A Comparative Fatigue Assessment of a Semi-Submersible FOWT Using Time- and Frequency-Domain Methodologies

This study presents a comparative fatigue analysis of a semi-submersible floating offshore wind turbine (FOWT), focusing on the HiveWind platform. Two distinct fatigue assessment methodologies—time-domain and frequency-domain—are applied to identical structural locations identified through a global fatigue screening involving over 16,000 hotspots. The time-domain analysis is carried out using high-fidelity simulations in time domain, incorporating full irregular sea states and turbine-generated time series, requiring over 120 TB of RAM and 150 cores for full-scale IEC-compliant simulations. In parallel, the frequency-domain method is implemented using Ansys and post-processed with Miner’s rule and environmental wave data, integrating damage equivalent loads (DELs) from aeroelastic turbine simulations. Results indicate that while frequency-domain analysis significantly reduces computational cost (by over 90%), it tends to underestimate fatigue damage, particularly in regions proximal to the wind turbine tower. As structural distance from the rotor increases, damage predictions between the two methods converge. The observed discrepancies are partially attributed to the DEL input simplifications inherent to the frequency-domain approach. This work underscores the importance of method selection in offshore fatigue analysis and suggests a location-sensitive hybrid strategy, leveraging time-domain simulations near critical dynamic zones and frequency-domain approximations in peripheral regions.