Utilization of a Probabilistic Function to Describe the Performance of In-Line Crack Inspection Systems and a Possible Application in Hydrogen Service

Abstract

A more result-driven, holistic view on the performance of an In-line inspection (ILI) system is desired by pipeline operators, namely an integrated view on the Probabilities of Detection, Identification and Sizing, i.e. POD, POI and POS respectively. It is self-evident to combine the product of the individual probabilities to a single value POX as proposed also by API 1163. The changes in material properties of existing pipelines related to the repurposing to Hydrogen service yields smaller acceptable anomaly dimension. Consequently the dependency of the POX vs. anomaly dimensions need to be considered as function. The current practice to document the performance of an ILI system is typically based only on the specific dimensions of minimum detectable crack-like anomaly. In the case of crack-like anomalies, the contribution of the dimensions (e.g. length, depth, orientation) and interactions of crack-like anomalies can be used to turn POX into a meaningful function/dependency depending on the anomaly dimensions instead of a single value. To establish a POX to anomaly dimension relation successfully for a specific ILI crack detection service, the relevance of field verifications needs to be taken into account as well. Previous publications as well as physics foundations of NDE demonstrate that an increase in anomaly dimensions typically yields a higher sensitivity of the underlying inspection system. This can be immediately linked to the POX function as product of POD, POI and POS. This paper provides access to an in-depth assessment of POX based on the results of recent field verification data. The paper focuses on the interaction between POX and different anomaly dimensions. This is supported by the most commonly applied standard for in-line inspection, API 1163, which implies the validation of inspection performance with use of field verifications based on the reported results. In addition, a performance specification can be derived exclusively based on a field verification campaign, in the absence of other comparable performance information. As an example, as part of a conversion of natural gas transmission pipelines to hydrogen service the sensitivity of inspection systems is frequently discussed. A potentially increased crack growth rate da/dN and a reduction in pipe steel toughness seem to require a reduction of minimum acceptable anomaly dimensions, in the absence of better statistical evidence of the actual ILI performance. The proposed POX assessment will be a credible means to establish optimal re-inspection intervals for existing in-line crack inspection technologies.