Influence Of Local Scour On Dynamic Response And Fatigue Life Of Offshore Wind Turbine With Pile Foundation

The complex and changeable marine environment makes the supporting structure design of wind turbine face new challenges with the development of wind power from land to sea.The pile foundation of offshore wind turbine is eroded by waves and currents for a long time to result in soil loss and form local scour pits.When the offshore wind turbine is subjected to wind,waves and seismic loads,the existence of local scour pits will inevitably have an important impact on the supporting structure of the wind turbine.In this paper,the numerical simulation of NREL-5MW offshore wind turbine with pile foundation is carried out.It is necessary to study the effects of local scouring on the dynamic response,extreme value response and fatigue life of offshore wind turbine with pile foundation.The main work and achievements are as follows:(1)The three-dimensional integrated solid model of offshore wind turbine tower pile soil is established by combining Turbsim,Open FAST and Abaqus,which can apply wind load,wave load and seismic load,and the rationality and accuracy of the model are verified.(2)The response simulation of offshore wind turbine under wind wave load,seismic load and wind wave seismic load is carried out respectively,and the influence of local scour on the natural frequency and dynamic response of wind turbine support structure is compared and analyzed.The results show that the increase of scour pit depth will reduce the natural vibration frequency of the wind turbine support structure,and when the scour pit depth reaches 3 times the pile diameter,the natural frequency of the supporting structure has been lower than 1P frequency.The existence of scour pit will increase the dynamic response of wind turbine support structure.At the rated wind speed,the scour depth of twice the pile diameter will cause the rotation angle at the initial mud surface of the pile foundation to exceed 0.25 ° specified in the specification;When the wind speed is cut out,the scouring depth of 1.5 times the pile diameter can cause the rotation angle at the initial mud surface of the pile foundation to exceed0.25 ° specified in the specification.Under the action of wind wave seismic load,when the wind speed is cut-in wind speed,the dynamic response of wind turbine support structure is dominated by seismic load;When the wind speed reaches the rated wind speed and cut-out wind speed,the dynamic response of wind turbine support structure is dominated by wind and wave load.Under the cut-out wind speed,the increase of scouring pit depth will cause the maximum value of dynamic response under wind wave seismic load to be less than that under wind wave load.(3)It is analyzed that the influence of local scouring on the short-term extreme value of Mises stress of pile foundation under wind wave load by using the global maximum method.The results show that the existence of scouring pit increases the short-term extreme value of stress,and the influence of scour pit depth on the short-term extreme value of stress decreases with the increase of wind and wave load.(4)It is analyzed that the influence of local scour on the fatigue life of pile foundation under wind wave load by using rain flow counting method,S-N curve and Miner linear fatigue cumulative damage theory.The results show that the fatigue life of pile foundation is mainly affected by wind speed,but the increase of scour pit depth will also shorten the fatigue life of pile foundation.At 3m/s wind speed,the increase of scouring pit depth has a significant impact on the fatigue life of the tower foundation connection.The scour depth of twice the pile diameter can shorten the fatigue life of the tower foundation connection by 84.2%.However,with the continuous increase of wind and wave load,the impact of scouring pit depth on fatigue life decreases.At 25m/s wind speed,the scour depth of twice the pile diameter can only shorten the fatigue life of the tower foundation connection by 14.5%.Compared with the depth of the scour pit,the width of the scour pit has little effect on the fatigue life of the tower foundation connection.The influence of the depth of the scour pit on the fatigue life of the pile foundation should be emphatically considered in the structural design of wind turbine.