Quantifying the effect of rock armour scour protection on eigenfrequencies of a monopile supported OHVS

It is common practice to install scour protection systems during the construction of offshore wind farms. This is because the guidelines allow for a more optimal structural design, as the potential effects of scouring may be ignored. However, multiple sources, as well as measurements, suggest that there is an additional gain in stiffness which is not being taken into account by current guidelines. Scour protection has the ability to provide extra foundational stiffness to the monopile, which results in higher natural eigenfrequencies and mode shapes. In this paper the focus will be on an offshore high voltage station (OHVS) located in the North Sea and the impact of a rubble riprap scour protection on the foundational stiffness of the structure. Furthermore, a proposal is made on how to take these effects into account when calculating for design. This site was equipped with a mobile measurement system containing a multi-axial accelerometer, which was active in a period both before and after installation of the scour protection rock armour layer. In case of similar environmental circumstances, the eigenfrequencies of the OHVS were noticeably higher after the scour protection was installed, thus substantiating the claim of increased foundational stiffness. To perform numerical studies on the impact of the scour protection system on structural dynamics, an OHVS support structure finite element model was built. As no formally agreed upon formulation to include scour protection exists, two possible methods are proposed. One is adding a global accretion layer to model the scour protection as an additional soil layer. The other calculates an overburden pressure due to the scour protection weight, which translates to the stiffening of the upper soil layers of the foundation. Both methods have their own strengths and weaknesses, but more research on different locations and offshore structures is needed to formalize the definitive description for modelling scour protection.