EMAT Reinspection Interval Considering POD and Undetected Cracks

Abstract Crack (or crack-like) anomalies are a major threat to the safety and structural integrity of oil and gas transmission pipelines. Pipeline operators commonly manage the crack threat by hydrostatic test (HT) and in-line inspection (ILI) using electromagnetic acoustic transducer (EMAT) or other inspection tools. As part of the ILI-based assessment, the determination of the re-inspection interval is critically important to assure the safe operation of pipelines. A common method to determine the reinspection interval is to deterministically grow a hypothetical crack anomaly and determine the time to reach critical size, where the initial size of crack is typically assumed based on EMAT vendor specification. The limitation of this method is that the re-inspection interval is highly impacted by the vendor tool detection specification. The potential for any undetected cracks larger than the detection threshold are not properly considered. The probability of detection (POD) of EMAT tools for cracks is not as high as magnetic flux leakage (MFL) tools for corrosion anomalies. Crack anomalies with size corresponding to 90% POD are usually more severe than corrosion anomalies with 90% POD. Therefore, the risk of missed (undetected) crack anomalies cannot be ignored when determining the re-inspection interval of EMAT runs. This paper describes a probabilistic method that considers the field verified tool POD to determine the EMAT re-inspection interval. The proposed method reflects reality better as it explicitly accounts for uncertainties associated with input parameters as well as the effect of missed cracks due to POD. To quantify the risk of undetected crack anomalies, this paper systematically reviews the company’s historical EMAT reported and field found cracks, and develops the actual EMAT performance specification, i.e., POD curve, as a function of crack size and the proximity to a weld. Log-Logistic regression is employed to fit the data considering the unbalanced sample size of deep and shallow cracks. Given the developed POD curve, various metrics are explored to characterize the severity of each ILI assessment path considering the EMAT reported crack population and size. As a result, a probabilistic metric is proposed to quantitatively characterize the risk due to EMAT undetected cracks, which can be used to prioritize the EMAT reinspection between lines more effectively. A safety check is also performed using the EMAT-field crack repair database. The proposed method could be used to determine the reinspection interval properly. The details of the proposed generic framework are included in the paper, which could be used by feeding updated or other company-specific data. In addition, the line-specific hydrostatic test results before or after EMAT and real-time updated dig findings could also be built into the framework as prior knowledge to better prioritize the EMAT reinspection.