Sinking of armour layer around a cylinder exposed to a current

The flow processes in a scour protection around a monopile in steady current are described in relation to transport of sediment in the scour protection based on physical model tests. The scour protection consisted of uniformly distributed coarse stones without filter layer. Transport of sediment in the scour protection may cause sinking of the scour protection.

This may reduce the stability of the monopile and change for instance the natural frequency of the dynamic response of an offshore wind turbine in an unfavourable manner. The most important flow process with regard to transport of sediment and sinking of the scour protection is found to be the horseshoe vortex.

It is found that a larger pile diameter relative to the size of the protection stones will cause a larger sinking and that two layers of stones will decrease the sinking relative to one layer of stones with the same size.