Photo: Al Grillo/AP Photo

In August, after a leak in a transit line caused BP to shut down its Prudhoe Bay operations, workers scrambled. The firm will now replace 16 miles of the decades-old pipeline.

Pipeline inspection technology was a ho-hum subject until August, when an oil spill from a leaking pipe forced BP Plc to shut down operations at Prudhoe Bay, Alaska, cutting U.S. domestic production capacity by nearly 8 percent. Traders bid up the price of oil futures, environmentalists added sloppy maintenance to their list of reasons not to develop reserves in sensitive areas elsewhere, and pundits began bandying arcane terms of trade, notably robotic pigs.

Separating hype from facts, it's plain that the Alaska incident points to serious lapses on BP's part. It also gives us a chance to analyze how up-and-coming technologies might help detect corrosion and other problems at other aging oil fields around the world.

Inspection cannot by itself keep pipes from corroding, but it can, at least, catch the problem before it gets bad enough to cause a rupture, which can result in fires, explosions, and widespread pollution. That way, oil companies can plan to close down and repair lines.

Inspection should have been even better at Prudhoe Bay than in many other oil fields, because its pipelines were visible, having been built, of necessity, above permanently frozen ground. But "humans aren't as rigorous as machines," as one NDE expert noted. Further, humans can’t see what’s going on inside a pipe.

BP says it used ultrasonic, radiographic, magnetic flux, guided wave, and electromagnetic NDE methods to see inside the pipes in Alaska. All were deployed by sensor-laden “smart” pigs to inspect them, but BP did not make regular use of the technology on the particular transit line that ruptured in August.

Rust Never Sleeps

Since corrosion is inevitable, oil-pipeline companies must take steps to minimize it. They have steadily improved the steel alloys in the pipes used to transport crude and gas, improving safety, according to a recent report by the Pipeline Research Council International Inc. Most pipelines now use fusion bond epoxy coatings, like those used on ship's hulls, to reduce corrosion.

Another approach, called cathodic protection, targets the electrochemical root of corrosion–a battery-like process in which differing electric potentials create a current that oxidizes iron. The result is the weak compound iron oxide, otherwise known as rust. One way to protect the pipe is by applying sacrificial anodes–metals that corrode more readily than iron and thus sop up the current. Another, more active way is to apply a small countercurrent to the pipe.

Maintenance workers can also add chemicals to control the microorganisms that feed on organic materials and secrete chemicals that speed corrosion. Such microbial action is currently thought to have caused the recent problem in Alaska.

Finally, there is mechanical cleaning by pigs. But to do it right, workers must first assess the pipeline’s condition. One way is by inserting small bars of pipeline material, called corrosion coupons, into the oil stream and then take them out for analysis after a set period. This method gives the overall rate of corrosion but cannot pinpoint its effects.

To do that job requires smart pigs with more advanced inspection techniques, such as magnetic flux leakage (MFL) and ultrasonic transduction (UT). An MFL pig uses magnets to induce a strong magnetic flux in the wall of the pipeline, senses any leakage of flux, and uses the data to deduce how much metal has been lost, and from which places. A UT pig sends sound waves through the pipe and compares the speed of their propagation with what would be expected in a pipe of the proper thickness. A UT pig generally has some 500 transducers and takes 625 readings per second, according to Alyeska Pipeline, the operator of the Trans Alaska Pipeline System.