Finite element analysis of a monopile under one-way and two-way lateral cyclic loading

Offshore Wind Turbines (OWT) are usually constructed on a large diameter monopile. These foundations are subjected to lateral loads and overturning moments due to wind and wave action. The lateral loads from a violent wind and waves are cyclic. In this paper, an attempt has been made to examine the influence of embedded length on monopile behaviour using finite element analysis (Plaxis 3D). The centrifuge test carried out on a monopile embedded in sandy soil was used to validate the constituent model. The numerical studies were performed on a 6m diameter monopile by varying the load amplitude and embedded length ratios (L/D = 4, 5, and 6). The monopile was subjected to both one-way and two-way symmetric lateral cyclic loading with an amplitude of 30%, 40%, and 50% of the monotonic ultimate capacity of the pile. The effect of embedded lengths and the load amplitudes on the accumulated displacements and the cyclic stiffness were studied. From the study, it is found that the rate of accumulated displacement increases by a factor of 0.3 to 0.7 when the load amplitude increased from 30% to 50%. Also, the load amplitude was observed to cause a linear increase in the accumulated displacements. The slope of attenuation value was found to be higher for an increase in the L/D ratio from 4 to 5 as compared to 5 to 6.