The migration of exploration and development activity into the National Petroleum Reserve-Alaska to the west of existing development on the North Slope demands some creative thinking into how to move heavy loads across the tundra many miles from the nearest road. And even although tundra transportation occurs during the winter season when snow and ice protect the delicate land surface, vehicles that can negotiate rough terrain while exerting little pressure on the ground surface provide critical support for off-road oil and gas operations.
Craig Tornga, general manager of Crowley Marine Services in Anchorage, told Petroleum News in May about the latest design improvements in his company’s massive CATCO all-terrain vehicles. Using innovative ideas through experience and new technologies, Bill Kuper, CATCO manager and his team, has designed a new electromotive CATCO vehicle to provide enhanced services to our customer, The company has a “patent pending” on the new power electromotive design.
Airbags instead of tires
CATCO vehicles have been in use for a number of years and use eight airbag tires with huge circular airbags instead of tires. The airbag tires spread the vehicle’s load across the ground, so that even when fully loaded the vehicle only exerts a pressure of 3 to 4 pounds per square inch on the ground surface, Tornga said. An individual yoke arm holds each wheel independently, thus enabling the wheels to move up and down as the vehicle clambers over rough terrain like a giant caterpillar.
The old design used an individual mechanical transmission through a drive shaft for each pair of wheels, with friction rollers rather than wheel axles turning the individual airbag tires. The roller drive effectively eliminates skid damage to the ground surface — the roller will skid on the bag before the bag skids on the ground.
“You have your weight right on your bag … so if you’re trying to pull a good load, all bags are turning,” Tornga said. “Then if your friction is less than the friction between the ground (and the bag) … your slippage is then between the roller and your bag, not between the bag and the ground.”
100,000-pound capacity
A strategy to increase the CATCO carrying capacity from 60,000 pounds to 100,000 pounds triggered the latest design. Increasing the payload reduces the number of trips required to carry gear to a remote site, thus reducing the transportation costs.
“We’re getting to 100,000 pounds so that we can reduce the cost per mile per ton for our customers,” Tornga said. The designers also tried to minimize the vehicle operating costs, Tornga explained.
Crowley used a technique called finite element analysis to analyze the stresses from that 100,000-pound load. This analysis led to a tractor-trailer configuration, rather like a fifth wheel travel trailer. The tractor unit has 10 air bag wheels, rather than the eight bags of the old design. The trailer has six bags.
“The existing trailer that we have is a 40-foot trailer with four bags,” Tornga said. “So our new design is a 65-foot trailer with six bags to get to the 100,000 pound capacity.”
Electric drives
Having determined the basic design of the vehicles, the designers turned their attention to the drive system. The initial concept involved a conventional mechanical drive, as in the existing CATCO vehicles. But recent electric motor innovations made an electric drive system, rather like the powering of a diesel-electric railroad locomotive, possible.
“About 10 years ago we had looked at going to electric drive, but the motors were so large at that time and they weighed so much and they were so costly that it just wasn’t feasible,” Tornga said. “We discovered that some of the new electric systems made it feasible to do,” he said.
The roller that drives each air bag tire will now have its own electric motor with its own controller, powered by a diesel generator in the tractor unit. The motors and controllers are attached to a chassis-like center rail that extends the length of the vehicle.
By putting strain gauges on existing CATCO vehicles the engineers could determine the size of electric motor that the new vehicles would need. When doing this testing the engineers made the unexpected discovery that the mechanical load applied from the vehicle’s engine did not apply uniformly to all of the wheels.
“We found that the mechanical system had peak loads to different drive rollers, and it wasn’t evenly distributed,” Tornga said.
That discovery led to a significant advantage in the electric drive, in that the controller for each motor can adjust the power applied to an individual air bagged tire. Not only that — the torque applied to each drive roller can be adjusted during a turn, thus allowing relatively tight turns without causing ground damage.
Low weight
The use of the electric motors also reduces the weight of the vehicle by eliminating the mechanical power train components.
“It really proved out in lightening our load quite a bit – we were taking almost 3,000 pounds out of the tractor,” Tornga said.
The design has further reduced vehicle weight by eliminating the need for two diesel engines — there’s now just one engine driving the generator in the tractor and powering the trailer.
“On the existing system we have two engines — one on the cab and one suspended under the back deck on the trailer,” Tornga said.
A decision not to include a battery bank for balancing the electrical load of the drive system, as in a railroad locomotive, also minimizes vehicle weight. Batteries could assist braking and when driving uphill and regenerate on downhill sections.
“It’s pretty flat up there (on the Slope), so you’re not really regenerating your batteries,” Tornga said.
Environmentally friendly
As well as providing an individually adjusted drive to each airbag tire, the electrical drive will reduce environmental impacts by eliminating drips and spills from the transmission fluids and lubricants needed for a mechanical system, Tornga said.
“We see environmentally it’s going to be a lot better too — we’ve reduced emissions, fuel consumption and reduced a lot of potential spills and still maintain the environmental protection the uniquely designed CATCO provides,” he said.
And, although some other modern vehicle designs for tundra travel involve the use of rubber tracks rather than wheels, Tornga thinks that the CATCO’s wheeled design reduces the likelihood of tundra damage during turns and enables faster travel.
The bottom line is that we have a lighter vehicle that’s able to carry a heavier load and ultimately reduced the costs per ton per mile for our customer while still maintaining the environmental protection of the tundra, Tornga said.
