Mooring Equipment Guidelines
3rd Edition
(MEG )
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
Issued by the
Oil Companies International Marine Forum
First Published 1992
Second 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
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accepted by the Oil Companies International Marine Forum (OCIMF), the membership or employees of
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Introduction
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
iii
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:
iv
••
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)
Contents
Contents
Introduction
iii
List of Figures
x
List of Tables
1
Principles of Mooring
1
1.1
General
3
1.2
Forces Acting on the Ship
4
1.2.1
4
Wind and Current Drag Forces
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
1.7.2
1.7.3
1.7.4
18
18
22
22
1.8
1.9
Operating Limits
Operating Guidelines/Mooring Limits
Joint Terminal/Ship Meeting and Inspection
Instrumented Mooring Hooks or Visual Inspection of Mooring Lines
Ship Mooring Management
23
1.8.1
23
Line Tending
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
1.11.2
1.11.3
1.11.4
2
xiv
Recommendations for Berth Designers
Recommendations for Terminal Operators
Recommendations for Ship Designers
Recommendations for Ship Operators
26
26
27
27
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
38
Most Probable Maximum (MPM) Wave Motions
v
Mooring Equipment Guidelines 3rd Edition
3
Mooring Arrangements and Layouts
39
3.1
Principal Objectives
41
3.2
Requirements at Piers and Sea Islands
43
3.2.1
3.2.2
3.2.3
3.2.4
43
44
44
50
3.3
Requirements at SPMs
53
3.4
Requirements for Emergency Towing, Escorting and Pull-Back
54
3.4.1
55
vi
Fittings for Tug Escort and Pull-Back
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
3.9.2
65
66
Requirements for Receiving Ship
Requirements for Discharge Ship
3.10 Arrangements at Cargo Manifolds
68
3.11 Mooring Augmentation in Exceptional Conditions
69
3.11.1
3.11.2
3.11.3
3.11.4
4
Number, Size and Type of Lines
Arrangements for Breast Lines
Arrangements for Spring Lines
Special Arrangements for Gas Carriers
Provision of Shore Moorings
Use of Shore-Based Pulley
Advantage of Pulley System
Disadvantage of Pulley System
69
69
69
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
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
4.4.2
4.4.3
4.4.4
4.4.5
4.4.6
4.4.7
81
81
81
82
82
83
83
Bitts (Double Bollards)
Single Cruciform Bollard (Single Bitt)
Recessed Bitt
Closed Chock (Fairlead)
Pedestal Fairlead and Rollers of Button-Roller Chocks
Universal Fairlead (4 Roller Type)
Universal Fairlead (5 Roller Type)
Contents
4.4.8
4.4.9
4.4.10
4.4.11
5
6
Emergency Towing Arrangement
Single Point Mooring Equipment
Mooring Winches
Comparison of Combined Stresses with the 85% of Yield Criterion
84
85
85
85
4.5
Strength Testing of Mooring Fittings
86
4.6
Marking of Mooring Fittings
87
4.7
General Recommendations
88
4.7.1
4.7.2
88
88
Recommendations for Ship Designers
Recommendations for Ship Operators
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
5.9.2
5.9.3
105
105
105
Rounded Gunwale Connection
Doublers versus Inserts
High Strength Steel Fittings
5.10 Certification and Inspection
106
Mooring Lines
107
6.1
109
General
6.1.1
6.1.2
6.1.3
6.1.4
6.2
6.3
General Safety Hazards
Strength Criteria
Elasticity
Record Keeping
109
111
112
113
Wire Mooring Lines
114
6.2.1
6.2.2
6.2.3
6.2.4
6.2.5
6.2.6
114
114
114
116
117
117
Material
Construction
Corrosion Protection
Bend Radius
Handling, Inspection and Removal from Service
Standard Specifications
Conventional Fibre Mooring Lines
118
6.3.1
118
General
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Mooring Equipment Guidelines 3rd Edition
6.3.2
6.3.3
6.3.4
6.4
6.5
7
121
121
123
High Modulus Fibre Mooring Lines
124
6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
6.4.6
6.4.7
6.4.8
124
125
125
126
126
127
128
130
General
Properties of High Modulus Synthetic Fibres
High Modulus Synthetic Fibre Materials
High Modulus Synthetic Rope Constructions
Characteristics
