Empirical design of scour protections around monopile foundations: Part 1: Static approach

Together with new opportunities, offshore wind farms raise new engineering challenges. An important aspect relates to the erosion of bottom material around the foundation of the wind turbines, caused by the local increase of the wave and current induced flow velocities by the pile's presence. Typically, the expected scour has a considerable impact on the stability and dynamic behavior of the wind turbine and a scour protection is placed to avoid erosion of the soil close to the foundation. Although much experience exists on the design of scour protections around bridge piers (which are placed in a current alone situation), at present, little design guidelines exist for the specific case of a scour protection around a monopile foundation subjected to a combined wave and current loading.This paper describes the derivation of a static design formula to calculate the required stone size for a scour protection around a monopile foundation in a combined wave and current climate. Due to the difficult physical processes involved in flow disturbance and displacement of bed protection material at the base of a foundation, the formula is based on the results of an experimental model study which is described in this paper. A linear relationship was found between the critical bed shear-stress τcr and the bed shear-stress caused by current τc and waves τw, respectively. When applying the formula for a typical situation in the North Sea, a significant reduction of the required stone size is obtained, compared to existing design criteria.In part 2, following this paper (De Vos et al., in preparation), an optimization of the design procedure is obtained by allowing limited stone motion for top layer stones. This is obtained by adding a damage factor to the design formula, which leads to significantly smaller stone diameters and thus a more economical approach.