Shaking table tests and numerical analysis of monopile-supported offshore wind turbines under combined wind, wave and seismic loads

Abstract

Currently, large-diameter monopile-supported offshore wind turbines (OWTs) are widely used in offshore areas of China. However, considerable numbers of wind farms are located in seismically active areas with liquefiable soil layers. Therefore, shaking table tests and numerical simulation based on a scaled model of Siemens SWT-4.0-130 OWTs were carried out to investigate the failure mechanism of the OWT system in liquefiable sand under combined wind, wave and seismic loads. The scaling law focusing on the governing physical mechanism of the OWT system considering multi-field coupling are presented and verified. The dynamic responses of the OWT and the excess pore pressure fluctuation around the soil-monopile domain in different operating states are discussed and compared. The results show that the proposed scaling laws can well restore the dynamic characteristics of the prototype. The seismic excitation is the dominant load for inducing the structural response under combined wind, wave and seismic loads. Due to the coupling effect, the dynamic responses under combined wind and wave loads were slightly less than the superposition of structural responses under separated wind or wave loads. Liquefaction of the shallow soil layer within the range of four times the pile diameter was detected when the PGA reached 0.4 g (three-dimensional input) or 0.64 g (one-dimensional input), which induced obvious tilt and settlement of the OWT model after the test. The sudden increase in the pore pressure resulted in the liquefaction of soil, which further led to a significant decrease in the natural frequencies and decay of the acceleration responses at the tower top. The above findings are expected to provide valuable experience for the 1 g underwater shaking table tests on OWT systems and insights for the practical design of OWTs in seismically active areas.