Development of an Engineers' Design Guide for Deepwater Fiber Moorings

Abstract Various operators are currently pioneering the use of synthetic fiber ropes for mooring platforms in deepwater. The first systems were installed in 1997 and there is now a demand for industry guidance to assist engineers to design and detail such moorings. The paper describes an industry-funded initiative to develop such a guide using the best available data. Design methodologies conventionally adopted for steel wire and chain moorings must be modified for the design of synthetic fiber rope moorings. This is mainly due to their very different density and stiffness characteristics. In addition fiber ropes raise questions of abrasion resistance, sensitivity to compression and other characteristics which are different from steel components. Finally the experience base for synthetic fiber moorings is more limited. The paper describes the principal content of the guide. It deals with the characteristics of fiber ropes and the simplifications that have been developed to incorporate their unique features into feasible analyses. The perceived limitations resulting from the current status of test data are described. This forms a fundamental step in identifying the feasibility of applying structural reliability techniques to the problem in order to rationalise the choice of factors of safety. Introduction and Background In January 1999 the "Engineers' Design Guide to Deepwater Fiber Moorings"1 (abbreviated below to the "Guide") was published on behalf of an international group. The group was formed in 1996 in support of a Joint Industry Project and consisted of Oil Operators, Fiber Manufacturers, Rope Makers, Design and Installation Contractors, Classification Societies and Government Agencies. The total number of supporting participants was 31 and the full list is provided in Table 1. The sponsoring participants contributed specialists toattend the steering committee meetings that reviewed the document and also funded the preparation and development of the drafts by the project secretariat. The idea of producing such a Guide had developed from an earlier joint industry project that looked at the possibility of using high technology fibers for deepwater moorings2. It included a major experimental programme to obtain data on fatigue endurance limits for a variety of fiber types, rope construction and termination types. The range of tests undertaken is listed in Table 2. The results from this project became available at the same time that the Oil Industry was increasingly looking at deepwater locations. Fiber tethers offer many advantages for such moorings and there was considerable enthusiasm in the industry to build on earlier experimental projects to develop a guide that could be used by designers. Objectives and Principles Several principles influenced the way in which the Guide was developed. It was the primary objective of the study to develop a document that would assist practising engineers faced with the challenge of designing deepwater fiber rope mooring systems. The document was specifically not intended to be an "official code of practice" with all the legal implications of such a claim. Rather it was to be a first, best attempt at laying out the features of fiber rope moorings, which are different from conventional mooring systems, and taking the designer through the stages that must be considered.