Stability and dynamic response analysis of a submerged tension leg platform for offshore wind turbines

This paper proposed a submerged tension leg platform (STLP) for an offshore wind turbine in moderate water depth (70-150 m). During the transportation, the platform is semi-submersible and self-stabilized because it has a relatively large water plane area. Therefore, it can be wet-towed out together with the wind turbine from the quayside to the offshore installation site. During the operation phase, the platform is submerged with a relatively small water plane area, which improves its hydrodynamic performance. Hence, this STLP wind turbine requires low transportation and installation cost. It can also achieve a potentially good dynamic behavior during the operation phase. In this paper, the stability of the STLP wind turbine during the transportation phase is first assessed without considering the mooring lines. The results show that the STLP wind turbine has a good stability to ensure a safe wet towing. The dynamic responses of the STLP wind turbine during the operation phase are then studied with emphasis on the effect of second-order wave loads, wind-wave misalignment and water depth. Based on fully coupled time domain simulations, it is found that the effect of second-order wave loads on the dynamics of the STLP wind turbine is slightly larger in an extreme sea state than that in a moderate sea state. The standard deviations of the surge, sway, roll and pitch motions and the tower base bending moment are dependent on the wind-wave misalignment, while those of the heave and yaw motions, the blade root bending moment are not. In addition, a larger water depth leads to larger standard deviations of the platform motions and a smaller standard deviation of the tower base bending moment. The effect of water depth on the blade root and tower top bending moments is negligible.