Dynamic characteristics of tower response of a FOWT under wind-wave misalignments and mooring line failures

Floating offshore wind turbines (FOWTs) enable energy harvesting from shallow and deep-water regions with high wind potential. However, the structural performance of their towers is sensitive to wind–wave misalignment and mooring line failure. This study investigates the tower response of a 5 MW spar-type FOWT subjected to combined wind–wave misalignment and single mooring line failure using fully coupled aero–hydro–servo–elastic time-domain simulations. Five wind-wave misalignment angles (0–120°) are considered over three wind regimes, resulting in a total of nine load cases. Tower-top fore–aft (FA) and side–side (SS) displacements, together with tower-base shear forces and bending moments, are analyzed under intact and failed mooring configurations. Results show that FA motion and associated base shear force decrease with increasing wind-wave misalignment angle due to reduced aerodynamic thrust at 90° with minimum variability, whereas SS displacement and lateral shear force increase significantly as lateral hydrodynamic forcing and sway–roll coupling become dominant. Mooring line failure amplifies variation in response and shifts equilibrium positions due to reduction in overall restoring stiffness, with downwind failures generally producing stronger dynamic amplification of tower top motion and tower base loads. Among the tower-base moment components, the yaw-related moment (Mz) exhibits the highest sensitivity to both wind–wave misalignment and mooring-line failure, increasing by up to 19.98% as the misalignment angle increases from 0° to 90° and by approximately 2.4 times following mooring-line failure. Tower-base bending moments exhibit pronounced sensitivity to combined misalignment and asymmetric restoring forces, leading to increased fatigue-critical load variability in shear forces and moments, particularly at 90°. These findings highlight the influence of wind-wave misalignment and mooring line failures on tower performance and provide useful insights for fatigue assessment and failure-tolerant design of spar-type FOWTs in the future.