Model test and finite element analysis results of a monopile in very dense sand under unidirectional horizontal cyclic loading

Abstract

This study is carried out to understand the response of a monopile for a large (8–9 MW) offshore wind turbine located at a 45m water depth under lateral cyclic loading. Firstly, a 1g large-scale model pile test was performed in saturated very dense sand under numerous cycles of lateral loading, N=2000. The test results show that the lateral pile head displacement and rotation increase while the secant stiffness of pile decreases with increasing N. The numerical model was then validated by comparing the pile head displacement results of the test and the finite element (FE) model analysis. The soil was modelled using the UBC3D-PLM constitutive model to understand the development of the excess pore pressure ratio (EPPR) around the monopile. The FE analysis was performed cycle by cycle up to N=5 for a large diameter “XL monopile” with diameter D=9m and embedded length L=50m. The results showed that the EPPR around the monopile is insignificant. Furthermore, the effect of the number of loading cycles on the evolution of the EPPR, pile head deformation and pile head rotation are insignificant. In addition, a relatively high maximum pile head rotation accumulation θmax≅0.13° was obtained during cyclic loading.