CONCRETE OFFSHORE GRAVITY STRUCTURES

TECHNICAL POLICY BOARD

CONCRETE OFFSHORE GRAVITY STRUCTURES GUIDELINES 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


16 Dec 08

1

RLJ


05 Oct 87

0

RLJ


Date

Revision

Prepared by

Authorised by

www.gl-nobledenton.com

CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION

PREFACE This document has been drawn with care to address what are likely to be the main concerns based on the experience of the GL Noble Denton organisation. This should not, however, be taken to mean that this document deals comprehensively with all of the concerns which will need to be addressed or even, where a particular matter is addressed, that this document sets out the definitive view of the organisation for all situations. In using this document, it should be treated as giving guidelines for sound and prudent practice on which our advice should be based, but guidelines should be reviewed in each particular case by the responsible person in each project to ensure that the particular circumstances of that project are addressed in a way which is adequate and appropriate to ensure that the overall advice given is sound and comprehensive. Whilst great care and reasonable precaution has been taken in the preparation of this document to ensure that the content is correct and error free, no responsibility or liability can be accepted by GL Noble Denton for any damage or loss incurred resulting from the use of the information contained herein.

© 2010 Noble Denton Group Limited, who will allow:  the document to be freely reproduced,  the smallest extract to be a complete page including headers and footers but smaller extracts may be reproduced in technical reports and papers, provided their origin is clearly referenced.

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CONTENTS SECTION 1 2 3 4

5

6

7

8

9

10

11

SUMMARY INTRODUCTION DEFINITIONS THE APPROVAL PROCESS 4.1 General 4.2 GL Noble Denton Approval 4.3 Certificate of Approval 4.4 Scope of work leading to an approval 4.5 Technical studies 4.6 Surveys 4.7 Limitation of approval HEALTH, SAFETY AND ENVIRONMENT 5.1 Introduction 5.2 Jurisdiction 5.3 Responsibilities 5.4 Risk Management 5.5 Qualification and Training 5.6 Safety Plan 5.7 Contingency and Emergency Planning and Procedures 5.8 Security and tracking system ORGANISATION, PLANNING AND DOCUMENTATION 6.1 Introduction 6.2 Organisation and communication 6.3 Quality assurance and administrative procedures 6.4 Technical procedures 6.5 Technical documentation 6.6 Certification WEATHER CRITERIA 7.1 Introduction 7.2 Metocean criteria 7.3 Weather restricted operations 7.4 Weather / Metocean forecast WEIGHT CONTROL 8.1 Introduction 8.2 Weight Class 8.3 Reserves 8.4 Not-to-exceed weight 8.5 Weight control audits 8.6 Dimensional control STRUCTURAL STRENGTH 9.1 Loadcases 9.2 Reinforced Concrete 9.3 Structural Steel 9.4 Compressed Air MOTION RESPONSES 10.1 Purpose 10.2 Motion Response determination 10.3 Human Limit STABILITY AND FREEBOARD 11.1 General

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PAGE NO. 5 6 7 10 10 10 10 10 11 12 12 13 13 13 13 13 14 14 14 14 15 15 15 15 15 16 16 17 17 17 19 20 21 21 21 21 21 21 21 22 22 22 22 23 24 24 24 24 25 25 Page 3

CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION 11.2 Inclining tests 11.3 Intact stability and freeboard requirements 11.4 Damage stability and freeboard requirements 12 BALLASTING AND COMPRESSED AIR SYSTEMS 12.1 General 12.2 Redundancy 12.3 Inlets 12.4 Pipework 12.5 Umbilicals 12.6 Instrumentation 12.7 Air Cushion 13 BUILDING BASIN 14 TOW-OUT FROM DRY-DOCK 14.1 Mooring and handling lines 14.2 Underkeel clearance for leaving basin 14.3 Side Clearances 14.4 Underkeel Clearance Outside Basin 14.5 Towage and marine considerations 15 CONSTRUCTION AND OUTFITTING AFLOAT 15.1 Introduction 15.2 Structural and stability limitations 15.3 Construction spread 16 OTHER PHASES – MOORINGS, DECK MATING, LIFTING & TOWAGES 16.1 Moorings 16.2 Deck mating 16.3 Lifting 16.4 Towages 17 INSTALLATION 17.1 General 17.2 Site location 17.3 Installation method 17.4 Positioning systems 17.5 Docking piles 17.6 Skirt penetration REFERENCES

26 26 27 29 29 29 29 29 29 30 30 31 32 32 32 32 32 32 33 33 33 34 35 35 35 35 35 36 36 36 36 37 37 37 38

TABLES Table 7-1 Table 7-2 Table 7-3 Table 9-1

17 18 19 22

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Return Periods Value of JONSWAP γ, ratio of Tp:Tz and Tp:T1 for each integer value of K Seastate Reduction Factor Load Factors

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1

SUMMARY

1.1

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 towed on their own buoyancy, both deeper draft structures, where much of the construction work is carried out 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 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]. These Guidelines are intended to lead to an approval by GL Noble Denton. This Revision 3 supersedes Revision 2 dated 31 March 2010 and the main changes are described in Section 2.7. 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

1.2 1.3 1.4

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

INTRODUCTION

2.1

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 to be applicable to structures that are towed on their own buoyancy, both deeper draft structures, where much of the construction work is carried out 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 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]. 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 project development and it is emphasised that discussions with GL Noble Denton at an early stage would be desirable. These Guidelines are intended to lead to an approval by GL Noble Denton. Such approval does not imply that compliance with national codes and legislation, or the requirements of other regulatory bodies, harbour authorities and/or any other parties would be given. This document covers specific aspects of the towages of the GBSs and completed platforms that are additional to the requirements of 0030/ND “Guidelines for Marine Transportations”, Ref. [4]. The towage of components and integrated decks is also covered by 0030/ND. Similarly deck-mating is covered in 0031/ND “Guidelines for Floatover Operations”, Ref. [5] and moorings in 0032/ND “Guidelines for Moorings, Ref. [6]. This report refers to, and should be read in conjunction with other GL Noble Denton Guideline documents, 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].

2.2

2.3

2.4

2.5 2.6

2.7

This Revision 3 supersedes Revision 2 dated 31 March 2010. Principal changes include: 

the addition of sections on definitions (Section 3), the approval process (Section 4), health, safety and environment (Section 5), organisation and documentation (Section 6), weight control (Section 8) and installation (Section 17).

 2.8

Mooring, deck-mating and towing are now covered in separate guidelines, Ref. [6], Ref. [5] and Ref. [4] respectively. Electronic versions of GL Noble Denton Guidelines are available on www.gl-nobledenton.com. Care should be taken when referring to any GL Noble Denton Guideline document that the latest revision is being consulted.

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3

DEFINITIONS

3.1

Referenced definitions are underlined.

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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 be considered to include vessel or ship where appropriate.)

Benign area

An area which is free of tropical revolving storms and travelling depressions, (but excluding the North Indian Ocean during the Southwest monsoon season and the South China Sea during the Northeast monsoon season). The specific extent and seasonal limitations of a benign area should be agreed with the GL Noble Denton office concerned.

Certificate of Approval

A formal document issued by GL Noble Denton stating that, in its judgement and opinion, all reasonable checks, preparations and precautions 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 perform marine 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 place inshore or offshore, onto a floating or a previously installed substructure.

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 any associated company which carries out the scope of work and issues a 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 the approval of a marine operation by a specified independent survey house..

JSA

Job Safety Analysis

Loadout

The transfer of a major assembly or a module onto a barge, e.g. by horizontal movement or by lifting.

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION Term or Acronym

Definition

MBL

Certified Minimum Breaking Load (of a wire).

Operational reference period

The planned duration of the operation, including a contingency period.

Platform

The completed platform after deck mating, consisting of the GBS and Topsides.

PNR

Point of No Return - the last point in time, or a geographical point along a route, at which an operation could be aborted or returned to a safe condition.

QRA

Quantified Risk Analysis

Reduced exposure operation

An unrestricted operation which has an exposure sufficiently short (typically under 30 days) for the design extremes to be reduced. The design extremes may be reduced below the 10 year seasonal return, to give the same probability of encounter as a 30 day exposure to a 10 year seasonal storm. In this case the “adjusted” design extremes are defined in terms of the 10% risk level (see Section 6.5 of 0030/ND, Ref. [4].

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 Class surveys.

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 Class survey.

SWL

Safe Working Load in tonnes. (see also Working Load Limit)

Vessel

A marine craft designed for the purpose of transportation by sea. See Barge.

Weather restricted operation

A marine operation which can be completed within the limits of an operational reference period with a favourable weather forecast (generally less than 72 hours), taking contingencies into account. The design environmental condition need not reflect the statistical extremes for the area and season. A suitable factor should be applied between the design weather conditions and the operational weather limits (see Section 7.3.3)

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION Term or Acronym

Definition

Weather un-restricted operation

An operation with an operational reference period greater than the reliable limits of a favourable weather forecast (generally less than 72 hours). The design weather conditions must reflect the statistical extremes for the area and season. The design weather is typically a 10 year seasonal storm, but subject to Section 7.2.2. An alternative concept is to consider conditions calculated to be exceeded during a number of simulated operations (see Section 6.5 of 0030/ND, Ref. [4]).

Working Load Limit (WLL)

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The maximum static load that the wire, cable or shackle is designed to 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 a Consultant, 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 APPROVAL

4.2.1

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. 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. 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 normally satisfied by the issue of a Certificate of Approval. Responsibility for interpreting the terms of the Warranty so that an appropriate Scope of Work can be defined rests with the Assured.

4.2.2 4.2.3

4.3

CERTIFICATE OF APPROVAL

4.3.1 4.3.2

4.3.4

The deliverable of the approval process will generally be a Certificate of Approval. 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 within acceptable limits, and an operation may proceed. The Certificate of Approval for a marine operation such as float out, towage or installation will normally be issued when all preparations including seafastening and ballasting are complete, marine equipment such as tugs and towing connections have been inspected, a readiness meeting has been held, and the actual and forecast weather are suitable for the operation to begin. Agreement is required on the end-point of each Certificate of Approval.

4.4

SCOPE OF WORK LEADING TO AN APPROVAL

4.3.3

In order to issue Certificates of Approval for a typical GBS, platform or structure, the topics GL Noble Denton will typically consider the following: 4.4.1

FLOAT OUT OF DRY-DOCK a. Moorings in dry dock after flooding and lift-off b. Tidal conditions c. Loads, stresses and deflections d. Hydrostatic and stability characteristics e. Side and underkeel clearances f. Handling and towing arrangements including wires, winches, tugs and connections g. Meteorological limitations h. Systems, including power generation, ballast and compressed air systems i. Tow route to inshore construction moorings or hand-over point j. Surveys of out-of-dock channel and tow route k. Connection into construction moorings, if appropriate l. Marine procedures.

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION 4.4.2

TOWAGES a. The towage route and surveys, side and underkeel clearances b. The design metocean parameters for the towage route and season c. Motion responses d. Resulting loads and stresses e. Stability f. Systems, including power generation, ballast and compressed air systems g. Towing resistance and required bollard pull h. Tug specification i. Towing connections and arrangements j. Navigational equipment k. Marine procedures.

