## 5 Laboratory Tests
### Testing Results  
First of all, the erosion around the monopole before using the \"fishnet\" technique was investigated. After the tank was filled with 1-m-deep water, the two propulsion systems started to accelerate the water in the tank until the average flow speed in the tank reached 0.22 m/s. The depth of erosion was then measured from four directions every 20 min. All measurement results obtained within 200 min are shown in Figure 18, where testing point 1 was located in front of the monopile, testing point 3 was behind the monopile, and testing points 2 and 4 were on either side of the monopile foundation. From Figure 18, it can be found that, with the increase in testing time, the erosion depths in all four directions generally showed an increasing trend. However, in contrast to the results measured from the front (testing point 1) and two sides (testing points 2 and 4), the depth of the scour pit behind the monopole (i.e., at testing point 3) developed much more slowly. This was attributed to the role of the counter-rotating vortices behind the monopile (see Figure 8). Among the measurement results in the four directions, the scour pit developed most rapidly on both sides of the monopole. The largest shear stresses that occurred at these two specific locations could account for this phenomenon. However, it was interestingly found that the development of the erosion depths even at these two locations also showed more or less a difference in trend, the presence of which should be related to the asymmetric vortices on both sides of the monopole (see Figure 10a). Then, the above tests were repeated after applying a 20-cm-long fishnet device, of which the mesh size was 1.5 cm and the diameter of the fishnet thread was 1.8 mm, to the monopole. After the test went on for 180 min, the erosion depths at testing points 1, 2, and 4 were measured. The measurement results are listed in Table 4, in which the corresponding erosion depths measured before using the \"fishnet\" technology are also listed for comparison. Figure 17: Schematic diagram for monitoring the effect of the scour around the foundation. Figure 18: The development of scouring effect versus time at different positions before using fishnet. From Table 4, it can be seen that, after using the fishnet device, the scouring depths measured at all three testing points decreased to different extents. In particular, the erosion depth measured at testing point 4 decreased most from 5.5 cm to 3.5 cm, and the corresponding reduction rate reached up to 36.36%. When considering the reduction in erosion depth in all three directions, the average reduction was about 23%. To facilitate understanding, the erosion results observed when the testing time was 180 min before and after using the fishnet device are shown in Figure 19.  
From Figure 19, it can clearly be seen that, after the fishnet device was installed, the erosion was significantly constrained not only in terms of erosion depth but also in terms of the size of the area being eroded. In the meantime, it was found that, despite the application of the fishnet device, the erosion depths in front and on both sides of the monopile were always much larger than the erosion depth behind the monopile. This was consistent with the results shown in Figure 18.  
Subsequently, the optimal design of the fishnet device was also investigated in the laboratory tests. As in Section 4.2, the influences of the size of the fishnet \(l\), the diameter of the fishnet thread \(d\), and the length of the fishnet device \(H\) on seabed erosion were investigated in the subsequent tests. Firstly, an experiment was performed to characterize the influence of the fishnet size on erosion depth. In the experiments, the water in the tank was still 1 m deep, and the average water flow rate was still kept at 0.22 m/s. When the fishnet device was 20 cm long, the diameter of the fishnet thread was 1.2 mm, and the sizes of the fishnets were 1 cm, 1.5 cm, 2 cm, 2.5 cm, and 3 cm, the erosion depths at testing points 1, 2, and 4 were measured.