Device for the connection of rods for rotational drive of a downhole pumping apparatus

A device for connecting sucker, or drive rods, consisting of a threaded coupling, one end of which has a right hand female thread and the other a left hand female, which joins the threaded pins of adjacent rods. The ends of the pins are equipped with dogs, or lugs, that interlock when the connection is made up, firmly linking the rods torsionally. The coupling serves only to keep the dogs, or lugs, engaged and to carry the tensional load on the connection. Such a connection provides a stronger torsional link between rods than connections currently available. The connection also does not require the special make-up procedure of the current systems and cannot over tighten or back off during operation.

BACKGROUND OF INVENTION

Field of the Invention

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, that 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.

SUMMARY OF INVENTION

The existing 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 lubricates 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 connection 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 connection between the rods, thereby removing the torsional loading of the threads and holding the rods end together via a right-left threaded coupling that cannot over-tighten nor back-off after make up.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

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.

The general configuration of the rod connection for both sucker rods and drive rods is shown inFIG. 1. At the end of the rod body11is the coupling assembly, consisting of the transition flare,13, the wrench flat,15, the rod shoulder,17, with the machined surface19facing the threaded rod pin,21. Note that the threads of the threaded rod pin are right-hand, as is the norm in the petroleum industry. The adjacent rod, shown asFIG. 2, has the same components of the coupling assembly, including right-hand threads on the threaded rod pin.

The two rods shown inFIG. 1andFIG. 2are joined via an internally threaded female coupling,23, shown in side-view inFIG. 3. Both ends of the internally threaded coupling23are machined flat surfaces25and27, and are designed to bear against the machined surfaces19and20of the rod shoulders17and18. The internal threads of the internally threaded female coupling23, shown as29and31, are shown inFIG. 4. Note that both internal threads29and31are right-hand threads to mate with the right-hand threads of the threaded rod pins21and22, respectively.

The connection between the two rods is affected by first screwing internally threaded female coupling23on to the threaded rod pin21ofFIG. 1rod until hand-tight, then screwing the adjacentFIG. 2rod into the internally threaded female coupling23, as shown inFIG. 5. OnceFIG. 2rod is fully threaded into internally threaded female coupling23, the two rods are then torqued to a desired value, and the connection is complete, as seen inFIG. 6.FIG. 6also shows, via a cut-away of the internally threaded female coupling23, the relative positions of the threaded rod pins21and22of the joined rods. Note that they are not in physical contact with one another.

As seen inFIG. 6, the threaded rod pins21and22are not in direct physical contact. The only connection between the rods for torque transmission is via the internally threaded female coupling,23. Internally threaded female coupling23is not fixedly connected to either rod, or through another mechanical means like a spline or other toothed connection. The only effective torsional connection internally threaded female coupling23has with either rod is via the friction between the machined flat surface ends25and27of the internally threaded female coupling, and the machined surfaces19and20of the rod shoulders17and18, respectively. This frictional connection is only effective if the aforesaid machined surfaces are completely clean and dry, so that the make-up of the connection results in a “locked” condition, held in place by static friction that is greater than what any expected torque could overcome.

Threaded connections, such as that shown inFIG. 6, with the aforesaid machined ends25and27, and machined surfaces19and20that are less than perfectly clean and dry, are prone to gradually tighten during operation, and particularly due to torque surges during start-up, finally causing the pin threads to fail in sheer, or the pin body to fail in tension. Less than clean connections also could loosen and back off during back-spin when the system is shut-down. All of these attendant problems of the existing rod connection when used in a rotational drive system can be eliminated by utilizing the present invention.

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 coupling23and the rod shoulders17and18. A better configuration would be to have a mechanical torsional connection between the rod ends. However, such a mechanical connection requires that the two rods cannot rotate freely relative to one another when connected, so utilizing the existing right hand threaded pin-coupling connection 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 gets around this problem by utilizing a coupling with both right and left hand threads, which engages with the threaded rod pins20and21with similar right and left hand thread, rather like a turnbuckle, to draw the rods together in a fixed rotational position relative to one another, so that a mechanical torsional connection of some sort can be engaged as the rods are being pulled together.

