New insights into local scour characteristics around vibrating monopiles under combined wave-current conditions.

Cyclic lateral vibrations on monopile foundations of offshore wind turbines throughout their service life may affect the local scour around the foundations. While previous studies have explored local scour around vibrating monopile foundations under current-only conditions, the local scour characteristics around these foundations under combined wave-current conditions remains unclear. This experimental study presents new insights into local scour characteristics around vibrating monopile foundations under combined wave-current conditions. A novel dimensionless parameter, the friction velocity-based Froude number (Fr∗), is proposed to quantify the effect of hydrodynamic intensity on the scour process. Based on the Fr∗ values obtained in the experiment, three distinct regimes were identified to characterize the impact of vibrations on the scouring process: In Regime 1 (Fr∗ < 0.055), scour dimensions are more pronounced, while in Regime 3 (Fr∗ > 0.06), they are less significant compared to the equilibrium scour hole around static monopile foundations. In the transitional phase, Regime 2 (0.055 < Fr∗ < 0.06), scour dimensions fluctuate by up to 10.9% relative to static conditions. Additionally, an estimation equation for equilibrium scour depth based on Fr∗ is developed, demonstrating good prediction accuracy and broad applicability. These insights advance the understanding of scour features and provide a valuable reference for predicting scour depth around vibrating monopile foundations in diverse hydrodynamic environments. • A new dimensionless parameter, the undisturbed friction velocity-based Froude number Fr ∗, has been developed to quantify the hydrodynamic intensities under combined wave-current conditions. • Three distinct regimes around vibrating monopile foundations are identified based on the Fr∗ number: Compared to static monopile foundations, the characteristic dimensions of equilibrium scour holes around vibrating monopile foundations are larger in Regime 1 and smaller in Regime 3. • An empirical equation based on Fr ∗ for estimating the equilibrium scour depth around vibrating monopile foundations under combined wave-current conditions has been established. [ABSTRACT FROM AUTHOR]