].

4.5.2 A Certificate of Approval for installation or any intermediate stage applies to the safety of the GBS or

platform during the defined installation period. It does not imply that the completed platform will be fitfor purpose.

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5 PROJECT SAFETY AND CONTROL

5.1 HEALTH, SAFETY AND ENVIRONMENT

5.1.1 Recommendations on HS and E appear in Section 5 of Ref. [1] “General Guidelines for MarineProjects”.

5.2 ORGANISATION, COMMUNICATION AND DOCUMENTATION

5.2.1 Recommendations on Organisation, Communication and Documentation appear in Section 6 of Ref. [1] “General Guidelines for Marine Projects”.

5.3 WEATHER / METOCEAN CRITERIA AND FORECASTS

5.3.1 Requirements and recommendations on Weather /Metocean criteria and forecasts appear in Section 7of Ref. [1] “General Guidelines for Marine Projects”.

5.4 WEIGHT CONTROL

5.4.1 Requirements and recommendations on Weight Control appear in Section 8 of Ref. [1] “GeneralGuidelines for Marine Projects”.

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6 STRUCTURAL STRENGTH

6.1 LOADCASES

6.1.1 The strength of the structure in the installed condition should be covered by the relevant certifyingauthority or classification society who will normally refer to a suitable offshore structural code or rulessuch as the GL Rules, Ref. [8]. Evidence shall be submitted to GL Noble Denton to confirm that thestrength is adequate in the temporary phases before it is safely installed for the following cases during:

a. float-out from building basin (with and without any air cushion)

b. the most critical construction afloat stages

c. any towages, with or without a deck

d. deep submergence for deck mating

e. installation on the seabed, including:

1. any impact with the seabed including any rocks or debris during installation

2. penetration and grouting phases3. any impact with scour protection during its placement.

f. Any other critical phase as agreed with GL Noble Denton.

6.1.2 Loadcases shall be derived by the addition of fluctuating loads resulting from wind, wind heel, waveaction and the effect of towline pull or mooring loads to the static forces resulting from gravity andhydrostatic loads. Accidental loadings shall also be considered. Specific loadcases shall be agreedwith GL Noble Denton. Adequate global and local strength shall be documented.

6.1.3 The unit shall be able to safely withstand a static heel angle of 10 degrees, or any greater anglerequired during construction, towage or installation. If it has damage stability the unit shall also be ableto withstand the static and dynamic loads caused by the flooding of any one compartment in a 25metres/second wind and associated waves. These would be regarded as extreme (ultimate limit state)conditions.

6.1.4 Hydrostatic loads on the substructure at the deepest draught during deck-mating can be the governingloadcase. It shall be demonstrated that a thorough independent check of the calculations covering thisloadcase has been carried out, and that the design and reinforcement details assumed in thecalculations concur with the as-built condition.

6.1.5 Any limitations on the maximum allowable duration of deep immersion, in relation to the structuralstability of the unit, should be established and the procedures planned accordingly.

6.2 STRUCTURAL CONCRETE & COMPRESSED AIR

6.2.1 Further details appear in Sections 9.7 (Concrete) and 9.8 (Compressed Air) of 0001/ND, Ref. [1].

6.2.2 Testing of concrete for permanent works should be covered by the certifying authority and testing for temporary works should follow the same requirements.

6.3 STRUCTURAL STEEL

6.3.1 Any steel which forms part of the primary structure, or any critical temporary structure used duringconstruction, deck-mating or deep submergence, towage or installation, shall be of high qualitystructural steelwork with full material certification. All welded connections shall be subject to agreedNDT inspection.

6.3.2 Steel shall be assessed using the methodology of a recognised and applicable offshore code includingthe associated load and resistance factors for LRFD codes or safety factors for ASD/WSD codes.Further details appear in Section 9.1 of 0001/ND, Ref. [1].

6.3.3 Any welded connections shall be subject to NDT inspection certificates showing appropriate levels of

inspection.

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6.4 PENETRATIONS

6.4.1 Any penetrations (e.g. for risers or J-tubes) below the water line shall be reinforced near to thepenetration against all ranges of hydrostatic pressures and accidental impact from dropped objectsand vessel impact if likely at any draft.

