Dynamic reconfiguration of a floating vertical axis wind turbine driven by progressive mooring stiffness degradation

Mooring systems are essential to the station-keeping and operational safety of floating wind turbines. However, how progressive mooring stiffness degradation affects platform dynamics and rotor performance before failure remains poorly understood. This study develops a coupled computational fluid dynamics (CFD) and finite element method (FEM) framework for a three-bladed floating vertical axis wind turbine (VAWT) on the DeepCwind semisubmersible platform from the Offshore Code Comparison Collaboration Continuation (OC4) project. STAR-CCM+ resolves rotor aerodynamics using an overset mesh, while ABAQUS/AQUA computes platform hydrodynamics, flexible-beam mooring responses and seabed contact. Six computational branches are generated from the same converged intact state by smoothly reducing the effective axial stiffness of either mooring line 1 (ML1) or mooring line 2 (ML2) by 30%, 60% and 90%, representing mild, moderate and severe stiffness loss in this study. Mean horizontal offset, six-degree-of-freedom motion statistics and top-tension statistics for all three mooring lines reveal nonlinear system reconfiguration. Horizontal migration remains limited at 30% and 60% stiffness loss but exceeds 5 m at 90% loss. Its magnitude reflects degradation severity, whereas its direction depends on the degraded line through changes in horizontal restoring-force balance. Motion changes are selective, with amplified sway and yaw but limited changes in heave and pitch. Platform re-equilibration modifies catenary geometry and redistributes mooring loads. Consequently, system-wide mean unloading can coexist with increased cyclic tension in intact lines, and the largest cyclic response does not necessarily occur at the greatest stiffness loss. Despite this dynamic reconfiguration, the mean power coefficient and thrust remain within 1% of their intact values over 200 to 500 s under prescribed-speed operation. Apparently stable aerodynamic performance can therefore conceal a markedly altered platform-mooring dynamic state. Condition monitoring should combine aerodynamic output with equilibrium position, platform motion and multi-line tension indicators.