HSEHealth & Safety

Executive

Floating production system

JIP FPS mooring integrity

Prepared by Noble Denton Europe Limitedfor the Health and Safety Executive 2006

RESEARCH REPORT 444

Crown copyright 2006

First published 2006

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to hmsolicensing@cabinet-office.x.gsi.gov.uk

HSEHealth & Safety

Executive

Floating production system

JIP FPS mooring integrity

Noble Denton Europe Limited No 1 The Exchange

62 Market StreetAberdeen

AB11 5PJ

The main objective of this report is to improve the integrity of the mooring systems on Floating ProductionSystems (FPSs). It is intended to be read and understood by non mooring specialists such as FPSOperational staff - so that the people who live and work on FPSs will be better able to become moreinvolved in the vital task of looking after their own mooring systems. Meanwhile the included feedback onthe actual performance of mooring systems in the field should assist designers and manufacturers toimprove future mooring designs. Hence, the report attempts to identify gaps in the existing knowledge ofmooring behaviour and components to provide a road map for future work. Appendix C includes a paperpresented at the 2005 Offshore Technology Conference (OTC) which represents a stand alone summaryof the key points of the JIP.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents,including any opinions and/or conclusions expressed, are those of the authors alone and do notnecessarily reflect HSE policy.

HSE BOOKS

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CONTENTS SECTION PAGE NO.

1 EXECUTIVE SUMMARY 11 2 INTRODUCTION AND SCOPE 17 3 MOORINGS OVERVIEW 22 3.1 Mooring Basics 22 3.2 Mooring Line Constituents 39 3.3 Determination of Minimum Break Load (MBL) & Maximum Stresses 58 4 CONTEXT SETTING - HISTORICAL INCIDENTS AND THEIR

SIGNIFICANCE 65 4.1 Long-Term Degradation Mechanisms 65 4.2 Multiple Line Failure Incidents 74 4.3 Petrojarl 1 Multiple Lines Failure (1994) 76 5 CONSEQUENCES OF MOORING LINE FAILURE 77 5.1 Single Line Failure 77 5.2 Multiple Line Failure 79 5.3 Danger of Hydrocarbon Release 81 5.4 Business Interruption Consequences - Two Case Studies 82 6 HANDLING, TRANSPORTATION/TRANSFER AND INSTALLATION 85 6.1 Transportation/Transfer 85 6.2 Installation of Mooring Lines and Connectors 86 6.3 Installation Watch Points from a Mooring Integrity Standpoint 93 7 CORROSION, FATIGUE AND WEAR (CASE STUDIES) 99 7.1 The Balmoral FPV An Industry Benchmark 99 7.2 Corrosion and Wear Allowance Discussion of Code Requirements 101 7.3 North Sea FPSO Apparent Corrosion and Wear Data 108 7.4 Sulphate Reducing Bacteria (SRB) Induced Pitting Corrosion 112 7.5 Stress Corrosion Fatigue 113 7.6 Wear Analysis (Shoup and Mueller Work) 118 8 UNBALANCED LINE PRE-TENSIONS (CASE STUDIES) 122 8.1 North Sea Semi-Submersible FPS 122 8.2 Line Payout/Pull-In Test 123 8.3 North Sea FPSO 124 9 MOORING BEHAVIOUR AT THE VESSEL INTERFACE (CASE STUDIES) 126 9.1 Permanently Stoppered Off Versus Adjustable Lines 126 9.2 Wear at Trumpet Welds Internal and External Turrets 130 9.3 Use of Bending Shoes 143 10 FURTHER MOORING CASE STUDIES 145 10.1 Wire Rope Systems 145 10.2 Unintended Line Disconnection 146 10.3 Excursion Limiting Weighted Chain Problems 151 10.4 Line Run Outs and Quick Releases 155 10.5 Windlass Failures 158 11 SPARS AND OFFLOADING BUOYS (CASE STUDIES) 160 11.1 Brent Spar Buoy 160 11.2 Floating Loading Platform (FLP) 163 12 TURRET MECHANICAL IMPLICATIONS FOR MOORING INTEGRITY 165 12.1 Introduction to Turrets and Failure Modes 165

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12.2 Implications of Mechanical Repairs 168 13 GENERAL TRENDS AND STATISTICS 169 13.1 Questionnaire Process 169 13.2 Summary Statistics for Unit Type and Geographical Area 171 13.3 HSE UK Sector and Norwegian Statistics 174 14 CONNECTORS AND TERMINATIONS 177 14.1 Background 177 14.2 What Type of Connectors Can be Considered for Long Term Mooring (LTM) 177 14.3 Terminations General 182 14.4 Connector/Termination Design Flow Chart 186 14.5 Detailed Design Guidance 189 14.6 Proof Load Testing and Its Impact on Fatigue 193 15 OUT OF PLANE BENDING CHAIN AND ROPES (FIBRE + WIRE) 197 15.1 Tension Bending at a Wildcat and its Effect on Fatigue 197 15.2 Tension Bending at Chainhawse 205 15.3 Tension Bending In Wire Rope 215 15.4 General Implications of Tension Bending Fatigue for the FPS Industry 219 15.5 Recommendations 222 16 FRACTURE MECHANICS AND CRITICAL CRACK SIZE 223 16.1 Required Data 223 16.2 Fracture Mechanics and Chains State of the Art Summary 224 16.3 Fracture Mechanics Critical Crack Size Implications 225 17 LINE STATUS MONITORING AND FAILURE DETECTION 226 17.1 Instrumentation Status - Survey Results 226 17.2 Existing Failure Detection Systems 227 17.3 Future Failure Detection Systems 232 18 INSPECTION, REPAIR & MAINTENANCE (IRM) 238 18.1 In Air-Inspection 238 18.2 Where to Inspect on a Mooring Line 239 18.3 In-Water Inspection 242 18.4 Marine Growth Removal 246 18.5 Manufacturing Tolerances and the Inspection Implications 247 18.6 Wildcat/Gypsywheel Inspection 247 18.7 Inspection Frequency Code Requirements 255 18.8 Outline Method To Break Test Worn Mooring Components 257 19 SPARING OPTIONS 261 19.1 Contingency Planning - Spares and Procedures 261 20 THE IMPORTANCE OF A COMPREHENSIVE MOORING DESIGN

SPECIFICATION 264 20.1 Installation Parameters 265 21 KEY CONCLUSIONS & FUTURE WORK RECOMMENDATIONS 267 21.1 Overview 267 21.2 Key Conclusions 268 21.3 Recommendations for Further Study 270 22 REFERENCES AND BIBLIOGRAPHY 272 23 APPENDIX A - SUMMARY OF PAST RELEVANT JIPS 278 24 APPENDIX B MOORING INTEGRITY QUESTIONNAIRE (EXCEL) 279 25 APPENDIX C 2005 OTC JIP PAPER 280 26 APPENDIX D HSE SAFETY NOTICE 3.2005 FLOATING PRODUCTION

AND OFFLOADING (FPSO) MOORING INSPECTION 281

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LIST OF TABLES

Table 1-1 - North Sea Mooring Line Failure Data, 1980 to 2001 [Ref. 1] ...................................12 Table 1-2 - Indicative Single Mooring Line Failure Costs ...........................................................13 Table 3-1 Summary of Chain Design Parameters (modified from Vicinay Chain Catalogue)..41 Table 3-2 Comparison of Manufacturing Parameters ................................................................41 Table 3-3 Chain Geometry Implications for Inspection and Maintenance ................................42 Table 3-4 - Summary of Ship or Marine Grade Chains [Ref. 13].................................................45 Table 3-5 Example of Indicative Surface Hardness Values for Various Chain Grades (courtesy

of Vicinay).............................................................................................................................46 Table 3-6 Illustration of Indicative Wire Rope Material Properties [Ref. 2] .............................49 Table 3-7 - Comparison of the Advantages of Spiral and Six Strand Wire (courtesy of Bridon) 50 Table 3-8 - Comparison of the Cons of Spiral and Six Strand Rope ............................................50 Table 3-9 - Wire Rope Recommendations for Varying Field Lives (courtesy of Bridon) ...........50 Table 3-10 - Stipulated MBL and Proof Load Values for Various Sizes and Grades of Chain

(courtesy of Vicinay).............................................................................................................62 Table 5-1 - Line Failure Cost Estimate, 50,00bpd North Sea FPSO ............................................83 Table 5-2 - Line failure Cost Estimate, 250,000bpd West African FPSO ....................................84 Table 7-1 - Example of Specified Corrosion and Wear Allowances from One Classification

Society.................................................................................................................................102 Table 12-1 - Summary of the Pros and Cons of Sliding and Roller Bearings [Ref. 48] .............167 Table 13-1 - Example of the First Page of the Questionnaire see appendix B for a Full Listing

