Abstract

To capture wind energy in the windiest parts of the ocean, floating wind turbines, which are designed to work in deep waters, have to be deployed. To support the wind turbines, floaters, such as spar-buoy, semisubmersible, and tension leg platforms, are commonly adopted, which are tethered to the seabed via mooring lines in order to restrain their motions. In this work, to evaluate dynamic response of floating offshore wind turbines (FOWTs) under the action of waves, an overset mesh based multi-phase flow solver is applied to model the wave structure interaction problem due to its proven capability to accurately capture large amplitude motions of structures. In the meantime, the quasi-static mooring line model is integrated into the overset mesh CFD model so the effects of both waves and mooring systems on FOWTs can be simulated. To validate the coupled model, a test case that involves a moored semisubmersible floating wind turbine model under the action of waves only is simulated and the predicted heave and pitch motions are compared with available experimental data and other numerical work. The validated overset CFD-mooring line model is then applied to investigate the dynamic motion response of the semi-submersible floater in waves of various steepness and periods including focused wave groups, demonstrating its accuracy and capability for FOWT applications.

INTRODUCTION

Continuously growing demand of clean energy from the ocean has driven a rapid development of both o shore renewable energy industry and research. One of the mostly popular technologies of utilising o shore renewable energy is to install the wind turbines on either fixed or floating structures in the ocean. For the floating structures in the deep sea, there are several types of platforms that have already been widely used in offshore oil and gas industry, such as semi-submersible, tension-leg, and spar platforms. Although the technology to be adopted for floating offshore wind turbines (FOWTs) is similar to that used in oil and gas industry, there are important di erences in the mass (size) and wave/wind loading characteristics between the two structures, hence the need for a specific evaluation of the hydrodynamic characteristics especially their stability and survivability under extreme conditions. To address these issues, a high-fidelity numerical model needs to be developed to accurately and simultaneously model hydrodynamics and aerodynamics of floaters and turbine rotors respectively, as well as mooring loads.

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