Selection Criteria
Installation
Inspection and Removal from Service
Synthetic Tails
131
6.5.1
6.5.2
6.5.3
6.5.4
131
131
134
134
General
Tail Length
Retirement Criteria
Methods of Connecting Tails
Winch Performance, Brake Holding Capacity and Strength Requirements
137
7.1
Function and Type of Mooring Winches
139
7.1.1
139
7.2
7.3
7.4
7.5
viii
Construction
Bend Radius
Handling and Storage of Synthetic Lines
Automatic Tension Winches
Winch Drums
140
7.2.1
7.2.2
7.2.3
140
141
141
Split Drums
Undivided Drums
Handling of SPM Pick-up Ropes
Winch Drives
143
7.3.1
7.3.2
7.3.3
7.3.4
143
144
144
144
Hydraulic Drives
Self-Contained Electro-Hydraulic Drives
Electric Drives
Steam
Winch Brakes
145
7.4.1
7.4.2
7.4.3
7.4.4
7.4.5
7.4.6
146
147
150
150
150
153
Layers of Mooring Line on Drum
Band Brakes
Disc Brakes
Input Brakes
Winch Brake Testing
Brake Holding Capacity
Winch Performance
154
7.5.1
7.5.2
7.5.3
7.5.4
7.5.5
154
154
154
154
154
Rated Pull
Rated Speed
Light-Line Speed
Stall Heaving Capacity
Drum Capacity
7.6
Strength Requirements
156
7.7
Winch Testing
157
Contents
7.8
8
7.7.1Rules Concerning Testing at Manufacturer’s Facility for the Acceptance
of the Manufacturer and Purchaser
157
7.7.2
157
Onboard Acceptance Test
Summary of Recommendations
158
7.8.1
7.8.2
158
158
Recommendations for Ship Designers
Recommendations for Ship Operators
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
AWind and Current Drag Coefficients for VLCCs and Gas Carriers and Example Force
Calculation (Including Symbols and Notations)
177
B
Rope Over-Strength
205
CGuidelines 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
ix
Mooring Equipment Guidelines 3rd Edition
List of Figures
x
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
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
6.6A) Fairing of Split Drum Edge
B) & C) Faired Split Drum Edge
129
129
6.7A) & B) Significant Wave Height and Mean Period, Upper Bound Limits for 50% Line Load
132
for 138,000 m3 LNG Carrier
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
xi
Mooring Equipment Guidelines 3rd Edition
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)
xii
233
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
xiii
Mooring Equipment Guidelines 3rd Edition
List of Tables
1.1
xiv
Maximum Longitudinal and Transverse Wind Forces on a 250,000 DWT Tanker
4
1.2Mooring 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
1.3Mooring 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
PR INC I P LE S O F MO OR I NG
SECTION
Mooring Equipment Guidelines 3rd Edition
2
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.
Mooring
Dolphin
Loading
Platform
Breast
Lines
Spring
Lines
Spring
Lines
Breast
Lines
Breasting Dolphins
Stern
Lines
Head
Lines
CL
Figure 1.1: Typical Mooring Pattern
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.
3
Mooring Equipment Guidelines 3rd Edition
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 Forces
The 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.
4
SECTION 1 - Principles of Mooring
Wind
Transverse Force
Offset Distance
Longitudinal Force
0˚
Resultant Force
Yawing Moment
90˚
Increasing
0
Wind Velocity
Approximate Force on Ship
0
Increasing
Force on Ship
Transverse
Force
Longitudinal
Force
(+)
0 Force
(–)
0˚
45˚
90˚
135˚
Direction of Wind off Bow
180˚
Figure 1.2: Wind Forces on a Ship
Figure
WindorForces
on a Ship
With the exception of wind that is dead
ahead1.2:
or astern
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.
5
Mooring Equipment Guidelines 3rd Edition
25 tonnes
40 tonnes
70 tonnes
118 kN
245 kN
392 kN
686 kN
0.5 x Draft
5 x Draft or more
Current
0.2 x Draft
1.6 x
Draft
12 tonnes
Assumes 2 knot current, 5˚ off the bow
Figure 1.3: Effect of Underkeel Clearance on Current Force
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 under­keel 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.
z
6
SECTION 1 - Principles of Mooring
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 moor­ing 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 pretension 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.
