Effects of rotor control on structural loading of floating wind turbines with pumped water ballast

This study develops a coupled aero–hydro–mooring–servo–pumping model to investigate the dynamic responses and structural loads of floating offshore wind turbines (FOWTs). The proposed quasi-static ballast-pumping model uses a PID controller with low-pass-filtered pitch feedback to maintain mean platform pitch near zero while ensuring gradual water-level adjustments and low pump-power demand. Ballast pumping primarily reduces the mean tower-base bending moment (TBBM), with associated reductions in peak TBBM and slight improvements in power output. To mitigate the negative damping associated with baseline rotor control, two alternative strategies incorporate gain-scheduled or constant-gain platform-motion feedback. Both suppress low-frequency platform pitch oscillations near rated wind speed, reducing peak TBBM and damage equivalent loads (DELs). TBBM decomposition reveals aerodynamic, gravitational, and inertial contributions, with responses strongly influenced by platform pitch and wave conditions. In contrast, blade-root bending moments are dominated by aerodynamic loads and show less sensitivity to wave conditions. Dynamic mooring loads are governed primarily by platform surge, with limited sensitivity to rotor control, while the quasi-static mooring model substantially underestimates mooring DELs compared with the dynamic model. Overall, the findings highlight the distinct roles of ballast pumping in reducing mean structural loads and of rotor control in mitigating dynamic load fluctuations. Given the rigid-body assumptions and limited experimental validation, the predicted load reductions and fatigue-related indicators should be interpreted as comparative numerical estimates for understanding load mechanisms and assessing control strategies.