## 3 Analysis of Discussion
### Analysis of Short-Term Effect Test Results of Extreme Wind, Wave and Current  
This section is based on the experimental conditions designed in Table 2 and the calculation parameters of wind, wave and current under the prototype sea conditions. Three groups of model tests were carried out on the mechanical properties of the offshore wind turbine monopile foundation in the ultimate load state stage (SLS). The dynamic response of the offshore wind turbine monopile foundation under extreme wind and wave currents is obtained. Figure 14a-c show the load-displacement test results of groups D1, D2, and D3 under extreme wind and wave currents. This group of tests simulates the short-term effects of extreme wind, wave and current loads in the marine environment. It can be seen from the figure that the development process of the pile body displacement is basically the same as the load form, indicating that the load action form has a great influence on the displacement development. At the same time, it can be seen that the density of the load-displacement curve does not change significantly at the initial stage of loading and the later stage of loading, indicating that the displacement of the pile body has maintained a rapid development trend under the short-term action of extreme wind, wave and current loads. In practical engineering, when the wind turbine structure is subjected to short-term extreme wind and waves, the displacement of the pile body will develop rapidly. Therefore, a shutdown strategy should be adopted in time to reduce the load on the blades, thereby reducing the displacement development rate of the pile foundation structure. Figure 13: The pore water pressure at different depthson side A on side B. Figure 15 shows the variation of pore water pressure at the position of No. 1 piezometer in the soil around the pile body in the model test. It can be seen from the figure that in the whole test process, the change trend of pore pressure is basically consistent with the change form of load. At the same time, it can be seen that the peak pore pressure at the position of the No. 1 pore pressure gauge has a certain increase with the action of the simulated wind, wave and current load. However, since the pore pressure gauge is located in the  
Figure 14: The load–displacement curves. (**a**) The load–displacement curve of Group D1; (**b**) the load–displacement curve of Group D2; (**c**) the load–displacement curve of Group D3. surface soil at a depth of 1 times the pile diameter from the soil surface, the increase in pore pressure caused by the drainage effect is not obvious. Figure 16 shows the change of pore water pressure in the soil around the pile measured by the No. 2 piezometer which is close to the pile body. Compared with the No. 1 piezometer, because the vertical distance from the pile body is shorter, the soil at this position has a greater response to the load, so the resulting displacement is more significant. The overall pore water pressure change amplitude is significantly higher than the monitoring value of No. 1 porosimeter. The overall fluctuation form of pore pressure is still basically consistent with the load form. Due to the short distance from the pile body, the reciprocating motion of the pile foundation will cause a drainage channel between the pile body and the soil body, resulting in contact seepage. Therefore, the accumulation of pore pressure is still not obvious, but the peak pore pressure has a certain increase. Figure 16: The results of No. 2 pore pressure gauge. Figure 15: The results of No. 1 pore pressure gauge. Figure 17 shows the variation of pore water pressure of the soil around the pile at the position of the No. 8 piezometer located in the deep soil in the model test. It can be seen from the figure that the accumulation of soil pore water pressure at No. 8 piezometer.