Physical modelling of dynamic scour protections: analysis of the damage number

In offshore wind turbines, the need for scour protection may represent a considerable part of the total investment required. Its optimisation is therefore a key aspect in offshore wind energy investments. The protection is commonly designed to be statically stable. An economically attractive possibility is to adopt a dynamic stability criterion that enables some degree of movement of the armour layer stones without allowing the filter placed beneath to be exposed at any time. This criterion is based on the acceptable damage number used to specify the mean diameter of the stones used in the armour layer. Little guidance exists on how to define the acceptable limits for this damage number. This paper elaborates on the effect of different hydrodynamic conditions on the damage number of scour protections. The behaviour of the damage number is studied by means of physical model testing of an offshore monopile foundation. The results show that dynamically stable profiles are developed for damage numbers ranging from 0·40 to 3·53. The accuracy of a predictive formula for the damage number is influenced by the water depth. This parameter described the stability of dynamic scour protections more accurately than the stability parameter used in former projects.