TAUT LEG SPREAD MOORINGS: A COST-EFFECTIVE STATIONKEEPING ALTERNATIVE FOR DEEPWATER PLATFORMS

Spread moored floating production platforms are employed worldwide in the exploitation of offshore hydrocarbons. To date they have all employed catenary spread mooring systems (CSMs) using chain or combination wire/chain components. In water depths to 1500 feet and beyond, such simple wire/chain systems become increasingly inefficient and costly. To improve cost efficiency, tighten watch circles and lower vertical load on the platform several innovations have been introduced such as the use of submerged spring buoys ground wire, and ropes made from synthetic aramid fibre. An aramid rope has yet to be employed in a permanently spread moored production platform as industry awaits a better understanding of the long term behaviour of this material. The installed cost of a deepwater CSM remains high despite these innovations and a need exists for even more efficient spread moorings as industry looks to water depths of 3000 feet and beyond. The most promising alternative to the CSM to emerge in recent years is the Taut Leg Spread Mooring (TLSM) system, with short scope legs and where vertical uplift on the anchors is permitted. This paper explains the operating principles of a TLSM, its performance sensitivity to variations of key parameters and shows the cost benefit versus a CSM by specific case studies.