Field Tests on Penetration Resistance of Offshore Wind Foundations in Scour Protection

Field tests were conducted to evaluate the penetration resistance of steel plates in scour protection systems, usually employed on bottom-fixed offshore foundations. Scour protection typically consists of loose rock, whose particle size is comparable to the thickness of the skirts or piles, introducing a grain-size effect. To investigate this effect, plates under plane-strain conditions with four different thicknesses were tested in limestone gravel. In addition, interaction between closely spaced skirts was examined through double-plate configurations. The results indicate that penetration resistance increases with both penetration depth and plate thickness, and the response followed four phases: (i) an initial sharp increase in resistance, (ii) a transient phase while the failure mechanism develops around the tip, (iii) a linear increase once the mechanism is fully mobilized, and (iv) a plateau at a critical depth where resistance becomes nearly constant. Interaction effects between double-plate skirts were negligible at shallow penetration depths. A comparison with previous tests in high-density gravel of similar mean particle size showed lower penetration resistance for the limestone material. Analytical predictions based on bearing capacity theory agreed with the data when grain-size effects were included using an equivalent plate thickness equal to the actual thickness plus the mean particle diameter.