Jacket foundation is typically the preferred choice for Offshore Wind Turbines (OWTs) erected in water depth varying from 40 m to 80 m. In this paper, an integrated dynamic analysis model is designed to study the coupling between aerodynamics, servodynamics, hydrodynamics, soil-structure interaction for piled jacket OWTs. The performances of the AeroDyn and ServoDyn modules are verified by FAST, showcasing their applicability under deterministic and stochastic environmental conditions. The OWT dynamic responses, especially for t-z modeling, stress-transfer mechanism and structural fatigue damage, are subsequently studied. The overall deformation of the jacket calculated by the nonlinear elastic t-z curve in the API guideline, is overwhelmed by the t-z curve formulated using bounding surface plasticity framework, due to the ignorance of the loading history effect. Accompanied by a compressed-released-recompressed stress-transfer process, the downwind tube would experience high stress level, hence necessitating more attention in the ultimate limit state design of piled jacket structure. Otherwise, the upwind tube seems to be more decisive to the fatigue limit state design of piled jacket structure, owing to severe fluctuation in structural stress caused by a tensed-released-re-tensed stress-transfer tendency.

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.