Analysis of Pipe Expansion Associated With Field Hydrostatic Testing Available to Purchase

Pipeline construction projects built in the United States to the recent requirements for operation at 80% SMYS are subjected to high stress pre-service hydrostatic tests. At very high stress levels, pipe diameter expansion, as measured by an ILI caliper tool, should not be a surprising result. In some cases, however, excessive pipe expansion has occurred and been reported by PHMSA in an industry advisory bulletin regarding potentially low yield strength results for X-70 and X-80 grade pipes. In response to this advisory notice, a detailed review was made of pipe expansion conditions for the 270 mile Southeast Supply Header (SESH) pipeline which was built in 2008. This review included a statistical analysis of the pipe yield strength and dimensional characteristics prior to installation, a review of hydrostatic test levels during the pre-service field hydrotest, and the corresponding diameter results measured by the multi-channel in-line inspection caliper tool. The results demonstrate that the pipe yield strength property distribution was well within good X-70 and X-80 process capability. The purpose of this paper is to demonstrate that explanations other than low yield strength pipe may be responsible for pipe expansion.