concrete offshore gravity structures

8/7/2019 CONCRETE OFFSHORE GRAVITY STRUCTURES

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TECHNICAL POLICY BOARD

www.gl-nobledenton.com

CONCRETE OFFSHORE GRAVITY STRUCTURESGUIDELINES FOR APPROVAL OF CONSTRUCTION AND

INSTALLATION

0015/ND

Once downloaded this document becomes UNCONTROLLED.Please check the website below for the current version.

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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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION

CONTENTSSECTION PAGE NO.

1 SUMMARY 5

2 INTRODUCTION 6 3 DEFINITIONS 7 4 THE APPROVAL PROCESS 10

4.1 General 10 4.2 GL Noble Denton Approval 10 4.3 Certificate of Approval 10 4.4 Scope of work leading to an approval 10 4.5 Technical studies 11 4.6 Surveys 12 4.7 Limitation of approval 12

5 HEALTH, SAFETY AND ENVIRONMENT 13

5.1 Introduction 13 5.2 Jurisdiction 13 5.3 Responsibilities 13 5.4 Risk Management 13 5.5 Qualification and Training 14 5.6 Safety Plan 14 5.7 Contingency and Emergency Planning and Procedures 14 5.8 Security and tracking system 14

6 ORGANISATION, PLANNING AND DOCUMENTATION 15 6.1 Introduction 15 6.2 Organisation and communication 15 6.3 Quality assurance and administrative procedures 15 6.4 Technical procedures 15 6.5 Technical documentation 16 6.6 Certification 16

7 WEATHER CRITERIA 17 7.1 Introduction 17 7.2 Metocean criteria 17 7.3 Weather restricted operations 19 7.4 Weather / Metocean forecast 20

8 WEIGHT CONTROL 21 8.1 Introduction 21 8.2 Weight Class 21

8.3 Reserves 21 8.4 Not-to-exceed weight 21 8.5 Weight control audits 21 8.6 Dimensional control 21

9 STRUCTURAL STRENGTH 22 9.1 Loadcases 22 9.2 Reinforced Concrete 22 9.3 Structural Steel 22 9.4 Compressed Air 23

10 MOTION RESPONSES 24 10.1 Purpose 24

10.2

Motion Response determination 24

10.3 Human Limit 24 11 STABILITY AND FREEBOARD 25

11.1 General 25

0015/ND Rev 3 Page 3


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11.2 Inclining tests 26 11.3 Intact stability and freeboard requirements 26 11.4 Damage stability and freeboard requirements 27

12 BALLASTING AND COMPRESSED AIR SYSTEMS 29 12.1 General 29 12.2 Redundancy 29 12.3 Inlets 29 12.4 Pipework 29 12.5 Umbilicals 29 12.6 Instrumentation 30 12.7 Air Cushion

13 BUILDING BASIN 31 14 TOW-OUT FROM DRY-DOCK 32

14.1 Mooring and handling lines 32 14.2 Underkeel clearance for leaving basin 32 14.3 Side Clearances 32

14.4 Underkeel Clearance Outside Basin 32 14.5 Towage and marine considerations 32

15 CONSTRUCTION AND OUTFITTING AFLOAT 33 15.1 Introduction 33 15.2 Structural and stability limitations 33 15.3 Construction spread 34

16 OTHER PHASES MOORINGS, DECK MATING, LIFTING & TOWAGES 35 16.1 Moorings 35 16.2 Deck mating 35 16.3 Lifting 35 16.4 Towages 35

17 INSTALLATION 36 17.1 General 36 17.2 Site location 36 17.3 Installation method 36 17.4 Positioning systems 37 17.5 Docking piles 37 17.6 Skirt penetration 37

REFERENCES 38

TABLESTable 7-1 Return Periods 17 Table 7-2 Value of JONSWAP , ratio of Tp:Tz and Tp:T1 for each integer value of K 18 Table 7-3 Seastate Reduction Factor 19 Table 9-1 Load Factors 22


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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 draft structures, where much of the construction work is carriedout afloat, as well as shallower draft structures where the construction of the Gravity Base Structure(GBS) can be essentially completed in dry dock. These guidelines do not apply to low freeboardstructures such as tunnel segments, dock gates, breakwater sections, etc. which should meet therequirements of GL Noble Denton 0030/ND Guidelines for Marine Transportations, Ref.[4].

1.2 These Guidelines are intended to lead to an approval by GL Noble Denton.1.3 This Revision 3 supersedes Revision 2 dated 31 March 2010 and the main changes are described in

Section 2.7.1.4 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

1.5 The following phases are covered in separate guidelines as follows: Towages of the GBS, completed platform, components or integrated decks in 0030/ND,

Guidelines for Marine Transportations Ref.[4]. Inshore or offshore deck mating in 0031/ND, Guidelines for Floatover Operations Ref.[5] Temporary moorings in 0032/ND, Guidelines for Moorings Ref [6]



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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 draft structures, wheremuch of the construction work is carried out afloat, as well as shallower draft structures where theconstruction of the Gravity Base Structure (GBS) can be essentially completed in dry dock. Theseguidelines 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.[4].

