Validation of Fatigue Models for ERW Seam Weld Cracking

In response to the National Transportation Safety Board (NTSB) Recommendation P-09-1, the Department of Transportation (DOT) Pipeline and Hazardous Material Safety Administration (PHMSA) initiated a comprehensive study to identify actions that could be implemented by pipeline operators to significantly reduce longitudinal seam failures in electric resistance weld (ERW) pipe. As part of the project, Task 4 in Phase II was designed to validate existing failure prediction models and, where gaps exist, refine or develop these models needed to assess and quantify defect severity for cold welds, hook cracks, and selective seam weld corrosion (SSWC) (the primary ERW/Flash Weld seam threats) for failure subject to loadings that develop both during hydrotests and in service. These models would then be used to develop software to support integrity management of seam welds with enough flexibility to benefit from the experience gained during this project. The purpose of this paper is to review the models used for fatigue crack growth rate (FCGR) calculations. Both the Willenborg Model, which is used to incorporate the retardation of crack growth after an overload occurs (such as a hydrostatic test in a pipeline), and the Walker Model, which is used to account for variation in stress ratio during the operation of a structure (i.e. pressure cycles in a liquid pipeline), will be discussed. Laboratory fatigue crack growth rate test results on several grades of pipe will be used to generate the constants employed in these models. The reports generated during the course of the project are publicly available and are located on the following PHMSA website: http://primis.phmsa.dot.gov/matrix/PriHome.rdm?pri=390.