Feasibility of a Dynamically Stable Rock Armour Layer Scour Protection for Offshore Wind Farms

Armour layer scour protections around offshore wind turbine foundations are commonly designed to provide a static protection in storm conditions, which means no or limited movement of rock is allowed (Den Boon et al., 2004, De Vos et al., 2011). This approach often results in large stone sizes and high scour protection costs. Therefore, a dynamic approach can be an interesting alternative. Such a dynamic design can be achieved by decreasing the armour stone size allowing movement of the stones and increasing the armour layer thickness to prevent filter layer exposure. A physical test program was conducted to investigate the feasibility and behaviour of such a dynamically stable scour protection. In this model, a monopile foundation exposed to typical North Sea combinations of unidirectional currents and waves was reproduced in a wave flume. The program included a number of test series each with different water depths. In each test series, the armour layer stone size and the armour layer thickness were varied, in order to obtain a reshaping scour protection, without filter material exposure. Damage and failure were assessed both visually and using a 3D-laser profiler. Because previous works on damage numbers of rock armour layer scour protections mainly focus on static design, a new damage number was introduced and compared to the visual observation. This allowed the definition of a ‘dynamic area’ between static design and failure. Scour pit development in time and equilibrium profiling were also analyzed. The results of the tests showed that the concept of a dynamically stable scour protection is feasible.