Development of the Frequency-Dependent Electrochemical Method as a Survey Technique for Determining the Integrity of Buried Pipelines Under Cathodic Protection, 20-R9760Printer Friendly Version
Inclusive Dates: 10/01/07 10/01/09
Background - The objective of this project was to test the applicability of a frequency-dependent electrochemical method to identify a coating defect site (also called a holiday in the pipeline industry) on a buried pipeline under cathodic protection. Presently available survey techniques estimate locations of external holiday sites with rather high degrees of uncertainty, leading to false positive detections and unnecessary excavation of sites. The project was undertaken because, if successful, the method could be used as a secondary inspection technique to accurately identify holiday sites. Before this investigation began, the method had been used to determine the micro- and macroscopic defects on coated coupons in a laboratory-scale setup. The method involves applying a sinusoidal potential signal to a working electrode and then measuring the response of a particular location by placing a two reference electrode probe in the electrolyte above that location. The study aimed to resolve scale-up issues related to inspection and identification of holidays on buried pipelines under cathodic protection using the method.
Approach - The project included three research and development activities: (1) determine critical parameters such as the probe distance from the pipeline surface, distance between the two reference electrodes in the probe, and frequency and amplitude of the applied signal; (2) conduct field experiments on a buried pipeline that has holidays at specific locations; and (3) analyze field data to identify holiday locations. The critical parameters were determined by conducting numerical simulations of physico-chemical process models and further refined during the field experiments. Field tests were conducted with a 120-ft-long, 0.5-ft-diameter underground pipeline buried at a 4-ft depth on SwRI premises. The field data were analyzed to calculate the difference between the potential signals collected through the references electrodes of the probe.
Accomplishments - Analyses of field data confirmed the applicability of the frequency-dependent electrochemical method for positively identifying holidays on buried pipelines. Based on these results, the method can be used as a secondary inspection technique to identify and confirm holiday locations on buried pipelines. In particular, the method can be used effectively to identify holidays on buried pipelines that have risers at a short spacing interval of a few hundred feet. Furthermore, it is potentially applicable for buried pipelines used in nuclear power plants and chemical plants because electrical attenuations, which limit the applicability of the method, will be minimal.