DENTS CAN OCCUR either during pipeline construction – for instance in the form of rock-induced dents or as a result of the handling and backfill processes – or in service, such as by excavator impact. If failure as a result of a dent is not immediate, it is possible that the induced dent and/or defect combination can deteriorate in service and cause failure at some time after the initial impact. Often incidents of dents go unreported and the challenge to the pipeline operator is the identification of those defects that may threaten the future integrity of the pipeline from those defects that are dormant and require no further action.
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.
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