4.4.3

CONSTRUCTION AFLOAT a. Construction schedule b. Meteorological criteria for mooring design c. Loads, safety factors and redundancy of mooring system d. Drafts and underkeel clearances in all conditions e. Structural strength at all stages of construction f. Stability at all stages of construction g. Marine spread requirements h. Lifting and installation of equipment and sub-assemblies.

4.4.4

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.

4.5

TECHNICAL STUDIES

4.5.1

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 his contractors, 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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4.6

SURVEYS

4.6.1

Surveys and attendances carried out as part of GL Noble Denton's scope of work typically include, as may 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 as barges, 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 ballasting operations, 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. [2], 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 Denton suitability 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 is suggested that a preliminary survey be undertaken before committing to the vessel use or charter.

4.7

LIMITATION OF APPROVAL

4.7.1 4.7.2

A Certificate of Approval is issued for a particular towage, voyage or operation only. A Certificate of Approval is issued based on external conditions observed by the attending surveyor of the jacket, hull(s), machinery and equipment, without removal, exposure or testing of parts. Any alterations in the surveyed items or agreed procedures after issue of the Certificate of Approval may render the Certificate void unless approved by GL Noble Denton. No responsibility is accepted by GL Noble Denton for the way in which a transportation is conducted after departure. Fatigue damage is excluded from a transportation Certificate unless specific instructions are received from the client to include it in the scope of work. A Certificate of Approval for transportation does not cover any moorings prior to the start of transportation, or at any intermediate shelter port or arrival port, unless specifically approved by GL Noble Denton. A Certificate of Approval for installation applies to the safety of the GBS or platform during the defined installation period. It does not imply that the completed platform will be fit for purpose.

4.7.3 4.7.4 4.7.5 4.7.6

4.7.7

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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 be managed in accordance with local jurisdiction, as well as appropriate guidelines and specifications regarding health, safety and the environment (HSE).

5.2

JURISDICTION

5.2.1

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 the environment. It should also be noted that a marine operation may involve more than one nation’s area of jurisdiction, and that for barges and vessels the jurisdiction of the flag state will apply. This document does not specifically cover platform removal, but it should be assumed that the design must allow technically and economically feasible removal operations, in accordance with international environmental legislation.

5.2.2

5.3

RESPONSIBILITIES

5.3.1

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 and environment. The owner may delegate the execution and follow-up of these issues to the contractor. If a part of the marine operations is to be carried out near other facilities or their surroundings, safety zones shall be defined by the owner.

5.3.2

5.4

RISK MANAGEMENT

5.4.1

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 hazards b. Preventative measures to avoid hazards wherever possible c. Controls to reduce the potential consequences of unavoidable hazards d. 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 should include personnel and organisations involved in the design of structures and systems, as well as those involved in the marine operation. 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 rigorous study. 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. 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. 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 to enable rational choices to be made between alternatives. Ideally, each of the various studies outlined above should be managed by a competent independent person familiar with the overall concept, but outside the team carrying out the relevant system or structure design or operational management.

5.4.3

5.4.4 5.4.5

5.4.6

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5.5

QUALIFICATION AND TRAINING

5.5.1

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. Adequate training appropriate to each individual’s function and situation should be given, including job training, site safety training and briefings, marine safety and survival training. Fire and evacuation alarms should be periodically tested, and drills should carried out periodically, or as required by safety legislation. Computer simulation and training, and/or model tests may give valuable information for the personnel carrying out the operation.

5.5.2 5.5.3 5.5.4

5.6

SAFETY PLAN

5.6.1

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 conditions b. Risks incurred by construction, transportation, installation and commissioning activities c. Risks to the environment.

5.7

CONTINGENCY AND EMERGENCY PLANNING AND PROCEDURES

5.7.1

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 limits d. Stability parameters approaching pre-set limits e. Failure of mechanical, electrical or control systems f. Fire g. Collision h. Leakage i. Pollution j. Structural failure k. Mooring failure l. Human error m. Man overboard n. Personnel accidents or medical emergencies.

5.7.2

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

5.8

SECURITY AND TRACKING SYSTEM

5.8.1

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. 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 also be reported to GL Noble Denton.

5.8.2

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6

ORGANISATION, PLANNING AND DOCUMENTATION

6.1

INTRODUCTION

6.1.1

This Section presents the organisation and documentation system that shall be set up for the performance of marine operations.

6.2

ORGANISATION AND COMMUNICATION

6.2.1

An appropriate Project organisation shall be set up, illustrating how the marine operations integrate with the rest of the project. Key responsibilities shall be clearly defined, and the responsibilities and reporting lines shown on the organisation chart. Organisation charts shall be drawn up for each marine operation, showing the reporting line into the project organisation, and, for example, those responsible for the reporting lines: a. Client's representative b. Overall project management c. Operation management d. Towing vessels e. Mooring systems and marine spread f. Ballast system operation g. Weather forecasting h. Support services i. Advisory panel providing expertise as required j. Safety k. Statutory, regulatory and approving bodies.

6.2.2

6.2.3 6.2.4

6.2.5 6.2.6

6.2.7

In each case the responsibilities and duties of each function shall be clearly defined, to minimise uncertainties and overlapping responsibilities. During on-going marine operations, the site team should be limited to those persons with defined roles during the operation. Back-up services, including emergency services, contingency assistance and technical advisory services shall be identified and appropriately located. Communication systems, including radio channels, telephone, telefax numbers, e-mail addresses and out-of-hours numbers shall be identified. Where personnel changes occur during the course of an operation because of shift changes, these shall be identified. Every effort should be made to avoid changes of personnel during critical stages of the operation. Where transfer of responsibility is involved, times of and procedures for hand-over from one organisation to another (e.g. from fabrication to marine operations, from on-shore to offshore) shall be identified.

6.3

QUALITY ASSURANCE AND ADMINISTRATIVE PROCEDURES

6.3.1

An approved quality management system in accordance with the current version of ISO 9000 or equivalent shall be adopted and be in place.

