Drive technology could replace nodding donkey pumps

Paul Boughton

Linear rod pump artificial lift system combines ac motor servo control, simple mechanics, and control software that is responsive to the individual characteristics of each well. Lucie Hodkova reports

Onshore oil fields traditionally use pumping systems based on rocking beams, nodding donkey or sucker-rod pumping (SRP) mechanisms. In fact, two-thirds of all the world’s oil producing wells rely on this type of artificial lift. These pumps include large moving metal parts that are hazardous to assemble and disassemble, potentially dangerous in use and which take up large amounts of space. While mechanically simple, they can have reliability issues resulting from running dry, which causes damage to expensive and hard-to-access components.

In past decades this was not a problem because oilfields tended to be remote and on low value real estate, but increasingly oil is being extracted from wells in environmentally sensitive areas and even urban locations. Also, the need to improve overall operating efficiency has climbed markedly over the years and so operators have long been searching for a reliable and accurate way to control SRPs and improve their productivity.

Now oil and gas extraction technology has been advanced thanks to a new development by Unico, part of Regal. Its LRP linear rod pump artificial lift system combines ac motor servo control, simple mechanics, and control software that is responsive to the individual characteristics of each well. As such it is a revolutionary departure from traditional rocking beam pump designs.

Compared to conventional pumps, the LRP Linear Rod Pump system developed by Unico is simple and compact, making it economical and easy to install, while its electronic controls advance the technology significantly. Transportation, site preparation, and installation costs are minimal, with the LRP system mounting directly onto the wellhead rather than requiring the construction of a large cement deck.

Unico has designed the LRP system so that it can be easily transported in a pickup truck, handled by two people, and installed within two-three hours. It is highly suitable for locations where regulations prohibit the use of beam pumps, such as residential neighbourhoods and car parks. Further, due to the quiet operation and small size of the system, it can be easily screened by trees or fences so that it doesn’t detract from the natural landscape in rural locations or disturb local residents.

All told, the LRP system opens up new opportunities to tap oil and gas reserves that might otherwise be unreachable. It can also be used as a portable, temporary or testing pump, moving easily from site to site as required.

The key technology of the LRP system is a flux vector inverter drive, which controls the motor powering the sucker rod through a rack and pinion mechanism. Basically, the sucker rod strokes up and down to lift the oil or gas from the well, and as such is similar to an old fashioned beam pump.

The difference is that, through the drive, the LRP system has precision control of the speed, position and power of the sucker rod at every point in its operating cycle. Thus during the down-stroke, the speed of travel is reduced so that the plunger enters the fluid gently. Once immersed the speed is increased through the remainder of the pump stroke to maximise production. This motion profile is programmable and can be set up to adjust on the fly to adapt to changing conditions so that performance is constantly self-optimising.

This direct control eliminates the cumbersome, high-inertia beam mechanics of other systems, while compared to hydraulic reversing systems, the LRP solution is much more elegant and capable.

The simplicity of the concept is reflected through into the mechanical design of the pump. The rod runs through a channel inside the rack and is suspended from the top by a conventional rod clamp so that it has some degree of lateral movement. The vector-controlled induction motor, coupled to the rack-and-pinion mechanism through a gearbox, cycles the rack up and down to reciprocate the rod. The rack is lubricated with each stroke by submersion into a fully contained oil bath.
By using a high-performance motor and line-regenerative drive high system, efficiency can be achieved, even on deep wells, without resorting to the massive counterweights so common in nodding donkey installations. In fact, this low-inertia design allows the LRP system to use a much smaller motor and gearbox than a conventional jack pump and avoids the need to oversize other transmission components.

A major part of the success of the LRP system is attributable to the motor control software which achieves motion profiles that are impossible through mechanical means alone. Pump fill is optimally regulated by independently adjusting the upstroke and down-stroke range and speed. Soft landing speed control minimises fluid impact shock loads while an automated valve check controls standing and travelling leakage.

The software, which is embedded into the flux vector drive integral controller also provides well data reporting, surface and downhole dynamometer plotting, remote access capability, automatic restarting, fault diagnostics, etc. The drive can also be interrogated remotely, via the web or telemetry, allowing unmanned monitoring and control of the pump to further optimise running costs.

Thus Unico’s new LRP Linear Rod pumping system advances extraction technology on several fronts and is opening up oil field locations that were previously considered to be non-viable, due to the compact size of the installation as well as the improved economic benefits of the system.

Lucie Hodkova is with Regal UK. For more information, visit

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