2.2 As each platform will differ in design, building location and destination, detailed recommendations willapply 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.

2.3

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.

2.4 This document covers specific aspects of the towages of the GBSs and completed platforms that areadditional to the requirements of 0030/ND Guidelines for Marine Transportations, Ref.[4]. Thetowage of components and integrated decks is also covered by 0030/ND.

2.5 Similarly deck-mating is covered in 0031/ND Guidelines for Floatover Operations, Ref.[5] andmoorings in 0032/ND Guidelines for Moorings, Ref.[6].

2.6 This report refers to, and should be read in conjunction with other GL Noble Denton Guidelinedocuments, particularly:a. 0013/ND Guidelines for Loadouts, Ref.[1]

b. 0021/ND Guidelines for the Approval of Towing Vessels, Ref.[2]c. 0027/ND Guidelines for Marine Lifting Operations, Ref.[3] d. 0030/ND Guidelines for Marine Transportations, Ref.[4]e. 0031/ND Guidelines for Floatover Operations , Ref.[5]f. 0032/ND Guidelines for Moorings, Ref.[6].

2.7 This Revision 3 supersedes Revision 2 dated 31 March 2010. Principal changes include: the addition of sections on definitions (Section3), the approval process (Section4), health,

safety and environment (Section5), or ganisation and documentation (Section6), weight control(Section8) and installation (Section17).

Mooring, deck-mating and towing are now covered in separate guidelines, Ref.[6], Ref. [5]andRef. [4]respectively.

2.8 Electronic versions of GL Noble Denton Guidelines are available onwww.gl-nobledenton.com . Careshould be taken when referring to any GL Noble Denton Guideline document that the latest revision isbeing consulted.

http://www.gl-nobledenton.com/http://www.gl-nobledenton.com/


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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.)

Benign area An area which is free of tropical revolving storms and travellingdepressions, (but excluding the North Indian Ocean during theSouthwest monsoon season and the South China Sea during theNortheast monsoon season). The specific extent and seasonallimitations of a benign area should be agreed with the GL NobleDenton office concerned.

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 an integrated topsides over a substructure,generally by floatover and ballasting. Deck mating may take placeinshore or offshore, onto a floating or a previously installedsubstructure.

DP Dynamic Positioning.

FMECA Failure Modes Effects and Criticality Analysis

Freeboard The height of the lowest downflooding point above the still water level.

GBS Gravity Base Structure.

GL Noble Denton Any company within the GL Noble Denton Group including anyassociated company which carries out the scope of work and issuesa Certificate of Approval, or provides advice, recommendations or designs as a consultancy service.

GM Initial metacentric height.

HAZID Hazard Identification Study

HAZOP 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 AnalysisLoadout The transfer of a major assembly or a module onto a barge, e.g. by

horizontal movement or by lifting.



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Term or Acronym Definition

Weather un-restrictedoperation

An operation with an operational reference period greater than thereliable limits of a favourable weather forecast (generally less than72 hours). The design weather conditions must reflect the statisticalextremes for the area and season.

The design weather is typically a 10 year seasonal storm, butsubject to Section7.2.2. An alternative concept is to consider conditions calculated to be exceeded during a number of simulatedoperations (see Section 6.5 of 0030/ND, Ref.[4]).

Working Load Limit (WLL) The maximum static load that the wire, cable or shackle is designedto withstand.


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4 THE APPROVAL PROCESS

4.1 GENERAL

4.1.1 GL Noble Denton may act as a Warranty Surveyor, giving Approval to a particular operation, or as aConsultant, providing advice, recommendations, calculations and/or designs as part of the Scope of Work. These functions are not necessarily mutually exclusive.

4.2 GL NOBLE DENTON APPROVAL4.2.1 GL Noble Denton means any company within the GL Noble Denton Group including any associated

company which carries out the scope of work and issues a Certificate of Approval.4.2.2 GL Noble Denton approval may be sought where an operation is the subject of an Insurance Warranty,

or where an independent third party review is required.4.2.3 An Insurance Warranty is a clause in the insurance policy for a particular venture, requiring the

approval of a marine operation by a specified independent survey house. The requirement is normallysatisfied by the issue of a Certificate of Approval. Responsibility for interpreting the terms of theWarranty so that an appropriate Scope of Work can be defined rests with the Assured.

4.3 CERTIFICATE OF APPROVAL4.3.1 The deliverable of the approval process will generally be a Certificate of Approval.4.3.2 The Certificate of Approval is the formal document issued by GL Noble Denton when, in its judgement

and opinion, all reasonable checks, preparations and precautions have been taken to keep risks withinacceptable limits, and an operation may proceed.

