Predicting loose rock scour protection deformation around monopiles using the relative mobility number and the Keulegan–Carpenter number

Abstract

Loose rock scour protections are widely applied as countermeasure for scour development at offshore foundations. This study presents a deformation model that predicts the deformation depth in loose rock scour protections around monopiles using two relatively simple parameters. The first parameter is the relative mobility, which is the ratio between the Shields number and the critical Shields number. The relative mobility is a measure of the hydraulic loading relative to the resistance of the material. The second parameter is the total Keulegan–Carpenter (KC) number, which is the ratio between the length of the orbital motion at the pile and the diameter of the pile. The total KC number accounts for the contribution of both a steady current and a wave-induced current. It is a measure of the wave stroke asymmetry (leading to a larger net displacement) and the differences in bed shear stress amplification around the pile (thus increasing the effect of the relative mobility). The deformation model is based on an experimental database consisting of roughly 380 individual tests spanning a wide range of parameters representative for typical North Sea conditions. The derived model can directly be applied in the design of loose rock scour protections in engineering practice.