Effect of axial force and bending moment on the limit internal pressure of dented pipelines

The extrusion and scraping action of rocks at the bottom of buried pipelines can lead to dent and scratch defects. Beside internal pressure, the pipelines should also bear other forms of loads during its operation, such as bending moment and axial force. The combination of internal pressure and these loads can make the pipe easy to damage. In order to study the influence of bending and axial loads on the limit internal pressure of dent-scratch pipelines, large numbers of finite element calculations are carried out in this paper. Through the comparison with experimental results, a failure criterion is proposed for finite element analysis (FEA). It is found in actual buried pipelines during pipe inspection that the existed maximum bending strain is equal to 0.4%. Take the bending moment which leads to the axial strain of 0.4% as the reference moment. A series of finite element calculations are conducted to analyze the influence of various bending loads on the limit internal pressure of pipelines with dent-scratch defects. In addition, the effect of axial compression loads on the limit internal pressure is also studied by FEA. According to the finite element results, it is concluded that both axial load and bending moment can decrease the limit internal pressure compared to one subjected to internal pressure only. Moreover, the tendency of the impact is approximately linear. Finally, an expression of limit internal pressure with respect to pipe material, pipe and defect dimensions and the magnitude of axial force is fitted, which provides a strong guidance for the safety assessment of the defective pipelines.