Recent close-interval potential surveys have raised concerns about the accuracy of potential measurements on some older, poorly-coated pipelines. Two significant sources of error were identified: metallic IR-drops and equalization currents. These resulted in measured on and off potentials which differed by hundreds of millivolts from the true potentials, even though the measurements were conducted in accordance with industry best practice.
Examples of these phenomena will be discussed and the electrical theory will be explained. Both interrupted test post surveys and close-interval potential surveys are susceptible and these errors can be difficult to detect. This can have a significant impact on interpretation of survey data, External Corrosion Direct Assessments, and remedial program decisions.
Guidance for identifying when these issues may be relevant and methods for detecting and compensating for these issues will be provided.
Close-interval potential surveys (CIPS) are used extensively in the cathodic protection industry to provide a more comprehensive view of the cathodic protection levels along a pipeline. These surveys may be used independently to guide remedial programs or may be leveraged as part of External Corrosion Direct Assessment (ECDA) or other assessment techniques. If the potential measurements are found to be inaccurate, this can distort the conclusions of these efforts, leading to lower confidence in a company’s programs, sub-optimal use of resources, and ultimately to increased risk profiles.
During several recent CIPS on older, poorly-coated pipelines in western Canada, anomalous potential measurements were observed despite the surveys being conducted using industry best practice. The cause of these problems was investigated, and two inter-related phenomena were identified: metallic IR-drop and equalization currents. All close-interval surveys and interrupted test post surveys are susceptible to these effects, with the degree of impact ranging from negligible to very significant.
Figure 1 shows an example of a dataset collected on a 15-km section of a pipeline where these effects were found to be significant. Along most of this section, this 1950’s era NPS 12 pipeline runs alone in the right-of-way and all influencing rectifiers that could be identified were interrupted. Rectifiers and test posts are shown for reference.