6.4.2 Penetrations shall be positively sealed to prevent the ingress of water whist the structure is afloat.

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7 MOTION RESPONSES

7.1 PURPOSE

7.1.1 The behaviour of the unit shall be determined by means of theoretical calculations and/or modeltesting, in order to determine the response to the design environmental loadings for construction,towage and installation, as appropriate. In particular, the responses should be determined, or shownto be negligible, for the following conditions:

a. Departure from building basin, and towage to floating construction site

b. All stages of construction afloat

c. Deck mating operations

d. Towage to offshore site, at all likely towage draughts

e. Installation at final location

f. Any condition where the motions may be critical to loss of freeboard, stability, position keepingor other considerations.

7.2 MOTION RESPONSE DETERMINATION

7.2.1 The guidance of Sections 7.7 and 7.8 of 0030/ND “Guidelines for Marine Transportations”, Ref. [5]shall be applied.

7.2.2 The motion responses should be determined for all relevant headings, and at speed zero for towagecases. The maximum responses should be based on a 3 hour exposure period.

7.3 HUMAN LIMIT

7.3.1 If the structure is manned for towage or installation, care be should taken to avoid accelerations inexcess of about 0.2g except for occasional short periods, to enable personnel to carry out their dutiesefficiently.

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8 STABILITY AND FREEBOARD

8.1 GENERAL

8.1.1 This Section expands on the stability section of 0030/ND “Guidelines for Marine Transportations”Ref. [5], and is mainly intended to cover “Condeep”-type gravity structures (see Section 2.1). However the principles will apply to most types of concrete gravity platforms.

8.1.2 Sufficient positive stability and reserve buoyancy shall be ensured during all stages of the marineoperations. Both intact and damage stability shall be evaluated, on the basis of an accurate geometricmodel.

8.1.3 In calculations of stability and reserve buoyancy / freeboard, due allowance shall be included for uncertainty in mass, buoyancy, volume, location of centre of gravity, density of liquid and solid ballast,and density of seawater.

8.1.4 The output of the weight control programme as described in Section 8 of 0001/ND, Ref. [1]) shall betaken into account.

8.1.5 Stability calculations should include corrections and allowances for:a. Free surface

b. Air cushion

c. Icing

d. Influence of moorings, including a check on the consequences of failure.

e. Cantilever Structures

f. Temporary Loads

8.1.6 The number of openings in buoyant elements adjacent to the sea shall be kept to a minimum. Wherepenetrations are necessary for access, piping, ventilation, electrical connections, etc. arrangementsshall be made to maintain watertight integrity. During construction phases, particular attention shouldbe paid to openings near the waterline, which will vary as construction proceeds.

8.1.7 Damage stability requirements shall be evaluated considering the operation procedure, environmentalloads and responses, the duration of the operation and the consequences of possible damage.Compartments that may be subject to flooding or partial flooding include:

a. Compartments adjacent to the sea

b. Compartments inside the structure, crossed by seawater filled pipes

c. Skirt compartments containing compressed air.

8.1.8 Special attention should be paid to flooding which may be caused by:

a. Impact loads from vessels

b. Damage to structure or pipework from dropped objectsc. Mechanical system failure

d. Human error.

8.1.9 The consequences of water ballast escaping from any compartments above the waterline, or theescape of air from any air cushion shall be evaluated where applicable.

8.1.10 Flooding as a result of vessel impact is assumed to occur in a zone bounded by two horizontal planesnormally positioned 5 metres above and 8 metres below the waterline. These levels should bereviewed if deep draught vessels are likely to be operating nearby.

8.1.11 For operations where the structure cannot meet damage stability criteria, measures shall be taken tominimise the risk, by:

a. Limiting the exposure periodb. Providing additional local structural strength

c. Providing additional protection, such as fendering

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d. Minimising vessel movements near the structure

e. Dedicated procedures and experienced personnel.

8.1.12 A risk assessment shall be carried out for operations where at any stage stability or reserve buoyancyis critical, or where damage stability cannot be obtained. The results of the risk assessment shall beagreed with GL Noble Denton. The duration of the critical condition should be minimised.Requirements for back-up or protection systems, or special procedures should be assessed.

8.2 INCLINING TESTS

8.2.1 Inclining tests should normally be performed at different stages during construction afloat, particularlyprior to any marine operation where the displacement, centre of gravity or stability may be critical.

8.2.2 Where a number of very similar units are constructed at the same place, the requirement for incliningtests on the later units will be determined after a study of weight variations (from displacement tests)and Centre of Gravity variations (from inclining tests) of the previous units.

