A Study on Maximum Stress of the Strength Test on Oil Pipelines

The fatigue propagation of residual cracks after the pipeline being pressure tested is one of the main factors affecting normal pipeline service life. Starting with fracture characteristics of the defects in pipeline, the paper analyzes the influence of test pressure stress on the propagation life of the residual cracks in pipeline. It points out that the overload peak of the high stress from test pressure has a considerable delay effect on crack propagation rate after pressure test: the higher the overload peak, the greater the delay effect; and that the overload peak enlarges the crack size, resulting in the reversal of pipe's load-bearing capacity and the accelaration of crack propagation. The quantitative analysis shows that at smaller lengthdepth ratio of surface cracks, the reversal of load-bearing capacity is minimum, and the influence on crack propagation rate is also quite small. And what to be surmounted through pressure test is the occurrence of large sized cracks. Therefore, it is concluded that the higher the stress from test pressure, the longer the crack life and the longer the pipeline service life. In the light of the calculation of crack's residual life and the actual conditions of the current pipes in China, the paper points out that the maximum stress of strength test on pipeline shall be the yield limit of the pipe.