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Rugged hydraulic motor clears oil-well tubing
Rolling-vane, serial-flow design by Gary Harris delivers high power at high efficiency
By Terrence Lynch, Contributing Editor -- Design News, March 3, 1997
Humble, TX--A newly patented serial hydraulic motor, relatively immune to high temperatures and capable of working with a variety of fluids, could dramatically lower energy-industry maintenance costs and revolutionize undersea dredging operations to boot.
The MacDrill Downhole Motor from Rotech Holdings was developed with backing from the European Commission's THERMIE energy program. This Euro-American design solves the problem of working oil wells that become clogged with sand and precipitants. Until now, crews cleared such obstructions with a rubber-bodied hydraulic motor-drill combination. But high temperatures in many wells, and the motor's incompatibility with solvents and other chemicals, made for very short motor life--where the system could be used at all.
In contrast, the MacDrill motor uses no rubber except for O-rings between sections. Properly configured, it can withstand temperatures to 600F. As Gary Harris, who developed the motor along with Hector Susman, explains: "At 600F, this motor can do things no one else's is able to do, like work in deep wells and even geothermal wells where maintenance is an especially costly problem."
The motor comprises a housing and concentric stator and rotor elements. It features four chambers defined by the stator's inner diameter, the egg-shaped rotor's outer surface, two bleed-pressure-loaded stator seal rods, and seal rods at the rotor's maximum radii.
Pressurized working fluid enters the motor from above, passing between the motor housing and the stator's outer diameter. Diametrically opposed holes in the stator wall allow the fluid to enter two of the stator-rotor chambers, imparting a couple that turns the rotor clockwise. As the rotor's seals pass the stator seals, the fluid exits the chamber through now-exposed exhaust ports in the rotor, through the rotor's center, and then further down along the motor. The fluid is diverted again to the outer housing-stator space and then into a second motor 90 degrees out of phase with its upper partner, ensuring continuous power output.
Harris estimates the motor's efficiency at 80 to 90%, depending on its size and seal condition. A 31/8-inch tool-size motor weighs 112 lbs, operates at a maximum 850 rpm at a flow rate of 90 GPM, and generates a maximum torque of 250 lb-ft. Versions of the motor are now at work in North Sea and Middle East oil fields.
In the hellish conditions for which it was designed, the motor's mean time between overhauls is 50 hours. That figure is low for most motors, but a lifetime for many oilfield tools. The unit's simple, rugged construction makes overhauls relatively sim-ple. "Its attractiveness comes from the fact that it withstands high temperatures, and that it can be used with solvents or any clean fluid," Harris says.
As an example of the motor's versatility, it was recently used in experimental subsea dredging applications. In this case, engineers fitted the motor with counter-rotating propellers to deliver a high-energy stream of water to blast a trench along the ocean floor. In 30 minutes, it produced a trench comparable to 20 hours' worth of conventional dredging. In addition to its efficacy, the design has the added benefit of being operated with seawater instead of hydraulic fluid or other potential pollutants. Harris notes that the MacDrill could be run in reverse, i.e., as a high-powered pump, opening the way to many other applications.
Additional details…Contact Gary Harris, ROTECH Holdings, 5902 Bent Tree Ct., Humble, TX 77346, (281) 852-4159, or Hector Susman, Whitemyres Ave., Mastrick Industrial Estate, Aberdeen, Scotland AB2-6HQ, (441) 224-698-698.
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