4.3.3 The Certificate of Approval for a marine operation such as float out, towage or installation will normallybe issued when all preparations including seafastening and ballasting are complete, marine equipmentsuch as tugs and towing connections have been inspected, a readiness meeting has been held, andthe actual and forecast weather are suitable for the operation to begin.

4.3.4 Agreement is required on the end-point of each Certificate of Approval.

4.4 SCOPE OF WORK LEADING TO AN APPROVALIn order to issue Certificates of Approval for a typical GBS, platform or structure, the topics GL NobleDenton will typically consider the following:

4.4.1 FLOAT OUT OF DRY-DOCKa. Moorings in dry dock after flooding and lift-off b. Tidal conditionsc. Loads, stresses and deflectionsd. Hydrostatic and stability characteristicse. Side and underkeel clearancesf. Handling and towing arrangements including wires, winches, tugs and connectionsg. Meteorological limitationsh. Systems, including power generation, ballast and compressed air systemsi. Tow route to inshore construction moorings or hand-over point j. Surveys of out-of-dock channel and tow routek. Connection into construction moorings, if appropriatel. Marine procedures.



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4.4.2 TOWAGESa. The towage route and surveys, side and underkeel clearancesb. The design metocean parameters for the towage route and seasonc. Motion responsesd. Resulting loads and stressese. Stabilityf. Systems, including power generation, ballast and compressed air systemsg. Towing resistance and required bollard pullh. Tug specificationi. Towing connections and arrangements j. Navigational equipmentk. Marine procedures.

4.4.3

CONSTRUCTION AFLOATa. Construction scheduleb. Meteorological criteria for mooring designc. Loads, safety factors and redundancy of mooring systemd. Drafts and underkeel clearances in all conditionse. Structural strength at all stages of constructionf. Stability at all stages of constructiong. Marine spread requirementsh. Lifting and installation of equipment and sub-assemblies.

4.4.4 INSTALLATIONa. Details and survey reports of installation locationb. Installation methodc. Loads and stresses during installationd. Stabilitye. GBS ballasting system and limitationsf. Mooring, positioning and handling systemsg. Installation procedures.

4.5 TECHNICAL STUDIES4.5.1 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.



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5 HEALTH, SAFETY AND ENVIRONMENT

5.1 INTRODUCTION

5.1.1 This Section contains general guidelines on safety and emergency issues. This subject must bemanaged in accordance with local jurisdiction, as well as appropriate guidelines and specificationsregarding health, safety and the environment (HSE).

5.2 JURISDICTION5.2.1 In addition to Approval by GL Noble Denton, construction and marine operations will be subject to

national and international regulations and standards on personnel safety and protection of theenvironment. It should also be noted that a marine operation may involve more than one nations areaof jurisdiction, and that for barges and vessels the jurisdiction of the flag state will apply.

5.2.2 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.

5.3 RESPONSIBILITIES5.3.1 Normally, the owner of the structure will have the overall responsibility for planning and execution of

the construction and marine operations, and hence also the safety of personnel, facilities andenvironment. The owner may delegate the execution and follow-up of these issues to the contractor.

5.3.2 If a part of the marine operations is to be carried out near other facilities or their surroundings, safetyzones shall be defined by the owner.

5.4 RISK MANAGEMENT5.4.1 Risk management should be applied to the project to reduce the effects of hazards and to limit the

overall risk. The preferred approach may be achieved by addressing the following functions:

a. Identification of potential hazardsb. Preventative measures to avoid hazards wherever possiblec. Controls to reduce the potential consequences of unavoidable hazardsd. Mitigation to reduce the impact of risk, should hazards occur.

5.4.2 Each major marine operation should be subject to detailed hazard studies. Those taking part shouldinclude personnel and organisations involved in the design of structures and systems, as well as thoseinvolved in the marine operation.

5.4.3 Each major system, essential to the performance and safety of marine operations, including, for example, the power generation, ballast and compressed air systems, should be subjected to a rigorousstudy. Those taking part should include personnel and organisations involved in the marine

operations, as well as those involved in the design and operation of the system.5.4.4 A variety of techniques including HAZID, HAZOP, FMECA, HIRA, JSA and Tool Box Talks may be

used as appropriate to monitor and control such hazards and to model their potential effects.5.4.5 QRA techniques may be used to compute the particular level of risk applicable to any operation, to

compare levels of risk between alternative proposals or between known and novel methods, and toenable rational choices to be made between alternatives.

5.4.6 Ideally, each of the various studies outlined above should be managed by a competent independentperson familiar with the overall concept, but outside the team carrying out the relevant system or structure design or operational management.



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5.5 QUALIFICATION AND TRAINING5.5.1 Operation supervisors shall possess thorough knowledge and have experience from similar

operations. Other key personnel shall have knowledge and experience within their area of responsibility. Before the start of an operation, all personnel involved shall be briefed by the

supervisors regarding responsibilities, communication, safety, and step by step procedures and tasks.5.5.2 Adequate training appropriate to each individuals function and situation should be given, including job

training, site safety training and briefings, marine safety and survival training.5.5.3 Fire and evacuation alarms should be periodically tested, and drills should carried out periodically, or

as required by safety legislation.5.5.4 Computer simulation and training, and/or model tests may give valuable information for the personnel

carrying out the operation.

