Mooring Equipment Guidelines - Clb Thuyền Trưởng...

The OCIMF mission is to be the foremost authority on the safe and environmentally responsible operation of oil tankers and terminals, promoting continuous

improvement in standards of design and operation.

Oil Companies International Marine Forum

Mooring Equipment Guidelines3rd Edition (MEG )

Issued by theOil Companies International Marine Forum

First Published 1992Second Edition 1997

Third Edition 2008

ISBN 978 1 905331 32 1

© Oil Companies International Marine Forum, Bermuda

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library.

The Oil Companies International Marine Forum (OCIMF) is a voluntary association of oil companies having an interest in the shipment and terminalling of crude oil and oil products. OCIMF is organised to represent its membership before, and to consult with, the International Maritime Organization and other governmental bodies on matters relating to the shipment and terminalling of crude oil and oil products, including marine pollution and safety.

Terms of Use

The advice and information given in this guide (´Guide`) is intended purely as guidance to be used at the user’s own risk. No warranties or representations are given nor is any duty of care or responsibility accepted by the Oil Companies International Marine Forum (OCIMF), the membership or employees of OCIMF or by any person, firm, corporation or organisation (who or which has been in any way concerned with the furnishing of information or data, the compilation or any translation, publishing, supply or sale of the Guide) for the accuracy of any information or advice given in the Guide or any omission from the Guide or for any consequence whatsoever resulting directly or indirectly from compliance with, adoption of or reliance on guidance contained in the Guide even if caused by a failure to exercise reasonable care on the part of any of the aforementioned parties.

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iii

Introduction

The shipping industry has always been concerned with safe mooring practices. A fundamental aspect of this concern entails the development of mooring systems that are adequate for the intended service, with maximum integration of standards across the range of ship types and sizes. To further this aim the Oil Companies International Marine Forum first published Mooring Equipment Guidelines in 1992 and this latest, third edition provides a major revision and update to the original content to reflect changes in ship and terminal design, operating practices and advances in technology.

Although numerous standards, guidelines and recommendations concerning mooring practices, mooring fittings and mooring equipment exist, where guidance is given it is often incomplete. For example, the number of hawsers and their breaking strength may be recommended without any advice on mooring winch pulling force or brake holding capacity. These guidelines provide an extensive overview of the requirements for safe mooring from both a ship and terminal perspective and embrace the full spectrum of issues from the calculation of a ship’s restraint requirements, the selection of rope and fitting types to the retirement criteria for mooring lines.

A broad-based working group was established by OCIMF to develop the text for this edition with the participation of OCIMF members and other industry associations, including the International Association of Independent Tanker Owners (INTERTANKO), the International Chamber of Shipping (ICS), the Society of International Gas Tanker and Terminal Operators (SIGTTO), the International Association of Classification Societies (lACS), the International Association of Ports and Harbors (IAPH), the Nautical Institute (NI) and the International Harbour Masters Association (IHMA). Valuable contributions were also received from representatives of rope manufacturers, winch manufacturers, equipment suppliers, shipyards and specialist consultants.

The following is an overview of some of the substantive changes included in this edition:

Wind and current drag coefficients have been included from earlier OCIMF and SIGTTO publications •that are now out of print. All coefficient data is now appended to the Guidelines

the guidance has been expanded to account for site-specific conditions at terminals and the impact •on mooring patterns, prompting consideration of the need for more rigorous analysis incorporating vessel motion and dynamic force calculations

reference has been made to the content of IMO MSC/Circ.1175 • Guidance on Shipboard Towing and Mooring Equipment and related IACS Unified Requirements. In addition, guidance on ship’s fittings associated with both emergency towing, escorting and pull-back and harbour towing includes relevant content from the OCIMF publication Recommendations for Ships’ Fittings for Use with Tugs

the concept of ´Design Basis Load` has been introduced for establishing the required strength of •ship’s mooring fittings. The treatment of geometric effects, such as wrap angle on a fitting, has been modified to align with practices in other industries and is no longer automatically included within quoted safety factors

it is recommended that all ship’s mooring fittings should be designed to carry the MBL of the •attached mooring. The recommendations concerning the strength of ship’s mooring fittings are based on the principle of rope failure before fitting failure and fitting failure before hull or foundation failure

recommendations on the marking of fittings are aligned with the requirements of IMO MSC/1175, as •adopted in SOLAS Chapter II – I, Regulation 3-8

full account has been taken of the introduction of new rope materials, such as those manufactured •from High Modulus Polyethylene (HMPE), and the related impact on equipment design and operation. Relevant content from the OCIMF publication Guidelines on the Use of High-Modulus Synthetic Fibre Ropes as Mooring Lines on Large Tankers has been included

guidance on mooring line tails has been revised in the light of industry experience, particularly with •regard to their use at exposed berths

revised guidance is appended on the inspection and maintenance of mooring lines. •

These guidelines represent best known mooring technology and practice. It is recognised that it may not always be practical to retrofit all aspects of this technology to existing mooring systems. For existing ships, where the mooring arrangement does not meet the recommendations described in these guidelines, both ship and terminal operators should be made aware of the limitations of the mooring system and have

Introduction

iv

Mooring Equipment Guidelines 3rd Edition

contingency plans drawn up to deal with them. The contingency plans should include (but not be limited to) predetermined environmental limits for berthing, stoppage of cargo loading or unloading, and departure from the berth.

Alternatives to the recommendations contained in these guidelines should only be introduced on the basis of a formal risk assessment and should be implemented through a proper change management process. The guidelines address ‘conventional’ and ‘alternative’ mooring systems, but this does not extend to arrangements and novel designs, such as those employing vacuum pads. In addition the guidelines are not intended to apply to vessels operating in extreme environments.

This publication attempts to refine, unify and update selected existing guidelines and to add essential information that has either been omitted or poorly defined. Care has been taken to ensure that the design performance of equipment is optimised, while not overlooking the equally important factors of ease of handling and safety of personnel.

These guidelines represent a recommended minimum requirement and are intended to be useful to ship and terminal designers and operators. They are not intended to inhibit innovation or future technological advances. Although primarily addressing tankers and gas carriers, many of the recommendations are considered to be equally applicable to other vessel types.

With the publication of this third edition, the following documents have been superseded and are removed from print:

OCIMF • Guidelines on the Use of High-Modulus Synthetic Fibre Ropes as Mooring Lines on Large Tankers (First Edition, 2002)

OCIMF • Recommendations for Ships' Fittings for Use with Tugs (First Edition, 2002)

OCIMF • Prediction of Wind and Current Loads on VLCCs (Second Edition, 1994)

OCIMF/SIGTTO • Prediction of Wind Loads on Large Liquefied Gas Carriers (First Edition, Reprinted 1995)

v

Contents

Contents

Introduction iii

List of Figures x

List of Tables xiv

1 Principles of Mooring 1

1.1 General 3

1.2 Forces Acting on the Ship 4

1.2.1 Wind and Current Drag Forces 4

1.3 Mooring Pattern 7

1.4 Elasticity of Lines 11

1.5 General Mooring Guidelines 14

1.6 Operational Considerations 16

1.7 Terminal Mooring System Management 17

1.7.1 Operating Limits 18 1.7.2 Operating Guidelines/Mooring Limits 18 1.7.3 Joint Terminal/Ship Meeting and Inspection 22 1.7.4 Instrumented Mooring Hooks or Visual Inspection of Mooring Lines 22

1.8 Ship Mooring Management 23

1.8.1 Line Tending 23

1.9 Emergency and Excessively High Mooring Load Conditions 24

1.10 Limitations on the Use of Tugs and Boats 25

1.11 General Recommendations 26

1.11.1 Recommendations for Berth Designers 26 1.11.2 Recommendations for Terminal Operators 26 1.11.3 Recommendations for Ship Designers 27 1.11.4 Recommendations for Ship Operators 27

2 Mooring Restraint and Environmental Criteria 29

2.1 General Considerations 31

2.2 Standard Environmental Criteria 32

2.3 Calculation of Forces 33

2.4 Mooring Restraint Requirements 34

2.4.1 Basic Principles of Mooring Calculations 34

2.4.2 Standard Restraint Requirements 36

2.5 Site-Specific Environmental Data and Mooring Line Loads 37

2.5.1 Most Probable Maximum (MPM) Wave Motions 38

vi


3 Mooring Arrangements and Layouts 39

3.1 Principal Objectives 41

3.2 Requirements at Piers and Sea Islands 43

3.2.1 Number, Size and Type of Lines 43 3.2.2 Arrangements for Breast Lines 44 3.2.3 Arrangements for Spring Lines 44 3.2.4 Special Arrangements for Gas Carriers 50

3.3 Requirements at SPMs 53

3.4 Requirements for Emergency Towing, Escorting and Pull-Back 54

3.4.1 Fittings for Tug Escort and Pull-Back 55

3.5 Requirements for Multi-Buoy Moorings 57

3.6 Requirements for Harbour Towing 59

3.7 Requirements for Barge Mooring 62

3.8 Requirements for Canal Transit 63

3.9 Requirements for Ship-to-Ship (STS) Transfer 64

3.9.1 Requirements for Receiving Ship 65 3.9.2 Requirements for Discharge Ship 66

3.10 Arrangements at Cargo Manifolds 68

3.11 Mooring Augmentation in Exceptional Conditions 69

3.11.1 Provision of Shore Moorings 69 3.11.2 Use of Shore-Based Pulley 69 3.11.3 Advantage of Pulley System 69 3.11.4 Disadvantage of Pulley System 69

