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Currently, there is a great interest to globally develop offshore wind energy due to the greenhouse effect and energy crisis. Great efforts have been devoted to develop reliable floating offshore wind energy technology to exploit the wind energy resources in deep seas. This paper presents a comparative study of the dynamic response of three different semisubmersible floating wind turbine structures. All the three platforms support the same 5MW wind turbine. The platforms examined are: a Vshaped Semi, an OC4-DeepCwind Semi and a Braceless Semi at 200 m water depth. A dynamic analysis is carried out in order to calculate and compare the performance of these platforms. The comparison is made on the rigid body motions of the semisubmersible platform and tensions of the mooring lines. The presented comparison is based on statistical values and spectra of the time series of the examined response quantities. Coupling effects are more significant for the V-shaped Semi platform. The V-shaped Semi and the Braceless Semi show a more rational motion response under the investigated load cases. The results of this analysis may help to resolve the fundamental design tradeoffs between among different floating system concepts. INTRODUCTION Offshore wind energy has experienced rapid development over the past ten years. In 2017, the investment in offshore wind technology in Europe reached up to 7.5 billion Euro. The 560 new offshore wind turbines were connected to the grid with an increased capacity of 3,148 MW [1]. However, most offshore wind turbines are installed in shallow water with bottom-fixed substructures, which includes monopiles, gravity foundations, tripods, tri-piles and jackets that are used in water depths of up to 50 m [2-4]. For greater water depths, the floating offshore wind turbine (FOWT) is a unique solution to harness the offshore wind energy many different countries. Numerous floating platform configurations are possible for use in offshore wind turbines technology, and much has been learned from the knowledge of the offshore oil and gas industry. Based on the fundamental principles adopted to achieve static stability, a floating platform can be classified into three primary concepts: a semisubmersible (Semi), a Spar buoy and a Tension Leg Platform (TLP). Many researchers have made great efforts to investigate the difference in the dynamic responses of a variety of floating wind turbine systems [5-7]. Compared to spar buoys and TLPs, the Semi platforms is more feasible in a variety of water depths and has lower costs of installation. There are several successful semisubmersible floating wind turbine projects over the world, including WindFloat [8], Fukushima FORWARD demonstration project [9] and OO Star Wind Floater [10]. Bayati [11] investigated the impact of second-order hydrodynamics on an OC4-DeepCwind semisubmersible offshore wind turbine. Moreover, the second order hydrodynamic force can stimulate the oscillation of the platform and further cause fatigue damage to the structure. Masciola and Robertson [12] used the coupled and uncoupled models on OC4DeepCwind semisubmersible FOWT to determine how the mooring systems influence the motion of the FOWT. Luan et al. [13] established a numerical model for a Bracessless semisubmersible platform and performed an extreme sea states analysis. The results showed that the platform has good stability under extreme seas and is a good design concept. Moreover, the

A Comparative Study on the Dynamic Response of Three Semisubmersible Floating Offshore Wind Turbines