How Do You Inspect a Natural-Gas Pipeline?

With pigs, of course.

San Bruno, California. Click image to expand.
Site of the San Bruno explosion

A natural-gas pipeline ruptured on Thursday in San Bruno, Calif., just south of San Francisco. The resulting explosion and fire killed at least four people, but the death toll will likely rise. A 2009 report by Pacific Gas & Electric warned that a segment of 30-inch-diameter pipe matching the description of the ruptured line might fail. How did PG&E know it was at risk?

Aerial photography, perhaps, or spectroscopy. A utility company can designate a segment of pipe “high-risk” for any number of reasons. A surge in local human activity is one cause for worry: Pipes are more likely to get dinged when there are more people—construction workers, landscapers, etc.—in the surrounding areas.To keep tabs on population density, companies have airplanes flying over their pipelines on a regular basis. Evidence of high-acidity water seeping down toward the pipe would also be a warning sign, as corrosion can weaken or thin its walls. Pipelines that are very old or have been used to transport significant quantities of impure natural gas are considered risky, as well. As of now, there’s no indication of what triggered PG&E’s concern over the San Bruno pipe.

Keeping tabs on corrosion in the pipes is a two-step process. Utility companies first perform geologic surveys of the surrounding land to determine how much water will be washing over their lines and how acidic that water will be. Then they perform a series of calculations to predict how long it will take for the pipes will corrode.

To check the integrity of a pipe directly, the company drops a robotic snake, known as a smart pig, into the line to test for defects. (The process is called “pigging” the pipeline.) The flow of gas propels the pig along as its odometer-equipped wheels keep track of its location. The pig’s magnets check to make sure the line is perfectly round, rust-free, and uniformly thick. It also collects small samples from inside of the pipes. Back in the lab, if spectroscopy tests reveal the presence of rust in the lines, the utility company has two choices: They can send another pig into the line to scrub it out, or they can dig up the pipe to inspect it manually. The latter process is laborious, expensive, and can result in significant service interruptions.

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Explainer thanks James Speight, co-author of Handbook of Natural Gas Transmission and Processing.

Related in Slate: Daniel Engber explained how to steal natural gas from a pipeline, how oil pipelines get corroded, and why pipelines were invented in the first place. Paul Roberts explained why the natural-gas market took off in the 1990s.

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