## 3 Analysis of Discussion
### Analysis of Long-Term Effects of Normal Wind, Wave and Current on Test Results  
This set of test conditions is to simulate the working state of the wind turbine foundation under the long-term action of normal wind and waves, so as to investigate the dynamic response of the wind turbine monopile foundation under the long-term cyclic action of wind, wave and current loads.  
Based on the load calculation results of the prototype fan, the amplitude of the final resultant force is kept close to the design amplitude of the cyclic load in this group of test conditions by adjusting the load parameters. The above cyclic loads were then applied to the model structure for approximately 5000 cycles.  
Taking the test results of L2 test group (cyclic load ratio \(\xi=20\%\)) as an example, the displacement development results of pile body are explained. It can be seen from Figure 19 that the accumulated displacement of the pile body develops rapidly in the early stage of loading (the first 500 s) due to the gradual displacement of the single-pile foundation to the A side under the action of the cyclic load. However, in the later stage of loading, the increasing rate of the cumulative displacement of the pile body gradually decreases, and the overall displacement curve tends to be flat. From the details of the displacement results, the development trend of the displacement is still consistent with the form of the load.  
Figure 19: Cumulative displacement development.  
Figure 18: Distribution of the maximum pore water pressure along depth.  
Figure 20 shows the hysteresis curve of horizontal load displacement when simulating normal wind, wave and current load for a long time, taking the L3 test group with the cyclic load ratio \(\xi\), as 30% as an example. It can be seen from the load-displacement curves of this group that with the progress of cyclic loading, the area of the hysteresis loop of the load-displacement curve gradually decreases under the same number of loads. It shows that the displacement development rate of the pile body will gradually decrease with the increase in the number of cycles under the long-term load.  
Figure 21 shows the relationship between the cumulative displacement of group L2 and the logarithm of the number of cycles. Due to the limited number of load actions in the model test, it is impossible to truly restore the high-cycle cyclic load action of the actual fan during operation. According to some current research conclusions, the cumulative displacement empirical formula can be used to predict the pile displacement development under the long-term action of wind and waves. Due to the irregularity of the load in this test (there are both large and small wave peaks), it is difficult to extract the peak-finding data, so only the relationship between the overall displacement at the loading point and the number of cyclic actions is analyzed.  
Figure 20: Load–displacement hysteresis curve of L2 groups under long-term load.  
Figure 22a-c, respectively, show the changes of excess pore water pressure monitored by No. 4 porosimeter in the soil under the three working conditions during the long-term circulation of wind, wave and current. It can be seen from the figure that in the initial period of loading, the pore pressures measured in the three groups of tests are all negative values. This is because the pile body vibrates under the load at the initial stage of the load, and a certain gap is generated between the pile and the soil, thereby generating a negative pore pressure. After that, with the action of long-term load, the overall pore water pressure of the soil in the middle of the pile showed an upward trend, but after a certain number of cycles, the increase in the pore pressure tends to a stable value, and the pore water pressure still cyclically fluctuates with the action of the load. The variation trend of pore water pressure of No. 4 porosimeter in the three groups of tests is similar, but the results of pore pressure are different due to different load amplitudes.  
Figure 22: _Cont._