## 5 Conclusions  
We have considered the complex working conditions of buried pipelines with internal corrosion defects in seasonally frozen soil regions, and a finite element model of a three-dimensional pipeline-soil thermo-mechanical coupling model was established. The temperature field has been verified and the thermo-mechanical coupling analysis has been carried out in this paper. The stress distribution characteristics of the inner wall of the buried pipeline without damage under different pipeline internal pressures were analyzed, and the maximum location of the Mises equivalent stress in the inner wall of the pipeline was determined. On this basis, the equivalent stress distribution characteristics  
Figure 19: (**a**) The comparison of calculation values by finite element model with values of fit formula; (**b**) Comparison curves for residual values. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & \multicolumn{2}{c}{**Corrosion**} & \multicolumn{2}{c}{**Corrosion**} & \multicolumn{2}{c}{**DNV-RP-**} & \multirow{2}{*}{**B31G**} & \multirow{2}{*}{**Blast Data**} & \multicolumn{2}{c}{**Fitting**} \\  & **Length (mm)** & **Width (mm)** & **Depth (mm)** & & & **F101** & & **Formula** \\ \hline
1 & 100 & 50 & 8.8 & 22.97 & 21.04 & 24.30 & 22.90 \\
2 & 200 & 50 & 4.4 & 23.10 & 21.3 & 24.11 & 24.17 \\
3 & 200 & 50 & 8.8 & 19.69 & 18.17 & 21.76 & 20.95 \\
4 & 200 & 50 & 13.1 & 13.78 & 13.72 & 17.15 & 16.80 \\
5 & 200 & 100 & 8.8 & 19.69 & 18.17 & 23.42 & 20.98 \\
6 & 200 & 200 & 8.8 & 19.69 & 18.17 & 22.08 & 20.99 \\
7 & 300 & 50 & 8.8 & 17.60 & 16.71 & 19.08 & 19.41 \\ \hline \multicolumn{7}{c}{Maximum error} & \multicolumn{2}{c}{3.727} & \multicolumn{2}{c}{5.250} & \multirow{2}{*}{} & 2.436 \\ \multicolumn{7}{c}{Minimum error} & \multicolumn{2}{c}{1.010} & \multicolumn{2}{c}{2.370} & \multirow{2}{*}{} & 0.058 \\ \multicolumn{7}{c}{Residual sum of squares} & \multicolumn{2}{c}{40.221} & \multicolumn{2}{c}{90.644} & \multirow{2}{*}{} & 9.984 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Verification of the results of the fitted equations with the pipeline bursting data, DNV-RP-F101 and B31G evaluation results. and residual strength of buried pipeline with internal corrosion defects were studied by orthogonal design method, and the following conclusions were obtained:  
The location of the maximum Mises equivalent stress on the inner wall of the non-destructive pipeline is near the junction of the transition section and the frost heave section. When the temperature is the lowest, the Mises equivalent stress is the largest. 1. The residual strength of pipelines with internal corrosion defects in permafrost regions can be evaluated safely and reliably by the orthogonal analysis method. The calculation is simple and convenient for engineering applications. 2. The corrosion depth of the pipeline in the frozen soil area is the main factor affecting the residual strength of the pipeline; as the depth of corrosion defects increases, the residual strength decreases. The corrosion length is the second; but when the corrosion length reaches 600 mm, its effect on the residual strength of the pipeline is no longer significant. The corrosion width has the least effect on the residual strength. 3. Based on the finite element numerical simulation data, a formula for calculating the residual strength of the pipeline with internal corrosion defects in seasonally frozen soil region was obtained by fitting. Compared with the existing corrosion evaluation specifications, the calculation results of the fitting formula obtained according to the stress concentration theory have small errors and uniform error distribution, which can better meet the prediction requirements of failure pressure of oil and gas pipelines with internal corrosion in seasonally frozen soil regions. Conceptualization, [PERSON] ([PERSON]), [PERSON] ([PERSON]), and L.H. ; methodology, [PERSON] ([PERSON]), G.C., [PERSON] ([PERSON]), [PERSON], and L.H. ; software, G.C. ; validation, [PERSON] ([PERSON]), and G.C.