An efficient approach for time-domain fatigue analysis for semi-submersible hulls of floating wind turbines

• An efficient decoupled methodology for time-domain fatigue analysis of semi-submersible floating wind turbine hulls is proposed. • A multi-segment floater model is developed to capture global structural responses and internal loads under combined wind and wave conditions. • Hot-spot stresses at critical hull locations are calculated from time-domain simulations and used for fatigue assessment with rainflow counting and S-N curves. • The decoupled approach reduces computational effort by more than 80% compared to the fully coupled method, while maintaining deviations within 12% at critical sections. • Long-term fatigue analyses across six offshore sites demonstrate the balance between accuracy and efficiency for practical design applications. This study evaluates an efficient time-domain fatigue analysis approach for welded joints in semi-submersible floating wind turbine (FWT) hulls. A fully coupled, time domain approach serves as reference for comparing simplified methods. In particular, a simplified decoupled method in which the global load effects due to wind and waves are determined separately and the corresponding hot spot stress for estimating the fatigue damage in representative points is determined by superimposing the time series of the stress due to global wave and wind effects. A 10-MW semi-submersible FWT at various North Sea and China offshore locations, is analyzed employing a multi-segment floater model to capture global structural responses. Fatigue damage of various positions of the hull is estimated using the rainflow cycle counting method and the SN-curve approach. The results indicate that the fully coupled method, while highly accurate in capturing real-time wind-wave interactions, implies significant computational costs. The decoupled approach enhances the efficiency and reduces the simulation requirements by over 80 %, while maintaining a reasonable accuracy, particularly for wind turbines in the Northern North Sea, i.e., with a deviation of <12 %. In addition, a highly simplified method is proposed, which is based on considering only the most probable wave conditions at different wind speeds. This method shows good agreement with fully coupled fatigue assessments in the Northern North Sea, but it leads to notable underestimation in the South China Sea. However, further investigations of this approach are needed to document its feasibility. The findings in this study highlight the trade-offs between computational cost and accuracy, and especially shows the potential of the decoupled method for an efficient long-term fatigue assessment of floating wind turbines. Future research should be carried out to document its applicability for various floating wind turbine designs and offshore locations.