Response of a wind turbine structure to strong ground motions and high velocity winds

The objective of the present study is to evaluate the performance of the wind turbine tower for earthquake and high velocity wind loading events. The performance was evaluated by probabilistic approach by employing fragility analysis. For earthquake fragility analysis four displacement based limit states: a) global buckling b) yielding c) permanent deformation d) foundation overturning were defined. The wind fragility analysis was performed by defining three limit states: a) global buckling b) yielding, c) foundation overturning. The demand data, as required for fragility analysis, comes from the full scale nonlinear analysis by applying earthquake and wind loads. The capacity data comes from the pushover analysis by applying loads along the different directions.;Few studies in the recent past include creating the simplified wind turbine model, and analyzing the particular type of response. In the present study, a full scale model of a wind turbine tower based on Vestas 1.65 MW was created with details such as door and cable opening at the bottom. The height of the tower was 80 m. The nacelle and rotor were simplified as concentrated mass at the top of the tower. The finite element model comprises of shell and solid elements for main body of the structure, the point mass elements for nacelle and rotor assembly. The turbine model was validated for geometric, engineering and dynamic properties.;The seismic loadings were applied as earthquake accelerations availed from PEER (Pacific Earthquake Engineering Research) center database. The earthquake loads were the time history acceleration records in orthogonal directions. The preliminary analysis performed by applying original earthquake loads did not fail the turbine tower. The earthquake records were scaled by the factor of 2.5 so as to fail the structure. The near and far fault acceleration loadings were applied at the base of the wind turbine tower. The wind loadings were applied as wind fields generated by TURBSIM for various mean wind velocities. The wind loads were applied with the attack angles of 0 and 30 degrees with respect to the rotor axis.;In the case of near fault earthquake loading, the maximum magnitude for the response spectral acceleration in the present study was 1.715 g. When the response spectral acceleration reaches to 1.715 g, the exceedance probability for global buckling was found to be 5.7%. The exceedance probability for yielding was found to be 68%. The exceedance probability for permanent deformation was found to be 26.4%, whereas the exceedance probability for foundation overturning was found to be 99.4%. In case of far fault loading, the maximum magnitude of spectral acceleration was 0.57 g. When the response spectral acceleration reaches this value the exceedance probability was found to be 0% for global buckling, 0.4% for yielding, 0.25 % for permanent deformation and 10.8% for foundation overturning limit state. For both the near and far fault loading, the foundation overturning was found to be the most critical limit state. This foundation was not designed for an earthquake prone region. The analysis was performed by improving the foundation by increasing the width from 15 m to 16.56 m. The exceedance probability rendering to failure was found to be reduced significantly.;In the case of high velocity wind, the maximum mean wind velocity was 70 m/s. In case of 0 degree wind direction, the exceedance probability for all the three limit states were found to be 0% even when the mean wind velocity reaches to the maximum value of 70 m/s. In case of 30 degree wind direction, when the mean wind velocity reaches to 70 m/s, the exceedance probability for global buckling was found to 0%, the exceedance probability for yielding was found to be 88%, the exceedance probability for foundation overturning was found to be 100%. This foundation also was adopted from a hazard-free region and shown to be susceptible to failure. The exceedance probability leading to the failure was found to be reduced when the foundation was improved by increasing the width from 15 m to 16.56 m.;For the wind turbine studied in this research, it could be concluded that the foundation overturning is the most critical limit state, and needs a due consideration while designing the wind turbine tower system. In case of earthquake, the near fault earthquake loading is the most detrimental to the stability of the wind turbine structures. The first mode failure governs the safety and satiability of the wind turbine structures. The 30 degree wind direction loading is more critical wind direction compared to 0 degree wind direction loading. Repeating the analysis for different wind turbine types, capacities and sizes would give more data, and more comprehensive conclusion could be drawn. (Abstract shortened by UMI.).