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This paper explains and defines the synthetic fiber rope stretch and stiffness change-in-length properties which are important for deepwater platform mooring system design. It introduces the concept of accumulated elastic stretch, which causes temporary stretching and stiffening of the rope during tension cycling. It proposes how these properties can be determined by rope testing and by computer modeling. It proposes how the change-in-length properties should be used in mooring system design and analysis. Background The offshore industry and the fiber rope manufacturers have now had some experience in using fiber ropes in deepwater platform moorings. Petrobras pioneered this in the mid 1990s. In the late 1990s several class societies 1 2 3 and then API 4 published guidelines for fiber rope deepwater moorings. Tension Technology International with others also published fiber rope design guidelines.5 Those guidelines were based on what was then perceived to be the best design methods and the best test methods for the rope properties needed to carry out those methods. Since those guides were published, several fiber rope mooring systems - Mad Dog 6 7 and Red Hawk - have been designed for the Gulf of Mexico. A number of large fiber ropes have been tested and analyzed for use in those and other projects. Also, findings are now available from several significant fiber rope research programs which were conducted while and after those guides were written. Introduction The unique stretch and stiffness characteristics of synthetic fiber ropes are important in the design and analysis of deepwater platform mooring systems. Mooring system designers are experienced in designing with wire rope and chain catenary mooring lines. The stiffness characteristics of wire rope and chain are essentially linear. These components do not experience permanent stretch until high tension. With these components, the principal mooring system restoring force is produced by the catenary effect. As mooring water depths increase, there are incentives for using fiber ropes, as discussed in other papers. But with fiber rope, the principal mooring system restoring force is produced by the tension vs. stretch (stiffness) characteristics of the rope instead of by the catenary effect. Mooring system designers are still trying to understand the unique stretch and stiffness characteristics of fiber ropes and to incorporate them into traditional mooring design procedures. The stretch and stiffness characteristics of typical synthetic fiber ropes are time dependent and non-linear. Fiber ropes experience both elastic and permanent stretch. Conflicting and confusing terminologies are sometimes used. Some important rope properties have only recently been adequately understood and explained. Here the term stretch refers to any change in rope length hich is produced by tension. Stiffness is the relationship between stretch and applied tension. Other terminology is introduced in the text and summarized in Nomenclature at the end of this paper.

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The Mad Dog Floating Production System (FPS) will be the first truss spar to use polyester for a permanent mooring system. The breaking strength of the polyester ropes is also the largest ever-made being in the 2000 mT range. As such, prototype testing to validate the breaking strength capacity of the spliced ropes was important along with gathering performance data of the ropes to be used in the mooring design and global performance analyses of the FPS. Since this is the first spar to use a polyester mooring, and since loop currents typically govern the mooring of a spar in the Gulf of Mexico, a better understanding of the "static drift stiffness" or extension of the rope was required. Thus far, polyester moorings have been predominately used by Petrobras in the Campos Basin for semi-submersible FPS and Floating Production, Storage and Offloading (FPSO) units and thus only the dynamic and drift stiffness has been important. Much data is available for these two stiffness, but very little on the static drift stiffness was available. Consequently, a model had to be developed along with procedures to test the ropes to derive this stiffness. This paper will discuss the prototype test plan, which basically follows API RP 2SM but with several deviations, in particular to obtain more dynamic stiffness and static drift stiffness (extension) data over a range of mean loads, load ranges and rate of loading. In addition, axial tensioncompression fatigue testing was conducted explicitly to the mean load, range of loading and number of cycles expected to occur to the mooring while in-service to confirm this is not a problem since tension did fall below 5% of MBL. Finally a stiffness model will be presented that can be used for mooring design / global performance analyses for FPS using a polyester mooring system. Information presented in this paper will help designers of polyester mooring systems and also should impact the future revision of API RP 2SM. Introduction Petrobras has designed and installed numerous polyester mooring systems to semi-submersibles FPS and Floating Production, Storage and Offloading (FPSO) systems (Costa, 2001). However, to date, polyester has not been used in a permanent mooring system outside of the Campos Basin. BP Exploration & Production Inc. ("BP") and the Mad Dog project partners changed that when the taut-leg polyester mooring system was installed on the truss spar in early 2004. The Mad Dog project was facing a significant hurdle in trying to keep development cost down so the company and the partners, BHP Billiton Petroleum (Deepwater) Inc. ("BHP Billiton"), and Union Oil Company of California ("Unocal"), could sanction the project. In order to control cost, it was important that the hull be fabricated and transported to the Gulf of Mexico as a single piece. The size and weight of the hull was already challenging the capabilities of the worldâ??s heavy lift vessels and in addition, payload was increasing to meet topsides requirements. Thus the project team investigated using a taut leg polyester mooring system.