3.12 Emergency Towing-off Pennants 70

3.13 Combination of Various Requirements 72

3.14 Safety and Operational Considerations 73

3.15 Equipment and Fitting Line-up 74

4 Design Loads, Safety Factors and Strength 75

4.1 General 77

4.2 Basic Strength Philosophy 78

4.3 Existing Standards and Requirements 79

4.4 Recommended Design Criteria 80

4.4.1 Bitts (Double Bollards) 81 4.4.2 Single Cruciform Bollard (Single Bitt) 81 4.4.3 Recessed Bitt 81 4.4.4 Closed Chock (Fairlead) 82 4.4.5 Pedestal Fairlead and Rollers of Button-Roller Chocks 82 4.4.6 Universal Fairlead (4 Roller Type) 83 4.4.7 Universal Fairlead (5 Roller Type) 83

vii

Contents

4.4.8 Emergency Towing Arrangement 84 4.4.9 Single Point Mooring Equipment 85 4.4.10 Mooring Winches 85 4.4.11 Comparison of Combined Stresses with the 85% of Yield Criterion 85

4.5 Strength Testing of Mooring Fittings 86

4.6 Marking of Mooring Fittings 87

4.7 General Recommendations 88

4.7.1 Recommendations for Ship Designers 88 4.7.2 Recommendations for Ship Operators 88

5 Structural Reinforcements 89

5.1 Basic Considerations 91

5.2 Mooring Winches 93

5.3 Chocks and Fairleads 94

5.4 Pedestal Fairleads 98

5.5 Bitts 101

5.6 Recessed Bitts 102

5.7 SPM Fittings and Smit Brackets 103

5.8 Tug Push Points 104

5.9 Special Considerations 105

5.9.1 Rounded Gunwale Connection 105 5.9.2 Doublers versus Inserts 105 5.9.3 High Strength Steel Fittings 105

5.10 Certification and Inspection 106

6 Mooring Lines 107

6.1 General 109

6.1.1 General Safety Hazards 109 6.1.2 Strength Criteria 111 6.1.3 Elasticity 112 6.1.4 Record Keeping 113

6.2 Wire Mooring Lines 114

6.2.1 Material 114 6.2.2 Construction 114 6.2.3 Corrosion Protection 114 6.2.4 Bend Radius 116 6.2.5 Handling, Inspection and Removal from Service 117 6.2.6 Standard Specifications 117

6.3 Conventional Fibre Mooring Lines 118

6.3.1 General 118

viii


6.3.2 Construction 121 6.3.3 Bend Radius 121 6.3.4 Handling and Storage of Synthetic Lines 123

6.4 High Modulus Fibre Mooring Lines 124

6.4.1 General 124 6.4.2 Properties of High Modulus Synthetic Fibres 125 6.4.3 High Modulus Synthetic Fibre Materials 125 6.4.4 High Modulus Synthetic Rope Constructions 126 6.4.5 Characteristics 126 6.4.6 Selection Criteria 127 6.4.7 Installation 128 6.4.8 Inspection and Removal from Service 130

6.5 Synthetic Tails 131

6.5.1 General 131 6.5.2 Tail Length 131 6.5.3 Retirement Criteria 134 6.5.4 Methods of Connecting Tails 134

7 Winch Performance, Brake Holding Capacity and Strength Requirements 137

7.1 Function and Type of Mooring Winches 139

7.1.1 Automatic Tension Winches 139

7.2 Winch Drums 140

7.2.1 Split Drums 140 7.2.2 Undivided Drums 141 7.2.3 Handling of SPM Pick-up Ropes 141

7.3 Winch Drives 143

7.3.1 Hydraulic Drives 143 7.3.2 Self-Contained Electro-Hydraulic Drives 144 7.3.3 Electric Drives 144 7.3.4 Steam 144

7.4 Winch Brakes 145

7.4.1 Layers of Mooring Line on Drum 146 7.4.2 Band Brakes 147 7.4.3 Disc Brakes 150 7.4.4 Input Brakes 150 7.4.5 Winch Brake Testing 150 7.4.6 Brake Holding Capacity 153

7.5 Winch Performance 154

7.5.1 Rated Pull 154 7.5.2 Rated Speed 154 7.5.3 Light-Line Speed 154 7.5.4 Stall Heaving Capacity 154 7.5.5 Drum Capacity 154

7.6 Strength Requirements 156

7.7 Winch Testing 157

ix

Contents

7.7.1 Rules Concerning Testing at Manufacturer’s Facility for the Acceptance of the Manufacturer and Purchaser 157

7.7.2 Onboard Acceptance Test 157

7.8 Summary of Recommendations 158

7.8.1 Recommendations for Ship Designers 158 7.8.2 Recommendations for Ship Operators 158

8 Mooring Fittings 161

8.1 Introduction 163

8.2 Mooring Bitts 164

8.3 Cruciform Bollards 165

8.4 Closed and Panama-Type Chocks 166

8.5 Roller Fairleads and Pedestal Fairleads 167

8.6 Universal Roller Fairleads 168

8.7 Selection of Fitting Type 171

8.8 Stoppers 172

Appendices 175

A Wind and Current Drag Coefficients for VLCCs and Gas Carriers and Example Force Calculation (Including Symbols and Notations) 177

B Rope Over-Strength 205

C Guidelines for Handling, Inspection and Removal from Service of Wire Mooring Lines 213

D Guidelines for Inspection and Removal from Service of Fibre Ropes 227

E Tanker Mounted SPM Fittings 235

F Strength of Chain Tensioned over a Curved Surface 243

Glossary of Terms and Abbreviations 259

Bibliography 265

Index 267

x


List of Figures

1.1 Typical Mooring Pattern 3

1.2 Wind Forces on a Ship 5

1.3 Effect of Underkeel Clearance on Current Force 6

1.4 Mooring Pattern Analysis 9

1.5 Effect of Hawser Orientation on Restraint Capacity 10

1.6 Effect of Mooring Elasticity on Restraint Capacity 12

1.7 Comparison of Steel Wire versus HMPE Mooring Lines with and without 11m Tails 15

1.8 Effect of Line Length on Tending Requirements 16

2.1 Generic Mooring Layout Used for Computational Purposes 36

3.1 Typical Mooring Arrangement of a Tanker 46

3.2 Tanker - Mooring Arrangement on the Forward Deck 48

3.3 Tanker - Mooring Arrangement on the Aft Deck 49

3.4 Special Arrangement for Aft Backsprings 50

3.5 Typical Mooring Arrangement of an LNG Carrier 51

3.6 LNG Carrier - Mooring Arrangement on the Forward Deck 52

3.7 LNG Carrier - Mooring Arrangement on the Aft Deck 52

3.8 Typical Emergency Towing Arrangement at Forward End 54

3.9 Typical Emergency Towing Arrangement at Aft End 55

3.10 Multi-Buoy Mooring (MBM) 58

3.11 Mooring Pattern During Ship-to-Ship Transfer 65

3.12 Rigging of Emergency Towing-off Pennant 70

3.13 Alignment and Maximum Fleet Angle for Mooring Winches 74

5.1 Typical Cantilevered Foundation in way of Rounded Gunwale 94

5.2 Example of a Cantilevered Fairlead Foundation 95

5.3 Roller Fairlead with Individual End Frames 96

5.4 Deck Reactions with Two Types of Universal Fairleads 97

5.5 Typical Foundation for Pedestal Fairlead 98

5.6 Deck Reinforcement for Pedestal Fairlead 99

5.7 Extended Reinforcement Example to Reduce Stress on Longitudinals 100

6.1 Examples of Potential Snap-Back Danger Zones 110

6.2 Load - Extension Characteristics - Wire and Fibre Ropes, New and Broken-In 112

xi

List of Figures

6.3 Wire Line Constructions 115

6.4 Effects of Bending on Wire Rope Strength 117

6.5 Construction of Conventional and High Modulus Synthetic Fibre Ropes 122

6.6 A) Fairing of Split Drum Edge 129

B) & C) Faired Split Drum Edge 129

6.7 A) & B) Significant Wave Height and Mean Period, Upper Bound Limits for 50% Line Load for 138,000 m3 LNG Carrier 132

C) Significant Wave Height and Mean Period, Upper Bound Limits for 50% Line Load for 267,000 m3 LNG Carrier 133

D) & E) Significant Wave Height and Mean Period, Upper Bound Limits for 50% Line Load for 107,000 DWT Tanker 133