8.2.3 The output from the inclining test should be used to check and calibrate the output from the weightcontrol programme.

8.2.4 Normally inclining tests should be performed:

a. Along two axes if the platform is not axi-symmetrical

b. For each direction in several steps out to maximum inclination and back, and with positive andnegative inclinations.

8.2.5 A sensitivity analysis of the parameters affecting the test results should be performed.

8.2.6 Procedures for the test should be developed considering:

a. The inclining angle should be sufficient to obtain an adequate degree of accuracy

b. The inclining angles should be measured by at least 2 independent devices

c. The preferred method of inclining is by shifting weights, without changing displacement

d. The preferred draught is such that the stability is minimum and the waterline intersects thestructure in a vertical wall-sided area

e. A detailed assessment of the state of construction and amount of temporary equipment andmaterials is essential. No construction activities or shifting of any construction equipment shallbe allowed during the test

f. The effects of external forces due to winds, waves, currents, moorings, tugs etc shall bemonitored. Maximum allowable wind and current speeds shall be determined

g. A statistical assessment of the results should be included in the test report.

8.3 INTACT STABILITY AND FREEBOARD REQUIREMENTS

8.3.1 INTACT STABILITY

8.3.1.1 The initial GM shall be positive, normally not less than 0.5.m (after allowing for all possibleinaccuracies in measuring it), and such that the maximum inclination of the GBS or platform does notexceed 5 degrees in the design storm conditions as defined in Section 7 of 0001/ND, Ref. [1]. Calculation of maximum inclination should take into account:

a. Maximum amplitude of pitch or roll motion in the design seastate, plus

b. Inclination due to design wind, plus

c. Inclination due to mooring line tensions or required towline pull.

8.3.1.2 During towing, the static inclination in still water when subjected to 50% of required towline pull shouldnot normally exceed 2 degrees. Differential ballasting may be used to reduce the static inclination

resulting from towline pull only by not more than 1 degree.8.3.1.3 The area under the righting moment curve shall be not less than 140% of the area under the

overturning moment curve. Both curves shall be bounded by the least of:

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a. The second intercept of the righting and overturning moment curves

b. The angle of downflooding

c. The angle which would cause any part of the GBS to touch bottom in the minimum water depthat the construction site or along the towage route. This requirement may be deleted for

installation at the offshore site.d. The angle at which allowable stresses are reached in any part of the structure, construction

equipment, topsides or topsides attachments, if applicable.

8.3.1.4 The wind used for overturning moment calculations should be the design wind for the operation, asdefined in Section 7.2.3 of 0001/ND, Ref. [1]. Short duration operations during construction or towagemay be considered as weather-restricted operations, provided the structure can achieve or be returnedto a safe condition, within the operational reference period

8.3.2 FREEBOARD

8.3.2.1 For towages and construction afloat, the freeboard as defined in Section 3 shall not be less than thegreater of:

a. 1 metre above the design wave crest height, with allowance for run-up, all around the structure,under the design storm loading from the most critical direction,

b. 6 metres in the intact condition, if the unit does not have one-compartment damage stability.

8.4 DAMAGE STABILITY AND FREEBOARD REQUIREMENTS

8.4.1 DAMAGE STABILITY FOR FLOAT-OUT AND TOWAGE TO INSHORE CONSTRUCTION SITE

8.4.1.1 For float-out from dry-dock, one-compartment damage stability is usually impractical, because of thelimited underkeel clearance, and is therefore not required.

8.4.1.2 For other inshore tows the structure shall have one-compartment damage stability, as defined inSections 8.1.7 through 8.1.10.

8.4.1.3 If one-compartment damage stability requirements cannot be fulfilled, the requirements for constructionafloat in Section 8.4.2.2 shall apply.

8.4.2 DAMAGE STABILITY DURING CONSTRUCTION AFLOAT

8.4.2.1 During the period of construction afloat, the platform shall possess one-compartment damage stability,for as much of the construction period as is practical.

