Re-analysis of the deformation of rock-based scour protections around monopiles exposed to waves and current

Publicly available data on deformation of scour protections around monopiles has been collected, reviewed, and re-analysed. The review showed that the studies published over the past two decades are comparable and provide results that are overall coherent. The reanalysed data, all based on physical model tests, has been analysed both in qualitative and quantitative ways. The qualitative analysis shows that the deformation patterns of the tests can be uniquely classified based on Shields number and wave-current ratio, while the effect of the, rather low, $KC$-numbers covered by the data was negligible. The quantitative analysis showed that the magnitude of the deformation follows the same trend as the qualitative analysis. Finally, an analysis of the time scale of the deformation indicates an overall trend with regards to wave-current ratio similar to what is reported for live-bed scour, although the time scales are longer. Based on the analysis a methodology of a design approach for an optimised armour rock design is outlined together with a discussion of potential issues with this method, primarily due to limited available data, and how to reduce the risks associated with these issues.