Referring toFIGS. 7 and 8, the adjacent ends of two rods to be connected are shown. Both rods are similar to those shown inFIGS. 1 and 2, having, respectively, a rod body33and34, wrench flat35and36, shoulder37and38and a threaded rod pin39and40. However, theFIG. 7threaded rod pin39has a left-hand thread, as opposed to the right-hand thread of threaded rod pin21. TheFIG. 8threaded rod pin40has a right-hand thread. Also, machined into the ends of both rods are the dogs of a two-lobe dog clutch assembly. The rod ofFIG. 7has dogs41and43(43not visible in this view), and the rod ofFIG. 8has dogs42and44. Each dog is a machined steel “lug”, with the quarter circle cross-sectional shape, with each rod having two dogs 180° apart, as seen in the end views of the rod ends shown asFIGS. 9 and 10. The dogs41and43on theFIG. 7rod, and the dogs42and44on theFIG. 8rod are machined to interlock and engage snugly together, as seen inFIGS. 11 and 12, forming mechanical torsional connection between the rods that is significantly stronger than the torsional limit of the rod body33and34.

FIGS. 11 and 12show how the dogs engage, with the female coupling45holding them together, not shown for clarity. The female coupling45is shown inFIG. 13, with the semi-transparent view showing the left-hand internal threads47on the left, matching the left-hand threads39of theFIG. 7threaded rod pin40, and the right-hand internal threads48on the right, matching the right-hand threads of theFIG. 8threaded rod pin40.

FIGS. 14, 15 and 16show how the connection between the adjacent rods is made-up. The rod ends are inserted into the adjacent ends of the female coupling45, which is rotated in the direction shown by the white arrow, A. The threaded rod pins39and40and the female coupling45engage, and the rotation of the female coupling45, as shown, draws the rods together, without causing either rod to rotate relative to one another. As they are drawn together, the dogs begin to engage, as seen inFIG. 11, and then completely engage, as shown inFIG. 12. Note that the female coupling45does not bear upon the shoulders37and38. Shoulders37and38are present to protect the threads from being damaged by the wrenches and other equipment that engage with the wrench flats35and36during installation.FIG. 17shows the made-up connection, with a cut-away in the coupling showing the fully engaged dog clutch members.

Once the dogs are engaged, the rods cannot rotate relative to one another, and the connection is secure. The only way it can come apart is if the female coupling45is 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 coupling45has only to carry the tensional load of the rod weight. No particular amount of torque is required to make up this connection, as there is no required friction between components to transmit torque. Because of this, the components do not have to be particularly clean or dry during assembly.

Since the female coupling45need not be made up with appreciable torque, there may be circumstances where, through vibration or rubbing against the inner tubing wall, the female coupling45may begin to unscrew if not restrained somehow. To be completely sure that the female coupling45remains firmly made up with the threaded rod pins39and40, one or both of the threaded rod pins39and40would be cut with a tapered thread so as to require some nominal torque to make up the connection between female coupling45and the threaded rod pins39and40. This nominal torque would serve to keep the female coupling45from 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 coupling45becoming inadvertently disconnected from the threaded rod pins39and40.

Although the embodiment shown inFIGS. 7 through 17is preferred, there are alternative ways to torsionally join the adjacent rods, while utilizing the same coupling concept of the preferred embodiment.FIGS. 18, 19 and 20show such an alternative system consisting of a spline connection between the rods. A splined stub shaft49, machined at the end of theFIG. 18rod, mates with a female spline receptacle51, machined at the end of the FIG.19rod.FIG. 20shows, via a semi-transparent view, the internal female spline53. This spline system would be more costly to manufacture, and not as strong as the preferred dog clutch system, but would function similarly.

It will be appreciated by those skilled in the art, upon reading this detailed description, may think of some other variations in structure and form to torsionally connect the adjacent rods, and such variations are within the contemplation of the invention as described and claimed in the following: