Influence of scour and scour protection on the stiffness of monopile foundations in sand

Offshore structures are subject to cyclic effects from wind and waves, as well as altered seabed. Local and global scour can form around the foundation elements, which in turn can affect the structures' response in terms of serviceability, eigenfrequency and fatigue limit state. If a scour protection is installed, a stiffening effect is to be expected. The paper investigates the influence of local scour and scour protection on the soil-structure interaction of an exemplary monopile foundation in sandy soil with a numerical finite element model, in which the soil behaviour is modelled by the advanced HSsmall material law. With that, the effect of the depth and the geometry of a scour hole on the load deformation behaviour and in particular on the stiffness of a monopile is quantified. It is shown that the exact geometry of a conical scour hole is of minor importance for the effect on the monopile stiffness. However, scour depths of around 1.5 times the pile diameter, which are often to be considered in design, strongly reduce the stiffnesses with reduction factors of around 2.5. Furthermore, the positive influence of a scour protection layer on the soil-structure interaction is included in the comparison. First order estimates for a typical offshore configuration reveal that an installed scour protection can increase the stiffnesses of the monopile-soil system under operation considerably, mainly dependant on the thickness of the protection layer. The study exemplarily quantifies that the presence of scour protection implies an increase in structural integrity as a so far underrated added value and may actually be an effective mitigation measure for dysfunctional monopile foundations in offshore wind industry.