8.4.2.2 When the platform does not possess one-compartment damage stability, then:

a. A means should be available to compensate inclination due to flooding of any compartment,and

b. The minimum freeboard in any intact case shall exceed 6 metres, and

c. There shall be sufficient structural strength in the outer walls to withstand impact loads from the

construction spread and vessels, which may be in close proximity to the platform, andd. Fendering may be used to reduce impact loads in critical areas, and

e. Lifting of heavy objects shall be carefully controlled. Protection shall be provided againstdropped objects. Any lifts which, if dropped, could endanger the platform shall be identified andadditional precautions taken, and

f. Any objects or equipment on barges alongside, which if dropped, could endanger the platformshall be similarly identified and additional precautions taken, and

g. Rigorous procedures shall be developed to minimise the risk of flooding. These shall includeconsideration of collision, leakage through the ballast or other systems, reliability andredundancy of pumping arrangements and power supplies, and

h. At all times there shall be adequately trained personnel on board the platform, and

i. A risk assessment of flooding shall be carried out, and the results agreed as acceptable by GLNoble Denton.

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8.4.3 DAMAGE STABILITY FOR TOWAGE TO OFFSHORE SITE AND INSTALLATION

8.4.3.1 When towing on the base or columns the platform shall possess one-compartment damage stability.

8.4.3.2 It is acknowledged that for an offshore tow, the above requirement might be impractical, in which case:

a. The structure shall be locally reinforced within the zone defined in Section 8.1.10, to withstand

impact from the largest towing or attending vessel, and/or

b. Rigorous procedures shall be developed to minimise the risk of flooding, and

c. A risk assessment of flooding shall be carried out, and the results agreed as acceptable by GLNoble Denton.

8.4.3.3 It is acknowledged that during installation, it might be impractical to provide reinforcement againstcollision over the full range of waterlines. Planning and risk assessment shall include a procedure toreturn the structure to the reinforced waterline should the installation operation be aborted.

8.4.4 FREEBOARD

8.4.4.1 For offshore towages, after damage, when subject to the design wind for the operation, a freeboard of not less than 5 metres shall remain above the design wave crest height, with allowance for run-up, all

around the structure, under the design storm loading from the most critical direction.

8.4.5 STRUCTURAL LIMITATIONS

8.4.5.1 The structure shall be demonstrated to meet the requirements of Section 6.1.3.

8.5 INSTRUMENTATION

8.5.1 Adequate instrumentation shall be installed to measure:

a. The water level in all compartments, quantity and percentage

b. Status of all valves

c. Pump status and flow rates

d. Main and emergency power supply status

e. Platform draught, heel and trim

f. Compartment air pressure

g. Compressor status

h. Air cushion pressure

i. Water seal level in skirt compartments

j. Status of access doors and manholes.

8.5.2 Essential instrumentation shall be supplied from an Un-interruptible Power Supply.

8.6 AIR CUSHION

8.6.1 The following recommendations apply to the use of underskirt compressed air used for additionalbuoyancy or for reducing bending moments in the structure:

a. All piping shall be secure, protected and of adequate capacity and strength

b. Supply lines shall have non-return valves

c. Back-ups shall be provided for all critical valves and piping

d. Adequate reserve compressors shall be onboard

e. A venting system shall be provided to guarantee that all air is removed after use, to ensure noresidual free surface remains

f. Sufficient water seal (bottom of air cushion above bottom of skirt) shall be available to preventair escaping

g. The air cushion should be isolated in separate compartments, so that failure of any part of thesystem does not cause a large heel or trim in addition to loss of buoyancy.

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9 CONSTRUCTION AND OUTFITTING AFLOAT

9.1 INTRODUCTION

9.1.1 This Section covers the marine operational aspects of construction and outfitting afloat. Constructionand outfitting afloat includes activities on the platform starting with tow-out from construction site,mooring at outfitting site, if different, and activities on the platform at the outfitting site until departurefor the offshore location.

9.1.2 Inshore deck mating, although a construction activity afloat, is excluded from this Section and iscovered in 0031/ND, Ref. [6].

9.1.3 Documentation and procedures for construction afloat shall follow the general principles of Section 6 of 0001/ND, Ref. [1]. Documentation and procedures must above all ensure that those planning,authorising and carrying out the work are fully informed about any limitations and constraints whichmay be placed on the work by factors outside their own discipline.

