FREQUENCY DEPENDENT IMPEDANCE ANALYSIS OF THE FOUNDATION-SOIL-SYSTEMS OF ONSHORE WIND TURBINES

Due to the growing use of wind energy, wind turbines need to be built on sites with poor load bearing character. To increase stability und durability of the turbines, dynamic investigation of the holistic system consisting of turbine, tower and foundation, including soil-structure-interaction (SSI) effects need to be carried out. The use of different foundations, shallow or pile foundations, on varying soil conditions affects the overall dynamic behavior of the slender wind turbine tower. Therefore, a frequency tuning might be necessary while designing the tower. The influence of the SSI effects on slender structures is well known in research and practice. However, the choice of adequate and problem oriented SSI calculation belongs to the experience of the structural engineer. The available methods vary from simple analytical models to highly sophisticated numerical methods, demanding high calculation costs. This study attempts to streamline this selection process, presenting an impedance catalogue for an onshore wind turbine, to be used by practical engineers to estimate the dynamic impedance and the vibration behavior of the system in few steps. Frequency dependent impedance functions are presented for shallow and pile foundations on homogenous and layered soils. These functions are computed using the substructure method, dividing the overall system into structure and soil model. While the structure of the wind turbine is modelled via the finite element method, the soil model is constructed using the boundary element method. Concluding the holistic model is exposed to wind load to examine the SSI influence during dynamic loading.

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