Pipeline Coating and CP Surveys in the Permian Basin

Long rights of way, sandy and rocky soil

Permian gathering and transmission lines run through remote rights of way, and a crew covers long distances between access points. Near Wickett the sandy and rocky soils suggested high resistivity, and a well coated line in high resistivity soil near power lines is the case where AC corrosion is most likely.

Short segments need their own planning. On the six segments near Carlsbad and Loving, from 1,100 to 7,000 feet long, mobilizing and setting up the current interrupters can take as long as the survey walk, so nearby segments were grouped into shared field days, and each segment got its own report, indication list and severity ranking.

Surveying new lines before they go into service

Three new crude oil lines in southeast New Mexico had no cathodic protection yet when their specifications called for a DCVG survey before closeout in 2020. At each connection point we set up a portable rectifier and temporary anodes, brought the instant off potential to at least -1.1 V against copper sulfate, and recorded the gradient every 5 to 7.5 feet. The lateral of about 7,600 feet near Carlsbad and the first phase of about 14.75 miles near Jal showed no indications. The second phase in Lea County, about 27.5 miles, had five coating defects, and we strongly recommended a cathodic protection system for that line.

Coatings without cathodic protection

Near Wickett, an ACVG survey of two gathering systems built in 2006 and 2007 found no coating defects, but DC potentials of -470 to -598 mV showed that the lines had been built without cathodic protection. A good coating with no CP will not stay safe as it ages, so we set out two ways to add protection: galvanic anodes at the valve sets, or an impressed current system with a deep groundbed on each system. Both start with isolating the lines from the well sites and the plant.

AC on lines that share corridors with power lines

Near Wink, eight data loggers recorded AC pipe-to-soil potentials every 20 seconds for 48 hours on a crude oil and gathering system where several lines parallel or cross high-voltage transmission and distribution lines. Average levels ran from about 0.5 to 4.5 volts, and on one 12-inch crude oil line the readings spiked to 12.5 volts. Data like that shows the operator where AC is highest and whether a mitigation design should follow.

From survey to dig list

A coating survey is an indirect inspection, and most of its value comes from what is done with the results. Indications are ranked by severity, the worst are excavated, and the coating and any metal loss found in the dig are compared with what the survey predicted. Pipe-to-soil potential data then shows whether the cathodic protection is keeping up where defects cluster.