DESIGN AND DYNAMIC ANALYSIS OF THREE- LEGGED ARTICULATED SUPPORTING TOWER FOR OFFSHORE WIND TURBINE

Abstract

Demand for renewable energy sources is rapidly increasing since they are able to replenish the depleting fossil fuels and their capacity to act as a carbon neutral energy source. A substantial amount of such clean, renewable and reliable energy potential exists in offshore winds. The major engineering challenge in establishing an offshore wind energy facility is the design of a reliable and financially viable offshore support for the wind turbine tower. An economically viable support for an offshore wind turbine is a compliant platform since it moves with wave forces and offer less resistance to them. Amongst the several compliant type offshore structures, articulated type is an innovative one. It is flexibly linked to the seafloor and can move along with the waves and restoring is achieved by the horizontal component of the large buoyancy force. An innovative concept, three-legged articulated support for an offshore wind turbine is designed in this thesis. The platform is designed to support the National Renewable Energy Laboratory (NREL) 5 MW reference turbine in a water depth of 144 m. Experimental and numerical investigations are done on the designed three-legged articulated structure supporting the above 5 MW wind turbine. The experimental investigations are performed on a 1: 60 scaled model in a 4 m wide wave flume at the Department of Ocean Engineering, Indian Institute of Technology, Madras. The experimental investigation includes free oscillation study and motion response under regular waves. The tests are conducted for regular waves of various wave periods and wave heights and for different orientations of the platform. The motion responses are presented in the form of Response Amplitude Operators (RAO). The results from the experimental study revealed that the proposed articulated structure is technically feasible in supporting the offshore wind turbine because the natural frequencies are away from ocean wave frequencies which make the RAOs relatively small and the tower always remains vertical. The numerical study is carried out using hydrodynamic software ANSYS AQWA. The natural periods computed and those obtained from free oscillation experiment are in good agreement, indicating that all principal effects are incorporated in the numerical model. The comparison of the experimental and numerical results for regular waves show that the surge

v

responses agreed well with the experiments conducted and is revealed through favourable comparison of Response Amplitude Operator in the predominant degree of freedom (surge). Thereafter, to investigate the complete behaviour of this compliant support system under the actual ocean environment, a comprehensive numerical investigation on the various aspects of dynamic response of the three-legged articulated wind tower under different sea states are evaluated for several waves as well as combined wind and wave cases. The results show that this three-legged articulated support is a promising concept for supporting an offshore wind turbine.