The influence of the pile Reynolds number on monopile scour prediction across experimental length scales under combined wave-current loading

ABSTRACT Monopiles are the most commonly used offshore wind foundation structures in Europe. Given how scour affect their stability and life-time performance, literature is rich in formulating and testing of equations capable of predicting equilibrium scour depth and time scale. However, considerable inaccuracies occur when applying current scour prediction approaches to larger scales. This mismatch is partly due to a lack in reliable data gained from large scale experiments. The influence of the pile Reynolds number has been previously overlooked and neglected from integration in design formulae, despite its influence on how the horseshoe and lee-wake vortex systems interact with the sediment bed. In this study, two new experimental data sets (with pile diameters of 0.12/0.20 m and 0.57 m) covering a pile Reynolds number range of for combined wave and current loading are presented and complemented with data from previous studies. Using the comprehensive data set that comprises more than 100 points, the proposed time scale equation for a pile Reynolds number greater than improves the from 0.14 to 0.70 when the pile Reynolds number is incorporated. Furthermore, an improved equilibrium scour depth equation is proposed, reaching an of 0.67 for all data points.