Research on fully coupled method for floating offshore wind turbines considering nonlinear time-domain hydrodynamic effects

Most existing engineering tools, relying on frequency-domain linearized assumptions, underestimate critical nonlinear hydrodynamic phenomena such as transient free-surface interactions, quadratic terms in Bernoulli and wave-structure coupling, leading to design inaccuracies of floating offshore wind turbines (FOWTs). This study developed a novel aero-hydro-servo-elastic full coupled simulation tool FAST2Wasim (F2W), integrating FAST code and nonlinear hydrodynamic solver Wasim via Dynamic Link Library technology. The framework enables real-time data exchange to resolve nonlinear hydrostatic restoring forces, instantaneous wetted surface variations, and nonlinear wave effects. Validation under free-decay, regular/irregular wave, and combined wind-wave conditions demonstrates that the nonlinear hydrodynamic model outperforms conventional linear methods, particularly in capturing asymmetric hydrostatic stiffness, transient pressure redistribution near the waterline and low-frequency motion resonances. Linear hydrodynamic models exhibit unphysical pressure predictions during large motions, while F2W's nonlinear solver aligns with realistic wave-structure interactions. The proposed tool bridges the gap between high-fidelity CFD and oversimplified linear tools, offering a computationally efficient tool for optimizing FOWT structural integrity under nonlinear hydrodynamic loads.