Scour around a monopile induced by directionally spread irregular waves in combination with oblique currents

The progressive expansion of offshore wind energy towards greater water depths demands for an optimization of foundation structure designs to a wider range of load conditions. In offshore waters, wind driven wave irregularity and directionality become important aspects of realistic sea states. To further improve the scour prediction in marine environment a novel experimental study was conducted to investigate the influence of directionally spread (3D) irregular waves on the scouring process. The tests were carried out in the 3D wave basin of the Ludwig-Franzius-Institute, Leibniz University Hannover, Germany. A monopile structure was simulated by a transparent pile made of acrylic glass. The study was set up to progress understanding and explore dissimilarities of scour development and patterns induced by directionally spread (3D) and unidirectional (2D) waves. The model tests were complemented by tests with superimposed oblique currents. Despite identical total wave energy in terms of m(0) between the directionally spread and unidirectional wave spectra, minor but distinct differences in scour depth and rate could be observed, which inevitably can only be attributed to the presence and role of the wave spread. For wave-only conditions final scour depths S/D induced by directionally spread waves were on average 33% smaller than those for unidirectional waves. Furthermore, final scour depths decreased with increasing wave spreading and displayed a growing dependency on KC numbers with increasing wave spreading. In combined wave and current conditions of up to U-cw < 0.62 scour depths were found to be slightly larger and scour progression over time faster for directionally spread than for unidirectional waves. Differences regarding the scouring rates and depths have been found to decline with flow conditions further approaching current dominated regime. A prediction approach to estimate the maximum scour depth induced by either directionally spread or unidirectional random waves is proposed. Reasonable results and insights of the present study help advancing the understanding of scour development under more realistic, i.e. natural sea-state, conditions for a more reliable design of marine and offshore infrastructure.