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Title: Dealing with internal inspection uncertainty in pipeline-integrity management
Downloadable: Yes 
Catalog No.: 2343s
Date of Publication: 2015-03-01
Price: $25.00 US
Authors: Susannah Turner
Abstract: INTERNAL INSPECTION IS a key element of best practice in pipeline-integrity management and strategic asset-management planning. The information that can be collected on the condition of a pipeline is invaluable and can be used for many purposes. The key uses are:

Immediate integrity assessment:

to identify any requirement for pressure reduction;
to identify locations requiring immediate local inspection and possible repair.

Remnant-life assessment for integrity management:

to identify locations that require local inspection and possible repair in the future;
to plan future internal inspections.

Remnant-life assessment for asset- management planning:

to estimate the useful remaining life of the pipeline;
to plan future developments and possible requirements for rehabilitation or replacement.

There are, of course, many other important uses of inspection data such as understanding the hazards affecting a pipeline, or determining the position of a pipeline. However for the purposes of this paper we will focus on the uses listed above, with reference to degradation mechanisms that can be monitored by inspection.

Inspection data are not perfect and, in particular, defects may be undersized or oversized. Depending on the inspection technology and the type of linepipe, these errors may be significant. For example the inspection accuracy for a standard magnetic-flux-leakage tool measuring an internal corrosion defect in seamless pipe close to a girth weld may be in excess of ± 30% of the pipe’s wall thickness at 80% confidence. These errors are real, and there is plenty of evidence and experience in the industry of defects found in pipelines that are significantly deeper or shallower than the best estimate provided by the internal- inspection contractor.

Best practice requires that, to ensure integrity and safety, these errors are considered when assessing defects. This can lead to what appear to be very conservative assessments particularly when considering the longer-term performance of a pipeline. It is proposed that – consistent with best practice – measurement errors should be explicitly considered to determine requirements for pipeline-integrity-management activities, including dealing with immediate integrity issues and planning the next internal inspection. This approach will prevent failures and allow effective risk management. However, separate, less- conservative assessments that are designed to give a best estimate of remnant life are more appropriate for longer-term asset-management planning.

In this paper, an approach for dealing with inspection errors in a rational and justifiable manner is discussed. The paper considers how measurement uncertainty can be dealt with in a way which will ensure safe operation whilst allowing realistic long-term planning. Illustrative examples are given.

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