Abstract
Accurate evaluation of the remaining strength of cracklike flaws identified via pipeline inline inspection (ILI) or in-ditch non-destructive examination (NDE) is critical to ensuring continued safe operation of liquid and gas transmission pipelines. Modern pipeline ILI tools have sufficient resolution to detect longitudinally overlapping crack-like flaws that exist in the same radial plane, referred to as stacked cracks. Depending upon the crack sizes and pressure loading, stacked cracks can interact to reduce burst pressure below that of any of the individual stacked cracks.
Closely located cracks are often evaluated using interaction criteria, such as those provided by API 579-1/ASME FFS-1 Part 9 (API 579) [1], which specify how and when multiple nearby cracks can be combined into a single crack for the purpose of an integrity assessment. When applied to stacked cracks, the interaction criteria can often lead to recategorizing stacked cracks as a through-wall crack, which requires urgent response from the pipeline operator.
Here, an improved interaction criterion was developed for stacked cracks based on the results of elastic-plastic finite element analysis (FEA) models of multiple combinations of stacked crack sizes and orientations, pipe material properties, and operating stress. These improved interaction criteria provide pipeline operators with an easy-to-apply methodology to analyze stacked cracks that reduces the excess conservatism associated with legacy methods.
