The present invention relates to improvements in connections between the individual segments of a rod string for the rotational drive of a downhole pump used in production wells to retrieve and deliver to the surface production fluids from subterranean deposits.
A common method of lifting fluid from an oil well, the progressive cavity pumping system, utilizes a string of steel rods attached to a progressive cavity pump at the bottom of the well, which are rotated by a drive mechanism at the surface to activate the pump. This string of rods is similar to that used in a beam, or sucker rod, pumping apparatus, sharing an identical method of connection between the individual sections of rod, but utilizes rotational, rather than reciprocating, motion to activate the downhole pump.
The type of connection between rod segments utilized in both sucker rod pumping systems and progressive pumping systems (as well as other rod rotational drive pumping systems) consists of threaded pins at the ends of the rod segments, that are joined via an internally threaded female coupling. The threaded pins of the two rods to be joined are screwed into the female coupling until the machined ends of the coupling are tightly made up against machined shoulders on the rods. This type of connection was developed for the sucker rod application, where the rod motion is reciprocation, and loads on the rods and rod connections are entirely tensional.
When the progressive cavity pump was developed, the widely available sucker rods were utilized for the rotating rod string to drive the downhole pump, despite the fact that the sucker rod connection was not designed to transmit the torsional loads of the progressive cavity pump drive. The existing system of joining rods for rotational drive functions satisfactorily when installed and operated properly, but remains the single greatest problem of the various rotational rod drive systems. The present invention addresses these problems with a new rod connection system that is stronger and much easier to install properly than the existing system and will be only slightly, if at all, more costly than the existing system.
The existing, prior art system for joining the individual rods that make up the rod string used to rotationally drive a downhole pump consists of threaded pins at the ends of the rods connected via a female threaded coupling. The rods are equipped with machined shoulders near the threaded pins, and the rods are screwed into the coupling until the rod shoulders make up tightly against the ends of the coupling. The torsional force of one rod is transmitted to the adjacent rod through the coupling via the friction between the machined surfaces of the rod shoulders and the ends of the coupling.
The principal problem of the existing rod connection for rotating rod systems like the progressive cavity pumps, is over tightening of the connection during operation, resulting in failure of either the threaded pin or coupling. This over tightening occurs because of grease or dirt contamination lubricating the machined surfaces of the rod shoulders or coupling ends, allowing the connection to gradually tighten until either the pin or coupling fails. The surfaces of the rod shoulders and coupling ends must be absolutely clean, dry and free of any contamination, so that when the threaded connection is made up to the prescribed torque, the surfaces are “locked” in place by static friction. This cleanliness requirement is a significant burden during rod string installation, as making sure that every connected surface is completely clean, in the naturally oily and dirty environment of a well service rig, requires constant vigilance. There only needs to be one less-than-clean connection out of hundreds to result in a rod string failure.
Another problem with the existing rod connection for rotating rod systems, is the threads of the connection are under both torsional and tensional loading, as the coupling must both transmit torsional load to the coupling, as well as carry the tensional loading due to rod weight. This problem is, at its worst, at or near the surface, as the tension on the rod pins is maximized due to the weight of the rods hanging below, and the rod pins can fail, particularly during start-up torque surges.
A further, but lesser, problem with the existing rod connection system is the backing-off separation of the rods. Since the existing connection consists typically of right-hand threaded members, back spinning of the rod string, which will occur with progressive cavity pumps whenever the surface drive is shut off, can result in the unscrewing of one or more of the connections, requiring a costly well service to reconnect the rods.
The present invention eliminates all of these problems with the existing rod connections by physically linking adjacent rods for torsional load transmission via a dog clutch, or similar torsional connection, between the rods. The threaded coupling provides only the tensional connection between the rods.
