Model tests are carried out on the large-diameter single pile and the conventional long pile under lateral cyclic loads in clay

which aims to investigate the behavior of pile foundations of offshore wind turbines under the action of cyclic loads from wind and wave. Based on the backbone by API and Masing rules

a cyclic p-y curve is formed

where p is load and y is displacement. The method is presented to calculate the cyclic degradation of a pile subjected to lateral cyclic loading

using a typical clay model in the literature

which modifies the undrained shear strength by accumulated absolute plastic shear strain. By relating cycle numbers and cyclic amplitudes of lateral load to the cyclic degradation of soil around pile

the method can consider various types of lateral cyclic load in offshore projects. The results by model tests and theoretical calculations demonstrate that

under the same cyclic load

the cyclic resistance capacity of a large-diameter single pile with larger stiffness is better than that of a conventional long pile. For the design of a large-diameter single pile foundation of the offshore wind turbine

it is more rational to consider the post cyclic lateral capacity of the foundation with the residual strength of clay.