A time-domain method is developed for modeling the dynamics of a floating truss-structure wind turbine with multiple rotors mounted on the deck of the platform. In its hydrodynamic aspect, a hybrid panel-stick model is built up incorporating the potential flow theory to evaluate the wave inertia force and a Morison strip method to evaluate the wave drag force. The proposed analysis model is validated against a 1/50 scale test of a semi-submersible floating wind turbine, which was carried out in Kyushu University. Good agreement between the simulation results and the experimental data confirms the validity of the developed method. Further numerical simulations are performed in a set of wind and wave conditions to investigate the effect of wave drag force on the dynamics of the floating wind turbine. The results show that applying a hybrid panel-stick model is fairly effective to reduce the unphysical large resonant responses.
Semisubmersible type is one mainstream type of FWTs (floating wind turbines). An important issue of the semisubmersible foundations is to predict their motion responses, among which the heave response may be of a particular concern. At the natural frequencies, when using a potential flow based method, the semisubmersible normally endures remarkable resonant responses. The reason lies in that, near the boundary layer of the submerged part of the floating structure, the waveinduced drag force (which is a viscous force) has not been accounted for as that is usually done in a Navier-Stokes equation based solver. To include the viscous effect, the potential flow based method needs to be modified to some extent. On the other hand, although there have been some good works on determining the wave-induced loads upon a semisubmersible platform (e.g., Hooft, 1972; Mathisen et al., 1982), they are primarily based on the strip theory (or pure Morison equation) neglecting three-dimensional wave interactions between the Morison elements. Considering the co-existence of large-diameter columns and small-diameter members, it is advisable to employ a hybrid approach combining the potential flow theory and the Morison equation (Li and Yu, 2012). Liu et al. (2016) have done such a hybrid modeling in the frequency domain. The present work extends the idea to the time domain for a complex floating structure.
