The authors propose a failure criterion for pipelines with surface and longitudinal flat through defects based on destructive testing of specimens with induced cracks over the width and thickness. The specimens are prepared from pipeline walls. The crack depth, the failure load, and the specimen metal deformation curve are used to build the rated rupture stress to crack depth function and to determine the failure toughness values for crack propagation over the wall thickness and for through crack propagation cases. The suggested criterion is found to be aligned with the ultimate crack resistance failure criterion. Assuming the stress-strain state at failure in a cracked specimen to be close to the stress-strain state at failure in the area of a longitudinal surface crack in a pipeline the suggested approach can be applied to assess the strength of a cracked pipeline, which has the wall thickness and the deformation curve similar to that of the specimen. The article considers the conditions for developing the so-called "leak to failure" in a pipeline depending on the failure toughness anisotropy coefficient. We have proposed and substantiated the approach to ranking flat defects in a pipeline depending on the severity level using the development of the leak to failure. The consequences of a leak are substantially less severe than that of a failure. This fact shall be taken into account when ranking flat defects by the severity level. Therefore, if the surface defect length is known from the smart pigging results the type of possible loss of tightness, i.e. rupture or leakage, can be assessed well before the accident. The defects which can lead to a rupture should be repaired as the first priority.
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DOI: http://dx.doi.org/10.1520/stp49657s
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