Aberdeen: DNV has endorsed Sigma Offshore’s new FPSO turret mooring system that can be installed without drydocking and which uses a fraction of the steel required for traditional turret systems. Suitable for harsh conditions, such as the Arctic, the new technology is expected to significantly reduce installation and operational expenses.

Installing a Sigma Smart Mooring System on an FPSO requires approximately 500 tonnes of additional steel whereas conventional fluid transfer systems use around 5,000 tonnes. Tank capacity is not reduced and only limited modifications are required to the vessel as hull invasion is minimal. The installation can be done quayside at approximately 30 per cent less cost than conventional systems and without disruption to any work being undertaken on the FPSO’s process equipment.

DNV sponsor Robbie Williamson says the Smart Mooring System utilises a unique gimbal system with bearings acting around three perpendicular axes. This achieves greater weather independence for the mooring as it allows the FPSO rotate about the centre canister while the inner trunnion ensures minimised bending moments occur in the risers and other structures during heavy seas.

Designed for small to medium fields where the production through 6-8 risers is 50,000 barrels a day, the system allows for full weather-vaning and, if needed, the FPSO can be disconnected from its turret and flow lines within seconds.

DNV’s endorsement of the technology involved application of their Qualification Procedures for New Technology. The process provides a systematic approach to ensuring the system functions reliably within its specified operational limits.

“DNV’s role included project management support, document review, design assessment, simulations and testing,” said Eugene Ogosi of DNV. During this time, DNV was able to propose design improvements to enhance the reliability of the new technology based on their extensive experience with offshore equipment and operation.

The system and its qualification process are described in a paper presented at Offshore Europe 2011 which was co-authored by Robbie Williamson of DNV and David Galbraith and John Donovan of Sigma Offshore Limited.