The Polyester Rope Taut Leg Mooring Concept: A Feasible Means For Reducing Deepwater Mooring Cost and Improving Stationkeeping Performance

Abstract

Abstract The polyester rope taut leg mooring system offers a unique opportunity to reduce deepwater mooring system cost, while simultaneously improving station keeping performance. These gains are over catenary or taut leg systems designed using all steel components. This paper builds upon work presented at prior OTC conferences and focuses on concept feasibility and implementation. Feasibility is addressed from a systems basis including fiber and rope selection, definition of mechanicalproperties, mooring system integration, and effects of long term usage. Implementation is believed practical based oncurrent technology and in-place manufacturing capability Available cyclic tension test results for polyester rope suggest a comparable fatigue performance to wire rope. The mostsignificant challenge facing application of the polyester taut leg mooring concept is the lack of in-service experience compared to conventional steel catenary mooring systems. Introduction The geometry of the polyester taut leg mooring system is compared in Figure 1 with a similar capability all-steel catenary mooring. The novelty of the polyester taut leg concept is that the restoring force in surge, sway, and yaw is derived primarily from line stretch. This method of developing restoring force differs markedly from conventional steel catenary systems which develop restoring force primarily through change in catenary shape. The feasibility of thepolyester taut leg mooring system is made possible because of the much lower modulus of polyester fiber compared to steel and other high-strength non-steel fiber materials. The line geometry, low in-water line weight, and restoring force generation of the polyester taut leg concept allow for a number of key cost and technical savings over all-steel systems including:Reduced line length and seafloor footprintReduced mean and low frequency platform offsetsReduced fairlead tensions and increased platform payload. In the taut leg geometry, the platform mooring lines are terminated at the seafloor with a vertically loaded anchor. The relatively small footprint compared to conventional catenary mooring designs may be desirable for multi-platformdevelopments or in cases when a there is a high density of infield sub sea equipment. Increased platform payload is made possible by the low in water weight of the polyester rope compared to steel wire and chain. The reductions in offset and dynamic line tension arepossible due to polyester having a relatively low axial stiffness, while the system having a high horizontal stiffness since all of the restoring force is developed from line stretch. These technical gains appear possible in conjunction with a reduced system cost as seen in the work of Chaplin and Del Vecchio (1992) and D'Souza et al. (1993). These papersprovide specific examples of the performance and cost advantage of the polyester taut leg mooring system over comparable steel and other synthetic fiber alternatives. Theobjective of this paper is to demonstrate overall concept feasibility and to illustrate its implementation using current technology and in-place manufacturing capability. Figure 1: Polyester taut leg mooring system. (Available in Full Paper)