7
Mooring Equipment Guidelines 3rd Edition
8
1
8.6
56.7
56.7
1
10.4
52.6
56.2
14
1
15.9
91.6
91.2
2
11.8
49.9
54.0
12
13
2
5.0
6.8
6.8
3
5.4
48.5
53.1
10
3
0
54.4
61.2
11
5
0
39.0
44.9
6
0
5.9
13.2
7
0
5.9
13.2
7
6
8
39.0
10.9
6.3
5
4
8.2
43.5
48.1
4
4.1
5.9
5.9
5
0
83.9
88.4
8
9
5
0
62.6
69.8
6
0
19.5
17.7
6
0
7.7
17.2
7
0
19.0
17.2
7
6
7
0
7.3
16.8
8
28.6
5.0
11.8
8
39.4
14.5
9.5
5
Mooring arrangements as above except that
lines 2, 4, 11 and 13 are polypropylene
4
0
34.9
39.9
8
9
2
9
28.6
5.0
12.2
4
9
39.4
15.0
9.5
9
39.5
11.3
6.3
4
3
3
1
10
0.9
36.7
70.3
2
10
0
37.6
67.1
10
0
25.9
43.5
1
11
0
30.4
49.9
11
3.6
5.4
5.9
11
0
25.4
42.6
12
0
40.8
70.3
12
0
50.3
88.0
12
0
34.0
57.1
13
0
24.9
46.3
13
2.7
5.4
6.3
13
0
24.9
51.2
14
0
24.0
45.8
14
0
33.6
73.0
14
0
23.6
47.6
Figure
1.4: Mooring
Analysis
Figure
1.4:
MooringPattern
Pattern
Analysis
Note: Computer Program assumes line does not yield or break. Examples are based on ballasted 250,000 dwt ship. Loads are for conditions shown.
Should the wind shift, lines without loads, as shown above, would assume some loadings, so all lines should be tended at all times
Line number
60 knot head wind
60 knot wind 45º off bow
60 knot beam wind
Showing effect on line
tensions as consequence
of non-ideal leads
CASE 3 Site-specific
All Wire Mooring
Line number
60 knot head wind
60 knot wind 45º off bow
60 knot beam wind
10
3
0
34.5
39.5
13 12
2
11.3
57.1
56.7
Illustrates lack of contribution of fibre lines
to overall mooring strength
CASE 2 Generic Mixed Moorings
Not Recommended
Line number
60 knot head wind
60 knot wind 45º off bow
60 knot beam wind
All lines 42 mm
MBL 115 tonnes
CASE 1 Generic All Wire Mooring 14 11
All loads are in tonnes
SECTION 1 - Principles of Mooring
9
10
B
B
A
Head/Stern Lines
A
25°
Spring Lines
25°
Spring Lines
= 2 x A x Sin 25° = 0.85A
= 1 x B x Cos 25° + 1 x A x Cos 25° x Cos 25° = 0.91B +0.82A
Head/Stern Lines
25°
A = Allowable Working Load in Head/Stern Lines
B = Allowable Working Load in Spring Lines
Transverse Restraint Capacity = 2 x A = 2A
Longitudinal Restraint Capacity = 1 x B = 1B
Transverse Restraint Capacity
* Longitudinal Restraint Capacity
Assume 0°
B
Assume 0°
B
Figure 1.5: Effect of Hawser Orientation on Restraint Capacity
Figure 1.5: Effect of Hawser Orientation on Restraint Capacity
* Longitudinal Restraint Capacity under a longitudinal force only; if a transverse force is present the longitudinal restraint will be further reduced
due to opposition of forces in head and stern lines. Also, elasticity effects have been neglected in this example which may further reduce the
longitudinal restraint capacity.
A
A
Assume ‘Flat’
Mooring Equipment Guidelines 3rd Edition
SECTION 1 - Principles of Mooring
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 shipto-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.