5.6 SAFETY PLAN5.6.1 A safety plan shall be included in the procedure manual for each operation. This plan consists of the

safety rules that apply to minimise the risks encountered during each operation:

a. Risks inherent from the metocean conditionsb. Risks incurred by construction, transportation, installation and commissioning activitiesc. Risks to the environment.

5.7 CONTINGENCY AND EMERGENCY PLANNING AND PROCEDURES5.7.1 Contingency and emergency planning should form part of the operational procedures. Plans should be

developed for all foreseeable emergencies, which may include:a. Severe weather b. Planned precautionary action in the event of forecast severe weather c. Structural parameters approaching pre-set limitsd. Stability parameters approaching pre-set limitse. Failure of mechanical, electrical or control systemsf. Fireg. Collisionh. Leakagei. Pollution j. Structural failurek. Mooring failurel. Human error

m.

Man overboardn. Personnel accidents or medical emergencies.

5.7.2 The procedures should detail alarm signals, reporting, communication, organisation and requiredequipment such as escape routes, personnel rescue means and fire fighting equipment.

5.8 SECURITY AND TRACKING SYSTEM5.8.1 A suitable security and tracking system should be in use to record personnel on the structure, to track

their whereabouts, and if required, to restrict access to certain areas to authorised personnel only.5.8.2 The contractor is responsible for accident reporting to the owner and regulatory bodies. Any incidents,

accidents or near-misses relevant to the safety of the structure or future marine operations shall alsobe reported to GL Noble Denton.



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7 WEATHER CRITERIA

7.1 INTRODUCTION

7.1.1 This Section refers to the metocean criteria applicable for marine operations.7.1.2 It may be impractical and/or uneconomic to design marine operations for extreme conditions.

However, if the operational design levels are set too low the waiting time for acceptable conditionscould be excessive.

7.1.3 A set of limiting metocean criteria should be established dependent on the duration of each discreetoperation, which may be a weather restricted or a weather unrestricted operation as defined inSection 3. Reference may be made to ISO 19901-1 "Metocean design and operationalconsiderations", Ref.[7] for weather unrestricted operations. For weather restricted operations seeSections starting with7.3.1.

7.2 METOCEAN CRITERIA

7.2.1 DESIGN AND OPERATIONAL CRITERIA7.2.1.1 For each specific phase of marine construction and critical operations in a marine operation, the design

and operational metocean criteria shall be defined.

7.2.2 RETURN PERIODS7.2.2.1 The return periods to be considered should be related to the duration of the marine operation. As

general guidance, the following criteria may be applied provided that the independent extremes areconsidered concurrently:

Table 7-1 Return Periods

Duration of use Design environmental criteria

Up to 3 days Specific weather window3 days to 1 month 10 year retur n, seasonal, or using a reduced exposure computation

(see Section 3) with minimum of 1 year return, seasonal

1 month to 1 year 50 year return, seasonal *

More than 1 year 50 year return, all year *

* If conditions have been determined using the joint probability of different parameters, then the 50year return period should be increased to 100 years.

7.2.2.2 Directionality of the environment should be considered.

7.2.3 WIND7.2.3.1 The design wind speed shall generally be the 1 minute mean velocity at a reference height of 10m

above sea level. A longer or shorter averaging period may be used for design depending upon thenature of the operation, the structure involved and the response characteristics of the structure to wind.

7.2.3.2 The relationship between wind velocity at the reference and actual elevations above sea level andbetween different averaging periods can be found in ISO 19901-1 "Metocean design and operationalconsiderations", Ref.[7].

7.2.4 WAVES7.2.4.1 The maximum design wave shall be the most probable highest individual wave in the design seastate,

assuming an exposure of 3 hours. The determination of the height, period and crest elevation of themaximum wave should be determined from an appropriate higher-order wave theory and account for shallow water effects.



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7.2.4.2 Seastates shall include all relevant spectra up to and including the design storm seastate for theconstruction site or towage route. Long-crested seas shall be considered unless there is a justifiablebasis for using short-crested seas. Consideration should be given to the choice of spectrum.

7.2.4.3 In the simplest method the peak period (Tp) for all seastates considered, should be varied as:

(13.Hsig) < Tp < (30.Hsig)where Hsig is in metres, Tp in seconds. The effects of swell should also be considered if not alreadycovered in this peak period range.However, this method incorrectly assumes that all periods are equally probable. As a result thismethod should generally produce higher design accelerations than would be the case when using themore robust Hsig-Tp method described in the following section.

