Control-oriented nonlinear benchmark for floating offshore wind turbines with oscillating water column chambers: A WEC-Sim framework

The design of advanced control strategies for floating offshore wind turbines (FOWTs) is limited by restricted access to platform motion states in widely used aero-hydro-servo-elastic simulation tools, where platform dynamics are embedded within servo modules and exposed through constrained interfaces. This limitation complicates closed-loop platform stabilization, particularly under high-wind and strongly coupled operating conditions. This paper presents a control-oriented nonlinear time-domain simulation framework that enables explicit platform pitch feedback and systematic analysis of controller-platform interactions. The framework is implemented in MATLAB using WEC-Sim’s multibody dynamics formulation and couples aerodynamic, hydrodynamic, mooring, and servo-dynamic subsystems within a unified time-domain model. The model is developed for the NREL 5-MW reference turbine mounted on an ITI Energy Barge platform incorporating an Oscillating Water Column (OWC) chamber. Platform geometry is defined in MultiSurf and processed hydrodynamics through WAMIT, while nonlinear mooring restoring forces are modeled using analytical catenary formulations. Nonlinear Froude-Krylov forces, viscous drag, quasi-static catenary mooring dynamics, and generator-converter dynamics are explicitly integrated, providing direct access to the full six-degree-of-freedom platform state vector throughout the simulation. Model fidelity is assessed through free-decay tests, Response Amplitude Operator analysis (RAOs), and benchmark comparison against OpenFAST. The results demonstrate the accurate prediction of natural periods, resonance behavior, and operational responses, with close agreement in rotor torque, generator power, and platform pitch dynamics under combined wind-wave loading. By prioritizing control transparency over computational efficiency, the proposed framework provides a robust numerical foundation for feedback control synthesis, stability analysis, and robustness studies of floating wind-wave systems. The model establishes a verified baseline for the development of advanced platform stabilization strategies and hybrid control architectures in floating offshore wind turbines.