.............................................................................................................................................169 Table 13-2 - UK Sector of the North Sea Data [Ref. 49]..........................................................174 Table 13-3 - UK Sector of the North Sea Data [Ref. 49]...........................................................174 Table 13-4 Number of Anchor Incidents in the Period of 1990-2003 in the Norwegian Sector

[Ref. 50] ..............................................................................................................................174 Table 15-1 Comparison between Chain Tension-Bending Fatigue Parameters Note that values

in italics are derived from BOMEL measured stress factor. ...............................................203 Table 15-2 : Wire Rope Fatigue Reduction Due to Tension Bending [Ref. 31].........................216 Table 15-3 - S-N Parameters for Mooring Chain Fatigue..........................................................220

LIST OF FIGURES

Figure 1-1 - Red Arrows Indicate Key Areas subject to Degradation on a ..................................14 Figure 2-1 - JIP Organisation ........................................................................................................19 Figure 2-2 - CTR Breakdown........................................................................................................19 Figure 2-3 Participants at the Steering Committee meeting in Paris .........................................21 Figure 3-1 Typical Turret Moored FPSO...................................................................................22 Figure 3-2 Shallow and Steep Mooring Line Angle Illustration................................................23 Figure 3-3 Line Heading Illustration..........................................................................................23 Figure 3-4 Definition of Windward and Leeward Lines + Environmental Offset.....................24 Figure 3-5 Offset Position and Tension Effect...........................................................................25 Figure 3-6 Illustration of Load Excursion Curve [Ref. 2]..........................................................25 Figure 3-7 - Typical Spread Moored Unit, Girassol FPSO offshore West Africa (courtesy of Stolt

Offshore) ...............................................................................................................................26 Figure 3-8 Illustration of Catenary System................................................................................28 Figure 3-9 - Typical Spread Moored Catenary System (Courtesy of Vryhof) .............................28 Figure 3-10 Illustration of Taut-Leg system ..............................................................................29 Figure 3-11 - Typical Spread Moored Taut-Leg System (Vryhof)...............................................29

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Figure 3-12 Illustration of Surge, Sway, Heave, Roll, Pitch and Yaw ......................................30 Figure 3-13 Example of Optimising the Stiffness of the Load offset Curve ............................32 Figure 3-14 - Illustration of Long Crested (Unidirectional) Seas .................................................35 Figure 3-15 - Illustration of Short Crested (Confused) Seas.......................................................35 Figure 3-16 - Example of a Wave Breaking on a Column of a Semi-Submersible ......................36 Figure 3-17 - Illustration of Deepwater Breaking Wave Types (Plunging Break on the Left and

Spilling Breaking on the Right) ............................................................................................37 Figure 3-18 - Illustration of the Damage Caused to Schiehallions Bow by an Unusually Steep

Wave (courtesy of BP) ..........................................................................................................37 Figure 3-19 - Model Illustration of the Effect of a Breaking Wave on a FPSO (Courtesy of APL

website) .................................................................................................................................38 Figure 3-20 - Isambard Kingdom Brunel in front of Studded Chain for the Great Eastern steam

ship, circa 1858 .....................................................................................................................39 Figure 3-21 Comparison of the Geometry of Modern Studded and Studless Chain..................40 Figure 3-22 - SPATE Contour Map of a 76mm Loose Stud Chain Link [Ref. 8].....................43 Figure 3-23 - Example of the Arrangement of an Asymmetric Stud ............................................44 Figure 3-24 Indication of the Manufacturing Tolerances of Studless Links (courtesy of

Vicinay).................................................................................................................................47 Figure 3-25 Studlink Manufacturing Tolerances (courtesy of Vicinay) ....................................48 Figure 3-26 - Illustration of the Make Up of Different Wire Rope Types (courtesy of Bridon) ..49 Figure 3-27 Chronology of Deep Star Funded Synthetic Mooring Studies ............................51 Figure 3-28 - Accurate Drag Anchor Placement by Crane in Good Weather Conditions (courtesy

of Stolt Offshore) ..................................................................................................................52 Figure 3-29 Installation and Normal (Vertical) Load Position (courtesy of Vryhof ) .................53 Figure 3-30 Anchor Pile + Chain Tail Deployed by a Twin Crane Construction Vessel (courtesy

of Stolt Offshore) ..................................................................................................................53 Figure 3-31 Suction Anchor Deployment (courtesy of Stolt Offshore).......................................54 Figure 3-32 - Example of a Tensile Test on a Steel Sample cut out from a Chain Link .............58 Figure 3-33 - Example of a Chain Sectioned for Material Testing..............................................59 Figure 3-34 - Example of Terminology during a Tensile Test (courtesy of Ashby & Jones, [Ref.

17]) ........................................................................................................................................59 Figure 3-35 - Stress Strain Curves for R3, R4 and R5 Chain Steel (Data courtesy of Vicinay)..61 Figure 3-36 Approximation of the Stress Distribution in a Typical Chain Link .......................63 Figure 3-37 - Illustration of a Finite Element Representation of a Chain Link ............................64 Figure 3-38 Finite Element Representation of a Shackle Body .................................................64 Figure 4-1 Illustration of some of the Main Factors which Influence Mooring Integrity..........65 Figure 4-2 - North Sea Pioneer on the Argyll Field .................................................................66 Figure 4-3 Fulmar SALM after Breakaway (courtesy of BBC film clip) ..................................68 Figure 4-4 Schematic of the layout of the Fulmar SALM .........................................................69 Figure 4-5 - Extract from On this Day BBC Website ...............................................................70 Figure 4-6 Illustration of a Typical Lightship Weathervaning Mooring ...................................71 Figure 4-7 - Helicopter Rescue from the Free Drifting North Carr Lightship after Mooring

Failure [Ref. 20] ....................................................................................................................72 Figure 4-8 - Illustration of the North Carr Link Failure Relative to a 1999 North Sea FPSO Link

Failure (fatigue cracking followed by ductile rip out) ..........................................................73 Figure 4-9 - Dutch Lightship Number 11 whose Mooring Failed in a Force 10 Gale in October

1991 which also broke a number of semi-sub moorings see Section 4.2...........................73 Figure 4-10 - Fifth Generation Deepwater Nautilus Broke free of all her Moorings during

Hurricane Ivan.......................................................................................................................75 Figure 4-11 - Petrojarl 1 which experienced two broken lines at the same time when hit by a

steep wave .............................................................................................................................76 Figure 5-1 - Summary of a Single Line Failure Scenario .............................................................78

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Figure 5-2 Illustration of Riser Stretch After Loss of Position Following Mooring Line Failure....................................................................................................................................79

Figure 5-3 - Potential Multiple Line Failure Scenario ..................................................................80 Figure 5-4 - Example of how Mooring Integrity Philosophy can affect Production ....................81 Figure 6-1 - Spooling Fibre rope onto a Powered Reel from Standard Containers ......................85 Figure 6-2 - Illustration of the Weight and Handling Issues Associated with Mooring

Components (Courtesy of Stolt Offshore) ............................................................................86 Figure 6-3 - Red Arrows Show Examples of Mooring Dog-Legs.............................................87 Figure 6-4 - Illustration of Twist on a FPSO Mooring Line during Recovery ............................90 Figure 6-5 Illustration of a Hockle in Spiral Strand Wire during Recovery of a FPSO

Mooring System ....................................................................................................................90 Figure 6-6 - Example of Damage to the Bend Stiffener on an Open Socket ................................91 Figure 6-7 Illustration of Spiral Strand Wire Kinking during Installation.................................91 Figure 6-8 - Mid Line Buoy Swivel Connection Link (courtesy of MoorLink AB). ...................92 Figure 6-9 Pre-Stretching Polyester lines During Installation to Minimise the Requirement for

Future Line Length Adjustments [Ref. 27] ...........................................................................96 Figure 6-10 - Illustration of the Potential Difficulty in offshore alignment of pins on large

Diameter Rope [Ref. 26] .......................................................................................................97 Figure 6-11 - Sledge used to Protect H Connector during Deployment over the Stern Roller

(Courtesy I. Williams)...........................................................................................................97 Figure 7-1 The Balmoral Benchmark FPV which has been continuously on station since 1986

(Courtesy of CNR) ................................................................................................................99 Figure 7-2 Plan View of Mooring Incidents at Balmoral........................................................100 Figure 7-3 Illustration of the Extent of General Corrosion on a Recovered Floating Production

Unit Mooring Line after 16 years service ...........................................................................103 Figure 7-4 Illustration of the Extent of Corrosion Pitting..........................................................104 Figure 7-5 Example of the Damage Caused to the Crown of the Links ..................................105 Figure 7-6 Arrow shows the Apparent Grinding Action on the Inner Face of One of the Links