A preliminary search of the patent literature was made and the following listed patents pertaining to the present invention are believed to be relevant:
Journeay, U.S. Pat. No. 1,547,759 (November 1930)
Mefferd, U.S. Pat. No. 4,821,818 (April 1989)
Hughes, U.S. Pat. No. 5,950,744 (September 1999)
Xin et al., U.S. Patent App. 2012/0088588 A1 (Pub. April 2012)
Journeay U.S. Pat. No. 1,547,759 describes a system to join sections of drill pipe, consisting of interlocking tongues and slots in the ends of the drill pipe, which are connected via a threaded sleeve, with a mechanical ratchet mechanism to prevent unthreading of the sleeve. The Journeay patent is specifically for joining tubular devices, which are required to provide a channel for flow of liquids during the process of well drilling, where the present invention is for the connection of solid rods entirely for the torsional drive of a downhole pump. Also, the principal function of the Journeay patent was to provide a drill string connection that would not unscrew during drilling operations, whereas the principal objective of the present invention is to prevent the drive rods from over-tightening, the opposite of unscrewing.
Mefferd U.S. Pat. No. 4,821,818 describes a system of connecting tubular members used an auger drilling that is similar to Journeay in the use of a tongue and groove connection. However, Mefferd connects the pipes via cylindrical collars, rather than a threaded sleeve, as in the case of Journeay. Like Journeay, the Mefferd patent is for joining tubular members, which are required to provide a channel for flow of liquids.
Hughes U.S. Pat. No. 5,950,744 describes a system of joining drill pipe, or tubing, that provides for a unique angular orientation of the entire string of pipe or tubing. The system is similar to Journeay in the use of a tongue and groove connection between the pipes, joined via a threaded sleeve, but with irregular spaced tongues and grooves, so that the pipes can be joined in only one orientation. Like Journeay and Mefferd, the system is for the joining of tubular members required to provide a channel of flow of fluids, and, in the case of Hughes, a passage for the running of instruments or other devices into a well.
Neither Journeay's nor Hughes' invention has found use in oil field operations, for two reasons. First, the tongue and groove connection between the pipes, as shown in the patent descriptions, would have less than half the torsional strength of the drill pipe, requiring a large increase in wall thickness within the connection to increase the strength to the required level. This would severely reduce the inside diameter of the drill pipe at each connection. Also, the threaded connection between the sleeve and the pipe ends would not provide the level of pressure seal required by drill pipe or tubing, where there can frequently be several thousands of pounds of pressure differential between the inside of the drill pipe or tubing, and the outside of the pipe or tubing.
The present invention does not suffer from either of these problems. Rod string connections are typically significantly larger in diameter than the rods, so there is ample material to make a torsional connection that is actually stronger than the torsional strength of the rods themselves. Also, since the rods are solid and are not required to provide a channel for high pressure fluid flow, the threaded connection need not provide a pressure seal, and can be designed purely for tensional strength.
Xin et al. patent publication 2012/0088588 A1 describes a rod connection involving different thread sizes on the adjacent rods, a wedge and groove torsional link between the joined rods, with the link being held in place via a complex coupling consisting of a partially threaded outer sleeve, a two piece inner threaded sleeve and an outer lock nut.
This rather complex system differs significantly from the present invention in the way the rods are coupled for tension, the connection make-up procedure, and the configuration of the torsion carrying members.
In the current invention, the rod tension is carried by the threaded ends of the adjacent rods via a single piece, internally threaded coupling, providing a much simpler connection make-up procedure. Also, instead of a wedge-groove torsional connection between rods, the rod ends of the present invention have identical dog clutch elements. The bearing surfaces of the dogs are parallel to the rod axis, so torsional loading causes no appreciable axial forces, and the coupling carries the rod tension only. This dog clutch configuration provides a connection with significantly greater torsional strength than the wedge-groove connection, as the dog clutch components are loaded purely in shear, where the wedge-groove connection results in significant bending loading of the groove members. Locking of the coupling of the present invention is affected via simple tapered threads over a short portion of one of the threaded rod ends, as the torque in the rod string has no appreciable affect on the coupling.
To appreciate the benefits of the present invention compared to the existing rod connection method, the details and dynamics of the existing system should be examined.
