Application of an Undrained and a Partially Drained Cyclic Accumulation Model for Monopile Design

Monopile design for offshore wind turbines has traditionally considered sand layers as drained in conventional beam spring analyses and in finite element analyses. However, with the large diameters commonly used in practice, the sand is generally close to undrained during one load cycle. The recently developed partially drained cyclic pore pressure accumulation model (PDCAM) has been implemented in the 3D FE program PLAXIS. This model accounts for the soil degradation under partially drained cyclic loading. For the clay layers, the undrained cyclic strain accumulation model (UDCAM), which accounts for soil degradation under undrained cyclic loading is used. The performance of both models is demonstrated by 3D finite element analyses of a monopile subjected to cyclic loading, with a soil stratigraphy that includes both undrained and partly drained material behavior. Results in terms of displacements of the pile and soil reactions are compared with traditional beam-spring analyses.