Tahiti Spar as a Wet Tree Concept

Abstract

This paper will address the advantages and challenges involved in designing a truss spar for wet tree application. The spar has traditionally been used as a dry tree concept with top tensioned production risers. Chevron's Tahiti project in the Gulf of Mexico will be the first wet tree truss spar. For Tahiti, the favorable motions characteristics of the spar enabled successful fatigue design of the sour service HPHT steel catenary risers without use of corrosion resistance alloys. Use of polyester mooring significantly improved hull vortex induced motions response to loop currents relative to traditional chain-wire-chain mooring and enabled SCR departure angles to be reduced. HPHT production riser were hung off using pull tubes, which simplified riser installation and eliminated uncertainties related to performance of flex joints under HPHT conditions. Challenges for a wet tree truss spar design include increased hull density and float-off requirements due to smaller moonpool dimensions than traditional spars with top tensioned risers. In addition, the large number of pull tubes created challenging VIV fatigue designs and as a result, stringent welding and NDE requirements were imposed for the pull tubes in the shipyard.