The Effect of Patch Repair on Stress Distribution in a Damaged Polyethylene Gas Pipe

In this work, finite element analysis is used to calculate the stress distribution in a patch repaired defected polyethylene (PE) gas pipe under internal pressure. The burial depth of the PE80B pipe was selected to be 125 centimeters while the patch material was assumed to be from high density polyethylene (HDPE). During loading process, surcharge load, temperature change, soil’s column weight and soil – pipe interaction were included in the analysis. Four types of patch arrangement were selected to repair the damaged pipe. According to the results, the semi-circular and saddle fusion patches reduce the peak von Mises stress in the pipe by almost the same amount, while the peak von Mises stress in the patch is the least for saddle fusion repair. The resulting stresses developed in the pipe and patches were compared with their corresponding reduced strengths for a life expectancy of fifty years. The shape of the defect hole in the pipe was considered to be circular or elliptic. For elliptic defects, various minor to major diameters, a/b, were selected to simulate a circular to a crack shaped defects. According to the results, for a saddle fusion repair, as the defect shape approaches that of a crack, the peak von Mises stress in the pipe almost doubles and exceeds the pipe allowable strength for a working life of 50 years. For larger values of a/b, the stress level in the patch repaired pipe is well below its allowable value for the same life expectancy.

Keywords:

Patch repair , buried gas pipe , HDPE , temperature variation