Large-diameter steel pipe pile foundations, typically known as monopiles, are currently the dominant foundation solution for supporting offshore wind turbines. The design of monopiles in sandy seabed is typically based on p-y curves derived for fully drained conditions. However, in reality, the drainage condition around a monopile under cyclic loading, at least during each single loading cycle, is generally undrained. To verify the applicability of the design methods based on fully drained condition, this study conducted a series of finite element analyses examining the effect of drainage condition on the monopile soil-pile interaction in sandy seabed. Based on the analyses in four sands which are of different relative densities and particle size distributions, it is found that, for medium dense to very dense sands that exhibit dilative response upon shearing, the effect of drainage conditions can be practically ignored within the range of load relevant for practical engineering. For loose sands or sands with considerable fines that exhibit contractive response upon shearing, the drainage conditions have negligible effect on the soil-pile interaction stiffness at low to modest load levels; however, the undrained conditions can lead to lower capacities. This implies that the current design approach which assumes fully drained soil response is still acceptable for the FLS design in such soil conditions. However, for the ULS design, assumption of drained soil response may lead to overestimation of the lateral bearing capacity and assessment of the actual drainage condition and its influence on soil-pile interaction on a project-specific basis is warranted for such cases.
