| As a kind of cheap renewable energy, wind energy is one of the large-scale development and utilization energy in every country in the field of new energy. Because offshore wind power has many distinctive advantages compared to onshore wind power and has huge potential of development,it has become the new direction of various countries in the world. Offshore wind turbine support structure is subjected to cyclic loads caused by both aerodynamic loads and hydrodynamic loads. During the whole design life, the cycle counts of the alternating stress are so large that it will easily lead to the fatigue damage of the structure. Therefore,it is very necessary to do some further research on the fatigue of the offshore wind turbine support structure.In this study,the support structure of tension leg platform for offshore wind turbine in one wind farm is demonstrated. The fatigue life of this structure is analyzed under aerodynamic loads and hydrodynamic loads according to S-N curve and linear fatigue cumulative damage theory. The primary research contents and relevant conclusions in this study are as follows:(1) The coupling effect of the component and control system in the nacelle considered,the multibody dynamic finite element model of the support structure of tension leg platform for offshore wind turbine is built. The coupling effect of the aerodynamic loads and hydrodynamic loads considered,dynamic characteristic analysis of wind turbine on the tension leg platform is conducted firstly. Then, dynamic response analysis of wind turbine on the tension leg platform under the normal power production case and parked case is conducted. The results show that the first order natural frequency of wind turbine overall structure avoid the rotor excitation frequency effectively, and resonance will not occur. Compared to the wind load, wave load is the main load, controlling the movement of the wind turbine foundation.The motion of wind turbine on the tension leg platform under the selected load cases are reasonable,and wind turbine can generate power steadily under the six selected normal power production cases.(2) The finite element model of the support structure of tension leg platform for offshore wind turbine is built with the software DNV/SESAM. The wind loads, the aerodynamic loads and wave loads is applied to the structure finite element model. Overall strength of wind turbine support structure is analyzed and checked based on ABS standard. And fatigue hot spot zone is confirmed in this paper. The calculation results show that the overall strength of wind turbine support structure meets the demands, and the higher stress area is mainly located in the joint of the column and the pontoon.(3) Local refined finite element model of the fatigue hot spot zone is built. Fatigue damage of wind turbine support structure is calculated according to S-N curve and linear fatigue cumulative damage theory under wind turbine loads and wave loads respectively. The effects of three different superposition method under wind turbine loads and wave loads on fatigue life are studied,including the linear superposition method,the quadratic superposition method and the out phase of equivalent fatigue stress superposition method. Finally,the recommended approach is provided. The results show that it is reasonable to use the out phase of equivalent stress superposition method, and the life of support structure is 22.22 years, which meets the demands. |
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