Assessment of the external coating condition of underground pipelines depends on the use of approved external corrosion direct assessment (ECDA) indirect inspection techniques such as alternating current voltage gradient (ACVG), alternating current attenuation (ACCA) and direct current voltage gradient (DCVG). The efficient application of these techniques (especially ACVG and DCVG) to ensure the optimal detection and prioritization of coating anomalies largely depends on a myriad of factors including depth of cover, probe spacing, current level and soil resistivity.
Research and industry experience have shown that the normalization of key factors, such as current level, probe spacing, and depth of cover, lead to the accurate detection and prioritization of coating anomalies for ECDA and coating assessment projects. However, there have been scenarios in which known coating faults have not been detected with DCVG and ACVG, even when key factors were considered during the indirect inspection.
This paper will present case studies that demonstrate instances in which the optimization of coating anomaly detection and prioritization methodologies led to the successful execution of ECDA projects and prevented unnecessary pipe bell-hole excavations. In addition, lessons learned from scenarios in which known coating faults were not detected by voltage gradient surveys are presented.
