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

This study compares time-domain and frequency-domain fatigue assessment methodologies for the HiveWind semi-submersible floating offshore wind turbine (FOWT). Identical structural regions were selected from a global fatigue screening covering more than 16,000 hot spots. The time-domain approach combines Bladed–OrcaFlex coupled simulations with a high-fidelity Reduced Basis Finite Element Method (RB-FEM) structural model, whereas the frequency-domain approach combines linear wave-induced structural transfer functions with damage-equivalent turbine loads. The two approaches were compared at three representative structural details located at different positions within the floater. The results show a strong dependence of the agreement on structural location: the largest discrepancies occur at the central column–bracing connection, close to the wind-turbine load path, whereas substantially better agreement is obtained at a more distant column and in the heave-plate regions. The frequency-domain methodology requires substantially fewer computational resources, but its underlying linearization and DEL representation do not retain load sequence, relative phase, transient peaks, or nonlinear coupling effects. These results support a location-dependent assessment strategy in which detailed time-domain analyses are concentrated on fatigue-critical regions, while frequency-domain methods are used for preliminary assessment and less dynamically sensitive locations.