UFC 4-159-03

3 October 2005

UNIFIED FACILITIES CRITERIA (UFC)

DESIGN: MOORINGS

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

 UFC 4-159-03
3 October 2005
UNIFIED FACILITIES CRITERIA (UFC)

DESIGN: MOORINGS

Any copyrighted material included in this UFC is identified at its point of use.
Use of the copyrighted material apart from this UFC must have the permission of the
copyright holder.

U.S. ARMY CORPS OF ENGINEERS

NAVAL FACILITIES ENGINEERING COMMAND (Preparing Activity)

AIR FORCE CIVIL ENGINEER SUPPORT AGENCY

Record of Changes (changes are indicated by \1\ ... /1/)

Change No. Date Location

_____________
This UFC supersedes Military Handbook 1026/4, dated July 1999.
 UFC 4-159-03
3 October 2005

UNIFIED FACILITIES CRITERIA (UFC)

NEW DOCUMENT SUMMARY SHEET

Description of Change: The UFC 4-159-03, DESIGN: MOORING represents another

step in the joint Services effort to bring uniformity to the planning, design and
construction of piers and wharves. This UFC contains extensive modifications in the
following areas:
Heavy Weather Mooring
Passing Ship Effects
Ship Generated Waves
Conversion from Mil-Hdbk to UFC and general updates and revisions

Reasons for Change: The existing guidance was inadequate for the following reasons:
Need to convert to UFC format
Incorporation of changes described above
Update to referenced documents

Impact: The following direct benefits will result from the update of 4-159-03, DESIGN:
MOORING:
Although primarily a U.S. Navy document, a single, comprehensive, up to
date criteria document exists to cover mooring design.
Eliminates misinterpretation and ambiguities that could lead to design and
construction conflicts.
Facilitates updates and revisions and promotes agreement and uniformity of
design and construction between the Services.
 UFC 4-159-03
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TABLE OF CONTENTS
Page
CHAPTER 1 INTRODUCTION
Paragraph 1-1 Purpose and Scope ............................................................. 1
1-2 Purpose of Criteria................................................................. 1
1-3 Definition................................................................................ 1
1-4 Cancellation ........................................................................... 1
1-5 Organizational Roles and Responsibilities............................. 1

CHAPTER 2 MOORING SYSTEMS

Paragraph 2-1 Introduction ........................................................................... 3
2-1.1 Purpose of Mooring ............................................................... 3
2-2 Types of Mooring Systems .................................................... 3
2-2.1 Fixed Mooring Systems ......................................................... 3
2-2.2 Fleet Mooring Systems .......................................................... 3

CHAPTER 3 BASIC DESIGN PROCEDURE

Paragraph 3-1 Design Approach ................................................................... 17
3-2 General Design Criteria ........................................................ 19
3-2.1 Mooring Service Types .......................................................... 20
3-2.2 Facility Design Criteria for Mooring Service Types ................ 21
3-2.3 Ship Hardware Design Criteria for Mooring Service Types.... 21
3-2.4 Strength ................................................................................. 22
3-2.5 Serviceability.......................................................................... 22
3-2.6 Design Methods..................................................................... 25
3-2.7 General Mooring Integrity ...................................................... 25
3-2.8 Quasi-Static Safety Factors ................................................... 25
3-2.9 Allowable Ship Motions.......................................................... 25
3-3 Design Methods..................................................................... 30
3-3.1 Quasi-Static Design ............................................................... 30
3-3.2 Dynamic Mooring Analysis..................................................... 32
3-4 Risk ..................................................................................... 32
3-5 Coordinate Systems .............................................................. 36
3-5.1 Ship Design/Construction Coordinates .................................. 36
3-5.2 Ship Hydrostatics/Hydrodynamics Coordinates ..................... 36
3-5.3 Local Mooring Coordinate System......................................... 36
3-5.4 Global Coordinate System ..................................................... 36
3-5.5 Ship Conditions...................................................................... 36
3-6 Vessel Design Considerations ............................................... 40
3-7 Facility Design Considerations............................................... 40
3-8 Environmental Forcing Design Considerations ...................... 42
3-8.1 Winds..................................................................................... 42
3-8.2 Wind Gust Fronts................................................................... 45
3-8.3 Storms ................................................................................... 50
3-8.4 Currents ................................................................................. 53

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TABLE OF CONTENTS (continued)
Page

3-8.5 Water Levels.......................................................................... 53

3-8.6 Waves.................................................................................... 53
3-8.7 Water Depths......................................................................... 54
3-8.8 Environmental Design Information......................................... 54
3-9 Operational Considerations ................................................... 57
3-10 Inspection .............................................................................. 58
3-11 Maintenance .......................................................................... 59
3-12 General Mooring Guidelines .................................................. 59

CHAPTER 4 STATIC ENVIRONMENTAL FORCES AND MOMENTS ON VESSELS

Paragraph 4-1 Scope .................................................................................... 63
4-2 Engineering Properties of Water and Air ............................... 63
4-3 Principal Coordinate Directions.............................................. 64
4-4 Static Wind Forces/Moments ................................................. 65
4-4.1 Static Transverse Wind Force................................................ 65
4-4.2 Static Longitudinal Wind Force .............................................. 75
4-4.3 Static Wind Yaw Moment....................................................... 79
4-5 Static Current Forces/Moments ............................................. 82
4-5.1 Static Transverse Current Force............................................ 82
4-5.2 Static Longitudinal Current Force for Ships ........................... 89
4-5.3 Static Longitudinal Current Force for Blunt Vessels............... 94
4-5.4 Static Current Yaw Moment ................................................... 95
4-6 Wind and Current Forces and Moments on Multiple Ships .... 96

CHAPTER 5 ANCHOR SYSTEM DESIGN PROCEDURES

Paragraph 5-1 General Anchor Design Procedure ........................................ 97
5-2 Drag-Embedment Anchor Specification................................. 105
5-3 Driven-Plate Anchor Design .................................................. 110

CHAPTER 6 FACILITY MOORING EQUIPMENT GUIDELINES

Paragraph 6-1 Introduction ............................................................................ 115
6-2 Key Mooring Components ..................................................... 115
6-2.1 Tension Members .................................................................. 115
6-2.2 Compression Members.......................................................... 115
6-3 Anchors ................................................................................. 115
6-4 Chain and Fittings.................................................................. 120
6-5 Buoys..................................................................................... 124
6-6 Sinkers................................................................................... 124
6-7 Mooring Lines ........................................................................ 124
6-7.1 Synthetic Fiber Ropes ........................................................... 124
6-7.2 Wire Ropes ............................................................................ 131
6-8 Fenders ................................................................................. 131

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TABLE OF CONTENTS (continued)
Page

6-9 Pier Fittings............................................................................ 134

6-10 Catenary Behavior ................................................................. 136
6-11 Sources of Information........................................................... 139

CHAPTER 7 VESSEL MOORING EQUIPMENT GUIDELINES

Paragraph 7-1 Introduction ............................................................................ 141
7-2 Types of Mooring Equipment ................................................. 141
7-3 Equipment Specification ........................................................ 141
7-4 Fixed Bitts .............................................................................. 142
7-5 Recessed Shell Bitts.............................................................. 142
7-6 Exterior Shell Bitts ................................................................. 142
7-7 Chocks................................................................................... 142
7-8 Allowable Hull Pressures ....................................................... 142
7-9 Sources of Information for Ships’ Mooring Equipment........... 142

CHAPTER 8 EXAMPLE PROBLEMS

Paragraph 8-1 Introduction ............................................................................ 149
8-2 Single Point Mooring - Basic Approach ................................. 149
8-2.1 Background for Example ....................................................... 151
8-2.2 Ship ..................................................................................... 151
8-2.3 Forces/Moments .................................................................... 152
8-2.4 Quasi-Static Design ............................................................... 152
8-2.5 Mooring Hawser Break .......................................................... 152
8.3 Fixed Mooring - Basic Approach............................................ 155
8-3.1 Background............................................................................ 155
8-3.2 Goal ..................................................................................... 155
8-3.3 Ship ..................................................................................... 155
8-3.4 Forces/Moments .................................................................... 156
8-3.5 Definitions .............................................................................. 156
8-3.6 Preliminary Analysis .............................................................. 156
8-3.7 Wharf Mooring Concept ......................................................... 161
8.4 Spread Mooring - Basic Approach ......................................... 166
8-4.1 Background for Example ....................................................... 166
8-4.2 Goal ..................................................................................... 166
8-4.3 Ship ..................................................................................... 166
8-4.4 Forces/Moments .................................................................... 168
8-4.5 Anchor Locations ................................................................... 169
8-4.6 Definitions .............................................................................. 170
8-4.7 Number of Mooring Legs ....................................................... 171
8-4.8 Static Analysis ....................................................................... 173
8-4.9 Dynamic Analysis .................................................................. 174
8-4.10 Anchor Design ....................................................................... 177
8-5 Mooring LPD-17, LHD-1 and LHA-1 ...................................... 181

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TABLE OF CONTENTS (continued)

Page

8-5.1 Mooring LPD-17..................................................................... 181

CHAPTER 9 PASSING SHIP EFFECTS ON MOORED SHIPS

Paragraph 9-1 Introduction ............................................................................ 190
9-2 Passing Ship Effects on Moored Ships .................................. 190

CHAPTER 10 SHIP WAVES

Paragraph 10-1 Introduction ............................................................................ 193
10-2 Ship Waves............................................................................ 193

CHAPTER 11 HEAVY WEATHER MOORING GUIDELINES

Paragraph 11-1 Introduction ............................................................................ 197
11-2 Discussion ............................................................................. 197
11-3 Heavy Weather Mooring Guidelines ..................................... 198
11-4 Action..................................................................................... 203
11-4.1 Planning................................................................................. 203
11-4.2 Analysis and Design .............................................................. 203
11-4.3 Maintenance .......................................................................... 203
11-4.4 Operations ............................................................................. 203
11-5 Environmental Design Criteria for Selected Sites .................. 203

APPENDIX A REFERENCES/BIBLIOGRAPHY........................................... A-1

FIGURES

Figure Title Page

1-1 DOD Organizations Involved With Ship Moorings ............................. 2

2-1 Single Ship, Offset From a Pier With Camels .................................... 5
2-2 Ship at a T-Pier (plan view) ............................................................... 5
2-3 Floating Drydock Spud Moored ......................................................... 6
2-4 Ships on Both Sides of a Pier (plan view) ......................................... 6
2-5 Two Ships on One Side of a Pier (plan view) .................................... 7
2-6 Ship at Anchor ................................................................................... 11
2-7 Single Point Mooring With Drag Anchors........................................... 12
2-8 Single Point Mooring With a Plate Anchor and a Sinker .................... 13
2-9 Bow-Stern Mooring Shown in Plan View ........................................... 14
2-10 Med-Mooring ..................................................................................... 14
2-11 Spread Mooring ................................................................................. 15
2-12 Two Inactive Ships Moored at a Wharf .............................................. 16
2-13 Spread Mooring ................................................................................. 16

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FIGURES (continued)
Figure Title Page

3-1 Risk Diagram ..................................................................................... 35

3-2 Ship Design and Hydrostatic Coordinates ........................................ 37
3-3 Local Mooring Coordinate System for a Ship..................................... 38
3-4 Local Mooring Coordinate System for a Ship..................................... 39
3-5 Ratio of Wind Speeds for Various Gusts............................................ 43
3-6 Typhoon OMAR Wind Chart Recording ............................................. 44
3-7 Sample Wind Gust Fronts on Guam .................................................. 46
3-8 Distribution of Guam Wind Gust Front Wind Angle Changes............. 47
3-9 Initial Versus Maximum Wind Speeds for Wind Gust Fronts.............. 48
3-10 Wind Gust Front Maxima on Guam 1982-1986 ................................. 49
3-11 Idealized Models of Chain Wear ........................................................ 60

4-1 Definition of Terms............................................................................. 66

4-2 Local Coordinate System for a Ship .................................................. 67
4-3 Sample Ship Profiles ......................................................................... 70
4-4 Shape Function for Transverse Wind Force ...................................... 71
4-5 Example ..................................................................................... 73
4-6 Blockage Effect for an Impermeable Structure Next to a Moored Ship 74
4-7 Sample Yaw Normalized Moment Coefficient.................................... 81
4-8 Examples of Ratios of Ship Draft (T) to Water Depth (d) .................. 83
4-9 Broadside Current Drag Coefficient ................................................... 84
4-10 Example of Transverse Current Drag Coefficients............................. 88

5-1 Example of a Drag-Embedment Anchor

(Stabilized Stockless Anchor) ......................................................... 99
5-2 Example of a Drag-Embedment Anchor (NAVMOOR Anchor) ......... 100
5-3 Driven-Plate Anchor........................................................................... 101
5-4 Anchor System Holding Capacity in Cohesive Soil (Mud) ................. 108
5-5 Anchor System Holding Capacity in Cohesionless Soil (Sand) ......... 109
5-6 Major Steps of Driven-Plate Anchor Installation................................. 113

6-1 Chain Fittings..................................................................................... 123

6-2 Synthetic Line Stretch ........................................................................ 127
6-3 SEA-GUARD Fender Information ...................................................... 132
6-4 SEA-CUSHON Fender Performance ................................................. 133
6-5 Pier and Wharf Mooring Fittings Shown in Profile and Plan Views .... 135
6-6 Sample Catenary ............................................................................... 137
6-7 Load/Deflection Curve for the Example Mooring Leg ........................ 138

7-1 Fixed and Recessed Shell Bitts ......................................................... 144

7-2 Recessed Shell Bitt (minimum strength requirements) ..................... 146

8-1 Some Types of Behavior of Ships at Single Point Moorings .............. 150

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FIGURES (continued)
Figure Title Page

8-2 Example Single Point Mooring........................................................... 153

8-3 Example Mooring Failure Due to a Wind Gust Front ......................... 154
8-4 Wind Forces and Moments on a Single Loaded CVN-68 for a
75-mph (33.5-m/s) Wind .................................................................. 157
8-5 Definitions ..................................................................................... 158
8-6 Optimum Ideal Mooring...................................................................... 159
8-7 Required Mooring Capacity Using the Optimum Ideal Mooring ......... 160
8-8 Efficiency of Ship Moorings Using Synthetic Lines at Piers and
Wharves ..................................................................................... 162
8-9 CVN-68 Wharf Mooring Concept (‘Model 2’) ..................................... 163
8-10 Component Analysis of Mooring Working Capacity ........................... 164
8-11 Mooring Line Tensions for a CVN-68 Moored at a Wharf with
75 mph (33.5 m/s) Winds (‘Model 2’) .............................................. 165
8-12 Aircraft Carrier Mooring Concept ....................................................... 167
8-13 Wind Forces and Moments on a Nest of Four DD 963 Class
Vessels for a Wind Speed of 78 mph (35 m/s) ................................ 170
8-14 Spread Mooring Arrangement for a Nest of Four Destroyers............. 172
8-15 End View of DD 963 Mooring Nest .................................................... 175
8-16 Plate Anchor Holding Capacity .......................................................... 180
8-17 LPD-17 USS SAN ANTONIO............................................................. 182
8-18 Approximate Safe Mooring Limits for LPD-17 with 28 Parts of
Mooring Line .................................................................................... 184
8-19 Sample LPD-17 Standard Mooring .................................................... 186
8-20 Sample LPD-17 Storm Mooring ......................................................... 187
8-21 Sample LPD-17 Heavy Weather Mooring .......................................... 188
8-22 Sample LPD-17 Mooring at a Double-Deck Pier................................ 189

9-1 Sample Passing Ship Situation.......................................................... 192

9-2 Example of Passing Ship Predictions ................................................ 193

10-1 Ship Waves ..................................................................................... 196

10-2 Ship Hull-Forms Tested ..................................................................... 197
10-3 Maximum Wave Height One Wavelength Away From the Sailing
Line for a SERIES 60 Hull.................................................................. 198

11-1 Example of a Standard LPD 17 Mooring and

Heavy Weather Mooring .................................................................... 202
11-2 Securing Two Parts of Heavy Weather Mooring Line ........................ 204

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TABLES

Table Title Page

2-1 Examples of Fixed Moorings.............................................................. 4

2-2 Examples of Fleet Moorings .............................................................. 8

3-1 Parameters in a Mooring Project ....................................................... 17

3-2 Basic Mooring Design Approach With Known Facility for a
Specific Site and a Specific Ship ..................................................... 18
3-3 Design Issues .................................................................................... 20
3-4 Mooring Service Types ...................................................................... 21
3-5 Facility Design Criteria for Mooring Service Types ............................ 23
3-6 Ship Mooring Hardware Design Criteria............................................. 24
3-7 Minimum Quasi-Static Factors of Safety............................................ 26
3-8 Recommended Practical Motion Criteria for Moored Vessels............ 27
3-9 Quasi-Static Design Notes................................................................. 31
3-10 Conditions Requiring Special Analysis .............................................. 34
3-11 Design Considerations – Ship............................................................ 40
3-12 Design Considerations – Facility........................................................ 41
3-13 Sample Distribution of Wind Gust Fronts on Guam (Agana NAS)
from 1982 to 1986............................................................................ 47
3-14 Storm Parameters.............................................................................. 50
3-15 Some Sources of Environmental Design Information ........................ 55
3-16 Mooring Operational Design Considerations ..................................... 58
3-17 Inspection Guidelines......................................................................... 59
3-18 Design Recommendations................................................................. 61

4-1 Engineering Properties of Air and Water............................................ 63

4-2 Sample Wind Coefficients for Ships................................................... 69
4-3 Recommended Ship Longitudinal Wind Force Drag Coefficients ...... 76
4-4 Recommended Values of θx ............................................................. 76
4-5 Normalized Wind Yaw Moment Variables.......................................... 80
4-6 Predicted Transverse Current Forces on FFG-7 for a Current Speed
of 1.5 m/s (2.9 knots) ...................................................................... 87
4-7 Area Ratio for Major Vessel Groups .................................................. 92
4-8 Example Destroyer ............................................................................ 93
4-9 Example Longitudinal Current Forces on a Destroyer ....................... 93
4-10 Current Moment Eccentricity Ratio Variables .................................... 96

5-1 Anchor Specification Considerations ................................................. 98

5-2 Anchor Characteristics....................................................................... 102
5-3 Drag Anchor Holding Parameters U.S. Customary............................ 106
5-4 Drag Anchor Holding Parameters SI Units......................................... 107
5-5 Driven-Plate Anchor Components ..................................................... 111
5-6 Major Steps in Driven-Plate Anchor Installation................................. 112

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TABLES (continued)

Table Title Page

5-7 Typical Driven-Plate Anchors............................................................. 114

6-1 Practical Experience With Anchors.................................................... 116

6-2 Stockless Anchors in the U.S. Navy Fleet Mooring Inventory ............ 118
6-3 NAVMOOR Anchors in the U.S. Navy Fleet Mooring Inventory......... 119
6-4 FM3 Mooring Chain Characteristics................................................... 121
6-5 Properties of FM3 Chain Anodes....................................................... 122
6-6 Foam-Filled Polyurethane Coated Buoys .......................................... 125
6-7 Stretch of Synthetic Lines .................................................................. 126
6-8 Double Braided Nylon Line ................................................................ 128
6-9 Double Braided Polyester Lines......................................................... 129
6-10 Some Factors to Consider When Specifying Synthetic Line or
Wire Rope ..................................................................................... 130
6-11 Commonly Used U.S. Navy Pier Mooring Fittings ............................. 134
6-12 Sources of Information for Facility Mooring Equipment...................... 139

7-1 Types of Ship Based Mooring Equipment.......................................... 143

7-2 Fixed Ship’s Bitts (minimum strength requirements) ......................... 145
7-3 Closed Chocks (minimum strength requirements) ............................ 147
7-4 Sources of Information for Ships’ Mooring Equipment ....................... 148

8-1 2nd LT JOHN P BOBO Parameters (Fully Loaded) .......................... 151

8-2 CVN-68 Criteria (Fully Loaded) ......................................................... 155
8-3 DD 963 Criteria (1/3 Stores) ............................................................. 168
8-4 Environmental Forces ........................................................................ 169
8-5 Quasi-Static Leg Tensions for the Spread Mooring at Various
Wind Directions With a Flood Tidal Current ..................................... 173
8-6 Peak Dynamic Chain Tensions for DD 963 Nest for Various Wind
Directions and a Flood Tidal Current ............................................... 176
8-7 DD 963 Nest Motions for Surge, Sway, and Yaw at Various Wind
Directions With a Flood Tidal Current .............................................. 179
8-8 LPD-17 Characteristics ...................................................................... 183

GLOSSARY ..................................................................................... 204

APPENDIX A REFERENCES ...................................................................... A-1

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CHAPTER 1

INTRODUCTION

1-1 PURPOSE AND SCOPE. This UFC provides design policy and
procedures for design of moorings for U.S. Department of Defense (DOD) vessels.

1-2 PURPOSE OF CRITERIA. The purpose of this UFC is to ensure quality,

consistency, and safety of DOD vessels, mooring hardware, and mooring facilities
throughout the world. Other criteria should not be used without specific authorization.

1-3 DEFINITION. A mooring, in general terms, is defined as a compliant

structure that restrains a vessel against the action of wind, wave, and current forces.
For the purposes of this UFC, the emphasis is on moorings composed of tension
members (chain, line, wire rope, etc.) and compression members (fenders, camels, etc.)
used to secure vessels (surface ships, submarines, floating drydocks, yard craft, etc.).
The term mooring in this UFC includes anchoring of ships.

1-4 CANCELLATION. This UFC cancels and supersedes MIL-HDBK-1026/4

Mooring Design, Naval Facilities Engineering Command, July 1999.

1-5 ORGANIZATIONAL ROLES AND RESPONSIBILITIES. Over the design

life of a mooring facility, many organizations are involved with the various aspects of a
facility. Personnel involved range from policy makers, who set the initial mission
requirements for vessels and facilities, to deck personnel securing lines. Figure 1
illustrates the DOD organizations that must understand the various aspects of moorings.
In addition, all these groups must maintain open communications to ensure safe and
effective moorings.

Safe use of moorings is of particular importance for the end users (the
ship's personnel and facility operators). They must understand the safe limits of a
mooring to properly respond to significant events, such as a sudden storm, and to be
able to meet mission requirements.

It is equally important for all organizations and personnel shown in Figure

1-1 to understand moorings. For example, if the customer setting the overall mission
requirement states "We need a ship class and associated facilities to meet mission X,
and specification Y will be used to obtain these assets" and there is a mismatch
between X and Y, the ship and facility operators can be faced with a lifetime of
problems, mishaps, and/or serious accidents.

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Figure 1-1. DOD Organizations Involved With Ship Moorings

PENTAGON, NAVSEA,
MSC, NAVFAC
MISSION
DEFINITION/ NAVSEA,
PLANNING NSWCCD-SSES,
NAVFAC MSC

FACILITY SHIP
DESIGN DESIGN

NAVFAC SHIPYARD
ROICC, SUPSHIP
USACOE FACILITY SHIP
CONSTRUCTION FABRICATION

PORT OPS, FLEET,

FACILITY SHIP
PWC MSC
OPERATION/ OPERATION/
MAINTENANCE MAINTENANCE

Organization Examples

Primary Users

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CHAPTER 2

MOORING SYSTEMS

2-1 INTRODUCTION. The DOD uses several types of mooring systems to

moor ships. These systems can be summarized into two broad categories of moorings:

a) Fixed Moorings - Fixed moorings are defined as systems that include

tension and compression members. Typical fixed mooring systems include moorings at
piers and wharves.

b) Fleet Moorings - Fleet moorings are defined as systems that include

primarily tension members. Mooring loads are transferred into the earth via anchors.
Examples of fleet moorings include fleet mooring buoys and ship’s anchor systems.

The more common types of moorings are discussed in this chapter.

2-1.1 PURPOSE OF MOORING. The purpose of a mooring is to safely hold a

ship in a certain position to accomplish a specific mission. A key need is to safely hold
the vessel to protect the ship, life, the public interest, and to preserve the capabilities of
the vessel and surrounding facilities. Ship moorings are provided for:

a) Loading/Unloading - Loading and unloading items such as stores,

cargo, fuel, personnel, ammunition, etc.

b) Ship Storage - Storing the ship in a mooring reduces fuel consumption

and personnel costs. Ships in an inactive or reserve status are stored at moorings.

c) Maintenance/Repairs - Making a variety of repairs or conducting

maintenance on the ship is often performed with a ship moored.

d) Mission - Moorings are used to support special mission requirements,

such as surveillance, tracking, training, etc.

Most DOD moorings are provided in harbors to reduce exposure to waves,

reduce ship motions, and reduce dynamic mooring loads. Mooring in harbors also
allows improved access to various services and other forms of transportation.

2-2 TYPES OF MOORING SYSTEMS. Examples of typical moorings systems

are given in this chapter.

2-2.1 Fixed Mooring Systems. Examples of typical fixed moorings are given in
Table 2-1 and illustrated in Figures 2-1 through 2-5.

2-2.2 Fleet Mooring Systems. Examples of typical fleet moorings are given in
Table 2-2 and illustrated in Figures 2-6 through 2-13.

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Table 2-1. Examples of Fixed Moorings

a. Single Vessel Secured at Multiple Points

MOORING TYPE FIGURE DESCRIPTION
NUMBER
Pier/Wharf 2-1 Multiple tension lines are used to secure a vessel
2-2 next to a pier/wharf. Compliant fenders, fender
piles and/or camels keep the vessel offset from
the structure. A T-pier may be used to keep the
ship parallel to the current, where the current
speed is high.
Spud Mooring 2-3 Multiple vertical structural steel beams are used to
secure the vessel, such as a floating drydock.
This type of mooring is especially effective for
construction barges temporarily working in shallow
water. Spud moorings can be especially
susceptible to dynamic processes, such as harbor
seiches and earthquakes.

b. Multiple Vessel Moorings

MOORING TYPE FIGURE DESCRIPTION
NUMBER
Opposite Sides of a Pier 2-4 Vessels can be placed adjacent to one another on
opposite sides of a pier to provide some blockage
of the environmental forces/moments on the
downstream vessel.
Multiple Vessels Next to 2-5 Vessels can be placed adjacent to one another to
One Another provide significant blockage of the environmental
forces/ moments on the downstream vessel(s).

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Figure 2-1. Single Ship, Offset From a Pier With Camels

STORM BOLLARDS
MOORING
LINE
CAMELS

LPD-17

Figure 2-2. Ship at a T-Pier (plan view)

CURRENT

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Figure 2-3. Floating Drydock Spud Moored (spuds are secured to a pier, which is
not shown, and the floating drydock rides up and down on the spuds; profile view
is shown)

Figure 2-4. Ships on Both Sides of a Pier (plan view)

CAMELS LINES

PIER

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Figure 2-5. Two Ships on One Side of a Pier (plan view)

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Table 2-2. Examples of Fleet Moorings

a. Vessel Secured at a Single Point

MOORING TYPE FIGURE
NUMBER DESCRIPTION
At Anchor 2-6 Typical configuration includes the ship deploying a
single drag anchor off the bow. This is usually a
temporary mooring used as a last resort in benign
conditions. A large amount of harbor room is
required for the ship swing watch circle. If the wind
changes direction dramatically then the anchor will
have to reset. Dynamic fishtailing, even under
steady winds and currents, may be a problem.
Putting out a second anchor in what is known as a
Hammerlock mooring may be required in storm
anchoring.
Single Mooring Buoy 2-7 A single point mooring (SPM) buoy is secured to
2-8 the seafloor typically with 1 to 12 ground legs and
either drag or plate anchors. The ship moors to
the buoy using an anchor chain or hawser. The
vessel weathervanes under the action of forcing,
which helps to reduce the mooring load. This type
of mooring requires much less room than a ship at
anchor because the pivot point is much closer to
the vessel. A vessel at a mooring buoy is much
less prone to fishtailing than a ship at anchor.
Many of the mooring buoys at U.S. Navy facilities
around the world are provided under the U.S.
Navy’s Fleet Mooring Program.

