Pipeline integrity study improves inspection process

Alyeska engineers have devised a better way to find hidden dents and other mechanical damage to the Trans Alaska Pipeline System (TAPS).

They developed a technique to review data collected by an ultrasonic in-line inspection tool, better known as a "pig," to efficiently discover and examine the extent of mechanical damage to pipeline walls. The technique improves TAPS safety and efficiency by allowing engineers to determine if damage is significant and in need of repair without having to dismantle or excavate sections of pipeline for visual inspection.

"Our engineers analyzed the ultrasonic pig data from 2001, which revealed small dents in the pipeline," said Janna Miller, Alyeska system engineering supervisor. "Nobody in the industry had ever used an ultrasonic pig to look at data this way."

Engineers identified 77 belowground pipeline sections as having possible mechanical damage and requiring visual inspection. Crews excavated about 40 of these sites between 2001 and 2004 to inspect and repair damage and to better understand the scanning data. Engineers now believe the remaining sites probably have very small dents that pose no significant threat to pipeline integrity, but they are still assessing the sites to be certain.

"After analyzing the data from the digs, we determined that it might not be necessary to complete the remaining digs but there was no method or industry guidelines that suggested we wouldn't have to do this work," said Miller.

The project team that worked on this pipeline integrity study included Alyeska engineers Elden Johnson, Alan Beckett and Bhaskar Neogi. They completed an extensive engineering assessment, conducted sensitivity analyses of the data and used a risk management analysis to determine which sites should be inspected.

Their analysis determined that 29 of the remaining 35 dents do not require excavation. This determination avoided substantial costs and eliminated risks associated with digging up the mainline pipe 29 times. Miller said six of the remaining locations are still being studied.

"Pipeline inspection can be very expensive, especially when excavation is required," said Miller. "A lot of equipment and personnel are needed to complete this complex and often hazardous type of work."

Alyeska uses three types of pigs to inspect and maintain the pipeline. Cleaning pigs are sent through the pipeline every week or two to scrape wax buildup off the pipe walls. Corrosion pigs are used every three years to scan the pipeline with either ultrasonic or magnetic flux leakage detection devices to monitor corrosion and evaluate pipeline integrity with respect to corrosion damage. And operators use geometry pigs every five years or when needed to evaluate the stresses in the pipe and to check for bending in the pipe wall by measuring the bending radius and pipe ovality.