Design and Hydrodynamic Analysis of a Semi-submersible with Two 5WM Wind Turbines
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- Institutt for marin teknikk 
Offshore wind energy has been experiencing a rapid development for last decade. High cost is still the main challenge for the offshore wind industry. Additionally, bottom fixed floating wind turbines may become costly for water depth larger than 50m. An effective way to reduce the cost could be to increase the amount of power generation by a single turbine structure. In this thesis, a semi-submersible platform has been designed to support two NREL 5MW wind turbines, whose cost is possibly decreased through a more effective utilization of the floater structure and mooring system and other auxiliary devices. The design is then checked against buoyancy, stability criteria. First order and second order hydrodynamics are analyzed. In addition to the floater, a mooring system has been designed based on the catenary theory. Spectrum analysis of the mooring line tension has been performed to identify the eigenfrequencies of the moored structure. In the hydrodynamic analysis, floating wind turbine models with different levels of complexity have been established and studied through the thesis. Viscous drag forces and second order wave forces have also been implemented. Finally a simplified time domain method to include the aerodynamic forces are proposed. Unfortunately, a mismatch of the software has not solved within the time limit. High chances are the aerodynamic forces can be simulated to an acceptable accuracy for response analysis by the proposed method.