6.4

TECHNICAL PROCEDURES

6.4.1 6.4.2

Technical procedures shall be in place to control engineering related to the marine activities. The objective of these procedures is to define applicable technical standards, to ensure agreement and uniformity, with production of bridging documents where necessary, on matters such as: a. International and national standards and legislation b. Certifying authority/regulatory body standards c. Marine warranty surveyor guidelines

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION d. e. f. g. h.

Project criteria Design basis Metocean criteria Calculation procedures Change management.

6.5

TECHNICAL DOCUMENTATION

6.5.1

An integrated document numbering system for the entire project is recommended, including documents produced by client, contractors, sub-contractors and vendors. Documents relating to marine operations should be grouped into levels according to their status, for example: a. Criteria and design basis documents b. Procedures and operations manuals c. Supporting documents, including engineering calculations, equipment specifications and systems operating manuals.

6.5.2

6.5.3

6.5.4

6.5.5 6.5.6

Procedure documents, intended to be used as an active tool during marine operations should include a section which clearly shows their references to higher and lower level documents, and should list all inter-related documents. An organogram format is often helpful. Essential elements to be included in or referred to by a marine operation document include: a. Introduction b. Reference documents c. Outline execution plan d. Organogram and lines of command e. Job-descriptions for key personnel f. Safety plan g. Authorities and permits including notification and approval requirements h. Contractual approvals and hand over i. Environmental criteria, including design and operational criteria j. Operational bar chart, showing the anticipated duration of each activity, inter-related activities, key decision points, hold points k. Specific step-by-step instructions for each phase of the operation, including sequence, timing, resources and check lists l. Reference to related calculations, e.g. environmental loads, moorings, ballast, stability, bollard pull m. Contingency and emergency plans n. Emergency Preparedness Bridging Document. Documents referred to by a marine operation procedure document should be available and accessible on board or on site close to the operation for reference by those involved. The document should define the On-Scene Commander in the event of an emergency situation and the interfaces between the various parties involved.

6.6

CERTIFICATION

6.6.1

Marine equipment proposed should generally be in possession of valid Certificates, in accordance with Table 5-2 in 0030/ND, Ref. [4] although some Certificates are not mandatory within inshore waters.

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7

WEATHER CRITERIA

7.1

INTRODUCTION

7.1.1 7.1.2

7.1.3

This Section refers to the metocean criteria applicable for marine operations. 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 conditions could be excessive. A set of limiting metocean criteria should be established dependent on the duration of each discreet operation, which may be a weather restricted or a weather unrestricted operation as defined in Section 3. Reference may be made to ISO 19901-1 "Metocean design and operational considerations", Ref. [7] for weather unrestricted operations. For weather restricted operations see Sections starting with 7.3.1.

7.2

METOCEAN CRITERIA

7.2.1 7.2.1.1

DESIGN AND OPERATIONAL CRITERIA 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 7.2.2.1

RETURN PERIODS 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 are considered concurrently: Table 7-1

7.2.2.2 7.2.3 7.2.3.1

7.2.3.2

7.2.4 7.2.4.1

Return Periods

Duration of use

Design environmental criteria

Up to 3 days

Specific weather window

3 days to 1 month

10 year return, 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 50 year return period should be increased to 100 years. Directionality of the environment should be considered. WIND 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 the nature of the operation, the structure involved and the response characteristics of the structure to wind. The relationship between wind velocity at the reference and actual elevations above sea level and between different averaging periods can be found in ISO 19901-1 "Metocean design and operational considerations", Ref. [7]. WAVES 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 the maximum wave should be determined from an appropriate higher-order wave theory and account for shallow water effects.

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION 7.2.4.2

7.2.4.3

7.2.4.4

7.2.4.5

7.2.4.6

Seastates shall include all relevant spectra up to and including the design storm seastate for the construction site or towage route. Long-crested seas shall be considered unless there is a justifiable basis for using short-crested seas. Consideration should be given to the choice of spectrum. 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 already covered in this peak period range. However, this method incorrectly assumes that all periods are equally probable. As a result this method should generally produce higher design accelerations than would be the case when using the more robust Hsig-Tp method described in the following section. In the alternative method, a contour is constructed within the Hsig-Tp plane that identifies equally probable combinations of Hsig & Tp for the design return period subject to theoretical constraints on wave breaking. This contour should also cover swell. The combinations should be tested in motion response calculations to identify the worst case response. The relationship between the peak period Tp and the zero-up crossing period Tz is dependent on the spectrum. For a mean JONSWAP spectrum (γ=3.3) Tp/Tz = 1.286; for a Pierson-Moskowitz spectrum (γ=1) Tp/Tz = 1.41. The following Table 7-2 indicates how the characteristics of the JONSWAP wave energy spectrum vary over the range of recommended seastates. The constant, K, varies from 13 to 30 as shown in the equation in Section 7.2.4.3 above. T1 is the mean period (also known as Tm). Table 7-2

7.2.4.7 7.2.4.8 7.2.4.9 7.2.5 7.2.5.1 7.2.5.2

Value of JONSWAP γ, ratio of Tp:Tz and Tp:T1 for each integer value of K

Constant K

Γ

Tp/Tz

Tp/T1

Constant K

γ

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

For operations involving phases sensitive to extreme sea states, such as temporary on-bottom stability or green water assessment, the maximum wave height and associated period should be used. 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. 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. CURRENT 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. Currents may be divided into two different categories: a. Tidal currents b. Residual currents that remain when the tidal component is removed, including river outflows, surge, wind drift, loop and eddy currents.

