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Root Cause Analysis of Dent With Crack: A Case Study

[+] Author Affiliations
Udayasankar Arumugam, Ming Gao, Ravi Krishnamurthy

Blade Energy Partners, Houston, TX

Rick Wang, Richard Kania

TransCanada Pipeline Ltd., Calgary, ON, Canada

Paper No. IPC2012-90504, pp. 735-746; 12 pages
  • 2012 9th International Pipeline Conference
  • Volume 2: Pipeline Integrity Management
  • Calgary, Alberta, Canada, September 24–28, 2012
  • Conference Sponsors: International Petroleum Technology Institute, Pipeline Division
  • ISBN: 978-0-7918-4513-4
  • Copyright © 2012 by ASME


A combined caliper and tri-axial MFL in-line inspection (ILI) reported a bottom side 2.7%OD dent associated with 76% metal loss. The reported dent depth is well below the 6% limit while strain analysis of this ILI dent profile showed a maximum equivalent strain of 17.4%, which exceeds the 6% strain limit for gas pipeline. Due to the high dent strain level, the raw signals of metal loss were revisited, which indicated this associated metal loss appears to be a crack rather than corrosion. In-field investigation revealed that this dent is indeed associated with branched cracks both at internal and external pipe surface but no leak was detected.

The primary objective of this case study is to determine the cause for cracking in the dent. As part of this study, detailed investigation was performed including LaserScan based strain analysis, lab pressure-cycle testing and fracture surface examination. An attempt was made to quantify the plastic strain damage of this dent and its susceptibility to cracking using the existing plastic damage models, namely, ductile failure damage indicator (DFDI), strain limit damage (SLD) and minimum elongation limit criterion. The investigation showed that the internal cracks were formed at the time of indentation while the external cracks formed by spring-back (elastic rebounding) due to the removal of rock constraint. Full size fatigue testing of this cracked dent showed leak failure modes rather than rupture. In this paper, the approach, results and the findings are summarized and discussed.

Copyright © 2012 by ASME



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