Analysis of the influence of scour depth on the dynamic response of offshore wind turbine towers under earthquake action

The operational environment of offshore wind turbine towers is complex, and the harsh service environment makes them more vulnerable to damage under conditions of complex stress such as sea water scouring. The scouring pit has a great influence on the vibration of wind turbine towers. It is of great importance to study the dynamic response to earthquakes of wind turbine towers under scouring depths. The research object of this study was a wind turbine tower at a wind farm in Jiangsu Province, which had a seven-degree seismic fortification in the area. Based on finite element simulation, on-site monitoring and a shaking table test of the offshore wind tower, and considering pile-soil interaction in a refined model, variation in the natural vibration period of the structure under different scouring depths and the influence of different scouring depths on the dynamic response of the structure under seismic excitation were studied. On-site monitoring results show that the #6 wind turbine structure is seriously eroded by sea water, and the vibration amplitude is clear compared with the #15 wind turbine built in the same period. These aspects indicate that the influence of scouring depth on the structure could not be ignored. Analysis of numerical simulation show that scouring depth has a great influence on the high-order mode of the structure, which lengthen the natural vibration period of the structure by a maximum of 33%. On account of scouring, constraints of the soil layer on the highly flexible structure were weakened, and the structure produced considerable vibration, which could lead to damage of structures such as wind towers. Results also indicate that when encountering a seven-degree rare earthquake, power generation should immediately be stopped. The variation curves of the shaking table test and the numerical simulation results were more uniform, and the trend coincided well, which fully verified the accuracy of the numerical simulation.