Abstract:
A drive rod string for a progressive cavity pump. The drive rod string includes a plurality of drive rods. Each drive rod has a pair of opposed ends, wherein each end terminates in a frustoconical pin having tapered threading and having a radially extending cylindrical shoulder. A plurality of connectors are provided to connect the drive rods. Each connector is attached to one of the ends of the drive rods, wherein each connector has a pair of opposed frustoconical threaded recesses which extend from a pair of opposed shoulders which mate with the cylindrical shoulders of the frustoconical pins. An internal secondary stop within said connector acts as a positive stop in each connector for the frustoconical pin.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to rotatable rods and connectors used with progressive cavity devices for pumping oil or other fluids out of wells. More specifically, the present invention relates to drive rods having novel pins and connectors specially and uniquely designed for high torque rotation and suited for use with progressive cavity pumps or motors.  
           [0003]    2. Prior Art  
           [0004]    For over 100 years, sucker rods have been utilized to pump crude oil and other liquids out of wells. Sucker rods typically have threaded pins on each end that are screwed into connectors. Connectors attach to a sucker rod on either end. Sucker rods vary in length but typically have standard, common gauged threaded pins manufactured to API Standard II B specifications.  
           [0005]    A string of sucker rods may extend several thousand feet into a well. They must, therefore be very strong. Because of this, they are typically made of metal. In addition, lighter sucker rods are typically included at the bottom of the string while the sucker rods at the top of the string are often heavier and stronger in order to support the string. The top of the sucker rod string is attached to a pump jack. The bottom of the string is attached to a pump. Pump jacks reciprocate the sucker rod string in an upward and downward motion and subjects the string to compression and tension forces. This reciprocating motion operates the pump located at the bottom of the well.  
           [0006]    It is highly desirable that sucker rods do not become unscrewed and detach from the connectors between them. If this occurs, the pumping action halts. It is also difficult and labor intensive to stop the pump jack and retrieve the portion of the sucker rod string in the bottom of the well. In order to prevent this, sucker rod pins and connectors have been designed so as not to come unscrewed when a reciprocating motion is applied. In order to minimize the risk of detachment, sucker rod pins have evolved into a three part structure. First, at the end of the pin is straight threading. This provides for maximum friction on the flanks of the threading. Just below the threading is an undercut, or stress relief. Finally, both the connectors and the sucker rods have shoulders designed to engage one another. The presence of the stress relief allows for greater friction between the engaged shoulders. The friction between the shoulders prevents the sucker rods and connectors from disengaging. The straight threading increases the ability of the sucker rod string to withstand the stress of the reciprocating motion.  
           [0007]    Recent advances in pump technology have made it desirable to utilize progressive cavity (“PC”) pumps in oil and water wells. PC pumps are well suited for very viscous liquids, such as crude oil and for liquids having solids, such as sand, therein They are also capable of operating under very high pressure. Examples of these pumps may be found as far back as U.S. Pat. No. 1,892,217; U.S. Pat. No. 2,085,115; and U.S. Pat. No. 2,483,370 issued to Moineau. These PC pumps have fewer moving parts than other pumps typically used in wells. They are both sturdier and less likely to malfunction.  
           [0008]    The same technology utilized in progressive cavity pumps may also be employed in progressive cavity motors.  
           [0009]    Existing sucker rods are well suiting to withstand the stress of reciprocating tension and compression motion. However, standard sucker rods are not ideal for use with PC pumps or motors. PC pumps require a rotational motion instead of a reciprocating motion. However, they are not ideally suited to withstand high torque rotation.  
           [0010]    It is, therefore, desirable to provide a drive rod and connector for progressive cavity pumps capable of withstanding high torque rotation.  
           [0011]    It is also desirable to provide a pin for a drive rod and a connector that will resist rotation between the pin and connector once the pin has been fully threaded into the connector.  
           [0012]    It is also known that rotatable movement of the threaded sucker rod pin in the connector will cause the pin to elongate, even after the pin is threaded into the connector.  
           [0013]    It is, therefore, desirable to provide a connector for a high torque rotatable progressive cavity drive rod that includes an internal secondary stop to halt elongation of the pin.  