9.1.4 All responsible parties must remember that the platform cannot be treated as a normal onshore

construction activity. Any activity can be constrained by factors which change on a daily basis, andwhich can be inter-related, including:

a. Structural loads and resistance

b. Draught, displacement, ballast condition and stability

c. Mooring loads and resistance

d. Marine spread requirements

e. Weather conditions

f. Other on-going activities and access restrictions.

g. Environmental conditions imposed by local authorities

9.2 STRUCTURAL AND STABILITY LIMITATIONS9.2.1 The following shall be taken into account when assessing the structural loading and stresses during

construction afloat:

a. Static loads

b. Hydrostatic loads

c. Tidal changes

d. Mooring loads

e. Differential ballasting

f. Environmental and environmentally-induced loads

g. Loads due to construction spread

h. Vessel impact loads

i. Contingency loads, including accidental flooding, mooring line breakage, dropped objects, asappropriate

j. Seasonal Loads as ice etc.

k. Shallow water effects

9.2.2 Any structural, stability or draught limitations which lead to constraints on construction operations mustbe clearly defined, and written into the relevant operational procedures. These may include:

a. Maximum and minimum draught

b. Differential ballast levels in adjacent buoyancy compartments, or any compartment and the sea

c. Weight distributiond. Structural limitations on heel or trim, which may therefore lead to limitations on draught, stability

or environmental conditions

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e. Free surface limitations

f. Phases during which one-compartment damage stability does not exist, and the requirementsgiven in Section 8.4.2.2 apply.

9.2.3 The age of any time-dependent construction material such as concrete should be taken into account inthe calculations and procedures.

9.3 CONSTRUCTION SPREAD

9.3.1 The construction spread may include barges and other floating equipment moored alongside or near the platform, to serve the following functions:

a. Storage for construction materials and equipment

b. Concrete mixing plant

c. Temporary power supply

d. Temporary ballast control

e. Offices

f. Workshops

g. Personnel reception area and security

h. Berthing and unloading area for ferries, transport barges and vessels

i. Safety and emergency facilities.

9.3.2 The number of barges moored alongside the platform should be kept to a minimum. Where practical,any redundant equipment should be removed from the spread.

9.3.3 The mooring and fendering system for each item of the spread should be designed in accordance withthe requirements of 0032/ND, Ref. [7]. Where such a design is impractical, then the design andoperational meteorological limits for the moorings should be clearly defined. Procedures should bedeveloped to close down the function of the affected equipment and remove it to a place of safety,

before the operational limit is reached. Adequate tugs and safe moorings should be available toperform this operation.

9.3.4 All equipment and material on barges shall be secured to minimise the risk of loss overboard. Anyequipment which, if lost overboard, could cause damage to the structure, shall be identified andhandled so as to minimise the hazard.

9.3.5 All floating equipment moored adjacent to the platform shall possess one-compartment damagestability. The need for contingency pumping equipment on site should be evaluated.

9.3.6 A hazard identification study and risk assessments should be carried out for the entire spread involvedduring the construction and outfitting afloat. The results should be submitted for GL Noble Dentonapproval.

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10 OTHER SYSTEMS & PHASES

10.1 BUILDING /CONSTRUCTION BASIN

10.1.1 See Section 10 of 0001/ND – General Guidelines for Marine Projects, Ref. [1]

10.2 TOW OUT FROM DRY-DOCK / BUILDING BASIN


10.3 TEMPORARY BALLASTING AND COMPRESSED AIR SYSTEMS


10.4 MOORINGS

10.4.1 See 0032/ND – Guidelines for Moorings, Ref. [7]

10.5 DECK MATING

10.5.1 See 0031/ND – Guidelines for Float-Over Installations /Removals, Ref. [6].

10.6 LIFTING

10.6.1 See 0027/ND – Guidelines for Marine Lifting Operations, Ref. [4].

10.7 TOWAGES

10.7.1 See 0030/ND – Guidelines for Marine Transportations, Ref. [5].

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11 INSTALLATION

11.1 GENERAL

11.1.1 This section describes the general requirements for the installation of a concrete gravity platform at itsfinal offshore location. The installation procedures will vary, depending on parameters including:

a. The size and design of the platform

b. Water depth

c. The positioning tolerances required in all 6 degrees of freedom

d. The positioning / station-keeping system proposed

e. Whether cranes, winches or external buoyancy is required for lowering and/or positioning

f. Whether the operation involves docking over a template, docking piles or other structures

g. Stability at all stages of immersion

h. Whether a vertical or inclined installation is required

i. Tolerances on differential ballast levels j. The skirt design, and penetration method

k. Whether under-base grouting is required

l. Whether solid ballast or scour protection is required.

11.2 SITE LOCATION

11.2.1 The position of the site location shall be given in both geographical and grid coordinates.

11.2.2 The water depth and bathymetric tolerances shall be determined.

11.2.3 When determining the extent of the survey area, the following shall be accounted for:

a. Tolerances on site survey position

b. Inaccuracy of position monitoring systems during installation

c. Operational tolerances

d. The approach corridor

e. Whether a holding location is required close to the site

f. Whether an inclined installation, with previous off-site touch-down is required

g. The proximity of any other platforms or subsea assets at or near the location.

11.2.4 The bottom topography shall be established by swathe bathymetry and checked by single beam echosounder, side scan sonar, magnetometer and ROV video. The extent of any required levelling or other seabed preparation should be decided at the design stage.

11.2.5 The seabed and sub-seabed conditions shall be established by coring, magnetometer, in-situ testing,lab testing and sub-bottom profiling.

11.2.6 Current surveys should be carried out at all depths.

11.2.7 The area should be checked to ensure that there are no travelling sand-waves or other seabed erosion/accretion that could affect the structure after installation.

11.2.8 A site survey of the installation area covering the full area of any anchor pattern, carried out not morethan 4 weeks before the start of installation, shall be provided to verify the location of all subseainfrastructure, debris and obstructions.

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11.3 SEABED PREPARATION

11.3.1 If the seabed has uneven levels or consistency then it may need preparation before the unit isinstalled. Typical methods include:

a. Controlled dumping and compacting of gravel before final levelling

b. Placing sand-bags

c. Excavating of unsuitable soils before replacing as in a. or b. above.

In all cases the required tolerances for level and compaction should be agreed with GL Noble Dentonat an early stage.

11.4 INSTALLATION METHOD PRINCIPLES

11.4.1 In general it is desirable for all installation phases to be reversible though this may not always bepossible, especially if there are temporary unstable phases.

11.4.2 The approval criteria will depend on the installation methods and in particular:

a. What external assistance (e.g. temporary buoyancy, winches, cranes etc) is required

b. Range of positive stability at all stages of installation.c. Length of weather windows required and sensitivity to bad weather or strong currents

d. If scour protection is required immediately after emplacement (see Section 11.8).

11.4.3 For structures towed on their side an Up –End procedure is required. Additional or alternativerequirements for this, or any other new or unusual operation, will be developed by GL Noble Dentonafter appropriate study of the proposals and risk assessments, preferably at an early stage in theproject.

11.4.4 In general, shallow draught platforms will be towed to site with freeboard on the base structure. Theseplatforms frequently undergo a phase of instability during submergence of the base, and an inclinedinstallation procedure must then be used. Sometimes it will be necessary to touch down on one edgeto achieve stability. In the event of an inclined installation the following shall be considered:

a. All machinery, systems and personnel, if aboard, must be able to work efficiently in the inclinedcondition

b. Monitoring of ballast levels, and allowable differential levels

c. Structural capacity of the skirt at touch down, and possible impact loads imposed

d. Skirt touch down, if on the final site, may disturb the seabed, and prejudice the final skirtpenetration or base slab bearing

e. If the skirt touch down is on the final site, accurate position control may be difficult in theinclined condition

f. If skirt touch down is remote from the final site, the deballast capability required by Section 12 of 0001/ND, Ref. [1], will be used.

11.4.5 Deep draught structures are normally towed to site floating on the columns, and installed by ballastingvertically.

11.5 POSITIONING SYSTEMS

11.5.1 The positioning system will normally be by means of the tug fleet. This will often consist of the towfleet, rearranged into a star configuration.

11.5.2 Where the position and orientation tolerances are not critical, the tugs may be in free floatingconfiguration.

11.5.3 Where more precise positioning is required, the tugs may be connected at the bow to pre-laid anchorsthough other mooring systems are possible.

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11.6 DOCKING PILES

11.6.1 If docking piles are used for positioning, then suitable analyses shall be presented to confirm thedynamics of the structure, the feasibility of engaging with the pile(s), the strength and elasticity of thepiles, the behaviour of the piles in the soil, the structure/pile interaction and loads.

11.6.2 A monitoring system for the docking stage, with adequate redundancy, is required.

11.7 SKIRT PENETRATION

11.7.1 Calculations shall be presented to demonstrate that the skirts will penetrate adequately under theapplied loads, whether gravity loads only are required, or whether negative pressure needs to beapplied.

