Effects of scour on horizontal bearing behavior of monopole foundations for offshore wind turbines

Abstract

Monopile is widely used as a typical foundation for offshore wind turbines (OWT). Local scour usually occurs around offshore pile foundations, which may bring the reduction of both the effective stresses in the surrounding soil and the effective pile embedment. Until now, the effects of local scour on the lateral pile-soil interaction have not been well understood. In this study, a three-dimensional finite element model for the lateral pile-soil interaction is proposed and verified with experimental results. The p-y curves describing the pile-soil interaction are extracted from a series of numerical results with various sandy soil conditions and pile parameters. A parameterized expression of the p-y curves for large-diameter monopile in sand is proposed. The results indicate that scour could induce a significant transition of pile behavior for a monopile typically employed for OWT. If the pile exhibits rigid structural behaviors after scour, the pile’s tip and shaft resistance may become a significant component of the whole soil resistance, which would render the traditional p-y approach inapplicable. For a given depth below the scour base, the p-y curves get significantly stiffer with increasing scour depth, especially at relatively shallow depths. The scour effects on the variation of p-y curves from the present numerical results are generally consistent with the existing centrifuge results. As the slope angle of scour holes decreases, the effect of the remaining sloping overburden soil above the level of the scour base reduces and the stiffening of the p-y curves at a given depth below the scour base is correspondingly alleviated.