Concept Research of a Countermeasure Device for Preventing Scour around the Monopile Foundations of Offshore Wind Turbines

Scouring has long been considered to be a major issue affecting the reliability of the monopile foundations of offshore wind turbines (OWTs) on sandy seabeds. To reduce the impact of scouring, several tons of rock/stone are usually placed around the foundations shortly after the installation of them. Such a measure is costly. Moreover, rock and stone may spread widely on the seabed during the long-term service period of OWTs. It has no doubt that recycling these rock and stone on the seabed is quite difficult in future decommission. For this reason, a new scour-countermeasure device (SEMCD) is proposed and studied in this paper. Considering that the major driver of scouring is horseshoe vortices around the monopile foundation, a hollow horn-like SEMCD with an arc surface profile is designed for weakening the horseshoe vortices. The SEMCD is made of either cement or other kinds of corrosion resistant materials. It is light in weight, and easy to install and decommission. In the paper, the working mechanism of the SEMCD is first explained. Then, its scouring mitigation effect (SME), i.e., its contribution to the reduction of horseshoe vortices and the mitigation of seabed erosion around the foundation, is studied through investigating its influences on down/up-flow and seabed shear stress. Finally, the optimal size of the SEMCD is discussed through investigating the impact of its size on the speeds of up and down flows and the shear stress on seabed surface. The calculation results have shown that the proposed SEMCD has great potential to prevent scouring and seabed erosion, so that it is of significance to improve the reliability of the monopile foundations of OWTs.