7.2.4.4 In the alternative method, a contour is constructed within the Hsig-Tp plane that identifies equallyprobable combinations of Hsig & Tp for the design return period subject to theoretical constraints onwave breaking. This contour should also cover swell. The combinations should be tested in motionresponse calculations to identify the worst case response.

7.2.4.5 The relationship between the peak period Tp and the zero-up crossing period Tz is dependent on thespectrum. For a mean JONSWAP spectrum (=3.3) Tp/Tz = 1.286; for a Pierson-Moskowitz spectrum(=1) Tp/Tz = 1.41.

7.2.4.6 The followingTable 7-2 indicates how the characteristics of the JONSWAP wave energy spectrumvary over the range of recommended seastates. The constant, K, varies from 13 to 30 as shown in theequation in Section7.2.4.3 above. T1 is the mean period (also known as Tm).

Table 7-2 Value of JONSWAP , ratio of Tp:Tz and Tp:T1 for each integer value of K

Constant K Tp /Tz Tp /T1 Constant K Tp /Tz Tp /T1

13 5.0 1.24 1.17 22 1.4 1.37 1.27

14 4.3 1.26 1.18 23 1.3 1.39 1.28

15 3.7 1.27 1.19 24 1.1 1.40 1.29

16 3.2 1.29 1.20 25 1.0 1.40 1.29

17 2.7 1.31 1.21 26 1.0 1.40 1.29

18 2.4 1.32 1.23 27 1.0 1.40 1.29

19 2.1 1.34 1.24 28 1.0 1.40 1.29

20 1.8 1.35 1.25 29 1.0 1.40 1.29

21 1.6 1.36 1.26 30 1.0 1.40 1.29

7.2.4.7 For operations involving phases sensitive to extreme sea states, such as temporary on-bottom stabilityor green water assessment, the maximum wave height and associated period should be used.7.2.4.8 For precise operations sensitive to small fluctuations of the sea level even under calm sea state

conditions, the occurrence of long period, small amplitude swell on the site should be checked.7.2.4.9 Attention should also be paid to particular site conditions prone to current acting against the waves

which could amplify the steepness of the sea state.

7.2.5 CURRENT7.2.5.1 The design current shall be the rate at mean spring tides, taking account of variations with depth and

increases caused by the design storm and storm surge.7.2.5.2 Currents may be divided into two different categories:

a. Tidal currentsb. Residual currents that remain when the tidal component is removed, including river outflows,

surge, wind drift, loop and eddy currents.



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7.2.5.3 Tidal currents may be predicted reliably, subject to long term measurement (at least one completelunar cycle at the same season of the year as the actual planned operation). Residual currents mayonly be reliably predicted or forecast using sophisticated mathematical models.

7.2.6 OTHER PARAMETERS7.2.6.1 Other factors including the following may be critical to the design or operations and should be

addressed:a. Water level including tide and surgeb. Sea icing, icing on superstructurec. Exceptionally low temperatured. Large temperature differencese. Water density and salinityf. Bad visibility.

7.3 WEATHER RESTRICTED OPERATIONS7.3.1 GENERAL7.3.1.1 Weather restricted operations (as defined in Section3) should be planned using reliable time history

data that not only indicates the probability of not exceeding the selected or limiting design criteria, butalso the persistence of such conditions for the season considered. In the more mature zones thisinformation will be available, but in other areas it can be difficult to obtain or may not be available. Thequality of data available will influence the assessment of overall risk.

7.3.2 IMPACT ON DESIGN7.3.2.1 During design the following should be considered:

a. Measures to speed up the operation and provide more margin on the weather windowb. Re-design the operation limitations to sustain higher metocean conditionsc. Contingency situations, and back-up and stand-by measuresd. Whether delays to previous activities could push the operation into an unfavourable season.

7.3.3 MARGINS ON WEATHER7.3.3.1 To undertake any operation, the operation criteria shall be less than the design criteria. The margin

is a matter of judgement, dependent on factors specific to each case.7.3.3.2 Unless agreed otherwise with GL Noble Denton, for marine operations with an operational duration of

no more than 24 hours the maximum forecast seastate shall not exceed the design seastate multipliedby the applicable factor fromTable 7-3 below. For operations with other durations alternative factorsapply and should be agreed with GL Noble Denton. The forecast wind and current shall be similarlyconsidered when their effects on the operation or structure are significant.

Table 7-3 Seastate Reduction Factor

Weather Forecast Provision (see Section 7.4) Reduction Factor

No project-specific forecast (in emergencies only) 0.50

One project-specific forecast source 0.65

One project-specific forecast source plus in-field wave monitoring (waverider buoy) 0.70

One project-specific forecast source plus in-field wave monitoring andoffshore meteorologist 0.75



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9 STRUCTURAL STRENGTH

9.1 LOADCASES

9.1.1 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.

9.1.2 The unit shall be able to withstand a static heel angle of 10 degrees. If it has damage stability the unitshall also be able to withstand the static and dynamic loads caused by the flooding of any onecompartment in a 25 metres/second wind and associated waves. These would be regarded asextreme (ultimate limit state) conditions.

