Fatigue assessment for mooring chain of floating offshore wind turbines in monsoon and typhoon regions

Offshore areas such as the Taiwan Strait have abundant wind energy resources but are influenced by both monsoons and typhoons, presenting unique challenges for floating offshore wind turbine (FOWT) mooring systems. This research evaluates the fatigue damage in mooring lines in a region plagued by monsoons and typhoons. Fatigue often governs mooring design when conventional curves are applied without considering mean tension effects. Clump weights, while commonly applied in shallow water to reduce offsets, have not been studied thoroughly for their impact on fatigue. This study develops a long-term fatigue assessment framework, incorporating regional metocean conditions, coupled hydrodynamic simulations in OrcaFlex, and fatigue damage estimation through Miner's rule and rainflow counting. Accounting for mean tension can significantly extend fatigue life and enable designs that satisfy criteria. Sensitivity analyses were also conducted on pretension, anchor radius, and the use of clump weights. Results indicate that clump weights can extend fatigue life by 47%, primarily by increasing gravitational restoring forces and introducing localized inertia that dampens dynamic tensions. This work also identifies the touchdown area as the critical fatigue hotspot in monsoon- and typhoon-prone regions, contrasting with wave-dominated regions where the top chain typically suffers more fatigue. These findings offer valuable insights for extending fatigue life in harsh shallow-water conditions for FOWT mooring systems.