Preserving the Evidence Boundary from ILI Measurement to Engineering Assessment

In-line inspection (ILI) and non-destructive testing generate measurements and engineering interpretations rather than a perfect physical replica of every pipeline feature. The value of those results depends on how clearly tool applicability, performance limits, feature identity, field verification, uncertainty and engineering assumptions remain visible as inspection data moves into integrity assessment.

This white paper presents a series-specific ILI Evidence Reconciliation Chain for preserving that evidence boundary from inspection measurement through engineering interpretation.

The chain examines six connected stages:

inspection applicability — whether the selected technology is suitable for the relevant threat, geometry and operating conditions;

tool performance — what detection, classification, sizing and location capability is actually justified;

analysis — how raw or processed inspection signals become reported features;

feature matching — how indications are mapped to the correct physical asset location and, where applicable, to prior inspection runs;

field verification — how excavation, NDT, direct examination and other independent evidence test the inspection result; and

engineering assessment — how inspection evidence is combined with material, geometry, pressure, operating history, environment and uncertainty to support an integrity conclusion.

The analysis draws on publicly available material including API Standard 1163 on In-line Inspection Systems Qualification, PRCI API 1163 performance-validation work, PHMSA integrity-management guidance and hard-spot advisory material, and DNV pipeline integrity-management guidance.

A central proposition is that an ILI result becomes defensible engineering evidence only when the reported measurement is interpreted together with tool applicability, tool performance, feature identity, verification evidence and the assumptions used in the final integrity assessment.

The paper distinguishes carefully between detection, identification, classification, sizing and location performance. These are different capabilities and should not be collapsed into a generic statement that an inspection tool is “accurate.” The evidence required also depends on the engineering decision: screening, feature prioritization, remaining-strength assessment and growth-rate estimation can require different levels of measurement confidence.

Verification digs and field NDT are treated as more than checks on individual anomaly depth. They can reveal systematic issues involving tool response, feature location, geometry, coating condition, interacting threats and field-measurement uncertainty. Verification programmes should therefore consider representativeness as well as severity.

The paper also addresses run-to-run comparison. Repeated inspections can support growth assessment only when feature matching, reference alignment, tool-performance differences, analysis changes and sizing uncertainty are sufficiently controlled. Changes in sensor technology, analysis software, reporting thresholds or reference chainage can otherwise create apparent growth that is partly methodological.

ILI evidence is further placed within the wider asset context. Material properties, seam type, manufacturing history, pressure history, stress state, coating condition, cathodic protection, environmental conditions, prior repairs and operating changes may all affect the final interpretation. Contradictory evidence should be reconciled rather than prematurely forced into agreement.

The ILI Evidence Reconciliation Chain is a practitioner synthesis, not a new inspection standard or defect-assessment method. It does not replace API 1163, PRCI research, PHMSA requirements, DNV guidance, owner procedures or specialist engineering judgement. It does not reproduce proprietary inspection-tool specifications and does not prescribe universal dig, repair or intervention thresholds.

The scope is deliberately limited to publicly discussable inspection evidence, analytical assurance and human engineering decision support. Automatic repair, isolation, pressure-change commands and other consequential physical actions remain outside the scope of this work.

This paper forms No. 24 of the From Project to Live Asset — Independent Practitioner Research Series, Phase III, extending the wider steel-pipe lifecycle research programme from AI-assisted integrity analysis into inspection-evidence reconciliation and defensible engineering assessment.

Author-written abstract, reproduced from the authoritative Zenodo DOI record.