Abstract: The design of offshore monopile is generally governed by its accumulated response to lateral cyclic load, e.g. loads induced by winds and waves. In order to investigate the characteristic of this accumulated response, a user subroutine of degradation stiffness model (DSM) is developed and incorporated into a commercial finite difference program. Based on this program, the effect of load cycle number and load character on the displacement development of monopile is quantified; and the applicability and reliability of two mostly used models for prediction of accumulated displacement are evaluated. The results show that: In dense sand ground, cyclic loading with larger load amplitude leads to a continuous increase in monopile displacement as the number of loading cycles increasing; and the maximum load in each load cycle is suggested to be less than 0.58 times its static load capacity to produce a plateau of accumulated displacement after a certain number of load cycles; logarithmic model is capable of predicting accumulated displacement induced by smaller load and underestimates the displacements for larger load levels; the power model shows its advantage over the logarithmic model for its validity under various load levels; and the value of model parameter α increases linearly with increase in the load levels. Finally, parameter values of power model are given for various load conditions.

Key words: offshore monopile, lateral cyclic loading, accumulated deformation, prediction model, numerical modelling