6.8 Typical Links for Connecting Lines with Tails 134

6.9 Cow Hitch 135

7.1 The Split Drum Winch 140

7.2 Jacketed High Modulus Fibre Moorings on Split Drum Winches 141

7.3 Calculation of Mooring Line MBL and Relationship to Winch Parameters 146

7.4 Effect of Applied Torque on Brake Holding Power 147

7.5 Spring-Applied Brake with Hydraulic Release 149

7.6 Spring-Applied Brake with Manual Setting and Release 149

7.7 Typical Winch Brake Test Equipment 151

7.8 Improper Fitting of Locking Nuts to Brake Tightening Screw 152

7.9 Simplified Brake Test Kit 153

7.10 Effect of Slippage on Final Brake Holding Load – Spring-Applied Brakes 153

8.1 Methods of Belaying a Rope on Bitts 165

8.2 Closed Chock 166

8.3 Types of Universal Roller Fairleads 169

8.4 Additional Chafe Plates for Type A Fairleads 170

8.5 Universal Fairleads with Additional Inboard Rollers 170

8.6 Stoppers 172

A1 Sign Convention and Coordinate System 182

A2 Longitudinal Wind Drag Force Coefficient (CXw) 183

A3 Lateral Wind Drag Force Coefficient (CYw) 184

A4 Wind Yaw Moment Coefficient (CXYw) 184

A5 Longitudinal Current Drag Force Coefficient (CXc)– Loaded Tanker (WD/T = 1.1) 185

xii


A6 Longitudinal Current Drag Force Coefficient (CXc) – Loaded Tanker (WD/T = 1.2) 186

A7 Longitudinal Current Drag Force Coefficient (CXc) – Loaded Tanker (WD/T = 1.5) 187

A8 Longitudinal Current Drag Force Coefficient (CXc)– Loaded Tanker (WD/T = 3.0) 188

A9 Longitudinal Current Drag Force Coefficient (CXc) – Loaded Tanker (WD/T > 4.4) 189

A10 Lateral Current Drag Force Coefficient (CYc) – Loaded Tanker 190

A11 Current Yaw Moment Coefficient (CXYc) – Loaded Tanker 191

A12 Longitudinal Current Drag Force Coefficient (CXc) – Ballasted Tanker (40% T) 192

A13 Lateral Current Drag Force Coefficient (CYc) – Ballasted Tanker (40% T) 193

A14 Current Yaw Moment Coefficient (CXYc) – Ballasted Tanker (40% T, Based on Midships) 194

A15 Variation in Bow Configuration 195

A16 Current Velocity Correction Factor (K) 196

A17 Longitudinal Wind Drag Force Coefficient (CXw) – Gas Carrier 199

A18 Lateral Wind Drag Force Coefficient (CYw) – Gas Carrier 200

A19 Wind Yaw Moment Coefficient (CXYw) – Gas Carrier 200

B1 Depiction of HMPE Mooring Line Residual Strength 207

C1 Proper Method of Locating Rope Anchorage Point on a Plain Drum 217

C2 Examples of Rope Damage with Broken Wires 223

C3 Reduction in Wire Rope Diameter 223

C4 Wire Rope Crushing Damage 224

C5 Rope Stretch Leading to Decreased Elasticity 224

C6 Cross Section Depicting Substantial Wear and Severe Lateral Corrosion 224

C7 Basket or Lantern Deformation 225

C8 An Open Kink and Examples of Damage Caused 225

D1 New Rope 229

D2 Used Rope 229

D3 Damaged Rope 229

D4 Residual Strength to Rope Damage Relationships 230

D5 Surface Abrasion 232

D6 Plucked Strand in Cover 232

D7 Single Cut Strand 233

D8 Multiple Cut Strands 233

D9 Glazed, No Fibre Damage (Bent Rope) 233

D10 Glazed, No Fibre Damage (Flat rope) 233

xiii

List of Figures

D11 Same Rope as Figures D9 and D10: After Flexing No Permanent Damage 233

D12 Actual Melting Damage 233

E1 Typical Tongue-Type Bow Chain Stopper 239

E2 Positioning of Forward Fairleads, Bow Chain Stoppers and Pedestal Roller Leads 240

F1 Three Cases of a Chain Bent over a Curved Surface 246

F2 Geometry of a Chain Bent over a Curved Surface 247

F3 Approximate Relation between Angle α and Angle ß 248

F4 Angle α as Function of D/d for Various Angles ß 248

F5 Free Body Analysis of Half Chain Link 249

F6 Non-Dimensional Stress Factor as a Function of D/d for Various Angles β 251

F7 Comparison of Grooved and Ungrooved Surface Cases 252

F8 Test Set-up. Test 15, α =135°, D/d = 4, 8 Links 253

F9 Results of Tests of Chain Tensioned over Curved Surface 256

xiv


List of Tables

1.1 Maximum Longitudinal and Transverse Wind Forces on a 250,000 DWT Tanker 4

1.2 Mooring Analysis Data - Tanker 107,000 DWT, 35 Knot Wind 315° (Offshore) and 045° (Onshore); 5 Knot Current 350°; and 2 Metre, 10 Second 045° Wave 20

1.3 Mooring Analysis Data - LNG Carrier 267,000m3, 35 Knot Wind 315° (Offshore) and 045° (Onshore); 5 Knot Current 350°; and 2 Metre, 10 Second 045° Wave 21

3.1 Emergency Towing-off Pennants – Recommended MBL and Length 71

4.1 Comparison Between Section 4.4 and MSC Circ. 1175 79

5.1 Typical Pad Width and Thickness 105

6.1 Strength Criteria 111

6.2 Typical MBLs of Steel Wire Rope 116

6.3 Typical Characteristics of Materials used for Conventional Synthetic Ropes 118

6.4 Minimum Breaking Forces in kN of Synthetic Ropes (New, Dry Ropes, Unspliced) 120

6.5 Typical Properties of High Modulus Synthetic Fibres and Steel Wire Ropes 125

6.6 Examples of High Modulus Synthetic Fibre Trade Names 126

6.7 Typical MBLs of High Modulus Synthetic Fibre Ropes 127

7.1 Performance Specification for Mooring Winches 155

8.1 Maximum Permissible Rope Loading of Bitts 164

A.1 Principal Dimensions/Characteristics of Typical Liquefied Gas Carriers 198

C.1 Summary of the Major Criteria for the Inspection and Discard of Wire Ropes 222

F.1 Chain Tensioned over Curved Surface, Properties of Chain Samples 253

F.2 Summary of Test Results, Chain Tensioned over Curved Surface 255

PRINCIPLES OF MOORING

SECTION

2


3

SECTION 1 - Principles of Mooring

1.1 General

The term ‘mooring‘ refers to the system for securing a ship to a terminal. The most common terminals for tankers are piers and sea islands. However, other shipboard operations such as mooring at Single Point Moorings (SPMs), Multi-Buoy Moorings (MBMs), Floating Production, Storage and Offloading vessels (FPSOs) and offshore loading facilities, emergency towing, tug handling, barge mooring, canal transit, ship-to-ship transfer and anchoring may fall into the broad category of mooring and so require specialised fittings or equipment. Anchoring equipment is covered by Classification Society rules and is therefore not included in these guidelines.

Figure 1.1 shows a typical mooring pattern at a tanker terminal.

Figure 1.1: Typical Mooring Pattern

The use of an efficient mooring system is essential for the safety of the ship, her crew, the terminal and the environment. The problem of how to optimise the moorings to resist the various forces will be dealt with by answering the following questions:

What are the forces applied on the ship? •What general principles determine how the applied forces are distributed to the mooring lines? •How can the above principles be applied in establishing a good mooring arrangement? •

Since no mooring arrangement has unlimited capability, to address these questions it will be necessary to understand precisely what the moorings of a ship are expected to achieve.

BreastLines

MooringDolphin

SternLines

SpringLines

BreastLines

SpringLines

Breasting Dolphins

HeadLines

LoadingPlatform

C L

Figure 1.1: Typical Mooring Pattern

4


1.2 Forces Acting on the Ship

The moorings of a ship must resist the forces due to some, or possibly all, of the following factors:

Wind •current •tides •surges from passing ships •waves/swell/seiche •ice •changes in draft, trim or list. •

This Section deals mainly with the development of a mooring system to resist wind, current and tidal forces on a ship at a conventional berth. Normally, if the mooring arrangement is designed to accommodate maximum wind and current forces, reserve strength will be sufficient to resist other moderate forces that may arise. However, if appreciable surge, waves or ice conditions exist at a terminal, considerable loads can be developed in the ship’s moorings. These forces are difficult to analyse except through model testing, field measurements or dynamic computer programs. Ships calling at such terminals should be made aware that the standard environmental condition may be exceeded and appropriate measures will need to be implemented in advance.

Forces in the moorings due to changes in ship elevation from either tidal fluctuations or loading or discharging operations must be compensated by proper line tending.

1.2.1 Wind and Current Drag ForcesThe procedures for calculating these forces are covered in Section 2 and Appendix A. Although the initial calculations were based on large ships, additional testing conducted for smaller ships has shown that the wind and current drag coefficients are not significantly different for most cases. Consequently, the large ship drag coefficients in Appendix A may be used for bridge-aft ships with similar geometry, down to 16,000 DWT in size.

Figure 1.2 demonstrates how the resultant wind force on a ship varies with wind velocity and direction. For simplicity, wind forces on a ship can be broken down into two components: a longitudinal force acting parallel to the longitudinal axis of the ship and a transverse force acting perpendicular to the longitudinal axis. The resultant force initiates a yawing moment.

