# Seismic Effect of Marine Corrosion and CFRP Reinforcement on Wind Turbine Tower
## 2 Establishment and Verification of FE Model
### Dynamic Characteristics and Model Verification of Wind Turbine Tower  
Based on the Lanczos method of ABAQUS, the mode shapes and natural frequencies of wind turbine tower structures are analyzed. The first four orders of mode vibration are shown in Figure 7. The first mode of vibration of the structure is left and right oscillation of the blade and the superstructure, the second mode is the front and rear vibration, the third mode is mainly the torsional deformation of the blade and the superstructure, and the fourth mode is the bending deformation of the structure.  
Figure 5: FE model with random corrosion.  
Figure 6: The wall thickness of the tower varies with time.  
To verify the correctness of the finite element model, the calculated natural frequency of the FE model is compared with the structural natural frequency data measured by the \"braking test\" of the wind turbine. Two wind turbine towers are selected as experimental objects, namely 6# and 15#. The \"braking test\" refers to the emergency shutdown of the running fan system. The excitation effect is generated by the emergency shutdown, so as to measure the vibration characteristics of the whole wind turbine tower. The excitation effect is generated by the emergency shutdown to measure the vibration characteristics of the entire wind turbine tower. The statistics of identified natural frequencies and damping ratios of the structure obtained through the \"braking test\" are shown in Table 2.  
Table 3 shows the comparison of natural frequencies of wind turbine. The first-order natural frequency of the structure measured in two \"braking tests\" of 6# wind turbine is 0.319 Hz and 0.318 Hz, respectively. The difference between the calculated value of the finite element model and the measured value is 9.1% and 9.4%, respectively. The first natural frequency of the structure, measured twice by 15# wind turbine, is 0.327 Hz and 0.331 Hz, respectively. The difference between the calculated value of the finite element model and the measured value is 5.7% and 6.8%, respectively, both within the allowable range of the project, which can prove the rationality of the finite element model.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multicolumn{3}{c}{**6\# Wind Turbine Tower**} & \multicolumn{3}{c}{**15\# Wind Turbine Tower**} \\ \cline{2-7}  & **Frequency** & **Natural Period** & **Damping Ratio (\%)** & **Frequency** & **Natural Period** & **Damping Ratio (\%)** \\ \hline
1 & 0.319 & 3.135 & 1.01 & 0.331 & 3.021 & 0.99 \\
2 & 0.318 & 3.145 & 1.02 & 0.327 & 3.058 & 1.00 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Statistics of identified natural frequencies and damping ratios.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Wind Turbine Tower** & **Test** & **FE Value (Hz)** & **Measured Value (Hz)** & **Difference Value (\%)** \\ \hline
6\# & 1 & 0.351 & 0.319 & 9.1 \\
2 & 2 & 0.351 & 0.318 & 9.4 \\ \hline
15\# & 1 & 0.351 & 0.331 & 5.7 \\
2 & 2 & 0.351 & 0.327 & 6.8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of natural frequencies of wind turbine.  
Figure 7: Model shapes of the tower.