New Parallel Wire Cable Tether Study for Deepwater TLP

ABSTRACT This paper describes New Prefabricated Parallel Wire Strand or shortly "New-PPWS", the result of joint industry research on the new concept of parallel wire cable for TLP. New-PPWS has a slightly twisted parallel wire construction with double layered polyethylene and a laminated lead film for water tightness. Theoretical and experimental studies were carried out to establish the technical and economical feasibility of the concept. The study includes fatigue test, anti-corrosion test, installation simulation through cable handling test and total system design. The inspection method to investigate the structural integrity during operation were also developed. Following these research works, a case study including structural design and installation procedure design with related equipment at water depth of 1,500 ft was carried out. The result of 2 year field test as a buoy mooring cable for 1,800 ft water depth is also discussed. INTRODUCTION A tether is regarded as the most vital component to the reliability of TLP. Conoco used the forged pipe tether in Hutton, however the material cost encouraged potential TLP users to investigate more cost effective alternatives. This study was initiated from this point of view, focusing on the structural performance of new parallel wire cable which has been developed for cable stayed bridges and expected to yield a less tether size and reduction of cost. The first research project was planned and carried out under the sponsorship of Getty Oil Company and Agip S.p.a. from 1982 to 1983. The purpose of the project is to establish the technical feasibility of New-PPWS as a tether component in sea water environment. Moreover, the New-PPWS tether system was evaluated in terms of tether manufacturing, structural design, installation design and total cost. Following these works, New-PPWS was firstly applied to the offshore use in Japan in 1984, which was for the mooring cable of the fish-gathering buoy. The cable was retrieved after two years of service and the integrity of the system was confirmed. BASIC CONCEPT OF NEW-PPWS FOR TLP New-PPWS shown in Fig. 1 is classified as parallel wire cable where all wires are twisted with the same pitch, thus forming the complete metal touch between the neighboring wires. Twisting is introduced to prevent snaking when it is reeled on a drum. To make use of this characteristic in sea water environment, various requirements as listed below are needed to be satisfied.Structural property High strength, high elastic modulus and long fatigue life under extremely severe loading conditions are required.Corrosion protection Perfect prevention from seawater intrusion into cable structure under high pressure is required. Mechanical strength of corrosion protection layer is another requirement, and the repairing method should be established.Inspection Detection of seawater intrusion into cable structure which may lead to wire breaks is necessary.Handling performance Easy manufacturing, transportation, installation and termination are desirable. Components of New-PPWS have following characteristics and fulfill above requirements.