Dynamic Response And Fatigue Damage Analysis For Floating Offshore Wind Turbine Structures

Wind is a renewable energy which is clean, safe and continuous. Generally, there are plenty of high-quality wind energy resources in deep sea, but with the increase of water depth, the marine environment becomes more complex and severely. Therefore, the research on the dynamic response and fatigue damage of floating offshore wind turbine (FOWT) is vital significance.In this thesis, American National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine, which is a representative utility-scale, multi-megawatt turbine, is chosen as design reference. Then the finite element (FE) models of three typical FOWTs are established. And the wind turbine operation loads are calculated based on aerodynamic theory. The dynamic response and fatigue damage for FOWT structures under the environmental conditions combined with long time statistical information of South China Sea. The scientific and technological achievements of this paper will provide theoretical guidance and technical support for the practical engineering design of the deep water floating offshore wind turbine structure. The main content of this thesis can be summarized as the followings:(1) Three typical FOWTs (Semi-submersible, Spar, TLP) structures are chosen as the target structures. And the dynamic responses of three FOWTs in both frequency and time domain and the tensions of mooring system are calculated under the same environmental conditions. Afterwards the sensitivity analysis of the structure on wave periods and wind speeds are performed. The primary rules of the motion responses of FOWT along with the marine environment are obtained.(2) The OC4-DeepCwind semi-submersible type FOWT is chosen as benchmark structure, which is developed for a U.S.-based project aimed at generating test data for use in validating floating offshore wind turbine modeling tools. The responses of system under environmental loads as wind, current and wave are calculated in frequency and time domains. The influences of various structural parameters such as distance between columns, diameter of columns, draught and water-entrapment plates on the dynamic response of the structure are investigated. The results can be applied for optimized design of structure.(3) The fatigue damage is apt to occur in FOWT which is a typical fatigue structure. Therefore, in order to ensure structural safety in the design working life, it is necessary to make accurate estimates of fatigue life for FOWT. Firstly, the structural fatigue damage under wind turbine loads and wave loads are calculated respectively. Secondly, under simultaneous wind turbine and wave loading, the fatigue damage rule is analyzed. Lastly, the results of the linear superposition are compared with coupling. The conclusions indicate that the coupling induction affects the regular pattern of structural fatigue damage. It can provide some references for the accurate assessment of the fatigue life for FOWT.