         SUMMARY OF THE INVENTION  
         [0014]    The rods and connectors of the present invention have frustoconical tapered, rather than straight, threading. In the existing art of sucker rods, tapered threading has generally been discouraged. When reciprocating motion is applied, it is typically easier to jar loose sucker rods having tapered threading. However, because the purpose of these sucker rods is to provide a superior rod string for use in high torque, rotational motion, tapered threading is actually preferred. The constant, high speed rotation ensures that the sucker rods and their connectors remain screwed together tightly.  
           [0015]    Another reason current sucker rods strings are not suitable for use with PC pumps that require rotation is that the connectors are not well designed for this type of stress. Though well designed to tolerate the stresses of reciprocating motion, the walls of a connector are generally too thin to withstand high torque. When exposed to high rotational pressure, these connectors will break. Because the present invention provides for tapered threading on the pins of the sucker rods, the walls of the connectors are thicker. Connectors having thicker walls are capable of withstanding greater torsional stress.  
           [0016]    To further facilitate transfer of the rotational motion between sucker rods and connectors, the undercut found on sucker rod pins is reduced in the present invention. The rotational motion of the string constantly tightens the pin within the connector, thereby reducing sucker rod&#39;s danger of disengagement. The need for the increased friction on the shoulders provided by an undercut is therefore eliminated. Furthermore, this additional threading provides for a greater surface area over which the rotational motion is transferred. This further enhances the sucker rod string&#39;s ability to withstand high torsional stress. The synergistic effect of tapered threading, thicker connectors, and reduction of the undercut provides rods and connectors having superior tolerance to rotational stress.  
           [0017]    In the present invention, the pins of sucker rods are modified so that they may better withstand the stress of high torque up to 1,750 foot pounds.  
           [0018]    It is therefore an object of the present invention to provide sucker rod strings capable of withstanding the stress of high torque rotation.  
           [0019]    Once the pin has been fully threaded in the connector, continued rotational force of the pin in the connector causes elongation of the pin. Accordingly, it is desirable to reduce movement between the pin and connector once the pin has been fully threaded into the connector. Increased friction between the pins and connectors is provided by increasing the outside diameter of the conenctor and the shoulder of the pin. Increased friction between the pins and the connectors is provided by roughening the surfaces of the shoulder of the pin and roughening the surfaces of the shoulder of the connector.  
           [0020]    Elongation of the pin once it has been fully threaded in the connector is also discouraged by a secondary internal stop within the connector. The internal stop is normally spaced from the end of the pin and will engage the pin if elongation continues past a desired point.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 shows a diagrammatic illustration of a drive rod string and connector being utilized to drive a progressive cavity pump downhole in a well;  
         [0022]    [0022]FIG. 1A shows an enlarged portion of the drive rod string of FIG. 1;  
         [0023]    [0023]FIG. 2 shows a side plan view of one end of a typical prior art sucker rod currently in use;  
         [0024]    [0024]FIG. 3 is a side plan view of the drive rod of the present invention;  
         [0025]    [0025]FIG. 4 is a cross sectional view of a drive rod connector of the present invention with secondary internal stop;  
         [0026]    [0026]FIG. 5 is a side view of the drive rod shown in FIG. 3 threaded into the connector shown in FIG. 4 partially cut-away for clarity; and  
         [0027]    [0027]FIG. 6 illustrates an outside perspective view of a connector shown apart from the pins of the connector.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    The present invention is an improved drive rod and connector capable of high torque rotation. The key features of the present invention work together synergistically to improve the amount of rotational stress that a drive rod and connector can withstand. These improvements include drive rod pins having tapered threading and reduced undercut. This also results in strengthened connectors having a larger outer diameter and thicker sidewalls. The reduced undercut increases the surface area by which rotational motion may be transferred from a drive rod to a connector to a subsequent drive rod.  
         [0029]    The present invention is especially useful for driving a progressive cavity pump or motor.  
         [0030]    Drive rods are comprised of approximately three components: The first, the body, is a long, usually metal, shaft typically about 25-30 feet in length. The other two components are the pins located at each end of the shaft. The pins are threaded so that they may be screwed into connectors. One rod is screwed into the bottom of a connector while another is screwed into the top. This is repeated until the rod string is of the desired length. This can sometimes be several thousand feet and require numerous rods.  
         [0031]    Recently, pumps known as progressive cavity pumps have been developed for use in pumping oil and other fluids out of wells. These pumps are especially suited for pumping thick viscous liquids such as crude oil. A relatively simple design makes them both sturdy and reliable. This has resulted in a need for a sucker rod better suited for rotational rather than reciprocating tension and compression motion.  
