Long-stroke sucker-rod pumping units have definite advantages as compared to their traditional counterparts. They produce greater liquid rates with less downhole pump problems and can also increase the life of the rod string due to the reduced number of stress reversals.
This article examines present-day long-stroke rod pumping methods and discusses the two main types of technologies available: the fully mechanically driven Rotaflex and the hydraulically driven DynaPump units.
Although several types of long-stroke pumping units were developed in the past with some success, today only two models are available.1, 2, 3 They work on different principles.
The Rotaflex Unit
After several attempts in the late 1980s to develop a completely mechanical long-stroke pumping unit, it was Lively’s patented version that became accepted in the oilfield.4 Its trade name is Rotaflex. By offering many advantages over traditional pumping units, it is a strong competitor to beam type pumping units and electric submersible pumping (ESP) equipment at the same time. All benefits of Rotaflex units are a direct consequence of its mechanical design. These units are getting more and more popular. The total number of Rotaflex installations was about 800 in 2002 but became more than 7,000 a decade later.5
Construction and Operation
The construction and the basic operation of Rotaflex units are explained in Figure 1, which shows a schematic drawing of the main parts and their functions. The unit is driven by a traditional pumping unit gearbox via the drive sprocket of a vertically arranged chain assembly with an idler sprocket situated vertically above the drive sprocket. The heavy-duty roller chain, as it is turned by the drive sprocket, drives a weight box (containing the counterweights of the unit), which is connected to one of the links of the chain. These components constitute the drive train of the unit.
On the well side, the polished rod is directly connected to an elastic load belt that runs on a drum situated higher up in such a way that polished rod loads generate only vertical forces in the belt. The other end of the belt, hanging vertically from the belt drum, is fixed to the weight box. The weight box, therefore, carries all operating loads—the pumping load on the polished rod and the counterbalance load. Since its movement is strictly vertical, it can always fully utilize the total weight of the counterweights in order to balance the load on the gearbox.
The structural components of a Rotaflex unit are shown in Figure 2. The derrick, or tower, supports and contains most of the machinery and stands above the wellhead. The polished rod is connected to the unit in the traditional way: using a carrier bar, a polished rod clamp and wireline hangers. The wireline hanger is fixed to a strong, flexible load belt hanging from the belt drum, which is situated at the top of the derrick structure. The other end of the load belt is fixed to the counterweight assembly (weight box) that travels in a vertical direction up and down inside of the derrick.
The Rotaflex pumping unit converts the prime mover’s rotation into the reciprocating movement required for pumping with the help of the chain and sprocket and the carriage assemblies. Therefore, the unit’s kinematic behavior can best be described through a thorough analysis of the movement of those components during the pumping cycle. The motion of the carriage assembly during a full pumping cycle is very symmetric. At the start of the polished rod’s upstroke the carriage is at the top of the idler sprocket, then follows the perimeter of the sprocket, then moves on a vertical line, then turns around the drive sprocket.
The kinematic parameters: position, torque factor, and acceleration of the carriage are expressed as functions of the sprocket angle, θ. This is the angle of rotation of the idler sprocket and is measured in the direction of the sprocket’s rotation starting at θ = 0 valid at the start of the upstroke.
The variation of the major kinematic parameters with time during a complete pumping cycle for an example Model 1151 Rotaflex unit is plotted in Figure 3. As seen, carriage position (and polished rod position) changes linearly with time and with sprocket angle for most of the up and downstroke. Acceleration values for a pumping speed of 2 strokes per minute (SPM) are also given in the figure. The long, quiet movement of the carriage results in zero acceleration for most of the pumping cycle. Torque factors are constant, except for two short periods around the two extremes of the stroke. Only their sign is different for the up- and the down-stroke.
The Rotaflex unit has an extremely quiet operation with very low accelerations and negligible dynamic effects and offers a kinematic behavior that very profoundly differs from that of any beam pumping unit. The polished rod of traditional beam pumping units continuously accelerates during the up-stroke and decelerates during the down-stroke, causing varying velocities. Those units, in contrast to Rotaflex units, experience high dynamic loads.
The DynaPump Unit
The DynaPump unit, a computer-controlled hydraulically driven long-stroke sucker-rod pumping unit was invented by A. Rosman at the end of the 1980s.6, 7 Since its commercial introduction to the oilfield in 2001 the technology underwent several improvements that have lead to the present-day models. Just like the Rotaflex units, DynaPump units can also compete with traditional rod pumping and ESP installations by offering greater system efficiencies and lower production costs. Due to their advantageous features DynaPump units are getting popular; the worldwide number of installations reached 500 in the year 2008.8
Construction and Operation
The two main components of a DynaPump installation are the pumping unit and the hydraulic power unit. The first converts the hydraulic power to lift the well load. The second provides the power fluid of a controlled volumetric rate to drive the pumping unit. A schematic drawing is given in Figure 4 that depicts the main parts of the system. The power and pumping units are connected by high-pressure hoses, not shown in the figure.