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Table 2-2. (continued) Examples of Fleet Moorings

b. Vessel Secured at Two Points

MOORING TYPE FIGURE
NUMBER DESCRIPTION
Bow-Stern Mooring 2-9 A vessel is moored with one buoy to the bow and
another to the stern. This system has a much
smaller watch circle than a vessel at a single
mooring buoy. Also, two moorings share the load.
However, the mooring tension can be much higher
if the winds, currents, or waves have a large
broadside component to the ship.

c. Vessel Secured at Multiple Points

MOORING TYPE FIGURE
NUMBER DESCRIPTION
Med-Mooring 2-10 The vessel bow is secured to two mooring buoys
and the stern is moored to the end of a pier or
wharf. This type of mooring is commonly used for
tenders or in cases where available harbor space
is limited. Commonly used in the Mediterranean
Sea. Hence, the term “Med” Mooring.
Spread Mooring 2-11 Multiple mooring legs are used to secure a vessel.
This arrangement of moorings is especially useful
for securing permanently or semi-permanently
moored vessels, such as floating drydocks and
inactive ships. The ship(s) are usually oriented
parallel to the current.

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Table 2-2. (continued) Examples of Fleet Moorings

d. Multiple Vessel Moorings

MOORING TYPE FIGURE DESCRIPTION
NUMBER
Nest Multiple tension members are used to secure
2-12 several vessels together. Separators are used to
2-13 keep the vessels from contacting one another.
Nests of vessels are commonly put into spread
moorings. Nested vessels may be of similar size
(as for inactive ships) or much different size (as a
submarine alongside a tender). Advantages of
nesting are: a nest takes up relatively little harbor
space and forces/moments on a nest may be less
than if the ships were moored individually.

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Figure 2-6. Ship at Anchor

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Figure 2-7. Single Point Mooring With Drag Anchors

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Figure 2-8. Single Point Mooring With a Plate Anchor and a Sinker

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Figure 2-9. Bow-Stern Mooring Shown in Plan View

Figure 2-10. Med-Mooring

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Figure 2-11. Spread Mooring

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Figure 2-12. Two inactive ships moored at a wharf (separators between ships not
shown)

Figure 2-13. Spread Mooring

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CHAPTER 3

BASIC DESIGN PROCEDURE

3-1 DESIGN APPROACH. Begin the design with specified parameters and
use engineering principles to complete the design. Types of parameters associated with
mooring projects are summarized in Table 3-1. The basic approach to performing
mooring design with the facility and ship known is given in Table 3-2.

Table 3-1. Parameters in a Mooring Project

PARAMETER EXAMPLES
1. Operational Parameters Required ship position, amount of
motion allowed
2. Ship Configuration Basic ship parameters, such as length,
width, draft, displacement, wind areas,
mooring fitting locations, wind/current
force, and moment coefficients
3. Facility Configuration Facility location, water depth,
dimensions, locations/type/capacity of
mooring fittings/fenders, facility
condition, facility overall capacity
4. Environmental Parameters Wind speed, current speed and
direction, water levels, wave conditions
and possibility of ice
5. Mooring Configuration Number/size/type/location of tension
members, fenders, camels, etc.
6. Material Properties Stretch/strain characteristics of the
mooring tension and compression
members

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Table 3-2. Basic Mooring Design Approach With Known Facility for
a Specific Site and a Specific Ship

STEP NOTES
Define customer(s) Define the ship(s) to be moored, the type of service
requirements required, the maximum allowable ship motions, and
situations under which the ship will leave.
Determine planning Define the impact/interaction with other facilities
requirements and operations, evaluate explosive arcs, determine
permit requirements, establish how the mooring is
to be used, review the budget and schedule.
Environmental Impact Prepare any required studies and paperwork.
Assessments
Define site and Determine the water depth(s), engineering soil
environmental parameters, design winds, design currents, design
parameters waves, design water levels, and evaluate access.
Ship characteristics Find the characteristics of the ship(s) including sail
areas, drafts, displacements, ship mooring fittings,
allowable hull pressures, and other parameters.
Ship forces/moments Determine the forces, moments, and other key
behaviors of the ship(s).
Evaluate mooring Evaluate the alternatives in terms of safety, risk,
alternatives cost, constructability, availability of hardware,
impact on the site, watch circle, compatibility,
maintenance, inspectability, and other important
aspects.
Design Calculations Perform static and/or dynamic analyses (if required)
for mooring performance, anchor design, fender
design, etc
Notifications Prepare Notice to Mariners for the case of in-water
construction work and notify charting authorities
concerning updating charts for the area.

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Table 3-2. (continued) Basic Mooring Design Approach With Known Facility for
a Specific Site and a Specific Ship

STEP NOTE
Plans/Specs Prepare plans, specifications, and cost estimates.
Permits Prepare any required environmental studies and
obtain required permits.
Installation planning Prepare instructions for installation, including safety
and environmental protection plans.
Installation monitoring Perform engineering monitoring of the installation
process.
Testing Perform pull tests of all anchors in mooring facilities
to ensure that they hold the required load.
Documentation Document the design and as-built conditions with
drawings and reports.
Instructions Provide diagrams and instructions to show the
customer how to use and inspect the mooring.
Inspection Perform periodic inspection/testing of the mooring
to assure it continues to meet the customer(s)
requirements.
Maintenance Perform maintenance as required and document on
as-built drawings.

3-2 GENERAL DESIGN CRITERIA. General design issues shown in Table 3-

3 should be addressed during design to help ensure projects meet customers’ needs.

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Table 3-3. Design Issues

CRITERIA NOTES
Vessel operating Under what conditions will the vessel(s) exit? What are the
conditions operating mission requirements for the ship? What is the
maximum allowable hull pressure?
Allowable motions How much ship motion in the six degrees-of-freedom will be
allowable for the moored ship? This is related to brow
positions and use, utilities, ship loading and unloading
operations, and other requirements. Note that most ships
have a very high buoyancy force and moorings should be
designed to allow for water level changes at a site.
User skills Is the user trained and experienced in using the proposed
system? What is the risk that the mooring would be
improperly used? Can a design be formulated for easy and
reliable use?
Flexibility How flexible is the design? Can it provide for new mission
requirements not yet envisioned? Can it be used with
existing facilities/ships?
Constructability Does the design specify readily available commercial
products and is it able to be installed and/or constructed
using standard techniques, tolerances, etc.?
Cost Are initial and life cycle costs minimized?
Inspection Can the mooring system be readily inspected to ensure
continued good working condition?
Maintenance Can the system be maintained in a cost-effective manner?
Special requirements What special requirements does the customer have? Are
there any portions of the ship that cannot come in contact
with mooring elements (e.g., submarine hulls)?

3-2.1 Mooring Service Types. Four Mooring Service Types are defined to help
identify minimum design requirements associated with DoD ships and piers, and
determine operational limitations. Facility and ship mooring hardware should
accommodate the service types shown in Table 3-4.

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Table 3-4. Mooring Service Types
MOORING SERVICE DESCRIPTION
TYPE
TYPE I This category covers moorings for mild weather
Mild Weather Mooring (sustained winds of less than 35 knots; below gale force)
and currents less than 1 knot. Mooring situations include
ammunition facilities, fueling facilities, deperming
facilities, and ports of call. Use of these moorings is
normally selected in concert with forecasted weather.
TYPE II This category covers moorings that are used through
storm conditions. Moorings include standard, storm and
nested configurations. Vessel will normally leave prior to
an approaching hurricane, typhoon, surge or other
extreme event. Naval ships intend to go to sea if 50 knot
winds are expected, but storms may come up quickly, so
higher design winds are recommended.
TYPE IIA Standard MST IIA covers mooring in winds of 50 knots or less in
Mooring broadside currents of 1-1/2 knots or less. The practice is
to provide for full pier operation for MST IIA.
TYPE IIB Storm MST IIB covers mooring in winds of 64 knot or less in
Mooring broadside currents of a 2 knots or less. This is the
intended Navy ship mooring design requirement. It is
encouraged for general home porting because sudden
storms can produce high winds on short notice. Pier
operations may be impacted for MST IIB if lines must be
run across a pier.
TYPE III This category covers moorings of vessels that cannot or
Heavy Weather may not get underway prior to an approaching hurricane
Mooring or typhoon. Moorings include fitting-out, repair,
drydocking, and overhaul berthing facilities.
TYPE IV This category covers moorings that are used to
Permanent Mooring permanently moor a vessel that will not leave in case of a
hurricane, typhoon, or surge. Moorings include inactive
ships, floating drydocks, ship museums, training berthing
facilities, etc.

3-2.2 Facility Design Criteria for Mooring Service Types. Mooring facilities
are designed conforming to the site specific environmental criteria given in Table 3-5.
Table 3-5 gives design criteria in terms of environmental design return intervals, R, and
in terms of probability of exceedence, P, for 1 year of service life, N=1. The ship usually
has the responsibility for providing mooring lines for Mooring Service Types I and II,
while the facility usually provides mooring lines for Mooring Service Types III and IV.

3-2.3 Ship Hardware Design Criteria for Mooring Service Types. Ship
mooring hardware needs to be designed to accommodate various modes of ship
operation. During Type II operation, a ship may be moored in relatively high broadside

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current and get caught by a sudden storm, such as a thunderstorm. Type III mooring
during repair may provide the greatest potential of risk, because the ship is moored for a
significant time and cannot get underway. There are several U.S. shipyards where
DOD ships can undergo major repairs. The area near Norfolk/Portsmouth, VA has
some of the most extreme design criteria, so ship’s hardware design should be based
on conditions derived from this site. Ship mooring hardware environmental design
criteria are given in Table 3-6. During Type IV mooring, the ship is usually aligned with
the current, extra padeyes can be welded to the ship hull for mooring, etc., so special
provisions can be made for long-term storage.

3-2.4 Strength. Moorings should be designed and constructed to safely resist

the nominal loads in load combinations defined herein without exceeding the
appropriate allowable stresses for the mooring components. Normal wear of materials
and inspection methods and frequency need to be considered. Due to the probability of
simultaneous maximum occurrences of variable loads, no reduction factors should be
used.

3-2.5 Serviceability. Moorings should be designed to have adequate stiffness

to limit deflections, vibration, or any other deformations that adversely affect the
intended use and performance of the mooring. At the same time moorings need to be
flexible enough to provide for load sharing, reduce peak dynamic loads and allow for
events, such as tidal changes.

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Table 3-5. Facility Design Criteria for Mooring Service Types

MOORING WIND* CURRENT** WATER WAVES

SERVICE TYPE LEVEL
TYPE I Less than 35 knots 1 knot or less mean lower N.A.
low to mean
higher high
TYPE IIA Vw=50 knots (max.) 1.5-knot extreme low to P=1 or
max. mean higher R=1 yr
high
TYPE IIB Vw=64 knots (max.) 2.0-knot extreme low to P=1 or
max. mean higher R=1 yr
high
TYPE III P=0.02 or P=0.02 or extreme low to P=0.02 or
R=50 yr R=50 yr mean higher R=50 yr
high
TYPE IV P=0.01 or P=0.01 or extreme water P=0.01 or
R=100 yr R=100 yr levels R=100 yr

*Use exposure D (UFC 1-200-01 Design: General Building Requirements; flat,

unobstructed area exposed to wind flowing over open water for a distance of at least 1
mile or 1.61 km) for determining design wind speeds. Note that min. = minimum return
interval or probability of exceedence used for design; max. = maximum wind speed
used for design.

**To define the design water depth for ship mooring systems, use T/d=0.9 for flat keeled
ships; for ships with non-flat hulls, that have sonar domes or other projections, take the
ship draft, T, as the mean depth of the keel and determine the water depth, d, by adding
0.61 meter (2 feet) to the maximum navigation draft of the ship (note, may vary
depending on sonar dome size)

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Table 3-6. Ship Mooring Hardware Design Criteria

a. Ship Anchor Systems*

VESSEL TYPE MINIMUM MINIMUM MINIMUM
WATER WIND CURRENT
DEPTH SPEED SPEED
Ships 240 ft 70 knots 4 knots
73 m 32.9 m/s 1.54 m/s

Submarines 120 ft 70 knots 4 knots

36.6 m 36.0 m/s 2.06 m/s

b. Ship Mooring Systems**

MOORING SERVICE TYPE MINIMUM WIND SPEED MINIMUM
CURRENT
SPEED
Type I 35 knots 1 knot
18.0 m/s 0.51 m/s
Type II*** 64 knots 2 knots
33.0 m/s 1.03 m/s
Type III 95 knots 2 knots
48.9 m/s 1.03 m/s

*Quasi-static design assuming wind and current are co-linear for ship and submarine
anchor systems (after NAVSEASYSCOM DDS-581).

**To define the design water depth, use T/d=0.9 for flat keeled ships; for ships with non-
flat hulls, that have sonar domes or other projections, take the ship draft, T, as the mean
depth of the keel and determine the water depth, d, by adding 0.61 meter (2 feet) to the
maximum navigation draft of the ship (note, may vary depending on sonar dome size).

***Ships need to carry lines suitable for MST IIB.

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3-2.6 Design Methods. All moorings should be designed by
skilled/knowledgeable professional personnel. Methods must be used that assure that
ships are safely moored. Below are some guidelines.

Mooring Service Type I and II moorings can often be designed using

quasi-static tools with 3 degrees-of-freedom (surge, sway and yaw). Examples of tools
include FIXMOOR (Note: FIXMOOR is available from NAVFAC Atlantic CIENG
WATERS TOOLBOX; hereinafter referred to as WATERS TOOLBOX), OPTIMOOR,
AQWA LIBRIUM, etc. Specialized tools need to be considered for cases of high
currents, high tidal ranges, passing ship effects, ship waves, multiple/nested ships,
situations that are likely to be dynamic and other specialized cases. It is valuable to
ships’ and port operations personnel to provide generalized mooring designs for
Mooring Service Types I and II.

Mooring Service Types III and IV must be designed on a case-by-case

basis using dynamic methods because of the extremely high loading that occurs during
extreme storms. It is recommended that NFESC be contacted concerning the design of
these types of moorings.

3-2.7 General Mooring Integrity. For multiple-member moorings, such as for a

ship secured to a pier by a number of lines, the mooring system strongly relies on load
sharing among several members. If one member is lost, the ship should remain
moored. Therefore, design multiple member mooring to ensure that remaining members
maintain a factor of safety at least 75 percent of the intact mooring factors of safety
shown in Table 3-7 with any one member missing.

3-2.8 Quasi-Static Safety Factors. Table 3-7 gives recommended minimum

factors of safety for “quasi-static” design based on material reliability.

3-2.9 Allowable Ship Motions. Table 3-8 gives recommended operational ship
motion criteria for moored vessels. Table 3-8(a) gives maximum wave conditions for
manned and moored small craft (Permanent International Association of Navigation
Congresses (PIANC), Criteria for Movements of Moored Ships in Harbors; A Practical
Guide, 1995). These criteria are based on comfort of personnel on board a small boat,
and are given as a function of boat length and locally generated.

Table 3-8(b) gives recommended motion criteria for safe working

conditions for various types of vessels (PIANC, 1995).

Table 3-8(c) gives recommended velocity criteria and Table 3-8(d) and (e)
give special criteria.

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Table 3-7. Minimum Quasi-Static Factors of Safety
COMPONENT MINIMUM NOTES
FACTOR OF
SAFETY
Stockless & balanced 1.5 For ultimate anchoring system holding capacity;
fluke anchors use 1.0 for ship's anchoring*
High efficiency drag 2.0 For ultimate anchoring system holding capacity
anchors use 1.0 for ship's anchoring*
Fixed anchors (piles & 3.0 For ultimate anchoring system holding capacity*
plates)
Deadweight anchors - Use carefully (see Naval Civil Engineering
Laboratory (NCEL) Handbook for Marine
Geotechnical Engineering, 1985)
3.0 For relatively straight lengths.
Chain 4.0 For chain around bends.
These factors of safety are for the new chain
break strength.
Wire rope 3.0 For the new wire rope break strength.
Synthetic line** 3.0 For new line break strength.
Ship bitts *** Use American Institute of Steel Construction
(AISC) code.
Pier bollards *** Use AISC & other applicable codes.
*It is recommended that anchors be pull tested.
**Reduce effective strength of wet nylon line by 15 percent.
*** For mooring fittings take 3 parts of the largest size of line used on the fitting; apply a
load of: 3.0*(minimum line break strength)*1.3 to determine actual stresses, σact.; design
fittings so (σact./ σallow.)<1.0, where σallow. is the allowable stress from AISC and other
applicable codes.

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Table 3-8. Recommended Practical Motion Criteria for Moored Vessels

(a) Safe Wave Height Limits for Moored Manned Small Craft
(after PIANC, 1995)

Beam/Quartering Seas Head Seas

Vessel Wave Maximum Wave Maximum
Length (m) Period (sec) Sign Wave Period (sec) Sign Wave
Height, Hs Height, Hs
(m) (m)
4 to 10 <2.0 0.20 <2.5 0.20
“ 2.0-4.0 0.10 2.5-4.0 0.15
“ >4.0 0.15 >4.0 0.20
10-16 <3.0 0.25 <3.5 0.30
“ 3.0-5.0 0.15 3.5-5.5 0.20
“ >5.0 0.20 >5.5 0.30
20 <4.0 0.30 <4.5 0.30
“ 4.0-6.0 0.15 4.5-7.0 0.25
“ >6.0 0.25 >7.0 0.30

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Table 3-8. (continued) Recommended Practical Motion Criteria for Moored
Vessels

(b) Recommended Motion Criteria for Safe Working Conditions1 (after PIANC, 1995)
Vessel Cargo Handling Surge Sway Heave Yaw Pitch Roll
Type Equipment (m) (m) (m) ( o) ( o) ( o)
Fishing Elevator crane 0.15 0.15 - - - -
vessels Lift-on/off 1.0 1.0 0.4 3 3 3
10-3000 Suction pump 2.0 1.0 - - - -
GRT2
Freighters& Ship’s gear 1.0 1.2 0.6 1 1 2
coasters Quarry cranes 1.0 1.2 0.8 2 1 3
<10000
DWT3
Ferries, Roll- Side ramp4 0.6 0.6 0.6 1 1 2
On/ Roll-Off Dew/storm ramp 0.8 0.6 0.8 1 1 4
(RO/RO) Linkspan 0.4 0.6 0.8 3 2 4
Rail ramp 0.1 0.1 0.4 - 1 1
General
cargo 5000- - 2.0 1.5 1.0 3 2 5
10000 DWT
Container 100% efficient 1.0 0.6 0.8 1 1 3
vessels 50% efficient 2.0 1.2 1.2 1.5 2 6
Bulk carriers Cranes Elevator/ 2.0 1.0 1.0 1.0 2 2 2 6 2
30000- bucket-wheel 1.0 0.5 - 2 - -
150000 Conveyor belt 5.0 2.5 3
DWT
Oil tankers Loading arms 3.05 3.0 - - - -
Gas tankers Loading arms 2.0 2.0 - 2 2 2

Notes for Table 3-8(b):

1
Motions refer to peak-to-peak values (except for sway,
which is zero-to-peak)
2
GRT = Gross Registered Tons expressed as internal volume of
ship in units of 100 ft3 (2.83 m3)
3
DWT = Dead Weight Tons, which is the total weight of the
vessel and cargo expressed in long tons (1016 kg) or metric
tons (1000 kg)
4
Ramps equipped with rollers.
5
For exposed locations, loading arms usually allow for 5.0-
meter motion.

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Table 3-8. (continued) Recommended Practical Motion Criteria for Moored
Vessels

(c) Recommended Velocity Criteria for Safe Mooring Conditions for Fishing Vessels,
Coasters, Freighters, Ferries and Ro/Ro Vessels (after PIANC, 1995)

Ship Surge Sway Heave Yaw Pitch Roll

Size(DWT) (m/s) (m/s) (m/s) (o/s) (o/s) (o/s)
1000 0.6 0.6 - 2.0 - 2.0
2000 0.4 0.4 - 1.5 - 1.5
8000 0.3 0.3 - 1.0 - 1.0

(d) Special Criteria for Walkways and Rail Ramps

(after PIANC, 1995)

Parameter Maximum Value

Vertical velocity 0.2 m/s
Vertical acceleration 0.5 m/s2

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Table 3-8. (Continued) Recommended Practical Motion Criteria for Moored
Vessels

(e) Special Criteria

CONDITION MAXIMUM NOTES

AMPLITUDE
VALUES
Heave - Ships will move vertically with any
long period water level change (tide,
storm surge, flood, etc.). The
resulting buoyancy forces may be
high, so the mooring must be
designed to provide for these motions
due to long period water level
changes.
Loading/unloading 0.6 m (2 Maximum ramp motion during
preposition ships feet) loading/unloading moving wheeled
vehicles.
Weapons 0.6 m (2 Maximum motion between the crane
loading/unloading feet) and the object being loaded/unloaded.

3-3 DESIGN METHODS

3-3.1 Quasi-Static Design. Practical experience has shown that in many

situations such as for Mooring Service Types I and II, static analysis tools, such as
FIXMOOR (WATERS TOOLBOX), OPTIMOOR and AQWA LIBRIUM, can be used to
reliably determine mooring designs in harbors. Winds are a key forcing factor in
mooring harbors. Winds can be highly dynamic in heavy weather conditions. However,
practical experience has shown that for typical DOD ships, a wind speed with a duration
of 30 seconds can be used, together with static tools, to develop safe mooring designs.
The use of the 30-second duration wind speed with static tools and the approach shown
in Table 3-9 is called “quasi-static” design.

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Table 3-9. Quasi-Static Design Notes

CRITERIA NOTES
Wind speed Determine for the selected return interval, R. For
typical ships use the wind that has a duration of
30 seconds at an elevation of 10 m.
Wind direction Assume the wind can come from any direction
except in cases where wind data show extreme
winds occur in a window of directions.
Current speed Use conditions for the site (speed and direction).
Water levels Use the range for the site.
Waves Neglected. If waves are believed to be
important, then dynamic analyses are
recommended.
Factors of safety Perform the design using quasi-static forces and
moments (see Chapter 4), minimum factors of
safety in Table 3-7, and design to assure that all
criteria are met.

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3-3.2 Dynamic Mooring Analysis. Conditions during Mooring Service Types
III and IV, and during extreme events can be highly dynamic. Unfortunately, the
dynamic behavior of a moored ship in shallow water can be highly complex, so
dynamics cannot be fully documented in this UFC. An introduction to dynamics is
provided in Chapter 8. Information on dynamics is found in: Dynamic Analysis of
Moored Floating Drydocks, Headland et. al. (1989); Advanced Dynamics of Marine
Structures, Hooft (1982); Hydrodynamic Analysis and Computer Simulation Applied to
Ship Interaction During Maneuvering in Shallow Channels, Kizakkevariath (1989); David
Taylor Research Center (DTRC), SPD-0936-01, User’s Manual for the Standard Ship
Motion Program, SMP81; Low Frequency Second Order Wave Exciting Forces on
Floating Structures, Pinkster (1982); Mooring Dynamics Due to Wind Gust Fronts,
Seelig and Headland (1998); and A Simulation Model for a Single Point Moored Tanker,
Wichers (1988). Some conditions when mooring dynamics may be important to design
or when specialized considerations need to be made are given in Table 3-10.

The programs AQWA DRIFT and AQWA NAUT (Century Dynamics, Houston, TX) are
examples of software tools that can be used to simulate highly dynamic mooring
situations.

3-4 RISK. Risk is a concept that is often used to design facilities, because the
probability of occurrence of extreme events (currents, waves, tides, storm surge,
earthquakes, etc.) is strongly site dependent. Risk is used to ensure that systems are
reliable, practical, and economical.

A common way to describe risk is the concept of ‘return interval’, which is

the mean length of time between events. For example, if the wind speed with a return
interval of R = 100 years is given for a site, this wind speed would be expected to occur,
on the average, once every 100 years. However, since wind speeds are probabilistic,
the specified 100-year wind speed might not occur at all in any 100-year period. Or, in
any 100-year period the wind speed may be equal to or exceed the specified wind
speed multiple times.

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The probability or risk that an event will be equaled or exceeded one or
more times during any given interval is determined from:

EQUATION: P = 100%* (1- (1- 1/ R) N ) (1)

where

P = probability, in percent, of an event

being equaled or exceeded one or more
times in a specified interval
R = return interval (years)
N = service life (years)

Figure 3-1 shows risk versus years on station for various selected values
of return interval. For example, take a ship that is on station at a site for 20 years (N =
20). There is a P = 18.2 percent probability that an event with a return interval of R =
100 years or greater will occur one or more times at a site in a 20-year interval.

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Table 3-10. Conditions Requiring Special Analysis

FACTOR SPECIAL ANALYSIS REQUIRED

Wind > 45 mph for small craft
> 75 mph for larger vessels
Wind waves > 1.5 ft for small craft
> 4 ft for larger vessels
Wind gust fronts Yes for SPMs
Current > 3 knots
Ship waves and passing ship effects Yes for special cases (see
Kizakkevariath, 1989; Occasion,
1996; Weggel and Sorensen, 1984 &
1986)
Long waves (seiches and tidal waves or Yes
tsunamis)
Berthing and using mooring as a break Yes (see MIL-HDBK-1025/1)
Parting tension member May be static or dynamic
Ship impact or other sudden force on the Yes (if directed)
ship
Earthquakes (spud moored or stiff Yes
systems)
Explosion, landslide, impact Yes (if directed)
Tornado (reference NUREG 1974) Yes
Flood, sudden water level rise Yes (if directed)
Ice forcing Yes (if a factor)
Ship/mooring system dynamically Yes (dynamic behavior of ships at
unstable (e.g., SPM) SPMs can be especially complex)
Forcing period near a natural period of the Yes; if the forcing period is from 80%
mooring system to 120% of a system natural period
Note: SPM = single point mooring

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Figure 3-1. Risk Diagram

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3-5 COORDINATE SYSTEMS. The various coordinate systems used for
ships and mooring design are described below.

3-5.1 Ship Design/Construction Coordinates. A forward perpendicular point

(FP), aft perpendicular point (AP), and regular spaced frames along the longitudinal
axes of the ship are used to define stations. The bottom of the ship keel is usually used
as the reference point or “baseline” for vertical distances. Figure 3-2 illustrates ship
design coordinates.

3-5.2 Ship Hydrostatics/Hydrodynamics Coordinates. The forward

perpendicular is taken as Station 0, the aft perpendicular is taken as Station 20, and
various cross-sections of the ship hull (perpendicular to the longitudinal axis of the ship)
are used to describe the shape of the ship hull. Figure 3-2 illustrates ship hydrostatic
conventions.

3-5.3 Local Mooring Coordinate System. Environmental forces on ships are

a function of angle relative to the vessel’s longitudinal centerline. Also, a ship tends to
move about its center of gravity. Therefore, the local “right-hand-rule” coordinate
system, shown in Figure 3-3, is used in this UFC. The midship’s point is shown as a
convenient reference point in Figures 3-3 and 3-4.

3-5.4 Global Coordinate System. Plane state grids or other systems are often
used to describe x and y coordinates. The vertical datum is most often taken as relative
to some water level, such as mean lower low water (MLLW).

3-5.5 Ship Conditions. Loading conditions are defined in NAVSEA

NSTM 096. There are three common conditions or displacements that a ship has at
various stages including:

• “Light Condition” – This is the ship condition after first launching.

• “One-Third Stores Condition” – This is the typical ship condition during ship
repair, as indicated in SUPSHIP docking/undocking records.

• “Fully Loaded Condition” – This is the ship condition during operations.

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Figure 3-2. Ship Design and Hydrostatic Coordinates

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Figure 3-3. Local Mooring Coordinate System for a Ship

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Figure 3-4. Local Mooring Coordinate System for a Ship

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3-6 VESSEL DESIGN CONSIDERATIONS. Some important vessel mooring
design considerations are summarized in Table 3-11. General information on ships can
be found in the Ships Characteristics Database (WATERS TOOLBOX) and in the
NAVSEA Hitchhikers Guide to Navy Surface Ships.

Table 3-11. Design Considerations - Ship

PARAMETER NOTES
Ship fittings The type, capacity, location, and number of
mooring fittings on the ship are critical in designing
moorings.
Ship hardware The type, capacity, location, and number of other
mooring hardware (chain, anchors, winches, etc.)
on the ship are critical.
Buoyancy The ship’s buoyancy supports the ship up in the
heave, pitch, and roll directions. Therefore, it is
usually undesirable to have much mooring capacity
in these directions. A large ship, for example, may
have over a million pounds of buoyancy for a foot of
water level rise. If an unusually large water level
rise occurs for a mooring with a large component of
the mooring force in the vertical direction, this could
result in mooring failure.
Hull pressures Ships are designed so that only a certain allowable
pressure can be safely resisted. Allowable hull
pressures and fender design are discussed in
NFESC TR-6015-OCN, Foam-Filled Fender Design
to Prevent Hull Damage.
Personnel access Personnel access must be provided.
Cargo Loading Ramps/sideport locations
Hotel services Provision must be made for utilities and other hotel
services.
Ship condition Ships are typically in the “Light”, “One-Third Stores”
or “Fully-Loaded” condition or displacement.

3-7 FACILITY DESIGN CONSIDERATIONS. Some important facility mooring

design considerations are summarized in Table 3-12.

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Table 3-12. Design Considerations - Facility

PARAMETER NOTES
Access Adequate ship access in terms of channels,
turning basins, bridge clearance, etc. needs
to be provided. Also, tugs and pilots must be
available.
Mooring fittings The number, type, location and capacity of
mooring fittings or attachment point have to
meet the needs of all vessels using the
facility.
Fenders The number, type, location, and properties of
marine fenders must be specified to protect
the ship(s) and facility.
Water depth The water depth at the mooring site must be
adequate to meet the customer’s needs.
Shoaling Many harbor sites experience shoaling. The
shoaling and possible need for dredging
needs to be considered.
Permits Permits (Federal, state, environmental,
historical, etc.) are often required for facilities
and they need to be considered.

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3-8 ENVIRONMENTAL FORCING DESIGN CONSIDERATIONS.
Environmental forces acting on a moored ship(s) can be complex. Winds, currents,
water levels, and waves are especially important for many designs. Specific
environmental design criteria for selected sites of interest can be found in the
Climatalogical Database (WATERS TOOLBOX)

3-8.1 Winds. A change in pressure from one point on the earth to another
causes the wind to blow. Turbulence is carried along with the overall wind flow to
produce wind gusts. If the mean wind speed and direction do not change very rapidly
with time, the winds are referred to as “stationary.”

Practical experience has shown that wind gusts with a duration of

approximately 30 seconds or longer have a significant influence on typical moored ships
with displacements of about 1000 tons or larger. Vessels with shorter natural periods
can respond to shorter duration gusts. For the purposes of this UFC, a 30-second wind
duration at a 10-meter (33-foot) elevation is recommended for the design for “stationary”
winds. The relationship of the 30-second wind to other wind durations is shown in
Figure 3-5.

If wind speed and/or direction changes rapidly, such as in a wind gust

front, hurricane or tornado, then winds are “non-stationary”. Figure 3-6, for example,
shows a recording from typhoon OMAR in 1992 at Guam. The eye of this storm went
over the recording site. The upper portion of this figure shows the wind speed and the
lower portion of the figure is the wind direction. Time on the chart recorder proceeds
from right to left. This hurricane had rapid changes in wind speed and direction. As the
eye passes there is also a large-scale change in wind speed and direction.

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Figure 3-5. Ratio of Wind Speeds for Various Gusts (after ASCE 7-95)
1.30

1.25

1.20

1.15

1.10

1.05
Vt/V30

1.00
NON-HURRICANE
0.95

0.90

0.85

0.80
HURRICANE
0.75

0.70
1 10 100 1000 10000
GUST DURATION, t (sec)

t Vt/V30 Vt/V30
(sec) Non-Hurricane Hurricane
1 1.182 1.221
2 1.160 1.196
3 1.145 1.175
5 1.124 1.147
10 1.080 1.097
20 1.030 1.034
30 1.000 1.000
40 0.977 0.971
50 0.955 0.950
60 0.938 0.932
70 0.924 0.917
100 0.891 0.879
200 0.846 0.822
400 0.815 0.780
1000 0.783 0.739
3600 0.753 0.706

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Figure 3-6. Typhoon OMAR Wind Chart Recording 3 October 2005

WIND SPEED

‘EYE’

TIME
1 hour

WIND DIRECTION

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3-8.2 Wind Gust Fronts. A particularly dangerous wind condition that has
caused a number of mooring accidents is the wind gust front (Mooring Dynamics Due to
Wind Gust Fronts, Seelig and Headland, 1998 and CHESNAVFACENGCOM, FPO-1-
87(1), Failure Analysis of Hawsers on BOBO Class MSC Ships at Tinian on 7
December 1986). This is a sudden change in wind speed that is usually associated with
a change in wind direction (Wind Effects on Structures, Simiu and Scanlan, 1996). The
key problems with this phenomena are: (1) high mooring dynamic loads can be
produced in a wind gust front, (2) there is often little warning, (3) little is known about
wind gust fronts, and (4) no design criteria for these events have been established.

A study of Guam Agana National Air Station (NAS) wind records was
performed to obtain some statistics of wind gust fronts (National Climatic Data Center
(NCDC), Letter Report E/CC31:MJC, 1987). The 4.5 years of records analyzed from
1982 through 1986 showed approximately 500 cases of sudden wind speed change,
which were associated with a shift in wind direction. These wind shifts predominately
occurred in 1 minute or less and never took longer than 2 minutes to reach maximum
wind speed. Figure 3-7 shows sudden changes in wind speed and direction that
occurred over a 2-1/2 day period in October 1982. These wind gust fronts seemed to be
associated with a nearby typhoon.

Table 3-13 gives the joint distribution of wind shifts in terms of the amount
the increase in wind speed and the wind direction change. Approximately 60 percent of
the wind gust fronts from 1982 through 1986 had wind direction changes in the 30-
degree range, as shown in Figure 3-8.

Based on the Guam observations, the initial wind speed in a wind gust
front ranges from 0 to 75 percent of the maximum wind speed, as shown in Figure 3-9.
On the average, the initial wind speed was 48 percent of the maximum in the 4.5-year
sample from Guam (NCDC, 1987).

Simiu and Scanlan (1996) report wind gust front increases in wind speed
ranging from 3 m/sec to 30 m/sec (i.e., 6 to 60 knots). Figure 3-10 shows the
distribution of gust front winds from the 4.5-year sample from 1982 through 1986 on
Guam. This figure shows the probability of exceedence on the x-axis in a logarithmic
format. The square of the wind gust front speed maximums was plotted on the y-axis,
since wind force is proportional to wind speed squared. Figure 3-10 provides a sample
of the maximum wind gust front distribution for a relatively short period at one site.
Those wind gust fronts that occurred when a typhoon was nearby are identified with an
“H.” It can be seen that the majority of the higher gust front maximums were associated
with typhoons. Also, the typhoon gust front wind speed maxima seem to follow a
different distribution that the gust front maxima associated with rain and thunderstorms
(see Figure 3-10).

Effects of winds and wind gusts are shown in the examples in Chapter 8 of
this UFC.

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Figure 3-7. Sample Wind Gust Fronts on Guam, 2-4 October 1982

45

40
40 40 40
40
30 20

35

30 Wind Dir.
40 15
WIND SPEED (knots)

Shift (deg)=

WIND SPEED (m/s)

40 30 20
25

20 10

50
15

10 5

0 0
0 12 24 36 48 60
TIME, (Hours; Start 02 Oct 82)

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Table 3-13. Sample Distribution of Wind Gust Fronts on Guam (Agana NAS) from
1982 to 1986

NUMBER OF OBSERVATIONS
WIND SPEED CHANGE WIND DIRECTION CHANGE
(knots) (m/s)

MIN. MAX. MIN. MAX. 20 30 40 50 60 70 80 90

deg deg deg deg deg deg deg deg

6 10 3.1 5.1 28 241 66 30 4 2

11 15 5.7 7.7 8 42 18 13 5 3 1 1

16 20 8.2 10.3 6 7 3 2 2

21 25 10.8 12.9 3 2 1

26 30 13.4 15.4 1

Figure 3-8. Distribution of Guam Wind Gust Front Wind Angle Changes

60
50 Percent of Observations
% OF SHIFTS

CLOCKWISE 62%
40
COUNTERCLOCKWISE 38%
30
20
10
0
20 30 40 50 60 70 80 90
WIND ANGLE CHANGE (deg)

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Figure 3-9. Initial Versus Maximum Wind Speeds for Wind Gust Fronts

25

20
INITIAL WIND SPEED (m/s)

48%

15

10

0
0 5 10 15 20 25
MAX WIND SPEED (m/s)

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Figure 3-10. Wind Gust Front Maxima on Guam 1982-1986

600

500 H
2

H
MAX WIND SPEED SQUARED (m/s)

400
H
H

300

200

100

0
0.1 1.0 10.0 100.0
PROBABILITY OF EXCEEDENCE

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3-8.3 Storms. Table 3-14 gives environmental parameters for standard storms.

Table 3-14. Storm Parameters

(a) Tropical Storms

LOWER WIND SPEED UPPER WIND SPEED

Storm (m/s) (mph) (knts) (m/s) (mph) (knts)
TROPICAL 9.8 22 19 16.5 37 32
DEPRESSION
TROPICAL STORM 17.0 38 33 32.6 73 63
HURRICANE 33.1 74 64 - - -

(b) Saffier-Simpson Hurricane Scale

WIND SPEED RANGE OPEN COAST STORM SURGE RANGE

LOWER UPPER LOWER UPPER
CATE- (m/s) (mph) (m/s) (mph) (m) (ft) (m) (ft)
GORY
1 33.1 74 42.5 95 1.22 4 1.52 5
2 42.9 96 49.2 110 1.83 6 2.44 8
3 49.6 111 58.1 130 2.74 9 3.66 12
4 58.6 131 69.3 155 3.96 13 5.49 18
5 69.7 156 - - 5.79 19 - -

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Table 3-14. (continued) Storm Parameters

(c) Beaufort Wind Force*

LOWER WIND SPEED UPPER WIND SPEED

BEAUFORT WIND (m/s) (mph) (knts) (m/s) (mph) (knts)
FORCE/
DESCRIPTION
0 CALM 0.0 0 0 0.5 1 1
1 LIGHT AIRS 0.5 1 1 1.5 4 3
2 LIGHT BREEZE 2.1 5 4 3.1 7 6
3 GENTLE GREEZE 3.6 8 7 5.1 12 10
4 MODERATE 5.7 13 11 8.2 18 16
BREEZE
5 FRESH BREEZE 8.8 20 17 10.8 24 21
6 STRONG BREEZE 11.3 25 22 13.9 31 27
7 MODERATE GALE 14.4 32 28 17.0 38 33
8 FRESH GALE 17.5 39 34 20.6 46 40
9 STRONG GALE 21.1 47 41 24.2 54 47
10 WHOLE GALE 24.7 55 48 28.3 63 55
11 STORM 28.8 65 56 32.4 73 63
12 HURRICANE 32.9 74 64 36.6 82 71
*After Handbook of Ocean and Underwater Engineers,
Myers et al. (1969). The above table should be used with caution,
because design conditions for a specific site could vary from the values shown.

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Table 3-14. (continued) Storm Parameters

(d) World Meteorological Organization Sea State Scale

Sign. Wave Height Sustained Wind Speed Modal

SEA STATE (ft) [m] (knts) [m/s] Wave
Period
Range
(sec)
0 CALM/GLASSY NONE NONE -
1 RIPPLED 0-0.3 [0-0.1] 0-6 [0-3] -
2 SMOOTH 0.3-1.6 [0.1-0.5] 7-10 [3.6-5.1] 3-15
3 SLIGHT 1.6-4.1 [0.5-1.2] 11-16 [5.7-8.2] 3-15.5
4 MODERATE 4.1-8.2 [1.2-2.5] 17-21 [8.7-10.8] 6-16
5 ROUGH 8.2-13.1 [2.5-4.0] 22-27 [11.3-13.9] 7-16.5
6 VERY ROUGH 13.1-19.7 [4.0-6.0] 28-47 [14.4-24.2] 9-17
7 HIGH 19.7-29.5 [6.0-9.0] 48-55 [24.7-28.3] 10-18
8 VERY HIGH 29.5-45.5[9.0-13.9] 56-63 [28.8-32.4] 13-19
9 PHENOMENAL >45.5 [>13.9] >63 [>32.4] 18-24

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3-8.4 Currents. The magnitude and direction of currents in harbors and
nearshore areas are in most cases a function of location and time. Astronomical tides,
river discharges, wind-driven currents, and other factors can influence currents. For
example, wind-driven currents are surface currents that result from the stress exerted
by the wind on the sea surface. Wind-driven currents generally attain a mean velocity
of approximately 3 to 5 percent of the mean wind speed at 10 meters (33 feet) above
the sea surface. The magnitude of this current strongly decreases with depth.

Currents can be very site specific, so it is recommended that currents be

measured at the design site and combined with other information available to define the
design current conditions.

3-8.5 Water Levels. At most sites some standard datum, such as mean low
water (MLW) or mean lower low water (MLLW), is established by formal methods.
Water levels are then referenced to this datum. The water level in most harbors is then
a function of time. Factors influencing water levels include astronomical tides, storm
surges, river discharges, winds, seiches, and other factors.

The design range in water levels at the site must be considered in the
design process.

3-8.6 Waves. Most DOD moorings are wisely located in harbors to help
minimize wave effects. However, waves can be important to mooring designs in some
cases. The two primary wave categories of interest are:

a) Wind waves. Wind waves can be locally generated or can be wind

waves or swell entering the harbor entrance(s). Small vessels are especially
susceptible to wind waves.

b) Long waves. These can be due to surf beat, harbor seiching, or other
effects.

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Ship waves may be important in some cases. The response of a moored
vessel to wave forcing includes:

a) A steady mean force.

b) First order response, where the vessel responds to each wave, and

c) Second order response, where some natural long period mode of

ship/mooring motion, which usually has little damping, is forced by the group or other
nature of the waves.

If any of these effects are important to a given mooring design, then a six-
degree-of-freedom dynamic of the system generally needs to be considered in design.
Some guidance on safe wave limits for moored manned small craft is given in Table 3-
8(a).

3-8.7 Water Depths. The bathymetry of a site may be complex, depending on

the geology and history of dredging. Water depth may also be a function of time, if
there is shoaling or scouring. Water depths are highly site specific, so hydrographic
surveys of the project site are recommended.

3-8.8 Environmental Design Information. Some sources of environmental

design information of interest to mooring designers are summarized in Table 3-15.

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Table 3-15. Some Sources of Environmental Design Information

a. Winds
Climatalogical Database (WATERS TOOLBOX)
UFC 1-200-01 Design: General Building Requirements
National Bureau of Standards (NBS), Series 124, Hurricane Wind Speeds in
the United States, 1980
Nuclear Regulatory Commission (NUREG), NUREG/CR-2639, Historical
Extreme Winds for the United States – Atlantic and Gulf of Mexico Coastlines,
1982
Hurricane and Typhoon Havens Handbooks, NRL (1996) and NEPRF (1982)
NUREG/CR-4801, Climatology of Extreme Winds in Southern California, 1987
NBS Series 118, Extreme Wind Speeds at 129 Stations in the Contiguous
United States, 1979
U.S. Navy Marine Climatic Atlas of the World, Ver 1.0

b. Currents
Climatalogical Database (WATERS TOOLBOX)
National Ocean Survey records
Nautical Software, Tides and Currents for Windows, 1995
U.S. Army Corps of Engineers records

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Table 3-15. (continued) Some Sources of Environmental Design Information

c. Water Levels
Climatalogical Database (WATERS TOOLBOX)
Federal Emergency Management Agency records
U.S. Army Corps of Engineers, Special Report No. 7, Tides and Tidal Datums
in the United States, 1981
National Ocean Survey records
Hurricane and Typhoon Havens Handbooks, NRL (1996) and NEPRF (1982)
Nautical Software (1995)
U.S. Army Corps of Engineers records

d. Waves
Hurricane and Typhoon Havens Handbooks, NRL (1996) and NEPRF (1982)
U.S. Army Corps of Engineers, Coastal Engineering Manual (current version)
gives prediction methods

e. Bathymetry
From other projects in the area
National Ocean Survey charts and surveys
U.S. Army Corps of Engineers dredging records

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3-9 OPERATIONAL CONSIDERATIONS. Some important operational design
considerations are summarized in Table 3-16.

Table 3-16. Mooring Operational Design Considerations

PARAMETER
NOTES
Personnel experience/ What is the skill of the people using the mooring?
training
Failure What are the consequences of failure? Are there
any design features that can be incorporated that
can reduce the impact?
Ease of use How easy is the mooring to use and are there
factors that can make it easier to use?
Safety Can features be incorporated to make the mooring
safer for the ship and personnel?
Act-of-God events Extreme events can occur unexpectedly. Can
features be incorporated to accommodate them?
Future use Future customer requirements may vary from
present needs. Are there things that can be done
to make a mooring facility more universal?

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3-10 INSPECTION. Mooring systems and components should be inspected
periodically to ensure they are in good working order and are safe. Table 3-17 gives
inspection guidelines.

Table 3-17. Inspection Guidelines

MOORING MAXIMUM NOTES

SYSTEM OR INSPECTION
COMPONENT INTERVAL
Piers and wharves 1 year Surface inspection
3 years Complete inspection - wood structures
6 years Complete inspection - concrete and steel
structures
See NAVFAC MO-104.2, Specialized
Underwater Waterfront Facilities
Inspections; If the actual
capacity/condition of mooring fittings on a
pier/wharf is unknown, then pull tests are
recommended to proof the fittings.
Fleet Moorings 3 years See CHESNAVFACENGCOM, FPO-1-
84(6), Fleet Mooring Underwater
Inspection Guidelines. Also inspect and
replace anodes, if required. More
frequent inspection may be required for
moorings at exposed sites or for critical
facilities.
Synthetic line 6 months Per manufacturer’s recommendations

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Table 3-17. (Continued) Inspection Guidelines

MOORING MAXIMUM NOTES

SYSTEM OR INSPECTION
COMPONENT INTERVAL
Ship’s chain 36 months 0-3 years of service
24 months 4-10 years of service
18 months >10 years of service
(American Petroleum Institute (API) RP
2T, Recommended Practice for Planning,
Designing, and Constructing Tension Leg
Platforms)
Wire rope 18 months 0-2 years of service
12 months 3-5 years of service
9 months >5 years of service
(API RP 2T)

3-11 MAINTENANCE. If excessive wear or damage occurs to a mooring

system, then it must be maintained. Fleet mooring chain, for example, is allowed to
wear to a diameter of 90 percent of the original steel bar diameter. As measured
diameters approach 90 percent, then maintenance is scheduled. Moorings with 80 to
90 percent of the original chain diameter are restricted to limited use. If a chain
diameter reaches a bar diameter of 80 percent of the original diameter, then the
mooring is condemned. Figure 3-11 illustrates some idealized models of chain wear

3-12 GENERAL MOORING GUIDELINES. Experience and practical

considerations show that the recommendations given in Table 3-18 will help ensure
safe mooring. These ideas apply to both ship mooring hardware and mooring facilities.

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Figure 3-11. Idealized Models of Chain Wear

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Table 3-18. Design Recommendations

IDEA NOTES
Allow ship to move with The weight and buoyancy forces of ships can be very
rising and falling water high, so it is most practical to design moorings to allow
levels ships to move in the vertical direction with changing water
levels. The design range of water levels for a specific site
should be determined in the design process.
Ensure mooring system A system is only as strong as its weakest segment; a
components have system with components of similar strength can be the
similar strength most economical. Mooring lines should not have a break
strength greater than the capacity of the fittings they use.
Ensure load sharing In some moorings, such as at a pier, many lines are
involved. Ensuring that members will share the load
results in the most economical system.
Bridle design In cases where a ship is moored to a single point mooring
buoy with a bridle, ensure that each leg of the bridle can
withstand the full mooring load, because one member may
take the full load as the vessel swings.
Provide shock absorbing Wind gusts, waves, passing ships, etc., will produce
in mooring systems transient forces on a moored ship. Allowing some motion
of the ship will reduce the dynamic loads. ‘Shock
absorbers’ including marine fenders, timber piles,
synthetic lines with stretch, chain catenaries, sinkers, and
similar systems are recommended to allow a moored ship
to move in a controlled manner.

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Table 3-18. (continued) Design Recommendations

IDEA NOTES
Limit the vertical angles Designing ships and piers to keep small vertical line
of lines from ship to pier angles has the advantages of improving line efficiency and
reducing the possibility of lines pulling off pier fittings.
Select drag anchors to Design mooring system that uses drag anchor, so that the
have a lower ultimate anchor will drag before the chain breaks.
holding capacity than
the breaking strength of
chain and fittings
Limit the loading on drag Drag anchors work on the principle of ‘plowing’ into the
anchors to horizontal soils. Keeping the mooring catenary angle small at the
tension seafloor will aid in anchor holding. Have at least one shot
of chain on the seafloor to help ensure the anchor will
hold.
Pull test anchors Pull testing anchors is recommended to ensure that all
whenever possible to facilities with anchors provide the required holding
the full design load capacity.

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CHAPTER 4

STATIC ENVIRONMENTAL FORCES AND MOMENTS ON VESSELS

4-1 SCOPE. In this chapter design methods are presented for calculating
static forces and moments on single and multiple moored vessels. Examples show
calculation methods.

4-2 ENGINEERING PROPERTIES OF WATER AND AIR. The effects of

water and air at the surface of the earth are of primary interest in this chapter. The
engineering properties of both are given in Table 4-1.

Table 4-1. Engineering Properties of Air and Water

Standard Salt Water at Sea Level at 15oC (59oF)

PROPERTY SI SYSTEM ENGLISH SYSTEM
Mass density, ρw 1026 kg/m3 1.9905 slug/ft3
Weight density, γw 10060 newton/m3 64.043 lbf/ft3
Volume per long ton (LT) 0.9904 m3/LT 34.977 ft3/LT
Kinematic viscosity, ν 1.191E-6 m2/sec 1.2817E-5 ft2/sec

Standard Fresh Water at Sea Level at 15oC (59oF)

PROPERTY SI SYSTEM ENGLISH OR
INCH-POUND
SYSTEM
Mass density, ρw 999.0 kg/m3 1.9384 slug/ft3
Weight density, γw 9797 newton/m3 62.366 lbf/ft3
Volume per long ton (LT) 1.0171 m3/LT 35.917 ft3/LT
Volume per metric ton 1.001 m3/ton 35.3497 ft3/ton
(ton or 1000 kg or 1 Mg)
Kinematic viscosity, ν 1.141E-6 m2/sec 1.2285E-5 ft2/sec

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Table 4-1. (continued) Engineering Properties of Air and Water

Air at Sea Level at 20oC (68oF)*

PROPERTY SI SYSTEM ENGLISH OR
INCH-POUND
SYSTEM
Mass density, ρa 1.221 kg/m3 0.00237 slug/ft3
Weight density, γa 11.978 newton/m3 0.07625 lbf/ft3
Kinematic viscosity, ν 1.50E-5 m2/sec 1.615E-4 ft2/sec
* Note that humidity and even heavy rain has relatively
little effect on the engineering properties of air (personal communication with the
National Weather Service, 1996)

4-3 PRINCIPAL COORDINATE DIRECTIONS. There are three primary axes

for a ship:

X - Direction parallel with the ships

Longitudinal axis
Y - Direction perpendicular to a vertical plane
through the ship’s longitudinal axis
Z - Direction perpendicular to a plane formed by
the “X” and “Y” axes

There are six principal coordinate directions for a ship:

Surge - In the “X”-direction

Sway - In the “Y”-direction
Heave - In the “Z”-direction
Roll - Angular about the “X”-axis
Pitch - Angular about the “Y”-axis
Yaw - Angular about the “Z”-axis

Of primary interest are: (1) forces in the surge and sway directions in the
“X-Y” plane, and (2) moment in the yaw direction about the “Z”-axis. Ship motions occur
about the center of gravity of the ship.

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4-4 STATIC WIND FORCES/MOMENTS. Static wind forces and moments on
stationary moored vessels are computed in this chapter. Figure 4-1 shows the definition
of some of the terms used in this chapter. Figure 4-2 shows the local coordinate
system.

4-4.1 Static Transverse Wind Force. The static transverse wind force is
defined as that component of force perpendicular to the vessel centerline. In the local
ship coordinate system, this is the force in the “Y” or sway direction. Transverse wind
force is determined from the equation:

EQUATION: Fyw = 0.5 ρa Vw 2 A y C yw f yw {θw } (2)

where

Fyw = transverse wind force (newtons)

ρa = mass density of air (from Table 4-1)
Vw = wind speed (m/s)
Ay = longitudinal projected area of the ship (m2)
C yw = transverse wind force drag coefficient
f yw {θ w } = shape function for transverse force
θ w = wind angle (degrees)

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Figure 4-1. Definition of Terms

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Figure 4-2. Local Coordinate System for a Ship

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The transverse wind force drag coefficient depends upon the hull and
superstructure of the vessel and is calculated using the following equation, adapted
from Naval Civil Engineering Laboratory (NCEL), TN-1628, Wind-Induced Steady Loads
on Ships.

[ ]
EQUATION: C yw = C * ((0.5(hS + h H )) / h R ) 2 / 7 A S + (0.5* h H / h R ) 2 / 7 A H / A Y (3)

where

C yw = transverse wind force drag coefficient

C = empirical coefficient, see Table 4-2
h R = 10 m = reference height (32.8 ft)
h H = A H / L wL = average height of the hull, defined as
the longitudinal wind hull area
divided by the ship length at the
waterline (m)
AH = longitudinal wind area of the hull
(m2)
L wL = ship length at the waterline (m)
hS = height of the superstructure above the
waterline(m)
AS = longitudinal wind area of the
superstructure (m2)

A recommended value for the empirical coefficient is C = 0.92 +/-0.1

based on scale model wind tunnel tests (NCEL, TN-1628). Table 4-2 gives typical
values of C for ships and Figure 4-3 illustrates some ship types.

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Table 4-2. Sample Wind Coefficients for Ships

SHIP C NOTES
Hull dominated 0.82 Aircraft carriers, drydocks
Typical 0.92 ships with moderate superstructure
Extensive 1.02 Destroyers, cruisers
superstructure

The shape function for the transverse wind force (NCEL, TN-1628) is
given by:

EQUATION: f yw {θ w }= + (sinθ w - 0.05* sin{5θ w }) / 0.95 (4)

where

f yw {θ w } = transverse wind coefficient shape function

θw = wind angle (degrees)

Equation 4 is positive for wind angles 0 < θw < 180 degrees and negative
for wind angles 180 < θw < 360 degrees. Figure 4-4 shows the shape and typical
values for Equation 4.

These two components were derived by integrating wind over the hull and
superstructure areas to obtain effective wind speeds (NCEL, TN-1628). The following
example illustrates calculations of the transverse wind force drag coefficient.

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Figure 4-3. Sample Ship Profiles

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Figure 4-4. Shape Function for Transverse Wind Force

θw (deg) fwy{θw} θw (deg) fwy{θw}

0 0.000 45 0.782
5 0.069 50 0.856
10 0.142 55 0.915
15 0.222 60 0.957
20 0.308 65 0.984
25 0.402 70 0.998
30 0.500 75 1.003
35 0.599 80 1.003
40 0.695 85 1.001
45 0.782 90 1.000

1.1

0.9

0.8

0.7
fyw{θw}

0.6

0.5

0.4

0.3

0.2

0.1

0
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180
W IND ANGLE (deg)

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EXAMPLE: Find the transverse wind force drag coefficient on the destroyer shown in
Figure 4-5.

SOLUTION: For this example the transverse wind force drag coefficient from Equation 3
is:

[ ]
C yw = C * ((0.5(23.9m + 6.43m))/10m)2/71203m 2 + (0.5 * 6.43m/10m)2/71036.1m 2 /2239m 2

C yw = 0.940 * C .

Destroyers have extensive superstructure, so a recommended value of C

= 1.02 is used to give a transverse wind force drag coefficient of Cyw = 0.940*1.02 =
0.958.

Note that for cases where an impermeable structure, such as a wharf, is

immediately next to the moored ship, the exposed longitudinal wind area and resulting
transverse wind force can be reduced. Figure 4-6 shows an example of a ship next to a
wharf. For Case (A), wind from the water, there is no blockage in the transverse wind
force and elevations of the hull and superstructure are measured from the water
surface. For Case (B), wind from land, the longitudinal wind area of the hull can be
reduced by the blocked amount and elevations of hull and superstructure can be
measured from the wharf elevation.

Cases of multiple ships are covered in par. 4.6.

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Figure 4-5. Example

AH hS
AS
hH=AH/LwL

T
LwL

PARAMETER VALUE VALUE

(SI UNITS) (ENGLISH)
Lw L 161.23 m 529 ft
2 2
AY 2239 m 24100 ft
2 2
AH 1036 m 11152 ft
2 2
AS 1203 m 12948 ft
hH = A H/Lw L 6.43 m 21.1 ft
hS 23.9 m 78.4 ft

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Figure 4-6. Blockage Effect for an Impermeable Structure Next to a Moored Ship

CASE (A)
CASE (B) WIND FROM WATER
WIND FROM LAND

Elevation Elevation

WHARF

END VIEW

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4-4.2 Static Longitudinal Wind Force. The static longitudinal wind force on a
vessel is defined as that component of wind force parallel to the centerline of the vessel.
This is the force in the “X” or surge direction in Figure 4-2. Figure 4-1 shows the
definition of winds areas.

The longitudinal force is determined from NCEL, TN-1628 using the

equation:

EQUATION: Fxw = 0.5 ρa Vw 2 A x C xw f xw (θw ) (5)

where

Fxw = longitudinal wind force (newtons)

ρa = mass density of air (from Table 4-1)
Vw = wind speed (m/s)
Ax = transverse wind area of the ship (m2)
C xw = longitudinal wind force drag coefficient
fxw (θw ) = shape function for longitudinal force
θw = wind angle (degrees)

The longitudinal wind force drag coefficient, Cxw , depends on specific

characteristics of the vessel. Additionally, the wind force drag coefficient varies
depending on bow ( CxwB ) or stern ( CxwS ) wind loading. Types of vessels are given in
three classes: hull dominated, normal, and excessive superstructure. Recommended
values of longitudinal wind force drag coefficients are given in Table 4-3.

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Table 4-3. Recommended Ship Longitudinal Wind Force Drag Coefficients

VESSEL TYPE CxwB CxwS

Hull Dominated (aircraft carriers,
submarines, passenger liners) 0.40 0.40
Normal* 0.70 0.60
Center-Island Tankers* 0.80 0.60
Significant Superstructure
(destroyers, cruisers) 0.70 0.80
*An adjustment of up to +0.10 to CxwB and CxwS should
be made to account for significant cargo or cluttered decks.

The longitudinal shape function also varies over bow and stern wind
loading regions. As the wind direction varies from headwind to tailwind, there is an
angle at which the force changes sign. This is defined as θx and is dependent on the
location of the superstructure relative to midships. Recommended values of this angle
are given in Table 4-4.

Table 4-4. Recommended Values of θx

LOCATION OF θx (deg)
SUPERSTRUCTURE
Just forward of midships 100
On midships 90
Aft of midships (tankers) 80
Warships 70
Hull dominated 60

Shape functions are given for general vessel categories below:

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CASE I SINGLE DISTINCT SUPERSTRUCTURE

The shape function for longitudinal wind load for ships with single, distinct
superstructures and hull-dominated ships is given below (examples include aircraft
carriers, EC-2, and cargo vessels):

EQUATION: f xw (θ w ) = cos (φ ) (6)

 90° 
where φ− =   θ for θ w < θx (6a)
 θx  w
 90° 
φ+ =   (θ − θ ) + 90° for θ w > θx (6b)
 180°−θx  w x

θ x = incident wind angle that produces no net

longitudinal force (Table 4-4)

θ w = wind angle

Values of f xw (θ w ) are symmetrical about the longitudinal axis of the vessel. So when
θ w > 1 8 0 ° , use 3 6 0 ° − θ w as θ w in determining the shape function.

CASE II DISTRIBUTED SUPERSTRUCTURE

 sin(5γ ) 
 sin ( γ ) - 
 10 
EQUATION: f xw (θ w ) = (7)
0.9

 90° 
where γ =  θ + 90° for θ w < θ x (7a)
−  θx  w
 90°    90°θx  
γ =  (θW ) +  180°−    for θ w > θ x (7b)
+  180°−θx    180°−θx  

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Values of f xw (θ w ) are symmetrical about the longitudinal axis of the vessel. So when
θ w > 1 8 0 ° , use 3 6 0 ° − θ w as θ w in determining the shape function. Note that the
maximum longitudinal wind force for these vessels occurs for wind directions slightly off
the ship’s longitudinal axis.

EXAMPLE: Find the longitudinal wind drag coefficient for a wind angle of 40 degrees
for the destroyer shown in Figure 4-5.

SOLUTION: For this destroyer, the following values are selected:

θx = 70o from Table 4-4

CxwB = 0.70 from Table 4-3

CxwS = 0.80 from Table 4-3

This ship has a distributed superstructure and the wind angle is less than the crossing
value, so Equation 7a is used to determine the shape function:

γ = ( 90o / ( 70o ) )40o + 90o = 1414

. o
−

 sin(5 * 141.4 o ) 
 sin (141.4 o ) - 
 10 
f xw (θ w ) = = 0.72
0.9

At the wind angle of 40 degrees, the wind has a longitudinal component on the stern.
Therefore, the wind longitudinal drag coefficient for this example is:

C xw f xw (θw ) = 0.8 * 0.72 = 0.57

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4-4.3 Static Wind Yaw Moment. The static wind yaw moment is defined as the
product of the associated transverse wind force and its distance from the vessel’s
center of gravity. In the local ship coordinate system, this is the moment about the “Z”
axis. Wind yaw moment is determined from the equation:

EQUATION: M xyw = 0.5 ρa Vw 2 A y LC xyw {θw } (8)

where

M xyw = wind yaw moment (newton*m)

ρa = mass density of air (from Table 4-1)
Vw = wind speed (m/s)
Ay = longitudinal projected area of the ship (m2)
L = length of ship (m)
C xyw {θw } = normalized yaw moment coefficient
= moment arm divided by ship length
θw = wind angle (degrees)

The normalized yaw moment coefficient depends upon the vessel type.
Equation 9 gives equations for computing the value of the yaw moment coefficient and
Table 4-5 gives empirical parameter values for selected vessel types. The normalized
yaw moment variables is found from:

θw * 180
EQUATION: C xyw {θw } = - a1 * sin( ) 0<θw<θz (9)
θz

C xyw {θw } = a2 * sin[(θw − θz ) * λ )] θz≤θw<180 deg (9a)

and symmetrical about the longitudinal axis of the vessel,

where

Cxyw {θw } = normalized wind yaw moment coefficient

a1 = negative peak value (from Table 4-5)
a2 = positive peak value (from Table 4-5)
θw = wind angle (degrees)
θz = zero moment angle (degrees) (from Table 4-5)

180 * deg
λ = (dimensionless) (9b)
[ (180 * deg - θz ) ]

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Table 4-5. Normalized Wind Yaw Moment Variables

SHIP TYPE Zero Negative Positive NOTES

Moment Peak (a1) Peak (a2)
Angle (θz)
Liner 80 0.075 0.14
Carrier 90 0.068 0.072
Tanker 95 0.077 0.07 Center island w/
cluttered deck
Tanker 100 0.085 0.04 Center island w/ trim
deck
Cruiser 90 0.064 0.05
Destroyer 68 0.02 0.12
Others: 130 0.13 0.025 stern superstructure
102 0.096 0.029 aft midships
superstructure
90 0.1 0.1 midships
superstructure
75 0.03 0.05 forward midships
superstructure
105 0.18 0.12 bow superstructure

A plot of the yaw normalized moment coefficient for the example shown in
Figure 4-5 is given as Figure 4-7.

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Figure 4-7. Sample Yaw Normalized Moment Coefficient

0.15

0.125
a2

0.1

0.075
Cxyw

0.05

0.025

0
θz
-0.025
-a1

-0.05
0 20 40 60 80 100 120 140 160 180
WIND ANGLE (deg)

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4-5 STATIC CURRENT FORCES/MOMENTS. Methods to determine static
current forces and moments on stationary moored vessels in the surge and sway
directions and yaw moment are presented in this section. These planar directions are
of primary importance in many mooring designs.

4-5.1 Static Transverse Current Force. The transverse current force is

defined as that component of force perpendicular to the vessel centerline. If a ship has
a large underkeel clearance, then water can freely flow under the keel, as shown in
Figure 4-8(a). If the underkeel clearance is small, as shown in Figure 4-8(b), then the
ship more effectively blocks current flow, and the transverse current force on the ship
increases. These effects are considered and the transverse current force is determined
from the equation:

EQUATION: Fyc = 0.5 ρ w Vc 2 LwL T C yc sinθ c (10)

where

Fyc = transverse current force (newtons)

ρw = mass density of water (from Table 4-1)
Vc = current velocity (m/s)
L wL = vessel waterline length (m)
T = average vessel draft (m)
C yc = transverse current force drag coefficient
θc = current angle (degrees)

The transverse current force drag coefficient as formulated in Broadside

Current Forces on Moored Ships, Seelig et al. (1992) is shown in Figure 4-9. This drag
coefficient can be determined from:

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Figure 4-8. Examples of Ratios of Ship Draft (T) to Water Depth (d)

(a) T/d = 0.25 (b) T/d = 0.8

SHIP
End View

T T
d

d
Current
Flow

seafloor

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Figure 4-9. Broadside Current Drag Coefficient

3.50

This figure is for current speeds of 1.5 m/s or

less and for ships moored in relatively large
3.00 harbors or channels w here the moored
ship does not significantly restrict the flow .
This figure uses:

C1 = 3.2 and K = 2
2.50
as discussed in the text.

2.00
Cyc

1.50
χ=
32

16
1.00
8
4

2
0.50

0.00
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
T/d

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EQUATION: Cyc = C 0 + (C1 - C 0 ) * (T / d) K (11)

where CO = deepwater current force drag coefficient for T/d ≈ 0.0;

this deepwater drag coefficient is estimated from:

EQUATION: C 0 =0.22 * χ (12)

where χ is a dimensionless ship parameter calculated as:

EQUATION: χ = LwL 2 * Am / ( B *V ) (13)

where LwL = is the vessel length at waterline (m)

Am = is the immersed cross-sectional
area of the ship at midsection (m2)
B = is the beam (maximum ship width at
the waterline) (m), and
V = is the submerged volume of the ship, (m3)
(which can be found by taking the
displacement of the vessel divided
by the unit weight of water, given
in Table 4-1).
C1 = shallow water current force drag coefficient
where T/d = 1.0; for currents of 1.5 m/s
(2.9 knots or 4.9 ft/sec) or less; C1 = 3.2 is recommended
T = average vessel draft (m)
d = water depth (m)
K = dimensionless exponent; laboratory data from
ship models shows:
K=2 Wide range of ship and barge tests; most all
of the physical model data available can be
fit with this coefficient, including submarines.
K=3 From a small number of tests on a fixed
cargo ship and for a small number of tests
on an old aircraft carrier, CVE-55
K=3 From a small number of tests on an old
submarine hull, SS-212

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The immersed cross-sectional area of the ship at midships, Am , can be
determined from:

EQUATION: Am = Cm * B * T (14)

Values of the midship coefficient, Cm , are provided in the Ships Characteristics

Database (WATERS TOOLBOX) for DOD ships.

The above methods for determining the transverse current force are
recommended for normal design conditions with moderate current speeds of 1.5 m/s
(2.9 knots or 4.9 ft/sec) or less and in relatively wide channels and harbors (see Seelig
et al., 1992).

If the vessel is moored broadside in currents greater than 1.5 m/s (2.9 knots or
4.9 ft/sec), then scale model laboratory data show that there can be significant vessel
heel/roll, which effectively increases the drag force on the vessel. In some model tests
in shallow water and at high current speeds this effect was so pronounced that the
model ship capsized. Mooring a vessel broadside in a high current should be avoided,
if possible.

Scale physical model tests show that a vessel moored broadside in a restricted
channel has increased current forces. This is because the vessel decreases the
effective flow area of a restricted channel, which causes the current speed and current
force to increase.

For specialized cases where:

(1) vessels are moored in current of 1.5 m/s

(3 knots or 5 ft/sec) or more, and/or

(2) for vessels moored in restricted channels

then the designer should contact NFESC.

Recent full-scale measurements with a floating drydock show the transverse current
force equations should also be used to compute the longitudinal drag forces for blocky
vessels.

EXAMPLE: Find the current force on an FFG-7 vessel produced by a current of θc=90
degrees to the ship centerline with a speed of 1.5 m/s (2.9 knots or 4.9 ft/sec) in salt
water for a given ship draft. At the mooring location, the harbor has a cross-sectional
area much larger than the submerged ship longitudinal area, LwL * T .

SOLUTION: Dimensions and characteristics of this vessel are summarized in the lower
right portion of Figure 4-10. Transverse current drag coefficients predicted using
Equation 11 are shown on this figure as a solid bold line. Physical scale model data

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(U.S. Naval Academy (USNA), EW-9-90, Evaluation of Viscous Damping Models for
Single Point Mooring Simulation) are shown as symbols in the drawing, showing that
Equation 11 provides a reasonable estimate of drag coefficients. Predicted current
forces for this example are given in Table 4-6.

Table 4-6. Predicted Transverse Current Forces on FFG-7 for a Current Speed of
1.5 m/s (2.9 knots)

d d Fyc Fyc
T/d (m) (ft) (MN)* (kips)**
0.096 45.7 150 0.55 123
0.288 15.2 50 0.66 148
0.576 7.62 25 1.03 231
0.72 6.096 20 1.30 293
0.96 4.572 15 1.90 427
* MN = one million newtons
**kip = one thousand pounds force

This example shows that in shallow water the transverse current force can be three
times or larger than in deep water for an FFG-7.

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Figure 4-10. Example of Transverse Current Drag Coefficients

3.20
Data taken from tests conducted at the US
3.00
Naval Academy at scales 1/24.75 and 1/80.
Some data taken at 5 and 6 knots is not
2.80
included. (Kreibel, 1992)
2.60

2.40

2.20

2.00 χ = 14.89
1.80
Cyc

1.60
FFG-7
1.40 Cm = 0.78
LwL = 124.36 m
1.20 B = 11.58 m
T = 4.389 m
1.00 D = 3590 long ton (LT)
3
0.80 V = 3590 LT * 0.9904 m /LT
3
= 3555.7 m
0.60 Am = 0.78 *B *T = 39.64 m
2

2
Model data points χ = LwL *Am/(B*V) = 14.89
0.40 C0 = 0.8489
C1 = 3.2
0.20
K =2
0.00
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
T/d

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4-5.2 Static Longitudinal Current Force for Ships. The longitudinal current
force is defined as that component of force parallel to the centerline of the vessel. This
force is determined for streamlined ship-shaped vessels from the following equation
(Naval Civil Engineering Laboratory (NCEL), TN-1634, STATMOOR – A Single-Point
Mooring Static Analysis Program):

EQUATION: Fxc = Fx FORM + Fx FRICTION + Fx PROP (15)

where

Fxc = total longitudinal current load (newtons)

FxFORM = longitudinal current load due to
form drag (newtons)
FxFRICTION = longitudinal current load due to skin friction (newtons)
FxPROP = longitudinal current load due to propeller drag (newtons)

The three elements of the general longitudinal current load equation, FxFORM , FxFRICTION ,
and FxPROP are described below:

FxFORM = longitudinal current load due to form drag

1
EQUATION: FxFORM = ρw Vc 2 B T C xcb cos(θc ) (16)
2

where

ρw = mass density of water, from Table 4-1

Vc = current speed (m/s)
B = maximum vessel width at the
waterline(m)
T = average vessel draft (m)
C xcb = longitudinal current form drag coefficient = 0.1
θc = current angle (degrees)

FxFRICTION = longitudinal current load due to skin friction

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1
EQUATION: FxFRICTION = ρw Vc 2 S C xca cos(θc ) (17)
2

where

ρw = mass density of water, from Table 4-1

Vc = current speed (m/s)
S = wetted surface area (m2); estimated using

 
 
EQUATION: S = 1.7 T L  D 
(18)
Tγ
wL +  
 
 w 

T = average vessel draft (m)

L wL = waterline length of vessel (m)
D = ship displacement (newtons)
γ = weight density of water, from Table 4-1
w
C xca = longitudinal skin friction
coefficient, estimated using:
2
 
 
EQUATION: C xca = 0.075 /  log R 
N− 2 (19)
 10  

R N = Reynolds Number

Vc LwL cos(θc )
EQUATION: RN = (20)
ν

ν = kinematic viscosity of water, from Table 4-1

θ c = current angle (degrees)

FxPROP = longitudinal current load due to fixed propeller drag

1
EQUATION: FxPROP = ρ V 2 A C cos(θC ) (21)
2 w c p PROP

where

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ρw = mass density of water, from Table 4-1
Vc = current speed (m/s)
A p = propeller expanded blade area (m2)
C PROP = propeller drag coefficient = 1.0
θC = current angle (degrees)

A Tpp A Tpp
EQUATION: Ap = = (22)
1.067 - 0.229 (p / d) 0.838

A Tpp = total projected propeller area (m2)

for an assumed propeller pitch ratio of p / d = 1.0

L wL B
EQUATION: A Tpp = (23)
AR

A R is a dimensionless area ratio for propellers. Typical values of this parameter for
major vessel groups are given in Table 4-7.

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Table 4-7. AR for Major Vessel Groups

SHIP AREA RATIO, A R

Destroyer 100
Cruiser 160
Carrier 125
Cargo 240
Tanker 270
Submarine 125

Note that in these and all other engineering calculations discussed in this
UFC, the user must be careful to keep units consistent.

EXAMPLE: Find the longitudinal current force with a bow-on current of θc=180 degrees
with a current speed of 1.544 m/sec (3 knots) on a destroyer in salt water with the
characteristics shown in Table 4-8.

SOLUTION: Table 4-9 shows the predicted current forces. Note that these forces are
negative, since the bow-on current is in a negative “X” direction. For this destroyer, the
force on the propeller is approximately two-thirds of the total longitudinal current force.
For commercial ships, with relatively smaller propellers, form and friction drag produce a
larger percentage of the current force.

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Table 4-8. Example Destroyer

PARAMETER SI SYSTEM ENGLISH OR

INCH-POUND
SYSTEM
LwL 161.2 m 529 ft
T 6.4 m 21 ft
B 16.76 m 55 ft
D, ship displacement 7.93E6 kg 7810 long tons
Cm; estimated 0.83 0.83
S; est. from Eq 18 2963 m2 31 897 ft2
AR; from Table 4-7 100 100
RN; from Eq 20 2.09E8 2.09E8
Cxca; est. from Eq 19 0.00188 0.00188
Ap; est. from Eq 22 32.256 m2 347.2 ft2

Table 4-9. Example Longitudinal Current Forces on a Destroyer

FORCE SI SYSTEM ENGLISH OR PERCENT OF

INCH-POUND TOTAL FORCE
SYSTEM
FxFORM; Eq 15 -13.1 kN* -2.95 kip** 22%
FxFRICTION; Eq 16 -6.8 kN -1.53 kip 12%
FxPROP; Eq 17 -39.4 kN -8.87 kip 66%
Total Fxc = -59.4 kN -13.4 kip 100%

* kN = one thousand newtons

**kip = one thousand pounds force

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4-5.3 Static Longitudinal Current Force for Blunt Vessels. The methods in
Chapter 4-5.2 are inappropriate for very blunt-bow vessels, such as floating drydocks.
For blunt-bow vessels use the methods and equations in Chapter 4-5.1 Static
Transverse Current Force for the longitudinal current force on the hull. In this case use
the appropriate parameters as input.

For example, take the case of a floating drydock 180 feet wide with a draft
of 67 feet moored in a water depth of 70 feet. A current of 1.2 knots is predicted (using
methods in Chapter 4-5.1) to produce a longitudinal current force of 144.9 kips on this
floating drydock. Full-scale measurements were made on the actual drydock for this
case and the measured longitudinal force was 143 kips. In this example the predicted
force is approximately 1% higher than measured.

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4-5.4 Static Current Yaw Moment. The current yaw moment is defined as that
component of moment acting about the vessel’s vertical “Z”-axis. This moment is
determined from the equation:

ec
EQUATION: M xyc = Fyc ( )L (24)
LwL wL

where

M xyc = current yaw moment (newton*m)

Fyc = transverse current force (newton)
ec
= ratio of eccentricity to vessel waterline length
LwL
ec = eccentricity of Fyc (m)
LwL = vessel waterline length (m)

ec
The dimensionless moment arm is calculated by choosing the slope and y-intercept
LwL
variables from Table 4-10 which are a function of the vessel hull. The dimensionless
moment arm is dependent upon the current angle to the vessel, as shown in Equation
25:

e
EQUATION: = a + b * θc θc=0° to 180° (25)
LwL

e
= a + (b * (360 deg − θ c )) θc=180° to 360° (25a)
LwL

where

ec
= ratio of eccentricity to vessel waterline length
LwL
a = y-intercept (refer to Table 4-10) (dimensionless)
b = slope per degree (refer to Table 4-10)
θc = current angle (degrees)

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The above methods for determining the eccentricity ratio are recommended for
normal design conditions with moderate current speeds of less than 1.5 m/s (3 knots or
5 ft/sec). Values provided in Table 4-10 are based upon least squares fit of scale model
data taken for the case of ships with level keels. Data are not adequately available for
evaluating the effect of trim on the current moment.

Table 4-10. Current Moment Eccentricity Ratio Variables

a b NOTES
SHIP Y-INTERCEPT SLOPE PER
DEGREE
SERIES 60 -0.291 0.00353 Full hull form typical of
cargo ships
FFG -0.201 0.00221 “Rounded” hull typical of
surface warships
CVE-55 -0.168 0.00189 Old attack aircraft
carrier
SS-212 -0.244 0.00255 Old submarine

4-6 WIND AND CURRENT FORCES AND MOMENTS ON MULTIPLE

SHIPS. If ships are moored in close proximity to one another then the nearby ship(s)
can influence the forces/moments on a given ship. The best information available on the
effects of nearby ships is results from physical model tests, because the physical
processes involved are highly complex. NFESC Report TR-6003-OCN provides scale
model test results of wind and current forces and moments for multiple identical ships.
From two to six identical ships were tested and the test results were compared with test
results from a single ship. Data are provided for aircraft carriers, destroyers, cargo
ships, and submarines.

Cases included in NFESC Report TR-6003-OCN include: individual ships,

ships in nests and ships moored on either sides of piers. Results are provided for the
effects of winds and currents in both tabular and graphical form.

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CHAPTER 5

ANCHOR SYSTEM DESIGN PROCEDURES

5-1 GENERAL ANCHOR DESIGN PROCEDURE. Anchor systems ultimately

hold the mooring loads in fleet mooring systems. Anchors are used on both ships and
in mooring facilities, so selection and design of anchors are included in this section.

The type and size of anchor specified depends upon certain parameters,
such as those shown in Table 5-1.

The most commonly used anchors in DOD moorings are drag-embedment

anchors and driven-plate anchors, so they will be discussed here. Other types of
specialized anchors (shallow foundations, pile anchors, propellant-embedment anchors,
rock bolts, etc.) are discussed in the NCEL Handbook for Marine Geotechnical
Engineering.

Figures 5-1 and 5-2 illustrate typical drag-embedment anchors. Figure 5-3 illustrates a
driven-plate anchor. Some characteristics of these two categories of anchors are given
in Table 5-2.

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Table 5-1. Anchor Specification Considerations

PARAMETER DESCRIPTION
Holding capacity The size/type of anchor will depend on the amount
of anchor holding required.
Soils Engineering properties and sediment layer
thickness influence anchor design.
Use If anchors will be relocated, then drag anchors are
most commonly used.
Weight The amount of weight that can be handled or
carried may control anchor specification.
Equipment The size and characteristics of installation
equipment are important in anchor specification.
Directionality Drag anchors may provide little uplift capacity and
primarily hold in one direction; driven plate anchors
provide high omni directional capacity.
Performance Whether anchor will be allowed to drag or not, as
well as the amount of room available for anchors
systems, will influence anchor specification.

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Figure 5-1. Example of a Drag-Embedment Anchor (Stabilized Stockless Anchor)

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Figure 5-2. Example of a Drag-Embedment Anchor (NAVMOOR Anchor)

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Figure 5-3. Driven-Plate Anchor

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Table 5-2. Anchor Characteristics

(a) Drag-Embedment Anchors

CHARACTERISTICS NOTES
Many basic designs and sizes are NAVMOOR-10 & -15 and stockless of 20 to
available from manufacturers. 30 kips are stocked by NFESC.
Works primarily in one horizontal Enough scope of chain and/or wire rope
direction. needs to be provided to minimize uplift
forces, which can pull the anchor out. If a
load is applied to a drag anchor at a
horizontal axis off the centerline of the
anchor, then the anchor may drag and reset.
Flukes should be set for the soil type. Anchor performance depends strongly on
the soil type. Fixing the maximum angle of
the fluke will help ensure optimum
performance. For mooring installations the
flukes should be fixed open and stabilizers
added for stockless anchors to help prevent
overturning.
Adequate sediment required. Sand layer thickness needs to be
approximately one fluke length and mud
needs to be 3 to 5 fluke lengths thick.
May not work in all seafloor types. May be unreliable in very hard clay, gravel,
coral, or rock seafloors; and in highly layered
seafloors.
May not work well for sloping If the seafloor has a slope of more than
seafloors. several degrees, then the anchor may not
hold reliably in the down-slope direction.

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Table 5-2. (continued) Anchor Characteristics

(a) Drag-Embedment Anchors (Continued)

Anchor can drag. If the anchor is overloaded at a slow enough
rate, then the anchor can drag, which
reduces the peak load. Anchor dragging can
be a problem if the room for mooring is
restricted. If adequate room is available then
anchor drag can help prevent failure of other
mooring components.
Anchors can be reused. Drag-embedment anchors can be recovered
and reused.
Proof loading recommended. Pulling the anchor at the design load in the
design direction will help set the anchor and
assure that the soil/anchor interaction
provides adequate holding.

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Table 5-2. (continued) Anchor Characteristics

(b) Driven-Plate Anchors

CHARACTERISTICS NOTES
Size and design of anchor are These anchors have been used in a variety of
selected to provide adequate holding, soils from soft mud to hard coral. A driving
to allow driving, and to provide analysis is recommended for hard soil, because
adequate structural capacity. the anchor must be able to be driven in order to
work.
Multi-directional. Can be used on short scope, since the anchor
resists uplift forces. One plate anchor may be
used to replace several drag anchor legs, since
the anchors are multi-directional.
Anchors designed for the soil type. Anchors designed for the soil engineering
characteristics at the site.
Adequate sediment required. A minimum of several fluke lengths of sediment is
required to provide for keying and allow the
anchor to hold (NFESC TR-2039-OCN, Design
Guide for Pile-Driven Plate Anchors).
Anchor is fixed. The anchor will not drag, so this type of anchor is
well suited to locations with limited mooring area
available. The anchors cannot be recovered or
inspected.
Proof loading recommended. Pulling the anchor at the design load in the design
direction will help key the anchor and assure that
the soil/anchor interaction provides adequate
holding.
Installation equipment. Mobilization can be expensive, so installing a
number of anchors at a time reduces the unit
installation cost.

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5-2 DRAG-EMBEDMENT ANCHOR SPECIFICATION. Drag-embedment
anchors are carried on ships and used in many fleet-mooring facilities. Key
considerations in selecting an anchor are: soil type, anchoring holding capacity, anchor
weight, anchor stowage, cost, availability, and installation assets. Note that in SI units
the anchor mass is used to characterize anchor size, while in U.S. customary units the
anchor weight as a force is used.

Drag-embedment anchor holding capacities have been measured in full-

scale tests, modeled in the laboratory, and derived from soil analyses. Empirical anchor
holding curves were developed from this information (Naval Civil Engineering
Laboratory (NCEL), TDS 83-08R, Drag Embedment Anchors for Navy Moorings).
Predicted static ultimate anchor holding is given by:

EQUATION: HM = HR (WA / WR ) b (26)

where

HM = ultimate anchor system static holding

capacity (kips or kN)
HR = reference static holding capacity
WA = weight of the anchor in air
(for SI units use anchor weight in kilograms;
for U.S. units use anchor weight in pounds
force)
WR = reference anchor weight in air
(for SI units use 4536 kg;
for U.S. units use 10000 lbf)
b = exponent

Values of HR and b depend on the anchor and soil types. Values of these
parameters are given in U.S. customary units in Table 5-3 and for SI units in Table 5-4.

Figures 5-4 and 5-5 give holding capacities of selected anchors for mud
and sand seafloors.

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Table 5-3. Drag Anchor Holding Parameters U.S. Customary

SOFT SOILS HARD SOILS

Anchor Type (a ) (Soft clays and silts) (Sands and stiff clays)
HR b HR b
(kips) (kips)
Boss 210 0.94 270 0.94
BRUCE Cast 32 0.92 250 0.8
BRUCE Flat Fluke Tw in Shank 250 0.92 (c) (c)
BRUCE Tw in Shank 189 0.92 210 0.94
Danforth 87 0.92 126 0.8
Flipper Delta 139 0.92 (c) (c)
G.S. AC-14 87 0.92 126 0.8
Hook 189 0.92 100 0.8
LWT (Lightw eight) 87 0.92 126 0.8
Moorfast 117 0.92 (i) 60 0.8
100 (d) 0.8
NAVMOOR 210 0.94 270 0.94
Offdrill II 117 0.92 (i) 60 0.8
100 (d) 0.8
STATO 210 0.94 250 (e) 0.94
190 (f) 0.94
STEVDIG 139 0.92 290 0.8
STEVFIX 189 0.92 290 0.8
STEVIN 139 0.92 165 0.8
STEVMUD 250 0.92 (g) (g)
STEVPRIS (straight shank) 189 0.92 210 0.94
Stockless (fixed fluke) 46 0.92 70 0.8
44 (h) 0.8
Stockless (movable fluke) 24 0.92 70 0.8
44 (h) 0.8
(a) Fluke angles set for 50 deg in soft soils and according to manufacturer' s specifications
in hard soils, except w hen otherw ise noted.
(b) " b" is an exponent constant.
(c) No data available.
(d) For 28-deg fluke angle.
(e) For 30-deg fluke angle.
(f) For dense sand conditions (near shore).
(g) Anchor not used in this seafloor condition.
(h) For 48-deg fluke angle.
(i) For 20-deg fluke angle (from API 2SK effective March 1, 1997).

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Table 5-4. Drag Anchor Holding Parameters SI Units

SOFT SOILS HARD SOILS

Anchor Type (a ) (Soft clays and silts) (Sands and stiff clays)
HR b HR b
(kN) (kN)
Boss 934 0.94 1201 0.94
BRUCE Cast 142 0.92 1112 0.8
BRUCE Flat Fluke Tw in Shank 1112 0.92 (c) (c)
BRUCE Tw in Shank 841 0.92 934 0.94
Danforth 387 0.92 560 0.8
Flipper Delta 618 0.92 (c) (c)
G.S. AC-14 387 0.92 560 0.8
Hook 841 0.92 445 0.8
LWT (Lightw eight) 387 0.92 560 0.8
Moorfast 520 0.92 (i) 267 0.8
445 (d) 0.8
NAVMOOR 934 0.94 1201 0.94
Offdrill II 520 0.92 (i) 267 0.8
445 (d) 0.8
STATO 934 0.94 1112 (e) 0.94
845 (f) 0.94
STEVDIG 618 0.92 1290 0.8
STEVFIX 841 0.92 1290 0.8
STEVIN 618 0.92 734 0.8
STEVMUD 1112 0.92 (g) (g)
STEVPRIS (straight shank) 841 0.92 934 0.94
Stockless (fixed fluke) 205 0.92 311 0.8
196 (h) 0.8
Stockless (movable fluke) 107 0.92 311 0.8
196 (h) 0.8
(a) Fluke angles set for 50 deg in soft soils and according to manufacturer' s specifications
in hard soils, except w hen otherw ise noted.
(b) " b" is an exponent constant.
(c) No data available.
(d) For 28-deg fluke angle.
(e) For 30-deg fluke angle.
(f) For dense sand conditions (near shore).
(g) Anchor not used in this seafloor condition.
(h) For 48-deg fluke angle.
(i) For 20-deg fluke angle (from API 2SK effective March 1, 1997).

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Figure 5-4.Anchor System Holding Capacity in Cohesive Soil (Mud)

1000

800

600
500

400

300
Anchor System Holding Capacity (kips)

R
200 OO
VM
NA

T*
100 LW
GS,
,
rt h
80 nfo
Da
)
ke
60 Flu
d
ixe
(F
ss
ckle
40 S to

)
ke
Flu
le
ab
20 ov
M
s(
kles
oc
St

10
1
01 2 3 4 5 6 8 10 20 30 40

Anchor Air Weight (kips)

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Figure 5-5 Anchor System Holding Capacity in Cohesionless Soil (Sand)

1000

800

600 Fluke angles set for sand

500 as per manuf acturers
specif icat ions.
400

300
Anchor System Holding Capacity (kips)

R
OO T
200 M
,L
W
AV
N
, GS
th
or
nf
Da
le
A ng
100 ke
o Flu
5
80 -3
ss
kle
oc
St
60 le
A ng
ke
o Flu
4 8
s-
40 kles
oc
St

*Anchors require special

20 handling to ensure fluke
tripping (possibly fixed
open flukes).

101
01 2 3 4 5 6 8 10 20 30 40

Anchor Air Weight (kips)

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5-3 DRIVEN-PLATE ANCHOR DESIGN. NFESC has found that various
types of plate anchors are an efficient and cost effective method of providing permanent
moorings. Detailed design procedures for these anchors are given in NFESC TR-2039-
OCN, Design Guide for Pile-Driven Plate Anchors. Additional information is given in
NCEL Handbook for Marine Geotechnical Engineering. An overview of plate anchor
design is given here.

A driven-plate anchor consists of the components shown in Figure 5-3 and discussed in
Table 5-5.

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Table 5-5. Driven-Plate Anchor Components

COMPONENT NOTES
Plate Size the area and thickness of the plate to hold
the required working load in the given soils. A
plate length-to-width ratio of L/B = 1.5 to 2 is
shown by practical experience to give optimum
performance.
I-Beam Size the beam to provide: a driving member;
stiffness and strength to the anchor; and to
separate the padeye from the plate to provide a
moment that helps the anchor key during proof
testing.
Padeye Size this structure as the point where the chain
or wire rope is shackled onto the anchor prior
to driving.
Follower Length and size specified so assembly can
safely be picked up, driven, and removed.
Hammer Sized to drive the anchor safely. In most cases
it is preferable to use an impact hammer. A
vibratory hammer may be used in cohesionless
soils or very soft mud. A vibratory hammer
may also be useful during follower extraction.
Template A structure is added to the side of the driving
platform to keep the follower in position during
setup and driving.

Installation of a plate anchor is illustrated in Figure 5-6. Installation consists of three key
steps, as outlined in Table 5-6.

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Table 5-6. Major Steps in Driven-Plate Anchor Installation

STEP DESCRIPTION
1 Moor installation platform, place anchor in follower,
shackle anchor to chain, place the follower/anchor
assembly at the specified anchor location and drive the
anchor to the required depth in the sediment (record
driving blow count).
2 Remove follower with a crane and/or extractor.
3 Proof load the anchor. This keys the anchor, proves that
the anchor holds the design load, and removes slack from
the chain.

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Figure 5-6. Major Steps of Driven-Plate Anchor Installation

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Examples of plate anchors that have been used at various sites are
summarized in Table 5-7.

Table 5-7. Typical Driven-Plate Anchors

SIZE/LOCATION SEAFLOOR DRIVING PROOF LOAD

TYPE DISTANCE
INTO
COMPOTENT
SEDIMENT
0.91 m x 1.22 m Hard Clay 9 m (30 ft) 670 kN
(3 ft x 4 ft) (150 kips)
Philadelphia, PA Vertical
0.61 m x 1.22 m Sand 8 m (27 ft) 890 kN
(2 ft x 4 ft) (Medium) (200 kips)
San Diego, CA Vertical
m x 1.83 m Coral 12 m (40 ft) 1000 kN
(5 ft x 6 ft) Limestone (225 kips)
Guam Vertical
m x 3.35 m Mud 21 m (70 ft) 890 kN
(6 ft x 11 ft) (200 kips)
Pearl Harbor, HI Horizontal

The recommended minimum plate anchor spacing is five times the anchor
width for mud or clay and 10 times the anchor width for sand.

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CHAPTER 6

FACILITY MOORING EQUIPMENT GUIDELINES

6-1 INTRODUCTION. Equipment most often used in mooring facilities is

discussed in this section.

6-2 KEY MOORING COMPONENTS. A mooring is a structure that holds a

ship in a position using tension and compression members. The resulting mooring loads
are transferred to the earth via anchors or some other members, such as pier piles or a
wharf structure.

6-2.1 Tension Members. The most commonly used tension members in

moorings are:

Chain
Synthetic line
Wire rope
Tension bar buoys

6-2.2 Compression Members. The most commonly used compression

members in moorings are:

Marine fenders
Fender Piles
Camels
Mooring dolphins
Piers
Wharves

6-3 ANCHORS. Anchors are structures used to transmit mooring loads to the
earth. Anchors operate on the basis of soil structure interaction, so their behavior can
be complex. Fortunately, the U.S. Navy has extensive experience with full-scale testing
of a number of different anchor types in a wide variety of soils and conditions (NCEL
Handbook for Marine Geotechnical Engineering). This experience provides a strong
basis for design. However, due to the complex nature of structure/soil interaction, it is
strongly recommended that anchors always be pull tested to their design load during
installation. Design and illustration of some of the common anchor types routinely used
are discussed in Chapter 5 of this UFC, and in NCEL Handbook for Marine
Geotechnical Engineering.

A brief summary of some anchor experience is given in Table 6-1.

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Table 6-1. Practical Experience With Anchors

ANCHOR TYPE DESCRIPTION

Low Efficiency Drag Reliable if stabilizers are added (see Figure 5-1).
Embedment Anchors Not very efficient, but reliable through ‘brute force’.
(i.e., Stockless) Extensive experience. A large number available in
the U.S. Navy Fleet Mooring inventory. Efficiency
increased by fixing the flukes for the type of soil at
the site. Should be set and proof tested during
installation. Can be used in tandem in various
configurations (NCEL TDS 83-05, Multiple
STOCKLESS Anchors for Navy Fleet Moorings).
Vertical angle of tension member should be
approximately zero at the seafloor.
High Efficiency Drag Very efficient, highly reliable and especially
Embedment Anchors designed so it can easily be used in tandem (NCEL
(i.e., NAVMOOR) TN-1774, Single and Tandem Anchor Performance
of the New Navy Mooring Anchor). Excellent in a
wide variety of soil conditions. These are available
in the U.S. Navy Fleet Mooring inventory. Should
be set and proof tested during installation. Vertical
angle of tension member should be approximately
zero at the seafloor in most cases.

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Table 6-1. (Continued) Practical Experience With Anchors

ANCHOR TYPE DESCRIPTION

Driven-Plate Anchors Extremely efficient, can be designed to hold
extremely high loads and will work in a wide variety
of soils from mud to limestone (NFESC TR-2039,
Design Guide for Pile-Driven Plate Anchors). Can
take loads at any angle, so short scope moorings
can be used. Extensive experience. Requires a
follower and driving equipment. Most cost effective
if a number are to be installed at one site at one
time. Should be keyed and proof tested during
installation.
Deadweight Anchors Very low efficiency. Full-scale tests (NCEL, Fleet
Mooring Test Program – Pearl Harbor) show
anchor-holding capacity dramatically decreases
after anchor starts dragging, just when the anchor
capacity required may be most needed. As a
result, use of this type of anchor can be dangerous.
Deadweight anchors should be used with caution.
NFESC TR-6037-OCN provides an Improved Pearl
Harbor Anchor design.
Other anchor types NCEL Handbook for Marine Geotechnical
Engineering gives extensive technical and practical
information on a wide variety of anchors and
soil/structure interaction.

A summary sheet describing the stockless anchors in the U.S. Navy Fleet
Mooring inventory is given in Table 6-2. NAVMOOR anchors in inventory are described
in Table 6-3.

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Table 6-2. Stockless Anchors in the U.S. Navy Fleet Mooring Inventory

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Table 6-3. NAVMOOR Anchors in the U.S. Navy Fleet Mooring Inventory

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6-4 CHAIN AND FITTINGS. Chain is often used in fleet moorings because
chain:

Is easy to terminate
Can easily be lengthened or shortened
Is durable
Is easy to inspect
Is easy to provide cathodic protection
Has extensive experience
Is available
Is cost effective
Provides catenary effects

DOD commonly uses stud link chain, with each chain link formed by
bending and butt-welding a single bar of steel. Chain used in fleet moorings is Grade 3
stud link chain specifically designed for long-term in-water use (Naval Facilities
Engineering Service Center (NFESC), FPO-1-89(PD1), Purchase Description for Fleet
Mooring Chain and Accessories). This chain is designated as FM3. Properties of FM3
carried in stock are shown in Table 6-4. Anodes for use on each link of FM3 chain,
designed for diver replacement, are described in Table 6-5. Note that oversized anodes
may be used to extend the anode life and increase the time interval required for anode
replacement.

Older ships may use Die-Lock chain (not shown), which was made by
pressing together male and female parts to form each link. Die-Lock is not
recommended for long-term in-water use, because water may seep in between the male
and female parts. The resulting corrosion is difficult to inspect.

Chain routinely comes in 90-foot (27.4-meter) lengths called ‘shots’. A

number of other accessories are used with chain, as shown in Figure 6-1. For example,
shots of chain are connected together with chain joining links. Anchor joining links are
used to connect chain to anchors. Ground rings provide an attachment point for
multiple chains. Buoy swivels are used to connect chain to buoys. Refer to NFESC
TR-6014-OCN, Mooring Design Physical and Empirical Data and NFESC FPO-1-
89(PD1) for additional information on chain and fittings.

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Table 6-4. FM3 Mooring Chain Characteristics
NOMINAL
SIZE (inches) 1.75 2 2.25 2.5 2.75 3.5 4
NUMBER
OF LINKS 153 133 119 107 97 77 67
PER SHOT
LINK
LENGTH(inches) 10.6 12.2 13.7 15.2 16.7 21.3 24.3
WEIGHT
PER SHOT 2525 3276 4143 5138 6250 10258 13358
IN AIR (lbf)
WEIGHT
PER LINK 16.5 24.6 34.8 48 64.4 133.2 199.4
IN AIR (lbf)
WEIGHT
PER FOOT 26.2 33.9 42.6 52.7 63.8 104.1 135.2
SUB. (lbs/ft)
BREAKING
STRENGTH 352 454 570 692 826 1285 1632
(thousands lbf)
WORKING
STRENGTH (FS=3) 117.2 151.2 189.8 230.4 275.1 427.9 543.5
(thousands lbf)

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Table 6-5. Properties of FM3 Chain Anodes

NOMINAL
SIZE (inches) 1.75 2 2.25 2.5 2.75 3.5 4
ANODE
WEIGHT (lbs) 0.80 1.10 1.38 1.70 2.04 3.58 4.41
SCREW
LENGTH 1.25 1.50 1.75 1.75 2.00 2.25 2.25
(inches)
ANODE
WIDTH (inches) 1.50 1.62 1.75 1.94 2.06 2.38 2.69
LINK
GAP (lbf) 3.74 4.24 4.74 5.24 5.74 7.48 8.48
ANODES PER
FULL DRUM 1106 822 615 550 400 158 122
WEIGHT PER
FULL DRUM 976 979 917 993 869 602 550
(approx. lbf)
NOTE: 1. ALL SCREWS ARE .375-16UNC-2A, GRADE 5, HEX CAP
2. 4.00 INCH ANODES FIT ALL CHAIN SIZES
3. ALL SCREW HEADS ARE 9/16 INCH

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Figure 6-1. Chain Fittings

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6-5 BUOYS. There are two buoys commonly used on U.S. Navy Fleet
moorings: an 8-foot diameter buoy and a 12-foot diameter buoy. These buoys have a
polyurethane shell, are filled with foam, and have a tension bar to transmit mooring
loads to the chain. Properties of these buoys are given in Table 6-6. Some of the key
features of these buoys are that they require little maintenance and they are self-
fendering. A variety of older steel buoys in use are being phased out, due to their
relatively high maintenance cost. Some of the factors to consider in selecting the type
of mooring buoy to use are: availability, size, cost, durability, maintenance, inspection,
disposal and similar factors.

6-6 SINKERS. Sinkers are placed on fleet moorings to tune the static and
dynamic behavior of a mooring. Sinkers are usually made of concrete or low cost
metal. Key sinker parameters that can be specified in design include:

Mass
Weight
Location
Number
Size
Design

Special care needs to be taken in the design and inspection of lifting eyes and
attachment points on sinkers to ensure that they are safe.

6-7 MOORING LINES. The most common tension member lines used are
synthetic fiber ropes and wire rope. Synthetic lines have the advantage of easy
handling and some types have stretch, which can be used to fine tune static and
dynamic mooring behavior and aid in load sharing between tension members. Wire rope
has the advantage of durability.

6-7.1 Synthetic Fiber Ropes. Mooring lines are formed by weaving a number
of strands together to form a composite tension member. Lines are made of different
types of fiber and various constructions. Stretch/strain properties of selected lines are
shown in Table 6-7 and Figure 6-2. Engineering characteristics of some double braided
nylon and polyester lines are given in Tables 6-8 and 6-9. Additional information is
provided in NFESC TR-6014-OCN, Mooring Design Physical and Empirical Data. The
size and type of synthetic line specified in a given design will depend upon parameters
such as those shown in Table 6-10. A discussion of the use of various mooring line
types is given in Appendix A.

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Table 6-6. Foam-Filled Polyurethane Coated Buoys
PARAMETERS 8-FOOT BUOY 12-FOOT BUOY
Weight in Air 4,500 lbs 10,400 lbs
Net Buoyancy 15,000 lbs 39,000 lbs
Working Buoyancy (24” FB) 6,150 lbs 20,320 lbs
Proof Load on Bar (0.6 Fy) 300 kips 600 kips
Working Load of Bar (0.3Fy) 150 kips 300 kips
Diameter Overall (w/fenders) 8 ft 6 in 12 ft
Diameter of Hull 8 ft 11 ft 6 in
Length of Hull Overall 7 ft 9 in 8 ft 9 in
Length of Tension Bar 11 ft 4 In 13 ft I in
Height of Cylindrical Portion 4 ft 4 in 5 ft 7 in
Height of Conical Portion 3 ft 5 in 3 ft 2 in
Bar Thickness (top/bottom) 4.5/3 in 5/3.5 in
Top Padeye ID (top/bottom) 3.5/3.5 in 4.5/5 in
Shackle on Top 3 inch 4 inch
Maximum Chain Size 2.75 inch 4 inch
Min. Recommended Riser Wt 1,068 lbs 7,500 lbs
Riser Wt for 24” freeboard 8,850 lbs 18,680 lbs
Max. Recommended Riser Wt 7,500 lbs 21,264 lbs
Moment to Heel 1 deg:
Min Riser Wt 108 ft-lbs 1,183 ft-lbs
Max Riser Wt 648 ft-lbs 2,910 ft-lbs

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Table 6-7. Stretch of Synthetic Lines

WIRE DOUBLE DOUBLE

ROPE & 12-STRAND KEVLAR BRAIDED NYLON BRAIDED
STEEL HMWPE 4- POLY- 8- NYLON
% Break CORE STRAND ESTER STRAND
Strength % Stretch % Stretch % Stretch % Stretch % Stretch % Stretch
(T/Tb) (1) (2) (3) (4) (5) (6)
0 0.000 0.000 0.000 0.000 0.000 0.000
5 0.076 0.697 1.922
10 0.151 0.250 1.084 1.275 3.335 4.250
20 0.302 0.434 1.656 2.863 5.798 7.353
30 0.453 0.691 2.025 5.776 7.886 9.821
40 0.605 0.915 7.890 10.210 11.950
50 0.756 1.126 2.495 9.528 11.987 13.610
60 0.907 1.395 11.012 13.745 14.999
70 1.058 1.593 12.338 15.472
80 1.850 13.793
90 2.126 15.054
100 2.356 16.197

(1) From Tension Technology, Inc.

(2) High Molecular Weight Polyethylene; Sampson Ropes
(3) VETS 198 Rope; Whitehill Mfg.
(4) Double Braided; Sampson Ropes; Mean of 10 & 11 in. cir. Data “2-in-1
Stable Braid”
(5) Broken in line; from Tension Technology, Inc.
(6) Double Braided; Sampson Ropes; Mean of 7, 10 & 12 in cir. Data; “2-in-2
Super Strong”

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Figure 6-2. Synthetic Line Stretch

100

90
(2)

80

(1)
70
% OF LINE BREAK STRENGTH

(4)
60
(5)
(3)
50
(6)
40

30

20

10

0
0 2 4 6 8 10 12 14 16 18
% LINE STRETCH

(1) From Tension Technology, Inc.

(2) High Molecular Weight Polyethylene; Sampson Ropes
(3) VETS 198 Rope; Whitehill Mfg.
(4) Double Braided; Sampson Ropes; Mean of 10 & 11 in. cir. Data “2-in-1
Stable Braid”
(5) Broken in line; from Tension Technology, Inc.
(6) Double Braided; Sampson Ropes; Mean of 7, 10 & 12 in cir. Data; “2-in-2
Super Strong”

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Table 6-8. Double Braided Nylon Line*

SINGLE LINE THREE PARTS LINE

DIA. CIR. Av Fb Av Fb AE AE Av Fb Av Fb AE AE
(in) (in) (kips) (E5 N) (kips) (E5 N) (kips) (E5 N) (kips) (E5 N)
1.0 3 33.6 1.495 118.9 5.29 100.8 4.48 356.8 15.87
1.1 3.5 45 2.002 159.3 7.09 135 6.01 477.9 21.26
1.2 3.75 52 2.313 184.1 8.19 156 6.94 552.2 24.56
1.3 4 59 2.624 208.8 9.29 177 7.87 626.5 27.87
1.4 4.5 74 3.292 261.9 11.65 222 9.88 785.8 34.96
1.6 5 91 4.048 322.1 14.33 273 12.14 966.4 42.99
1.8 5.5 110 4.893 417.0 17.32 330 14.68 1168.1 51.96
1.9 6 131 5.827 463.7 20.63 393 17.48 1391.2 61.88
2.1 6.5 153 6.806 541.6 24.09 459 20.42 1624.8 72.27
2.2 7 177 7.873 626.5 27.87 531 23.62 1879.6 83.61
2.4 7.5 202 8.985 715.0 31.81 606 26.96 2145.1 95.42
2.5 8 230 10.231 814.2 36.22 690 30.69 2442.5 108.65
2.7 8.5 257 11.432 909.7 40.47 771 34.30 2729.2 121.40
2.9 9 285 12.677 1008.8 44.88 855 38.03 3026.5 134.63
3.2 10 322 14.323 1139.8 50.70 966 42.97 3419.5 152.11
3.5 11 384 17.081 1359.3 60.46 1152 51.24 4077.9 181.39
3.8 12 451 20.061 1596.5 71.01 1353 60.18 4789.4 213.04
4.1 13 523 23.264 1851.3 82.35 1569 69.79 5554.0 247.05
4.5 14 599 26.645 2120.4 94.32 1797 79.93 6361.1 282.95
4.8 15 680 30.248 2407.1 107.07 2040 90.74 7221.2 321.22

*After Sampson, dry, cyclic loading; reduce nylon lines by 15% for wet conditions

Note: Dia. = diameter, Cir. = circumference, Av Fb = average break strength,

AE = cross-sectional area times modulus of elasticity (this does not include the highly
nonlinear properties of nylon, shown in Figure 6-2)

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Table 6-9. Double Braided Polyester Lines*

SINGLE LINE THREE PARTS LINE

DIA. CIR. Av Fb Av Fb AE AE Av Fb Av Fb AE AE
(in) (in) (kips) (E5 N) (kips) (E5 N) (kips) (E5 N) (kips) (E5 N)
1.0 3 37.2 1.655 316.6 14.08 111.6 4.96 949.8 42.25
1.1 3.5 45.8 2.037 389.8 17.34 137.4 6.11 1169.4 52.02
1.2 3.75 54.4 2.420 463.0 20.59 163.2 7.26 1388.9 61.78
1.3 4 61.5 2.736 523.4 23.28 184.5 8.21 1570.2 69.85
1.4 4.5 71.3 3.172 606.8 26.99 213.9 9.51 1820.4 80.98
1.6 5 87.2 3.879 742.1 33.01 261.6 11.64 2226.4 99.03
1.8 5.5 104 4.626 885.1 39.37 312 13.88 2655.3 118.11
1.9 6 124 5.516 1055.3 46.94 372 16.55 3166.0 140.83
2.1 6.5 145 6.450 1234.0 54.89 435 19.35 3702.1 164.68
2.2 7 166 7.384 1412.8 62.84 498 22.15 4238.3 188.53
2.4 7.5 190 8.452 1617.0 71.93 570 25.35 4851.1 215.79
2.5 8 212 9.430 1804.3 80.26 636 28.29 5412.8 240.77
2.7 8.5 234 10.409 1991.5 88.59 702 31.23 5974.5 265.76
2.9 9 278 12.366 2366.0 105.24 834 37.10 7097.9 315.73
3.2 10 343 15.257 2919.1 129.85 1029 45.77 8757.4 389.55
3.5 11 407 18.104 3463.8 154.08 1221 54.31 10391.5 462.24
3.8 12 470 20.907 4000.0 177.93 1410 62.72 12000.0 533.79
4.1 13 533 23.709 4536.2 201.78 1599 71.13 13608.5 605.34
4.5 14 616 27.401 5242.6 233.20 1848 82.20 15727.7 699.60
4.8 15 698 31.049 5940.4 264.24 2094 93.15 17821.3 792.73

*After Sampson, dry, cyclic loading

Note: Dia. = diameter, Cir. = circumference, Av Fb = average break strength,

AE = cross-sectional area times modulus of elasticity

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Table 6-10. Some Factors to Consider When

Specifying Synthetic Line or Wire Rope

PARAMETER
Safety
Break strength
Diameter
Weight
Buoyancy and hydrodynamic properties
Ease of handling
Equipment to be used
Stretch/strain properties
Load sharing between lines
Dynamic behavior
Reliability
Durability
Fatigue
Exposure
Chaffing/abrasion
Wet vs. dry condition
Experience
Ability to splice
Ability to provide terminations
Inspection
Cost
Availability

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6-7.2 Wire Ropes. Wire rope is composed of three parts: wires, strands, and a
core. The basic unit is the wire. A predetermined number of wires of the proper size
are fabricated in a uniform geometric arrangement of definite pitch or lay to form a
strand of the required diameter. The required number of strands are then laid together
symmetrically around a core to form the rope. Refer to NAVSEA
NSTM 613 for additional information. Some of the features to consider when specifying
wire rope are listed in Table 6-10.

6-8 Fenders. Fendering is used between ships and compression structures,

such as piers and wharves, in fixed moorings. Fenders act to distribute forces on ship
hull(s) and minimize the potential for damage. Fendering is also used between moored
ships. A wide variety of types of fenders are used including:

Wooden piles
Cylindrical marine fenders
Hard rubber fenders
Mooring dolphins
Specially designed structures
Composite fender piles
Plastic fender piles
Pre-stressed concrete fender piles

Camels are wider compression structures used, for example, to offset a ship from a pier
or wharf.

The pressure exerted on ship hulls is a key factor to consider when specifying fenders.
Allowable hull pressures on ships are discussed in NFESC TR-6015-OCN, Foam-Filled
Fender Design to Prevent Hull Damage.

Behaviors of some common types of cylindrical marine fenders are shown

in Figures 6-3 and 6-4.

Refer to MIL-HDBK-1025/1 and NAVSEA NSTM 611 for detailed

information on fenders.

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Figure 6-3. SEA-GUARD Fender Information

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Figure 6-4. SEA-CUSHON Fender Performance

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6-9 PIER FITTINGS. Standard pier and wharf mooring fittings, as shown in
Figure 6-5, include:

Bollards
Bitts
Cleats

Cleats are not recommended for ships, unless absolutely necessary,

because they are low capacity.

Some of the fittings commonly used on U.S. Navy piers are summarized in
Table 6-11. Guidance for placing pier fittings in pier/wharf design is given in MIL-HDBK-
1025/1.

Table 6-11. Commonly Used U.S. Navy Pier Mooring Fittings

DESCRIPTION SIZE BOLTS WORKING
CAPACITY (kips)
SPECIAL Height=48 in. 12 x 2.75-in. dia. Horz. = 660
MOORING Base 48x48 in. @45 deg = 430
BOLLARD “A” Nominal = 450
SPECIAL Height=44.5 in. 8 x 2.25-in. dia. Horz. = 270
MOORING Base 39x39 in. @45 deg = 216
BOLLARD “B” Nominal = 200
LARGE BOLLARD Height=44.5 4 x 1.75-in. dia. Horz. = 104
WITH HORN Base 39x39 in. @45 deg = 66
Nominal = 70
LARGE DOUBLE Height=26 in. 10 x 1.75-in. dia.
BITT WITH LIP Base 73.5x28 in. Nominal = 75*
LOW DOUBLE Height=18 in. 10 x 1.625-in. dia.
BITT WITH LIP Base 57.5x21.5 in. Nominal = 60*
42-INCH CLEAT Height=13 in. 6 x 1.125-in. dia.
Base 26x14.25 in. Nominal = 40
30-INCH CLEAT Height=13 in. 4 x 1.125-in. dia. Nominal = 20
Base 16x16 in.
*Working capacity per barrel; after NAVFAC Drawing No. 1404464

It is recommended that all mooring fittings be clearly marked with their

safe working capacities on a plate bolted or welded to the base of the mooring device.
Additional information concerning the sizes and working capacities of pier and wharf
mooring fittings is found in NFESC TR-6014-OCN, Mooring Design Physical and
Empirical Data and in MIL-HDBK-1025/1. Also, NFESC assesses the condition of all
mooring fittings during its routine pier/wharf inspections U.S. Navy waterfront facilities.
Contact NFESC 55 (alex.viana@navy.mil orJason.bretzke@navy.mil) for inspection
results and reports for specific U.S. Navy waterfront facilities.

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Figure 6-5. Pier and Wharf Mooring Fittings Shown in Profile and Plan Views

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6-10 CATENARY BEHAVIOR. It is not desirable or practical to moor a ship
rigidly. For example, a ship can have a large amount of buoyancy, so it usually must be
allowed to move with changing water levels. Another problem with holding a ship too
rigidly is that some of the natural periods of the ship/mooring system can become short,
which may cause dynamic problems.

A ship can be considered a mass and the mooring system as springs.

During mooring design, the behavior of the mooring ‘springs’ can be controlled to fine
tune the ship/mooring system behavior to achieve a specified performance. This can be
controlled by the weight of chain or other tension member, scope of chain, placement of
sinkers, amount the anchor penetrates the soil, and other parameters. The static
behavior of catenaries can be modeled using the computer program CSAP2 (NFESC
CR-6108-OCN, Anchor Mooring Line Computer Program Final Report, User’s Manual
for Program CSAP2). This program includes the effects of chain and wire rope
interaction with soils, as well as the behavior of the catenary in the water column and
above the water surface.

As an example, take the catenary shown in Figure 6-6. This mooring leg
consists of four sections. The segment next to the anchor, Segment 1, consists of wire
rope, followed by three segments of chain. Sinkers with the shown in-water weight are
located at the ends of Segments 2 and 3. In this example, a plate anchor is driven 55
feet (16.8 meters) into mud below the seafloor. The chain attachment point to the ship
is 64 feet (19.5 meters) above the seafloor. The mooring leg is loaded to its design
horizontal load of H = 195 kips (8.7 E5 newtons) to key and proof load the anchor soon
after the anchor is installed. The keying and proofing corresponds to a tension in the top
of the chain of approximately 210 kips. Figure 6-6 shows the shape of the chain
catenary predicted by CSAP2 for the design load.

The computed load/deflection curve for the design water level for this
mooring leg, after proofing, is shown in Figure 6-7. The shape of this and the other
mooring legs in this mooring, which are not shown, will strongly influence the static and
dynamic behavior of the ship/mooring system during forcing.

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Figure 6-6. Sample Catenary

SEGMEN TYPE DIA WEIGHT LENGT SINKER

T H
(inches) (lbf/ft) (ft) (kips)
1 W 3.00 13.15 30 0
2 C 2.75 62.25 156 13.35
3 C 2.75 62.25 15 17.8
4 C 2.75 62.25 113 0

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Figure 6-7. Load/Deflection Curve for the Example Mooring Leg

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6-11 SOURCES OF INFORMATION. Detailed NAVFAC information, including
drawings, specifications, and manuals, is available in the National Institute of Building
Sciences, “Construction Criteria Base.” Further information can be obtained from the
Naval Facilities Engineering Command Criteria Office and the NFESC Moorings Center
of Expertise. A list of sources for information on facility mooring equipment is provided
in Table 6-12.

Table 6-12. Sources of Information for Facility Mooring Equipment

ITEM SOURCE
Standard fittings for waterfront NAVFAC Drawing No. 1404464
structures
Marine fenders UFGS 02395 “Prestressed Concrete Fender
Piling”
UFGS 02396 “Resilient Foam-Filled Marine
Fenders”
UFGS 02397 “Arch-Type Rubber Marine
Fenders”
Camels MIL-C-28628C(YD) “Camel, Wood, Marine;
Single Log Configuration, Untreated”
“Standard Aircraft Carrier Mooring Camel”
NAVFAC Drawings SD-1404045A to 52 and
NAVFAC Standard Spec. C39
“Standard Submarine Mooring Camel” NAVFAC
Drawings SD-1404943 to 47 and NAVFAC Spec.
C46
“Standard Attack Submarine Mooring Camel”
NAVFAC Drawings SD-1404667 to 70 and
NAVFAC Standard Spec. C49
Mooring lines Cordage Institute Technical Manual
Foam buoys NFESC purchase descriptions of Mar. 1988,
Dec. 1989 and May 1990.
Stud link chain and fittings NFESC purchase description of Mar. 1995.
NAVMOOR anchors NFESC purchase description of Nov. 1985 and
drawing package of July 1990.
Stud link chain anodes NFESC purchase description of June 1990.

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CHAPTER 7

VESSEL MOORING EQUIPMENT GUIDELINES

7-1 INTRODUCTION. A vessel must be provided with adequate mooring

equipment to meet its operational and design requirements. This equipment enables
the ship to anchor in a typical soil under design environmental conditions. In addition,
the ship can moor to various piers, wharfs, fleet moorings, and other facilities.
Equipment on board the ship must be designed for Mooring Service Types I, II and III,
as discussed in Chapter 3-1. Additional mooring hardware, such as specialized
padeyes, mooring chains, wire ropes, and lines, can be added for Mooring Service Type
IV situations.

7-2 TYPES OF MOORING EQUIPMENT. Basic shipboard mooring

equipment is summarized in Table 7-1. Additional information is provided in NAVSEA
NSTM Chapters 581, 582, 611 and 613; from Naval Sea Systems Command drawings
and publications; Cordage Institute, Cordage Institute Technical Manual; Guidelines for
Deepwater Port Single Point Mooring Design, Flory et al. (1977); The Choice Between
Nylon and Polyester for Large Marine Ropes, Flory et al. (1988); A Method of Predicting
Rope Life and Residual Strength, Flory et al. (1989); Fiber Ropes for Ocean
Engineering in the 21st Century, Flory et al, (1992a); Failure Probability Analysis
Techniques for Long Mooring Lines, Flory et al. (1992b); Modeling the Long-Term
Fatigue Performance of Fibre Ropes, Hearle et al. (1993); Oil Companies International
Marine Forum (OCIMF), Mooring Equipment Guidelines (1992); OCIMF
Recommendations for Equipment Employed in the Mooring of Ships at Single Point
Moorings (1993); OCIMF Prediction of Wind and Current Loads on VLCCs (1994);
OCIMF Single Point Mooring Maintenance and Operations Guide (1995); and Fatigue of
SPM Mooring Hawsers, Parsey (1982).

7-3 EQUIPMENT SPECIFICATION. Whenever possible, standard equipment

is used on board ships as mooring equipment. The specification, size, number, and
location of the equipment is selected to safely moor the ship. Some of the many factors
that need to be considered in equipment specification are weight, room required,
interaction with other systems, power requirements, reliability, maintenance, inspection,
and cost.

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7-4 FIXED BITTS. Bitts provide a termination for tension members. Fixed
bitts, Figure 7-1, are typically placed in pairs within a short distance forward or aft of a
chock location. They are often placed symmetrically on both the port and starboard
sides, so that the ship can moor to port or starboard. Capacities of the bitts are based
on their nominal diameter. Table 7-2 provides fixed bitt sizes with their associated
capacities. NFESC TR-6047-OCN “The Capacity and Use of Surface Ships’ Double
Bitts” (in preparation) provides additional information on ship’s bitts. The basic
philosophy for bitts use is that mooring lines should part well below the structural yield
of the double bits in Mooring Service Types I and II to minimize the chance that ship’s
mooring fittings need to be repaired. In Mooring Service Type III ‘Heavy Weather
Mooring’ is to keep the ship moored as safely as possible, so the working capacities of
the mooring lines can approximately equal the working capacities of the ship’s double
bitts.

7-5 RECESSED SHELL BITTS. Recessed shell bitts, Figure 7-2, are inset
into ships’ hulls well above the waterline. These bitts are used to moor lighterage or
harbor craft alongside. They also assist in mooring at facilities. The NAVSEA shell bitt
has a total working capacity of 92 kips (4.27 E5 newtons) with two lines of 46 kips
maximum tension each.

7-6 EXTERIOR SHELL BITTS. Aircraft carriers have exterior shell bitts,
Drawing No. 600-6601101, that are statically proof loaded to 184 kips (8.2 E5 newtons).
This proof load is applied 11 inches (280 mm) above the base. This testing is described
in the Newport News Shipbuilding testing report for USS HARRY S TRUMAN Bitts,
Chocks and Mooring Rings.

7-7 CHOCKS. There are many types of chocks, such as closed chocks,
Panama chocks, roller chocks, and mooring rings. Closed clocks are often used and
characteristics of these fittings are shown in Table 7-3.

7-8 ALLOWABLE HULL PRESSURES. As a ship berths or when it is

moored, forces may be exerted by structures, such as fenders, camels, and dolphins,
on the ship hull. NFESC TR-6015-OCN, Foam-Filled Fender Design to Prevent Hull
Damage provides a rational design criteria to prevent yielding of vessel hull plating.

7-9 SOURCES OF INFORMATION FOR SHIPS’ MOORING EQUIPMENT.

Additional information is available from the Naval Sea Systems Command (NAVSEA
03P), NSWCCD-SSES, Military Sealift Command (MSC), and the U.S. Coast Guard
(USCG). Table 7-4 provides a list of selected referenced materials.

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Table 7-1. Types of Ship Based Mooring Equipment*

EQUIPMENT DESCRIPTION
Drag embedment anchors One or more anchors required. See Chapter 7 for
anchor information.

Anchor chain Stud link grade 3 chain (see Chapter 6.4) is used.
Anchor windlass/wildcat and Equipment for deploying and recovering the
associated equipment anchor(s), including the windlass(s), hawse pipe(s),
chain stoppers, chain locker, and other equipment.
Bitts Bitts for securing mooring lines.
Chocks, mooring rings and Fittings through which mooring lines are passed.
fairleads
Padeyes Padeyes are provided for specialized mooring
requirements and towing.
Mooring lines Synthetic lines for mooring at piers, wharfs, and
other structures. See Chapter 6.7 for information.
Capstans Mechanical winches used to aid in handling
mooring lines.
Wire ropes Wire rope is sometimes used for mooring tension
members.
Fenders Marine fenders, as discussed in Chapter 6.8, are
sometimes carried on board.
Winches Winches of various types can support mooring
operations. Some ships use constant tension
winches with wire rope automatically paid out/pulled
in to adjust to water level changes and varying
environmental conditions. Fixed-length synthetic
spring lines are used in pier/wharf moorings that
employ constant tension winches to keep the ship
from ‘walking’ down the pier.
Other Various specialized equipment is carried to meet
needs (such as submarines).
*See NAVSEASYSCOM Naval Ships’ Technical Manual for additional information and
Chapter 3.1 for design criteria.

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Figure 7-1. Ship’s Fixed Double Bitts

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Table 7-2. Fixed Ships’ Bitts (minimum strength requirements)*

NAVSEA FIXED BITTS (after 804-1843362 REV B OF 1987)

NOMINAL SIZE (inches) 4 8 10 12 14 18

MAX. LINE CIR. (inches) 3 5 6.5 8 10 12
MAX. LINE DIA. (inches) 1.0 1.6 2.1 2.5 3.2 3.8
MAX. MOMENT (lbf-in x 1000) 134 475 1046 1901 3601 6672
MAX. CAPACITY (lbf x 1000)* 26.8 73.08 123.1 181 277 417
A - BASE LENGTH (inches) 16.5 28.63 36.75 44.25 52.5 64
B - BARREL DIA. (inches) 4.5 8.625 10.75 12.75 14 18
C - BARREL HT. (inches) 10 13 17 21 26 32
D - BASE WIDTH (inches) 7.5 13.63 17.25 20.25 22.5 28

MAX. LINE CIR. (mm) 76 127 165 203 254 305

MAX. CAPACITY (newton x 100000)* 1.19 3.25 5.47 8.05 12.32 18.55
A - BASE LENGTH (inches) 419 727 933 1124 1334 1626
B - BARREL DIA. (inches) 114 219 273 324 356 457
C - BARREL HT. (inches) 254 330 432 533 660 813
D - BASE WIDTH (inches) 191 346 438 514 572 711
* force applied at half the barrel height

*Note that the design of these bitts has changed over the years, so different classes of ships may have
different designs. The way the bitts are used may also influence their working capacity. Contact
NAVSEASYSCOM for additional information.

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Figure 7-2. Recessed Shell Bitt (minimum strength requirements)

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Table 7-3. Closed Chocks (minimum strength requirements)

NAVSEA CLOSED CHOCKS (from Drawing 804-1843363)

CHOCK SIZE (inches) 6 10 13 16 20 24

MAX. LINE CIR. (inches) 3 5 6.5 8 10 12
LINE BREAK (lbf x 1000) 26.8 73 123 181 277 417
A - HOLE WIDTH (inches) 6 10 13 16 20 24
B - HOLE HEIGHT (inches) 3 5 6.5 8 10 12
C - HEIGHT (inches) 8.5 11.25 13.88 16.75 25.75 25.25
D - BASE THICKNESS (inches) 5.25 6.5 7.5 9 16 13.5
E - LENGTH (inches) 13 19 23 28 38.75 40

MAX. LINE CIR. (mm) 76 127 165 203 254 305

LINE BREAK (newton x 100000) 1.19 3.25 5.47 8.05 12.32 18.55
A - HOLE WIDTH (mm) 152 254 330 406 508 610
B - HOLE HEIGHT (mm) 76 127 165 203 254 305
C - HEIGHT (mm) 216 286 352 425 654 641
D - BASE THICKNESS (mm) 133 165 191 229 406 343
E - LENGTH (mm) 330 483 584 711 984 1016

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Table 7-4. Sources of Information for Ships’ Mooring Equipment

ITEM SOURCE
Information on existing U.S. Navy Ships Characteristics Database
ships, drawings, and mooring (WATERS TOOLBOX)
hardware
General ship information NAVSEA Hitchhikers Guide to Navy
Surface Ships
Chocks NAVSEA Drawing No. 804-1843363 &
S1201-921623 (Roller Chock)
Panama chocks NAVSEA Drawing No. 804-1843363
Fixed bitts NAVSEA Drawing No. 804-1843362
Recessed shell bitts NAVSEA Drawing No. 805-1841948
Exterior shell bitts Newport News Shipbuilding Drawing No.
600-6601101
Cleats NAVSEA Drawing No. 804-2276338
Capstans/gypsy heads NAVSEA Drawing No. S260-860303
& MIL-C-17944
Hawser reels NAVSEA Drawing No. S2604-921841 &
42
Mooring lines Cordage Institute Technical Manual

*Contact NSWCCD-SSES Code 9732 at FriesJl@nswccd.navy.mil for drawings and

additional information on ship’s mooring equipment.

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CHAPTER 8

EXAMPLE PROBLEMS

8-1 INTRODUCTION. The design of mooring systems is illustrated through

the use of several examples in this section. The emphasis of this UFC is on statics, so
static results are shown. However, the marine environment can be dynamic, so
dynamic effects are illustrated in the examples.

8-2 SINGLE POINT MOORING - BASIC APPROACH. Design of single point

fleet moorings (SPMs) is illustrated here.

Let us first assume that the wind is coming from a specified direction and
has stationary statistical properties. The current speed and direction are constant. In
this case there are three common types of ship behavior, shown in Figure 8-1, that a
vessel at a single point mooring can have:

a) Quasi-static. In this case the ship remains in approximately a fixed

position with the forces and moments acting on the ship in balance. For quasi-static
behavior, the tension in the attachment from the ship to mooring will remain
approximately constant. Quasi-static analyses can be used for design in this case.

b) Fishtailing. In this case the ship undergoes significant surge, sway,

and yaw with the ship center of gravity following a butterfly-shaped pattern. The
mooring can experience high dynamic loads, even though the wind and current are
constant.

c) Horsing. In this case the ship undergoes significant surge and sway
with the ship center of gravity following a U-shaped pattern. The mooring can
experience high dynamic loads.

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Figure 8-1. Some Types of Behavior of Ships at Single Point Moorings

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These cases show that the type of behavior of a given ship at a given
single point mooring in a given environment can be very complex (Wichers, 1988), even
though the wind and current are steady. It is recommended that a dynamic stability
analysis first be conducted (Wichers, 1988) at the early stages of single point mooring
design. Then the type of analysis required can be determined. The results from this
analysis will suggest what type of method should be used to design a single point
mooring. These methods are complex and beyond the scope of this UFC. Behavior of
single point moorings is illustrated by example.

8-2.1 Background for Example. In this example two moorings were designed
and installed. The original designs were based on quasi-static methods. Ships moored
to these buoys broke their mooring hawsers when a wind gust front struck the ships. In
this example, the design and hawser failures are reviewed. The effects of wind
dynamics on a single point mooring are illustrated.

8-2.2 Ship. A single 2nd LT JOHN P. BOBO (T-AK 3008) class ship was
moored at each of two fleet mooring buoys. Table 8-1 gives basic characteristics of the
ship.

Table 8-1. 2nd LT JOHN P BOBO Parameters (Fully Loaded)

PARAMETER DESIGN BASIS DESIGN BASIS

(SI units) (English units)
Length
Overall 193.2 m 633.76 ft
At Waterline 187.3 m 614.58 ft
Between Perpendiculars 187.3 m 614.58 ft
Beam @ Waterline 9.80 32.15 ft
Draft 9.75 m 32 ft
Displacement 4.69E7 kg 46111 long tons
Line Size (2 nylon hawsers) - 12 inches

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8-2.3 Forces/Moments. In this case the design wind speed is 45 knots (23
m/s). Currents, waves, and tidal effects are neglected for these ‘fair weather’ moorings.
The bow-on ship wind drag coefficient is taken as the value given for normal ships of
0.7, plus 0.1 is added for a clutter deck to give a drag coefficient of 0.8. Methods in
Section 4 are used to compute the forces and moments on the ship. The computed
bow-on wind force is 68.6 kips (3.0 E5 newtons) for 45-knot (23-m/s) winds, as shown in
Figure 8-2 .

8-2.4 Quasi-Static Design. Quasi-static design procedures place the ship

parallel to the wind for this example, because in this position the forces and moments
on the ship are balanced out. Two mooring hawsers were specified for this design.
Extra factor of safety was specified for the two 12-inch nylon mooring hawsers, which
had a new wet breaking strength of 406 kips (1.8 E6 newtons), to account for poor load
sharing between the two hawsers.

8-2.5 Mooring Hawser Break. The ships were moored and faced into 15-knot
winds. The weather was unsettled, due to two nearby typhoons, so the ships had their
engines in idle. A wind gust front struck very quickly with a wind speed increase from
15 to 50 knots. As the wind speed increased, the wind direction changed 90 degrees,
so the higher wind speed hit the ships broadside. The predicted peak dynamic tension
on the mooring hawsers was 1140 kips (5.07 E6 newtons), (Seelig and Headland,
1998). Figure 8-3 is a simulation predicting the dynamic behavior of the moored ship
and hawser tension. In this case, the mooring hawsers broke and the predicted factor
of safety dropped to less than 1. In this event, the peak dynamic tension on the
mooring hawser is predicted to be 13.5 times the bow-on wind force for 50-knot (25.7-
m/s) winds.

This example shows that single point moorings can be susceptible to

dynamics effects, such as those caused by wind gust fronts or other effects.

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Figure 8-2. Example Single Point Mooring

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Figure 8-3. Example Mooring Failure Due to a Wind Gust Front

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8-3 FIXED MOORING - BASIC APPROACH. Development of a design
concept for a fixed mooring, a mooring that includes both tension and compression
members, is illustrated here.

8-3.1 Background. Several new aircraft carrier berthing wharf facilities are
being programmed. Users expressed concerns regarding the possibility of excessive
ship movement. Wind is the major environmental parameter of concern. Assume the
proposed sites have small tidal ranges and tidal currents.

8-3.2 Goal. Develop a concept to moor USS NIMITZ (CVN-68) class ships at
newly constructed wharves. Assume the Mooring Service Type is II and the design
wind speed is 75 mph (33.5 m/s).

8-3.3 Ship. Fully loaded USS NIMITZ (CVN-68) class ships are used in this
example. Table 8-2 gives some ship parameters. Additional information is found in the
Ships Characteristics Database (WATERS TOOLBOX).

Table 8-2. CVN-68 Criteria (Fully Loaded)

PARAMETER DESIGN DESIGN BASIS

BASIS (English units)
(SI units)
Length
Overall 332.8 m 1092 ft
At Waterline 317.0 m 1040 ft

Beam @ Waterline 40.8 134 ft

Draft 11.55 m 37.91 ft
Displacement 9.317 kg 91,700 long tons
Bitt Size - 12 inches
Line Size (nylon) - 8 and 9 inches

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8-3.4 Forces/Moments. Methods in Section 4 are used to compute the forces
and moments on the ship. These values are summarized in Figure 8-4.

8-3.5 Definitions. In this example we define a global coordinate system with

“X” parallel to the wharf, as shown in Figure 8-5. Then “Y” is a distance perpendicular
to the wharf in a seaward direction and “Z” is a vertical distance. Let “Pt 2” be the ship
chock coordinate and “Pt 1” be the pier fitting. A spring line is defined as a line whose
angle in the horizontal plane is less than 45 degrees and a breasting line whose angle
in the horizontal plane is greater than or equal to 45 degrees, as shown in Figure 8-5.

8-3.6 Preliminary Analysis. The first step for fixed mooring design is to analyze
the mooring requirements for the optimum ideal mooring shown in Figure 8-6.
Analyzing the optimum ideal arrangement is recommended because: (1) calculations
can be performed by hand and; (2) this simple arrangement can be used as a standard
to evaluate other fixed mooring configurations (NFESC TR-6005-OCN, EMOOR - A
Quick and Easy Method for Evaluating Ship Mooring at Piers and Wharves.

The optimum ideal mooring shown in Figure 8-6 consists of two spring
lines, Lines 1 and 4, which are assumed to resist longitudinal forces. There are two
breast lines, Lines 2 and 3, which are assumed to resist lateral forces and moments for
winds with directions from 0 to 180 degrees. Fenders are not shown. All lines are
assumed to be parallel to the water surface in the ideal mooring.

A free body diagram is made of the optimum ideal mooring for a loaded
CVN-68 in 75-mph (33.5-m/s) winds. It is found that the sum of the working mooring
capacity required for Lines 1 and 4 is 174 kips (7.7 E5 newtons) and the sum of the
working mooring capacity required for Lines 2 and 3 is 1069 kips (4.76 E5 newtons), as
shown in Figure 8-7. Note that no working line capacity is required in the ‘Z’ direction,
because the ship’s buoyancy supports the ship. The sum of all the mooring line working
capacities for the optimum ideal mooring is 1243 kips (5.53 E6 newtons).

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Figure 8-4. Wind Forces and Moments on a Single Loaded CVN-68 for a 75-mph
(33.5-m/s) Wind

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Figure 8-5. Definitions

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Figure 8-6. Optimum Ideal Mooring (Lines are parallel to the water surface and
breasting lines are spaced one-half ship’s length from midships)

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Figure 8-7. Required Mooring Capacity Using the Optimum Ideal Mooring

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8-3.7 Wharf Mooring Concept. Camels and fenders are located between the
wharf and ship to offset the ship in this design. Also, the wharf breasting line bollards
are set back from the face of the wharf, so that the vertical angles of the breasting lines
are approximately 10 degrees. Figure 8-8, from a study of a number of ship moorings at
piers and wharves (NFESC TR-6005-OCN) is used to estimate that a mooring system
using synthetic lines will have an efficiency of approximately 0.67 for the case of
breasting lines with a 10-degree vertical angle. The estimated total required working
mooring line capacity is the working line capacity of the optimum ideal mooring divided
by the efficiency. In this case, the estimated working line capacity required is 1243
kips/0.67 or approximately 1855 kips.

For extra safety, the selected concept ‘Model 2’ is given 11 mooring lines
of three parts each of aramid mooring line, as shown in Figure 8-9. A single part of line
is taken as having a break strength of 215 kips (9.2 E5 newtons). These lines have a
combined working strength of 11*3*215/3 = 2365 kips with a factor of safety of 3. These
lines are selected to provide extra safety. A component analysis, Figure 8-10, suggests
that this mooring concept has adequate mooring line capacity in the surge and sway
directions.

Quasi-static analyses are performed by computer using a fixed mooring

software program (W.S. Atkins Engineering Sciences, AQWA Reference Manual).
Analyses are performed for various wind directions around the wind rose. Results show
that the mooring line factors of safety are larger than the required minimum of 3 (i.e.,
line tensions divided by the new line break strength is less than 0.33), as shown in
Figure 8-11. In this concept the spring lines are especially safe with a factor of safety of
about 10. These analyses show ship motions of approximately 1 foot (0.3 meter) under
the action of the 75-mph (33.5-m/s) design winds.

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Figure 8-8. Efficiency of Ship Moorings Using Synthetic Lines at Piers and
Wharves (after NFESC TR-6005-OCN)

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Figure 8-9. CVN-68 Wharf Mooring Concept (‘Model 2’)

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Figure 8-10. Component Analysis of Mooring Working Capacity

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Figure 8-11. Mooring Line Tensions for a CVN-68 Moored at a Wharf With 75 mph
(33.5 m/s) Winds (‘Model 2’)

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Further quasi-static analyses show this concept is safe in up to 87-mph (38.9-m/s)
winds with a factor of safety of 3 or more on all the mooring lines. The computed
mooring efficiency for ‘Model 2’ at this limiting safe wind speed is 0.705, which is slightly
higher than the estimated value of 0.67, as shown in Figure 8-5.

These preliminary calculations show that this fixed mooring concept could
safely secure the ship. Figure 8-12 illustrates the mooring concept in perspective view.
Further information on this example is provided in NFESC TR-6004-OCN, Wind Effects
on Moored Aircraft Carriers.

8-4 SPREAD MOORING - BASIC APPROACH. Design of a spread mooring

for a nest of ships is illustrated in this section.

8-4.1 Background for Example. SPRUANCE class (DD 963) destroyers are
scheduled for inactivation and a mooring is required to secure four of these vessels
(NFESC SSR-6119-OCN, D-8 Mooring Upgrade Design Report). These ships are
inactive and cannot go out to sea, so the mooring must safely secure the vessels in a
hurricane using Mooring Service Type IV design criteria. At this location, wind is the
predominant environmental factor of concern. At this site the tidal range and tidal
current are small. Soil conditions at the site consist of an upper soft silty layer between
50 to 80 feet in depth (15 to 24 meters) over a stiff clay underneath. Water depth at the
site ranges between 31 to 35 feet (9.4 to 10.7 meters) MLLW.

8-4.2 Goal. Develop a concept to moor four DD 963 class destroyers in a

spread mooring. Use Mooring Service Type IV criteria and a design wind speed of 78.3
mph (68 knots or 35 m/s).

8-4.3 Ship. The ships are assumed to be at one-third stores/cargo/ballast

condition, since DD-963 vessels are unstable in the light condition. Table 8-3 gives
some ship parameters. Additional information is found in the NAVFAC Ships
Characteristics Database.

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Figure 8-12. Aircraft Carrier Mooring Concept (perspective view)

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Table 8-3. DD 963 Criteria (1/3 Stores)

PARAMETER DESIGN BASIS DESIGN BASIS

(SI units) (English units)
Length Overall 171.9 m 564 ft
At Waterline 161.2 m 529 ft
Beam @ Waterline 16.8 m 55 ft
Average Draft 6.5 m 21.2 ft
Draft at Sonar Dome 8.8 m 29 ft
Displacement 9.07E6 kg 8928 long tons
Chock Height From Baseline 10.7m stern 35 ft
15.9m bow 52 ft

8-4.4 Forces/Moments. Methods in Chapter 4, as well as data in NFESC

Report TR-6003-OCN, are used to compute the forces and moments on the ships.
These values are summarized in Figure 8-13. Wind angles are based on the local
coordinate system for a ship shown in Figure 4-2.

Note that wind tunnel model tests show that there is significant sheltering
in the transverse direction of downwind ships in this nest of identical ships, as shown in
NFESC Report TR-6003-OCN. However, there is little wind sheltering in the
longitudinal direction. Table 8-4 summarizes the environmental force calculations used
for this example.

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Table 8-4. Environmental Forces

Condition Load (Metric) Load (US) Comments

Single DD 1663.8 kN 374 kips Transverse Wind
963 257 kN 57.82 kips Longitudinal Wind
35972 m-kN 26531 ft-kips Wind-Yaw Moment
104.2 kN 23.4 kips Transverse Current
2.5 kN 0.56 kips Longitudinal Current
1216 m-kN 863.1 ft-kips Current Yaw Moment
4 ea DD 963 1989.9 kN 447.4 kips Transverse Wind
1028.7 kN 231.3 kips Longitudinal Wind
64595 m-kN 47643 ft-kips Wind-Yaw Moment
190.6 kN 42.8 kips Transverse Current
9.8 kN 2.2 kips Longitudinal Current
3342 m-kN 2372.7 ft-kips Current Yaw Moment

8-4.5 Anchor Locations. Driven-plate anchors are selected as a cost-effective

method to safely moor the nest of ships. The soils at the site are soft harbor mud of
depths between 50 to 80 feet (15 to 24 meters), so a chain catenary will form below the
seafloor (in the mud) as well as in the water column, as illustrated in Figure 6-6 (Section
6-10). A horizontal distance of 100 feet (30 meters) between the anchor location and
the chain daylight location (point where the anchor leg chain exits the seafloor) is
estimated based on Chain Soil Analysis Program (CSAP) modeling of the chain
catenary in the soil and in the water column.

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Figure 8-13. Wind Forces and Moments on a Nest of Four DD 963 Class Vessels
for a Wind Speed of 78 mph (35 m/s)

CURVE GRAPH MAX.

LIMITS VALUE
(1) Transverse E5 N 447.4 kips
Wind Force
(2) Longitudinal E5 N 231.3 kips
Wind Force
(3) Yaw Moment E6 N 47643 kip-ft

To ensure the mooring legs are efficient in resisting the imposed

environmental horizontal forces, a target horizontal distance of 170 feet (52 meters) is
chosen between the predicted daylight location (where the chain exits the soil) and the
attachment point on the ship for each of the mooring legs. Therefore, anchor locations
are established at a horizontal distance of 270 feet (82 meters) away from the vessel.

8-4.6 Definitions. In this example, a local ship and a global coordinate system
are defined. The local ship coordinate system is used to determine environmental loads
at the various wind and current attack angles, as shown in Figure 4-2, with the origin of

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the “Z” direction at the vessel keel. A global coordinate system for the entire spread
mooring design is selected with the point (0,0,0) defined to be at a specific location. For
this example, the origin is selected to be in the middle of the vessel nest and 164 feet
(50 meters) aft of the stern of the vessels. The origin for the “Z” direction in the global
coordinate system is at the waterline. This global coordinate system is used by the
various analysis programs to define the “chain daylight” locations and the location of the
vessel center of gravity within the spread mooring footprint.

8-4.7 Number of Mooring Legs. It is estimated that eight 2.75-inch chain

mooring legs are required, based on the safe working load of the chain (289 kips or 1.29
E6 newtons) and the applied environmental forces and moments on the nest of ships.
Four legs are situated on both sides of the nest and each mooring leg is angled to be
effective in resisting the longitudinal wind forces, as well as lateral wind forces and
moments, from winds approaching at angles other than broadside. Legs are also
placed toward the ends of the nest to be effective in resisting the yaw moment. To help
control ship motions, two 20-kip (9000-kg) sinkers are placed on each mooring leg
approximately midway between the vessel attachment point and the predicted chain
daylight location. A schematic of the planned spread mooring arrangement is shown in
Figure 8-14.

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Figure 8-14. Spread Mooring Arrangement for a Nest of Four Destroyers

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8-4.8 Static Analysis. A quasi-static analysis is performed on the mooring
system using a mooring analysis program (W.S. Atkins Engineering Sciences, AQWA
Reference Manual; other approved mooring analysis programs could have been used).
Each mooring leg is initially pretensioned to a tension of 4.4 kilo-newtons (10 kips).
Quasi-static analysis is performed for various combinations of wind and current
directions. Quasi-static results for various wind directions in conjunction with a 60-
degree flood tidal current of 0.6 knots (0.31 m/s) are shown in Table 8-5.

Table 8-5. Quasi-Static Leg Tensions for the Spread Mooring at Various Wind
Directions With a Flood Tidal Current

Wind Direction
LEG 0 300 600 900 1200 1500 1800
kN
1 52.49 214.99 447.01 609.02 945.05 866.04 541.00
2 - 62.50 347.99 486.02 769.03 927.03 571.03
3 693.00 941.04 844.02 588.02 560.00 343.99 93.50
4 668.00 808.04 611.02 387.00 255.02 45.60 -
5 622.01 490.03 84.52 - - - -
6 563.02 454.00 64.72 - - - -
7 - - - - - 220.99 449.01
8 - - - - - 309.02 564.00
Kips
1 11.8 48.33 100.49 136.91 212.45 194.69 121.62
2 - 14.05 78.23 109.26 172.88 208.4 128.37
3 155.79 211.55 189.74 132.19 125.89 77.33 21.02
4 150.17 181.65 137.36 87 57.33 10.25 -
5 139.83 110.16 19 - - - -
6 126.57 102.06 14.55 - - - -
7 - - - - - 49.68 100.94
8 - - - - - 69.47 126.79
- Indicates that the leg does not get loaded

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A maximum load of 943 kilo-newtons (212 kips) occurs on Leg 1 at a wind

direction of 120 degrees. This provides a quasi-static factor of safety of approximately 4
to the breaking strength of 2.75-inch FM3 chain.

8-4.9 Dynamic Analysis. A dynamic analysis is performed on the mooring

system to evaluate peak mooring loads and vessel motions using a mooring analysis
program (W.S. Atkins Engineering Sciences, AQWA Reference Manual). The initial
location of the vessel nest is based on the equilibrium location of the vessel nest
determined in the quasi-static analysis. An Ochi-Shin wind spectrum is used to simulate
the design storm (Wind Turbulent Spectra for Design Consideration of Offshore
Structures, Ochi-Shin, 1988). This simulation is performed for a 60-minute duration at
the peak of the design storm.

Figure 8-14 shows that the four vessels in the nest are close together and
Figure 8-15 shows that the ships have a large ratio of ship draft to water depth. In this
case it is estimated that the ships will capture the water between them as the ships
move. Therefore, the nest of moored ships was modeled as a rectangular box having a
single mass with the dimensions of 161.2 meters (length of each ship at the waterline),
71.62 meters wide (four ship beams + 5 feet spacing between ships), and 6.5 meters
deep (average vessel draft). Added mass for sway and surge was computed as if the
nest was cylindrical in shape with a diameter equal to the average draft. Damping as a
function of frequency was estimated from a diffraction analysis (W.S. Atkins Engineering
Sciences, AQWA Reference Manual).

Dynamic analyses were performed for various combinations of wind and

current directions using a wind speed time history that simulated the design storm.
Results showing the instantaneous peak tensions for various wind directions in
conjunction with a flood tidal current of 0.6 knots (0.31 m/s) are shown on Table 8-6.

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Figure 8-15. End View of DD 963 Mooring Nest

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Table 8-6. Peak Dynamic Chain Tensions for DD 963 Nest for Various Wind
Directions and a Flood Tidal Current

Wind Direction
LEG 0 300 600 900 1200 1500 1800
KN
1 167.05 288.9 634.03 828.68 2246.2 1848.5 731.73
2 55.089 174.58 430.31 545.27 1067.2 1152.1 720.54
3 1202.5 1625 995.98 818.81 1370 647.78 210.62
4 1362.2 1651.7 653.82 480.82 486.77 240.56 -
5 1284.2 1356.4 219.12 - - - -
6 938.06 901.87 217.04 - - - -
7 - - - - 55.019 374.91 514.26
8 - - - - 170.54 485.43 834.54
Kips
1 37.55 64.95 142.53 186.29 504.95 415.55 164.50
2 12.38 39.25 96.74 122.58 239.91 259.00 161.98
3 270.33 365.31 223.90 184.07 307.98 145.62 47.35
4 306.23 371.31 146.98 108.09 109.43 54.08 -
5 288.69 304.92 49.26 - - - -
6 210.88 202.74 48.79 - - - -
7 - - - - 12.37 84.28 115.61
8 - - - - 38.34 109.13 187.61

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Modeling shows that the instantaneous peak chain tension of 2246 knots
(505 kips) is predicted on Leg 1 as the moored vessel nest responds to wind gusts.
This provides a peak instantaneous factor of safety of 1.5 on the breaking strength of
the selected chain size. For this example, the peak dynamic chain tension during the 1
hour at the peak of the design storm is 2.4 times the quasi-static tension in the mooring
leg with the highest tension, Leg 1.

Nest motions for surge, sway, and yaw are provided in Table 8-7. This
table shows that the maximum surge of the vessel nest is approximately 7.4 meters
(24.3 feet) from its equilibrium condition at no loading. Maximum sway and yaw of the
vessel nest is 3.2 meters (10.5 feet) and 1.59 degrees clockwise, respectively. During a
dynamic analysis simulation, nest motions oscillated up to 5.4 meters (17.7 feet) in
surge (for a wind direction coming from the stern), 1.9 meters (6.2 feet) in sway (for a
wind direction 30 degrees aft of broadside), and 2.1 degrees in yaw (for a wind direction
30 degrees off the stern).

8-4.10 Anchor Design. Using the quasi-static design mooring leg tension,
anchor capacity and loads on the embedded plate anchor are calculated using
procedures outlined in NFESC TR-2039-OCN, Design Guide for Pile-Driven Plate
Anchors and NFESC CR-6108-OCN, Anchor Mooring Line Computer Program Final
Report, User's Manual for Program CSAP2. Due to the lower shear strengths of the soft
silty upper layers at the site, a 6-foot by 11-foot mud plate anchor is specified (this
anchor is summarized in the lower line of Table 5-7). A design keyed depth of 55 feet is
selected for the plate anchor. This will provide an estimated static holding capacity of
1913 kN (430 kips).

CSAP is used to predict the mooring leg tension at the anchor. Input
requirements of CSAP include: (1) mooring leg configuration between the anchor and
the buoy or chock; (2) water depth or height of chock above the seafloor; (3) soil profiles
and strength parameters; (4) location and size of sinkers; (5) horizontal tension
component of the mooring leg at the buoy or chock; (6) horizontal distance or total
length of the mooring leg between anchor and buoy or chock; and (7) anchor depth.
Output provided by CSAP includes: (1) chain catenary profile from the anchor to the
buoy or chock attachment point; (2) angle of the mooring leg from the horizontal at the
anchor, the seafloor, and the buoy or chock; (3) tension of the mooring leg at the
anchor, seafloor, and at the buoy or chock; (4) predicted daylight location for the
mooring leg; and (5) length of mooring leg required or horizontal distance between
anchor and buoy or chock.

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Table 8-7. DD 963 Nest Motions for Surge, Sway, and Yaw at Various Wind
Directions with a Flood Tidal Current

Wind Direction
Motion 00 300 600 900 1200 1500 1800
Surge (meters)
Origin 98.17 98.17 98.17 98.17 98.17 98.17 98.17
Start 105.6 105.4 103.6 98.1 93.7 89.2 88.1
Max 106.9 106.8 103.9 98.8 95.1 93.4 93.5
Min 102.3 102.3 102.4 98.1 93.7 89.2 88.1
Diff 4.6 4.5 1.5 0.7 1.4 4.2 5.4
Sway (meters)
Origin 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Start 0.84 1.49 2.39 2.97 1.27 2.02 1.14
Max 0.84 1.49 2.65 3.13 3.22 2.50 1.45
Min 0.52 0.83 0.93 1.35 1.27 1.43 1.11
Diff 0.32 0.66 1.72 1.78 1.93 1.07 0.34
Yaw (degrees)
Origin 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Start 0.76 1.09 1.43 0.64 -0.08 -0.74 -0.89
Max 0.76 1.18 1.59 0.80 -1.22 -1.49 -1.12
Min 0.38 0.27 0.43 -0.25 0.76 0.54 -0.83
Diff 0.38 0.91 1.16 1.05 1.96 2.03 0.29

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For this example, a keyed anchor depth of 55 feet was selected. Input
data included: (1) configuration of the mooring leg (30 feet of 3-inch wire attached to 2+
shots of 2.75-inch chain); (2) height of seafloor to vessel chock (46 feet stern and 64
feet bow); (3) soil profile and strength for the site (shear strength increases linearly at 10
pounds per ft2 per foot of depth); (4) information on the sinkers (2 each 20-kip sinkers
placed a horizontal distance of 170 feet away from the anchor; (5) horizontal tension
component of the mooring leg from the quasi-static results (195 kips); (6) horizontal
distance between anchor and chock (280 feet) from the quasi-static results; and (7)
depth of anchor (55 feet).

CSAP results for this design leg at this anchor depth indicate that the
predicted daylight location of the mooring leg is approximately 99 feet (30 meters) from
the anchor location and the leg tension at the anchor is 166 kips. A profile of this leg is
shown in Figure 6-6. Note that the interaction between the chain and the soil accounts
for a 25 percent reduction in tension on the mooring leg at the anchor. This gives a
predicted quasi-static anchor holding factor of safety of 2.6.

Based on the CSAP results, 6-foot by 11-foot plate anchors are specified.
Based on predicted keying distances required for this anchor, as outlined in NFESC TR-
2039-OCN, Design Guide for Pile-Driven Plate Anchors, the anchors should be installed
to a tip depth of 70 feet (21 meters) below the mudline to ensure that the anchor is
keyed at a minimum depth of 55 feet (16.8 meters). Figure 8-16 provides a comparison
between tip depth, keyed depth and ultimate capacity for this size anchor.

Further information concerning this design is provided in NFESC SSR-

6119-OCN.

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Figure 8-16. Plate Anchor Holding Capacity (6-foot x 11-foot anchor with keying
flaps in soft mud)

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8-5 MOORING LPD-17, LHD-1 AND LHA-1. As funding and time permit we
are preparing general guidelines and recommendations for mooring and/or anchoring
various classes of U.S. Navy ships. Work to date includes:

Reference Title
NFESC TR-6020-OCN Mooring USS WASP (LHD 1) Class Ships
NFESC TR-6028-OCN Mooring USS TARAWA (LHA 1) Class Ships
NFESC TR-6045-OCN LPD-17 USS SAN ANTONIO Class Berthing, Mooring and
Anchoring

Detailed information on mooring these three classes of ships is provided in the reports
listed above.

8-5.1 Mooring LPD-17. Examples of mooring LPD-17 (Figure 8-17 and Table
8-8) are illustrated in this section.

Figure 8-18 is a summary of the estimated safe mooring limits for LPD-17
based on the EMOOR planning tool for the ship moored with 28 parts of ship’s lines.
Figure 8-18 (upper) is for a water depth of 45 feet (i.e. T/d = 0.5) and Figure 8-18
(lower) is for a water depth of 25.2 feet (i.e. T/d = 0.9). The diagonal lines on the graph
correspond to various broadside current speeds, Vc. The dashed horizontal lines are
for the cases of 50-knot and 64-knot winds. The X-axis on each diagram is the mean
vertical angle of the breasting lines. The Y-axis is the estimated maximum safe wind
speed.

For example, take the case of the ship moored at a berth with a water
depth of 25 feet and a broadside current of 1.5 knots. For a mean vertical breasting line
angle of 30 degrees, the maximum estimated safe wind speed from Figure 8-18 (lower)
is 53 knots.

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Figure 8-17. LPD-17 USS SAN ANTONIO (Upper – hull form and mooring fitting
locations, Lower – profile view)

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Table 8-8. LPD-17 Characteristics

Parameter Metric English

26,750 US tons
Displacement (Full Load) 2.43 X 107 kg
23,880 longtons
Draft (Full Load) 6.9 m 22.64 ft
Length Overall 208.5 m 684 ft
Length at Design Waterline 201.5 m 661 ft
Bow Overhang from DWL Perpendicular 6m 19.68 ft
Stern Overhang from DWL Perpendicular 2 m 6.52 ft
Breadth, Molded Maximum 31.9 m 104.67 ft
Breadth, Amidships at DWL 29.46 m 96.67 ft
Lowest Projection above DWL 6.81 m 22.33 ft
Highest Projection above DWL 48.16 m 158 ft
Deck Elevation (01 Level) above DWL 12 m 39.33 ft
Cm = Cx Midships Coefficient 0.937 0.937
Cb Block Coefficient 0.601 0.601
Cp Prismatic Coefficient 0.641 0.641
Wetted Surface Area 6,340 m2 68,246 ft2
Waterplane Area 4,722 m2 50,830 ft2
Vertical Center of Gravity**** 11.13 m 36.5 ft
Natural Period in Roll 13.37 sec 13.37 sec
Vertical Center of Pressure **** 12.32 m 40.4 ft
Anchors (two-fluke, balanced fluke type) 13,800 kg** 30,425 lbs**
Chain (ABS Grade 3 stud link)*** 85 mm 3-3/8 inch
Ship’s Mooring Lines:
8.00 X 105 N 180 kips
18 with length of 600 feet; break =
12.46 X 105 N 280 kips
3 with length of 600 feet; break =
Main Double Bitt Size 305 mm 12 inches
Secondary Double Bitt Size 356 mm 14 inches
2
Maximum allowable hull pressure 145 kN/m 21 psi
* nominal size; ** delivered weight, min. specified 27,550 lbs, one port and one
starboard; *** 11 shots port and 13 shots starboard; **** above baseline

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Figure 8-18. Approximate Safe Mooring Limits for LPD-17 with 28 Parts of
Mooring Line

100
LPD-17 Fully Loaded
90 Water Depth = 45 ft (13.8 m) ; T/d = 0.5
14 Mooring Lines x 2 Parts Each
80
28 Parts of Line; FS=3.0
Line Break Strength = 180,000 lbs
MAXIMUM SAFE WIND SPEED (knots)

70

60 64-KNOT WINDS

50
50-KNOT WINDS

40
MST II Vc = 0
30 1 knot
1.5 knots
2 knots
20

10
EMOOR NO LINE TENDING
LPD-17.xls
0
0 10 20 30 40 50 60 70 80 90
MEAN VERTICAL BREASTING LINE ANGLE (deg)

100
LPD-17 Fully Loaded
90 Water Depth = 25.2 ft (7.7 m) ; T/d = 0.9
14 Mooring Lines x 2 Parts Each
80
28 Parts of Line; FS=3.0
Line Break Strength = 180,000 lbs
MAXIMUM SAFE WIND SPEED (knots)

70

60 64-KNOT WINDS

50
50-KNOT WINDS
40
MST II
30
Vc = 0
1 knot
20 1.5 knots
2 knots

10
EMOOR NO LINE TENDING
LPD-17.xls
0
0 10 20 30 40 50 60 70 80 90
MEAN VERTICAL BREASTING LINE ANGLE (deg)

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Six degree-of-freedom quasi-static analyses using the program AQWA LIBRIUM are
performed on various LPD-17 mooring configurations to determine safe mooring limits
as follows:

Figure 8-19 shows a sample Standard Mooring (Mooring Service Type IIa)
for the ship broadside to a current pushing the ship off the pier. In this example the
water depth is 45.3 feet for a ship draft to water depth ratio of T/d = 0.5. The maximum
safe wind speed for various current speeds is illustrated with a factor of safety of 3.0 or
higher on all mooring lines.

Figure 8-20 shows a sample Storm Mooring (Mooring Service Type IIb) for
the ship broadside to a current pushing the ship off the pier. For the case of a major
approaching storm the lines are run across the pier to the bollards on the opposite side
to significantly improve mooring efficiency. In this example the water depth is 30.2 feet
for a ship draft to water depth ratio of T/d = 0.75. The maximum safe wind speed for
various current speeds is illustrated with a factor of safety of 3.0 or higher on all mooring
lines.

Figure 8-21 shows a sample Heavy Weather Mooring (Mooring Service

Type III) for the ship broadside to a current pushing the ship off the pier. For the case of
a major approaching storm or hurricane the lines are run across the pier to the bollards
on the opposite side to significantly improve mooring efficiency. Ship’s lines are not
adequate for heavy weather mooring. Therefore, double braided polyester lines are
used because they have excellent fatigue resistance and some stretch to help improve
load sharing. In this example the water depth is 40 feet for a ship draft to water depth
ratio of T/d = 0.57. The maximum safe wind speed for various current speeds is
illustrated with a factor of safety of 2.5 or higher on all mooring lines. These dynamic
analyses were performed with the six-degree-of-freedom AQWA DRIFT. Figure 8-22
illustrates LPD-17 mooring at a double-deck pier.

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Figure 8-19. Sample LPD-17 Standard Mooring

(Mooring Service Type IIa)

Ship’s Mooring Lines

9 Lines of 2 Parts Each = 18 parts of line
1-Foot Slack Added to Lines 2, 7 & 8
Factor of Safety = 3.0a
Water Depth = 45.3 feet (T/d = 0.5)
Maximum Safe Wind Speed for:

Broadside
Current Speed Maximum Safe Wind Speed
0-knots 55 knots
1-knot 48 knots
1.5-knots 44 knots
2-knots 37 knots
a
Factor of safety on new lines with all lines intact. With the most
heavily loaded line broken,
the factors of safety will likely be reduced.

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Figure 8-20. Sample LPD-17 Storm Mooring

(Mooring Service Type IIb)

Ship’s Mooring Lines

10 Lines of 3 Parts Each = 30 parts of line
Factor of Safety = 3.0 (see note on Fig. 8-19)
Water Depth = 30.2 feet (T/d = 0.75)
Maximum Safe Wind Speed for:

Broadside
Current Speed Maximum Safe Wind Speed
0 Knots 85 knots
1 knot 81 knots
1.5 knots 76 knots
2 knots 68 knots

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Figure 8-21. Sample LPD-17 Heavy Weather Mooring
(Mooring Service Type III)

Double Braided Polyester Lines Fb=180 kips

14 Lines of 3 Parts Each = 42 parts of line
Factor of Safety = 2.5 (see note on Fig. 8-19)
Water Depth = 40 feet (T/d = 0.57)
Maximum Safe Wind Speed for:

Broadside
Current Speed Maximum Safe Wind Speed
0 knots 118 knots
1 knot 117 knots
1.5 knots 115 knots
2 knots 113 knots

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Figure 8-22. Sample LPD-17 Mooring at a Double-Deck Pier

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CHAPTER 9

PASSING SHIP EFFECTS ON MOORED SHIPS

9-1 INTRODUCTION. A ship moving through the water generates several

types of waves that may have an effect on moored ships, structures, shoreline erosion,
etc. One type of wave generated is a long-period pressure wave that may be very
small. However, this long wave (together with Bernoulli effects) may have a major
influence on moored ships.

9-2 PASSING SHIP EFFECTS ON MOORED SHIPS. A ship navigating in a

harbor or channel can produce major forces/moments on a nearby moored ship. The
moored ship is forced through a combination of wave, long-wave and Bernoulli effects
(NFESC TR-6027-OCN). NSTB Marine Accident Report, PB91-916404, NSTB/MAR-
91/04, for example, discusses a case where a nearby passing ship caused a moored
tanker to break its mooring and fuel lines. The resulting fire caused loss of life, in
addition to total loss of the pier and tanker.

The forces and moments acting on the moored ship depend upon a great
number of parameters including, relative size of the two ships, water depth, as well as
passing ship speed and separation from the moored ship. Figure 9-1 shows an
example passing ship case for parallel ships. This figure shows that the forces and
moments acting on the moored ship are highly time-dependant. Therefore, dynamic
programs, such as AQWA DRIFT (Century Dynamics, Houston, TX), are used to
determine the response of the moored ship to the passing ship.

Parallel passing ships are discussed in detail in NFESC TR-6027-OCN. An

associated spreadsheet PASS-MOOR.XLS is available that aids in predicting passing
ship forces and moments. Figure 9-2 illustrates example predictions. These examples
show that peak forces and moments applied to the moored ship in various degrees of
freedom occur at different times and highly dynamic.

A study is now underway (2004) at the U.S. Naval Academy to further refine
predictions of forces and moments on moored ships due to passing ships.

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Figure 9-1. Sample Passing Ship Situation

F X+
Pier or
Wharf

M+

L1
FY+

x Ship 1
Moored
V

Ship 2 L2
Moving

eta

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Figure 9-2. Example of Passing Ship Predictions

600 60000

400 40000

Fx Fy

MOMENT (thousands foot-pounds)

FORCE (thousands pounds)

200 20000

0 0

-200 -20000

-400 -40000

-600 -60000
0 20 40 60 80 100 120 140 160 180
TIME (SEC)

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CHAPTER 10

SHIP WAVES

10-1 INTRODUCTION. A ship moving through the water generates several

types of waves that may have an effect on moored ships, structures, shoreline erosion,
etc.

10-2 SHIP WAVES. Two of the most noticeable waves generated by moving
ships are the diverging (or bow wave) and transverse wave (Figure 10-1). These waves
intersect to form a cusp line and then the size of the highest generated wave tends to
decrease as the distance from the sailing line increases. Characteristics of the ship-
generated waves is a complex function of ship shape, water depth, ship speed, etc.
NFESC TR-6022-OCN summarizes measurements and recent findings on ship waves
for ship hull-forms, such as those illustrated in Figure 10-2.

A spreadsheet SHIP-WAVE.XLS is available for making ship wave

predictions. This spread sheet takes the measurements from a wide range of physical
ship wave measurements and uses one type of Froude scaling to predict maximum
wave height one wave length away from the ship sailing line for a ship of interest.

Figure 10-3, for example, shows the minimum ship speed required to
generate a given maximum wave height one wave length away from the ship sailing
line. The x-axis of this figure is water depth. The y-axis of this figure is ship speed.
Contours are for selected maximum wave heights. For deep water (the right side of this
figure), the wave height generally increases as the ship speed increases for the range
of conditions shown. In shallow water the wave height contours are much closer
together at higher speeds. This shows that in shallow water a small increase in ship
speed produces a dramatic increase in wave height.

The spreadsheet SHIP-WAVE.XLS and NFESC report TR-6022-OCN

provides additional information on waves generated by surface ships.

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Figure 10-1. Ship Waves (after Sorenson, 1997)

CUSP LOCUS LINE

y 19.47
o

V
SAILING LINE 0
TRANSVERSE Cd
WAVE DIVERGING WAVE

5
SERIES 60 Cb = 0.6 USNA (2000)
SHIP LENGTH = 1.524 M (5 FT)
4 T/d = 0.0168

2
Elevation (cm)

Hmax

-1

-2

-3 SHIP SPEED = 4.38 KNOTS (8.5 FPS)

DIST FROM SAILING LINE=1.848 M (6 FT)

-4
10 11 12 13 14 15
Time (sec)

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Figure 10-2. Ship Hull-Forms Tested (after Sorenson, 1997)

19 HAY TUG

18 HAY BARGE

17 HAY AUX

2, 58 USCG 40-FT CUTTER

8 to 12 CHEESE

13 WEINBLUM

14, 22 MARINER

15, 28 to 31, 32 SERIES 60 Cb=0.6

7, 16 MOORE DRYDOCK TANKER

47,48,50,51 CAPE BRETON MINER

49 CAPE BRETON MINER

TRIM, SQUAT AND RESISTANCE ONLY

32 DDG-51

39 FFG-7

45 BB-61

46 CVN-68

38 USCG HAMILTON WHEC 715

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Figure 10-3. Maximum Wave Height One Wavelength Away From the
Sailing Line For a SERIES 60 Hull

30
Hr1=3.5 m
SERIES 60 Cb=0.6 =3.0 m
& MARINER Hr1=2.0 m Hr1=2.5 m
(SHIP NOS.
25
14, 15, 22, 28, 29, 30)
Hr1=1.5 m

20 Hr1=3.5 m
Vr (knots)

15

10

Hr1=0.25 m 0.5 m 1.0 m

WSHIP2-OT2n.XLS

0
1 10 100 1000
dr (m)

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CHAPTER 11

HEAVY WEATHER MOORING GUIDELINES

11-1 INTRODUCTION. The purpose of this guidance is to advise facility

engineers, planners, and maintenance personnel of minimum facility planning and design
criteria necessary to ensure safe mooring of naval vessels during Mooring Service Type
III (Heavy Weather). Use this UFC to validate existing sites for heavy weather use and to
design new or modified facilities and moorings for heavy weather. Existing facilities and
moorings should not be used for heavy weather if they do not meet all criteria noted
herein unless provisions are made according to COMNAVSEASYSCOM msg R 130351Z
Jul 95 YZB . Since no Building Code or non-government standard exists for Mooring
Service Type III design, this UFC provides the relevant safety requirements and criteria
for facility aspects of ship mooring.

11-2 DISCUSSION. This criteria is a compilation of lessons learned from

Heavy Weather mooring studies for COMNAVSURFLANT by COMNAVFACENGCOM,
NFESC, and COMNAVSEASYSCOM. This criteria has been extensively coordinated
with COMNAVSEASYSCOM and internally within NAVFACENGCOM,
COMNAVSURFLANT, CINCLANTFLT N37, N46, and CINCPACFLT N37, N46. It
includes state of the art technology in vital facility areas such as mooring and risk
assessment. Congressional support for all new construction is dependent on specific
planning and design criteria applied consistently throughout the Navy - hence, the need
to formalize this criteria. Upgrades to existing facilities likewise require documentation
of new “code” requirements. A great deal or work has been performed to ensure that
ships are safely moored in Heavy Weather conditions. For example, below is a list of
some of the NFESC reports on Heavy Weather mooring.

NFESC REPORT TITLE

SSR-6078-OCN A Preliminary Assessment of Hurricane/Severe Storm Mooring
At Naval Station Mayport/Jacksonville, FL
SSR-6107-OCN Heavy Weather Mooring of USS Inchon (MCS-12) at U.S. Naval
Station, Ingleside, Texas
SSR-6112-OCN Heavy Weather Mooring of Ships Under Repair in the Hampton
Roads Area in 1997
SSR-6137-OCN Heavy Weather Mooring Analyses of Selected Ships
Under Repair, 1998
SSR-6145-OCN Heavy Weather Mooring of USS INCHON (MCS-12) at U.S.
Naval Station, Ingleside, Texas
SSR-6148-OCN Heavy Weather Mooring Design Report of Avenger and Osprey
Class Vessels, U.S. Naval Station, Ingleside, TX
SSR-6150-OCN SURFLANT Heavy Weather Mooring Program, Phase I
Completion Report

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SSR-6176-OCN Heavy Weather Mooring of FFG-7 and DDG-51 Ships at

Subase Kings Bay, Ga
SSR-6183-OCN Concept Study – Mooring Service Type III For a CVN-68 at
Navsta Mayport, Fla
SSR-6260-OCN Hurricane Mooring of Ships and Craft at Naval Coastal Systems
Center, Panama City, FL
SSR-6266-OCN Plate Anchor Concept for Heavy Weather Mooring of CVN, LHD
and LHA, Berth 42/42 Norfolk Naval Shipyard, Portsmouth, VA
SSR-6282-OCN Heavy Weather Mooring, NAVSTA Pascagoula, MS
SSR-6342-OCN Heavy Weather Mooring of USS JOHN F KENNEDY (CV 67)
Naval Station, Mayport, FL
SSR-6368-OCN Heavy Weather Mooring and Berthing-
Findings/Recommendations for Selected Berths
TM-6001OCN Risk Analysis for Ships Moored at Piers – Generalized
Evaluation of USS Tarawa (LHA-1) Mooring
TM-6015-OCN DD(X) Mooring Concepts
TR-6004-OCN Wind Effects on Moored Aircraft Carriers
TR-6012-OCN Rev B U.S. Navy Heavy Weather Mooring Safety Requirements
TR-6020-OCN Mooring USS WASP (LHD-1) Class Ships
TR-6023-OCN Dynamic Analyses of a CVN-68 in a Heavy Weather Mooring
TR-6028-OCN Mooring USS TARAWA (LHA 1) Class Ships
TR-6045-OCN LPD-17 USS SAN ANTONIO Class Berthing, Mooring and
Anchoring

11-3 HEAVY WEATHER MOORING GUIDELINES. Ships under repair in

graving docks may be relatively safe, since the ships are relatively protected from the
potentially high wind forces and are out of current and waves effects. However, ships at
piers and wharves may be subjected to high winds, wind gusts, wind gust fronts,
currents, waves, storm surges, etc. It is common practice for U.S. Navy ships to exit
port prior to arrival of hurricanes and other forecasted extreme weather conditions. This
practice is normally executed when destructive winds (sustained wind speed above 50
knots) are expected in the local area. However, ships in availability (i.e. under repair)
may not be able to go to sea. Therefore, these ships must be moored safely during
heavy weather or be moved to nearby safe facilities before storm arrival.
COMNAVSEASYSCOM msg R 130351Z Jul 95 YZB provides operational
recommendations to mitigate many effects of Heavy Weather. The effectiveness of
these mitigation measures is difficult to quantify. Therefore, facilities are often relied
upon to resist the loads. In each home porting region, only a portion of all berthing
facilities must be capable of heavy weather mooring, since only a portion of the ships

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cannot go to sea. Ships carry enough lines to moor in Mooring Service Type II as
defined below, but not for Type III. Also, facilities are generally designed for Type II and
not Type III.

This section provides some general guidelines on Heavy Weather

mooring. For additional information, see the publications listed above. For safe Heavy
Weather mooring:

Ensure that the facility, mooring fittings and fenders are adequate (see
UFC 4-150-07 Maintenance of Waterfront Facilities, UFC 4-150-08 Inspection of
Mooring Hardware, UFC 4-151-10 General Criteria for Waterfront Construction, MIL-
HDBK-1025/1 Piers and Wharves, etc.).

Identify alternative piers or wharves that the ship could be towed to that
may be safer and work out ahead of time all the logistics necessary to ensure that the
needed berth would be available and that the ship could arrive and be safely moored in
adequate time.

The facility needs to provide heavy weather mooring lines, since ship’s
lines are generally inadequate. Double braided polyester lines are recommended for
Heavy Weather mooring, because the lines have excellent fatigue resistance. These
lines also have some stretch which aids in load sharing between lines and helps
accommodate water level changes.

It is standard practice to first put the ship in a Standard of Storm Mooring

(i.e. Mooring Service Type Iia or Iib) when a ship comes to a facility for repair (Figure
11-1 upper). These types of moorings are used since the mooring lines can be secured
to mooring fittings near the ship, so the mooring lines have minimum interference with
repair work.

It is then standard practice to put the ship in a Heavy Weather mooring

(i.e. Mooring Service Type III) if storms, hurricanes or other threatening conditions are
expected (Figure 11-1 lower). In Heavy Weather mooring, for example, mooring lines
may be run

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Figure 11-1. Example of a Standard LPD 17 Mooring (Upper) and
Heavy Weather Mooring (Lower)

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across the pier to improve mooring efficiency and increase the number of mooring lines
that are used.

Detailed structural analyses show that ship’s double bitts have the
maximum safe working capacity when equal load is applied to each of the two barrels of
the bitts. It is recommended that methods be used to provide for equal loading to the
barrels in Heavy Weather mooring. This can be accomplished by various methods:

Using two parts of mooring line. Put an eye over the barrel of a set of bitts
and run to the shore fitting. Repeat for the second barrel. Carefully tend the lines so
both parts of line have the same pretension.

Using one line in two parts (Figure 11-2). Put an eye over one barrel and
run to the shore fitting. Run the line around a sheave, equalizer or tie down the loop of
line going around the shore fitting. Then tie off the end of the line to the second barrel
of the ship’s double bitts.

Detailed Heavy Weather mooring designs need to be prepared for each

ship at each berth. These designs need to consider the site-specific design criteria and
special circumstances. As a general rule-of-thumb:

Provide adequate numbers of breasting lines to safely secure the ship.

Place the lines towards the bow and towards the stern so they resist both lateral loads
and moments.

Breasting lines should be approximately perpendicular to the ship’s

centerline (to resist loads and moments), be of similar length (to help improve load
sharing) and should all have low vertical angles (to improve mooring efficiency and help
account for water level changes).

Provide adequate numbers of spring lines, which are approximately

parallel to the ship’s centerline. Equal numbers of spring lines should be run towards
the bow and stern. These lines should have rather low vertical angles to account for
water level changes.

It is recommended that preliminary designs be developed with quasi-static

designs and factors of safety of 2.5 or higher on all components. Dynamic methods can
then be used to refine and verify the designs. The various references cited in this
section provide examples of Heavy Weather mooring designs. Chapter 8 of this UFC
also provides Heavy Weather mooring examples.

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Figure 11-2. Securing Two Parts of Heavy Weather Mooring Line

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11-4 ACTION

11-4.1 Planning. NAVFAC Field Components, and Activities should assist

Claimants, Regional Commanders, and Shipyards to determine the number, location,
and critical ship class requirement for moorings used locally during Mooring Service
Type III. Mooring Service Type should be identified during the planning phase of
waterfront structures. Recommendations are provided in TR-6012-OCN Rev B U.S.
Navy Heavy Weather Mooring Safety Requirements.

11-4.2 Analysis and Design. Engineers at NAVFAC Field Components should

analyze moorings according to climatological criteria stated in TR-6012-OCN Rev B
U.S. Navy Heavy Weather Mooring Safety Requirements. Commercial enterprises
providing mooring for US Navy ships should likewise conform to the criteria contained
herein. Lines should provide a factor of safety against breaking of 2.5. Design pier and
wharf fittings for a working load equal to the break strength of the largest lines expected
to use the fitting. Moorings used for Type III service are subjected to significant
dynamic wind loads and should be analyzed accordingly. NFESC has capability of
providing this type of analysis on a reimbursable basis. Aramid and nylon lines also
respond differently to dynamic loads and should be properly modeled in the analysis.
Engineers should also verify the capacity of ship fittings for Type III Moorings. Dynamic
analysis indicated that in order to moor a CVN in heavy weather a ship alt is required as
well as facility upgrades at NNSY and NAVSTA Mayport. Ship alts, if required, should
be coordinated with NAVSEA.

11-4.3 Maintenance. Maintenance personnel should inspect moorings to ensure

acceptable performance during heavy weather. UFC 4-150-07 Maintenance of
Waterfront Facilities and UFC 4-150-08 Inspection of Mooring Hardware, provide
inspection guidance.

11-4.4 Operations. Activities should provide additional mooring lines to

supplement ship mooring lines for use during Mooring Service Type III.

11-5 ENVIRONMENTAL DESIGN CRITERIA FOR SELECTED SITES. A risk-

based approach is used in Heavy Weather mooring design to help ensure that ships are
safe no matter where they are moored. Site-specific design criteria (i.e. winds, water
levels, waves, etc.) associated each berth are used to help ensure that all ships are
safely moored. These criteria are contained in TR-6012-OCN Rev B U.S. Navy Heavy
Weather Mooring Safety Requirements.

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GLOSSARY

AISC. American Institute of Steel Construction.

API. American Petroleum Institute.

DDS. Design Data Sheet

DOD. Department of Defense.

DM. Design manual.

MLW. Mean low water.

MLLW. Mean lower low water.

MSC. Military Sealift Command.

NAVFAC. Naval Facilities Engineering Command.

NAVSEA. Naval Sea Systems Command.

NBS. National Bureau of Standards.

NCDC. National Climatic Data Center.

NFESC. Naval Facilities Engineering Services Center.

NUREG. Nuclear Regulatory Commission.

OCIMF. Oil Companies International Marine Forum.

PIANC. Permanent International Association of Navigation Congresses.

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APPENDIX A
REFERENCES
GOVERNMENT PUBLICATIONS UFC 1-200-01, Design: General
1. Unified Facilities Criteria Building Requirements

http://65.204.17.188//report/doc_ufc.html UFC 4-150-07, O&M: Maintenance of

Waterfront Facilities

UFC 4-150-08, O&M: Inspection of

Mooring Hardware

UFC 4-151-10, General Criteria for

Waterfront Construction

Mil-Hdbk 1025/1 Piers & Wharves

2. U.S. Navy, Office of the Chief of Naval OPNAVINST 3140.24E, Warnings and
Operations Conditions of Readiness Concerning
Hazardous or Destructive Weather
Phenomena, 21 December 1993

3. Naval Facilities Engineering Command CHESNAVFAC FPO-1-84(6)

(NAVFACENGCOM) Engineering Innovation Fleet Mooring Underwater Inspection
and Criteria Office Guidelines
6506 Hampton Blvd
Norfolk, Va 23508-1278 CHESNAVFAC FPO-1-87(1)
Failure Analysis of Hawsers on BOBO
http://www.lantdiv.navfac.navy.mil Class MSC Ships at Tinian on
7 Dec 1986

MIL-HDBK 1025/1 Piers and Wharves

MO-104.2 Specialized Underwater

Waterfront Facilities Inspections

4. Naval Facilities Engineering Service CR-6108-OCN, Anchor Mooring Line

Center Computer Program Final Report,
1100 23rd Avenue User’s Manual for Program CSAP2

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Port Hueneme, Ca 93043

CR-6129-OCN, Added Mass and
http://www.nfesc.navy.mil Damping Characteristics for Multiple
Moored Ships

FPO-1-89(PD1), Purchase Description

for Fleet Mooring Chain and
Accessories

Handbook for Marine Geotechnical

Engineering (NCEL, 1985)

SSR-6078-OCN, A Preliminary
Assessment of Hurricane/Severe Storm
Mooring At Naval Station
Mayport/Jacksonville, FL

SSR-6107-OCN, Heavy Weather

Mooring of USS Inchon (MCS-12) at
U.S. Naval Station, Ingleside, Texas

SSR-6112-OCN, Heavy
Weather Mooring of Ships Under
Repair in the Hampton Roads
Area in 1987

SSR-6119-OCN, D-8 Mooring Upgrade

Design Report

SSR-6137-OCN, Heavy Weather

Mooring Analyses of Selected Ships
Under Repair, 1998

SSR-6145-OCN, Alternative Heavy

Weather Mooring of USS INCHON
(MCS-12) at U.S. Naval Station,
Ingleside, Texas

SSR-6148-OCN, Heavy Weather

Mooring Design Report of Avenger and
Osprey Class Vessels, U.S. Naval
Station, Ingleside, TX

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SSR-6150-OCN, SURFLANT Heavy

Weather Mooring Program, Phase I
Completion Report

SSR-6176-OCN, Heavy Weather

Mooring of FFG-7 and DDG-51 Ships at
Kings Bay, Ga

SSR-6183-OCN, Concept Study –

Mooring Service Type III For a CVN-68
at Navsta Mayport, Fla

SSR-6260-OCN, Hurricane Mooring of

Ships and Craft at Naval Coastal
Systems Center, Panama City, FL

SSR-6266-OCN, Plate Anchor Concept

for Heavy Weather Mooring of CVN,
LHD and LHA, Berth 42/42 Norfolk
Naval Shipyard, Portsmouth, VA

SSR-6282-OCN, Heavy Weather

Mooring, NAVSTA Pascagoula, MS
SSR-6342-OCN, Heavy Weather
Mooring of USS JOHN F KENNEDY
(CV 67) Naval Station, Mayport, FL

SSR-6368-OCN, Heavy Weather

Mooring and Berthing-
Findings/Recommendations for
Selected Berths

TDS 83-05, Multiple STOCKLESS

Anchors for Navy Fleet Moorings
(NCEL)

TM-6001-OCN, Risk Analysis for Ships

Moored at Piers – Generalized
Evaluation of USS Tarawa (LHA-1)
Mooring

TDS 83-08R, Drag Embedment

Anchors for Navy Moorings (NCEL)

TM-6010-OCN, Some LHA(R )

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Mooring Concepts

TN-1628, Wind-Induced Steady Loads

on Ships (NCEL)

TN-1634, STATMOOR – A Single Point

Mooring Static Analysis Program
(NCEL)

TN-1774 Single and Tandem Anchor

Performance of the New Navy Mooring
Anchor (NCEL)

TR-2018-OCN, Assessment of Present

Navy Methods for Determining Loads at
Single Point
Moorings

TR-2039-OCN, Design Guide for

Pile Driven Plate Anchors

TR-6003-OCN, Wind and Current

Forces/Moments on Multiple Ships

TR-6004-OCN, Wind Effects on

Moored Aircraft Carriers

TR-6005-OCN (Rev B), EMOOR – A

Quick and Easy Method of
Evaluating Ship Mooring at Piers and
Wharves

TR-6012-OCN Rev B, U.S. Navy Heavy

Weather Mooring Safety Requirements

TR-6014-OCN, Mooring Design

Physical and Empirical Data

TR-6015-OCN, Foam-Filled Fender

Design to Prevent Hull Damage

TR-6020-OCN, Mooring USS WASP

(LHD-1) Class Ships

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TR-6022-OCN, Ship-Generated Waves

TR-6023-OCN, Dynamic Analyses of a

CVN-68 in a Heavy Weather Mooring

TR-6027-OCN, Passing Ship Effects on

Moored Ships

TR-6028-OCN, Mooring USS TARAWA

(LHA 1) Class Ships

TR-6037-OCN, Improved Pearl Harbor

and Kings Bay Anchor Designs

TR-6045-OCN, LPD-17 USS SAN

ANTONIO Class Berthing, Mooring and
Anchoring

5. United States Naval Academy EW-9-90, Evaluation of Viscous

Annapolis, MD 21402-5018 Damping Models for Single Point
Mooring Simulation

Viscous Drag Forces on Moored Ships

in Shallow Water (D. Kriebel, 1992)

6. David Taylor Research Center DTNSRDC/SPD-0936-01, User’s

Annapolis, MD 21402-5087 Manual for the Standard Ship Motion
Program, SMP81

7. Naval Environmental Prediction TR-82-03 Hurricane Havens Handbook

Research Facility
Monterey, CA

8. Naval Research Laboratory NRL/PU/7543-96-0025

4555 Overlook Ave Typhoon Havens Handbook
Washington, DC 20375 for the Western Pacific and Indian
Oceans

9. Naval Sea Systems Command Naval Ships Technical Manual

Washington, DC NSTM Chapers: 096, 581, 582, 611,
and 613

Design Data Sheet, DDS-581,

“Calculations for Mooring Systems”
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Hitchhikers Guide to Navy Surface

Ships, 1997

10. National Bureau of Standards Series 118, Extreme Wind Speeds at

129 Stations in the Contiguous United
States, 1979

Series 124, Hurricane Wind Speeds in

the United States, 1980

11. National Transportation Safety Board PB91-916404, NSTB/MAR-91/04

Washington, DC Marine Accident Report
Explosion and Fire Aboard the
U.S. Tankship Jupiter
Bay City, Michigan
September 16, 1990

12. National Climatic Data Center E/CC31:MJC

151 Patton Avenue Letter Report of 8 Dec 87
Asheville, NC 28801-5001

13. Nuclear Regulatory Commission NUREG/CR-2639

Washington, DC Historical Extreme Winds for the United
States – Atlantic and Gulf of Mexico
Coastlines

NUREG/CR-4801
Climatology of Extreme Winds in
Southern California

NUREG-0800
Tornado Resistant Design of Nuclear
Power Plant Structures, Nuclear Safety,
Volume 15, No. 4, Regulatory Guide,
July 1974

14. U.S. Army Corps of Engineers Special Report No. 7, Tides and Tidal
Datums in the United States, 1981

Shore Protection Manual, 1984

Coastal Engineering Manual (CEM)

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NON-GOVERNMENT PUBLICATIONS
1. American Institute of Steel Construction Manual of Steel Construction

2. American Petroleum Institute API RP 2T

1220 L Street NW
Washington, DC 20005

3. Cordage Institute Cordage Institute Technical Manual

350 Lincoln Street
Hingham, MA 02043

2. Oil Companies International Marine Mooring Equipment Guidelines (1992)

Forum (OCIMF)
Recommendations for Equipment
Employed in the Mooring of Ships at
Single Point Moorings (1993)

Prediction of Wind and Current Loads

On VLCCs (1994)

Single Point Mooring and Maintenance

Guide (1995)

5. Permanent International Association Criteria for Movement of Moored Ships

of Navigation Congresses (PIANC) in Harbors: A Practical Guide, 1995

AUTHORED PUBLICATIONS

Flory, J.F., M.R. Parsey, and H.A. McKenna. The Choice Between Nylon and Polyester
for Large Marine Ropes, ASME 7th Conference on Offshore Mechanics and Arctic
Engineering, Houston, TX, Feb 1988.

Flory, J.F., M.R. Parsey, and C. Leech. A Method of Predicting Rope Life and Residual
Strength, MTS Oceans’ 89, Sep 1989.

Flory, J.F., H.A. McKenna, and M.R. Parsey. Fiber Ropes for Ocean Engineering in the
21st Century, ASCE, C.E. in the Oceans, Nov 1992a.

Flory, J.F., J.W.S. Harle, R.S. Stonor, and Y. Luo. Failure Probability Analysis
Techniques for Long Mooring Lines, 24th Offshore Technology Conference
Proceedings, Offshore Technology Conference, Houston, TX, 1992b.

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Headland, J., Seelig, W., and C. Chern. Dynamic Analysis of Moored Floating
Drydocks, ASCE Proceedings Ports 89, 1989.

Headland, J., Seelig, W. Mooring Dynamics Due to Wind Gust Fronts, 1998.

Headland, J., Seelig. W., and Kreibel, D., Broadside Current Forces on Ships,
Proceedings Civil Engineering in the Oceans V, ASCE, 1992.

Hearle, J.W.S., M.R. Parsey, M.S. Overington, and S.J. Banfield. Modeling the Long-
Term Fatigue Performance of Fibre Ropes, Proceedings of the 3rd International
Offshore and Polar Engineering Conference, 1993.

Hooft, J.P., Advanced Dynamics of Marine Structures, John Wiley & Sons, New York,
New York, 1982.

Kizakkevariath, S. Hydrodynamic Analysis and Computer Simulation Applied to Ship

Interaction During Maneuvering in Shallow Channels, PhD Thesis, Virginia Polytechnic
Institute and State University, May 1989.

Myers, John J., et al. Handbook of Ocean and Underwater Engineers, McGraw-Hill
Book Company, New York, NY, 1969.

Occasion, L.K., The Analysis of Passing Vessel Effects on Moored Tankers, UCLA
Report PTE-490x, December 1996.

Ochi-Shin, Wind Turbulent Spectra for Design Consideration of Offshore Structures,

1988

Parsey, Fatigue of SPM Mooring Hawsers, 1982

Simiu, E. and Scanlan, R., Wind Effects on Structures, Third Edition, John Wiley &
Sons, 1996

Weggel, J.R. and Sorensen, R. M., Development of Ship Wave Design Information,
Proceedings of the International Conference on Coastal Engineering, ASCE, 1984

Wichers, J., A Simulation Model for a Single Point Moored Tanker, Maritime Research
Institute, Netherlands, Publication No. 797, 1988.

213