Remaining Useful Life Estimation of SPM and FPSO Mooring Chains Under Corrosion Effects

Floating production, storage and offloading (FPSO) and single-point mooring (SPM) calm buoy mooring chains represent the single point of failure for most offshore floating terminals. However, the recent rise in mooring line failures on FPSO and SPM units, particularly in West Africa, has underscored a critical industry blind spot: the underestimation of corrosion effects in remaining useful life calculations (RUL). Traditionally, remaining strength models assume pristine geometric conditions or apply a uniform corrosion allowance. However, in-service inspections of SPM and FPSO chains frequently reveal accelerated wastage in the splash zone, none of which is adequately captured by linear degradation models. This paper addresses this gap by developing a dedicated RUL estimation methodology centred on corrosion-driven deterioration. We analyzed in situ data from 76 chain samples to characterize corrosion rate distributions specific to the R3 mooring chain grade. The resulting model demonstrates how localized corrosion reduces the net cross-sectional area of the mooring chains' links. By moving from a time-based replacement philosophy to a corrosion-informed RUL prognosis, this work provides operators with the tools to optimize data interpretations, plan for chain replacements and avoid being faced with multiple line failures or domino effects and ultimately system failure while in operation. Over approximately 20 years of service, an operator replaces the top chains of its FPSO mooring chains twice, despite being originally selected for a 20-year service life. This occurs after the first 10 years and, secondly, after the second 10 years due to the excessive corrosion wastage on the FPSO and SPM mooring chains in West Africa. These decisions were reached through detailed mooring chains surveys, measurements and assessments using class society equations. The quantitative results showed that the reductions in the mooring chains' minimum breaking strength (MBS) fall within the range of 10-20%.