.............................................................................................................................................105 Figure 7-7 Example of the Damage Caused to a Hanging Shackle Pin on a FPSO Mooring Line

.............................................................................................................................................106 Figure 7-8 Finite Element Stress Contour Plot (compare red areas with Figure 7-6) [Ref. 8] ..106 Figure 7-9 - Example of Thrash Zone Wear ..............................................................................107 Figure 7-10 - Illustration of the Extent of Pitting Corrosion......................................................108 Figure 7-11 - Example of Wear and Pitting Corrosion on the Shackle Pin ...............................109 Figure 7-12 -Test Rig Set Up for Break Testing of Mooring Components (Studless Chain in the

instance) ..............................................................................................................................111 Figure 7-13 Illustration of Biologically Induced Pitting Corrosion in a Ballast Tank.............112 Figure 7-14 - Crack Growth per Cycle versus Stress Intensity Range [Ref. 2] ..........................113 Figure 7-15 Illustration of Excessive Chain Wear on a CALM Buoy [Ref. 34]......................115 Figure 7-16 Typical Temperature and Salinity Profile in the Tropical Oceans .......................116 Figure 7-17 Indicative Oxygen Concentration versus Water Depth (courtesy of BP).............116 Figure 7-18 Gulf of Mexico Snap Shot of Bottom Oxygen Concentration .............................117 Figure 7-19 - Measured Wear Rates of U3 and U4 Chain at 8,170lbs (300 tonnes equivalent)

[Ref. 34] ..............................................................................................................................119 Figure 8-1 Illustration of Line Tension Variations during a Payout/Pull-In Test....................123 Figure 9-1 - Turret Design in which Chain Lengths can be Adjusted (courtesy of Chevron-

Texaco)................................................................................................................................127 Figure 9-2 Generic Turret Design in which the Chains are Stoppered off at the Turret Base

(courtesy of Bluewater) .......................................................................................................127 Figure 9-3 - Spread Moored FPSO Single Axis Chain Stopper (courtesy of SBM)...................128

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Figure 9-4 - External Cantilever Turret which experienced Chain wear at the Trumpet Welds which was halted by use of UMPHE (courtesy of Shell)....................................................130

Figure 9-5 - Example of the Level of Inspection Detail which can be achieved using a Typical Workclass ROV (courtesy of I.Williams) ...........................................................................131

Figure 9-6- Test Tank Mock-Up of Micro-ROV inspection of Chain Emerging from Turret Trumpet (courtesy of I. Williams) ...................................................................................132

Figure 9-7 - Micro-ROV Photograph of Chain Wear Notches where Chain Emerges at the Trumpet Bell Mouth (courtesy of I. Williams) ...................................................................132

Figure 9-8 - Indication of the Extent of the Wear ......................................................................133 Figure 9-9 - Artificially Introduced Notch on to Spare Chain Links, note also Red Circular

Infrared Target (courtesy of I. Williams) ............................................................................134 Figure 9-10 - Example of Stretched Chain during Break Testing, the Blue Mark Shows the

Location of a Typical Notch (courtesy of I. Williams) .......................................................134 Figure 9-11 - Example of a Special Cobalt Chromium Anti-Wear Coating (courtesy of I.

Williams).............................................................................................................................135 Figure 9-12 - Photograph of a Recovered Link Showing a Wear Notch (courtesy of I. Williams)

.............................................................................................................................................136 Figure 9-13 - An Example of the Chain Damage noted after the Notched Chains had been

recovered back to Shore (courtesy of I. Williams)..............................................................136 Figure 9-14 - Turret Arrangement where the Chain Stopper (in red) is Behind the Rotation Point

(2 black concentric circles) .................................................................................................137 Figure 9-15 Illustration of Potential Wear at Metal to Metal Contact (courtesy of I. Williams)

.............................................................................................................................................138 Figure 9-16 - Fairlead Chain Stopper where the Chain Stopper is in Front of the Rotation Point

(used on some Spread Moored FPSOs) (courtesy of Maritime Pusnes) .............................138 Figure 9-17 - As Installed Photo Graph of the Design Shown in Figure 9-16 (courtesy of

Maritime Pusnes).................................................................................................................139 Figure 9-18 Typical CALM Buoy Chain Stopper (courtesy of The Professional Divers

Handbook [Ref. 38])..........................................................................................................140 Figure 9-19 - Amoco CALM Buoy- Note Inclusion of Rubber Casting (courtesy of [Ref. 38])140 Figure 9-20 - Comparison of Alternative Fairlead Arrangements (courtesy of Bardex) ...........142 Figure 9-21 Example of a Wire Rope Bending Shoe (courtesy of API RP25K) .....................143 Figure 9-22 - Example of a Chain Bending Shoe Design [Ref. 39]............................................143 Figure 9-23 - Bending Shoe Design which includes an Angle Sensor [Ref. 40]........................144 Figure 10-1 Examples of the Subjectivity Associated with Assessing IWRC Rope Conditions

[Ref. 43] ..............................................................................................................................145 Figure 10-2 - Illustration of the Mooring Layout and Connections............................................146 Figure 10-3 - Photograph of Disconnected Socket on the Sea-Bed (courtesy of BP/Stolt

Offshore) .............................................................................................................................147 Figure 10-4 - Note End Plate also seems to be Falling Off on the Right Hand Side (courtesy of

BP/Stolt Offshore)...............................................................................................................147 Figure 10-5 - End Connection Detail .........................................................................................148 Figure 10-6 - Illustration of Socket Minus End Plate .................................................................148 Figure 10-7 - Repair Utilised Bigger Bolts and Allowed the Socket Pin to Rotate ....................149 Figure 10-8 - Example of Retrofitted Anodes to Control Corrosion Rate ..................................150 Figure 10-9 - Example of Disconnected Anodes after approximately 12 months of Service.....150 Figure 10-10 - Example of Detached Clump Weight on the Sea-Bed ........................................151 Figure 10-11 - Example of Recovered Clump Weights ..............................................................151 Figure 10-12 Illustration of Where the Damage Occurred on the Mooring Catenary .............152 Figure 10-13 - Example of a Parallel Chain Excursion Limiter (courtesy of I. Williams) .........152 Figure 10-14 - Weighted Chain Option Utilising Parallel Chain Sections (courtesy of

N.Groves) ............................................................................................................................153

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Figure 10-15 - Red Arrow Illustrates the Local Wear can take place when utilising Parallel Chain (courtesy of N. Groves) ............................................................................................153

Figure 10-16 - Example of Mid-Line Buoy Failures on a European FPSO................................154 Figure 10-17 - Gripper chock showing chain damage ................................................................156 Figure 10-18 - Upper Gypsy Wheel Arrangement before Failure ..............................................156 Figure 10-19 - Gypsy wheel structure after failure, i.e. Gypsy Wheel No Longer Present ........156 Figure 10-20 - Illustration of a New Design of Kenter Shackle intended to have improved

Fatigue Performance ...........................................................................................................157 Figure 10-21 - Example of Windlass Crack (Red Arrow) due to Stress Raiser caused by Sharp

Corner (courtesy of BP) ......................................................................................................158 Figure 11-1 - General Arrangement of the Brent Spar Mooring System (courtesy of Shell) .....160 Figure 11-2 - Brent Spar Fairlead Chain Stopper in the Hull (courtesy of Shell).......................161 Figure 11-3 - Close Up of the Stopper (courtesy of Shell) .........................................................161 Figure 11-4 - Indentation from where the chain bore down on the Stopper (courtesy of Shell) 162 Figure 11-5 Red Arrow Illustrates wear on the chain, where it sat on the stopper (courtesy of

Shell) ...................................................................................................................................162 Figure 11-6 - Brent Spar Wire Sample Y1 prior to cleaning [Ref. 41].......................................163 Figure 11-7 FLP Mooring General Arrangement (courtesy of Shell)......................................163 Figure 11-8 - Example of Short Trumpets on a Long Term Moored Floating Loading Platform

(courtesy of Shell) ...............................................................................................................164 Figure 13-1 - Comparison of Mooring Line Inspection Periods for Different FPS Categories.173 Figure 13-2 Historical Failure Rates for Different Types of Units .........................................176 Figure 14-1 - Special Joining Shackle (courtesy of Vicinay Catalogue) ....................................179 Figure 14-2 - H Shackle Pin Configuration (courtesy of I. Williams) ....................................180 Figure 14-3 Illustration of Subsea Connectors which have been used on Pre-Installed Mooring

Lines ....................................................................................................................................181 Figure 14-4 - Example of a Special Joining Plate - Note Electrical Isolating Bush ...................181 Figure 14-5 Example of the Make Up of a Typical Closed Spelter Socket (courtesy of Bridon)

.............................................................................................................................................182 Figure 14-6 - Example of an Open Socket ..................................................................................183 Figure 14-7 - Example of a Closed Socket .................................................................................183 Figure 14-8 - Connector or Termination Design Flow Diagram - Initial Phase ........................187 Figure 14-9 - Connector (Termination) Detailed Design Flow Chart.......................................188 Figure 14-10 Illustration of a Purpose Designed connector allowing limited compliance in Two

Planes ..................................................................................................................................190 Figure 14-11 - Example of a Dynamic Analysis to Estimate the Angle for the V Slot Size on

the H Shackle...................................................................................................................191 Figure 14-12 - Example of Material with a Non Clearly Defined Yield Point ..........................194 Figure 15-1 Broken Link from Fairlead.....................................................................................197 Figure 15-2 Mechanical Damage on Fairlead Link................................................................... 197

Figure 15-3 - Support of a Link in a Wheel Fairlead .................................................................198 Figure 15-4 - Photograph of Test Link Showing Bearing Plates [Ref. 10]................................. 199 Figure 15-5 - General View of Tension Bending Test Rig (protective screens removed for

clarity) [Ref. 10]................................................................................................................. 199 Figure 15-6 - Broken Hardened Plates at the end of the First Test [Ref. 10] .............................200 Figure 15-7 - Twisted Link Due to Mis-aligned Butt Weld [Ref. 10] ........................................201 Figure 15-8 - Simple Out of Flatness Twist Measurement Jig [Ref. 10] ....................................201 Figure 15-9 - Illustration of Failed Link Due to Tension Bending [Ref. 10]..............................204 Figure 15-10 - Girassol Offloading Buoy [Ref. 55]...................................................................205 Figure 15-11 - Girassol Offloading Buoy Failure in Chain Link 5 [Ref. 55] .........................206 Figure 15-12 - Girassol Offloading Buoy Failure in Polyester Rope [Ref. 55] ......................206 Figure 15-13 - Chainhawser Arrangement and Location of Critical Link [Ref. 55] .................207

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Figure 15-14 - Out of Plane Bending Mechanism (See Section 25 [Ref. 56].........................208 Figure 15-15 - Schematic of SBM Test Rig [Ref. 55] ...............................................................209 Figure 15-16 - Photograph of SBMs Test Rig [Ref. 55]............................................................210 Figure 15-17 - Typical FPSO Chain Stopper Arrangement ........................................................211 Figure 15-18 Illustration of Wire Rope Failure Modes (courtesy of Bridon)..........................217 Figure 15-19 - The 1.0MN Wire Rope Bending-Tension Fatigue Test Machine .......................218 Figure 15-20 - Tension Bending at Wheel Fairlead (Bearing Load Eccentricity) and Tension

Bending from Interlink Friction (Torque at Contact)..........................................................219 Figure 15-21 - Comparison between Various Mooring Chain S-N Curves...............................221 Figure 17-1 - Sonar Fish for Deployment through Turret (courtesy Chevron Texaco) .............227 Figure 17-2 Sonar Fish Deployment Method (courtesy Chevron Texaco)..............................227 Figure 17-3 - Sonar Display Screen Showing 12 Mooring Lines and 2 Risers Close to the Centre

(courtesty Chevron Texaco) ................................................................................................228 Figure 17-4 - Simple Pre-Installed Inclinometer with + or 1 Degree Accuracy ......................229 Figure 17-5 - Illustration of a Football Sized ROV (Courtesy of I. Williams) .......................229 Figure 17-6 - Instrumented Load Pin Shackle Link (courtesy of BMT/SMS).........................230 Figure 17-7 - Indication of the Data Available from Instrumented Mooring Lines (courtesy of

BMT/SMS)..........................................................................................................................231 Figure 17-8 - Illustration of a New Sonar System due to be Installed in the North Seas (courtesy

of I. Williams) .....................................................................................................................233 Figure 17-9 - Close Up of the Proposed Sonar Head (courtesy of Ian Williams)......................233 Figure 17-10 - Response Learning Without Line Tension Input ................................................234 Figure 17-11 - Illustration of Riser Monitoring Instrumentation (courtesy of 2H) ....................236 Figure 18-1 - Red Arrows and Black Line Indicate Key Areas subject to Degradation on a

Mooring System (leeward likely to have worst wear) ........................................................239 Figure 18-2 - Example of a Weight Discontinuity which may Result In Enhanced Wear .........240 Figure 18-3 - Typical Turret Cross Section Illustrating that the key Mooring Components are

Submerged...........................................................................................................................241 Figure 18-4 - Chain Stopper View Prior to Chain Installation with Pull in Rigging Present

(compare to Figure 18-3).....................................................................................................242 Figure 18-5 - Illustration of ROV Deployed Optical Calliper Measurement System (courtesy

of Welaptega Marine Ltd) ...................................................................................................244 Figure 18-6 Illustration of Heavy Marine Growth on Long Term Deployed Chain...............246 Figure 18-7 - In-Situ Inspection of a Wildcat Pocket by Abseillers ..........................................248 Figure 18-8 - Close Up Of Fairlead Pocket Note Slight Lip on the Right ...............................248 Figure 18-9 - Example of Chain Wear From Sitting in a Wildcat Pocket ..................................249 Figure 18-10 - Red Zones Highlight the Importance of Checking all Relevant Structural

Connections (Courtesy of CNR) .........................................................................................249 Figure 18-11 - Example of a Parted Lubrication Line Feeding a Submerged Wildcat or Gypsy

Wheel (Courtesy of CNR)...................................................................................................250 Figure 18-12 - Example of a Non Flat Link................................................................................251 Figure 18-13 - Buchan FPS Wire Rope NDT Inspection Head .................................................252 Figure 18-14 - Proposed Wire Rope Inspection Toll Delpoyed from a ROV............................253 Figure 18-15 - Example of a Difficult Area to Inspect ..............................................................256 Figure 18-16 - Partially Buried Shackle Illustrates the Difficulties in checking locking pins

(courtesy of ENI).................................................................................................................256 Figure 18-17 - Example of the Wheel Tappers Approach Used for Detecting Cracks on Railway

Carriages and Locomotives.................................................................................................258 Figure 18-18 - Example of Anchor Handling and Heading Control Tugs during a Mooring Line

Repair Operation (courtesy of I. Williams)........................................................................259 Figure 18-19 - Use of Divers from a RIB to open up the Chain Stopper during a FPSO Mooring

Line Repair (coutesy of I. Williams)...................................................................................260

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Figure 19-1 - Example of a Plate Shackle which may be useful for a Temporary Repair (courtesy of Balmoral Marine)............................................................................................................262

Figure 19-2 - Temporary Mooring Line Winch Deck on a Gulf of Mexico Spar.......................263

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1 EXECUTIVE SUMMARY

1.1 Overview of the Report The main objective of this report is to improve the integrity of the mooring systems on Floating Production Systems (FPSs). It is intended to be read and understood by non mooring specialists such as FPS Operational staff - so that the people who live and work on FPSs will be better able to become more involved in the vital task of looking after their own mooring systems. Meanwhile the included feedback on the actual performance of mooring systems in the field should assist designers and manufacturers to improve future mooring designs. Hence, the report attempts to identify gaps in the existing knowledge of mooring behaviour and components to provide a road map for future work. Appendix C includes a paper presented at the 2005 Offshore Technology Conference (OTC) which represents a stand alone summary of the key points of the JIP.

1.2 Introduction Unlike trading ships, FPSs stay at fixed positions year after year without regular dry docking for inspection and repair. Since they cannot move off station they must withstand whatever weather comes their way. Hence, depending on location, at times their mooring systems need to withstand high storm loadings. Typically, during design for harsh environments, mooring systems do not have much reserve capacity above what is required to withstand survival conditions. Therefore, deterioration of the lines over time can increase the likelihood of single or multiple line failures. Multiple line failure could conceivably result in a FPS breaking away from the moorings and freely drifting in the middle of an oil field as has been seen in the past see Section 4. Failure of two adjacent mooring lines mooring lines at the same time due to wave shock loading has been seen and this could have serious implications if the risers are pressurised at the time.

This JIP is concerned with assessing how mooring systems have performed in the field to identify the level of degradation which has taken place. Hence, the JIP has looked at FPSOs, Semi-submersible production units and Spars through out the world. From the survey it has become apparent that certain, potentially significant, problems have occurred and thus the JIP wishes to publicise these so that they can be taken account of during inspection of existing units and during the design of future units.

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1.3 Indicative Mooring Statistics At the beginning of the project it was hoped that it would be possible to gain data on the mooring performance on most of the FPSs (turret and spread moored FPSOs, production semis and Spars) in the world. In practice the best data which has been obtained is for North Sea Units, partly due to local contacts and also the rigorous reporting regime in this area. In the absence of more comprehensive information, it thus seems prudent to consider official statistics for this region to be the best available indicator of the likelihood of mooring failure. Exactly how these statistics can be related to milder environments is difficult to quantify based on the presently available data set.

