Dynamic response and viscous effect analysis of a TLP-type floating wind turbine using a coupled aero-hydro-mooring dynamic code

This paper presents a coupled dynamic motion response analysis of a floating wind turbine using an in-house code, CRAFT (Coupled Response Analysis of Floating wind Turbine). Viscous drag forces on horizontal pontoons are carefully calculated, and a nonlinear spectral method is applied to efficiently solve the coupled tendon dynamics. Viscous drag forces and tendon dynamics are two important factors when assessing a tension-leg platform (TLP)-type floating wind turbine in a time-domain simulator. The analysis object is the NREL 5 MW Wind Turbine, which is supported by a three-leg mini-TLP platform. Simulations of the free decay and response amplitude operator (RAO) tests are conducted using CRAFT as well as FAST, another commonly used code. The obtained results are compared with experimental results to verify the capability of CRAFT. Viscous drag force induces higher harmonic pitch resonance, which is most prominent when the wave period is three times the natural period of the pitch and the wave height reaches a threshold. Springing motion is identified and found to be caused by this resonant pitch motion. Time-domain statistics show that extreme increases in tendon loads caused by springing as well as pitch and tendon tension probability distributions are non-Gaussian in random sea states. In addition, the resonant pitch motion is significantly reduced by aerodynamic damping.