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

7.2.6 7.2.6.1

OTHER PARAMETERS Other factors including the following may be critical to the design or operations and should be addressed: a. Water level including tide and surge b. Sea icing, icing on superstructure c. Exceptionally low temperature d. Large temperature differences e. Water density and salinity f. Bad visibility.

7.3

WEATHER RESTRICTED OPERATIONS

7.3.1 7.3.1.1

GENERAL Weather restricted operations (as defined in Section 3) should be planned using reliable time history data that not only indicates the probability of not exceeding the selected or limiting design criteria, but also the persistence of such conditions for the season considered. In the more mature zones this information will be available, but in other areas it can be difficult to obtain or may not be available. The quality of data available will influence the assessment of overall risk.

7.3.2 7.3.2.1

IMPACT ON DESIGN During design the following should be considered: a. Measures to speed up the operation and provide more margin on the weather window b. Re-design the operation limitations to sustain higher metocean conditions c. Contingency situations, and back-up and stand-by measures d. Whether delays to previous activities could push the operation into an unfavourable season.

7.3.3 7.3.3.1

MARGINS ON WEATHER 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. 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 multiplied by the applicable factor from Table 7-3 below. For operations with other durations alternative factors apply and should be agreed with GL Noble Denton. The forecast wind and current shall be similarly considered when their effects on the operation or structure are significant.

7.3.3.2

Table 7-3

Seastate Reduction Factor

Weather Forecast Provision (see Section 7.4)

0015/ND Rev 3

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 (wave rider buoy)

0.70

One project-specific forecast source plus in-field wave monitoring and offshore meteorologist

0.75

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION 7.3.4 7.3.4.1

OPERATION DURATION When defining the weather window required for a time-critical marine operation the schedule should be as realistic as possible. The window duration should include contingencies for: a. Inaccuracy in operation schedule. Schedules based on field experience gained from previous similar operations may need less contingency b. Technical / mechanical / operational delays. Operations using vulnerable or critical equipment need extra contingency c. Inaccuracy in the timing and length of window predicted by the metocean forecast. Extra contingency may be needed for operations where conditions are difficult to predict because of the area or the time of the year.

7.3.4.2

Generally the forecast window duration should be in excess of the required window duration including contingencies.

7.3.5 7.3.5.1

POINT OF NO RETURN (PNR) Weather restricted operations may be divided into sequences where the operation may be aborted and brought to a safe condition within the remainder of the existing weather window. The Point of No Return (PNR) should be defined as the last point in time, or a geographical point along a route, at which an operation could be aborted or returned to a safe condition. For the critical operation period between any PNR and the structure reaching a safe condition, the reliability of the weather window is crucial.

7.3.5.2 7.3.5.3

7.4

WEATHER / METOCEAN FORECAST

7.4.1 7.4.1.1

GENERAL Project specific forecasts shall be obtained before and during all marine operations. The forecast shall be issued at suitable regular intervals dependent on the operation, but not exceeding 12 hours. For complex and/or longer weather restricted operations, the forecaster(s) should preferably be present at site to check the local situation and provide regular weather briefings. Reference may be made to the Handbook of Forecasting Services, Ref. [9].

7.4.2 7.4.2.1

FORECAST PARAMETERS The forecast should cover short, medium term and outlook periods and include relevant parameters such as: a. Synopsis, barometric pressure, temperature b. Wind direction and velocity c. Waves and swell - significant and maximum height, direction and period d. Visibility, rain, snow, sleet, icing and sea ice e. Confidence level of the forecast.

7.4.2.2

Other conditions like current, tide and surge may be relevant to certain operations. Such conditions may need real time measurement as well as regular forecasting before and during operations. Attention should be paid to local current phenomenon, for instance: a. Loop-eddy currents in the Gulf of Mexico (unlikely to be relevant to GBSs) b. Soliton currents in the South China Sea.

7.4.3 7.4.3.1

ON SITE MONITORING Where operations are sensitive to local environmental conditions, or to changes in these, means for onsite monitoring should be considered (see Table 7-3).

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8

WEIGHT CONTROL

8.1

INTRODUCTION

8.1.1 8.1.2

This Section describes the procedures for weight control which should be in operation throughout construction and outfitting. Weight control shall be performed by means of a well-defined, documented system, complying with the ISO International Standard ISO 19901-5 – Petroleum and Natural Gas Industries – Specific requirements for offshore structures – Part 5: Weight control during engineering and construction, Ref. [8].

8.2

WEIGHT CLASS

8.2.1

Unless it can be shown that a particular structure and all its marine operations are not weight or CoG sensitive, then Class A weight control definition will be needed, as shown in Ref. [8].

8.3

RESERVES

8.3.1

For calculation purposes conservative values of weight and CoG should be used. It should be noted that some calculation results, e.g. transportation motions, may be more conservative with a lower value of weight and lower CoG. Where an operation is likely to be CoG sensitive, the extremes of the CoG envelope shall be considered in calculations. An adequate reserve shall be added to the 50/50 weight estimate, as defined in Ref. [8] to suit each particular operation and the weight and CoG sensitivity and the maturity of the project design. In general, the reserve applied to the final 50/50 weight estimate should be not less than 5%. Where structures or components are weighed, a reserve of not less than 3% should be applied to the final weighed weight, unless it can be demonstrated that the weighing method and equipment have an overall accuracy better than this figure.

8.3.2 8.3.3

8.3.4

8.4 8.4.1

NOT-TO-EXCEED WEIGHT In addition to any in-place design and operational considerations, the following factors may control the not-to-exceed weight: a. b.

Draft and stability for float-out, towages, matings and installation Allowable stresses in the structure for marine operations.

8.5

WEIGHT CONTROL AUDITS

8.5.1

During construction afloat, periodic draft measurements and weight control audits and inclining tests shall be carried out when a particular structure and its marine operations are weight or CoG sensitive, particularly prior to critical operations such as deck mating. See also Section 11.2.

