UKOPA Dent Assessment Algorithms: A Strategy for Prioritising Pipeline Dents

Abstract

in-line inspection tools improve, dents that would have been below the detection and reporting levels of previous inspections are now being detected and reported to pipeline operators. Consequently, operators are being faced with large numbers of dents in ILI reports that require further consideration and are left with the problem of how to prioritize these dents for further investigation and repair. Although code guidance is clear on the relative severity of dents associated with other features or those based on a depth or strain criteria, this may still leave a significant number of dents in the pipeline which fall within codified static dent assessment criteria, but which may still pose a threat, particularly from fatigue. Many transmission pipelines in the UK are now 30–40 years old and fatigue failures at dent locations are starting to be reported. Such occurrences have raised technical concerns with regulators regarding the perceived conservatism of current dent assessment methods as the dents in question were within the code limits and were reported through standard ILI technologies, however, they were not identified as significant. There is therefore a requirement to develop best practice guidance for the safe and economic operation of dented pipelines. The UK Onshore Pipeline Association (UKOPA) recognized that further guidance was needed in order that operators could identify dents which can be safely left in the pipeline and those for which further excavation is required. They have consequently developed a series of algorithms to allow pipeline operators to prioritize the dents for repair based on ILI results. This paper describes the background research to these algorithms as well the algorithms themselves, demonstrating their use with ILI dent data from operators of onshore oil and gas pipelines. The paper concludes with comments on the current conservatisms in the analysis of dent fatigue and proposes a way forward to allow pipeline operators to manage large numbers of dents for which the dent fatigue life is critical.Copyright © 2010 by ASME