PSI - Issue 64

Serdar Soyoz et al. / Procedia Structural Integrity 64 (2024) 484–491 Soyoz et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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4. Seismic fragility analysis To obtain seismic fragility curves; firstly, stress value at which local buckling occurs was determined by pushover analysis and then, seismic fragilities were developed by time history analysis under seismic and wind loading. To perform pushover analysis, a nonlinear finite element model of the wind turbine was developed in ANSYS software with parameters found in the updating process. Displacement controlled pushover analysis was performed in two steps. In the first step, the tower was analyzed under dead load which already created stresses on the tower due to the eccentric location of the center of mass of nacelle + rotor masses. Dead load analysis also creates base shear in the direction of loading and pushover curve starts from a negative value. In the second step, lateral displacement in the fore-aft direction was applied incrementally to the top of the wind turbine tower. Stress distribution through height is provided in Fig. 7 for the steps just before and after buckling which occurs at 2.15 m of top displacement. Pushover analysis results show that buckling starts when the minimum principle stress (absolute maximum stress in the compressive side) reaches - 392 MPa.

Fig. 7. Stress distribution just before and after buckling.

For time history analyses, 11 earthquakes compatible with the design spectrum with a 475 years return period were selected. Seismic fragility curves of the wind turbine tower were developed under different wind loading conditions as 5 m/s (mean speed),15 m/s, 25 m/s (cut-off speed) and parked condition as shown in Fig. 8. Rotor thrust (force time history) acting on tower top was obtained by FAST software which is an aero dynamic simulation tool developed by Jonkman and Buhl (2005).

Fig. 8. Fragility curves.