Detailed observations of edge scour of riprap scour protection for offshore-wind monopile and its effect on monopile’s bearing capacity

Large-diameter offshore monopiles are frequently subjected to scour hazards, necessitating the implementation of scour protection measures to mitigate soil erosion around the pile foundation. Riprap, the most widely used scour protection, has been extensively studied with respect to particle size design in armor and filter layers; however, the subsequent edge scour process – particularly its impact on the long-term protection stability and pile bearing capacity – remains poorly understood. In the present study, a series of tests were carried out to investigate the effects of key protection parameters (flow intensity, protection width, thickness, and stone size) on equilibrium edge scour depth and morphological characteristics. A predictive model for maximum edge scour depth is developed based on experimental observations from both this study and that of Petersen et al. (2015a), which has an overall model inaccuracy within ±25%. Furthermore, 3D finite element analysis of pile-seabed interaction incorporating the edge scour effects reveals that riprap installation effectively compensates for scour-induced reductions in lateral pile capacity, with its protection width exhibiting a greater influence than thickness. These findings provide valuable references for riprap design in offshore engineering applications.