8.6

DIMENSIONAL CONTROL

8.6.1

Where the balance between weight and buoyancy is critical to the draft, stability or floating behaviour, the dimensional control and monitoring shall be maintained to an appropriate degree of accuracy.

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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, wave action and the effect of towline pull or mooring loads to the static forces resulting from gravity and hydrostatic loads. Accidental loadings shall also be considered. Specific loadcases shall be agreed with GL Noble Denton. Adequate global and local strength shall be documented. The unit shall be able to withstand a static heel angle of 10 degrees. If it has damage stability the unit shall also be able to withstand the static and dynamic loads caused by the flooding of any one compartment in a 25 metres/second wind and associated waves. These would be regarded as extreme (ultimate limit state) conditions. Hydrostatic loads on the substructure at the deepest draft during deck-mating can be the governing loadcase. It shall be demonstrated that a thorough independent check of the calculations covering this loadcase has been carried out, and that the design and reinforcement details assumed in the calculations concur with the as-built condition. Any limitations on the maximum allowable duration of deep immersion, in relation to the structural stability of the unit, should be established and the procedures planned accordingly.

9.1.2

9.1.3

9.1.4

9.2

REINFORCED CONCRETE

9.2.1 9.2.2 9.2.3

The strength of concrete during floating phases shall comply with a recognised concrete design code. Stresses in structural steelwork shall be within those permitted by the latest edition of a recognised and applicable offshore structures code. Any time-dependent properties of the material shall be taken into account.

9.3

STRUCTURAL STEEL

9.3.1

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 the methodology of a recognised and applicable offshore code including the associated load and resistance factors for LRFD codes or safety factors for ASD/WSD codes. Traditionally AISC has also been considered a reference code. If the AISC 13th edition is used, the allowables shall be compared against member stresses determined using a load factor on both dead and live loads of no less than those detailed in the following Table 9-1.

9.3.2

Table 9-1

9.3.3 9.3.4

Load Factors

Type

WSD option

LRFD Option

SLS:

1.00

1.60

ULS:

0.75

1.20

Any load case may be treated as a normal serviceability limit state (SLS) / Normal operating case. 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 where this cannot be avoided by means of a suitable WPS, it may be necessary to increase the strength or impose a reduction on the design/permissible seastate.

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9.4

COMPRESSED AIR

9.4.1

Compressed air may be used to resist hydrostatic head on internal or external walls during ballasting, or for reducing overall bending moments by air cushions in skirt cells under well controlled conditions. However its absence should not result in structural collapse i.e. it should only be used to increase structural safety factors.

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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 model testing, 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 shown to 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 drafts e. Installation at final location f. Any condition where the motions may be critical to loss of freeboard, stability, position keeping or other considerations.

10.2

MOTION RESPONSE DETERMINATION

10.2.1

The guidance of Sections 7.7, 7.8 of GL Noble Denton 0030/ND “Guidelines for Marine Transportations”, Ref. [4] shall be applied. 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.

10.2.2

10.3

HUMAN LIMIT

10.3.1

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 duties efficiently.

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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 Transportations” Ref. [4], and is intended to cover “Condeep”-type gravity structures (see Section 2.1). 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 geometric model. 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. The output of the weight control programme as described in Section 8, shall be taken into account. 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.

11.1.2

11.1.3

11.1.4 11.1.5

11.1.6

11.1.7

The number of openings in buoyant elements adjacent to the sea shall be kept to a minimum. Where penetrations are necessary for access, piping, ventilation, electrical connections, etc. arrangements shall be made to maintain watertight integrity. During construction phases, particular attention should be paid to openings near the waterline, which will vary as construction proceeds. Damage stability requirements shall be evaluated considering the operation procedure, environmental loads 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.

11.1.8

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 objects c. Mechanical system failure d. Human error.

11.1.9

The consequences of water ballast escaping from any compartments above the waterline, or the escape of air from any air cushion shall be evaluated where applicable. 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 be reviewed if deep draft vessels are likely to be operating nearby. For operations where the structure cannot meet damage stability criteria, measures shall be taken to minimise the risk, by: a. Limiting the exposure period b. Providing additional local structural strength c. Providing additional protection, such as fendering d. Minimising vessel movements near the structure e. Dedicated procedures and experienced personnel.

11.1.10

11.1.11

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A risk assessment shall be carried out for operations where at any stage stability or reserve buoyancy is critical, or where damage stability cannot be obtained. The results of the risk assessment shall be agreed 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 TESTS

11.2.1

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. The output from the inclining test should be used to check and calibrate the output from the weight control programme. 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 and negative inclinations.

11.2.2 11.2.3

11.2.4 11.2.5

A sensitivity analysis of the parameters affecting the test results should be performed. 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 the structure in a vertical wall-sided area e. 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 shall be allowed during the test f. The effects of external forces due to winds, waves, currents, moorings, tugs etc shall be monitored. 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 11.3.1.1

INTACT STABILITY 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 not exceed 5 degrees in the design storm conditions as defined in Section 7.2. 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.

11.3.1.2

During towing, the static inclination in still water when subjected to 50% of required towline pull should not 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.

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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: 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 depth at 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.

11.3.1.4

The wind used for overturning moment calculations should be the design wind for the operation, as defined in Section 7.2.3. Short duration operations during construction or towage may be considered as weather-restricted operations, provided the structure can achieve or be returned to a safe condition, within the operational reference period.

11.3.2 11.3.2.1

FREEBOARD For towages and construction afloat, the freeboard as defined in Section 3 shall not be less than the greater 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.