         [0032]    [0032]FIG. 1 illustrates a typical well arrangement having a progressive cavity pump wherein the present invention is employed. An oil well  10  has a PC pump  28  located at the bottom of it. At the top of the well is a powered drive head  12 . The drive head  12  is rotated by a motor. This results in rotation of the polished rod  14  which projects downward through a stuffing box  16 . The stuffing box  16  creates a water proof seal such that pumped fluids will not exit through the top of stuffing box  16 . Directly below the stuffing box is a flowline  18 . Pumped oil or other fluid exits the well through this flowline. Below this is a tubing head  19  located directly above casing head  20 . Those skilled in the art of well drilling will be familiar with tubing head  19  and casing head  20  as well as the casing  26  itself. The polished rod  14  is connected to the top end of drive rod string  22 . Drive rods in the string are held together by connectors  24  as will be described.  
         [0033]    [0033]FIG. 1A shows an enlarged view of sucker rod string  22  and a connector  24  holding  2  rods  30  and  32  together. For simplicity, this diagram shows use of only 2 sucker rods. Those skilled in the art will appreciate that in practically all wells, the rod string is comprised of several rods and connectors. The pins of drive rods  30  and  32  are not seen in FIG. 1 or FIG. 1A. This is because the pins themselves have been screwed into the connector and are not visible.  
         [0034]    It will be recognized that various other arrangements are possible within the spirit and scope of this invention.  
         [0035]    [0035]FIG. 2 shows a pin of a typical rod common in the art prior to introduction of the present invention. Sucker rod pin  36  is comprised of a cylindrical threaded portion  38  and an undercut portion  40 . Cylindrical threaded portion  38  has a constant diameter and is sometimes known in the art as straight threading. Undercut  40  has a diameter slightly less than that of threaded portion  38  and extends to shoulder  42 . Shoulder  42  has a greater diameter than the other portions of the shaft. It has an engaging portion  44  that comes in firm contact with a complimentary shoulder on the connector (to be described below).  
         [0036]    The friction created by the contact between engaging portion  44  or shoulder  42  and the shoulder of a connector discourages the unscrewing of a sucker rod from a connector. The presence of an undercut  40  allows threaded portion  38  to be screwed into a connector more tightly, thus increasing the static friction created by the shoulders. Without undercut  40 , the connector and sucker rod are connected more loosely and are more likely to unscrew. Therefore, the present art teaches away from reducing or eliminating undercut  40 . Underneath shoulder  42  is a square bolt portion of  46 . This is the portion of the shaft that is engaged by tools that tighten the sucker rods&#39; connections to the connectors. A main body  48  of the sucker rod is substantially cylindrical and extends the length of the sucker rod. An identical pin, not shown, is located on the other end of the sucker rod main shaft  48 .  
         [0037]    Prior art connectors  24  are essentially elongated nuts having a bore and an interior threading complimentary to threaded portions  38 . The bore has a length that is at least slightly greater than the length of two pins  36 . This prevents pins  36  from contacting each other and allows the rods to be screwed into the connectors more tightly.  
         [0038]    [0038]FIG. 3 shows a drive rod of the present invention from a similar angle as the prior art in FIG. 2. It shows modified pin  60  having a threaded portion  62 . As clearly shown in FIG. 3, threaded portion  62  is slightly tapered and frustoconical. In a preferred embodiment, the threaded portion tapers slightly inward at about 3.5°. In this particular embodiment, there is a very narrow undercut  64 . This particular embodiment shows slight undercut  64  in order to illustrate that it is possible in the present invention to utilize an undercut. It is desirable to have undercut  64  as small as possible. It is however substantially easier to machine a pin having a slight undercut than machining a pin having no undercut at all. Tapered threading offers a variety of advantages. Threaded tapering offers better balance of strength from the connection, easier stabbing during make-up, quicker release on break-out and greater ease of re-working.  
         [0039]    In the past, the art has taught away from such tapering because they are easier to pull apart and become disconnected. However, when the sucker rods are rotated, the tapered threading and the pin tightens its engagement to the connector resulting in a very strong connection. Furthermore, tapering causes the pin  60  to be placed in radial and circumferential compression and the socket to be placed in radial compression and circumferential tension. This radial tension allows rotational movement to be transferred from a rod to a connector to a subsequent rod in a smoother fashion. This increases the amount of torque and rotational stress that the sucker rod string may be subjected to without failure.  
