ABSTRACT
The foundations of offshore wind turbines are typically designed and installed with scour protection to prevent removal of the adjacent seabed, and commonly comprises rock dump around the foot of the foundation in several layers (armour- and filter layer). In more recent years, the scour protection around monopile foundations for offshore wind turbine foundations have been installed with a single-graded solution comprising wide graded rock, omitting several layers at installation point. The solution of installing a single-graded scour protection may comprise considerable cost savings, as one installation procedure is removed from the process.
The stability of single-graded scour protections in steady current depends on similar dynamic responses as for two-layer solutions (Stability of armour stones, winnowing failure, edge scour), and have been studied in an experimental campaign comprising steady current in a wave/current flume at the facilities of the Technical University of Denmark.
The results of the experimental campaign to explain the stability of single-graded scour protections in steady current is presented. The governing process in currents is the armouring of the top layers of the stone layer as the fines are washed out by winnowing. Over time the single-graded stone material makes a transition to a two layer configuration consisting of a protective armour layer and a underlying filter layer. The interface between these two layers, termed the erosion level EL (see Fig. 1), is presented in design diagrams based the mobility parameter, gradation of the stone material and cover stone size.
