The coupled nonlinear motion and the influence of vortex induced loads on the motion of Spar type FOWT is studied based on the aero-hydro-vortex-mooring coupled model. The first order and second order difference-frequency wave loads are calculated based on 3D potential theory; the aerodynamic loads on the rotor are calculated based on blade element momentum theory; the vortex induced loads are simulated with CFD approach; the mooring forces are solved by the catenary theory and the nonlinearity of restoring forces are also considered. The nonlinear coupled model is set up and a numerical code is developed for solving the motion of OC3-Hywind Spar-type FOWT in time domain. The responses of FOWT are calculated under different load cases. It is shown that the amplitudes of sway and roll are excited by vortex excited force, and the roll amplitudes can reach to pitch value order in some circumstances. Due to the coupling effects, the heave motion is also influenced by vortex-induced forces. When vortex-shedding frequency is close to roll natural frequency, not only the roll but also the motions in other DOFs are increased and the super-harmonic resonance response occurs in heave motion.

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