Scour reduction performance of multi-layer collars around a cylindrical monopile

Local scour around offshore wind monopiles can reduce foundation embedment and threaten long-term structural performance. This study experimentally investigates single-, double-, and triple-layer collar systems under steady clear-water currents, with emphasis on the effects of collar width, elevation, vertical arrangement, layer number, and flow intensity. The main test matrix is conducted at a flow intensity of 0.8, while representative single- and double-collar configurations are additionally tested at flow intensities of 0.7 and 0.9. Quasi-equilibrium bed topographies are reconstructed and evaluated using the maximum scour depth, scour area, and scour volume. For the unprotected monopile at a flow intensity of 0.8, the dimensionless maximum scour depth, area and volume are 0.86, 5.62 and 2.47, respectively. A 24 cm bed-attached single-collar reduces these values to 0.18, 0.32, and 0.03, whereas its effectiveness decreases rapidly when the collar is elevated. The best double-collar configuration consists of a 24 cm-wide bed-attached lower collar and a 12 cm-wide upper collar separated vertically by 2.5 cm. It reduces the three indicators to 0.15, 0.18, and 0.01, corresponding to reductions of 82.6%, 96.8%, and 99.6%. A bed-attached triple-collar system provides comparable, but not superior, protection. Across flow intensities of 0.7-0.9, the representative double-collar system retains depth-, area-, and volume-reduction efficiencies of 78.8%-82.9%, 94.5%-96.8%, and 92.0%-99.6%, respectively. The results identify the lower-collar elevation and width as the primary design parameters, while upper collars provide supplementary control of residual scour.