Full-Scale Tests of Skirt Penetration Resistance in Gravel for Offshore Wind Structures

Full-scale tests were performed to study the penetration resistance of steel plates (skirts), which are common foundation elements of jacket offshore wind structures. Penetration resistance is well-studied in clays and sands, but there is limited information on penetration in gravels, which are typically used as the scour protection of critical elements like offshore wind substations. Direct extrapolation of penetration resistance in sands is not possible because of the grain-size effect. Two steel plate thicknesses and two gravel sizes were used to study the influence of the grain-size effect on the penetration resistance. Moreover, the importance of tip shape has also been evaluated by means of beveled tip tests. The results of the experimental tests showed a strong dependence of penetration resistance on penetration depth as expected, but also a significant dependence on skirt thickness and gravel grain size. A good agreement with experimental results has been found when analytically interpreting the penetration resistance using traditional bearing capacity formulas, but with an equivalent skirt thickness equal to the real skirt thickness plus the mean grain size, to account for the grain-size effect.