Effects of flow intensity and vibration–flow alignment angle on riprap protection around a vibrating monopile: An experimental study

Abstract

Riprap is one of the most widely adopted methods for protecting offshore wind turbine monopile foundations against local scour. However, its performance under monopile vibration remains insufficiently understood. This study presents a series of physical model experiments investigating riprap stability and scour development under three flow intensities and three alignment angles between monopile vibration and steady current. The results demonstrate that flow intensity has a direct influence, with higher flow intensities leading to deeper scour. In contrast, the vibration–flow alignment angle exhibits different levels of influence on various scour metrics. In particular, the dynamically enlarged blockage area is shown to play a dominant role in controlling scour depth. Based on the experimental data, empirical equations are proposed to predict multiple scour metrics. The proposed equations demonstrate acceptable accuracy for engineering applications and provide practical guidance for designing riprap protection around vibrating monopile foundations. These findings contribute to improving the reliability of scour protection strategies for offshore wind energy infrastructure.