Machine Design | Pipe crawlers keep creeping, thanks to SmartMotors
Four small servomotors, one per wheel, now keep pipe crawlers balanced as they travel hundreds of miles atop oil pipelines looking for dangerously corroded sections of pipe.
Over 1,000 miles of pipe on Alaska’s North Slope feed oil to the 800-mile-long Trans-Alaska Pipeline that transports crude oil from Prudhoe Bay to the southern coastal port of Valdez. Harsh arctic conditions, along with the water, sulfur, carbon dioxide, and microorganisms in crude oil, can lead to a pipeline losing up to 70% of its mass to corrosion during its lifetime. Unfortunately, some sections of pipe corrode faster than others, raising the probability of oil spills in the arctic tundra. The crawlers ferret out corroded sections using nondestructive testing (NDT) such as gamma radiation and digital X-rays.
Envision CmosXray LLC, Anchorage, Alaska, is a pioneer in the design, development, and use of these robotic gymnasts. It’s not uncommon to see a crawler traveling along the pipeline performing inspections at temperatures below –40°F.
Early crawlers only operated on a small range of pipe diameters, so each pipe size needed its own machine. Engineers at Envision wanted to come up with a single machine that could handle pipe diameters ranging from 4 in. to 4 ft. To complicate the task, support structures holding the pipelines above the ground add several layers of steel and additional obstacles to the crawler’s journey.
Placing a motor on each wheel lets the crawler nimbly maneuver over supports while carrying the radiation source and expensive imaging equipment needed for testing. Each motor must be in sync with the other three so the crawler doesn’t fall off the pipe. With limited space on the crawler for motion-control hardware, Envision engineers chose the relatively small SmartMotor from Moog Animatics, in Santa Clara, Calif. Each SmartMotor has an external differential encoder that creates a ratio of speeds in the drive wheels for steering the pipe crawler. Controls built into the SmartMotor helped simplify the onboard and remote motor-control system. Operators controlling the crawlers must use remote operation because the freezing winds, icy and uneven ground, and heavy equipment traveling overhead generate safety concerns. Specialized wireless communications lets crawlers operate up to 1,500 ft from their operator.
Alaskan pipelines also rise and fall with the landscape. So pipe crawlers must cope with these elevation changes while maintaining precise speeds for their imaging system as corrosion analysis relies on accurate imaging. An inclinometer senses the rise and fall of the pipe, as well as the position of the crawler on top of the pipe. SmartMotors read the inclinometer signals, letting each adjust the wheel’s velocity and acceleration to keep crawler speed constant despite obstacles, winds, and surface conditions like ice and snow.
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