Evaluation of Advanced Fibers for Deepwater Synthetic Fiber Mooring Systems

Abstract Mooring systems utilizing polyester ropes have facilitated exploration and production of deepwater fields and represent a step-change in mooring system design. As the industry expands into deeper and deeper water, the next logical step is to determine the effectiveness of additional advancements in mooring technology. One of the possibilities to enhance mooring system performance is the utilization of advanced fiber ropes constructed from such materials as aramids, high-modulus polyethylene (HMPE) and polyethylene naphthalate (PEN). These advanced fiber ropes have axial stiffness (EA) properties up to three times an equivalent polyester rope. The effect of mooring ropes with different levels of stiffness on the overall mooring system performance are evaluated for a semi-submersible floating production system located in water depths ranging 5000 feet to 15000 feet (1524 m to 4572 m). The basis for the semi-submersible floating production system (FPS) used in this study was developed previously as part of DeepStar CTR 7404, as reported in OTC 18467 [1]. The key learning from this study is that the steel components in the mooring leg create a catenary effect between fiber rope segments, so the mooring system is not a perfectly taut line. As a result the axial stiffnesses of the rope segments are less of a factor affecting mooring system response than may be expected. The results shown in this paper are part of a study performed for the Minerals Management Service (MMS) [2]. Introduction Over the past twenty years polyester rope mooring systems have developed into the preferred technology for deepwater mooring systems, especially in Brazil and the Gulf of Mexico. The light weight and high strength of polyester ropes allow a mooring system to provide the horizontal restoring force necessary to maintain station-keeping while minimizing the vertical forces imparted to the vessel. The natural desire is to evaluate the potential benefits that advanced fiber ropes may provide. The term "advanced fiber" generally refers to improved thermal, chemical and mechanical properties of higher-modulus "stiffer" fibers than polyester. Advanced materials evaluated include, for instance, aramids, HMPE and PEN. This paper summarizes some of the results developed for a study commissioned by the Minerals Management Service (MMS) [2]. The focus of this paper is to evaluate the effect of fiber rope stiffness on the vessel motion response of a floating production system. The floating production system described in OTC 18467 [1] was used as the basis for this study. This system consists of a four column semi-submersible with a ring pontoon located in a water depth of 10000 ft (3048 m). The mooring system described in the reference was modified for water depths of 5000 ft (1524 m) and 15000 ft (4572 ft). The system response was determined for seastates ranging from small fatigue seastates to a 1000-year hurricane event. The effect of fiber rope stiffness was evaluated by adjusting the rope stiffness by a factor of two and three above the nominal polyester rope stiffness. The fiber rope stiffness levels are described as low, medium, and high throughout this paper. A summary of the cases evaluated are shown in Table 1. The primary results of interest presented are the mooring tension response and the vessel motion response (offset, heel angle, and heave). The static response of the mooring system is also evaluated with static analyses. A discussion of the effect of rope stiffness on the overall mooring response is also provided at the end of this paper.