Ingenuity And Innovation To Boost Pipeline Pump Efficiency
The drive for equipment efficiency also is changing the way the trans-Alaska Pipeline System will operate.
Engineering is completed, piles have been driven and construction on site is about to begin to upgrade and automate the pump stations. But a glitch in the design of a critical motor, combined with the challenge of construction on active industrial sites, has thrown the $250-million project five to six months off schedule. Originally scheduled for start-up in March 2006, the pump stations will switch over to the new operations by the end of next summer.
Alyeska Pipeline Service Co., Fairbanks, Alaska, is converting four essential pump stations and a pressure-relief station on the pipeline it operates for oil producers on the North Slope to automated operation powered by electricity (ENR 1 /12/04 p. 14). Since construction in the 1970s, Stations 1, 3 and 4, north of the Brooks Range, have been powered by natural-gas generators. Station 9, 100 miles south of Fairbanks, and Station 5, the relief station at the foot of the Brooks Range on the south, use diesel fuel. All except Station 5 also require staff on site for operation and maintenance.
After the Strategic Reconfiguration Project, Pump Stations 1 and 9 will tie into the local utility grid. Stations 3 and 4 will replace their aging gas and diesel turbines with modern skid-mounted gas-turbine generators. Pumps will be converted to operate with electric motors using variable-frequency drives.
The VFDs will allow the pipeline to accommodate the volume of crude oil being transported. TAPS was built for 2 million bbl per day, but currently carries half that, and the volume has been falling for years. Electrification will allow the stations to be remotely operated without on site personnel.
VFD is a relatively new technology. "Because we have such a huge variation in throughput, the pumps have a wide range of speeds. The motor has to operate at that range," says Ian Livett, Alyeska project manager. "Even though they are proven motors, they haven't been operated at such variable speed ranges. There's a resonant vibration at motor speeds that exceeded the specifications."
A constant-speed motor can be tuned to eliminate the frame's natural frequency, explains Craig Wylie, project manager for Electric Machinery Co. Inc., Minneapolis, the manufacturer. In Alyeska's VFDs, the core excited the frame's natural frequency. The manufacturer got around the problem by isolating the core from the frame with a suspension system to attenuate the frequency. "It was fairly major. We essentially reconstructed the whole frame," he says.
"They did an excellent job to recover," says Livett. But the problem put the project "a couple of months behind schedule," pushing some of the scheduled outdoor work into cold weather, which begins about October, he says. "We made a conscious decision to slow down implementation of construction," he says.
Construction on an active industrial site presented another challenge. "Because we're installing at pump stations, there's a lot of underground utilities," Livett says.
"The specifications almost require hand digging to avoid damage." Ground-penetrating radar located anomalies as deep as 100 ft. Workers then excavated with an air knife, a 1/4-in. pipe with an air nozzle that cuts frozen earth but not steel or cable. With such methods, workers drove 1,070 steel piles at the five stations. "We didn't hit anything unexpected," says Livett.
The harsh climate makes it necessary to modularize construction to the maximum. The Anchorage shops of Veco Corp. and ASRC Energy Services Co. are fabricating 56 truckable modules weighing up to 100 tons for VFDs, pumps, motors, lube-oil skids, controls, power distribution and diesel generators. Veco is the prime contractor for major installations at pump stations 1 and 3. ASRC Energy Services is doing the same for stations 4, 5 and 9. More than 50 other Alaska-based contractors also are working on the project, and the Edmonton, Alberta, office of SNC Lavalin Group Inc. is performing engineering, procurement and construction management under a $28-million contract.
By Thomas F. Armistead