Wind force on the ship also varies with the exposed area of the ship. Since a head wind only strikes a small portion of the total exposed area of the ship, the longitudinal force is relatively small. A beam wind, on the other hand, exerts a very large transverse force on the exposed side area of the ship. For a given wind velocity the maximum transverse wind force on a VLCC is about five times as great as the maximum longitudinal wind force. For a 50 knot wind on a light 250,000 DWT tanker, the maximum transverse forces are about 300 tonnes (2,943 kN), whereas the ahead longitudinal forces are about 60 tonnes (589 kN).

Mean Draft metres

Astern tonnes

Ahead tonnes

Transverse tonnes

6 47.8 68 303

7 47.2 66.7 283

8 46.7 65.3 263

9 46.1 63.9 244

Table 1.1: Maximum Longitudinal and Transverse Wind Forces on a 250,000 DWT Tanker, 5 m Trim, 50 Knot Wind

If the wind hits the ship from any quartering direction between the beam and ahead (or astern), it will exert both a transverse and longitudinal force, since it is striking both the bow (or stern) and the side of the ship. For any given wind velocity, both the transverse and longitudinal force components of a quartering wind will be smaller than the corresponding forces caused by the same wind blowing abeam or head on.

5


Figure 1.2: Wind Forces on a Ship

With the exception of wind that is dead ahead or astern or dead abeam, the resultant wind force does not have the same angular direction as the wind. For example, for a 250,000 DWT tanker, a wind 45° off the bow leads to a resultant wind force of about 80° off the bow. In this case, the point of application of the force is forward of the transverse centre line producing a yawing moment on the ship.

It should be noted that the sign conventions used in this Section relate to the normal interpretation used by mariners, whereby a force from right ahead is considered to be from 0° and the compass angles proceed in a clockwise direction. This is different to the sign convention used by the scientific community, such as research establishments and designers, where a force from right astern is considered to be from 0° and the compass angles proceed in an anti-clockwise direction. This latter convention is adopted in Section 2 and Appendix A when discussing wind and current forces.

Transverse Force

Figure 1.2: Wind Forces on a Ship

Offset Distance

Wind

0̊

90˚

Wind Velocity 0˚ 45 ̊ 90 ̊ 135 ̊ 180˚

Increasing

Incr

easi

ng

Forc

e on S

hip

00

Appro

xim

ate

Forc

e on S

hip

Direction of Wind off Bow

Longitudinal Force

Transverse Force

0 Force

Yawing MomentResultant Force

(+)

(–)

Longitudinal Force

6


Figure 1.3: Effect of Underkeel Clearance on Current Force

Current forces on the ship must be added to the wind forces when evaluating a mooring arrangement. In general, the variability of current forces on a ship due to current velocity and direction follows a pattern similar to that for wind forces. Current forces are further complicated by the significant effect of clearance beneath the keel. Figure 1.3 shows the increase in force due to reduced under keel clearance. The majority of terminals are oriented more or less parallel to the current thereby minimising current forces. Nevertheless, even a current with a small angle (such as 5°) off the ship’s longitudinal axis can create a large transverse force and must be taken into consideration.

Model tests indicate that the current force created by a 1 knot head current on a loaded 250,000 DWT tanker with a 2 m underkeel clearance is about 5 tonnes (49 kN), whereas the load developed by a 1 knot beam current for the same underkeel clearance is about 230 tonnes (2,256 kN). For a 2 knot current, the force created would be about 14 tonnes (137 kN) when from ahead and 990 tonnes (9,712 kN) when on the beam.

12 tonnes

118 kN

25 tonnes

245 kN

40 tonnes

392 kN

70 tonnes

686 kN

Current

5 x

Dra

ft o

r m

ore

1.6

x

Dra

ft

0.5

x D

raft

0.2

x D

raft

Figure 1.3: Effect of Underkeel Clearance on Current Force

z

Assumes 2 knot current, 5 ̊off the bow

7


1.3 Mooring Pattern

The term ‘mooring pattern’ refers to the geometric arrangement of mooring lines between the ship and the berth. It should be noted that the industry has previously standardised on the concept of a generic mooring layout (see Figure 2.1), taking into account standard environmental criteria. The generic mooring layout is mainly applicable to a ‘multi-directional’ environment and to the design of ship’s mooring equipment. ‘Multi-directional’ is where no single direction dominates or where any of the environmental forces become a dominant factor.

For terminals with a ‘directional environment’, i.e. one with a high current, wind or swell waves, a site-specific layout such as one including head and stern lines and/or extra breast and spring lines may be more efficient. For ships regularly trading to these terminals, consideration may be given to the provision of additional or higher capacity mooring equipment.

The most efficient line ‘lead’ for resisting any given environmental load is a line orientated in the same direction as the load. This would imply that, theoretically, mooring lines should all be oriented in the direction of the environmental forces and be attached at such a longitudinal location on the ship that the resultant load and restraint act through one and the same location. Such a system would be impractical since it has no flexibility to accommodate the different environmental load directions and mooring point locations encountered at various terminals. For general applications, the moor ing pattern must be able to cope with environmental forces from any direction. This can best be approached by splitting these forces into a longitudinal and a transverse component and then calculating how to most effectively resist them. It follows that some lines should be in a longitudinal direction (spring lines) and some lines in a transverse direction (breast lines). This is the guiding principle for an effective mooring pattern for general application, although locations of the actual fittings at the terminal will not always allow it to be put into practice. The decrease in efficiency caused by deviating from the optimum line lead is shown in Figures 1.4. and 1.5 (compare Cases 1 and 3 in Figures 1.4, where the maximum line load increases from 57 (559 kN) to 88 tonnes (863 kN)).

However, it should be noted that for a 60 knot head wind the highest loaded line for the generic layout is 39.5 tonnes, whereas it is 28.6 tonnes for the specific layout. Therefore, for terminals located where the environment is directional, the specific layout is actually more efficient. Refer to Sections 1.5, 1.6, 1.7, 2.4 and 2.5 for further details.

There is a basic difference in the function of spring and breast lines, which must be understood by designers and operators alike. Spring lines restrain the ship in two directions (forward and aft); breast lines essentially deployed perpendicular to the ship restrain in only one direction (off the berth), restraint in the on-berth direction being provided by the fenders and breasting dolphins. Whereas all breast lines will be stressed under an off-berth environmental force, only the aft or the forward spring lines will generally be stressed. For this reason the method of line-tending differs between spring and breast lines (as explained in Section 1.8.1). It is important to recognise that, if spring lines are pre-tensioned, the effective longitudinal restraint is provided by only the difference between the tension in the opposing spring lines. Therefore, too high a pre-tension can significantly reduce the efficiency of the mooring system. Likewise, differences in vertical angles between forward and aft springs can lead to ship surge along the jetty.

Mooring patterns for a directional environment may incorporate head and stern lines that are orientated between a longitudinal and transverse direction. This optimises restraint for the longitudinal direction where the dominant environmental force acts, while maintaining some lateral restraint for the less dominant lateral environmental directions.

Another option for mooring layouts with dominant longitudinal forces is to add more spring lines.

Furthermore, the effectiveness of a mooring line is influenced by two angles, the vertical angle the line forms with the pier deck and the horizontal angle the line forms with the parallel side of the ship. The steeper the orientation of a line, the less effective it is in resisting horizontal loads. As an example, a line orientated at a vertical angle of 45° is only 75% as effective in restraining the ship as a line orientated at a 20° vertical angle. Similarly, the larger the horizontal angle between the parallel side of the ship and the line, the less effective the line is in resisting a longitudinal force.

8


9


Figure

1.4

: M

ooring P

atte

rn A

nal

ysis

1413

1112

109

54

31

2

14

1311

12

109

543

2

1

Line

num

ber

1

2

3

4

5

6

7

8

9

10

11

12

13

14

60 k

not

hea

d w

ind

8.6

11.3

0

0

0

0

0

39.0

39.5

0

0

0

0

0

60 k

not

win

d 4

5º

off b

ow

56.7

57.1

34.5

34.9

39.0

5.9

5.9

10.9

11.3

25.9

25.4

34.0

24.9

23.6

60 k

not

bea

m w

ind

56.7

56.7

39.5

39.9

44.9

13.2

13.2

6.3

6.3

43.5

42.6

57.1

51.2

47.6

CA

SE 1

Gen

eri

c A

ll W

ire M

oori

ng

All

lines

42 m

mM

BL

115 t

onnes

Line

num

ber

1

2

3

4

5

6

7

8

9

10

11

12

13

14

60 k

not

hea

d w

ind

15.9

5.0

0

4.1

0

0

0

39.4

39.4

0

3.6

0

2.7

0

60 k

not

win

d 4

5º

off b

ow

91.6

6.8

54.4

5.9

62.6

7.7

7.3

14.5

15.0

37.6

5.4

50.3

5.4

33.6

60 k

not

bea

m w

ind

91.2

6.8

61.2

5.9

69.8

17.2

16.8

9.5

9.5

67.1

5.9

88.0

6.3

73.0

CA

SE 2

Gen

eri

c M

ixed

Mo

ori

ng

s N

ot

Rec

om

men

ded

Illu

stra

tes

lack

of co

ntr

ibution o

f fibre

lin

es

to o

vera

ll m

ooring s

tren

gth

Line

num

ber

1

2

3

4

5

6

7

8

9

10

11

12

13

14

60 k

not

hea

d w

ind

10.4

11.8

5.4

8.2

0

0

0

28.6

28.6

0.9

0

0

0

0

60 k

not

win

d 4

5º

off b

ow

52.6

49.9

48.5

43.5

83.9

19.5

19.0

5.0

5.0

36.7

30.4

4

0.8

24.9

24.0

60 k

not

bea

m w

ind

56.2

54.0

53.1

48.1

88.4

17.7

17.2

11.8

12.2

70.3

49.9

70.3

46.3

45.8

CA

SE 3

Sit

e-s

peci

fic

All

Wire

Mooring

Show

ing e

ffec

t on lin

e te

nsi

ons

as c

onse

quen

ce

of non-i

dea

l le

ads

All

load

s ar

e in

tonnes

Mooring a

rran

gem

ents

as

above

exc

ept

that

lin

es 2

, 4,

11 a

nd 1

3 a

re p

oly

pro

pyl

ene

Note

: Com

pute

r Pr

ogra

m a

ssum

es lin

e does

not

yiel

d o

r bre

ak.