Sucker rods and drive rods used in the petroleum industry are usually 25 feet or 30 feet long. Both ends of each rod are identical and configured similar to that shown in
The two rods A and B are shown in
The connection between the two rods is affected by first screwing internally threaded female coupling 23 on to the threaded rod pin 21 of the
As seen in
Threaded connections, such as that shown in
The most serious problem of the existing system is that the rods are not physically connected for torsion, except via the friction between the internally threaded female coupling 23 and the rod shoulders 17 and 18. A better configuration would be to have a mechanical, torsional, connection between the rod ends. However, such a mechanical connection requires the two rods to remain rotationally fixed relative to one another when connected, so utilizing the existing right hand threaded pin-coupling connection method is not feasible. To make up such a connection, the two rods must rotate relative to one another, and if they are mechanically, torsionally connected, this relative rotation is not possible. The present invention overcomes this problem, as described by the following.
The dog clutch elements 43, 45 and 44, 46, shown in
Once the dog clutch elements are fully engaged, the now connected rods cannot rotate relative to one another, and the connection is secure. The only way it can come apart is if the female coupling 47 is unscrewed. Rotation of the rod string has no effect on the integrity of the coupling threaded connection with the rods, as the torque in the system is transmitted entirely via the dog clutch connection between the rods. The female coupling 47 has only to carry the tensional load of the rod weight. Only a minor amount of torque is required to make up this connection, as there is no required friction between components to transmit torque.
In the forgoing embodiment, the dog clutch elements 43 and 45 of rod C and 44 and 46 of rod D, have a cross-sectional angular size of 90° and are spaced 180° apart, as seen in
In the practical world of oil field operations, it would be beneficial if the rod connection process did not have the risk of damaging the threaded components of the rods due to mismatched engagement of the dog clutch. There are several ways this risk of incorrect assembly could be eliminated or greatly reduce.
For the symmetrical two-element dog clutch connection described above, a double-start thread configuration could be utilized. Double-start threads are two independent sets of parallel threads cut into the rod pins, with two thread-entry points 180° apart. The female coupling would be similarly threaded. Such a configuration would allow proper match-up of threads in the two-element dog clutch configuration no matter which of the two possible options of engagement was attempted. If a symmetrical three-element dog clutch connection were used, configured similarly to the two-element dog clutch connection detailed above, a triple-start threading scheme would allow thread match in any of the three potential angular orientations the dog clutch assembly could be engaged in.
The practical limit to the number of dog clutch elements, in a dog clutch-type connection is three. Dog clutches of two and three elements will have perfect load sharing among the elements. A four or more-element dog clutch will have guaranteed load sharing between only two of the elements, with partial loading of some of the others. This uneven loading will result in excess stress on the loaded elements and possible premature failure.
The simplest alternative to prevent improper thread match-up of a symmetrical dog clutch assembly, similar to that shown in
As a further alternative, and one that would also allow the use of single-start threading, the dog clutch elements could be machined in unequal angular size, and spaced such that they can be engaged in only one rotational orientation, an unsymmetrical dog clutch configuration.
The disadvantage of the asymmetrical dog clutch configuration is a reduction in connection torsional strength compared to the symmetrical configuration. That is, the element that is reduced in size, 57 in
In all of the forgoing connection schemes, the female coupling 47 need not be made up with appreciable torque, and there may be circumstances where, through vibration or rubbing against the inner tubing wall, the female coupling 47 may begin to unscrew if not restrained somehow. To be completely sure that the female coupling 47 remains firmly made up with the threaded rod pins 39 and 40, the threaded rod pin 39 would be cut with a tapered thread 53 over the last few threads, so as to require some nominal torque to make up the connection between the female coupling 47 and the threaded rod pin 39. This nominal torque would serve to keep the female coupling 47 from backing off in every circumstance. There are several other well known methods to lock threaded connections, and it is envisioned that any one or more of these alternative methods could be utilized in the present invention to prevent the female coupling 47 becoming inadvertently disconnected from the threaded rod pin 39.
It will be appreciated by those skilled in the art, upon reading this detailed description, one may think of some other variations in structure and form to torsionally connect the adjacent rods. Such variations are within the contemplation of the invention as described and claimed in the following: