Post-Construction DCVG Survey of a New 12-Inch Gas Line and an 8-Inch Crude Oil Line Near Carlsbad, New Mexico (2020)

In January 2020 we ran a DCVG survey on two new pipelines laid side by side near Carlsbad, New Mexico, about 16 miles of pipe in all. One is a 12-inch natural gas line and the other an 8-inch crude oil line. The project specifications called for the survey before closeout, and the contractor that built the lines asked us to do it.

A temporary circuit for each line

Neither line had a cathodic protection system yet, only test stations for monitoring, so each survey started with a temporary circuit. On the gas line we connected a portable rectifier at a test station near a compressor site and used a metal culvert under the site driveway as the ground. On the crude line the connection was made at a pig launcher and receiver skid, with temporary anodes as the ground. In both cases we raised the output until the instant off potential at the connection point was at least -1.1 V against a copper sulfate electrode.

The gas line took 120 mA. It moved from -0.58 V native to -1.18 V instant off, and the IR drop at the connection fell only from 5.7 V to 5.3 V over the survey. The crude line took 90 mA, moved from -0.61 V to -1.21 V, and its IR drop went from 2.6 V to 2.3 V. A signal that holds that steady over miles of pipe on so little current says the coating went into the ground in excellent condition.

Walking the lines

The gas line was walked from January 20 to 22 and the crude line from January 22 to 24. We carried a field data logger and two copper sulfate electrodes and recorded the voltage gradient every 5 to 7.5 feet. A peak of 5 mV or more is treated as a possible coating holiday, and each one was checked by setting the electrodes in four quadrants around it to confirm that the current pointed toward the same spot. Every confirmed indication was located by GPS, marked in the field and recorded with its total voltage to remote earth.

Three small indications

The survey found one indication on the gas line and two on the crude line. Each is graded by its percent IR, which compares the voltage measured at the defect with the total IR drop on the pipe at that point. The gas line indication read 25 mV, or 0.5 percent IR. The two on the crude line read 24 mV, or 1.0 percent, and 120 mV, or 4.8 percent. The grading most of the industry uses is a guideline, not a published standard, and it puts anything under 15 percent IR in the small category. All three were graded minor.

With defects that small, we said that excavating and recoating them may not be warranted. We did recommend a cathodic protection system and a maintenance program for both lines. Small holidays still leave bare steel in contact with the soil, and test stations only measure the pipe; they do not protect it.

Related work

Later in 2020 we surveyed three more new crude oil lines in the same part of the state, described in our write-up on DCVG acceptance surveys on new crude oil lines.

The method is described in more detail on our page for DCVG surveys.