7_Attachment 27 - NSWCCD-23-TM-2011-38 RevA Mooring Fitting Design and Verification Process Guidance.pdf

Naval Surface Warfare Center Carderock Division West Bethesda, MD 20817-5700

NSWCCD-23-TM-2011/38 Rev A November 2011

Ship Systems Integration and Design Department

Technical Memorandum

MOORING FITTING DESIGN AND VERIFICATION

PROCESS GUIDANCE

by Combatant Craft Division (Code 231)

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited

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M o o ri n g F it ti n g D e s ig n a n d

V e ri fi c a ti o n

P ro c e s s G u id a n c e

MOORING FITTING DESIGN AND VERIFICATION

PROCESS GUIDANCE

NSWCCD-23-TM-2011/38 Rev A

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Mooring Fitting Design and Verification Process

Guidance

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NSWCCD Code 231

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NSWCCD-23-TM-2011/38

Naval Surface Warfare Center

Carderock Division

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Virginia Beach, VA 23459-3239

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Attn PEO SHIPS, PMS 325G

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Naval Sea Systems Command

1333 Isaac Hull Avenue, SE Building 197

Washington Navy Yard, DC 20376

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PEO SHIPS, PMS 325G

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DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

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PAO approval received from NSWC Carderock, Congressional and Public Affairs

Branch, Code 3830 on 29 November 2011.

14. ABSTRACT

This document was developed to guide designers in industry and the U.S. Navy in the design, installation, testing and verification of safe and effective mooring fittings for installation on boats and craft.

15. SUBJECT TERMS

Mooring Fittings Design Guidance

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See 12.

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Malcolm W. Whitford

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Contents

Page

Figures ........................................................................................................................................... iv

Tables ............................................................................................................................................ iv

Administrative Information ......................................................................................................... v

Acknowledgements ....................................................................................................................... v

Summary

Introduction

Background

Objective Scope

Mooring System Design

Category of Service Category 1

Category 2

Category 3 Mooring Restraint Loads

Craft Weight Based Mooring Restraint Load Determination

Wind and Current Mooring Restraint Load Determination Arrangements

Mooring Line Sizing Mooring Fitting Strength

Fitting Factors of Safety Commercial-Off-The-Shelf Cleats Material Selection Material Properties

Design Details Load Path into Craft Structure Design Calculations

Fabrication and Installation Welding Surface Finish Mechanical Fasteners FRP Craft Metal Craft iv

Verification

Mooring Fittings

Documentation and Labeling

Specification Inputs

Documentation Boat Information Book (and other craft documentation)

References

Symbols, Abbreviations, and Acronyms

Figures

Page

Figure 1. Mooring Lines Poorly Adjusted Figure 2. Typical Mooring Arrangement

Figure 3. Typical Mooring Bitts Figure 4. Typical Open (left) and Closed Cleats (right) Figure 5. Typical Closed Chock and Fairlead

Figure 6. Proper Cleat Hitch Figure 7. Cleat with Properly Rigged Doubled Line

Figure 8. Load on Chock for 2.5-inch Circumference Breast Line Figure 9. Failure of Deck Mounted Bitt

Tables

Table 1. Minimum Breaking Strength for Double Braided Nylon Line v

Administrative Information

This work was performed by the Combatant Craft Division (Code 231) of the Ship

Systems Integration and Design Department at the Naval Surface Warfare Center, Carderock

Division (NSWCCD). This effort was sponsored by the US Navy Small Boats Program Office, PEO SHIPS, PMS 325G.

Acknowledgements

The Combatant Craft Division (Code 231) wishes to thank Mr. J.M. Coughlin, Program

Manager at the US Navy Small Boats Program Office (PEO Ships, PMS 325G), for sponsoring the development of this guide, Mr. Denis Bushey, PMS 325G for his overall management and guidance. Mr. Richard Wilson, Senior Naval Architect, provided significant information for the guide based on his experience with development of safe and effective craft systems. Mr. Jason

Marshall, PE, Senior Craft Design Manager, and Mr. Thomas Kush, Waterfront Operations

Facilities Manager provided helpful feedback and suggestions. Mr. Michael Riley, The Columbia

Group, served as technical editor. Mr. George Sasse of Seaward Services provided photographs of numerous installations and important input based on his knowledge of marlinspike seamanship. Their collective support is sincerely appreciated.

Summary

This document provides design guidance to assist boat builders, designers and engineers in the design, fabrication, installation, inspection, and verification testing of safe and effective mooring fittings on all U.S. Navy boats and craft. References for both Department of Defense and civilian design standards are provided, typical contract requirements are discussed, and best practices for mooring fitting design, material selection, and installation are presented.

Acceptance verification methods and boat builder information packages are discussed, and design safety factors established to mitigate the risk of unsafe conditions are presented. This guide provides a useful tool set, but it should not preclude the use of other detailed engineering practices or processes that can also deliver safe and effective capabilities. While not contractually binding, unless required in the Contract, it presents a collection of Navy preferences and expectations that will be updated as new lessons are learned, and as new materials or techniques are developed. The following list is an outline of the mooring system design process presented in this guide.

1. Determine ‘Category of Service’ for mooring

2. Determine mooring restraint loads

Craft weight based

Wind and current based

3. Arrangements

Select number and types of mooring fittings

Arrange mooring fittings

4. Mooring line size

Determine mooring line loads

Select rope size for mooring lines

5. Mooring fitting strength

Fitting Factors of safety

Commercial-Off-The-Shelf Cleats

Select materials

Identify material properties

Design details

Determine load path into the craft structure

Design calculations

6. Fabrication and Installation

Welding

Surface finish

Mechanical fasteners

Fiber reinforced plastic craft

Metal craft

7. Verification

8. Documentation and Labeling

Specification Inputs

Documentation

Introduction

Background

Mooring involving U.S. Navy craft is a critical safety evolution. Flaws in mooring fitting design or fabrication can result in catastrophic failures, loss of the craft, damage to property, and personnel injuries or death. Historically, these critical fittings had been engineered by the

Government and a detailed design provided to the boat builder. In the past two decades, the design of mooring fittings on many boats and craft has been entrusted to boat builders. However, government technical evaluation of mooring fittings and design methods provided by boat builders has identified problems in the basic assumptions and engineering approaches used.

There was a need for a single reference that describes a mooring fitting design and integration process from a blank sheet of paper to final safety testing that could be applied consistently across the various types of boats and craft.

Objective

This design guide provides general guidance to assist boat builders, designers and engineers in the design, fabrication, installation, inspection and verification of safe and suitable mooring systems for U.S. Navy craft.

The layout of this guide includes technical information as well as “TIPS” that highlight lessons learned and cautions for avoiding common issues. Typical contract requirements are presented, best practices are summarized, and lessons learned related to mooring system designs are presented.

TIP: Successful acquisition and delivery of a new craft relies on collaborative teams of knowledgeable individuals. Ask the Technical Point of Contact (TPOC) immediately if there are questions related to design requirements, applicable standards, or acceptance criteria.

Scope

This guide is applicable to conventional aluminum, Fiber Reinforced Plastic (FRP), and steel construction, and includes minimum best practice approaches to arrangement, material assumptions, and engineering methods to ensure adequate mooring fittings and arrangements.

This design guide is not intended to provide detailed procedures for the design of mooring fittings and their supporting structure.

The breadth of technical issues addressed in this design guide is as complete as possible;

however this guidance is not exhaustive and does not cover all aspects of design or all situations.

Design of the mooring fittings and their integration into the craft requires the technical skills and judgment of an experienced engineer, and no guide can capture all the factors that must be considered.

Mooring System Design

Design of a safe and effective mooring system requires consideration of the restraint loads that the mooring lines and fittings are required to resist, the arrangements of the mooring components on the craft necessary to accomplish safe mooring in the required conditions, selection of mooring lines, selection of the craft fittings and hardware, and analysis of the strength of the mooring system components and the craft structure.

High-strength fittings for Navy craft are designed to minimize the chance of failure under foreseeable (not just normal) service conditions throughout the craft’s service life. This is because high strength fittings present the potential for a single-point failure with severe consequences, including injury or death resulting from, for example, a loaded line pulling a mooring fitting out of a deck. This approach to failure is feasible because the cost and weight penalty of designing high strength fittings for zero failure is low, compared to the cost and weight penalty of zero failure design for hull structure or other systems that typically do not present the potential for catastrophic single-point failures.

As an added precaution, it is generally desired that the line will fail before the fitting or craft structure fails to minimize the potential for damage and injury. An exception to this philosophy occurs on smaller craft when the mooring line can withstand loads substantially larger than those possible from wind and sea.

It is important to integrate the design and development of mooring fittings into the initial design process of a boat. Early integration of mooring fittings into the boat design is required to avoid structural and arrangements problems requiring redesign and rework. The general design practices discussed herein are important to achieving safe and functional mooring fittings and arrangements. Calculations and design of mooring fittings and their integration into the craft structure should follow these guidelines and sound engineering practice.

Category of Service

The first step in the mooring system design process is to determine the Category of

Service. Craft are grouped into three ‘Categories of Service’ to describe their mooring needs based on how the fleet will handle them under severe mooring conditions. The Category of

Service for mooring should be specified in the Contract or by the TPOC.

Category 1

Category 1 applies to craft less than 12m (39-feet 4-inches) in overall length, where the craft will be removed from the water when ‘hazardous winds’, greater than 35 knots, are expected. The craft must have a dedicated trailer or cradle for this category to apply.

Category 2

Category 2 applies to craft that are expected to be removed from the water when

‘destructive winds’, greater than 50 knots, are expected. The craft must either be pulled from the water onto a trailer or cradle, or head to sea or to a protected mooring site to avoid winds greater than 50 knots.

Category 3

Category 3 applies to craft that are not likely to be pulled from the water when

‘destructive winds’ are expected. These craft are generally too large to pull onto a trailer or cradle, too slow or small to run before the storm, and have no sufficiently protected mooring locations available.

An example of this category is the Ferry Boat used to service the Arizona Memorial in

Hawaii. These boats are typically not pulled when a typhoon is expected since the local cranes are busy elsewhere. The boats are pulled to as protected a mooring site as possible, but there are no alternative mooring locations that are likely to be out of the path of a storm that is headed for the island of Oahu.

Mooring Restraint Loads

The second step in the mooring system design process is to determine the basis for the required mooring restraint loads. Most mooring systems are designed to withstand the expected mooring loads due to wind and current. A potential exception to this philosophy occurs on smaller U.S. Navy craft when minimum mooring line sizes are used.

The minimum mooring line size for all US Navy craft is 2-inch circumference (5/8-inch diameter) double braided nylon. This is the minimum size line that can be handled securely without feeling the need to take a turn of line around a sailor’s hand (a very unsafe practice) for a good grip when the line is under load. The minimum breaking strength of 2-inch circumference double braided nylon line is 12,000 lbs based on reference [1].

When the minimum mooring line size is used, it can be significantly stronger than needed to withstand expected wind and current loads. Rather than over-designing the mooring fittings to withstand the breaking strength of the line under this circumstance, as an alternative, the fitting can be designed based on the weight of the boat. The reasoning behind this method is illustrated in Figure 1, where a boat was nearly suspended from its mooring fittings when the draft of the fuel barge changed as its fuel was offloaded.

Craft Weight Based Mooring Restraint Load Determination

All craft in ‘Category 1’ use the minimum 2-inch circumference (5/8-inch diameter) mooring line size to determine mooring fitting design loads. In Categories 2 and 3 for craft weighing less than 17,000 lbs, the craft weight may be used to determine mooring fitting strength requirements. The basis for this number will be explained later in the guide. For craft weighing more than 17,000 lbs, calculations may show that the minimum line size is appropriate for some craft. For example, for a 20,000 pound craft designated for Category 2 or 3 mooring conditions, calculations may indicate that a smaller line than the minimum line does not exceed safety factors. In this case the strength of the minimum line size (12,000 lbs) should be used as the mooring fitting strength requirement. Equations for computing minimum breaking strength are presented later in this guide.

Figure 1. Mooring Lines Poorly Adjusted

Wind and Current Mooring Restraint Load Determination

Calculations for wind and current forces should be completed for craft in mooring

Categories 2 and 3 that weigh more than 17,000 lbs. Category 2 Service provides for mooring in conditions of up to 50 knots of wind speed and 3 knots of current in accordance with the ‘Heavy

Weather’ section of reference (1). For craft in Category 3 service, wind speed should be 70 knots and current speed 5 knots, unless it is known that the craft will be moored in more severe conditions. Higher than default wind or current velocities should either be specified in the

Contract or provided by the TPOC.

The calculation methodology of reference (1) was developed for larger ships where standardized dock fitting locations and ranges of depth of water under the keel are parameters that are known during the design process. While this is not the case for small boats and craft, the computational process may still be used with simplifying assumptions that account for unknown fitting arrangements on docks, craft to craft mooring assumptions, and depth of water under keel.

These assumptions will be explained later in the guide. In addition, for small boats and craft, the wind speed is not reduced for potential masking effects of the dock, nor is it reduced for the height above the waterline. This provides a factor of safety for gusts above the steady wind speed. The current effects are similarly maximized. Calculations should be performed with both the wind and current from directly abeam. If the vessel has large frontal area above or below the waterline, roughly half the beam profile or more, the bow/stern profile forces should also be checked.

Windage areas and underwater drag profiles may either be calculated directly, using computer software, or approximated using the techniques in reference [1]. Deck equipment, lifelines, and masts should be included in the calculations. As an approximation, the windage area of lifelines may be taken as 10% of their overall height and length. Truss structure mast areas are typically approximated as 30% of their overall height and length. Effective bottom side drag profile may be approximated by multiplying the waterline length by the full load draft of the craft. If actual calculations are performed, any thin vertical underwater members, such as rudders, fins or skegs, should be calculated as twice their actual area to account for their higher drag coefficient compared to the rest of the hull. The total mooring force is the sum of the windage and current forces. Windage force is calculated by the equation (1).

00338.0 WWW VAF Equation (1)

Where: Fw is the windage force in pounds

Aw is the cross section of area presented to the wind in square feet

Vw is the wind velocity in knots (50 knots for Cat. 2, 70 knots for Cat. 3)

Current force is calculated by the equation (2).

67.5 CCC VAF Equation (2)

For DraftWaterDepth 5.1

Where: Fc is the current force in pounds

Ac is the cross section of area presented to the current in square feet

Vc is the current velocity in knots (3 knots for Cat. 2, 5 knots for Cat. 3)

Arrangements

The next step in the mooring system design process is to develop an arrangement for the mooring fittings on the craft. The arrangements for mooring and the design of the mooring fittings and their integration into the craft design should minimize risks to persons carrying out mooring procedures. To the extent practical, mooring fittings and their attachment to the hull and the supporting structure should be designed and arranged so that failure of the fitting or mooring line does not result in injury to personnel aboard the craft, or catastrophic damage to the craft

(e.g., flooding).

A typical U.S. Navy mooring configuration for craft employs two breast lines and two spring lines. Providing additional lines to a dock will typically result in an indeterminate loading condition where the mooring line loads will vary each time the craft is rigged, and where there is likely to be progressive failure of lines from sequential overloading. For example, if three breast lines were required to take the load and rigged: one forward, one aft and one just forward of midship. If the midship line were tighter than the forward line, it would take most of the forward load, overload, and then snap. The two remaining lines would not be strong enough to take the remainder of the load.

The breast lines are run from craft fittings to the dock at an angle as close to perpendicular to the dock as possible to restrain athwartships movement of the craft. The spring lines are run nearly parallel to the dock, forward and aft from their fitting(s), to restrain longitudinal movement of the craft. Fenders are used, as required, to prevent damage to the craft from rubbing against the dock. A typical arrangement is shown in Figure 2.

Figure 2. Typical Mooring Arrangement

The number and types of fittings and their arrangements will vary for different craft.

Typically, a minimum of three bitts or cleats are required on each craft (1 forward and 2 aft) to minimize obstruction of the craft by the mooring lines. Two fittings may be considered if it can be shown that a proper mooring configuration can be developed while mooring alongside a craft of similar size, with spring lines rigged, without obstructing required craft functions or maintenance. More cleats or bitts are typically installed on larger craft. An example of a mooring bitt is shown in Figure 3. Examples of open and closed mooring cleats are shown in

Figure 4. If cleats are used, they should be of the open type. Chocks or fairleads, shown in

Figure 5, may be required Consideration should be given to providing chocks or fairleads as needed to: 1) avoid obstructions (such as stanchions, lifelines, life rails, bulwarks, deck machinery, or deck house structure), 2) minimize any effects of mooring lines on personnel passageways that may be in use while the craft is moored, and 3) prevent an up angle on the mooring line from pulling the line off of a bollard without horns.

Figure 3. Typical Mooring Bitts

Figure 4. Typical Open (left) and Closed Cleats (right)

Figure 5. Typical Closed Chock and Fairlead

Fittings for breast lines should be located as far forward and aft as practical, with the centroid of the fittings (the midway point between the two fittings) being at or slightly forward of amidships, if possible. If the centroid of the two fittings is too far forward or aft, it will result in additional load on the fitting closer to amidships. Fittings should be arranged to allow for the full potential range of angles to dock fittings without the lines rubbing on obstructions. The range of angles for breast lines is up to 45 forward or aft, and up to 45 angled upward from the fitting. The downward angle will be limited by the deck edge. The range of angles for spring lines should be from nearly parallel to centerline up to 45 outboard, and up to 45 upward from the fitting. Obstructions should be minimized within these paths.

Mooring fittings should be arranged to provide fair lead of the mooring lines during all operations to reduce chafing of the mooring lines, damage to the craft or its equipment, or injury to personnel.

Based on the arrangement, and the number of fittings, breast and/or spring lines should be assigned to each mooring fitting. A maximum of two lines shall be assigned to any mooring fitting.

Mooring Line Sizing

The next step in the mooring system design is to determine mooring line loads and select mooring line size.

Safety factors for mooring lines are established based on the method for determining the load on the lines. Based on the American Boat and Yacht Council (ABYC) guidelines, reference

[2], the minimum mooring line size should be adequate for all craft in Category 1. For all craft in Mooring Category 1, and craft in Category 2 weighing less than 17,000 lbs, the 2-inch circumference (5/8-inch diameter) line is considered adequate with no further consideration of safety factors. For other craft in Mooring Category 2 and for all craft in Category 3, the calculated force in combined wind and current is multiplied by 3.0 to provide a safety factor

(when compared to the minimum breaking strength of the line).

As previously discussed, the locations of the fittings on the dock will generally not be known. It is assumed that the direction of pull will be within 45 of the ideal direction of pull whether vertical, horizontal or some combination, therefore the required mooring line minimum breaking strength (MBS) is typically determined by multiplying the ideal mooring line restraint load by √2 (i.e., 1.414) to account for mooring line angle.

For craft where wind and current loads are used to determine the mooring restraint loads, equations (3) and (4) apply for computing required minimum breaking strength for spring lines and breast lines.

For spring lines (for Fw and Fc in the fore – aft direction)

CWCWspring FFFFMBS 24.423 Equation (3)

For breast lines, for Fw and Fc in the athwartship direction, the minimum breaking strength is divided by 2 to reflect the division of the load between the forward and aft fittings, assuming balanced arrangements.

CW

CW

breast FF

FF

MBS

12.2

Equation (4)

Equations (3) and (4) should be used when the fittings for the breast lines are reasonably balanced forward and aft, and the abeam restraint mooring loads are evenly split between them.

If the fittings are not reasonably balanced forward and aft, the restraint load on the fitting closer to midship must be increased. In the case where the loads on the lines and fittings are based on craft weight, the centroid (mid-point between the forward and aft breast line fittings) should be within +/- 5% of the length of the craft from the longitudinal center of gravity. In the case where the loads on the lines and fittings are derived from winds and current loads, the centroid should be between midship and 10% of the length of the craft forward of midship for normally proportioned craft. If the center of above water or underwater features is unusually far forward or aft, the actual center of force should be calculated, and the mooring line loads adjusted accordingly. Once the minimum MBS for the spring and breast lines is determined, Table 1 should be used to select the line size for double braided nylon mooring lines. The actual MBS of the line must be larger than the calculated minimum MBS. A single diameter (or circumference) of line should be used for all mooring lines, based on the highest loads from the breast line and/or spring line calculations. This will reduce logistics supply issues and help minimize confusion regarding which line to use on the part of the crew. For monohull craft of normal proportions, sizing the spring lines the same as the breast lines should be adequate for withstanding the bow or stern direction loads. Check to ensure that spring line sizes are adequate for multi-hulls or other wide hulls.

Table 1. Minimum Breaking Strength for Double Braided Nylon Line

Circumference

(in)

Approximate

Diameter

(in)

Minimum

Breaking

Strength (lb)

2 5/8 12,000

2 - 1/2 3/4 18,400

2 - 3/4 7/8 22,500

3 1 26,500

3 - 1/2 1 - 1/8 36,000

4 1 - 1/4 48,000

Double braided nylon is the preferred choice of mooring line because it is flexible, has better chafing and wearing characteristics than 3-strand nylon, and has good flexibility for absorbing loads. Low stretch lines should be avoided, unless large safety factors are practical. When a boat starts moving and is pulled up short on a low stretch line, the loads transmitted into the craft and dock fittings are much higher than for double braided nylon lines that stretch. Quasi-static calculations are typically performed to determine loads, but they incorporate large safety factors to account for the dynamic environment. Should low stretch lines be considered for use, it is recommended that the strength requirement for the mooring lines and fittings be increased based on the ratio of the modulus of the material used compared to that of nylon. For example, with a Spectra line with 10 times the modulus of nylon, the Spectra line should be 10 times stronger than the nylon line for the given application, and the mooring fitting should be able to withstand 10 times the load calculated based on nylon line. This large design factor renders low stretch lines a very poor choice for mooring applications.

TIP: Polypropylene line is a high stretch material, but it degrades rapidly in sunlight, it has low breaking strength, and it has reduced knot holding characteristics. It is considered good practice to avoid the use of polypropylene line for mooring US Navy craft.

TIP: Line design strength should be based on the MBS of new, dry rope. The MBS of line is less than the average breaking strength. If only average or typical breaking strength data are available, reduce the average value by 15% (i.e., MBS =

0.85 x average breaking strength) to approximate the minimum breaking strength.

Mooring lines should be provided with an eye splice on one end, and large enough to go over intended cleat or bitt horns without undue force, but not so large that they will easily slip off. For craft in Category 1 with a length to beam ratio of 2.5 or greater, mooring line length should be no less than the overall craft length. For other craft, mooring line lengths should be based on the intended mooring arrangement, with allowance for doubled lines, proper rigging on fittings, fender widths, dock fittings 3-feet back from the edge of the dock, and the line path up to 45 from the ideal angle.

Mooring Fitting Strength

The next step in designing the mooring system is to determine the strength requirements for the fittings and their foundations. Mooring fitting strength is based on either: 1) the weight of the craft for craft weighing less than 17,000 lbs, or 2) based on the MBS of the mooring lines attached to it. Mooring fittings may share use as anchoring points or fairleads, reference [3], towing points or fairleads, reference [4], or as hoisting fittings, reference [5]. Fittings that may be used in other services must meet all of the requirements for all of the intended forms of service. Where structural loads or other properties appear to conflict, the higher criteria is generally applied. Consult with the TPOC if there are questions.

The criteria change for craft weighing less than 17,000 lbs is based on the following. The mooring fittings and craft should not break when the craft is hanging from the breast lines, per

Figure 1. If the breast lines are 5/8-inch diameter, with a MBS of 12,000 lbs, run at an angle of to the dock, each line will support 12,000 / √2 = 8,485 lbs. If the lines are reasonably balanced forward and aft, two lines can support a craft weight of 17,000 lbs in a normal mooring configuration, typically based on the Full Load weight of the craft less any loads that would not normally be on board during overnight mooring (e.g., the number of personnel onboard might be reduced either to zero, or to the number onboard for watch, etc.). The mooring lines part on a craft weighing more than 17,000 lbs if it is improperly moored, thus protecting the craft and mooring fittings.

Fittings should be physically sized to take the assigned number of lines in a ‘doubled’ configuration, with an eye splice passed through an open cleat or over a bitt, run out to the dock, and back to the cleat or bitt with a proper cleat hitch. Figure 6 illustrates a proper cleat hitch. A minimum of two line diameters of the bitt or cleat ‘horns’ should be left after all required doubled lines are tied to the fitting to ensure that the last cleat hitch is secure. Figure 7 shows a cleat with a doubled line properly tied off. Bitts, cleats and chocks should also be sized so that the required line configuration can be established without undo force or difficulty in threading the lines. If the mooring configuration also requires fenders to be attached to the mooring fittings, this should also be factored into the space requirements for the fitting. While the cleats, bitts and chocks should be physically sized to handle ‘doubled’ mooring lines, the fittings need only be structurally designed to handle loads based on ‘singled’ lines without exceeding yield strengths. Doubled lines will often be used by the US Navy to help mitigate the risk of line chafing during a storm (the yield strengths of recommended marine structural materials will allow the fitting to survive the doubled line without ultimate strength failure).

Figure 6. Proper Cleat Hitch

Figure 7. Cleat with Properly Rigged Doubled Line

Mooring fittings and their foundations must be designed to withstand the loads of all the mooring lines attached through their potential angles of pull. For example, a single 2.5-inch circumference breast line attached to a cleat without an accompanying chock should be able to withstand the pull of the MBS of the line through horizontal angles from 45 forward of athwartships to 45 aft of athwartships, and vertical angles from 45 above horizontal down to the lead angle to the deck edge. If the cleat had an accompanying chock, the angle from the cleat could be considered fixed in the direction of the chock. The chock would need to incorporate the load of the portion of the line coming directly from the cleat, as well as the load on the line with varying angle to the dock. A sample case for chock loading of a single line is shown in Figure 8.

Figure 8. Load on Chock for 2.5-inch Circumference Breast Line

TIP: If chocks are used for one line, both components of the line tension (coming into and going out from the chock) must be added (using vector math) to develop the net force on the chock. If two lines go through the chock, all four components of line tension must be added to develop the net force on the chock.

Fitting Factors of Safety

A factor of safety is applied to the estimated operational load (e.g., calculated MBS load) on each component of the mooring system. The purpose of the factor of safety is to address unknowns in the load and material properties of the mooring system components to assure that the components do not fail when subjected to the estimated operational load. The factors of safety on the mooring lines are selected so that they will not break in normal use, even when the effects of load and materials uncertainty are considered. The factor of safety on the mooring fittings (bitts, cleats, chocks and fairleads) and supporting structure is based on the breaking strength of the mooring lines so that the mooring lines fail before the fittings. The exception to this is when the weight of the boat is the limiting factor. In that case, the probability of exceeding the calculated line load is minimal. This avoids the danger of possible craft damage or personnel injury due to a failed fitting (propelled by the strain energy of a mooring line).

Mooring lines are selected such that their minimum breaking strength is greater than the expected operational load. Mooring fittings and foundations must be designed with a safety factor no less than 1.01 on yield. Should the mooring line fail, some damage to the craft is probable. Repair or replacement of a fitting or foundation that has some risk of deforming under these conditions is considered to be acceptable.

Marine grade aluminum (5086 and 5083 alloys) and steel have defined yield points.

Appropriate yield strengths must be used for aluminum and steel fittings and foundations, including using ‘as welded’ properties for welded members. Fiber reinforced plastic (FRP) materials do not have well defined yield points. In order to get a similar margin of safety, FRP structures should use a minimum factor of safety of 2.0 on the worst case loads generated by the mooring lines.

TIP: The factor of safety is not intended to compensate for damaged fittings or line.

Fittings and line should be inspected periodically (based on the Boat Information

Book for craft in service), and repaired or replaced before testing or use if damage exceeds specified tolerances.

The bending moment used for design of bitts, cleats and chocks is the product of the design load on the line multiplied by the height above the base of the top of the bitt crossbar, or the height of the line path at the cleat or chock. Components of the mooring fittings such as bitt crossbars or cleat horns must be designed for the maximum bending moment produced by a belayed mooring line, to the same factor of safety. The effect of nonstandard belays that may be used by inexperienced crew should be considered where this might increase the bending moment.

TIP: Many commercial off-the-shelf cleats and bitts have fastener holes that will not accommodate bolts large enough to achieve the same strength as the fitting. These holes must not be enlarged since this may result in failure of the fitting in way of the enlarged hole. Mooring fittings, whether fabricated or off-the-shelf, must be designed so that all parts provide the required strength.

TIP: While bolt shear, bearing and tensile strength for fasteners on commercial off-the-shelf fittings are important, they are typically bolted into weaker materials

(aluminum or FRP). It is typically more important to analyze the craft structural interface for bearing and tear-out stresses.

In the structural analysis of mooring fittings and supporting structure, all potential stress must be analyzed, including: tensile, compressive, bearing and shear, as appropriate. Buckling and other structural stability effects must also be considered.

Commercial-Off-The-Shelf Cleats

If COTS cleats are used, they should be the type with four countersunk machine screws attaching them to the craft. The 2-screw cleats will not provide sufficient resistance to side loading. It was assumed that the limiting factor for these cleats was the machine screws and the craft structure bearing interface with the machine screws. The machine screws must have a solid shank through the thickness of the craft plating or laminate to develop the strengths listed herein.

Calculations for ripping the countersunk heads out of the cleat were not performed. The greatest strength for the cleat is developed when the load is directed roughly 10 o from the centerline of the long axis of the cleat. A chock or fairlead would be required to maintain this axis. Example calculations for Commercial-Off-The-Shelf (COTS) 316 stainless steel open cleats are presented in the following paragraphs.

Any COTS 316 stainless steel open cleat larger than ten inches in length is capable of physically holding one 2-inch circumference double braided nylon line in the proper configuration (doubled line configuration with the eye splice through the open cleat and finished with a proper cleat hitch). None of these cleats, up through the 15-inch cleat, has adequate bolting to withstand the full pull of one 2-inch circumference double braided nylon line in any direction. Assuming that chocks or fairleads are not used, craft with a maximum moored weight of 9,900 lbs. can utilize 10-inch COTS 316 stainless steel open cleats if sufficient cleats are provided to allow for one (doubled) line per cleat. However a 10-inch cleat does not have sufficient physical size to permit two doubled lines on the cleat with an adequate amount of horn protrusion to keep the lines from slipping off. If 15-inch cleats are installed, the moored craft weight should be limited to 16,600 lbs if each cleat is required to hold one line, and moored craft weight should be limited to 8,400 lbs if two lines are required per 15-inch cleat. Custom made bitts or cleats (typically using 2-inch SCH 80 or larger pipe to provide adequate bend radii for the mooring lines) are recommended for craft weighing more than these limits.

The COTS cleats will be bolted into aluminum or FRP. In order to achieve the full strength of the fittings through the bolted connections, the aluminum or FRP must be thick enough to take the bolt bearing loads. A 10-inch cleat with 4 x 5/16-inch solid shank machine screws would need craft plating thicknesses of: 0.20-inch for 5086 aluminum or 0.66-inch for a standard ABS FRP laminate (alternating woven roving and chopped strand mat with up to 65% polyester resin by weight). The 15-inch cleat with 4 x ½-inch solid shank machine screws would need craft plating thicknesses of: 0.21-inch for 5086 aluminum or 0.70-inch for a standard ABS

FRP laminate. A higher strength FRP laminate could result in lower skin thickness. Plating or laminate thickness can be reduced proportionally by the weight of the craft if it is less than the maximum value for the specific cleat size and number of lines. The machine screws must have solid shanks to develop the strengths listed, both in shear and bearing. Metal backing plates may be required to maintain the solid shank through the craft plating thickness to allow the transition to threaded shank after the plating thickness without having to provide custom fasteners.

Threaded machine screws have substantially lower shear and bearing strengths than those with solid shanks.

Material Selection

Mooring fittings on aluminum or FRP craft may be of stainless steel or aluminum, unless otherwise specified.

Mooring fittings on steel craft should be fabricated of stainless steel to prevent corrosion that would occur on steel surfaces due to wear of the protective paint.

Stainless steel used in welded fittings should be AISI 316L (UNS S31603). Cast stainless steel fittings should be 316 alloy (UNS S31600).

Aluminum used in welded mooring fittings should be a suitable marine alloy of the 5000 series. Aluminum plate should conform to ASTM B928 (reference [6]).

TIP: 5086 or 5083 are the preferred alloys for fabrication of welded aluminum mooring fittings, since they offer good post-weld strength retention and are more resistant to corrosion in a marine environment than other readily available 5000 series alloys. If extrusion availability in these alloys is limited, contact the contract

TPOC for alternate material consideration.

Cast aluminum fittings should use alloys suitable for use in a marine environment that will prevent corrosion in service, with particular attention given to intergranular corrosion and stress corrosion cracking. Effective quality controls and inspection methods should be used to avoid deleterious defects in cast fittings, such as porosity, cracks, or other material defects.

Molded plastic mooring fittings should not be used. FRP mooring fittings should not be used without specific TPOC approval.

Material Properties

As-welded material properties should be used for welded components of mooring fittings and their supporting structure.

Properties of FRP laminates used for design of mooring fitting supporting structure should be verified by testing. In the design of bolted fittings, core materials should not be considered to support bolt bearing loads.

Design Details

Mooring bitts and cleats should be sized to accommodate typical belays using the mooring line size required by the design calculations, specified by the contract, or provided by the builder, whichever is largest. Sizing should permit all lines designated to the fitting to be doubled, with an eye splice and cleat hitch for each line, with at least two line diameters remaining on each cleat or bitt horn to prevent the final cleat hitch from slipping off. Chocks should be designed so that the opening will pass an eye splice or bight of line in addition to all of the required double lines. Where it is expected or specified that chafe protection will be used, chocks should in addition be sized to accommodate such protection.

Mooring fittings should be welded to craft structure if the fitting and hull metal are compatible. If bolted fittings are used, access should be provided to both sides of bolted mooring fittings for installation and inspection. Where required, fitting installations should be re-designed or moved as needed to provide access to both sides for installation and inspection. On

FRP craft where only one side of the fitting is accessible and no other solutions are feasible, with permission of the TPOC, standard nuts should be tack welded to the backing plate and bolts with prevailing torque self locking inserts should be used. Inaccessible bolted connections should not be used in metal boats, since welded metal fittings are an alternative.

TIP: To the extent practical, mooring fittings should be designed so that bolts are loaded primarily in shear and bearing, not tension. This reduces the potential for failure resulting from, e.g., a cracked bolt, since the remaining length of the bolt may still provide some load-bearing capability by acting as a pinned connection. This is not always achievable due to load direction and practical limitations on the geometry of the mooring fitting.

Load Path into Craft Structure

The design of the load path from mooring fittings to the supporting craft structure should be direct and linear, avoiding stress concentrations and other unsound design practices. Insert plates, doublers, reinforcing framing and other structure should be designed and fabricated so as to avoid hard points and stress risers.

FRP structure should have brackets, doublers, inserts, and/or special framing installed as necessary to distribute mooring fitting loads. Areas of cored structure in way of mooring fittings should have the core replaced by a high-density foam insert and the laminate suitably increased in thickness to prevent compressive or other failures due to bolting forces or service loads.

Alternatively, the core may be deleted and the laminate thickness suitably increased in way of the fitting.

Design Calculations

Design calculations should account for the loads and stresses on all mooring components throughout the load path, including the mooring lines, fittings and supporting craft structure.

Hand calculations are preferred since they encourage simplified design and direct load paths.

Fabrication and Installation

Welding

Only qualified weld procedures, materials and personnel should be used in the fabrication of welded components for mooring fittings. Welding and qualifications should be to NAVSEA

TO300-AU-SPN-010A, reference [7], and NAVSEA 0900-LP-060-4010, reference [8], or other military standards, American Bureau of Shipping (ABS) or American Welding Society (AWS) standards as specified for the craft. The builder is responsible for maintaining appropriate procedures, materials and personnel qualifications and record keeping to show compliance, or if welding is subcontracted, for imposing the same requirements on the supplier.

Surface Finish

The edges of all metal components should be rounded and smoothed to prevent chafe or other damage to the mooring lines. All potential structure in contact with the mooring lines shall be smoothed and rounded, including the deck edge next to fittings. A radius of three times the rope diameter wherever possible is preferred. Under no circumstance may the bend radius be less than the rope diameter. Where mooring lines might bear on rigid inflatable boat (RIB) tubes, sponsons, or other obstructing equipment or structure; chafing gear should be provided to minimize wear of the mooring lines or RIB tubes. The belaying surfaces of cleats and riding surfaces of bitts, cleats and chocks should have a surface roughness average value, Ra, measured in accordance with ANSI/ASME B46.1, not greater than 3.2 µm (125 micro-inches).

Mechanical Fasteners

Where mechanical fasteners are used for mooring fittings, they should be 316 stainless steel conforming to ASTM-F593 (bolts) and F594 (nuts), Group 2, Condition A (see references

[9] and [10]). Note that Condition A is a solution annealed condition that reduces susceptibility to intergranular corrosion. Washers should be of the same material as the nuts.

While stainless steel fasteners using less common alloys with higher strength than 316 are available (e.g., duplex stainless steels), for logistics support and safety reasons, it is generally preferred that 316 fasteners be used, since this alloy is commonly specified for all Navy boat fasteners. This assures that replacement fasteners are readily available, and reduces the chance that 316 fasteners would be inadvertently substituted for high strength fasteners during maintenance. Use of stainless steel fastener materials with higher strength than 316 alloy must have at least equal corrosion resistance, and such use must be specially requested by the builder and approved by the TPOC. Higher strength alloys typically provide no benefit, since the materials they are fastening typically limit the overall strength of the connection. Higher strength materials should be clearly documented in the Boat Information Book (BIB) and integrated logistics support (ILS) documentation to avoid inadvertent substitution of lower-strength replacement fasteners during the service life of the craft.

TIP: Stainless steel type 304 or 18/8 fasteners are commonly available and are sometimes improperly substituted by suppliers when type 316 is specified. Type 304 or 18/8 stainless steel fasteners do not provide adequate resistance to inter-granular corrosion and stress corrosion cracking in a marine environment and their use is prohibited.

Bolts should be used with one flat washer under the nut and one under the head of the…