The pumping unit of the DynaPump system drives a traditional downhole pump attached to the sucker rod string. The polished rod is lifted by the usual carrier bar attached to two wireline cables. The other ends of the cables are fixed to the unit base and the cables run on two sheaves to form a 2:1 pulley system. The sheaves are situated at the top of a plunger that protrudes from a special hydraulic three-chamber cylinder. The vertical position of the sheaves is the function of the hydraulic fluid rate received from the power unit. To counterbalance the variation of the well load during the pumping cycle gas pressure also acts in the three-chamber cylinder. The gas pressure is provided by a high-volume gas (usually nitrogen) storage cylinder. Because of the pulley system the displacement of the polished rod is exactly twice as much as the vertical movement of the sheaves caused by the three-chamber cylinder. For the same reason the load on the cylinder at any time equals twice the polished rod load.
In contrast to beam pumping units, DynaPump units allow the operator to select the variation of most of the kinematic parameters during the pumping cycle. These can be input at the unit’s controller.
The unit provides constant polished rod velocities for long portions of both the up-, and down-stroke periods. This feature greatly decreases dynamic forces and reduces energy requirements for most of the stroke. Up- and down-stroke speeds are selected independently of each other.
Both the up-, and the downstroke of the polished rod’s movement include periods of acceleration, constant speed, and deceleration. The lengths of the acceleration and deceleration periods (four in total) can be independently selected on the DynaPump, allowing excellent control of the unit’s dynamics at the stroke reversals.
A typical DynaPump unit’s velocity and acceleration pattern during the pumping cycle is depicted in Figure 5. As shown, big portions of both the up-, and downstroke are performed at constant polished rod velocities and the accelerations at the top and bottom stroke reversals are fully controlled by the operator for an optimum dynamic performance.
Polished rod position is found by integration of the velocity with respect to time. For the given case, calculated positions during the pumping cycle are given in Figure 6. The figure reveals the different phases of the movement of the unit. From point 1 to 2 during the upstroke acceleration period, the polished rod reaches the set upstroke speed. Then movement with a constant velocity follows from point 2 until the top switch position is reached at point 3. Now the deceleration period comes until point 4 where the polished rod’s travel reaches its maximum. Downstroke is starting next with the downstroke acceleration period from point 4 to 5. From here downward, travel continues with a constant velocity. This velocity is less than that during the upstroke. Finally, from point 6 belonging to the bottom switch position, the unit slows down to reach the bottom of the downstroke at point 7.
Comparison of Pumping Units
Rotaflex units are manufactured in four sizes with a minimum stroke length of 288 inches. Maximum stroke length is 366 in while maximum polished rod capacity is 50,000 pounds. Table 1 shows other operational data.
DynaPump units are classified according to the size (in inches) of the three-chamber cylinder’s plunger. The models are named accordingly. Table 2 shows main technical data of available models.
Power units come in seven different power capacities in the range of 15 to 150 HP. Each power unit can work with different pumping units, but bigger models usually require more power. Proper selection of the combination of power-, and pumping units is important.
The production capacities of the biggest Rotaflex and DynaPump units are compared in Figure 7, based on data received from Beck and Rosman.9, 10 The two units can produce very similar liquid rates from the same pumping depths.
Finally, Table 3 presents an interesting aspect of comparing long-stroke and conventional pumping units. The table contains main operational parameters like maximum polished rod load and stroke length of beam, DynaPump and Rotaflex units of comparable capacities along with their approximate weights. Since the weight of machines made of steel is a good indicator of their price, investment costs of the three units are easily compared. According to the data contained in the table, beam and Rotaflex units are in the same weight range. DynaPump units of comparable capacities have weigh about 50 percent less.
Today there are only two kinds of long-stroke pumping units that are widely used: Rotaflex and DynaPump. This paper demonstrates the different features of these units and compares their features and capabilities. Main findings are the following:
- Although both units provide very similar advantages their operational principles are widely different.
- Lifting capacities of the two units are comparable so they can be used under the same conditions.
- Kuhns, J. P. – Rizzone, M. L.: “Well-Pumping Apparatus.” US Patent 3,285,081, 1966.
- Gault, R. H.: “Longstroke Pumping Apparatus for Oil Wells.” US Patent 4,076,218, 1978.
- Tait, H. C.: ”A Rod Pumping System for California Lift Requirements.” Paper SPE 11747 presented at the California Regional Meeting held in Ventura, California, March 23-25, 1983.
- Lively, G. R.: “Long Stroke Well Pumping Unit with Carriage.” US Patent 4,916,959. 1990
- “Rotaflex Long Stroke Pumping Units.” Weatherford Artificial Lift Systems brochure. 2002.
- Rosman, A. H.: “Hydraulically Operated Lift Mechanism.” US Patent 4,801,126. 1989.
- Rosman, A. H.: “Oil-Well Pumping System or the Like.” US Patent 4,848,085. 1989.
- Huard, W.: “Intelligent Long Stroke Hydraulic Pumping System Reduces Well Intervention Costs while Maximizing Energy Efficiency.” Paper presented at the 4th Annual Sucker-Rod Pumping Workshop. Houston Texas, September 9-12, 2008.
- Beck, S.: Technical consultation. Production Systems, Weatherford International. November 2013.
- Rosman, A. H.: Personal communication. November 2012.