Influence of Pile-Soil Interaction on Dynamic Properties and Response of Offshore Wind Turbine with Monopile Foundation in Sand Site

In this study, a sophisticated 3D finite element model of NREL 5MW wind turbine (WT) superstructure is developed using Abaqus, which includes necessary functional features, such as rotor rotation, drivetrain, pitch, servo-control, etc. A multi-step validation process has been conducted in order to verify the reliability of the blade, the Rotor-Nacelle Assembly (RNA) and the entire WT model. In the time domain, a software framework of ABA-OWT is developed for the dynamic analysis of offshore wind turbine (OWT) based on the Abaqus WT structure model and FAST submodules. The framework is capable of computing wind loads, wave loads, and servo-control commands in a fully coupled way. The pile-soil interaction (PSI) is modeled by nonlinear p-y, t-z and Q-z springs and the dashpots parallel with them. A typical sandy site is considered, while several identical analyses are performed for both numerical models including fixed base and PSI. After calculating the eigenvalues of the WT structure and analyzing its dynamic response to the combined action of wind and wave, three representative wind-wave load cases are chosen for subsequent seismic response study. The important conclusions are as follows. Firstly, the PSI reduces the natural frequencies of FA1, SS1 and FA2 by about 12% and SS2 by 23.5%. Secondly, under the combined action of wind and wave, the PSI significantly increases the tower-top peak motion responses in normal operating cases, but its influence on the mudline peak internal force responses is almost negligible; For parking cases, the PSI dramatically enlarges the peaks of the tower-top motion responses and the mudline bending moment, and the effect of PSI on the structural peak responses in parking cases is greater than that in normal operating cases. Finally, under the conjoint action of wind, wave and seismic, the PSI increases the tower-top peak motion responses and reduce the mudline peak internal force responses, and because of the aeroelastic effects, the influence of PSI on the structural peak responses in normal operating cases is always the smallest. In a word, the PSI has a remarkable impact on the natural frequency, dynamic response and seismic response of offshore wind turbine (OWT) with monopile foundation, which should be considered in a reasonable way in the practical offshore wind power project.