Exa

mple

s ar

e bas

ed o

n b

alla

sted

250,0

00 d

wt

ship

. Lo

ads

are

for

conditio

ns

show

n.

S

hould

the

win

d s

hift,

lin

es w

ithout

load

s, a

s sh

ow

n a

bove

, w

ould

ass

um

e so

me

load

ings,

so a

ll lin

es s

hould

be

tended

at

all tim

es

96 6

987

87

Figu

re 1

.4: M

oori

ng P

atte

rn A

naly

sis

10


Figu

re 1

.5:

Effec

t of H

awse

r Ori

enta

tion

on

Rest

rain

t Cap

acit

yFi

gure

1.5

: E

ffec

t of H

awse

r O

rien

tation o

n R

estr

aint

Cap

acity

AB

Ass

um

e 0°

Ass

um

e ‘F

lat’

T

ransv

erse

Res

trai

nt

Cap

acity

= 2

x A

x S

in 2

5°

= 0

.85A

* L

ongitudin

al R

estr

aint

Cap

acity

= 1

x B

x C

os

25°

+

1 x

A x

Cos

25°

x Cos

25°

= 0

.91B +

0.8

2A

* L

ongitudin

al R

estr

aint

Cap

acity

under

a longitudin

al f

orc

e only

; if a

tra

nsv

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pre

sent

the

longitudin

al r

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aint

will

be

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her

red

uce

d d

ue

to o

pposi

tion o

f fo

rces

in h

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tern

lin

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Als

o, e

last

icity

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cts

hav

e bee

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cted

in t

his

exa

mple

whic

h m

ay furt

her

red

uce

the

lo

ngitudin

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estr

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ity.

AB

AA

Ass

um

e 0°

BB

Hea

d/S

tern

Lin

es

25°

Hea

d/S

tern

Lin

es

Tran

sver

se R

estr

aint

Cap

acity

= 2

x A

= 2

ALo

ngitudin

al R

estr

aint

Cap

acity

= 1

x B

= 1

B

Spring L

ines

25°

Spring L

ines

A =

Allo

wab

le W

ork

ing

Lo

ad

in

Head

/S

tern

Lin

es

B =

Allo

wab

le W

ork

ing

Lo

ad

in

Sp

rin

g L

ines

25°

25

°

11


1.4 Elasticity of Lines

The elasticity of a mooring line is a measure of its ability to stretch under load. Under a given load, an elastic line will stretch more than a stiff line. Elasticity plays an important role in the mooring system for several reasons:

High elasticity can absorb higher dynamic loads. For this reason, high elasticity is desirable for ship- •to-ship transfer operations, or at terminals subject to waves or swell

high elasticity also means that the ship will move further in her berth and this could cause problems •with loading arms or hoses. Such movement also creates additional kinetic energy in the mooring system

a third and most important aspect is the effect of elasticity on the distribution of forces among •several mooring lines. The simple four-line mooring pattern shown in the upper portion of Figure 1.5 is insensitive to the elasticity of the lines but is suitable only for tugs, small barges and very small ships such as coasters. Larger ships require more lines resulting in load sharing and interaction between lines. This becomes more complicated as the number of mooring lines increases. Optimum restraint is generally accomplished if all lines, except spring lines, are stressed to the same percentage of their breaking strength. Good load-sharing can be accomplished if the following principles are understood.

The general principle is that if two lines of different elasticity are connected to a ship at the same point, the stiffer one will always assume a greater portion of the load (assuming the winch brake is set) even if the orientation is the same. The reason for this is that both lines must stretch an equal amount and, in doing so, the stiffer line assumes a greater portion of the load. The relative difference between the loads will depend upon the difference between the elasticities, and can be very large.

The elasticity of a mooring line primarily depends upon the following factors:

Material and construction •length •diameter. •

Figure 1.6 demonstrates the significance of each of the above factors on load distribution. The most important points to note are the appreciable difference in elasticity between wire lines and fibre ropes and the effect of line length on elasticity. Case A shows an acceptable mooring where lines of the same size and material are used. Case B indicates the sharing of loads between lines of the same material but of different size and each line is stressed to approximately the same percentage of its breaking strength. However, Cases C and D are examples of mooring arrangements that should be avoided.

Wire mooring lines are very stiff. The elongation for a 6 x 37 construction wire line at a load where the material begins to be permanently deformed is about 1% of wire length. Under an equivalent load a polypropylene rope may stretch 10 times as much as a wire. Therefore, if a wire is run out parallel to a conventional fibre line, the wire will carry almost the entire load while the fibre line carries practically none. Elasticity also varies between different types of fibre lines and, although the difference is generally not as significant as that between fibre line and wire, the difference will affect load distribution. High modulus polyethylene or aramid fibre lines, for example, have much less elasticity than other synthetic fibre lines and would carry the majority of the load if run out parallel to conventional synthetic lines.

The effect of material on load distribution is critical and the use of mixed moorings for similar service, e.g. forward springs, is to be avoided. In some cases the fibre lines may carry almost no load, while at the same time some of the wires are heavily loaded, possibly beyond their breaking strength. The same could be true of mixed fibre lines of varying elasticity although the differences would generally not be as great unless the moorings also include high modulus synthetic ropes.

12


Figure 1.6: Effect of Mooring Elasticity on Restraint CapacityFigure 1.6: Effect of Mooring Elasticity on Restraint Capacity

300 mm Polyamide =50 tonnes (490 kN)

200 mm Polyamide = 25 tonnes (245 kN)

ACCEPTABLE

150 tonnes (1471 kN)

B) Effect of Mooring Line Size

Note: All Loads Are Approximate

Steel = 71 tonnes (696 kN)

Polypropylene =3 tonnes (29 kN)

Polyamide = 1 tonne (10 kN)

NOTACCEPTABLE


C) Effect of Mooring Line Material

NOTACCEPTABLE


D) Effect of Mooring Line Length

60 m

30 m

Same Size & Type Mooring Line

25 tonnes (245 kN)

50 tonnes (490 kN)

300 mm Polyamide =37.5 tonnes (368 kN)

ACCEPTABLE


A) Ropes of Same Size and Material

13


The effects of mixing wire and synthetic fibre lines are shown in Figure 1.4, by comparison of Cases 1 and 2. (Note the low loads in fibre lines 2, 4, 11 and 13 and the increase in wire loads from a maximum of 57 tonnes (559 kN) to a maximum of 88 tonnes (863 kN)).

The effect of line length (from securing point on board to shore bollard) on load distribution must also be considered. Line elasticity varies directly with line length and has a significant effect on line load. A wire line 60 m long will assume only about half the load of a 30 m parallel and adjacent line of the same size, construction and material.

Elasticity of a given type of line also varies with its diameter, construction and age. Usually this factor is not an important consideration since the load relative to a line’s strength is the governing factor rather than the absolute load. Conventional fibre ropes lose some elasticity with age.

14


1.5 General Mooring Guidelines

Consideration of the principles of load distribution in Figure 1.4 leads to the following mooring guidelines. These assume that the moored ship may be exposed to strong winds or current from any direction.

Mooring lines should be arranged as symmetrically as possible about the midship point of the ship. •(A symmetrical arrangement is more likely to ensure a good load distribution than an asymmetrical arrangement)

breast lines should be orientated as perpendicular as possible to the longitudinal centre line of the •ship and as far aft and forward as possible

spring lines should be orientated as parallel as possible to the longitudinal centre line of the ship. •

Head and stern lines are normally not efficient in restraining a ship in its berth. Mooring facilities with good breast and spring lines allow a ship to be moored most efficiently, virtually ‘within its own length’. The use of head and stern lines requires two additional mooring dolphins and decreases the overall restraining efficiency of a mooring pattern when the number of available lines is limited. This is due to their long length and consequent higher elasticity and poor orientation. They should only be used where required for manoeuvring purposes or where necessitated by local pier geometry, surge forces or weather conditions. Small ships berthed in facilities designed properly for larger ships may have head and stern lines because of the berth geometry.

