ACVG Coating Survey of the Main Trunks of Two Gathering Pipelines, Wickett (2024)

A clean ACVG survey is good news about a coating. In February 2024, near Wickett, it came with a second finding that mattered more.

The survey

An operator asked us to check the coating on two gathering systems built in 2006 and 2007, starting with the main trunk north of its plant and spending the rest of the time on the main trunk running south and east. Over three days a two-person crew walked the lines with an A-frame receiver. A transmitter impressed an AC signal on the pipe from three valve sets, using temporary anodes as the ground and 500 to 1,000 mA of signal current. Over a coating defect, the signal leaking to the bare steel builds a voltage gradient in the soil, and the receiver points toward it from both sides. We found none. The fusion bonded epoxy appeared to be in good condition.

What the potentials showed

At 20 aboveground valve sets we also read DC and AC pipe-to-soil potentials, since the lines run beside and across power lines in several places. The DC readings, -470 to -598 mV against copper/copper sulfate, are about what bare steel reads in soil with no current applied. The lines had been built without cathodic protection.

A good coating with no CP is not a safe state forever. Coatings age, and the first holidays would corrode freely. The sandy and rocky soils we saw suggest high resistivity, and a well coated line in high resistivity soil near power lines is the case where AC corrosion is most likely, so we recommended data logging to measure it.

Two ways to add protection

We set out two options, each needing the lines isolated from the well sites and the plant first. Sacrificial anodes at each valve set, through new test stations, are low maintenance and unlikely to disturb foreign lines, and may help with AC by grounding the pipe, but last 15 to 20 years and make later interrupted surveys impractical. An impressed current system with a rectifier and deep groundbed on each system lasts 25 to 40 years and can be adjusted, but needs power drops, bimonthly checks and attention to interference and AC mitigation.

Related services

The survey method is described on our page for ACVG surveys. Adding protection to an existing line starts with cathodic protection design.