ABSTRACT

A semi-spar type two-body floating platform has been proposed in this study. It consists of an upper floater and a hanging weight, connected through multiple suspension ropes. By adjusting the ballast water in the hanging weight, the floating system can be wet-towed to the installation site as a semi-submersible wind turbine, which makes the transportation process much convenient. A fully coupled aero-hydro-servo-elastic analysis of the novel FOWT system is carried out to examine its dynamic behavior. Two main aspects are investigated through numerical simulations, one is the effect of turbulent wind and turbulence intensity on the motion response of the FOWT, and the other one is the tension variation of semi-spar suspension ropes when the wind and waves are in different directions. The simulation results show that the turbulence intensity has significant impact on the surge and pitch motion of floating platform in low frequency range, and influence the tower base bending moment at natural vibration frequency as well. The magnitude of the tension variation of suspension ropes shows a strong correlation with the wave direction.

INTRODUCTION

Offshore wind power has undergone considerable development in recent years, mostly concentrated in shallow waters with fixed foundations. However, as offshore wind power moves from shallow waters to deep-sea area, the complexity and expense on constructing fixed offshore wind turbines has increased dramatically. Therefore, floating offshore wind turbines (FOWTs) anchored with moorings has been developed and gained much attention.

Currently, floating platforms are primarily classified according to the methods adopted to achieve stability, which include spar, semi-submersible and tension leg platforms (Jonkman and Matha, 2010). The spar type achieves stability by filling the bottom of the platform with amount of ballast to obtain a lower center of gravity than the center of buoyancy. Therefore spar-type platforms usually have a large draft, which leads to inconvenience in installation and transportation. Semi-submersible platforms are balanced by large waterplane area, and they normally have a shallow draft, which facilitates offshore transportation.

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