Table 1-1 summarises failure statistics for North Sea operations for different floating units covering the period 1980 to 2001. It is clear from these statistics that the probability of line failure per operating year is relatively high.

Type of Unit Number of Operating Years per Failure

Drilling Semi-submersible 4.7

Production Semi-submersible 9.0

FPSO 8.8

Table 1-1 - North Sea Mooring Line Failure Data, 1980 to 2001 [Ref. 1]

Given the safety critical nature of mooring lines and the likelihood of failure one might imagine that they would be heavily instrumented with automatic alarms which would go off in case of line failure. The following indicative statistics, based on data from the majority of North Sea based FPSOs, give an indication that instrumentation is not as prevalent as might be expected for such a heavily regulated region:

50% of units cannot monitor line tensions in real time, 33% of units cannot measure offsets from the no-load equilibrium position, 78% of units do not have line failure alarms, 67% of units do not have mooring line spares available, 50% of units cannot adjust line lengths.

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1.4 The Cost of Mooring Line Failure If a multiple mooring line failure should occur in storm conditions the potential cost and the implications for the whole FPS industry could be extremely high, depending on circumstances. Even a single mooring line failure would be costly as is illustrated in Table 1-2 for two different case studies, further details can be found in Section 5.4.

Description Approx. Cost of Single Line Failure

50,000bpd N. Sea FPSO 2M

250,000bpd W. African FPSO 10.5M

Table 1-2 - Indicative Single Mooring Line Failure Costs

1.5 Key Findings from the Survey

During the course of the project a few common themes emerged which are outlined below:

Wear where the Lines Connect to the Surface Platform Achieving material compatibility at the key turret interface is vital see Section 0. Whether the trumpet pivot point should be in board or outboard of the chain stopper needs further consideration for new designs. In addition, whether rotation should be permitted in two planes, rather the one which is typically the case at present also requires addressing based on further in field experience. This may have particular implications for spread moored FPSOs. Access for inspection of these areas also needs to be improved and this should be specified in the mooring design criteria see Section 20.

Wear/Corrosion Allowance for Long-Term Moored Units On two North Sea FPSs chain wear and corrosion has been found to be significantly higher than what is specified by most mooring design codes. More in field inspection data is needed to find out if this is a general finding, which could have long-term implications for other FPSs in the North Sea and elsewhere.

Excursion Limiting Weighted Chain Designs A number of excursion limiting weighted chain systems have experienced problems see Section 10.3. Great care is needed in the design of such systems; particularly if they are due to operate in adverse environmental conditions. Parallel chains seem to have worked well, as opposed to clump weights (lump masses) or hung off chain tails. Clean catenaries, i.e. without buoys or clump weights seem to work best, although water depth, riser offset limits and environmental conditions may mean that this is not always possible.

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Local Design of Connectors Connectors are vital components on any mooring system and they need to be very carefully designed if they are to prove reliable over a long field life. Certain mooring problems have been due to the local design of the connectors. Section 14.4 includes a summary of key items which should be considered during detailed connector design. There is an emerging need for the development of a fatigue resistant connector suitable for use with mooring chains during repair/overhaul operations.

Friction Induced Bending Friction induced bending fatigue appears to be a significant issue which has been somewhat neglected and warrants further investigation. This was less of an issue for catenary systems in moderate water depths. Deep water taut moored units seem to be potentially particularly susceptible see Section 15.2.

1.6 Key Areas to Check on a Mooring System Based on the survey results,

Figure 1-1 illustrates the key areas which should be inspected on a mooring line. The FPS has been displaced by environmental forces, thus illustrating both windward and leeward mooring lines.

Figure 1-1 - Red Arrows Indicate Key Areas subject to Degradation on a Mooring System (leeward likely to have worst wear)

From a number of units it has become clear that the less loaded leeward lines can be subject to greater degradation than the windward lines. This seems to be due to greater relative rotation on leeward lines since the line is typically under lower tension.

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1.7 What are the Best Ways to Detect Line Failures? It is vital to detect line failures promptly or else there is a danger of a unit entering a storm while still producing and thus being at an increased danger of loosing another mooring line. Detecting a line failure in the mud is difficult since the catenary shape, depending on sea-bed conditions, may not change that much. Section 17 summarises the key detection techniques available at present. It is clear that in-field trials are required to identify what systems prove to be reliable over the long-term. Hence, this is an on going issue which requires monitoring, assessment and publicising of the key findings.

1.8 Inspection Technologies

Inspecting moorings lines in situ is desirable due to the danger of damage during line recovery to the surface and also during re-installation. There is also a significant cost involved in mobilising intervention vessels to recover lines to the surface and then re-install them.

In-water line inspection is difficult, particularly with respect to identifying possible cracks. Despite this it has become clear that many possible problems can be identified early on, using tweaks to existing technology. This has been successful as long as suitably experienced people are involved in planning the inspection process and examining the results.

Section 18 summarises the present available inspection technologies and includes a prioritised list of possible future improvements.

1.9 Key Conclusions and Recommendations

The survey of past and presently operating FPS units has shown that serious incidents have occurred in the past including loss of station. The survey has also shown that even for more up to date designs, deterioration of certain areas of the moorings system may be more rapid than expected. As well as the detail issues there is a more general issue that requires addressing, namely the manner in which mooring integrity is managed and audited on an on-going basis.

Since moorings are category 1 safety critical systems, if the deterioration is not detected early and monitored/rectified the consequences could be severe. Hence, it is vital that whoever offshore is responsible for the day to day operation and inspection of FPS moorings should have a strong marine background, such as a Deck Officer or Marine Engineer. Such personnel have a suitable mindset in that they really understand the importance of moorings and their likelihood to deteriorate significantly over time. It is important that these personnel should be provided with sufficient resources so that they can be pro-active with regard to inspection and any possible repairs which may be required.

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Semi-submersible units have accumulated hundreds of years of mooring experience for varied world wide locations. A key point to learn from such units is that chains, wire ropes, gypsy wheels, stoppers and connectors have finite lives and do wear out. Although drilling rigs deploy and recover lines fairly regularly, which can cause damage, the wear seen on production semis is still significant see Section 7.1. However, most large scale FPS with 20+ year design lives seem to have been built on the expectation that the mooring lines will last for the life of the field and that safety will not be compromised towards the tail end of the field life, when production rates have dropped. If production rates have dropped there is less money available for mooring line repairs. Hence, assessments should be undertaken during the field life to assess whether line change outs may be required in the future and if so contingency money should be allowed for to cover this later expenditure.

In general, moorings should be thought of as relatively vulnerable primary structural members subject to constant dynamic motion. Expecting such systems to last for 20+ years without overhaul may prove to be optimistic. The commercial risks associated with a line failure during the field life justify the selection of top quality equipment from the outset. This equipment then needs to be regularly inspected and repaired as required to ensure that it is still fit for purpose.

Availability of mooring line spares including connectors is extremely variable. Given the several month lead-time associated with procuring new components, it is recommended that each operator should identify short term remedial measures to repair a line if it fails. This would involve identifying commonly available components which can be obtained at short notice from marine equipment rental companies. Outline procedures including the type of intervention vessel required should also be developed.

Mooring systems are not as simple as they first appear and they need careful management through out their design lives. Thus a life cycle approach to mooring design and operation is recommended. In this way designers can feedback their inspection requirements to Operators and then learn from whatever is found during inspection. Manufacturers should also be included in this feedback loop, since they may be best placed to implement improvements to their products. Hence over time mooring design and manufacturing should improve. At present designers and manufacturers are not always involved with the in field behaviour mooring systems. Therefore, they may not be aware of operational or inspection type issues. In general there seems to be a need for periodic Mooring Audits to re-assess original design parameters and review inspection records to assess whether the system is still fit for purpose.

It is clear from this state of the art review that to continue to improve mooring integrity a number of topics still require further investigation. A bullet point list is included in Section 21.3.

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2 INTRODUCTION AND SCOPE

2.1 The Need for a JIP

The number of Floating Production Systems (FPSs) operating in the world increased substantially during the 1990s and there is now an ever-increasing body of FPS operational experience. In 2001 Noble Denton was commissioned by the UK Offshore Operators Association (UKOOA) to review available operational data from the British sector. The key results to emerge from this study were as follows:

There has been one FPSO line failure for every 5.4 operating years (this figure has been updated during this study);

Several cases occurred in which there was systematic damage to more than one line;

Particular problems have been experienced at connectors and interfaces; In no cases was the damage recognised immediately; Long-term failure rates remain uncertain.