The vertical angle of the mooring lines should be kept to a minimum. •

The ‘flatter’ the mooring angle, the more efficient the line will be in resisting horizontally-applied loads on the ship.

A comparison of Cases 1 and 3 in Figure 1.4 demonstrates that a ship can usually be moored more efficiently within its own length. Although the same number of lines is used in each situation, Case 1 results in a better load distribution, minimising the load in any single line.

Generally, mooring lines of the same size and type (material) should be used for all leads. If this is not •possible, all lines in the same service, ie breast lines, spring lines, head lines, etc. should be the same size and type. For example, all spring lines could be wire and all breast lines synthetic.

‘First lines ashore’ are sometimes provided on very large ships to assist in the initial approach and positioning of the ship alongside. These lines often have high elasticity and are unlikely to add to the final restraining capacity of the system unless all lines in that group are of the same material.

Synthetic tails are often used on the ends of wire lines to permit easier handling and to increase line elasticity. Tails may also be used to increase the elasticity of low stretch ropes made from high modulus polyetheylene or Aramid fibres (see Section 6.5).

If tails are used, the same size and type of tail should be used on all lines run out in the same service. •

The effect of attaching 11 metre long tails, made from both polyester and polyamide, to steel wire and HMPE mooring lines is shown in the following graph. It should be noted that longer tails will have a significant impact on the assemblies’ elasticity.

15


Figure 1.7: Comparison of Steel Wire versus HMPE Mooring Lines with and without 11 Metre Tails (References 8 and 9)

Mooring lines should be arranged so that all lines in the same service are about the same length •between the ship’s winch and the shore bollard. Line elasticity varies directly with line length and shorter lines will assume more load.

Figure 1.7: Comparison of Steel Wire Versus HMPE Mooring Lines with and without 11m Tails

Steel Wire - 42 mm dia, 121t MBL with:No TailParallel strand polyester tail - 64 mm dia, 162t MBLPolyamide double braid grommet tail - 70 mm dia, 178t MBL

HMPE - 42 mm dia, 127t MBL with:No tailParallel strand polyester tail - 64 mm dia, 162t MBLPolyamide double braid grommet tail - 70 mm dia, 178t MBL

140

120

100

80

60

40

20

00 1 2 3 4 5

Load

(t)

Extension (%)

x x x x x x x x x x x x x x x x x xx

x x x x x x x x

x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

x x x x

x x x x

16


1.6 Operational Considerations

The mooring guidelines in Section 1.5 were developed to optimise load distribution to the moorings. In practice, final selection of the mooring pattern for a given berth must also take into account local operational and weather conditions, pier geometry and ship design. Some pilots, for example, desire head and stern lines to assist ships moving into, along, or out of a berth, while others may use spring lines for this purpose. Head and stern lines would be advantageous at berths where the mooring points are too close to the ship and good breast lines cannot be provided, or where the bollards are located so that the lines will have an excessive vertical angle in the light condition. These excessive angles would result in considerably reduced restraint capability.

High winds and currents from certain directions might make it desirable to have an asymmetrical mooring arrangement. This could mean placing more mooring lines or breast lines at one end of the ship.

The other factor to consider is the optimum length of mooring lines. It would be desirable to keep all lines at a vertical angle of less than 25°. For example, if the ship’s chock location is 25 m above the shore mooring point, the mooring point should be at least 50 m horizontally from the chock.

Long lines are advantageous both from a standpoint of load efficiency and line-tending. However, where conventional fibre ropes are used, the increased elasticity can be a disadvantage by permitting the ship to move excessively thereby endangering loading arms. Figure 1.8 illustrates the effects of line lengths on line-tending requirements.

Figure 1.8 : Effect of Line Length on Tending Requirements

Loaded Ship

Rise =15.2 m

Tend lines at these points (2)

Tend lines at these points (8)

15.2 m Winch

62.5 m

61 m

34.1 m

30.9 m

30.5 m

30.5m

61 m

Distance fromBollard to Ship’s Side

30.5 m45.7 m61 m

Minimum No. TendingsDue to 15.2 m Ship Rise

842

Notes • Assume Wire Lines. Yield Strain = 1% Length = 0.46 m for 45.7 m Hawser • Assume No slack in Wires • Assume Lines Tended when they reach Maximum Allowable Load

Figure 1.8: Effect of Line Length on Tending Requirements

Light Ship

17


1.7 Terminal Mooring System Management

Good mooring management requires the application of sound principles, well maintained equipment, trained personnel and, most importantly, proper co-ordination and interaction between ship and shore.

Terminals are responsible for the provision of mooring equipment on their berths that is appropriate, in both size and number, for the full range of ship sizes and types using the berths. Mooring bollards, mooring hooks or rollers/pulleys should be positioned and sized for the ships being handled. The optimum arrangement and SWL of mooring equipment should be based on the output of engineering analysis using site-specific environmental data (see Section 2.5).

While the safety of the ship and its proper mooring is the prime responsibility of the Master, the terminal, because of its knowledge of the operating environment at its site and its equipment, should be in the best position to advise the Master regarding mooring line layout and operating limitations. The mooring analysis should be used to provide information on recommended mooring arrangements for the range of ships using each berth. Based on this information, the terminal should produce standard mooring diagrams for each generic ship size depicting the recommended number, size, and service of moorings. The information should also include details of operating limitations (see Section 1.7.2).

The responsibilities and arrangements for the mutual checking of moorings, cargo transfer and other aspects of the ship/shore interface should be addressed under the provisions of the Ship/Shore Safety Check-List.

The mooring equipment of existing ships varies widely, ranging from synthetic mooring ropes, mixed moorings (synthetic ropes and wire lines), all wire moorings (with and without synthetic tails) to systems using high modulus synthetic fibre ropes. Rated brake capacities, winch and fairlead locations can vary significantly from ship to ship. Ship crews will have varying degrees of expertise in mooring matters and varying philosophies concerning maintenance and/or replacement of critical items of mooring equipment.

The terminal can utilise a number of concepts in modern mooring management to reduce the possibility of ship break-out. These are:

To run computer analysis of the mooring with site-specific environment to establish the optimum •pattern, vessel movement, fender and mooring line loads

18


based on this analysis, develop guidelines for the safe mooring of ships for the operating •environment existing at the terminal together with recommended mooring plans

to ensure that terminal mooring equipment is positioned and sized for the range of ships being •handled, is properly maintained and clearly marked with its SWL

to obtain information about the ship's mooring equipment prior to its arrival •to examine the ship’s mooring equipment after berthing to determine what modification, if any, •must be made to standard guidelines in view of the state of maintenance, training of crew, etc.

to check on the effectiveness of line tending periodically, either visually or by the instrumentation of •mooring hooks

to take whatever action is deemed appropriate to ensure stoppage of cargo transfer, dis connection •of loading arms or transfer hoses and removal of the ship from the berth should the ship fail to take appropriate measures to ensure safety of mooring or should environmental conditions reach or exceed the operating limits as agreed and documented in the Ship/Shore Safety Check-List.

1.7.1 Operating LimitsAnother important aspect in restraining the ship at its berth is the movement of the ship. No simple formula can be offered for the ship movement, although this is generally included in the output of computer calculations. Movement of the ship due to environmental loads can exceed loading arm or transfer hose operating limits before the strength limits in the mooring lines are reached. Similarly, limits and requirements may apply to gangways, particularly shore-based equipment incorporating a tower or a long span from the jetty to the ship. This is especially true for synthetic line systems. Under worsen ing environmental conditions, the loading arms and gangways may therefore have to be disconnected at lesser wind and current conditions than those used as a design basis for the mooring system.

Environmental operating limits should be established for each berth and should be detailed on the Ship/Shore Safety Check-List. In addition, ship’s staff should be advised of any limitations on ship movement due to the operating envelopes of shore equipment such as hard arms, hoses, fenders (compression limits) and gangways, and the actions to be taken should these be reached.

The concept of ‘manageable escalating events’ is applied when establishing environmental limits and the following illustrates this principle:

The loading arms may typically be drained and purged if necessary and disconnected when the wind •reaches 30 knots (15 metres/second) and preparations made to leave the berth

tugs may be requested to hold the ship alongside up to wind speeds of 35 knots (18 metres/second) •the gangway will be stowed and the ship will be ready to leave the berth at the Master’s judgement •when the wind reaches 35 knots (18 metres/second)

the ship’s mooring lines should be able to hold the ship in position with wind speeds of 60 knots •(31 metres/second) and the maximum tension in any one line should not exceed 55% of the MBL (wires), 50% MBL (synthetic ropes) or 45% MBL (polyamide). However, this may exceed the limits of the terminal’s mooring system and a lesser wind speed may be appropriate when establishing environmental limits

at wind speeds above 60 knots (31 metres/second), line tensions will exceed 60-65% MBL and winch •brakes will start to render (see Section 7.4). The ship will be in a potentially dangerous situation.

For ships moored at an SPM, the practical safety limitation may well be related to physical ability of the crew to handle hoses and work safely rather than either ship movement or mooring loadings.

