Platform Pitch Velocity Coupling in a Two-Body Aero-Hydro-Servo-Elastic Model for Floating Offshore Wind Turbines

Floating Conventional models for floating offshore wind turbines (FOWTs) typically treat the floating platform and turbine assembly as a single rigid body, obscuring the dynamic coupling between platform motions and rotor aerodynamics. This simplification limits the accuracy of power prediction and control design under offshore operating conditions. To overcome this limitation, this study develops a time-domain two-body aero-hydro-servo-elastic (AHSE) model for a spar-buoy FOWT that explicitly separates the floating foundation (Body 1) from the turbine assembly (Body 2), enabling a more accurate representation of load transfer and motion interactions. A key contribution of the proposed model is the explicit incorporation of platform pitch-induced blade-element velocity into the relative wind velocity calculation at the rotor plane, with its dependence on blade radial position and azimuth angle, thereby improving the prediction of rotor speed and power generation under platform motion. Simulations performed under realistic offshore conditions, with a mean wind speed of 18 m/s and a JONSWAP wave spectrum characterized by a significant wave height of 1.5 m and a peak period of 20 s, predict a mean power output of 15.1 MW, within 3% of the rated power, while the rotor speed stabilizes at 7.6 rpm. In addition, a quantitative code-to-code comparison was performed against OpenFAST for the same 15 MW spar configuration and operating condition. For the translational DOFs, the obtained RMSE values were 1.1631 m for Body 1 surge, 1.835 m for Body 1 sway, and 0.0907 m for Body 1 heave. For Body 2, the corresponding values were 1.8128 m for surge, 3.9700 m for sway, and 0.1456 m for heave. These results provide a quantitative assessment of the agreement between the proposed reduced-order MATLAB/Simulink formulation and the OpenFAST reference for the investigated operating condition. The proposed framework therefore provides a transparent and computationally manageable platform for studying coupled FOWT dynamics and for control-oriented investigations.