A Study on the Conservatism of the Dent Screening Criteria in the Canadian Standards Association (CSA) Z662:19 Oil and Gas Pipeline Systems Standard
Authors: Cousart Krystin , Holliday Chris 
Published: December 2022

Dents are a common anomaly type found in many pipelines, regardless of diameter, pipe type, pipe grade, environment, etc. It is not feasible or necessary to repair every dent found in a pipeline. In order to determine which dents do require mitigation, standards such as CSA Z662:19, provide a number of screening criteria for the identification of deformations that require further investigation, assessment or remediation.

The screening criteria outlined in Section 10.10.4 of the 2019 version of CSA Z662 includes both a depth and a sharpness criteria (i.e. length to depth ratio). For the dents reported by in-line inspection (ILI) that exceed either the dent depth or sharpness criteria, a pipe wall curvature strain-based assessment can be completed to determine their acceptability based on the likelihood of cracking that initiated with the formation of the dent. These screening criteria are intended to be conservative and were designed to identify the dents that are most likely to have unacceptable curvature strain values. However, in November 2018, the American Society of Mechanical Engineers (ASME) corrected the equations given in ASME B31.8 Gas Transmission and Distribution Piping Systems in the non-mandatory Appendix R which is used to calculate equivalent strain on the internal and external pipe surfaces.

A previous study involved a comparison of the static curvature strain results using the equivalent strain equations from both the 2016 version of ASME B31.8 and the latest 2020 version (which utilizes the corrected strain equations introduced in the 2018 edition). Over 2,500 dents, in a variety of pipelines were included in the comparison. The study identified that there is an increase in the number of dents with equivalent strains considered unacceptable based on the strain limits stipulated in CSA Z662:19. In addition, the study identified that the current length to depth ratio limit of 20 (the dent sharpness criteria) may not be sufficiently conservative; dents with equivalent strains that exceeded the allowable strain limits were considered acceptable based on the depth and sharpness screening criteria. It was noted that these dents, however, had complex dent geometry as they were either multi-apex or dents interacting with other dents.

The scope of the work reported in this paper was to expand on the previous study by increasing the sample size and considering the added strain limits for plain dents outlined in ASME B31.8-2020 (e.g. half the minimum elongation), in addition to the strain limits stated in CSA Z662:19. The study also explored the equivalent strain results for multiple peak dents and dents interacting with other dents, to determine whether the dent screening criteria within CSA Z662:19 are appropriate for dents with complex geometry.

The aim of the work was to determine whether the criteria in CSA Z662:19 are still conservative following the correction to the ASME B31.8 strain equations, while further considering additional strain limits, as well as, complex dent geometries. It was concluded that changes to the CSA Z662:19 dent depth and sharpness screening criteria would be prudent to ensure conservatism.