Dynamic Analysis and Development of Floating Offshore Wind Turbines

Abstract

A floating offshore wind turbine offers tremendous potential benefits in near future with major challenges. Various research centers, institutes, and universities pay attention to and perform researches on FOWTs. Currently, the efficient performance and the economics of the floating wind systems are paid more attention. The South Korean Government releases an ambitious plan, known as “Renewable Energy 3020”, to increase new renewable energy source power generation by 48.7 GW by 2030. The target is to provide 16.5 GW from wind energy which includes 13 GW from offshore wind. Focusing on a water depth of 150 m in the East Sea gas field off the coast, Ulsan, Korean, floating offshore wind turbines are considered to be the best candidate to fulfill the target. The main types of floating foundations are the semi-submersible, spar, and tension leg platforms. Usually, a semi-submersible platform is composed of three or four slender columns and those are connected to each other through braces. The semisubmersible relies mainly on the waterplane area of columns and the distances between each column to achieve stability. The TLP type is stabilized by a high-tension mooring system; for this reason, the anchor system for TLP is complex. The spar type usually uses heavy ballast materials such as concrete at the bottom to provide a lower center of gravity for stabilization, however, the acceleration at the nacelle and the tower base bending moment are large. Several semi-submersibles and spar types were tested in scaled models in the Ocean Engineering Wide Tank of the University of Ulsan. Numerical models of these types were built and validated based on model test results. The NREL FAST code was used to conduct the fully coupled numerical simulation of floating offshore wind turbines. A set of in-house codes generate potential hydrodynamic coefficients to coupling with FAST code. The finding here is that the semi-submersible platform has a strong effect from second-order wave loads, but the spar platform has little effect from second-order wave loads. This thesis aims to develop a spar-type platform to support the NREL-5-MW reference wind turbine for 150 m water depth of the East Sea gas field off the coast, Ulsan, Korean. The Spar-type platform includes a moonpool at the center. Design optimization processes are three steps, the first step is using a spreadsheet to calculate the platform dimension, the second step is the frequency-domain analysis which calculates the responses in regular waves and the last is the fully coupled simulation time-domain analysis to obtain dynamic responses in combined wind, wave, and current conditions. By having a water column inside the open moonpool, the dynamic responses of the FOWT system are reduced significantly. The reduction of motions will lead to reducing the nacelle acceleration and tower base bending moment as well. The spar-type platform combined with a moonpool has good performances in both operational conditions and extreme conditions. Compared with another floating type, the proposed model promises a feasibility to apply for the offshore wind farm in the East Sea gas field off the coast, Ulsan, Korean.