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| Pipeline
Inspection in Earthquake Country The Hetch Hetchy water system transports water more than 160 miles from the Sierra Nevada mountain range in California to the San Francisco Bay Area. The system delivers 260 million gallons of water per day to 770,000 customers in San Francisco and 1.6 million customers in nearby cities. The project was started in 1914 and has since undergone major upgrades and expansions to improve capacity, water purity, and earthquake readiness. William Melia is a senior engineer for the City of San Francisco Public Utilities Commission. His department maintains and repairs three pipelines that run 47 miles each through the San Joaquin Valley. Q: What makes inspecting the San Joaquin pipeline so challenging? A: We’re in charge of a pipeline that was built in different stages over the last 80 years. Each section has different corrosion characteristics depending on when it was built and the various earth conditions along the route. There’s no way to evaluate the corrosion without digging up the pipe and doing surface measurements. Q: How is AIS different from your manual inspection? A: We measure pipe wall thickness. Normally we have a technician using a UT scope, sweeping the pipe by hand and looking at a screen. His assessment is very subjective, and he doesn’t have a way to pinpoint and record the position of his observations. The first time we used the AIS system, it was on a section of pipe that we already scanned manually. Our technicians missed a particular characteristic of corrosion that I had to point out to them afterwards. Then we brought in AIS to look at the same section of pipe. Their engineer was barely a third of the way through the joint inspection when he pointed out the same unusual characteristics of corrosion on the C-scan. It was a circumferential corrosion line. I was impressed. I had technicians working on this for months and no one had ever seen it. And the reason why we don’t see corrosion like this with the UT scope is because the technician is sweeping back and forth, looking at the A-scan. He doesn’t know where his hand is positioned and his mind can’t map all the data. He has no XY record, except for a mental one. With the AIS system, the technician sees both the waveform and the B-scan being painted on the display continuously. The XY of every reading is saved. Thickness data is recorded and visualized on a contour map. Q: How would you compare your AIS with the manual UT inspection? A: When we don’t have direct contact with a pipe, we do manual UT inspection. When we replace mortar coating on pipes, this gives us the perfect opportunity to use AIS, which is contact dependent. When that happens, we’re using the machine 24 hours a day. If you’re inspecting for corrosion, as we are, there’s absolutely no comparison. AIS is far better and faster. When a technician, who’s sweeping the pipe by hand, hits an indication, he has to stop and make a note of it. Then we take out some paint and mark it. We do this several times and literally try to draw the corrosion limits on the surface of the pipe and locate the low point. With the AIS system, you’re long gone by this time. You have all the data mapped on-the-fly. How much faster is it? If you have a lot of indications, it’s performs up to 10 times faster than manual inspection. Q: What is your cost justification for using AIS? A: The AIS system pays for itself pretty quickly. I think it paid for itself on our first job – an 11-mile section of pipeline. Keep in mind, the whole process of inspection is costly. Just digging up a joint could run twenty thousand dollars by itself. So, it’s important to have an accurate, reliable and fast inspection system ready to go. With AIS, we discover defects faster and with more certainty. And I can think of no better tool to help the managers of public utilities make important decisions about maintenance and replacement. These are huge investments. We can hold up the B-scan, which looks like a contour map, and show visually why a certain action has to be taken. You tell them that red indicates severe corrosion and blue represents full pipe wall. Everyone understands it. Q: Can you comment on the quality of service AIS provides? A: When we first got the machine, there were start up problems. It didn’t help that we were running it continuously, seven days a week. But AIS made themselves available, even fetching parts for us to keep us going. They’ll do everything possible to provide good service. They’re also very good trainers. The technicians and laborers that we assign to the system pick it up very quickly. The learning curve for someone who hasn’t had exposure to this type of testing in the past is very low. |