Frequently-Asked Question FAQ #68

FAQ-68: Tool Tolerances in Evaluating Repair Criteria

Question: Must tool accuracy be considered when determining if an anomaly detected by in-line inspection meets repair criteria?

Answer: Yes. Operators are required to integrate relevant information on the condition of the pipeline in making decisions on excavation timing and other mitigative actions. Tool accuracy should be considered as part of the data integration process.

Accounting for tool accuracy is most important for immediate repair anomalies. Immediate repair conditions may not be discovered (because the ILI tool "undercalled" the defect), even if the tool functioned within its published accuracy specifications, if tool accuracy is not considered. Information on tool accuracy should be used to assure that defects requiring early excavation and mitigative action are properly identified and characterized. This does not necessarily mean simply adding the vendor-supplied accuracy specification to reported depth of metal loss indications. Several sources of data may be used, in conjunction with vendor-supplied tool specifications, to characterize pipeline defects. These include results of previous excavations, confirmation digs, results of concurrent inspections, and comparison to prior inspections. Uncertainties in this data should also be considered.

In addition, information on tool accuracy may be incorporated in engineering analysis such as "probability of exceedance" to help operators prepare a comprehensive defect remediation plan and schedule future assessments. Pipeline operators have the flexibility to apply processes specific to their unique risks by utilizing these techniques when evaluating specific pipeline defects.

Tool accuracy specifications are not the only uncertainty associated with assessment results, and are therefore not the only factor to be considered in evaluating the quality of internal inspection data and in making excavation timing and mitigation decisions. Defect characterization should consider all relevant uncertainties to assure that defects posing a potential integrity threat, including those meeting the criteria in 192.933, are promptly identified. The operator must document its approach for dealing with ILI accuracy and uncertainty per 192.947(d).