Abstract
This research work analysed the effects of seabed topography on the fatigue life of a steel catenary riser in the Gulf of Guinea. A fatigue analysis was carried out for two risers namely PFL-06 and PFL-11 on the Portside and starboard sides of an FPSO respectively. A fatigue analysis was run for a flat seabed and a 3-D seabed using Orcaflex’s 3-D seabed feature using the same set of input data. The metocean and environmental data were obtained from Field X riser design document. The fatigue life on the touch-down point for PFL-06 on a flat seabed was calculated as 267 years. On a 3-D modelled seabed, the fatigue life on the touch-down point of PFL-06 is 304 years. For riser PFL-11, the fatigue analysis gave 1231 years and 425 years for a flat and a 3-D seabed respectively. It was observed that since PFL-06 has an outer diameter of 0.324m and almost the same water depth, there was no much difference in the fatigue analysis for a flat seabed and a 3-D seabed. PFL-11 with a smaller outer diameter of 0.273m gave a far lower fatigue life of 425 years on the touch-down point for a 3-D seabed as against a fatigue life of 1231 years on a flat seabed. The maximum effective tension on the TDP for the two risers were lower for a 3-D seabed than a flat seabed. The research found out that assumptions of a flat seabed in the fatigue analysis of the touch down zone of steel catenary risers could have adverse effects in the analysis.
