Abstract

The growth in deepwater application of synthetic fiber rope moorings has created a need for an industry design code for such mooring systems. American Petroleum Institute (API) and the DeepStar consortium took the initiative by having the American Bureau of Shipping (ABS) consolidate existing design guides, joint industry research results, and industry operation experience into a new API Recommended Practice (RP) -- 2SM [Ref. 1]. This paper presents an overview on the development of this RP.

Synthetic fiber ropes have many advantages for deepwater moorings, but their unique properties and influence on the mooring system performance must be properly accounted for in the design. Furthermore, rope manufacturing, testing, and deployment require special considerations. This RP is therefore developed to address the unique features associated with mooring design and analysis; rope design, testing and manufacturing; rope handling and installation; and rope inspection and maintenance. It is hoped that use of the RP can help achieve synthetic fiber rope mooring systems that have similar reliability as their steel counterparts, and can improve efficiencies in deepwater field developments.

The paper outlines the development basis and principles, the participation of an Ad-Hoc API Work Group consisting of operators, designers, regulatory and government agents, rope manufacturers, and installation contractors. It also discusses the key technical issues identified and resolved, which include design concerns and factors of safety for overloading, fatigue, stiffness, creep rupture, minimum tension, rope damage evaluation, and discard criteria.

Introduction and Background

DeepStar has been actively involved in the study and testing of alternative deepwater mooring components since its inception in 1992 (Fig. 1), most notably polyester moorings. The DeepStar funded polyester taut leg mooring (TLM) test system was installed by Aker Marine Contractors in 1996 and retrieved in 1998 after a 27-month service in the Gulf of Mexico. The intent of the test program was to gain installation and handling experience, to determine extent and rate of creep of rope samples and bedding-in requirements, and to study other effects on the rope due to exposure to a realistic environment.

In January 1998, a workshop on synthetic fiber mooring rope was co-sponsored by the Minerals Management Service (MMS) and the Composites Engineering and Application Center (CEAC) of the University of Houston [Ref. 2]. During that meeting, participants acknowledged that use of synthetic fiber ropes is probably the only practical solution for ultradeep water moorings. MMS also pointed out the need for an industry standard for such mooring systems, and that an API/ISO code is preferred over other design guidelines.

As a fitting way to culminate several years of development in synthetic mooring technology, and to take advantage of the collective learning and expertise of DeepStar members and the industry as a whole, DeepStar volunteered to undertake this task. In September 1998, API and DeepStar initiated the present work by having ABS consolidate existing design guides, joint industry research results, and industry operation experience into the new API RP 2SM.

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