Correlating an ILI feature with field UT measurements means establishing that the ILI call and the field examination represent the same physical feature, then comparing their dimensions using a common reference basis.
The work involves more than matching an ILI feature number to a dig number. A defensible correlation considers the pipeline segment, tool run direction, reference girth welds, measured distance from those references, clock position, axial location, feature geometry, and the measurement basis used by both the ILI system and the field examination.
Differences between ILI and field measurements are not automatically evidence that either dataset is incorrect. The comparison should account for location tolerance, measurement uncertainty, feature-boundary definitions, surface preparation, examination coverage, and the possibility that the field condition contains multiple interacting or adjacent features.
Start with a common location reference
Import the ILI feature with its run direction, reported odometer or distance, feature type, clock position, axial extent, circumferential extent, depth and confidence information. Before comparing it with field data, identify the reference used by the dig crew: stationing, milepost, girth-weld number, wheel count, GPS or another operator-defined datum.
Girth welds are often useful because they are visible, repeatable reference points. Record the upstream and downstream reference welds, the direction of travel and the measured distance from each reference to the anomaly. If the field team used a wheel or odometer, preserve the count and the wheel circumference or calibration basis when required by the procedure. Do not convert a field distance to an ILI distance without recording the conversion and its assumptions.
Reconcile stationing and odometer data
Make the reconciliation explicit. A useful comparison table includes the ILI reported location, field station, upstream and downstream weld references, field odometer or wheel count, GPS where collected, clock position and the final correlated location. Differences may come from odometer drift, tool speed, run direction, weld identification, coating access, feature clustering or a different definition of the feature start and end.
- Confirm pipeline segment, inspection run, direction and feature identifier.
- Match the closest physical references and record their identifiers.
- Compare ILI and field distances from the same datum, using the same units.
- Check clock position and axial orientation before comparing dimensions.
- Document the accepted correlation and the uncertainty or unresolved difference.
Compare dimensions on the same basis
ILI depth is commonly reported as a percentage of nominal wall or as a calculated depth, while UT may provide thickness readings and a minimum wall value. Convert values only when the nominal wall, calibration, temperature and measurement basis are known. Record whether the field grid captured a peak pit, a broader corrosion patch or a profile that extends beyond the ILI interaction box.
Compare maximum depth, minimum wall, axial length, circumferential width and clock position separately. A tool may report a single feature where the field examination identifies multiple pits, or field preparation may expose a larger corrosion area than the original reported boundary. Preserve both datasets and explain whether the difference is a sizing difference, a location difference, an interaction decision or a limitation of access.
Use the correlated record for assessment
The final correlated record should point to the original ILI call, field UT grid or profile, photos, reference measurements and any calculation that uses the verified values. The Pipeline Integrity workflow is designed to carry ILI correlation, UT grids, wall-loss profiles, B31G/Modified B31G/RSTRENG inputs and repair documentation together. SketchMapp inspection drawing and CML mapping can provide an additional spatial reference when the dig or asset drawing needs a report-ready location view.
Questions inspectors ask
Should ILI and UT boundaries be forced to match?
No. Compare the boundaries, document the reason for the difference and use the procedure-approved feature definition for assessment.
What if the field crew cannot find the exact ILI location?
Record the search limits, reference features, methods and unresolved uncertainty. Keep the original ILI call visible rather than overwriting it.
Can the minimum UT wall be used without a profile?
It can be an input where the selected assessment method permits it, but the engineer must decide whether the measurement coverage and geometry support the assessment.