Introduction  This paper conducts study based on the design of monopile foundations for offshore wind turbines in sandy seabed, to analyze the influence of drainage conditions on soil-foundation interaction, and compare the initial stiffness of soil-foundation interaction obtained by prevailing design methods.

  Method  The finite element method was adopted to analyze the soil-foundation interaction of laterally loaded large diameter monopile foundations. An simple anisotropic sand constitutive model, i.e. SANISAND constitutive model based on the bounding surface theory was used for numerical simulations. Analysis of laterally loaded large diameter monopile foundations under different drainage conditions was performed to study the influence of drainage conditions on soil-foundation interaction. And the numerically derived p-y curves were compared with those predicted by the API method for comparison of the initial stiffness of soil-foundation interaction.

  Result  It is found that the influence of drainage conditions on the soil-foundation interaction of laterally loaded large diameter monopile foundations is dependent on the load level. At a relatively low load level, the difference in drainage conditions has negligible influence on soil-foundation interaction stiffness. However, as the load level increases, such influence becomes significant. It is also found that the API p-y curves overestimate the soil-foundation interaction stiffness of laterally loaded large diameter monopile foundations in sandy seabed.

  Conclusion  The influence of drainage condition on the soil-foundation interaction of laterally loaded large diameter monopile foundations is dependent on the load level, under the load level concerned in engineering design, the influence of drainage conditions on the stiffness of soil-foundation interaction of large-diameter monopile foundations is negligible, and it is viable to design monopile foundations based on fully drained conditions. The API p-y curves over-predict the stiffness of soil-foundation interaction.