L51643 Development and Validation of a Ductile Flaw Growth Analysis for Gas Transmission Line Pipe

Hydrostatic testing is widely used to demonstrate pipe-line integrity. The test has been used to validate the maximum allowable operating pressure (MAOP) of lines being commissioned. As existing pipelines age, the test is also being used to revalidate integrity following repairs. Repeated hydrotesting over some interval of time is also being considered as a regulatory measure. Both the initial hydrotest and subsequent repeated hydrotesting introduce the possibility for flaw growth, with low cycle fatigue being an obvious concern. Historically established safe operation points to the adequacy of hydrotesting to operating pressure ratios that range in the U.S. from 1.10 to 1.5 times MAOP. Federal regulations limit the peak MAOP for gas transmission pipelines to a value corresponding to 72 percent of the specified minimum yield stress (SMYS) of the material and set the minimum test pressure at 1.25 times the maximum operating pressure (MOP).

This report is the third in a series of topical reports that document the development and validation of a model that simulates the ductile growth of axial part-through-wall (PTW) flaws in linepipe. This program is performed under the auspices of the Pipeline Research Committee of PRCI and control of the NG-18 Structural Integrity Subcommittee chaired by Dr. Brian Rothwell. The objective of this study was to develop a validated model to characterize ductile growth of axial PTW flaws due to the effects of a hydrotest or repeated hydrotesting.