Unpiggable Does Not Mean Unassessable: Matching Integrity Methods to Pipeline Constraints

A pipeline that cannot accept a conventional inline inspection tool is not, on that basis alone, unassessable. An assessment feasibility map identifies which segments are constrained, which threats are credible, and how much uncertainty remains once a method is applied. The next step — the focus of this article — is matching a specific assessment method to a specific constraint and threat combination, so the resulting integrity decision is actually defensible.

"Unpiggable" is often treated as if it points to one substitute technology. In practice, no single method — inline inspection, direct assessment, guided wave screening, hydrotesting, or targeted excavation — answers every integrity question. Each has a defined scope, a defined set of threats it suits, and defined limits on what it can confirm. Applying the wrong method, or relying on the right method beyond its scope, can produce a record that looks complete while leaving the governing threat unresolved.

Method Selection Follows Constraint Mapping

Once a feasibility map has identified the access constraints, credible threats, and required confidence for a segment, method selection becomes a matching exercise — not a default to whichever tool is available, familiar, or commercially convenient. Selecting first and rationalizing afterward tends to produce inspection records that satisfy a checklist without closing the actual engineering question.

What Each Method Actually Confirms

• Inline inspection (conventional or modified) — where geometry and conditions permit, ILI gives the broadest continuous coverage, but only for the threats the selected tool is built to detect. An MFL run addresses metal loss, not stress corrosion cracking.
• ECDA — confirms external corrosion through indirect surveys and prioritized direct examination. Suited where coating/CP history supports the methodology; does not address internal corrosion or cracking.
• ICDA — addresses internal corrosion in specific flow regimes by identifying likely electrolyte accumulation points. Depends on flow assumptions matching actual operating history.
• SCCDA — applicable only where SCC susceptibility factors are credible; requires more conservative validation than ECDA/ICDA given the consequence profile of crack-like flaws.
• LRUT — screens accessible-from-one-end sections for gross cross-sectional change. A triage tool: it indicates where follow-up is warranted, not a substitute for flaw sizing or FFS.
• Hydrostatic testing — demonstrates pressure containment at the time of test. Does not characterize remaining wall thickness, corrosion growth rate, or subcritical flaws that survived the test.
• Targeted excavation — the most direct method, used to validate indirect findings or examine priority locations. Coverage is limited to the excavated footprint, so site selection matters as much as the examination.
• FFS / ECA — not inspection methods themselves, but the engineering step that converts findings from any of the above into a run/repair/monitor decision under API 579-1/ASME FFS-1 or BS 7910.

Matching Methods to the Type of Constraint

Three constraint types call for different logic. Geometric or access constraints (tight bends, sleeves, casing) point toward LRUT screening followed by targeted excavation where an anomaly is flagged. Data or threat-uncertainty constraints (unclear damage mechanism, incomplete history) point toward direct assessment — but only the DA variant matched to the confirmed credible threat. Confidence-requirement constraints (high-consequence area, regulatory scrutiny, prior failure) point toward combining methods rather than accepting one screening-level result as final.

Practical Example: Matching Method to Constraint

The same "unpiggable" label can apply to segments that need very different strategies.

A short cased or sleeved road crossing with limited excavation access is primarily an access problem — LRUT screening, followed by targeted excavation and FFS only where an indication is flagged, is a reasonable approach.

A low-flow, wet-gas, or intermittently operated segment is a different problem: the question is where electrolyte is likely to accumulate, not access. This calls for ICDA-style thinking — operating history review, low-point identification, and directed UT or excavation — rather than defaulting to an external corrosion assessment because the line happens to be unpiggable.

A segment showing coating degradation and CP anomalies points to external corrosion as the credible threat, better matched to ECDA-style surveys and direct examination than to a generic screening pass.

A segment with a suspected crack-like threat should not be treated as a wall-loss problem by default. It needs NDT capable of sizing crack-like flaws, defensible material and toughness assumptions, and an ECA — not a metal-loss-oriented assessment.

The label is the same in each case. The strategy is not. The constraint, the credible threat, and the confidence required are what should drive method selection — not the word "unpiggable" itself.

Common Mismatches and What They Cost

A few patterns recur: LRUT results treated as final rather than a screening output; a direct assessment variant selected by habit rather than confirmed threat; a hydrotest pass treated as equivalent to crack detection, when it only confirms pressure containment at one point in time; and one method applied uniformly across a segment that actually contains several different constraint-and-threat combinations. Each can produce a documented inspection history that looks complete while leaving the governing threat only partially addressed.

Documenting the Selection Logic

A defensible integrity record shows why a given method was selected for a given segment, what confidence it provides, and what uncertainty remains after it is applied. This traceability supports the integrity management program expectations under CSA Z662 and ASME B31.8S, and lets an operator demonstrate that method selection was threat-driven rather than default-driven.

How TES Canada Can Help

TES Canada supports operators in matching assessment methods to pipeline constraints as part of a broader integrity engineering approach — not as an extension of any single inspection technology. Our role is to help confirm credible threats, evaluate which methods are technically suited to each constraint, and integrate results into engineering-supported run, repair, or further-assessment decisions, including FFS and ECA support where flaws are identified.