Study on Fatigue Assessment Considering Mooring Chain Wear for Floating Offshore Wind Turbines

Abstract For the achievement of a carbon-neutral society by 2050, floating offshore wind turbines (FOWTs) have high expectations, especially in regions deeper than a water depth of 60 m because of the vast potential in Japan. For station keeping of FOWTs, the application in such a region generally requires mooring chains. Since the reliability assessment of mooring chains is one of the key issues to ensure the integrity of FOWTs during the design life, the mooring design limit state shall be properly evaluated. For investigation on the effect of wear and corrosion on fatigue damage, some researchers consider the only uniform decrease in chain diameter, which is insufficient to represent a change in the stress distribution due to local deformation. The local diameter loss could increase local stress and lead to the unexpected early failure of the mooring chain. In order to estimate the fatigue life accurately, it is necessary to estimate the local diameter loss due to the interlink wear considering the motion of floating structures. In this study, such fatigue assessment method is investigated by assuming the installation of UMaine VolturnUS-S reference platform developed for the IEA wind 15MW wind turbine at Ishikari Bay, Hokkaido, Japan. This is based on a method to estimate interlink wear quantitatively by finite element analysis considering the plastic deformation caused by the proof load test, and coupled dynamic analysis of a floating structure and mooring system using OpenFAST. As a result, the estimated wear amount at the fairlead has been much larger than the specified allowance. Additionally, from the estimated fatigue damage it has been found that mooring chains cannot withstand design life without replacement, and deterioration due to interlink wear and corrosion could lead to unexpected early failure of mooring chains.