The study concluded that the potential for multiple line failure is greater than is commonly perceived, and this should be a major cause for concern. The main reasons for this situation are:

Available inspection and maintenance provisions can allow long periods in which single or multiple defects can remain undetected;

Most UK sector FPSOs can not detect if they have lost a mooring line; The risk of mooring line failure is often underestimated and the majority

of operators do not carry spares or have systems in place for dealing with a line failure;

Design codes and standards give little guidance on terminations, connections, fair leads and stoppers which is where the majority of failures has been seen;

Similarly there is limited guidance on inspection, repair and maintenance.

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2.2 Scope Development

The JIP scope was developed to extend the previous UKOOA study to include international experience, and reassess the conclusions of the UK sector study in more global terms. In addition, a follow up has been carried on the recommendations of the UKOOA study to investigate the levels of exposure to duty holders, and developing measures to reduce the associated risks.

Specifically the work has covered the following:

Disseminate data gathered from international experience, Develop guidance for designing mooring connectors and interface

elements,

Provide guidance on mooring line inspection, Summarise the pros and cons of line failure detection methods, Take a look to future deepwater and taut leg applications Investigate and report illustrative case studies

The JIP scope has been adjusted during the project to take into account results found to date and also the difficulties experienced in obtaining international data.

2.3 JIP Objectives

The basic objectives of the JIP are to:

Improve safety Help to safeguard reputation of FPSO/FPS industry Feedback operational and inspection to mooring designers Publicise the importance and potential vulnerability of mooring systems

This report is intended to be read and understood by non mooring specialists such as FPS Operational staff. In this way the people who live and work on FPSs will be better able to become involved in the vital task of looking after their own mooring systems.

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2.4 Project Organization

The project organisation is illustrated in Figure 2-1.

Figure 2-1 - JIP Organisation

The scope of work was broken down into Cost, Time, and Resource Modules [CTRs], which were organized as follows:

Figure 2-2 - CTR Breakdown

Martin Brown Project Manager

Consultants: I.D. Williams, R Stonor, R Nataraja, D. Orr, R.V. Ahilan

ND Group Resources & Subcontractors

STEERING COMMITTEE

Nigel Robinson NDE Project Director

DESIGN AND CONSTRUCTION ISSUES

CTR 2 : Transportation, Handling & Installation Challenges

CTR 3 : Design of Connectors & Interfaces

DESIGN AND CONSTRUCTION ISSUES

CTR 2 : Transportation, Handling & Installation Challenges

CTR 3 : Design of Connectors & Interfaces

INTERNATIONAL SURVEY OF MOORING PROBLEMS

CTR 1 : Survey of International FPSO/ FPS ExpCTR 4 : Consequences of Line Failure

INTERNATIONAL SURVEY OF MOORING PROBLEMS

CTR 1 : Survey of International FPSO/ FPS ExperienceCTR 4 : Consequences of Line Failure

INTEGRITY MANAGEMENT

CTR 5 : Status Monitoring and Failure DetectionCTR 6 : Inspection, Repair & Maintenance, inc In

Water Survey CTR 7 : Sparing Options

DISSEMINATION OF RESULTS

(CTR 10)

Lessons Learned

Detailed Report

Integrity Check List

DISSEMINATION OF RESULTS

(CTR 10)

Lessons Learned Bulletins/Steering Committee briefings OTC paper Detailed Report

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2.5 Project Participants/Sponsors

The following list details the organisations which have sponsored the JIP plus the personnel nominated to the Steering Committee. It is worth noting that the Steering Committee meetings provided an excellent mechanism to obtain and distribute data. Thanks are given to all members of the committee and the Chairman for their participation.

1. ABS, Rod Yam and Ernesto Valenzuela 2. Ansell Jones 3. Balmoral Group, Doug Marr 4. Bluewater, Simon Stauttener 5. BP, Richard Snell, Peter Gorf and Steve Barron 6. Bureau Veritas, Frank Legerstee and Michel Franois 7. Chevron Texaco, Matthew Brierley, Paul Devlin, and Jim Hughes

(corresponding member)

8. ENI (Agip), Les Harley and Bill Nicol 9. Hamanaka Chains, Yoshiyuki Kawabe 10. HSE, Martin Muncer and Max English 11. IMS/Craig Group, Alan Duncan and Mark Prentice 12. Lloyds Register, Douglas Kemp, Richard Bamford and Alwyn McLeary 13. MARIN, Henk van den Boom and Johan Wichers 14. Maersk Marine Contractors, Graham Kennedy and Vere MacKenzie 15. National Oilwell/Hydralift-BLM, Philippe Gadreau 16. Norsk Hydro, Tom Marthinsen 17. Offspring International, Nigel Grainger and Russell Glen 18. Petro Canada, Sherry Power and Scott OBrien 19. SBM, Philippe Jean (Chairman) 20. Statoil, Kjell Larsen 21. Vicinay Cadenas, Dave Nicol and Eduardo Lopez 22. Welaptega Marine, Tony Hall

Many people from various organisations helped out through out the JIP by providing information. It is impossible to list them all, but their combined support has been crucial in enabling a comprehensive picture to be pulled together. Particular thanks are, however, given to Amerada Hess/Wood Group and Mr Ian Williams for making highly relevant data readily available to the JIP. Thanks also to Diane for all her assistance with the layout and editing of this document.

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2.6 Steering Committee Meetings

The Steering Committee met four times during the course of the JIP in Monaco, Aberdeen, Paris and Houston, all being well attended. The meetings in Monaco and Paris were part of the FPSO Forum/JIP Week. The Aberdeen meeting was a standalone meeting.

The final meeting in Houston was at the end of the 2005 Offshore Technology Conference (OTC).

Figure 2-3 Participants at the Steering Committee meeting in Paris

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3 MOORINGS OVERVIEW 3.1 Mooring Basics 3.1.1 Restoring Forces

To appreciate how to preserve the integrity of a mooring system it is helpful to have a basic understanding of the different types of mooring systems and how they work. This subject is covered in this chapter, which also includes a simple introduction to how such systems can be analysed.

The primary purpose of a mooring system is to maintain a floating structure on station within a specified tolerance, typically based on an offset limit determined from the configuration of the risers. The mooring system provides a restoring force that acts against the environmental forces which want to push the unit off station. In the following diagrams the main components of mooring system restoring force are explained.

The connection between the mooring system and the body of the vessel is where the restoring force of the mooring system acts, see Figure 3-1. At this connection point there are two force components present; horizontal and vertical. The horizontal component of the mooring lines tension acts as a restoring force. The vertical component acts as a vertical weight on the vessel. In deep water the vertical force can be quite considerable. For some designs of FPS, with limited payload capacity, the vertical mooring force can have significant design implications.

Figure 3-1 Typical Turret Moored FPSO

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It is informative to understand the significance of the mooring line angle as it departs the point of connection to the vessel. A low angle to the vertical will generate a low restoring force, with significant vertical load on the vessel. If the angle here is large, then the restoring force will be increased while the vertical load on the vessel will be reduced. This relationship can be seen in Figure 3-2. The vessel needs to be able to support the applied vertical loading.

Figure 3-2 Shallow and Steep Mooring Line Angle Illustration

The relationship outlined in Figure 3-2 is adequate for considering a 2 dimensional scenario. The mooring of a vessel, however, is a 3 dimensional problem and to this end it is necessary to consider the angle of the mooring line in the plane of the sea-surface. With reference to Figure 3-3 it can be seen that the tensions in a mooring line are split into two components; the restoring force that opposes the environmental loading, and the lateral force, which may balanced by another mooring line.

Figure 3-3 Line Heading Illustration

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3.1.2 Environmental Loading When there is no external loading on the system the vessel will not move from its static equilibrium position. When environmental loading does occur an imbalance in the system will occur. To restore equilibrium the mooring system restoring force must become equal to that of the environmental load. This is achieved through the vessel offsetting from its original position. As this occurs the windward lines will pick up tension and the leeward lines will shed tension. This is shown in Figure 3-4.

Figure 3-4 Definition of Windward and Leeward Lines + Environmental Offset

The vessel will offset until the windward lines have generated a restoring force that balances the environmental loading. This means that the distance between the anchor and fairlead will increase, and thus the tension at the fairlead will also increase. This is shown in Figure 3-5.

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The relationship between environmental load and vessel offset is often represented in a Load Excursion curve, as shown in Figure 3-6. This figure illustrates the load excursion characteristics of a 1,200m long, 76mm nominal diameter chain in 100m water depth with a working or pretension tension of 100te. The plot emphasizes the need to model the axial elasticity, even for chains, in order to get realistic results. Axial elasticity depends on geometry and material. Since there are new materials and geometries available in the market, it is important that designers should confirm with manufacturers that the values they are using agree with full scale testing values.

