A Feasibility Study for Using Fishnet to Protect Offshore Wind Turbine Monopile Foundations from Damage by Scouring  
[PERSON]  
1 Department of Mechanical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China; [EMAIL_ADDRESS].  
reduce the depth of the pile foundation into the soil, thereby changing the natural frequency of the wind turbine foundation and causing an adverse effect on the dynamic response, reliability, and safety of the whole wind turbine structure. Research showed that scour pits can reduce the ultimate bearing capacity of the monopile by 35% and increase the bending moment of the monopile by 48%-60% [7].  
In fact, the scouring phenomenon was long noted in bridge engineering, and much effort was made before in order to address such issues suffered by bridge piers [8; 9; 10; 11]. The achieved techniques can be roughly classified into two categories. The first category of techniques reduces scour mainly by adding sacrificial piles [12; 13] or changing the flow direction via guide panels [14], surface guide panels [15; 16], slots through the piers [15], and so on. The second category of techniques reduces scour using the approach of bed armoring. For example, hard engineering materials, such as blocks, rack rirpaps [17; 18; 19], and gravel bags [20], are applied on the seabed around the pier to prevent excessive scouring. However, these protective measures have limitations, and not all of them are suited for application to protecting offshore wind turbines. For example, the effectiveness of those technologies developed for changing flow direction is very sensitive to the direction of flow. They may work very well in river, where the water direction is relatively constant over time. However, the direction of tidal current in the marine environment may change all the time, and the riprap technology is difficult to maintain in river. It becomes more difficult to apply this technology in the harsh offshore environment. Moreover, the use of professional vessels to transport and place large quantities of stones and sandbags on the target seabed causes additional costs, let alone the challenges of recycling them in decommission; a slot or hole can be opened on a bridge pier. However, it is definitely unacceptable to do this on the monopile foundation of an offshore wind turbine, as that may threaten the safety of the turbine structure. In summary, although much effort was made to reduce the scouring around bridge piers, few of the achieved techniques are applicable to protecting offshore wind turbine foundations. This motivated scholars to develop erosion reduction techniques dedicated to offshore wind turbine foundations. For example, a unique countermeasure device was developed in Reference [3]. However, the application of this technique still involves the installation of a large device on the seabed, which is costly. Moreover, its application needs a flat surface of the seabed. Such a requirement cannot be always satisfied in real life. All these constraints limit the application of that technique. Therefore, it can be said that, to date, there is no cost-effective erosion reduction technique that can be readily applied to offshore wind turbines.  
Inspired by the spiral technology [21], a new countermeasure device, named \"fishnet\", is proposed in this paper. Its effectiveness in reducing the scouring around the monopile foundation of offshore wind turbines was tested by both numerical studies and laboratory testing approaches. As compared to the existing erosion reduction measures, \"fishnet\" is more cost-effective and easier to be applied to monopile foundations of offshore wind turbines.