         [0040]    A shoulder  66  has a greater diameter than the other portions of the shaft. It has an engaging portion or surface  68  that comes in contact with complimentary shoulder on the connector.  
         [0041]    In the present invention, rotational motion may be transferred not only by the shoulder  66 , as in traditional sucker rods, but also by the entire pin itself. This increase in surface area over which torque is transferred reduces overall stress on the pin and drive rod as a whole. Those skilled in the art will appreciate that by utilizing the entire pin as well as the shoulder to transfer rotational movement, overall strain on the drive rod is reduced. Typical one inch rod connections cannot withstand more than 1,100 foot pounds of torque. One inch drive rod connections according to the present invention, however, have been found to withstand up to 1,750 foot pounds of torque. This provides sturdier, more reliable, and faster withdrawal of oil or other fluids from a well.  
         [0042]    [0042]FIG. 4 shows a connector  70  of the present invention suitable for use with a drive rod disclosed in FIG. 3. Connector  70  is essentially an elongated nut having a cylindrical outside wall  72  penetrated by bore  80 . The bore  80  has tapered threading  74  complimentary to pin  60  in FIG. 3. In this particular embodiment, a slight undercut  76  is included in bore  80  in order to accommodate the slight undercut  64  on pin  60  shown in FIG. 3. As stated above, this undercut is unnecessary but the invention may be easier to machine in this fashion. In this particular embodiment, connector  70  is approximately 4 inches long. This is more than long enough to adequately accommodate two pins  60  which are each approximately 1.75 inches long. This allows the threading inside the connector  70  to tighten upon rotation of the sucker rods string. One of the advantages of the design of the present invention is that connector wall  72  is thicker than those of more traditional connectors without increase of the outside diameter. This allows connector  70  to withstand more torsional stress than a standard connector. The added thickness of wall  72  adds strength to the connector. In addition, the tapered threading allows the connector to transfer a rotational motion through both shoulder  82  and bore  80 . As explained above, spreading the transfer of rotational force over a larger surface area allows the connector, like the pin, to withstand more force.  
         [0043]    Additionally, connector  70  has a pair of opposed shoulders  82 . Each shoulder  82  has a roughened surface which mates with surface  82  which is similarly roughened. This significantly increases the coefficient of friction. The mating of the roughened surfaces helps transfer rotational movement of the pin to the connector. The mating of the roughened surfaces also prevents the pin from further rotation once the surfaces mate with each other.  
         [0044]    Finally, FIG. 5 illustrates a side view with a connector  70  partially cut-away. The pin  60  has been threaded into and fully engaged with the connector  70 . The connector  70  also includes an internal secondary stop  90  which has a diameter less than the diameter of the pins at their farthest most end. When each of the pins  60  is threaded into the connector  70 , the end of the pin is spaced slightly from the internal stop  90 . In the event that the pin continues to rotate with respect to the connector  70 , it has been found that it may cause the threaded portion of the pin to continue to elongate. In the event that the pin continues to elongate with further rotation, the internal stop  90  acts as a secondary, positive stop for the pin and restricts elongation past a certain permitted length. Accordingly, the mating of the shoulder of the pin with the shoulder of the connector acts as the initial stop for the pin. Additionally, the internal secondary stop  90  acts as a backup or secondary stop for rotation and elongation of the pin.  
         [0045]    In one non-limiting example, the pin may be permitted to elongate {fraction (10/1000)} of an inch before the end of the pin engages the internal secondary stop.  
         [0046]    [0046]FIG. 6 illustrates an outside perspective view of a connector  70  shown apart from the pins of the connector. The roughened surface of the shoulder  82  is visible on one end of the connector.  
         [0047]    Although these modifications may not appear significant, those skilled in the art will appreciate that the present invention has unique mechanical properties not found in existing sucker rods. Modifications made to the pins and connectors allow at least 25% more torque to be safely supplied through a rod string. Those skilled in the art will appreciate that this significantly improves the use of progressive cavity pumps.  
         [0048]    The present progressive cavity rod will provide greater torque values than existing sucker rod strings as may be observed from the following table:  
                                             TYPICAL RECOMMENDED TORQUE VALUES       FOR ROD STRING CONNECTORS       All torque values are in foot pounds            Rod Size   Grade D (78)   PC Rod               1″   1100   1,750*        1⅛″   1500   2,000**       1¼″   2100   2,500**                                  
 
         [0049]    Whereas, the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention.