Hawser System Design for Single-Point Moorings

Abstract

This paper presents design data for mooring hawsers for single-point moorings (SPM)-Properties of materials and constructions commonly used are discussed and compared. Factors that affect new and used hawser strength are discussed. Recommendations are made for SPM hawser material, construction, and safety factors for U.S. deepwater ports. Introduction During 1977, Exxon Research and Engineering Co. conducted a study for the U.S. Coast Guard to prepare guidelines for evaluating single-point moorings (SPM) for U.S. deepwater ports. As part of the study, SPM hawser systems were examined. This paper covers the major findings pertaining to hawsers.The hawser system serves to moor tankers to the SPM. An SPM hawser assembly consists of a large synthetic rope spliced to form eyes at both ends, or spliced end-to-end to form a loop, referred to as a strop. The eyes or ends of the strop are placed on thimbles, and chafing chains are attached to the thimbles. Flotation devices are provided to keep the chafing chains and hawsers afloat when a tanker is not moored. A typical SPM and hawser system with a moored tanker is shown in Fig. 1.Our examination of SPM hawser systems focused mainly on design. Information from many sources was gathered and analyzed in a comprehensive study of synthetic rope properties of interest in SPM hawser design. Data also was gathered for analyzing the associated hawser components, such as thimbles, flotation units, and chafing chains. Based on this study, guidelines were prepared for designing SPM hawser systems. Flory et al. presented a complete study of SPM hawser systems, along with studies on other aspects of SPM design. They included information on SPM hawser materials, constructions, breaking strengths, elasticities, safety factors, and assemblies. Supportive information presented here is not always complete. In some areas, we can give little more than conclusions and recommendations. Flory et al. give a more complete understanding of and more background to support our conclusions and recommendations. SPM Hawser Materials Nylon, polyester, and polypropylene synthetic fibers commonly are used for SPM hawsers. Other synthetic and natural rope fibers have properties detrimental for use in SPM'S. Fig. 2 gives comparative load-elongation curves for new nylon, polyester, and polypropylene ropes of the same construction. Table 1 shows comparative breaking strengths for large ropes made of each material.Fiber manufacturers often coat synthetic fibers to obtain desirable characteristics. Fibers of different coatings are differentiated by grade. Fibers used to manufacture rope are coated for maximum strength and to resist abrasion. Fiber manufacturers consider these fiber coatings proprietary, so it is difficult to compare the qualities of coatings used by different manufacturers. The characteristics imparted by the coating greatly influence the performance of an SPM hawser. Nylon Nylon is the most commonly used fiber for SPM hawsers. JPT P. 917