Figure 3-5 Offset Position and Tension Effect

Figure 3-6 Illustration of Load Excursion Curve [Ref. 2]

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3.1.3 Mooring Configuration The most common mooring configurations are Spread Moored and Single Point Mooring systems, which are taken to include turret systems. The key attributes of each are discussed in this section.

Spread Mooring This conventional mooring approach is widely adopted for semi-submersible drilling/flotel/production units. For floating production applications, spread moorings are used primarily with semi-submersibles and non-weathervaning FPSOs (i.e. no turret) see Figure 3-7. Since the wave loading on a semi-submersible is relatively insensitive to direction, a spread mooring system can be designed to hold a semi on location regardless of the direction of the environment, although there is probably an optimum heading. However, a spread system can also be applied to ship-shaped vessels, which are more sensitive to environmental directions, as long as the environmental conditions are relatively benign and the weather direction is fairly uniform without strong cross currents. In a location such as the North Sea, the forces which can be generated on the beam of a spread moored FPSO, plus the motions in such conditions, effectively prohibit such a mooring arrangement.

The mooring lines can be chain, wire rope, fibre rope or a combination of the three. Either conventional drag anchors or anchor piles can be used to terminate the mooring lines.

Figure 3-7 - Typical Spread Moored Unit, Girassol FPSO offshore West Africa

(courtesy of Stolt Offshore)

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Spread moorings are typically cheaper than turret moorings since they are mechanically far less complicated. However, they are limited to where they can be used and they can make offloading operations by a shuttle tanker somewhat more involved.

Single Point Moorings (SPMs) Single point moorings (SPMs), such as internal or external turrets, are used primarily for ship shaped units see Figure 3-1. They allow the vessel to weathervane, which is necessary to minimise environmental loads on the vessel by heading into the prevailing weather. There is a wide variety in the design of SPMs, but they all perform essentially the same function.

3.1.4 Catenary and Taut Leg Moorings

Two main types of mooring system can be used for either the Spread or Single Point system; Taut-Leg and Catenary. Both methods allow the system to withstand the applied forces, but through different mechanisms.

A catenary system generates restoring force through the lifting and lowering of the line onto the seabed, plus a limited amount of line stretch. This is shown in Figure 3-8 with a typical arrangement shown in Figure 3-9.

A taut-leg system makes use of the material properties of the mooring line, namely its elasticity, as shown in Figure 3-10. A typical taut-leg arrangement is shown in Figure 3-11. Taut-leg moorings are relatively new and are typically used in deep water to limit FPS offsets.

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Figure 3-8 Illustration of Catenary System

Figure 3-9 - Typical Spread Moored Catenary System (Courtesy of Vryhof)

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Figure 3-10 Illustration of Taut-Leg system

Figure 3-11 - Typical Spread Moored Taut-Leg System (Vryhof)

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3.1.5 Vessel Dynamics Waves will cause a vessel to move in all six degrees of freedom; surge, sway, heave, roll, pitch and yaw. These degrees of freedom are illustrated in Figure 3-6.

The motion of the vessel to individual waves is called its wave frequency or first-order response. As a mooring line moves through the water it will be subject to dynamic line drag and inertia loading and sometimes a whipping effect. It is possible to take this into account by undertaking a dynamic mooring analysis, but this does increase computing time significantly.

Figure 3-12 Illustration of Surge, Sway, Heave, Roll, Pitch and Yaw

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The compliance of a mooring system is such that conventionally the presence of the mooring system is not considered to affect the wave frequency response. The overall mooring system stiffness and associated natural frequency will influence its second order or low frequency slow drift response.

In deep water for certain floating objects, such as deep draft Spars, the wave frequency motion is attenuated to a certain extent by the mooring system due to the higher system stiffness. Hence, a coupled analysis is sometime undertaken. The general conclusion from this type of analysis appears to be that the mooring quasi-static tension has an impact on a floater's wave frequency response, which in turn will affect the mooring dynamic tension. On the other hand, the effect of dynamic tension is less important to a floater's wave frequency response. For deep water the effect of risers on the vessel response becomes increasingly important and this should be taken into account.

The coupled wave frequency motion of a floater can be calculated in the time domain using the wave force, wave frequency added mass and damping, and mooring force at each time step. Usually a convolution method needs to be adopted in the radiation force calculation. Although the coupled wave frequency motion calculation in the time domain is slower than the Response Amplitude Operator (RAO) based wave frequency motion calculation, it is still acceptable. Typically a 3 hour simulation will take a few minutes. However if there is very high mooring stiffness or if a mooring dynamic analysis is performed, then the computing time will be high.

3.1.6 Mooring Design

The tensions experienced by a mooring system at any time are driven by the following:

Static component from Wind, Mean Wave Drift and Current, Wave frequency component, caused by 1st order wave frequency motions and

drag/inertia effects on the line, Low frequency component, due to 2nd order low frequency waves and wind

dynamics.

The essence of mooring design is to optimise the behaviour of the mooring system such that the excursions of the surface vessel do not exceed the allowable flexible riser offsets, while at the same time ensuring that the line tensions are within their allowable values. Thus the mooring system load offset curve should not be too hard or too soft see Figure 3-13. Hence, considerable iteration work may be required to optimise a system for a particular location.

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It is worth noting that spring buoys (mid water buoys) and clump weights can also be used to obtain an optimised mooring system stiffness by extending the resistive forces over greater distances, hence allowing clearance over subsea features. However, their use should be treated with caution, particularly in areas subject to harsh environmental conditions, where they have been known to come adrift see Section 9.3. Buoys and clump weights are also likely to introduce bending effects which may have an undesirable impact on the fatigue life see Section 15.

Figure 3-13 Example of Optimising the Stiffness of the Load offset Curve

3.1.7 Mooring Analysis Calibration with Full Scale Behaviour The determination of maximum tensions for a multiple line system requires application of specialist computer programmes, which in many cases have been under continuous development for a number of years. Despite this, there are still uncertainties in estimating mooring loads using analysis software and model tests. Hence, it would be desirable to compare the behaviour of a full scale FPS in known weather conditions versus predictions. Surprisingly little work has been done on this topic, although this is partly due to the difficulties associated with obtaining reliable weather and instrumentation readings.

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3.1.8 Active Winching and Thruster Assistance Since there are now hundreds of years of accumulated mooring experience from semi-submersible rigs, it is informative to understand the basis of their mooring operations. This is reviewed in this section, which considers active winching and thruster assistance.

Active Winching Active winching can be undertaken on semi-submersible production, drilling and accommodation units. There are two basic options, namely:

1. Leeward line slackening, 2. All round length adjustment, including windward lines, so that the tensions are as

well balanced as possible at the limit of vessel surge.

If the leeward lines are slackened down too much this can result in greater yawing/surging and reduced direction control which can lead to higher line tensions. In other words, if there is too much slack in the system, there is an increased danger of high line snatch loadings. Windward line tension optimisation can also be problematic. To quote from Robert Ingliss informative 1992 paper [Ref. 3]:

in practise rig operators are reluctant to adjust windward line tensions in severe weather conditions and usually restrict adjustments, if any, to slackening leeward lines. This is partly to do with limitations in winch stall capacity and the risk of a winch or brake failure, but most importantly the majority of rigs are not provided with suitable tension monitoring devices and computerised winch control systems which would make extensive line tension optimisation a realistic possibility. The general situation is that analysts frequently utilise line optimisation to reduce tensions to meet acceptance criteria but these line tension optimisation procedures are almost never implemented in practice on a rig.

Based on this type of feedback the latest mooring design codes (e.g. ISO [Ref. 4] + OS E301 [Ref. 5]) do not permit either windward or leeward active wincing to minimise mooring line tensions apart from going from one operational state to another.

Thruster Assistance A number of semi-submersibles and a relatively small number of FPSOs are equipped with thruster assistance. The thruster assistance can be categorised as either Thruster Assistance (TA) or Automatic Thruster Assistance (ATA). TA is based on manual joystick thruster control. ATA makes use of automatic remote control algorithm system to control the behaviour of the thrusters.

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It has been found that operation of the thrusters can be very effective in reducing peak line tensions; even though the thrust delivered can be modest. Typically in a mooring analysis the thrusters are considered to reduce the mean load applied to the mooring system. However, thrusters also seem to damp down the magnitude of the slow drift second order offsets. They can also be helpful with respect to heading control. This can be particularly useful on a production vessel, if a small change in heading can result in reduced vessel motions, thus improving the efficiency of the oil/water separation process.

In practical terms, when operating in manual thruster mode, high line snatch loads can be avoided by applying thrust as the wave train approaches. This will tend to push the vessel in the direction of the advancing sea. As the wave passes it is necessary to ease down on the thrust to avoid over slackening the windward lines. If these become too slack there is an increased danger of snatch loading when the next wave train passes through.

