Project Detail

Advancing the Art of Corrosion Control

Piedmont Natural Gas enlists the cooperation of another operator to solve the mystery of pipe-to-low potentials on two of its lines.

Don Mitchell, Piedmont Natural Gas, Dudley, North Carolina

Norm Moriber, P.E., Mears Group Inc., San Ramon, California

Hazardous liquids and gas pipeline for underground metallic pipelines, the most common approach to external corrosion control is the application of a non-conductive protective coating which isolates the pipe metal from its surrounding environment. This application is usually combined with cathodic protection to address the inevitable coating flaws.

Cathodic protection (CP) has been defined as a technique to reduce the corrosion rate of a metal surface the cathode of an electrochemical cell. In impressed current CP, a voltage is applied (usually by a rectifier) between the metal surface and an external anode so that protective current flows through the soil electrolyte to the cathode. One of the real-world circumstances that can complicate attempts to control corrosion is stray direct current, that is, current flowing through paths other than the intended circuit.

Because the driving voltages associated with stray current are often greater than those involved in typical soil corrosion cells, mitigation can present special challenges. The two broad categories of stray current are static, with little or no variations over time, and dynamic, with relatively large, often sudden variations over time. The case study below describes a situation of static stray current where the source is the cathodic protection system for neighboring pipelines.

As a part of a large natural gas piping network Piedmont Natural Gas (PNG) operates two parallel pipelines (12-in. and 16-in. diameters) near Stanley Station in North Carolina. These lines run approximately east-to-west, with the newer 16-in. line to the south, and the lines traverse through several High Consequence Areas (HCAs). As part of their pipeline integrity management program, PNG engaged Mears Integrity Services to perform External Corrosion Direct Assessment (ECDA) indirect inspection testing for these lines in the HCAs. The testing included AC Attenuation, coating holidays, and cathodic protection surveys; and the data collected included piping outside the limits of the HCA to reach the nearest test station. It was noted that some of the pipe-to-soil potentials outside the HCAs were less negative than the established CP criterion, a pipe-to-soil potential at least as negative as 0.850 Volt with protective current sources interrupted. PNG authorized an investigation of this situation.

Subsequent testing identified an area approximately 2,500 feet in length where potentials for both PNG lines failed to satisfy CP criteria and were consequently, out of compliance with regulatory requirements and could be susceptible to external corrosion.

Near the center of this low potential area, the PNG lines are crossed by a corridor of four pipelines operated by Williams Gas Pipeline – Transco (WGP). These crossing lines include SGP A,B, and C, lines in addition to a Public Service North Carolina (PSNC) line that is interconnected with the WGP lines outside the vicinity of the crossings. It was learned that WGP had recently installed an impressed current CP System for this area based on linear anodes. The linear anode concept uses a cable with a conductive outer layer, centered in coke breeze backfill and installed parallel to the pipeline. This provides a closely coupled system that can distribute protective current efficiently along a pipeline. With the cooperation of WGP personnel, the rectifiers owned by PNG, WGP, and PSNC were cycled in groups and individually to determine what influences they had on the PNG potentials. In addition, an existing bond between PNG and WGP at Williams’ Stanley Station (approximately one-half mile away) was cycled separately.

The stray current investigation identified the following influences:

  • Current from the PNG sources was protective, but could not provide effective CP Levels to the test area because of the large distances involved.
  • Current from the SGP sources was generally protective to the PNG lines, with the greatest influences from the linear anode rectifier.
  • An active portion of one of the linear anode circuits crossed the two PNG lines and appeared to contribute to detrimental circulating stray current in the immediate vicinity of the crossings. Pipe-to-soil potential spikes more negative than -3.0 volts were observed at the crossing with this anode circuit.
  • Current from the PSNCE sources depressed the pipe-to-soil potentials at the crossings.
  • The existing bond at Stanley Station was an important factor in reducing interference effects from WGP at the crossings.

WGP personnel agreed to temporarily cut the linear anode circuit that crossed the PNG lines, disconnecting approximately 65 feet of the circuit. After the testing was completed, the following was learned:

  • Peak potentials at the crossing with the WGP linear anode were reduced to approximately -2.0 Volts. Because cutting of this circuit did not actually remove any of the anode or backfill, it was suspected that some current leakage continued.
  • With protective current from PNG and WGP rectifiers interrupted, the least projected potentials for the 12-in. line were positive: between +0.080 Volt and +0.120 Volt at the crossings with the WGP B and C lines.
  • With protective current from PNG and WGP rectifiers interrupted, the least protected potentials for the 16-in. line were positive: between +0.150 Volt and +0.200 Volt at the crossings with the WGP B and C lines.

