Efficient time-domain hydrodynamic model for floating offshore wind turbines using rational approximations

High-fidelity dynamic models of floating offshore wind turbines (FOWTs) must consider the fluid-structure interaction regarding the hydrodynamic loads. The linear hydrodynamic loads can be superposed by contributions from waves (diffraction), from platform motions (radiation) and from buoyancy (hydrostatics). The radiation force vector is related to the frequency-dependent added mass and damping matrices in the frequency domain and solved by time-consuming convolution integrals. This paper investigates the use of rational approximations to obtain the radiation force vector in the time domain for efficient simulations of the coupled FOWT-fluid system. The idea is to fit the numerically obtained hydrodynamic transfer functions by rational functions, which enables replacing the convolution integrals by a state-space model with limited numbers of internal degrees of freedom. Comparisons regarding the hydrodynamic loads andFOWTresponses are performed, using the rational approximations, the convolution integral as well as the Morison equation at different wind and wave states.