Most commonly, ILI metal-loss and geometry tools (and, in some cases, ILI crack-detection tools) are used to detect and report the characteristics and dimensions of dent defects. The ILI tools can provide information on the location and shape of the deformation, the nature of the damage, and its association with other features (metal loss, cracks, long seam or girth welds). Indeed, dents are found in the majority (> 80%) of pipeline miles inspected, with more than 50% of pipelines containing 10 or more dents.

Although the pipeline industry does recognize the potential threat from dents, much of the published guidance is limited to a combination of the nature of the damage (such as the presence of metal loss, stress risers, location, etc.) and a simple depth-based assessment of the deformation. In the US, prescriptive rules of this type are in place to provide operators with the timescale for the investigation and remediation of different forms of dent.

However, with unintentional releases still occurring in service from dents (from excavator impact damage and of construction origin) the current industry thinking and research supports the use of more-advanced assessment techniques (beyond the depth-based rules). These enhanced-assessment techniques make use of the detailed dent profile information obtained from high-resolution geometry tools and other supporting information on the presence and severity of stress risers from ILI tools. Indeed, the US regulations do allow operators an option to engage such techniques to re-evaluate the prescriptive timescales for certain dent categories.

This paper describes two levels of enhanced dent assessment that can be utilized to rank dents in order of severity and to assess their significance and need for remediation, and discusses their application, supported by real case study information.

References

1. Baker, 2004. Department of Transportation, Office of Pipeline Safety, TTO Number 10, Integrity management program - dent study. Delivery Order DTRS56-02-D-70036, Final Report, November.
2. Rosenfeld, 2001. Proposed new guidelines for ASME B31.8 on assessment of dents and mechanical damage. GRI, May.
3. ASME B31.8, 2003. Gas transmission and distribution piping systems.
4. Kiefner, 2006. Presentation on pipeline incidents caused by mechanical damage given at the Mechanical Damage Technical Workshop, Feb 28 to Mar 1, Houston.
5. Gelb, 1974. Applied optimal estimation. MIT Press.
6. Stroud, 1995. Engineering mathematics. 4th edn, Macmillan.
7. ASME B31.4, 2002. Pipeline transportation systems for liquid hydrocarbons and other liquids.
8. Rosenfeld et al., 1998. Strain estimation using Vetco deformation tool data. IPC 98, Calgary, June.
9. Alexander and Kiefner, 1997. Effects of smooth rock dents on liquid petroleum pipelines. API Publication 1156, Nov.
10. Alexander and Kiefner, 1999. Effects of smooth rock dents on liquid petroleum pipelines (Phase 2): Addendum to API Publication 1156, Oct.
11. Rinehart and Keating, 2002. Length effects on fatigue behaviour of longitudinal pipeline dents. IPC02, September, Calgary.
12. Fowler and Ayers, 1993. Acceptability of plain dents for offshore pipelines. PRCI/EPRG 9th Joint Technical Meeting on Line Pipe Research, May.
13. PDAM, 2003. Pipeline defect assessment manual.
14. DIN 2413, 1974. Part 1: Steel tubes. Computation of the wall thickness of steel tubes against internal pressure, Beuth Publishing.

Bibliography

API 1160, 2001. Managing system integrity for hazardous liquid pipelines.
AS2885.3, 2001. Pipelines - gas and liquid petroleum, Part 3: operation and maintenance. CSA Z662-03, 2005. Oil and gas pipeline systems, June.
DOT Liquid Rule, 2000. Part III Department of Transport, 49 CFR Part 195 Pipeline safety: pipeline integrity management in high consequence areas (hazardous liquid operators).
DOT Gas Rule, 2001. Part II Department of Transport, 49 CFR Part 192 Pipeline safety: pipeline integrity management in high consequence areas (gas transmission pipelines).
Noronha et al., 2005. The use of B-splines in the assessment of strain levels associated with plain dents. Instituto Brasileiro de Petroleo e Gas, Brazil, October.