Empirical design of scour protections around monopile foundations. Part 2: Dynamic approach

Abstract

When building offshore wind turbines, a static scour protection is typically designed, allowing no movement of top layer stones under design conditions, leading to an often conservative design. Furthermore, little design guidelines exist for a scour protection around a monopile foundation under combined wave and current loading.In part 1 (De Vos et al., 2011), preceding this paper, a static design guideline for a scour protection around a monopile is suggested, based on a combined wave and current flow field. By allowing some movement of the top layer stones of the scour protection, a more economical design is obtained. This paper describes the derivation of a dynamic design formula to calculate the required stone size for a scour protection around a monopile foundation in a combined wave and current climate. The formula is based on the results of an experimental model study, described in this paper. The formula gives an expected damage level to the scour protection, based on the wave orbital velocity, wave period, steady current velocity, water depth, relative stone density and stone size.When applying the formula for a typical situation in the North Sea, a reduction of 20% to 80% of the required stone size is obtained, compared to the static design approach, described in part 1.