9.1.3 Hydrostatic loads on the substructure at the deepest draft 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.

9.1.4 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.

9.2 REINFORCED CONCRETE9.2.1 The strength of concrete during floating phases shall comply with a recognised concrete design code.9.2.2 Stresses in structural steelwork shall be within those permitted by the latest edition of a recognised and

applicable offshore structures code.9.2.3 Any time-dependent properties of the material shall be taken into account.

9.3 STRUCTURAL STEEL9.3.1 The primary structure shall be of high quality structural steelwork with full material certification and

NDT inspection certificates showing appropriate levels of inspection. It shall be assessed using themethodology of a recognised and applicable offshore code including the associated load andresistance factors for LRFD codes or safety factors for ASD/WSD codes.

9.3.2 Traditionally AISC has also been considered a reference code. If the AISC 13th edition is used, theallowables shall be compared against member stresses determined using a load factor on both deadand live loads of no less than those detailed in the followingTable 9-1.

Table 9-1 Load Factors

Type WSD option LRFD Option

SLS: 1.00 1.60

ULS: 0.75 1.20

9.3.3 Any load case may be treated as a normal serviceability limit state (SLS) / Normal operating case.9.3.4 The infrequent load cases, generally limited to survival and damaged cases, may be treated as

ultimate limit state (ULS) / Survival storm cases. This does not apply to: Steelwork subject to deterioration and/or limited initial NDT unless the condition of the entire

loadpath has been verified, for example the underdeck members of a barge or vessel. Steelwork subject to NDT prior to elapse of the recommended cooling and waiting time as

defined by the Welding Procedure Specification (WPS) and NDT procedures. In cases wherethis cannot be avoided by means of a suitable WPS, it may be necessary to increase thestrength or impose a reduction on the design/permissible seastate.



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10 MOTION RESPONSES

10.1 PURPOSE

10.1.1 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 siteb. All stages of construction afloatc. Deck mating operationsd. Towage to offshore site, at all likely towage draftse. Installation at final locationf. Any condition where the motions may be critical to loss of freeboard, stability, position keeping

or other considerations.

10.2 MOTION RESPONSE DETERMINATION10.2.1 The guidance of Sections 7.7, 7.8 of GL Noble Denton 0030/ND Guidelines for Marine

Transportations, Ref.[4]shall be applied.10.2.2 The motion responses should be determined for all relevant headings, and at speed zero for towage

cases. The maximum responses should be based on a 3 hour exposure period.

10.3 HUMAN LIMIT10.3.1 If the structure is manned for towage or installation, care be should taken to avoid accelerations in

excess of about 0.2g except for occasional short periods, to enable personnel to carry out their dutiesefficiently.



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11 STABILITY AND FREEBOARD

11.1 GENERAL

11.1.1 This Section expands on the stability section of 0030/ND Guidelines for Marine TransportationsRef. [4], and is intended to cover Condeep-type gravity structures (see Section2.1).11.1.2 Sufficient positive stability and reserve buoyancy shall be ensured during all stages of the marine

operations. Both intact and damage stability shall be evaluated, on the basis of an accurate geometricmodel.

11.1.3 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.

11.1.4 The output of the weight control programme as described in Section8, shall be taken into account.11.1.5 Stability calculations should include corrections and allowances for:

a. Free surfaceb. Air cushionc. Icingd. Influence of moorings, including a check on the consequences of failure.

11.1.6 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.

11.1.7 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 seab. Compartments inside the structure, crossed by seawater filled pipesc. Skirt compartments containing compressed air.

11.1.8 Special attention should be paid to flooding which may be caused by:a. Impact loads from vesselsb. Damage to structure or pipework from dropped objectsc. Mechanical system failured. Human error.

11.1.9

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.11.1.10 Flooding as a result of vessel impact is assumed to occur in a zone bounded by two horizontal planes

normally positioned 5 metres above and 8 metres below the waterline. These levels should bereviewed if deep draft vessels are likely to be operating nearby.

11.1.11 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 strengthc. Providing additional protection, such as fenderingd. Minimising vessel movements near the structuree. Dedicated procedures and experienced personnel.



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11.1.12 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.

11.2 INCLINING TESTS11.2.1 Inclining tests should normally be performed at different stages during construction afloat, particularly

prior to any marine operation where the displacement, centre of gravity or stability may be critical.11.2.2 The output from the inclining test should be used to check and calibrate the output from the weight

control programme.11.2.3 Normally inclining tests should be performed:

a. Along two axes if the platform is not axi-symmetricalb. For each direction in several steps out to maximum inclination and back, and with positive and

negative inclinations.

