Finite Element Modelling of Long-Term Cyclic Response of Monopiles in Sand Using a Stiffness Degradation Approach

Abstract

Prediction of the permanent mudline rotation caused by long-term cyclic loading is of vital importance for the design of offshore wind turbine supported by monopile foundations. Modelling the cyclebycycle pile response using advanced soil models, considering the lifetime cyclic loads, in 3D finite element analysis is prohibitive due to the model complexity and computational cost. A simple stiffness degradation methodology is presented which adopts a practical equivalent degradation approach. This method includes a bounding surface model that captures the stress-strain relationship of sand in the first load cycle and a new equivalent degradation approach scheme for modelling strain accumulation in subsequent cycles. Cyclic strain accumulation in the soil is modelled using a new equivalent stiffness degradation approach, in which the plastic modulus and dilatancy relationship are scaled based on a strain accumulation law. The new cyclic approach outlined in this study can be applied to any monotonic constitutive model and importantly allows decoupling of the cyclic accumulated volumetric and deviatoric strains. The approach presented can therefore easily be implemented and added to other monotonic constitutive models. Within this study the model is calibrated to monotonic and cyclic triaxial tests data on sand and used in finite element modelling of monopile response considering realistic cyclic loading.