Spectral Fatigue Assessment of Agbami FPSO Hull for Onsite and Seagoing as per ABS’s SFA Guidance

Agbami FPSO is a floating facility for production, storage and offloading of oil at OPL 216/217 offshore Nigeria of the central Niger delta. The FPSO is positioned with spread moorings in water depth of about 1500 m for the service life of 20 years. As per the design requirement, the hull of FPSO shall be designed to meet ABS’s SFA (Spectral Fatigue Analysis) notation and the seagoing condition. Seagoing is a mandatory condition additionally required by Client, assuming a navigating vessel of North Atlantic. But, to conservatively assess Agbami for 60 years at its Onsite except specific details was taken into account, which is related to the uncertainties in S-N data & Palmgren-Miner’s damage rule and etc. Normally the off-western Nigerian sea has swell-governed environment and the multi-peaked spectral characteristics with wave and swells. For the spectral fatigue analysis, Jonswap spectrum for wind wave & Gaussian spectra for swells were taken into consideration as a representative in case of Onsite. Thus, in order to achieve a single damage, the combined spectral method for Onsite was introduced on the basis of EMDC’s FPSO FMS (Fatigue Methodology Specification). On the contrary, Walden’s wave scatter diagram as recommended by ABS for Seagoing was applied. The forward speed of FPSO in Seagoing case was also taken into account by WASIM, a hydrodynamic tool of DNV and compared to the results of PRECAL, ABS’s hydrodynamic code to ensure the validity of the analysis. The post-process to calculate fatigue damage was carried out using the in-house program to analyze fatigue of FPSO. The results from the analyses were found that the Seagoing led consequentially critical fatigue damage for most of hull structures more than Onsite that has moderated sea state.