Corrosion Basics: Locating Pipeline Coating Defects with a Pearson Survey

One of the first successful techniques for locating coating defects (holidays) on buried pipelines using surface electrical measurements is the Pearson survey, named after its inventor. Once these defects have been identified, the protection levels afforded by the cathodic protection (CP) system can be investigated at these critical locations in more detail. Decisions regarding coating rehabilitation can also be developed on the basis of specific information.

Several variations of this test method have been developed since J. M. Pearson introduced it in 1941. The procedure described here is essentially Pearson’s original concept. For piping with thick-film coatings, an alternating current signal of approximately 1,000 Hz is imposed (or thin-film coatings, a frequency of 175 Hz is typical) by means of a transmitter, which is connected to the pipeline and a temporary remote earth ground (often established with one or more earth spikes). Two survey operators make earth contact either through aluminum poles or metal cleats fastened to their shoes. A distance of several meters (typically 6 to 8 m [20 to 25 ft]) separates the operators. Essentially, the signal measured by the receiver, tuned to the transmitter frequency, is the potential gradient over the distance between the two operators. Defects are indicated by a change in the potential gradient, which translates into a change in signal intensity.

The measurements are usually recorded while walking directly over the pipeline. As the lead operator approaches a defect, increasing signal intensity is noted. As the lead person moves away from the defect, the signal intensity drops and later picks up again as the rear operator approaches the defect. The interpretation of signals requires some skill and practice, as indications can become confusing when several defects are located between the two operators. The location of a holiday indication can be refined by reducing the distance between the operators and resurveying the area.

In principle, a Pearson survey can be performed with an impressed current CP (ICCP) system remaining energized. Sacrificial anodes and bonds to other structures should be disconnected, because they can appear as very large earth contacts that may mask actual coating defects. An additional person is usually required to locate and mark the pipeline, place defect markers into the ground, and reposition the transmitter periodically.

By walking the entire length of the pipeline, an overall inspection of the right-of-way can be made together with the measurements. In principle, all significant defects and metallic conductors causing a potential gradient will be detected. There are no trailing wires and the ICCP current does not have to be interrupted or deactivated.

The limitations associated with Pearson surveys are similar to those associated with the close interval potential survey (CIS)—a method where the potential profile of a pipeline is recorded over its entire length by collecting potential readings at intervals of approximately 1 m. With a Pearson survey, the entire pipeline also has to be walked and contact established with the soil electrolyte. The technique is therefore unsuitable for deepwater crossings and inaccessible areas. Reasonable results have been obtained using earth contact through drilled holes in paving, similar to CIS.

Modern variations on the classical Pearson survey include the use of audio signals, signal null indications, single-surveyor techniques, and the lateral Pearson survey, where one operator walks over the pipeline while the second maintains a constant lateral separation. While each of these offers advantages in specific situations, the original Pearson survey remains an important technique; and it is included in NACE International Standard TM0109, “Aboveground Survey Techniques for the Evaluation of Underground Pipeline Coating Condition.”

This article is adapted by MP Technical Editor Norm Moriber, Mears Group, Inc., from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 509-511.

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