For the past several years the Pipeline Research Committee of the American Gas Association has sponsored research at Battelle's Columbus Laboratories with the objective of obtaining a better understanding of the behavior of defects in pressurized pipe. This objective is being pursued by means of full-scale experiments on line pipe specimens containing both artificial and actual defects. These experiments have led to the development of semiempirical equations for predicting the ductile failure stress levels of through-wall flaws and surface flaws. Although these equations have been presented before, the supporting data and analyses are described in this paper. The through-wall flaw equation is analogous to fracture mechanics criteria for plane stress fracture; but because it has been adapted to ductile line pipe materials, it contains the Dugdale Model for plastic flow in the material and a correction for the bulging stress resulting from pressure acting on the curved pipe walls. The surface flaw equation evolved from the experimental results on surface-flawed pipe specimens. It accounts for both length along the axis of the pipe, depth through the wall, and the bulging which also takes place at surface flaws. Both equations have been shown to give reliable prediction of failure stress levels for not only steel line pipe materials but for stainless steel and aluminum pressure vessels as well. The usefulness of these equations extends over a wide range of material toughness and strength levels, because they embody both tensile strength parameters and the notch-toughness as determined from the ductile shelf energy of Charpy V-notch impact specimens. The experimental results upon which the equations are based are presented and discussed herein as are the utility and degree of reliability of the equations.

Presented is a discussion of an hypothesized analytical explanation of ductile fracture initiation, propagation, and arrest in cylindrical pressure vessels and piping. The hypothesized analytical treatment is an attempt to predict initiation and arrest conditions for ductile fractures using Charpy V-notch plateau energy as a means of determining the toughness of the material. Data from a number of full-scale experiments on gas transmission pipe, nuclear reactor piping, and other cylindrical vessels are presented and are shown to be in agreement with the hypothesis.

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Pipelines comprised of materials manufactured prior to about 1980 are more likely than those comprised of newer materials to contain manufacturing or transportation-induced defects. These defects may become enlarged and fail in service because of pressure-cycle-induced fatigue crack growth. While such defects do not account for a large number of service failures, they clearly are a potential threat to pipeline integrity. In fact, the current U.S. pipeline integrity management regulations require seam-integrity assessments for certain types of pipe materials that appear to be particularly susceptible to this risk. To manage the risk of failure from pressure-cycle-induced fatigue a pipeline operator may need to carry out periodic seam-integrity assessments via either hydrostatic testing or in-line inspection using a reliable crack-detection tool. The appropriate period for reassessment depends on the sizes and growth rates of potential defects that may still exist (just-surviving defects) after an initial hydrostatic test or in-line inspection. The pressure cycles applied to the pipeline may cause the just-surviving defects to grow at a rate inherent in the material and its environment. Long-established principles can be used to predict the times to failure if the effective crack growth rate is known. A pipeline operator can use these principles to plan timely re-assessments to prevent failures. This paper describes one approach to predicting reassessment intervals. This approach has evolved over a period of more than 10 years. The authors have discovered some pitfalls and blind alleys that can lead to inappropriate predictions. The purpose of the paper is to show that while the well-known and widely available basic principles are sound, their application to pipeline integrity management requires an in-depth understanding of the particular pipeline being subject to assessment.