Influence of bounding surface plasticity-based soil-structure interaction model on integrated dynamic behaviour of jacket offshore wind turbines

The increasing mean sea depths have necessitated wind turbine foundation to have larger moment resistance capacity, from early design of monopiles to recent piled jackets. Design-oriented pile-soil interaction model (API t-z model) is modified for cyclic loading with a simple correction factor, with little attention paid to stiffness degradation and displacement accumulation caused by cyclic shakedown and ratcheting. Assisted by a bounding surface plasticity-based cyclic t-z model, this study aims to investigate the influence of t-z modeling on integrated analyses of jacket offshore wind turbines through modifying the open-source OpenFAST software. Demonstrated by the NREL 5 MW offshore wind turbine supported by piled jacket, the results show that the cyclic weakening of pile-soil interface leads to an upright load transfer from the vertical interface of the pile with degraded t-z resistance, to its lateral interface by mobilizing more p-y resistance. Ignorance of the stiffness degradation and displacement accumulation would mis-estimate modal properties, cumulative deformation, loading sharing behavior and stress transfer mechanism significantly, suggesting the model's merits in deformation control and stress transfer for piled jacket in feature design.