11
Mooring Equipment Guidelines 3rd Edition
150 tonnes (1471 kN)
A) Ropes of Same Size and Material
150 tonnes (1471 kN)
300 mm Polyamide =
37.5 tonnes (368 kN)
ACCEPTABLE
300 mm Polyamide =
50 tonnes (490 kN)
200 mm Polyamide =
25 tonnes (245 kN)
B) Effect of Mooring Line Size
ACCEPTABLE
Steel =
71 tonnes (696 kN)
150 tonnes (1471 kN)
Polypropylene =
3 tonnes (29 kN)
Polyamide =
1 tonne (10 kN)
NOT
ACCEPTABLE
25 tonnes (245 kN)
30 m
Same
Size &
Type
Mooring
Line
60 m
C) Effect of Mooring Line Material
150 tonnes (1471 kN)
D) Effect of Mooring Line Length
50 tonnes (490 kN)
NOT
ACCEPTABLE
Note: All Loads Are Approximate
Figure 1.6: Effect of Mooring Elasticity on Restraint Capacity
Figure 1.6: Effect of Mooring Elasticity on Restraint Capacity
12
SECTION 1 - Principles of Mooring
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.
13
Mooring Equipment Guidelines 3rd Edition
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.
14
SECTION 1 - Principles of Mooring
140
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Load (t)
x
1
x
0
x
x
x
x
2
x
x
x
x
x
x
0
x
x
x
x
x
20
x
x
x
x
x
x
40
x
x
x
x
x
60
x
xx
x
x
x
80
x
x
x
100
x
x
120
3
4
5
Extension (%)
Steel Wire - 42 mm dia, 121t MBL with:
No Tail
x x x x Parallel strand polyester tail - 64 mm dia, 162t MBL
Polyamide double braid grommet tail - 70 mm dia, 178t MBL
HMPE - 42 mm dia, 127t MBL with:
No tail
x x x x Parallel strand polyester tail - 64 mm dia, 162t MBL
Polyamide double braid grommet tail - 70 mm dia, 178t MBL
Figure 1.7: Comparison of Steel Wire versus HMPE Mooring Lines with and without 11 Metre Tails
(References
8 and
9)
Figure 1.7: Comparison of Steel
Wire
Versus
HMPE Mooring Lines with
and without 11m Tails
••
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.
15
Mooring Equipment Guidelines 3rd Edition
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 linetending requirements.
15.2 m
Winch
Tend lines at these points (2)
.1
34
m
62.5
m
m
30.9
30.5 m
61 m
30.5m
61 m
Tend lines at
these points (8)
Light Ship
Loaded Ship
Distance from
Bollard to Ship’s Side
30.5 m
45.7 m
61 m
Notes
Minimum No. Tendings
Due to 15.2 m Ship Rise
8
4
2
• 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
Figure1.8:
1.8 : Effect
Figure
Effect of
ofLine
LineLength
Lengthon
onTending
TendingRequirements
Requirements
16
Rise =
15.2 m
SECTION 1 - Principles of Mooring
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
17
Mooring Equipment Guidelines 3rd Edition
••
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, dis­connection
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 Limits
Another 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 worsen­ing 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 Limits
In 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
18
SECTION 1 - Principles of Mooring
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.
19
Mooring Equipment Guidelines 3rd Edition
Mooring line on
winch and tail
description
Wind
direction
Ship movement at manifold
(metres)(1)
Highest line
tension %(1)
Fwd
Aft
Out
Fender load
(tonnes)(1)
Steel wire with
11 m polyamide
tail
41
56
NS(2)
0.2
0.2
0.1
0.2
0.3
0.5
283
315
Steel wire with
22 m polyamide
tail
30
38
47
0.2
0.2
0.2
0
0.2
0.3
0.4
0.5
0.6
284
314
323
37
51
60
0.2
0.2
0.2
0.1
0.2
0.3
0.3
0.5
0.6
283
315
323
HMPE with 22 m
polyamide tail
29
36
45
0.2
0.2
0.2
0
0.2
0.3
0.4
0.5
0.6
283
314
323
pp/polyester or
100% polyester
25
28
33
0.2
0.2
0.2
0
0.2
0.3
0.4
0.5
0.6
282
315
323
Steel wire with
11 m polyamide
tail
37
52
60
0.1
0.2
0.2
0.1
0.2
