Fatigue damage assessment of mooring lines under the effect of wave climate change and marine corrosion

Fatigue is one of the major failure modes of offshore structures typically operating continuously for 20-30 years at sea. Gradually varying factors, such as wave climate change and marine corrosion, can contribute significantly to the fatigue damage assessment during the entire design life; however, these factors are generally ignored. This study aims to propose a new fatigue damage assessment method, wherein the effects of wave climate change and marine corrosion on the fatigue damage assessment of mooring lines are considered by introducing a reasonable model into the hull/mooring system coupled dynamics. The fatigue damage of the mooring lines used for a semi-submersible platform is assessed in the time domain using the proposed method. The results indicate that wave climate change and marine corrosion have a significant influence on the distribution of the mooring lines' tension ranges and fatigue damage assessment. When the effects of both wave climate change and marine corrosion are ignored, the cumulative fatigue damage during the entire design life is underestimated by up to 35%. Thus, it can be concluded that a design without the consideration of the gradually varying factors poses significant security risks.