11.2.4 A sensitivity analysis of the parameters affecting the test results should be performed.11.2.5 Procedures for the test should be developed considering:

a. The inclining angle should be sufficient to obtain an adequate degree of accuracyb. The inclining angles should be measured by at least 2 independent devicesc. The preferred method of inclining is by shifting weights, without changing displacementd. The preferred draught is such that the stability is minimum and the waterline intersects the

structure in a vertical wall-sided areae. A detailed assessment of the state of construction and amount of temporary equipment and

materials 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.

11.3 INTACT STABILITY AND FREEBOARD REQUIREMENTS

11.3.1 INTACT STABILITY11.3.1.1 The initial GM shall be positive, normally not less than 0.5.m (after allowing for all possible

inaccuracies 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 Section7.2. Calculation of maximuminclination should take into account:a. Maximum amplitude of pitch or roll motion in the design seastate, plusb. Inclination due to design wind, plusc. Inclination due to mooring line tensions or required towline pull.

11.3.1.2 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 inclinationresulting from towline pull only by not more than 1 degree.



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

11.4.3 DAMAGE STABILITY FOR TOWAGE TO OFFSHORE SITE AND INSTALLATION

11.4.3.1 When towing on the base or columns the platform shall possess one compartment damage stability.11.4.3.2 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 Section11.1.10, to withstandimpact from the largest towing or attending vessel, and/or

b. Rigorous procedures shall be developed to minimise the risk of flooding, andc. A risk assessment of flooding shall be carried out, and the results agreed as acceptable by GL

Noble Denton.

11.4.3.3 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 to the reinforced waterline should the installation operation be aborted.

11.4.4 FREEBOARD11.4.4.1 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, allaround the structure, under the design storm loading from the most critical direction.

11.4.5 STRUCTURAL LIMITATIONS11.4.5.1 The platform shall be demonstrated to meet the requirements of Section9.1.2.



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12 BALLASTING AND COMPRESSED AIR SYSTEMS

12.1 GENERAL

12.1.1 Regardless of any requirement to change draft during construction, towage or installation operations,floating concrete structures shall be fitted with a means of pumping out water from all compartments.12.1.2 All ballasting procedures shall be reversible so that, in case of emergency, the unit can be returned to

a safe draft within 24 hours.

12.2 REDUNDANCY12.2.1 The design of the ballasting system shall be such that the failure of any one valve to open or close, or

the fracture of any pipe, will not cause flooding of the unit, or failure to flood when required.12.2.2 All remotely controlled valves shall be capable of operation by a secondary, preferably manual system.

Any automatic or radio controlled system shall have a manual override system.12.2.3 The secondary valve operation system may be by ROV, provided that ROV access and a suitable

ROV are available at all stages of the operation.12.2.4 Sufficient redundancy of pumps, power supplies, cross-over pipework and instrumentation shall be

provided so that essential operations can continue in the event of failure of any one component or system.

12.3 INLETS12.3.1 All internal and external inlets shall be adequately protected to prevent damage by entering debris and

cables. All internal compartments must be cleaned of debris before immersion or towage starts.12.3.2 Except when in use for inlet or discharge, all openings to sea shall be protected by a double barrier.12.3.3 Any external valves and pipework shall be protected against collision and fouling by towlines, mooring

lines or handling wires.

12.4 PIPEWORK12.4.1 All essential pipework shall be of permanent-type construction and hydrostatically tested to a minimum

of 1.3 times the design pressure.12.4.2 Temporary flexible hoses are not generally permitted. Where their use cannot be avoided, for instance

for supply of back-up compressed air from a compressor barge alongside, then a risk assessment shallbe carried out to demonstrate the acceptability of the system and submitted for GL Noble Dentonapproval.

12.5 UMBILICALS12.5.1 If umbilicals are necessary to provide power and/or hydraulic services during any marine operation,

adequate back-up capability shall be provided, and fail-safe systems shall be incorporated into criticalcontrols.



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12.6 INSTRUMENTATION12.6.1 Adequate instrumentation shall be installed to measure:

a. The water level in all compartments, quantity and percentageb. Status of all valvesc. Pump status and flow ratesd. Main and emergency power supply statuse. Platform draft, heel and trimf. Compartment air pressureg. Compressor statush. Air cushion pressurei. Water seal level in skirt compartments j. Status of access doors and manholes.

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

12.7 AIR CUSHION12.7.1 The following recommendations apply to the use of underskirt compressed air:

a. All piping shall be secure, protected and of adequate capacity and strengthb. Supply lines shall have non-return valvesc. Back-ups shall be provided for all critical valves and pipingd. Adequate reserve compressors shall be onboarde. A venting system shall be provided to guarantee that all air is removed after use, to ensure no

residual free surface remainsf. Sufficient water seal (bottom of air cushion above bottom of skirt) shall be available to prevent

air escapingg. The air cushion should be isolated in separate compartments, so that failure of any part of the

system does not cause a large heel or trim in addition to loss of buoyancy.



