Abstract

Periodic inspection is a proven approach to structural integrity management of transportation systems. This is as true for pipelines as it is for aircraft and railways. Setting the re-inspection interval to ensure imperfections cannot grow to critical dimensions prior to the next inspection is a foundational requirement of this maintenance methodology.

API 1176 delineates two methods for re-inspection interval criteria for pipeline crack threat management: (1) maintaining a safety factor of 1.10 and at least a 30% of wall thickness remaining ligament depth until the next inspection, or (2) inspect at the half-life of the feature with the lowest remaining life taking the end of life being a 1.00 safety factor.

Recent proposed regulatory documents and draft rules have down-selected to Method (1), or at least demonstrating compliance to Method (1), which will require some operators who have only been using Method (2) to safely manage this change.

The two methods are compared for every asset of a large North American operator under current actual operating conditions. The relative conservatism of the two methods is directly compared. Sensitivity to a minimum remaining ligament requirement less than the recommended 30% of wall thickness is explored, and leak/rupture threat differentiation is considered. Implications of the change for a liquids pipeline operator in North America are described.

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