Wave- and Wind-Induced Dynamic Response of a Spar-Type Offshore Wind Turbine

This paper addresses coupled wave and wind-induced motions of spar-type 5-MW wind turbines in harsh and operational environmental conditions. Global dynamic motion responses have been analyzed by aero-hydro-servo-elastic time-domain simulations. The aerodynamics is based on an advanced blade element momentum theory. Panel method and Morison formula accounting for the instantaneous position of the structure are applied for hydrodynamics. Hydrodynamic drag and considering geometrical updating introduce nonlinearities. Hydrodynamic nonlinearities were found to cause excitation of the natural frequencies in the low frequency range more than in the wave frequency range. Extrapolation methods are applied to estimate the maximum responses. A previous study showed that the uncertainty of such an extrapolation for the present concept is less than 2%. In this study it is found that the mean values of the dynamic responses are primarily wind induced and the standard deviations of the responses are primarily wave induced. However, the standard deviation of the nacelle surge motion under operational conditions is primarily wind induced. The maximum of the responses under operational and survival conditions are wind induced and wave induced, respectively. For the present turbine, a power of 5 MW can be achieved at a mean wind speed greater than 15 m/s, whereas the rated wind speed for the original land-based wind turbine is approximately 11 m/s. It is found that the wind turbulence does not affect the dynamic motion and structural responses significantly. However, turbulence affects power production. The ratio of the bending moment and the shear force in the tower-spar interface under a 100-year environmental condition to the corresponding responses associated with the rated wind speed can be as high as 1.5 and 2.3, respectively. It is therefore important to consider survival environmental conditions in the ultimate limit state checks of floating turbines, whereas operational conditions are normally most critical for land-based wind turbines. Moreover, it is shown that under survival conditions, the normalized maximum responses, i.e., (maximum mean)/(standard deviation), corresponding to an upcrossing rate of 0.0001 based on five 1-h analyses varies between 4 and 6, whereas for operational cases the normalized maximum varies between 2 and 4.