0.3
0.2
0.4
0.5
302
320
321
Steel wire with
22 m polyamide
tail
25
34
44
0.1
0.1
0.2
0.1
0.2
0.3
0.2
0.4
0.5
302
320
322
34
47
62
0.1
0.2
0.2
0.1
0.2
0.3
0.2
0.4
0.5
302
320
322
HMPE with 22 m
polyamide tail
25
33
42
0.1
0.1
0.2
0.1
0.2
0.3
0.2
0.4
0.5
302
320
322
pp/polyester or
100% polyester
20
26
32
0.1
0.1
0.2
0.1
0.2
0.3
0.2
0.4
0.5
302
320
322
HMPE with 11 m
polyamide tail
Offshore
HMPE with 11 m
polyamide tail
Onshore
Scale
<N
A
100 m
B
C
Shore Target
12
10
13 11 9 D E
aa bb
15
14
F
H
8
G 7 6
cc dd
I
5 4 3
2
1
89t
2m
Wave
5 knot
Current
35 knot
Wind
Offshore
35 knot
Wind
Onshore
J
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
Insert to table
1.2 Current 350°; and 2 Metre, 10 Second 045° Wave
(Reference 10)
20
SECTION 1 - Principles of Mooring
Mooring line on
winch and tail
description
Wind
direction
Ship movement at manifold
(metres)(1)
Highest line
tension %(1)
Fwd
Aft
Out
Fender load
(tonnes)(1)
Steel wire with
11 m polyamide
tail
38
49
60
0
0
0
0.4
0.5
0.6
0.2
0.2
0.3
292
318
322
Steel wire with
22 m polyamide
tail
30
36
43
0
0
0
0.6
0.7
0.9
0.2
0.2
0.3
289
317
322
HMPE with 11 m
polyamide tail
36
45
56
0
0
0
0.4
0.5
0.6
0.2
0.2
0.3
291
318
322
29
35
41
0
0
0
0.6
0.7
0.9
0.2
0.2
0.3
289
317
322
HMPE with 11 m
pp/polyester or
100% polyester tail
41
62
NS(2)
0
0
-
0.3
0.4
-
0
0.1
-
309
324
-
HMPE with 22 m
pp/polyester or
100% polyester tail
37
48
60
0
0
0
0.4
0.5
0.6
0.2
0.2
0.3
283
316
322
Steel wire with
11 m polyamide
tail
34
45
60
0
0
0
0.5
0.6
0.7
0
0.1
0.2
309
324
324
Steel wire with
22 m polyamide
tail
26
32
39
0
0
0
0.7
0.8
0.9
0
0.1
0.2
309
324
324
HMPE with 11 m
polyamide tail
31
42
56
0
0
0
0.5
0.6
0.7
0
0.1
0.2
309
324
325
25
31
51
0
0
0
0.7
0.7
0.8
0
0.1
0.2
310
324
324
HMPE with 11 m
pp/polyester or
100% polyester tail
34
47
60
0
0
0
0.5
0.5
0.7
0.1
0.2
0.2
291
318
322
HMPE with 22 m
pp/polyester or
100% polyester tail
35
47
60
0
0
0
0.4
0.5
0.6
0.1
0.1
0.2
308
324
324
HMPE with 22 m
polyamide tail
HMPE with 22 m
polyamide tail
Scale
<N
A
Offshore
Onshore
100m
B
C
Shore Target
12
10
13 11 9 D E
aa bb
15
14
F
H
8
G 7 6
cc dd
I
5 4 3
2
1
89t
2m
Wave
5 knot
Current
35 knot
Wind
Offshore
35 knot
Wind
Onshore
J
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°
Insert to table
1.3
(Onshore); 5 Knot Current 350°; and 2 Metre, 10 Second 045° Wave
(Reference 10)
21
Mooring Equipment Guidelines 3rd Edition
1.7.3 Joint Terminal/Ship Meeting and Inspection
As 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 Lines
The 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.
22
SECTION 1 - Principles of Mooring
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 Tending
The 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.
23
Mooring Equipment Guidelines 3rd Edition
1.9
Emergency and Excessively High Mooring Load Conditions
Overloading of mooring lines is evidenced in a number of ways; for example, by direct measure­ments 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.
24
SECTION 1 - Principles of Mooring
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-com­pression 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.
25
Mooring Equipment Guidelines 3rd Edition
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
26
••
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
SECTION 1 - Principles of Mooring
••
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
arrange­ments 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
27
Mooring Equipment Guidelines 3rd Edition
28
••
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.