Design and analysis of a semi-submersible vertical axis wind turbine
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- Institutt for marin teknikk 
Wind energy are deployed by two types of wind turbines. They are Horizontal Axis Wind Turbine (HAWT) and Vertical Axis Wind Turbine (VAWT), classified according to their axis of rotation. In recent years, offshore wind energy playing a vital role in the wind turbine industry due to high intensity of air, less turbulent and comparatively clean and easily employed in large area which is difficult to manage for onshore or near-shore. The advantages of HAWTs are now facing different challenge in the offshore field due to its cost effectiveness . For this reason, VAWTs have the potential to reduce the cost of producing per unit power. Hence, the emergence and growing interest for VAWTs for offshore application. The availability of fully coupled simulation tools are extremely limited and more sophisticated tools are necessary to carry out the simulation in a fully coupled manner. In this thesis, we used SIMO-RIFLEX-AC for the fully coupled simulation developed by NTNU/MARINTEK. SIMO is capable to calculate the rigid body hydrodynamics forces and different moments on the floater which is designed to support the VAWT. RIFLEX is used to model the tower, blades, struts, mooring lines as flexible finite elements. To calculate the aerodynamic loads acting on the wind turbine we used Actuator Cylinder (AC) model. In addition, it accounts the effect of wind shear and turbulence, dynamic stall by using BL (Beddoes-Leishman) model and dynamic inflow. Then this code was coupled with SIMO-RIFLEX to carry out the integrated analysis. To carry out the fully coupled simulation in time domain, a model was developed in HydroD and analyze the response in the frequency domain. The model was then modified in SIMO and used for coupled simulation. This model is used to study the various cases such as decay test, steady wind test and turbulent and irregular wave test. Also, we carried out a study on the second order mean drift force to check the response of the system. To calculate the second order effect, we use Newman approximation, otherwise to evaluate the second order transfer function is really time consuming. Then we compare our data with different types of vertical wind turbine such as OC4 semi-submersible and landbased wind turbine and carried out a detail study. We focus our study on motion response; performance of the wind turbine such as power, rotor speed, thrust, torque; mooring line tension and tower base bending moment. We also carried out power spectral analysis to see response in different frequency ranges. Considering three wind load cases such as one is below rated speed, one is the rated speed and the last one is above rated speed. The later study was carried out by using the WAFO which is used as a MATLAB routines. One of the focus of the study is to optimize the original OC4 semi-submersible used to support the NREL 5MW wind turbine. The optimization was carried out in terms of reducing the weight and reducing the principal parameters of the platform which was carried out in the project. The optimization was satisfactory in terms of weight and its behavior because the modified OC4 semi-submersible preserve the main characteristics such as natural frequency and damping ratio. In general, a fully coupled analysis was carried out to observe the dynamic characteristic of the wind turbine and applied the results to compare its characteristics with other floating and landbased VAWTs. This will in turn helps us to reveal the advantages and disadvantages of using vertical axis wind turbine than horizontal axis wind turbine. Also, it will help to understand dynamic characteristics and behavior of different wind turbines.