Close Interval pipe-to-soil Surveys (CIS), Alternating Current Voltage Gradient (ACVG), and In-Line Inspection (ILI) data suggested corrosion activity on a buried gas transmission pipeline. An analysis of the survey results and additional testing in the field determined a foreign operator was responsible for static DC interference on the pipeline. This paper is a case study for the testing, analysis, and design for the interference.
EN Engineering (ENE) completed an AC interference study over 68 miles of an operator's transmission line. In addition, an ACVG survey of approximately one (1) mile of the pipeline was conducted at the take-off point of the 8" line. The first 5000' surveyed via ACVG can be seen in Figure 1.
As a result of this AC interference study, evidence of elevated AC corrosion risk has not been found on the operator's pipeline. However, a review of the provided data shows evidence of DC interference due to a nearby foreign rectifier groundbed at the take-off point of the operator's line.
Previous annual test point surveys completed by the operator along the pipeline have shown AC potentials of up to 8 VAC. These readings are located near powerline crossings. Mitigation measures were designed by a third-party and have been installed. These mitigation measures include galvanic anodes connected to the pipeline through solid-state decouplers (SSD). These locations can be seen in Figure 2.
Within operator's station at the take-off point of the 8" line, there are seven (7) foreign rectifiers with one (1) previously identified as influencing the operator's line. There are two (2) rectifiers which protect the foreign mainline, one (1) also protects the station piping. The groundbed for this rectifier was installed by HDD method as a conventional horizontal arrangement. It is located within 70-ft of the operator's pipeline. An aerial image of the foreign mainline, rectifier groundbed. An aerial image of the foreign mainline, rectifier groundbed, operator's line, and ILI metal loss locations can be seen in Figure 3.