Experimental study of scour mitigation around a monopile foundation using a semi-open annular inclined-pile array

Local scour threatens the stability and serviceability of offshore monopile foundations. This study experimentally evaluates a semi-open annular inclined-pile array as a scour countermeasure under steady unidirectional clear-water conditions using uniform quartz sand with d 50 = 0.25 mm. Tests in a large recirculating flume examine the effects of flow intensity, water depth, the vertical-radial position of the inclined-pile array, pile number, and array orientation. Scour evolution is monitored at four circumferential locations, and equilibrium bed topographies are reconstructed photogrammetrically. The maximum relative scour depth along the central monopile surface is adopted as the primary performance indicator. The array maintains lower scour depths throughout most of the development process and reduces the equilibrium maximum scour depth while modifying the spatial distribution of residual erosion. Under the representative condition of h = 0.40 m and F I ≈ 0.9, the maximum relative scour depth decreases from 1.12 to 0.81, corresponding to a 27.7% reduction. Protection performance depends on hydraulic and geometric configuration, with lower and more inward vertical-radial positions generally providing greater mitigation. Because the experiments are limited to steady-current, clear-water conditions and one sediment size, the findings should be interpreted within the tested parameter range.