3.1.9 Metocean Parameters and their Impact on Mooring Integrity For relatively benign environments, such as off West Africa, there is a much smaller difference between operational and survival sea states compared to say the North Sea. This means that if the metocean parameters, or the response of the vessel due to these parameters, is underestimated, there is significantly less of an in built safety margin compared to harsher climates, particularly with regard to fatigue.

The degree of spreading of the waves (see Figure 3-14 and Figure 3-15) can also affect mooring analysis results. The geographic area and fetch distance will influence the type of waves likely to be encountered in practice. Conventionally, short crested seas are considered to result in reduced wave frequency response and hence reduced mooring line tensions - see section 3.3.2 of [Ref. 6]. However, recent model test results at DHI in Denmark has shown that for certain vessel sizes the mooring loads in short crested waves can be higher than in long crested waves [Ref. 7]. Thus the key point is to ensure that the response of the system is thoroughly evaluated for the worst expected conditions (ie short or long-crested) both from a fatigue and a strength point of view.

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Figure 3-14 - Illustration of Long Crested (Unidirectional) Seas

Figure 3-15 - Illustration of Short Crested (Confused) Seas

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3.1.10 Rogue/Steep Breaking Waves and Shock Loading Mariners have used phrases such as Freak Waves, Rogue Waves, Walls of Water or even Holes in the Sea, to describe some of the conditions they have experienced at sea. Trading vessels are typically weather routed to avoid the worst of predicted weather conditions. However, permanently moored FPSs have to ride out whatever weather is thrown at them.

From a statistical sense the longer a FPS is on station the more likely it is to experience 100 year + conditions. If an elderly FPS with a mooring system which has seen wear, corrosion and has accumulated some hair line cracks is subject to such conditions, the likelihood of single or even multiple line failure is increased.

Very occasionally an unusually steep wave slam load could occur at the same time that a floating structure is around its maximum slow drift offset. The resulting shock or spike load on the mooring might be quite considerable. How much this shock loading is transferred to the mooring lines will depend to a significant extent on the degree of structural damping in the hull structure, the vessel inertia, how long the load acts and where the moorings are relative to where the wave impacts. For a semi, where you might get wave slam/slap right into one of the corners (see Figure 3-16), the amount of structural damping might well be less than compared say to a FPSO with an internal turret (see Figure 3-19). Hence the loading could be higher.

Figure 3-16 - Example of a Wave Breaking on a Column of a Semi-Submersible

In deep water steep elevated wave fronts with breaking or near breaking crests can occur see Figure 3-17. In addition, a "Three Sisters" wave group can occur in which the second wave is generally the highest and is often preceded by a long trough. Hence, a moored object may ride the first wave, but then plunge submerged into the base of the second steep fronted wave that then inflicts the greatest shock loading.

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Figure 3-17 - Illustration of Deepwater Breaking Wave Types (Plunging Break on the Left and Spilling Breaking on the Right)

In November 1998 the Schiehallion FPSO was struck by a wave which was felt throughout the vessel. The wave caused tears in the forward shell plating of the forecastle superstructure, buckling of supporting stiffeners and permanent deformation of the forecastle tween deck see Figure 3-18. Production was shut down and non essential personnel were evacuated to a nearby drilling rig. In this instance no damage was reported to the mooring system, but it illustrates the danger presented by infrequent steep breaking waves.

Figure 3-18 - Illustration of the Damage Caused to Schiehallions Bow by an Unusually Steep Wave (courtesy of BP)

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Present day standard mooring analysis tools do not evaluate this potential shock load effect on the mooring systems. Hence it is difficult to quantify. But there is a possibility, based on the wave description, that it could have been a factor which led to the virtually instantaneous multiple line failures experienced by Petrograd 1 in the early 1990s (see Section 4.3). This might also be a factor in the relatively frequent mooring line failures experienced by semi-subs. It is recommend that this topic should be investigated further and that appropriate cross checks should be made with the real life recorded response of FPSs in severe/steep sea weather conditions. However, it also should be noted that such weather conditions do not occur very often.

Figure 3-19 - Model Illustration of the Effect of a Breaking Wave on a FPSO (Courtesy of APL website)

The right hand side photograph of Figure 3-19 is perhaps an example of the type of wave conditions which could impart a shock loading to the moorings, depending on the FPSO offset at the time. If a mooring line had already broken and its failure had not been detected (due to a lack of failure of instrumentation) the chance of additional line failures would be high in these conditions.

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3.2 Mooring Line Constituents

3.2.1 Introduction

Various different materials can be used to assemble a mooring line. This section provides a brief description of the main components that typically constitute a mooring line. The pros and cons of the various types of line components are explained. This helps to aid understanding when considering how actual systems have performed in situ. Connectors and terminations are considered separately in Section 14.

3.2.2 History of Studded and Studless Chain

Early mooring lines tended to make use of simple links without studs. Development of this design led to usage of studded links, see for example Figure 3-20. Ease of handling and avoidance of kinking were the primary reasons for the introduction of studs. The resulting link geometry (see Figure 3-21) took advantage of the ability of the stud to resist some of the bending loads in the links. The studded link standard geometry of length of 6 x Bar Diameter (D) and breadth of 3.6 x D was approved by the British Admiralty in the 1860s.

Historically anchor chain used on ships was, in general, only required to meet intermittent short term loading and therefore, even over a long ship service life, fatigue was unlikely to be a problem.

Figure 3-20 - Isambard Kingdom Brunel in front of Studded Chain for the Great Eastern steam ship, circa 1858

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Studless Link Studded Link

Figure 3-21 Comparison of the Geometry of Modern Studded and Studless Chain

[Note: DNV Cert Note 2.6, states 3.3D to 3.4D for the of studless link width]

Fairly recent long-term applications of chains in the moorings of floating production systems have brought about the development of studless chain. The studless chain link has been redesigned with a smaller breadth to reduce the bending loads. These designs are increasingly used for long-term moorings because loose and missing stud problems are eliminated. Unfortunately, however, the fatigue life of studless chain has been shown to be half that of comparable studded chain, based on the results of fatigue testing [Ref. 8]. In other words the fatigue endurance of studded chains is twice that of studless if the studs remain tight. Of the 70 fatigue failures reported in the Houston JIP, 52% occurred at an inner Half-Crown position, 34% at an inner Crown position and 14% at a mid leg position. The Crown refers the area of maximum bend and Half-Crown essentially refers to the area of the link where bending commences.

The studless link standard geometry of length of 6 x D and breadth of 3.35 x D came to the market after 1989 as consequence of collaboration between DNV and Vicinay for the Veslefrikk B project. For this chain the first tentative specification went out in 1995 with the DNVs Certification Note 2.6. More recent developments include customised chain geometries also known as Variable Geometry and Weight (VGW) as discussed in OTC paper 8148, 1996 [Ref. 9]. VGW provides flexibility to modify link geometry and weight to suit a particular application see for example Section 18.8.2.

Table 3-2 and Table 3-3 summarise the relative merits of studless and studded chain in terms of design, manufacturing, inspection and maintenance.

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Requirement Recommended Chain Reason

Lower static or dead weight in the catenary Studless Lower weight per metre

Access for shackle and accessory connection

Studless More interior link space

Versatility, similar to end links Studless Completely open links

Greater safety factor with same weight per metre in the catenary (strength to weight ratio)

Studless Larger possible diameter with less weight per metre

Greater stiffness in the mooring line Studlink Higher elasticity modulus

Higher Break Load Both Same break load, but different proof loads

Transition through windlasses and fairleads

Both But studless more likely to knot or twist

Long fatigue life Open to discussion See previous page

Table 3-1 Summary of Chain Design Parameters (modified from Vicinay Chain Catalogue)

Requirement Recommended Chain Reason

Better inspection of weld and crown area Studless Greater access due to lack of stud

Elimination of stud locating problems Studless Lack of stud

Oversizing of the link in the weld zone Studless Elimination of the flattening and material expansion in the weld zone

No links with stud looseness Studless Lack of stud

Table 3-2 Comparison of Manufacturing Parameters

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Requirement Recommended Chain Reason

Eliminate premature fatigue due to loose studs

Studless No stud, therefore no notch effect

Reduce inspection/repair costs Studless Easier access + no loose studs to repair

Eliminate galvanic reaction between the stud and the link

Studless No stud, therefore no possibility of reaction

Increase reliability of the chain over time To be determined Although there are no loose studs issues with studless, the fatigue performance of studless is less good than that of studded

Handling and connectability with D shackles and hooks

Studless Better access for the through pin. Minimal requirements and restrictions

Early indication of system degradation Studli