11.7.2 Vent piping shall be provided to allow water in the skirt compartments to escape, or to allow negativepressure to be applied.

11.7.3 Design of the pipework should take into account the requirements for removal on decommissioning.

11.7.4 Structural loads on the skirts shall be shown to be acceptable.

11.7.5 If negative pressure is applied, then it shall be demonstrated that an adequate seal can be obtained at

the skirt tip, with minimal risk of “piping” between outside and inside.

11.8 ANTI-SCOUR PRECAUTIONS

11.8.1 All locations, especially with high current speeds, should be investigated to see if scour is likely to be aproblem.

11.8.2 Possible solutions to scour include:

a. Controlled rock dumping or placing sand-bags immediately after the unit is installed. Care mustbe taken to avoid any damage to the unit especially near penetrations, pipelines, cables or other sub-sea assets. Scour may start immediately after emplacement, especially in badweather.

b. Artificial seaweed or other seabed stabilisation methods. This solution needs to be

demonstrated to be successful under these conditions.

c. Increasing any skirt lengths, though this should have been determined at an early design stage.

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REFERENCES

[1] GL Noble Denton 0001/ND “General Guidelines for Marine Projects” [2] GL Noble Denton 0013/ND “Guidelines for Load-Outs”.

[3] GL Noble Denton 0021/ND “Guidelines for the Approval of Towing Vessels”.

[4] GL Noble Denton 0027/ND “Guidelines for Marine Lifting & Lowering Operations”.

[5] GL Noble Denton 0030/ND “Guidelines for Marine Transportations”.

[6] GL Noble Denton 0031/ND “Guidelines for Float-Over Installations /Removals”

[7] GL Noble Denton 0032/ND “Guidelines for Moorings”.

[8] GL “Rules for Classification and Construction IV Industrial Services, 6 Offshore Technology, 4 StructuralDesign and 3 Fixed Offshore Installations

All GL Noble Denton Guidelines can be downloaded from http://www.gl-nobledenton.com/en/rules_guidelines.php





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APPENDIX A - INITIAL INFORMATION REQUIRED FOR APPROVAL

The following describes the initial information normally required for an approval of construction, towageor transportation and installation of a concrete gravity structure by GL Noble Denton. However it may

vary depending on the size and complexity of the project. Many items may need to be developed by,or agreed with, GL Noble Denton once sufficient information is available, especially when innovativeequipment or procedures are proposed.

A.1. GENERAL INFORMATION REQUIRED A.1.1 Where approval is required, the following shall be submitted to GL Noble Denton for information and/or

review, consisting of:

a. The project management structure, including contractors and subcontractors.

b. Proposed project drawing and document register including risk assessments and marinemanuals /procedures as in Sections 5.4 and 6 of 0001/ND, Ref. [1].

c. Proposed timetable of events.

A.2. STRENGTH, MOTION RESPONSE & STABILITY (ALL PHASES) A.2.1 Information to cover the topics in Sections 6, 7 and 8.

A.2.2 Information on weight control of weight-sensitive items as in Section 8 of 0001/ND, Ref. [1].

A.3. CONSTRUCTION IN DRY-DOCK OR BASIN (if applicable) A.3.1 Information to cover the topics in Section 10 of 0001/ND, Ref. [1].

A.4. MOORINGS FOR CONSTRUCTION AFLOAT (if applicable)

A.4.1 See Appendix A of 0032/ND, Ref. [7]

A.5. FLOAT-OUT & TOW TO INSHORE CONSTRUCTION SITE (if applicable) A.5.1 Information to cover the topics in Section 11 of 0001/ND, Ref. [1].

A.6. CONSTRUCTION / OUTFITTING AFLOAT (if applicable) A.6.1 Information to cover the topics in Section 9.

A.7. HEAVY LIFTS (if applicable) A.7.1 See Appendix A of 0027/ND, Ref. [4].

A.8. TEMPORARY BALLASTING AND COMPRESSED AIR SYSTEMS (if applicable) A.8.1 Information to cover the topics in Section 12 of 0001/ND, Ref. [1].

A.9. DECK MATING (if applicable) A.9.1 See Appendix A of 0031/ND, Ref. [6].

A.10. TOW /TRANSPORTATION TO INSTALLATION SITE A.10.1 See Appendix A of 0030/ND, Ref. [5].

A.11. INSTALLATION

A.11.1 Information to cover the topics in Section 11.

Documents

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