Hawser System Design For Single Point Moorings

Abstract

This paper presents design information for mooring hawsers for single point moorings. It is based on a recent study, conducted by Exxon Research and Engineering for the U.S. Coast Guard, which will serve as a guide for the evaluation of SPMs used at U.S. deepwater ports. Information from many sources has been drawn together and analyzed to present a comprehensive study of synthetic rope properties of interest in SPM design. Much of the information has not been published before. Properties of nylon, polypropylene, and polyester fibers, and the types of rope construction used at SPMs are discussed. The elasticities of ropes of various materials and constructions are compared. The methods which are used to rate large-rope breaking strengths are evaluated. Factors which affect new and used SPM hawser strength are discussed. Chafing chains, hawser floatation devices, and the methods by which hawsers are attached to the tanker are also discussed. Recommendations are made for SPM hawser material and construction. Factors of safety for SPM hawser design for U.S. deepwater ports are also recommended. These factors of safety are higher than those previously adopted by the industry. The data and findings presented in this paper will be of interest to SPM designers and operators and to other users of large-diameter synthetic rope. Introduction During 1977, Exxon Research and Engineering Company conducted a study for the U.S. Coast Guard to prepare guidelines for evaluating Single Point Moorings (SPMs) for U.S. Deepwater Ports. As part of the study, SPM hawser systems were examined, and 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 end-to-end spliced 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. Floatation 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 tanker moored is shown in Figure 1. The examination of SPM hawser systems focused mainly on their design. Information from many sources was drawn together and analyzed in a comprehensive study of synthetic rope properties of interest in SPM hawser design. Information was also gathered and analyzed on the associated hawser components, such as thimbles, floatation units, and chafing chains. Based on the study, guidelines were prepared for the design of SPM hawser systems. The complete study of SPM hawser systems, along with studies on other aspects of SPM design, is presented in "Guidelines For Deepwater Port Single Point Mooring Design" (l) available to the public through the National Technical Information Service, Springfield, Virginia 22151.