Global Strength And Fatigue Life Analysis Of Three Pontoons Wind Turbine Foundation

Compared with offshore area,wind energy resources are more abundant in deep sea area.Traditional offshore fixed wind turbine could only be applied to the shallow water area,which couldn't develop to the deeper sea area.Therefore,the emergence of offshore floating wind turbine provides the possibility of deep sea wind power generation.The difference between the offshore floating wind turbine and the offshore platform is that the offshore floating wind turbine will be subjected to the wind turbine load caused by blades under the condition of normal power generation.During the whole design life,the cycle counts of the alternating stress caused by wave load and wind turbine load are so large that it will easily lead to the fatigue damage of the floating foundation.But in the past,the research on the fatigue life of the floating foundation of offshore wind turbine is often neglected or simplified the wind turbine load,which is not consistent with the actual situation.Therefore,the study on the fatigue life of floating foundation under the combined effects of wave load and wind turbine load is of great significance to the better evaluation of the fatigue life of offshore floating wind turbines.In this paper,a three pontoons wind turbine foundation is taken as the research object.First,the global strength analysis of the three pontoons wind turbine foundation under the extreme environment is carried out.Then a multibody dynamic model of the three pontoons offshore wind turbine is built and the resonance analysis of the three pontoons offshore wind turbine is carried out,and the wind turbine load is calculated.Based on them,the transient response analysis of the combined effects of dynamic wind turbine load,wave load and other external loads is carried out and the stress time history curves of the three pontoons wind turbine foundation are calculated.Finally,the rain flow counting program is compiled and the fatigue life of the floating foundation is analyzed.The main research work and conclusions are as follows:(1)Based on the long-term prediction design wave method,global strength of the three pontoons wind turbine foundation under the extreme environment is analyzed and is checked based on the relevant standards.The results show that the global strength of the three pontoons wind turbine foundation under extreme environment meets the requirements.(2)A multibody dynamic model of the three pontoons offshore wind turbine is built.Based on the modal analysis method,the resonance of the three pontoons offshore wind turbine is analyzed,and the dynamic wind turbine load under the condition of normal power generation is calculated.The results show that the natural frequency of the three pontoons offshore wind turbine could effectively avoid the rotor excitation frequency,and the resonance will not occur.The dynamic load of the wind turbine is the basis for the following transient response analysis.(3)The transient response analysis with the combined effects of wind turbine load,wave load,wind load and current load is carried out by full method,and the stress and strain of the floating foundation with time are obtained.The results show that the position of the maximum stress of the structure at different time is different,but all high stress areas appear in the connecting position of rods and lower floating bodies.(4)Based on the stress time history curves of the three pontoons wind turbine foundation,which are obtained by the transient response analysis,the fatigue life of the hot spots is calculated by the self compiled program of the rain flow counting,combined with the S-N curve and the linear fatigue cumulative damage theory.The results show that under the combined effects of dynamic wind turbine load,wave load,wind load and current load,the fatigue damage value of each hot spot is satisfied with the standard.