Dynamic response analysis of a floating platform for offshore wind turbines

In recent years, studies demonstrating the potential of the offshore wind energy have shown that the installation site of the wind turbines needs to be extended to areas farther from the shore. However, bottom founded type offshore wind turbines have limited the installation site to the near-shore areas, so far. Making more use of the wind energy source is possible through the floating type offshore wind turbines. Due to the extreme loads, comprehensive response analysis of the floating offshore wind turbines under combined wave and wind excitation is essential during the initial design. In this study, the response analysis of the large-scale floating platform carrying two wind turbines is performed under combined wave and wind loads. It is well accepted that the investigation of the rigid body motions of the floater is sufficient in response analysis; however, the elastic motions of the floater should be considered in present study due to its dimensions. In order to analyze the aero-hydro-elastic response of the floater, firstly the dynamic characteristics are determined by the finite element analysis by considering that the response of the structure can be represented by its dynamic characteristics. The rotor-nacelle assembly of the wind turbine is imitated by a concentrated mass with rotational inertia in finite element analysis. Hydrodynamic radiation and wave forces in frequency domain are computed according to the dynamic characteristics by using the boundary element method and transformed to their time domain representation by the convolution of the radiation impulse response function and using the sine transformation, respectively. The sea state is modeled by using the Pierson-Moskowitz wave spectra. The aerodynamic forces are obtained by considering the steady wind condition and using the blade element momentum theory. The time domain response of the structure is acquired by the time integration of the equation of motion.