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 pigs --machines that move inside pipes with flowing oil and inspect or clean as they go.
Separating the hype from the 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 that inspect pipes instead of scouring them. Pigging has been practiced in one form or another for decades, but BP did not make regular use of it 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 corrosiona 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 anodesmetals 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.
Smart pigs do have their limitations. They can be as long as a car, and cannot navigate many pipes. To remedy these faults, the industry envisages smaller, svelter robots that move under their own power and go wherever the operator desires, no matter the direction of the flow of oil (or natural gas, in gas pipelines). These robots will require much lighter sensors, and researchers are looking at a number of techniques.
J. Bruce Nestleroth and Richard J. Davis, of Battelle, based in Columbus, Ohio, describe one sensing method in an article published on August 30 in the journal Nondestructive Testing and Evaluation International. They use a device that moves through a pipeline while rotating pairs of permanent magnets around a central axis, stirring up powerful ”eddy currents” in the surrounding metal. Variations in these currents can paint a detailed picture of the pipeline’s walls.
Another method, remote field eddy current (RFEC) testing, uses a coil of wire carrying low-frequency alternating current to induce the eddy currents. Such coils can be made quite narrow, and can thus be used to inspect ”unpiggable” pipes from inside. The Department of Transportation is funding projects to test this idea in natural gas pipelines.
The best corrosion-detecting technologies can be effective only if they are implemented , and they aren’t cheap. Before the Prudhoe Bay disaster, BP expected to spend US $72 million on corrosion control. Now, it forecasts it will spend $195 million in 2007 on major Prudhoe Bay maintenance.
Add to that cost the ill will generated by the bad press and the embarrassment of having a former BP corrosion manager plead the Fifth Amendment during a U.S. House of Representatives Energy and Commerce subcommittee hearing. As any risk-management expert will tell you, risk mitigation is less expensive than crisis management. That message is certainly resonating loudly through the executive offices of many an energy company these days.
About the Author
A former energy journalist and commentator, Shirley S. Savage is a Maine-based freelance writer who covers energy, science, and technology.