Load and response analysis of wind turbines under atmospheric icing and controller system faults with emphasis on spar type floating wind turbine
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- Institutt for marin teknikk 
Deep water offshore wind is a vast, reliable and economical energy source. Floating offshore wind turbines are potential systems to capture this energy which need to be analysed and designed in order to achieve economic productions and deployments, reliable operation, and adequate service life. Application of floating offshore wind turbines has been limited to research area yet and there are still many challenges and questions that need to be answered by researchers and industries. The primary challenge is the electricity production cost, which needs to be lowered to make it competitive. This can be achieved by increasing the capacity factor, availability and reliability of the sub-systems and the total life-time of the system. In this regards floating offshore wind turbines have to be designed for all the normal and abnormal operational and environmental conditions. In the work presented in this thesis, the effects of two abnormal conditions on the performance and responses of a land-based and a spar-type floating wind turbines have been studied. These include: atmospheric icing as an abnormal environmental condition and malfunction in the controller sub-system as a system fault. The aero-hydro-servoelastic code HAWC2 was used as a main analysis tool for this study. The NREL 5-MW reference wind turbine was considered as a test case in this work. To study the first problem, the atmospheric ice accretion on the wind turbine blades was simulated with the NASA panel code LEWICE. A sensitivity analysis was performed to investigate the role of different atmospheric and system parameters on the ice profile geometry and ice mass distribution. A 24-hours unsteady atmospheric ice accretion was simulated on the NREL 5-MW wind turbine blades. The aerodynamic degradation of airfoil sections due to icing were estimated with the CFD code FLUENT. The numerical results for a few samples were validated against wind tunnel tests. The effect of atmospheric icing on the power production, load and responses and short-term fatigue damage of a land-based wind turbine were studied. Further the effect of atmospheric icing on the overall performance and extreme responses of a spar-type floating wind turbine, during power production, normal and emergency shut-down, extreme operation gust and survival conditions were studied. To address the second problem, the author has studied the published information on reliability analysis of different wind turbine sub-systems. The controller system is found to be one of the most unreliable sub-systems in a wind turbine. To evaluate the effect of controller malfunctions on the responses of the wind turbines, a few fault cases in the wind turbine controller system were modelled and the responses of both a land-based and a spar-type floating wind turbines were simulated under these fault conditions and compared. The faults include: three fault cases in the rotary encoders and two fault cases in the pitch actuator. Effects of faults on the wind turbine responses at different operational condition were studied. The extreme responses and fatigue damage under the fault conditions were compared with normal operational and extreme conditions defined in IEC 61400-3. The dissertation is based on articles published or accepted for publication or submitted in/to scientific conferences and journals, together with a synopsis explaining the background for the under taken research and methods used.