Substructure flexibility effects on semi-submersible floating wind turbine fatigue

Fatigue, driven by long-term stress concentrations arising from continuous dynamic loading, is a key cause of damage and potential failure in offshore structural connections. This issue is further amplified in floating offshore wind turbines (FOWTs) due to their dynamic sensitivity and exposure to coupled aero- and hydro-dynamic loading, emphasising the need for reliable fatigue assessment methodologies. This study investigates the influence of global member flexibility and local joint flexibility on the dynamic and fatigue response of the OC4 semi-submersible FOWT, with fatigue assessments focused on representative critical hotspots including the tower base, brace connections, and mooring lines. Local joint flexibility is modelled using the Buitrago parametric formulation, enabling representation of inter-member deformation mechanisms not captured in conventional rigid-body or beam-only flexible models. The fatigue evaluation framework integrates stress concentration factor formulations, rainflow cycle counting, S–N and T–N methodologies, and Palmgren–Miner linear damage accumulation with sensitivity checks for Goodman, Soderberg, and Gerber mean stress corrections. Environmental loading conditions are derived from joint probabilistic wind–wave representations for the North Sea, complemented by Norwegian Sea conditions, with coupled aero–hydro–moor–servo–elastic simulations performed in Orcina’s OrcaFlex in accordance with IEC 61400 standards using stochastic time-domain wind and wave loading. Diffraction-based inertia loads combined with Morison-type drag formulations are employed to capture both global and local hydrodynamic effects. Results show that rigid-body floater models can overestimate tower-base fatigue damage, predicting a fatigue life of 62 years compared to over 300 years for flexible configurations, while flexibility-induced axial loading reduces brace fatigue life from thousands of years to only a few hundred. Structural flexibility lowers mooring line tension by approximately 6% and extends fatigue life, with studded chains exhibiting up to 68% longer lifetimes than studless configurations. The findings demonstrate that structural flexibility and local joint compliance can significantly influence fatigue-sensitive regions within semi-submersible floating wind platforms, highlighting the importance of modelling fidelity when assessing long-term structural response.