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

15.3 CONSTRUCTION SPREAD

15.3.1

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 equipmentb. Concrete mixing plantc. Temporary power supplyd. Temporary ballast controle. Officesf. Workshopsg. Personnel reception area and securityh. Berthing and unloading area for ferries, transport barges and vessels

i. Safety and emergency facilities.15.3.2 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.15.3.3 The mooring and fendering system for each item of the spread should be designed in accordance with

the requirements of 0032/ND, Ref [6]. 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.

15.3.4 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.

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

15.3.6 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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16 OTHER PHASES MOORINGS, DECK MATING, LIFTING & TOWAGES

16.1 MOORINGS

16.1.1 See 0032/ND Guidelines for Moorings, Ref [6]

16.2 DECK MATING

16.2.1 See 0031/ND Guidelines for Floatover Installations, Ref. [5].

16.3 LIFTING

16.3.1 See 0027/ND Guidelines for Marine Lifting Operations, Ref.[3].

16.4 TOWAGES

16.4.1 See 0030/ND Guidelines for Marine Transportations, Ref.[4].



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17 INSTALLATION

17.1 GENERAL

17.1.1 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 platformb. Water depthc. The positioning tolerances required in all 6 degrees of freedomd. The positioning / station-keeping system proposede. Whether the operation involves docking over a template, docking piles or other structuresf. Stability at all stages of immersiong. Whether a vertical or inclined installation is requiredh. Tolerances on differential ballast levels

i. The skirt design, and penetration method j. Whether under-base grouting is requiredk. Whether solid ballast or scour protection is required.

17.2 SITE LOCATION17.2.1 The position of the site location shall be given in both geographical and grid coordinates.17.2.2 The water depth and bathymetric tolerances shall be determined.17.2.3 When determining the extent of the survey area, the following shall be accounted for:

a. Tolerances on site survey positionb. Inaccuracy of position monitoring systems during installationc. Operational tolerancesd. The approach corridor e. Whether a holding location is required close to the sitef. Whether an inclined installation, with previous off-site touch-down is requiredg. The proximity of any other platforms or subsea assets at or near the location.

17.2.4 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.

17.2.5 The seabed and sub-seabed conditions shall be established by coring, magnetometer, in-situ testing,

lab testing and sub-bottom profiling.17.2.6 Current surveys should be carried out.

17.3 INSTALLATION METHOD17.3.1 The overall installation method will depend on the configuration of the platform.



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17.3.2 In general, shallow draft 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 be used. Sometimes it will be necessary to touch down on one edge toachieve 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 inclinedconditionb. Monitoring of ballast levels, and allowable differential levelsc. Structural capacity of the skirt at touch down, and possible impact loads imposedd. Skirt touch down, if on the final site, may disturb the seabed, and prejudice the final skirt

penetration or base slab bearinge. If the skirt touch down is on the final site, accurate position control may be difficult in the

inclined conditionf. If skirt touch down is remote from the final site, the deballast capability required by Section12.1

will be used.

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

17.4 POSITIONING SYSTEMS17.4.1 The positioning system will normally be by means of the tug fleet. This will often consist of the tow

fleet, rearranged into a star configuration.17.4.2 Where the position and orientation tolerances are not critical, the tugs may be in free floating

configuration.17.4.3 Where more precise positioning is required, the tugs will probably be connected at the bow to pre-laid

anchors.

17.5 DOCKING PILES17.5.1 If docking piles are used for positioning, then suitable analyses shall be presented to confirm the

dynamics 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.

17.6 SKIRT PENETRATION17.6.1 Calculations shall be presented to demonstrate that the skirts will penetrate adequately under the

applied loads, whether gravity loads only are required, or whether negative pressure needs to beapplied.

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

17.6.3 Design of the pipework should take into account the requirements for removal on decommissioning.17.6.4 Structural loads on the skirts shall be shown to be acceptable.17.6.5 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.



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REFERENCES

[1] GL Noble Denton 0013/ND Guidelines for Loadouts.[2] GL Noble Denton 0021/ND Guidelines for the Approval of Towing Vessels.[3] GL Noble Denton 0027/ND Guidelines for Marine Lifting Operations.[4] GL Noble Denton 0030/ND Guidelines for Marine Transportations.[5] GL Noble Denton 0031/ND Guidelines for Floatover Operations[6] GL Noble Denton 0032/ND Guidelines for Moorings.[7] ISO 19901-1 Petroleum and Natural Gas Industries Specific requirements for offshore structures Part 1:

Metocean Design and Operational Considerations".[8] ISO 19901-5 Petroleum and Natural Gas Industries Specific requirements for offshore structures Part 5:

Weight control during engineering and construction.

[9] World Meteorological Organisation, Marine meteorology and related oceanographic activities, Report no 36,Handbook of offshore forecasting services, prepared by The Offshore Weather Panel, WMO/TD-NO 850,1998.

All GL Noble Denton Guidelines can be downloaded fromwww.gl-nobledenton.com
http://www.nobledenton.com/http://www.nobledenton.com/http://www.nobledenton.com/

concrete offshore gravity structures

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