11.4

DAMAGE STABILITY AND FREEBOARD REQUIREMENTS

11.4.1 11.4.1.1

DAMAGE STABILITY FOR FLOAT-OUT AND TOWAGE TO INSHORE CONSTRUCTION SITE For float-out from dry-dock, one-compartment damage stability is usually impractical, because of the limited underkeel clearance, and is therefore not required. For other inshore tows the structure shall have one-compartment damage stability, as defined in Sections 11.1.7 through 11.1.10. If one-compartment damage stability requirements cannot be fulfilled, the requirements for construction afloat in Section 11.4.2.2 shall apply.

11.4.1.2 11.4.1.3 11.4.2 11.4.2.1 11.4.2.2

DAMAGE STABILITY DURING CONSTRUCTION AFLOAT During the period of construction afloat, the platform shall possess one-compartment damage stability, for as much of the construction period as is practical. 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, and d. 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 against dropped objects. Any lifts which, if dropped, could endanger the platform shall be identified and additional precautions taken, and f. Any objects or equipment on barges alongside, which if dropped, could endanger the platform shall be similarly identified and additional precautions taken, and g. Rigorous procedures shall be developed to minimise the risk of flooding. These shall include consideration of collision, leakage through the ballast or other systems, reliability and redundancy of pumping arrangements and power supplies, and h. At all times there shall be adequately trained personnel on board the platform, and

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CONCRETE GRAVITY STUCTURES - GUIDELINES FOR APPROVAL OF CONSTRUCTION AND INSTALLATION i.

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

11.4.3 11.4.3.1 11.4.3.2

DAMAGE STABILITY FOR TOWAGE TO OFFSHORE SITE AND INSTALLATION When towing on the base or columns the platform shall possess one compartment damage stability. 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 11.1.10, 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 GL Noble Denton.

11.4.3.3

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

11.4.4 11.4.4.1

FREEBOARD 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.

11.4.5 11.4.5.1

STRUCTURAL LIMITATIONS The platform shall be demonstrated to meet the requirements of Section 9.1.2.

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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. All ballasting procedures shall be reversible so that, in case of emergency, the unit can be returned to a safe draft within 24 hours.

12.1.2

12.2

REDUNDANCY

12.2.1

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. 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. 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. 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.

12.2.2 12.2.3 12.2.4

12.3

INLETS

12.3.1

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. Except when in use for inlet or discharge, all openings to sea shall be protected by a double barrier. Any external valves and pipework shall be protected against collision and fouling by towlines, mooring lines or handling wires.

12.3.2 12.3.3

12.4

PIPEWORK

12.4.1 12.4.2

All essential pipework shall be of permanent-type construction and hydrostatically tested to a minimum of 1.3 times the design pressure. 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 shall be carried out to demonstrate the acceptability of the system and submitted for GL Noble Denton approval.

12.5

UMBILICALS

12.5.1

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 critical controls.

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12.6

INSTRUMENTATION

12.6.1

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 draft, 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.

12.6.2

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

12.7

AIR CUSHION

12.7.1

The following recommendations apply to the use of underskirt compressed air: a. All piping shall be secure, protected and of adequate capacity and 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 no residual free surface remains f. Sufficient water seal (bottom of air cushion above bottom of skirt) shall be available to prevent air escaping g. 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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13

BUILDING BASIN

13.1

The surrounding walls of the construction basin shall be designed in accordance with accepted civil and geotechnical engineering practice. Where materials are used whose stability characteristics can be affected by a change in pore water pressure, suitable monitoring devices shall be installed and the retrieved data analysed by competent geotechnical engineers to ensure continuing stability of the walls throughout the period of the platform construction. Meteorological design criteria for the basin design shall be in accordance with the return periods specified in Section 7.2.2. Consideration shall be given to the design of the basin walls, including but not limited to the following: a. The integrity of the walls shall remain stable when subjected to:  The highest astronomical tide, plus  Storm surge, plus  Maximum wave crest height, corrected for shoaling if applicable. b. The walls shall be of adequate height to prevent overtopping, except by spray, in the above conditions c. Walls shall be protected against the effects of collision, propeller scour and wave action d. The effects of ice loading shall be considered when applicable.

13.2

13.3 13.4

13.5

Pumping capacity shall be adequate for maximum seepage, rainfall (including run-off from surrounding land) and spray overtopping.

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14

TOW-OUT FROM DRY-DOCK

14.1

MOORING AND HANDLING LINES

14.1.1 14.1.2

If the structure is to remain afloat at moorings inside the basin, then the moorings shall be designed in accordance with 0032/ND, Ref [6], taking into account the weather criteria of Section 7.2.2. Handling lines and winching equipment shall be designed to withstand the design loads arising, assuming that handling operations are weather restricted operations. All wires shall be designed to a safety factor on certified breaking load of not less than 3.0. Connections to the structure and to the shore shall be designed in accordance with 0032/ND, Ref [6].

14.2

UNDERKEEL CLEARANCE FOR LEAVING BASIN

14.2.1

14.2.3 14.2.4

If the unit is likely to be at moorings in the basin at maximum draft for any significant time, the effects of siltation and negative surge shall be considered. When moored in, or leaving the building basin, the unit shall have a minimum underkeel clearance, using tidal rise if necessary, of at least 0.5 metres for a period adequate for float out, including contingencies. This would normally be of the order of 3 hours before, to 3 hours after high water. The planned float out operation, excluding contingencies, should normally be planned to be completed before high water. Compressed air may be used to reduce draft if required, subject to the requirements of Section 12.7. A tide gauge shall be installed on site to check that actual tidal levels correspond to those predicted.

14.3

SIDE CLEARANCES

14.3.1

14.3.2 14.3.3

Depending on the positional control of the unit during the exit of the basin, the channel width at full depth should normally be not less than: 1.2 x B when inside the basin, and 2.0 x B when out of the basin, where B = maximum structure dimension, normal to the direction of travel. The required channel width may be reduced if the unit is winched out along a fendered guide. The required channel width may need to be increased if tugs are used.