Because the CIS potential surveys rely on reference electrodes that travel over the surface of the soil, they are subject to measurement errors associated with current flowing through the soil from various sources. Interruption of the protective current sources is one way to reduce these errors. As a check on the CIS potential values, PNG installed new test stations, including soil access tubes to permit placement of reference electrodes close to the surface of the buried pipes. These potential measurements were in good agreement with the values obtained during the CIS testing. In addition, the test stations installations offered the opportunity to confirm that no significant corrosion damage has occurred.

As a result of the cooperative testing described above, a plan to remediate the DC stray current influences was developed. The plan involved installing a supplemental CP system local to the area. In addition, WGP was asked to permanently deactivate the 65-ft segment of their linear anode circuit by removing five feet of anode and backfill north of the PNG lines, and disconnecting the south end of the circuit from the junction box. This would be followed by system adjustment and testing to determine whether further action was required.

At the direction of PNG, Mears designed and installed a linear anode system for the 2,500-ft area of low potentials. The linear anode included two circuits with a total length of 1,179 feet to the west of the crossings and one circuit with a length of 1,113 feet to the east of the crossings. An additional circuit combined four short segments with a total length of 203 feet positioned in the larger gaps between WGP lines so that no active anode was within ten feet of a WGP line. Initial activation of this system and completion of the remediation plan resulted in the following:

  • WGP personnel cut and remove five feet of their anode circuit to deactivate the portion the crosses the Piedmont lines; a reduced potential peak remains.
  • PNG’s linear anode circuits were initially activated at a total output of 1.07 amps, which was eventually adjusted to a total of 3.12 amps.
  • At the adjusted outputs, pipe-to-soil potentials along the PNG lines outside the immediate vicinity of the pipeline crossings were improved and satisfied the -0.850 Volt polarized CP criterion at almost all locations.
  • Pipe-to-soil potentials within the immediate vicinity of the pipeline crossings were somewhat improved; however, these potentials did not satisfy any recognized CP criterion. The least protected potentials continued to be at the crossings with the WGP B and C lines. With protective current from the PNG and WGP rectifiers interrupted, the least protected instant off potential for the 16-in. improved to -0.600 Volt, while the least protected instant off potential for the 12-in. improved to -0.760 V.
  • The influence of PNG’s linear anode operation was protective to all four foreign pipelines.

It was decided to allow the CP system to operate and stabilize while designing and installing control rheostats for balancing of circuit outputs. Installation of these rheostats and adjustments to the system resulted in the following:

  • The additional time for polarization improved the least protected instant off potential of the PNG 12-inc. from -0.754 V to 0.808 V, while the lowest instant off potential for the 16-in. changed from -0.626 to -0.512 V.
  • The combined current output for the short anode segments near the crossings was adjusted from 230 mA to 470 mA. Low potentials improved to 0.880 V and 0.542 V for the 12-in. and 16-in. respectively.

It was concluded that the potential for the 16-in. could not be brought into compliance without a bond to WGP.

Following testing to properly size a temporary bond, a resistive bond to properly size a temporary bond, a resistive bond of 1.2 ohms was inserted to return 185 mA to WGP’s B Line with all rectifiers operating. With this temporary bond in place, the least negative instant off potentials were -1.039 V and -0.929 V for the 12-in. and 16-in., respectively. Short CIS performed over each of the WGP lines at the crossings showed that the operation of PNG’s linear anode rectifier was protective to these lines with the bond in place. WGP personnel gave permission for installation of the bond at the tested levels. This installation was completed in a bond box at the B-Line crossing station.

Acknowledgement

The authors wish to express their gratitude to the WGP personnel, whose cooperation was exemplary during all phases of this project. PNG would especially like to recognize the efforts of Mr. Bill Deaton, P.E., and Mr. Charlie Harrell for helping make the success of this project possible.

The authors

Mr. Don Mitchell is the Cathodic Protection and Pipeline Integrity Specialist for Piedmont Natural Gas in Dudley, North Carolina.

Mr. Norman Moriber, P.E., is a Chief Corrosion Engineer for Mears Integrity Services, in San Ramon, California.