Selection of FPSO Spread Mooring Systems in (Deep Water) Offshore Nigeria

Selecting FPSO spread mooring composition for use in deep water environments entails evaluating the fatigue performance of the composition together with other very important factors. In this work, 4x3 spread mooring composition and 4x2 spread mooring composition were compared for suitability in deep water offshore Nigeria on basis of fatigue performance and the following objective criteria; Cost, Restoring characteristics, Tandem offloading, Environmental applicability, Loads on the mooring line, Length of line, Redundancy and Maintenance ease. The hydrodynamic analysis was performed using Orcaflex software with appropriate metocean data typical to conditions offshore Nigeria for JONSWAP spectrum. Fatigue results derived from Orcaflex (hang-off and TDP) for the two compositions with similar material configurations of chain-wire-rope-chain indicated high values of (867 years: hang-off, 3232years: at the touch down point) for the 4x3 composition and less values of (349 years: hang-off, 975 years: TDP) for the 4x2 composition. The outcome of the analysis shows that fatigue results at TDP (1626 m) and hang-off (0.00 m) satisfy the minimum acceptable fatigue requirement of 200 years.