Reliability of Mooring Systems: Application to Polyester Moorings

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Abstract The trend to deep water oilfields has accelerated the development of mooring systems. Future moorings requirements represent a considerable step out on current practice, Polyester moorings offer considerable economic and operational advantages. The authors have previously used experience gained orr an FPS in combination with reliability analyses tn predict the failure probability of two harsh enviromnent FPSO Riser/Mooring systems. Recent work has built on this experience to predict the failure probability of polyester mooring systems and compare it with steel wire mooring performance. The paper outlines the previous work which provides the common basis for comparative reliability predictions of polyester moorings. The new polyester mooring reliability predictions are presented which make a significant contribution to demonstrating the acceptability of polyester moorings for production systems. Introduction The application of polyester to the moorings of production systems brings considerable potential economic advantages. Firstly the lower in water weight compared to steel wire rope systems significantly reduces the buoyancy in the production system required to support the mass of the mooring system with associated wave load reductions. For a 2000m water depth FPSO mooring in the GoM the still water mooring weightload is of the order of 220 tonne compared to a polyester line pre-tension of 130 tonne. Secondly the reduced offset afforded by taut polyster mooring system can provide considerable benefits in the design of the risers. For very deep water risers steel pipe rather than flexibles is required. For Spars and Semi's the riser bending forces and fatigue are a direct function of offset. Thirdly, the use of taut polyester lines reduces the mooring system footprint thus providing greater flexibility in subsea achitecture. For instance, the ability to route the flowlines so that these may be brought up to the FFSO in clusters can be economically achieved when the footprint is small. Steel wire rope and chain mooring systems have become accepted practice largely as a result of decades of experience and development of the materials for shipping and land based purposes and the subsequent development of oil industry specific materials and mooring design standards with appropriate design margins. (Refs. l&2). Polyester does not have the option of decades of development including the trial and error process inherent in early applications. The development of polyesler as a mooring material was driven primarily by scale aud fill sealc laboratory investigations of competing materials in the UK. The laboratory work, whilst providing a sound understanding of extreme load fatigue capability and the effect of slack or local compression in a rope, was not able to fully characterise long term nonlinear stiffness.