## 2 Working Mechanism of \"Fishnet\"  
As mentioned earlier, when the tidal current reaches the monopile of an offshore wind turbine, downflows are produced in front of the monopile, which accelerate the current flow on both sides of the monopile. Then, horseshoe vortices and annular acceleration current flow occur on both sides of the monopile, and pairs of counter-rotating vortices also occur behind the monopile. The vortices erode the seabed constantly, stirring the sand and soil on the seabed around the monopile foundations. After the suspended soil and sand are taken away by the accelerated current flow, a scour pit is finally produced around the monopile.  
The threaded pile anti-scouring technology borrowed from this paper was not initially applied to a monopile foundation of offshore wind turbines. Rather, it was applied to suppress the vortex-induced vibration of the cylinder structures in the offshore oil and gas industry and reduce the damage of these tubular structures by fatigue [22], as shown in Figure 1.  
Changes in the surface roughness of the monopile (Figure 1) are one of the important measures for reducing vortices. For example, the threaded pile technique, wherein the spiral line attaches to the surface of the monopile, blocks the transmission of the circular acceleration water flow and interferes with the flow of downflow, thereby reducing the cyclic lateral force acting on the pile and limiting the vortices behind it. In theory, if these measures are applied to the surface of a wind turbine monopile foundation, they should also play an equal role in reducing the strengths of downflow and annular accelerated water flow. However, threaded technology has some limitations in practical applications. For example, its efficiency is sensitive to the angle of the threaded pile [21]. If a rope is directly tied to the monopile, the rope may be displaced and deformed due to the water pressure and water flow, which changes the thread angle of the rope and consequently reduces the efficiency of the device. Such an issue can be avoided if the spiral lines are welded on the monopile foundation of the offshore wind turbine, but welding operations in marine environment are very difficult and costly. This motivated the proposal of the \"fishnet\" technique in this paper. It is shown in Figure 2.  
The fishnet increases the surface roughness of the monopile foundation and hinders the current flow's separation from the surface of the monopile. Thus, the \"fishnet\" can successfully reduce the shedding of current flow to a certain extent, which is beneficial for constraining the vortices around the monopile. In the meantime, as the nets disturb the path of local flow, the turbulent flow is generated locally, which somewhat consumes the energy of the vortices around the monopile foundation and, thus, reduces their intensities. Therefore, in theory, the \"fishnet\" technology has the ability to reduce scouring. Compared with the traditional anti-scour device, the \"fishnet\" technology shows a few unique advantages. Firstly, the fishnet is simple and easy to maintain. Secondly, the non-degradable composite fishnet material minimizes issues concerning corrosion and environmental pollution. Ultimately, the cost of the fishnet is low, which means that the scour mitigation cost can be greatly reduced.  
Encouraged by these unique advantages, a numerical study of the proposed technique is conducted Section 3 to investigate its effectiveness in different scenarios.  
Figure 1: Helical strakes for controlling vortex-induced vibration.  
Figure 2: “fishnet” technology: (**a**) before using the fishnet; (**b**) after using the fishnet.