A Sample Project
Selection of Indirect Inspection tools is part of the Pre-Assessment (Step 1). An important inspection tool used in the ECDA process is the close interval survey (CIS) where Mears’ crews measure the interrupted pipe-to-soil potential every ten feet along the pipeline. For cathodic protection (CP) pipe to be considered effective, NACE and the Code of Federal Regulations specify that the pipe needs to be polarized to -0.850 VDC or more negative with respect to a copper-copper sulfate reference electrode. This polarized potential may be indicated by the instant-off potential or the current-applied potential with consideration of voltage drops. In addition, a second complementary tool must be utilized and integrated to identify areas of highest concern. The two complementary tools that are used most frequently are coating condition surveys: the pipe current mapper (PCM), which operates on the principle of alternating current attenuation, and direct current voltage gradient (DCVG).
This sample project demonstrates how a rigorous root cause analysis can have a significant impact on understanding the pipeline’s integrity assessment. For this example, the Indirect Inspection (Step 2) tools, selected by another party, were CIS and PCM.
The Original Assessment Pinpoints a Problem
The plot from the CIS of the pipeline is shown below where there is a large, 0.475 V, depression (less negative) in the pipeline’s potential profile that fails to satisfy the -0.850 V CP criterion. In the ECDA process, this is typically considered a severe indication. The PCM survey in this asphalt-covered section didn’t indicate significant coating degradation.
The Incomplete Solution
With the data available for integration, the location with the least negative potential was chosen for the direct examination of the pipeline’s condition. However, the direct examination (Step 3), performed by Mears, did not find any large areas of missing coating that would have caused the significant depolarization and only found minor corrosion. Under these circumstances, our client chose the typical response of localized recoating and concluded that further action was not justified. Because the amount of corrosion that was found was considered minor, the Post-Assessment (by others) did not include a detailed root cause analysis. Unfortunately, that might not have addressed the actual threat.
During the Post-Assessment (Step 4), several questions should have been asked and investigated to better understand the threat, prevent further degradation, meet regulatory requirements, and avoid expensive rework.
Fast Forward to the Next Assessment
In accordance with the ECDA calculation of the reassessment interval, the next assessment was performed seven years later. The indirect inspection tools CIS and DCVG were selected, and the asphalt-covered areas were drilled to provide soil contact for the reference electrodes. In this example, the persons performing the previous Post-Assessment had not verified successful remediation of the issues in this area. When the data from the CIS and DCVG were integrated, this location also became an immediate excavation priority.
Since an immediate priority was still present, another excavation was required. This excavation, located at a DCVG coating flaw indication, coincided with a crossing under a foreign pipeline. More severe corrosion was discovered but again with minimal coating loss. After addressing the damage and backfilling, the potentials remained below criteria. What is impacting the potentials on this pipeline?
The Root Cause and Post-Assessment
With the significant damage identified, a root cause analysis was performed. A rigorous root cause analysis, further investigated the unusual conditions: very low polarization with little corresponding coating damage, insufficient polarization after the recoat and so on. It became clear that there had to be some factor, other than coating damage and aggressive soils that was driving down the polarization level of the pipe.
The rigorous assessment of the root cause started with a review of all documentation regarding the pipeline and the previous assessments performed. That research showed that the Mears CIS from the second assessment identified a previously undocumented foreign pipeline crossing at the same location as the lowest CIS potential. The One Calls for the CIS and DCVG pavement drilling provided key data pointing to the source of the threat.
The foreign line was found to be interfering with the subject transmission line’s cathodic protection system. Corrosion can occur as cathodic protection current leaves a pipeline and jumps onto a foreign line. In this instance, this loss of electric current caused the external corrosion and the depression in CP potentials. Repair of the coating in the first excavation moved the dominant current discharge point to the smaller coating flaw nearer to the crossing. By performing a rigorous root cause analysis, the actual threat was identified and it was possible to take appropriate action for mitigation. As a result, the threat of accelerated corrosion was controlled and a possible leak was averted.
As shown in this example, it is essential that the root cause of any corrosion is understood. To perform an adequate integrity assessment the operator should ensure:
- Surveyors collect other local data, such as foreign crossings, that could support root cause analyses.
- Root cause of the corrosion has been identified and mitigated, regardless of the amount of corrosion found.
- CP potentials meet criteria after the direct examinations are completed.
- The engineer is rigorous, considers all available information and reconciles all test results.