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

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 prevailing scour prediction approaches to larger scales. This mismatch is partly due to a lack in reliable data gained from large scale experiments. A parameter that significantly increases in large scale experiments is the pile Reynolds number. 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 1.4×10⁴to4.4×10⁵ 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 2.5×10⁴ improves the R² from 0.14 to 0.70 when the pile Reynolds number is incorporated. Furthermore, an improved equilibrium scour depth equation is proposed, reaching an R² of 0.67 for all data points.