Fibre rope tether moorings offer many advantages for deepwater floating platforms. A moored system will probably be cheaper and more reliable than a dynamically positioned (DP) system and the high strength to weight ratio of synthetic fibres offers particular advantages over steel wire rope or chain. The low immersed weight of fibre ropes removes the conventional catenary shape of the mooring and replaces it with the taut leg configuration in which an understanding of the rope elongation properties are critical to a successful design. As a result the design methodologies conventionally adopted for steel wire and chain moorings must be modified. Fibre ropes also raise questions of abrasion resistance, sensitivity to compression and other characteristics that are different from steel components. Finally the experience base for synthetic fibre moorings is more limited. The authors describe some of the stages involved in such a deepwater mooring design with reference to the recently available guidance documents prepared for designers by Joint Industry Projects.
During the preparation of the recently published Engineers Design Guide to Deepwater Fibre Moorings (1999) the drafting team prepared a number of worked examples to illustrate some of the features of designing deepwater fibre moorings using synthetic fibre ropes. This design guide (referred to below as the "Guide") has been developed as a result of a Joint Industry Project sponsored by an international group (see Table 1). It followed the Joint Industry Project Fibre Tethers 2,000 (1995) which included a major fatigue testing programme of fibre ropes.
In order to illustrate some of the characteristics of fibre moorings, which make their design different from conventional moorings this paper presents a design sequence based on one of the examples published with the Guide.