1.7.2 Operating Guidelines/Mooring LimitsIn the past, operating guidelines and mooring limits have generally been developed empirically. With the advent of computers and the ready availability of specialised programs, in combination with the development of more accurate wind and current drag coefficients, guidelines can be developed systematical-ly that can provide the limits for various classes of ships with varying mooring capabilities. It is recommended that mooring analyses are undertaken for facilities to validate recommended mooring arrangements and plans.

The following tables depict how data from a mooring analysis may be presented to assist ship and terminal staff understand and implement operating guidelines. In the examples shown in Tables 1.2 and 1.3, the maximum line and fender load, and ship surge and sway at the manifold, is given for an oil tanker and a

19


very large LNG carrier. Where mooring loads exceed the 50% of MBL (synthetic) and 55% of MBL (steel wire) limitations, additional shore lines or very small reductions in weather criteria may bring the mooring under the tension limit. Conditions shown as ‘not safe’ would require a very large reduction in weather criteria and would probably result in unacceptable increases in downtime.

The information generated can be used for a number of purposes:

To decide whether a given ship can be moored at a given berth under the expected weather •conditions

to determine when to discontinue cargo transfer and to disconnect loading arms •to advise the ship when it would be desirable to take on ballast to reduce its freeboard •to advise the ship when it would be desirable to have tugs available to assist in maintaining the ship’s •position at the jetty while preparations are made to vacate the berth.

Three significant wave heights are considered in the examples shown in the tables to establish the sensitivity to line tension and ship excursion over the range 1.0 m, 1.5 m and 2.0 m. These wave heights cover the typical range that would be experienced up to the practical limit of 2.0 m. It can be seen that at the higher wave heights the 11 m tails are inadequate and that longer 22 m tails are required. Conversely, at lower wave heights the 11 m tails are adequate. Another very important factor is the elasticity of the tail where the high stretch polyamide provides lower tensions than the lower stretch polpropylene/polyester and 100% polyester tails.

It should be noted that, when tail length is doubled, surge and sway does not increase by the same amount. As an example, for the large LNG carrier in Table 1.3, the HMPE mooring with 11 m tail, 2 m significant wave height and offshore wind, produces 0.6 m aft surge and 0.3 m sway out from the jetty. With a 100% increase in tail length to 22 m, there is no increase in sway and only a 50% increase in surge. For the tanker in Table 1.2, there is no change in surge and sway, even though the tail length has doubled.

20


Mooring line on winch and tail

description

Wind direction

Highest line tension %(1)

Ship movement at manifold (metres)(1) Fender load

(tonnes)(1)

Fwd Aft Out

Steel wire with 11 m polyamide

tail

Offshore

4156

NS(2)

0.20.2

0.10.2

0.30.5

283315


tail

303847

0.20.20.2

00.20.3

0.40.50.6

284314323

HMPE with 11 m polyamide tail

375160

0.2 0.20.2

0.10.20.3

0.30.50.6

283315323


293645

0.20.20.2

00.20.3

0.40.50.6

283314323

pp/polyester or 100% polyester

252833

0.20.20.2

00.20.3

0.40.50.6

282315323


tail

Onshore

375260

0.10.20.2

0.10.20.3

0.20.40.5

302320321


tail

253444

0.10.10.2

0.10.20.3

0.20.40.5

302320322


344762

0.10.20.2

0.10.20.3

0.20.40.5

302320322


253342

0.10.10.2

0.10.20.3

0.20.40.5

302320322

pp/polyester or 100% polyester

202632

0.10.10.2

0.10.20.3

0.20.40.5

302320322

Insert to table 1.2

Scale 100 m

B C H I

D E F G

Shore Target

13 91087

21

6 5 3414 11

aa bb cc dd

J

89t

2 mWave

5 knotCurrent

35 knotWind

Offshore

A

12

15

<N

35 knotWind

Onshore

NOTES FOR TABLE

(1) Ref Highest Line Tension: top row 1.0 m, middle row 1.5 m, lower row 2.0 m significant wave heights

2) NS= not a safe condition due to many lines overloading

Table 1.2: Mooring Analysis Data - Tanker 107,000 DWT, 35 Knot Wind 315° (Offshore) and 045° (Onshore); 5 Knot Current 350°; and 2 Metre, 10 Second 045° Wave

(Reference 10)

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Mooring line on winch and tail

description

Wind direction

Highest line tension %(1)

Ship movement at manifold (metres)(1) Fender load

(tonnes)(1)

Fwd Aft Out


tail

Offshore

384960

000

0.40.50.6

0.20.20.3

292318322


tail

303643

000

0.60.70.9

0.20.20.3

289317322


364556

000

0.40.50.6

0.20.20.3

291318322


293541

000

0.60.70.9

0.20.20.3

289317322

HMPE with 11 m pp/polyester or

100% polyester tail

4162

NS(2)

00-

0.30.4

-

00.1

-

309324

-


100% polyester tail

374860

000

0.40.50.6

0.20.20.3

283316322


tail

Onshore

344560

000

0.50.60.7

00.10.2

309324324


tail

263239

000

0.70.80.9

00.10.2

309324324


314256

000

0.50.60.7

00.10.2

309324325


253151

000

0.70.70.8

00.10.2

310324324


100% polyester tail

344760

000

0.50.50.7

0.10.20.2

291318322


100% polyester tail

354760

000

0.40.50.6

0.10.10.2

308324324

Insert to table 1.3

Scale 100m

B C H I

D E F G

Shore Target

13 91087

21

6 5 3414 11

aa bb cc dd

J

89t

2mWave

5 knotCurrent

A

12

15

<N

35 knotWind

Offshore

35 knotWind

Onshore

NOTES FOR TABLE

(1) Ref Highest Line Tension: top row 1.0 m, middle row 1.5 m, lower row 2.0 m significant wave heights

2) NS= not a safe condition due to many lines overloading

Table 1.3: Mooring Analysis Data - LNG Carrier 267,000 m3, 35 Knot Wind 315° (Offshore) and 045° (Onshore); 5 Knot Current 350°; and 2 Metre, 10 Second 045° Wave

(Reference 10)

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1.7.3 Joint Terminal/Ship Meeting and InspectionAs soon as practicable after berthing, it is recommended that terminals have their representative board the ship to establish contact with the Master or his designated representative. At this meeting the Terminal Representative should provide information relating to shore facilities and procedures. In addition he should, in concert with the Ship Representative:

Complete the Ship/Shore Safety Check-List in line with guidance given in ISGOTT (Reference 4) and, •where appropriate, physically check items before ticking off

obtain details of moorings and winches, including state of maintenance •review forecasted weather and arrange for the Master to be advised of any expected changes •assess freeboard limitations •determine the conditions at which cargo transfer will be discontinued and loading arms, hoses and •gangway will be disconnected and agree the precautions to be taken under high mooring load situations. Document operating limits on the Ship/Shore Safety Check-List.

1.7.4 Instrumented Mooring Hooks or Visual Inspection of Mooring LinesThe terminal should monitor the ship’s line tending activity by visual inspection of the mooring lines, particularly during cargo transfer and periods of changing environmental conditions.

In addition to the above, where an appropriate need has been identified and depending on the physical environment at the berth, it may be desirable to install mooring line load measurement apparatus. This equipment has been installed at a number of large tanker berths and at many LNG berths. It measures the line loads and has a central read-out in the terminal operation’s control room. Should the line loads become high or the lines become slack, the terminal operator can advise the ship.

In some terminals mooring tension information is transmitted to a shipboard fixed or portable display, for direct access by ship’s staff. In any case, the terminal should inspect lines periodically. If poor line tending by ship’s staff is observed, the terminal should notify the ship.

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1.8 Ship Mooring Management

Good ship mooring management requires a knowledge of good mooring principles, information about the mooring equipment installed on the ship, proper maintenance of this equipment and good seamanlike line tending.

Officers in charge of line tending and personnel assigned to tend lines should be aware of the capabilities of the equipment installed on their ship. Winches should be marked to show the design holding capacity. The torque required on the hand-wheel or lever to achieve the required brake rendering should be documented. Specifications of the mooring lines should also be available.

Recommendations concerning the proper direction of reeling or pay-out of the mooring line on the winch drum should be followed and the drum should be marked to prevent any possibility of error (see Section 7.4.2.6).

1.8.1 Line TendingThe objective of good line tending is to ensure that all lines share the load to the maximum extent possible and ship’s movement is limited in the berth (off or alongside the pier face). Pre-tensioning of lines (i.e. loading a line with a winch prior to the application of environmental forces) reduces ship movement and improves the load distribution when lines of different lengths and elasticities are being used.

To prevent excessive movement of the ship along the pier face it is very important to tend spring lines differently from breast lines. Tending head or stern lines presents a special problem (which is one more reason why they are not recommended). They must be tended like either spring or breast lines, depending on whether longitudinal or transverse restraint is more critical. For example, if a high longitudinal current on the bow is expected, the bow line should be pre-tensioned while the stern line is tensioned only to take up any slack. The following general rules apply to line-tending:

Generally, slack lines should be hauled in first. Slack lines may permit excessive movement of the ship •when there is a sudden change in the environment

only one line should be tended at a time. Anytime a line is tended, it temporarily changes the load •in other lines and may increase it. The simultaneous tending of two lines may therefore give erratic results or even an overload

whenever a spring line is tended the opposite spring must also be tended, but not simultaneously. •Rendering or heaving-in on only one spring line may cause excessive movement of the moored ship along the pier face

fender compression should be observed during discharge or during a rising tide. Fender com- •pression may be caused by over-tight breast lines. If there is high fender compression that is not caused by onshore winds or currents, the breast lines must be slackened.

