Edge scour at scour protections around offshore wind turbine foundations

When building offshore wind turbines with mono-pile foundations, scour protection is typically placed to avoid scouring of the soil close to the mono-pile. An important aspect is that the scour protection itself causes erosion, inflicted by the local increase in the hydrodynamic field and in turn increased bed shear stresses. Scour of the edge material alongside the scour protection may cause deformations and failure of the scour protection. This can reduce the stability of the stone layer and cause exposure of cables running between the mono-piles. The early stage results of an extensive experimental campaign to explain the edge sour process in steady current is presented. The three-dimensional flow field around the pile and scour protection is resolved by particle image velocimetry, showing a local increase in the hydrodynamic field leading to increased sediment transport and scour. The governing process in steady current is a pair of symmetrical counter-rotating vortices emerging in the near bed region in the wake of the pile and scour protection, causing a significant downstream scour hole.