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

Offshore wind turbines are often built on large-diameter monopile, which are exposed to lateral loads and overturning moments from wind and wave action, which is cyclic in nature. In this work, an attempt was made to examine the influence of embedded length on monopile behaviour using finite-element analysis (Plaxis 3D). The developed model was validated using a centrifuge test conducted on monopile embedded in sand. Numerical studies were conducted on a 6 m diameter monopile by varying the load amplitude and embedded length ratios. 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 pile monotonic ultimate capacity. The effects of embedded lengths and load amplitude on the accumulated displacements and the cyclic stiffness were studied. It was found that the rate of accumulated displacement increased by a factor of 0.3-0.7 when the load amplitude increased from 30% to 50%. An increase in load amplitude was also observed to cause a linear increase in accumulated displacements. The slope of the attenuation value was found to be higher for an increase in the L/D ratio from 4 to 5 as compared to an increase from 5 to 6.