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1.9 Emergency and Excessively High Mooring Load Conditions

Overloading of mooring lines is evidenced in a number of ways; for example, by direct measure ments of mooring line loads, by direct observation of the moorings by experienced personnel or by predictions made by those having a knowledge of the effects of wind and current on the ship mooring system or by winch slippage.

In general, ship’s moorings should not be subjected to environmental forces in excess of the designed environmental limits. In the event of mooring lines being, or likely to be, subjected to excessive loads, consideration should be given to immediately departing the berth. Should this not be practicable, the following precautions should be considered:

Discontinue cargo operations •call out crew, linemen, mooring boats, tugs and put the ship’s engines on readiness •ensure that winch brakes are tightened to the correct setting. •Do not release brakes and attempt to heave in

disconnect loading arms and gangways •should time and ship condition permit, consider taking-on ballast to reduce freeboard if loads are •due to high wind conditions

run extra moorings as available together with any shore mooring available to augment the ship’s •equipment.

In a developing potential emergency situation, the point at which the ship leaves the berth may be dictated by limits, such as hard arm or hose handling capability, the use of tugs and work boats and not solely mooring line loads or ship movement. It must be emphasised that the ship’s Master is responsible for the safety of the ship and he must decide whether it is safe to vacate the berth or whether, by making a hurried unberthing manoeuvre, he will in fact place his ship or personnel in greater danger. There are also certain berths where tidal conditions or manoeuvring areas may be such that the unberthing of the ship is prevented at certain times.

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1.10 Limitations on the Use of Tugs and Boats

Tugs can perform a very useful function in holding the ship against the berth to reduce the strain on moorings while preparations are made to vacate the berth. However, in deteriorating weather conditions the ready availability of tugs may be compromised.

Care should be exercised when high horsepower tugs are engaged to keep the tanker alongside a jetty. Hoses or cargo arms should be disconnected. The application of excessive power can result in over-com pression of the fenders and damage to the ship’s side. To minimise the possibility of damage, tug push points should be clearly marked on the ship’s hull. It must also be recognised that tugs have certain operating limits and that, particularly in berths subject to waves, these limits are likely to be exceeded.

In the case of MBMs, boats may be required to release mooring lines from buoys. At jetties, boats may be required to put line handlers on detached mooring dolphins. As with tugs, the boats will have operating limits that may be exceeded under extreme conditions.

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1.11 General Recommendations 1.11.1 Recommendations for Berth Designers

The mooring facilities provided at the berth should permit the largest ship that is to be •accommodated to remain safely moored alongside in the maximum environmental limits established for the specific site

the wind and current forces on the ship should be calculated for the wind and current conditions •under which the ship may remain moored at the berth, using the procedures covered in Section 2 of these guidelines. At exposed locations, the impact of dynamic loads will need to be considered in addition to the calculation of static loads. Most Probable Maximum (MPM) loads will need to be assessed when establishing allowable load criteria for moorings (see Section 2.5)

allowable loads in any wire mooring line should not exceed 55% of its Minimum Breaking Load •(MBL). For synthetic lines, except polyamide, loads should not exceed 50% of the line’s MBL. For polyamide, loads should not exceed 45% of the line’s MBL to allow for strength loss when wet (see Section 1.7.1)

the following principles should be applied when designing the layout of mooring facilities for the •berth:

mooring points should be disposed as nearly as possible symmetrically about the centre point • breast moorings should emanate from points near the fore and aft ends of the ship and as •nearly as possible perpendicular to the fore and aft line of the ship

the length of mooring lines at conventional berths should be within the range 35 to 50 m •and, where intended for the same service and practicable, be equal. (Note: when adopting a mooring pattern for a directional environment, it may not be possible to meet the recommended length criteria, see Section 1.3)

sufficient mooring points should be installed to provide a satisfactory spread of moorings for •the range of ship sizes that the berth is to accept. It is preferred that ships are moored by breast lines and spring lines only, although on berths designed to accept a range of ship sizes the mooring points will inevitably be such that smaller ships may need to use head lines and stern lines in addition to breast lines

the heights of mooring points should be such that vertical angles will be as small as practical •and, if possible, should not exceed 25° from the horizontal

breasting dolphins should, by preference, be positioned at distances apart of one third of the •overall length of the ship. At berths accommodating a range of ship sizes, the spacing of breasting dolphins should be such as to provide a breasting face between 25% and 40% of the ship’s overall length about the ship’s midship point to ensure compatibility with the ship’s parallel mid-body and balanced mooring forces. For fine-lined ships, lesser distances may be required to ensure that dolphins are within the parallel body. In such cases, care should be taken to ensure that the reduced spacing of the dolphins does not result in high yaw movements of the ship alongside, or excessive fender compression forces

berth mooring points should be provided with a SWL not less than the MBL of the largest rope •anticipated and be supplemented by capstans or winches and fairleads to enable the handling of ship’s moorings

shore based mooring equipment should be provided to augment shipboard equipment when the •operating conditions at the berth exceed the Standard Design Criteria or design environmental conditions.

1.11.2 Recommendations for Terminal Operators

Terminal operators should have a good understanding of mooring principles, the design of the •mooring system for the berth, the loads likely to be experienced in the mooring system under varying conditions of wind and current, and a clear appreciation of the operating limits applying to the various types of ships and mooring systems that may utilise the berth

terminal mooring equipment, including bollards, mooring hooks and/or rollers and pulleys should be •clearly marked with their SWL

terminal operators should recognise the problems likely to arise from the use of mixed moorings and •be aware of the need for effective application of winch brakes and good mooring management while ships are moored

27


ship-to-shore liaison should be established by the terminal operator prior to a ship's arrival where the •mooring plan, mooring procedures and continuing liaison on mooring matters during the time the ship is in the berth have been communicated and agreed on. Particular attention must be paid to the procedures to be followed in managing escalating events and emergencies.

1.11.3 Recommendations for Ship Designers

The mooring facilities provided on the ship should permit the ship to remain safely moored under •the Standard Environmental Criteria alongside a berth that is provided with a standard arrangement of mooring points

wind and current forces on the ship should be calculated applying the Standard Environmental •Criteria, the drag coefficients contained in Appendix A and by using the methods described in these guidelines. This calculation will determine the number, size and disposition of moorings required onboard

loads in any wire mooring line should not exceed 55% of the line’s MBL. For synthetic lines, except •polyamide, loads should not exceed 50% of the line’s MBL. For polyamide, loads should not exceed 45% of the line’s MBL to allow for strength loss when wet

mixed moorings, comprising full length synthetic ropes used in conjunction with wires, are not •recommended

wire or HMPE ropes should be the standard mooring equipment for all large tankers and gas carriers. •It is recognised that wire ropes of greater than 44 mm diameter may require special handling arrange ments in terminals

synthetic ropes may be used as the first line ashore for positioning the ship at either end, preferably •by means of handling and storage winches. These ropes should not be considered as contributing to the restraint of a ship moored principally with wires

when tails are fitted to mooring ropes they should have an MBL at least 25% higher than that of the •mooring lines to which they are attached. Polyamide tails should have a 37% higher MBL than the mooring line to take account of loss of strength when wet (see Section 6.5.1). In general, tails should have a length of not less than 11 m and be subject to rigorous examination and renewal procedures, as recommended in Section 6

winches for handling mooring ropes may be either of the split drum or undivided drum type; the •relative merits of the two types are described in Section 7.2

automatic winches are not recommended, but if fitted must have a capability to disengage the •automatic operational features

winch brakes should be designed to hold 80% of the line’s MBL and have the capability to be •adjusted down to 60% of the line’s MBL at which level they should be set in-service (see Section 7.4.6). They should be properly maintained and routinely tested

the layout of moorings should be such as to provide: • symmetry about the mid length and to provide the design numbers of moorings on each side •of the ship

breast lines sited as near as possible to the end of the ship • moorings used in the same service, as nearly as possible, the same length inboard of the ship •suitable chocks and fairleads in order to ensure correct alignment of moorings •bitts positioned for supplementary moorings. •

minimum safety factors listed in Table 4.1 are based upon the appropriate design criteria and loading •assumptions, and should be incorporated in all new equipment and mooring fittings

all equipment and fittings should be clearly marked with their SWL. • 1.11.4 Recommendations for Ship Operators

The principles of good mooring, including the dangers associated with mixed moorings, should be •understood by ship operators. Particular attention should be given in ship’s instructions to the proper application of winch brakes, the maintenance of moorings and winch brakes, good line tending procedures and the practices to be observed in the case of mooring emergencies

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each ship should be provided with information on the design of the mooring system with examples •to show the loads likely to be experienced under particular conditions and to illustrate those situations under which the limit of the system is likely to be reached

strength loss with mooring hours for tails is generally significant and a residual strength discard •criteria should be established for a particular rope construction and material.

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