14.4

UNDERKEEL CLEARANCE OUTSIDE BASIN

14.4.1

Once outside the building basin the minimum underkeel clearance shall be as required by Section 14 of 0030/ND, Ref. [4] until final emplacement.

14.5

TOWAGE AND MARINE CONSIDERATIONS

14.5.1

Tugs, towage and marine considerations for tow out of dock shall be in accordance with 0030/ND, Ref. [4].

14.2.2

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15

CONSTRUCTION AND OUTFITTING AFLOAT

15.1

INTRODUCTION

15.1.1

This Section covers the marine operational aspects of construction and outfitting afloat. Construction and 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 departure for the offshore location. Inshore deck mating, although a construction activity afloat, is excluded from this Section and is covered in 0031/ND, Ref. [5]. Documentation and procedures for construction afloat shall follow the general principles of Section 6. Documentation and procedures must above all ensure that those planning, authorising and carrying out the work are fully informed about any limitations and constraints which may be placed on the work by factors outside their own discipline. 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, and which can be inter-related, including: a. Structural loads and resistance b. Draft, displacement, ballast condition and stability c. Mooring loads and resistance d. Marine spread requirements e. Weather conditions f. Other ongoing activities and access restrictions.

15.1.2 15.1.3

15.1.4

15.2

STRUCTURAL AND STABILITY LIMITATIONS

15.2.1

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, as appropriate

15.2.2

Any structural, stability or draft limitations which lead to constraints on construction operations must be clearly defined, and written into the relevant operational procedures. These may include: a. Maximum and minimum draft b. Differential ballast levels in adjacent buoyancy compartments, or any compartment and the sea c. Weight distribution d. Structural limitations on heel or trim, which may therefore lead to limitations on draft, stability or environmental conditions e. Free surface limitations f. Phases during which one-compartment damage stability does not exist, and the requirements given in Section 11.4.2.2 apply.

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The age of any time-dependent construction material such as concrete should be taken into account in the 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 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.

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. 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 and operational meteorological limits for the moorings should be clearly defined. Procedures should be developed 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 to perform this operation. All equipment and material on barges shall be secured to minimise the risk of loss overboard. Any equipment which, if lost overboard, could cause damage to the structure, shall be identified and handled so as to minimise the hazard. All floating equipment moored adjacent to the platform shall possess one-compartment damage stability. The need for contingency pumping equipment on site should be evaluated. A hazard identification study and risk assessments should be carried out for the entire spread involved during the construction and outfitting afloat. The results should be submitted for GL Noble Denton approval.

15.3.3

15.3.4

15.3.5 15.3.6

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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 its final 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 the operation involves docking over a template, docking piles or other structures f. Stability at all stages of immersion g. Whether a vertical or inclined installation is required h. Tolerances on differential ballast levels i. The skirt design, and penetration method j. Whether under-base grouting is required k. Whether solid ballast or scour protection is required.

17.2

SITE LOCATION

17.2.1 17.2.2 17.2.3

The position of the site location shall be given in both geographical and grid coordinates. The water depth and bathymetric tolerances shall be determined. 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.

17.2.4

17.2.6

The bottom topography shall be established by swathe bathymetry and checked by single beam echo sounder, side scan sonar, magnetometer and ROV video. The extent of any required levelling or other seabed preparation should be decided at the design stage. The seabed and sub-seabed conditions shall be established by coring, magnetometer, in-situ testing, lab testing and sub-bottom profiling. Current surveys should be carried out.

17.3

INSTALLATION METHOD

17.3.1

The overall installation method will depend on the configuration of the platform.

17.2.5

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In general, shallow draft platforms will be towed to site with freeboard on the base structure. These platforms frequently undergo a phase of instability during submergence of the base, and an inclined installation procedure must be used. Sometimes it will be necessary to touch down on one edge to 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 inclined condition 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 skirt penetration or base slab bearing e. If the skirt touch down is on the final site, accurate position control may be difficult in the inclined condition f. If skirt touch down is remote from the final site, the deballast capability required by Section 12.1 will be used.

17.3.3

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

17.4

POSITIONING SYSTEMS

17.4.1

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. Where the position and orientation tolerances are not critical, the tugs may be in free floating configuration. Where more precise positioning is required, the tugs will probably be connected at the bow to pre-laid anchors.

17.4.2 17.4.3

17.5

DOCKING PILES

17.5.1

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 the piles, the behaviour of the piles in the soil, the structure/pile interaction and loads.

17.6

SKIRT PENETRATION

17.6.1

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 be applied. Vent piping shall be provided to allow water in the skirt compartments to escape, or to allow negative pressure to be applied. Design of the pipework should take into account the requirements for removal on decommissioning. Structural loads on the skirts shall be shown to be acceptable. 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.

17.6.2 17.6.3 17.6.4 17.6.5

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REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

GL Noble Denton 0013/ND “Guidelines for Loadouts”. GL Noble Denton 0021/ND “Guidelines for the Approval of Towing Vessels”. GL Noble Denton 0027/ND “Guidelines for Marine Lifting Operations”. GL Noble Denton 0030/ND “Guidelines for Marine Transportations”. GL Noble Denton 0031/ND “Guidelines for Floatover Operations” GL Noble Denton 0032/ND “Guidelines for Moorings”. ISO 19901-1 – Petroleum and Natural Gas Industries “Specific requirements for offshore structures – Part 1: Metocean Design and Operational Considerations". ISO 19901-5 – Petroleum and Natural Gas Industries “Specific requirements for offshore structures – Part 5: Weight control during engineering and construction”. 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 from www.gl-nobledenton.com

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CONCRETE OFFSHORE GRAVITY STRUCTURES

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