Abstract:
The invention relates to downhole rotary pumping arrangements and more particularly to drivehead gimbals and seals used in the driveheads of downhole rotary pumps. The gimbal and seal system described enables the wobble of a rotating shaft to be absorbed without compromising the seal around the shaft.

Description:
FIELD OF THE INVENTION 
     The invention relates to downhole rotary pumping arrangements and more particularly to drivehead gimbals and seals used in the driveheads of downhole rotary pumps. The gimbal and seal system described enables the wobble of a rotating shaft to be absorbed without compromising the seal around the shaft. 
     BACKGROUND OF THE INVENTION 
     Downhole rotary pumps, such as progressing cavity pumps, are used for the conveying of different types of fluids, but are especially well suited for the pumping of very viscous or thick liquids such as crude oil laden with sand. A downhole rotary pump is operated by a drive string, generally consisting of a rod or tube string that extends down the well bore. The drive string rotates in a stationary production tubing and is suspended from and rotated by a drivehead assembly which is associated with the wellhead. The drivehead assembly is generally mounted to the top of a wellhead which is attached to the top end of the production tubing. A polished rod is attached to the upper end of the drive string and extends through the drivehead to facilitate sealing around the drive string and backwashing of the pump. The pump includes a stator connected to the bottom end of the production tubing and a rotor attached to the bottom end of the drive string. Upon actuation of the pump by rotation of the drive string, the pumped liquid is forced to the ground surface through the annular space provided between the drive string and the production tubing. 
     In conventional downhole rotary pump driveheads, a stuffing box is used to seal the annular space between the tubing and the drive string in the drivehead and above the pumped liquid take-off valve of the wellhead. The stuffing box is generally mounted in a bottom end of the drivehead to tightly seal around the rotating polished rod. In order to reduce wear of the polished rod and stuffing box components and to provide adequate lubrication and cooling of the polished rod/stuffing box interface, the fit of the stuffing box around the string must be adjusted such that a controlled leakage of about 2 to 3 drops of pumped liquid per minute is achieved. This constant leakage, although at a relatively low rate, can result in the accumulation of significant amounts of spilled liquid over long periods of operation. Recently, political pressure by environmental groups and the enactment of laws in many jurisdictions which require “clean” wellheads, have forced well operators to reduce the stuffing box leakage rate. However, the resulting lesser lubrication causes considerably higher stuffing box and polished rod wear. Furthermore, wobble of the polished rod, which is especially prevalent with Moineau type pumps must be compensated by the stuffing box if a reliable seal is to be achieved, places additional stress on the sealing material respectively used. Thus, a reliable and durable seal is desired, which will not allow unacceptable leakage as described in Canadian Patent 2,162,311 issued Dec. 22, 1998 and U.S. Pat. No. 5,639,227 and incorporated herein by reference. 
     However, as a further improvement on the seal, and in order to further compensate for the wobble of a rotating shaft, there has been a need for seal system incorporating a gimbal and seal system. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a seal and gimbal system for use in a drivehead of a downhole rotary pump is provided, which pump is operated by a drive string suspended from the drivehead for rotation in a production tubing about a longitudinal axis, the drivehead having a bore in fluid communication and coaxial with the production tubing and sized to permit the passage of a polished rod attached to a top end of the drive string, the seal and gimbal system for sealing an annular space between the production tubing and polished rod from the drivehead, the seal and gimbal system comprising: 
     a top seal assembly fixed to the polished rod and rotatable with the polished rod, the top seal assembly also for providing a seal between the annular space and drivehead; 
     a gimbal assembly operatively connected to the top seal assembly and rotatable with the top seal assembly, the gimbal assembly having first and second portions enabling pivotal displacement of the first and second portions with respect to one another during rotation of the polished rod and a gimbal seal between the first and second portions; 
     a bearing assembly operatively connected to the gimbal assembly enabling rotation of the gimbal assembly and the polished rod with respect to the drivehead; 
     a bottom seal assembly operatively connected to the bearing assembly for providing a seal between the drivehead and annular space. 
     In further embodiments, the first portion includes an inner spherical member having a convex and spherical outer surface and the second portion includes a base member having a concave semi-spherical inner surface engageable with a retaining ring having a concave semi-spherical inner surface, the gimbal seal includes at least one o-ring seated between the first and second portions, and/or the bottom seal assembly includes an inner sleeve radial to the polished rod and at least one rotating seal selected from any one or a combination of a pressure seal, mechanical seal, debris exclusion seal and labyrinth seal between the rotating inner sleeve and drivehead. 
     In a further embodiment, the system also includes a bushing within the drivehead for stabilizing the polished rod. 
     In a more specific embodiment, the seal and gimbal system comprises: 
     a top seal assembly fixed to the polished rod and rotatable with the polished rod, the top seal assembly also for providing a seal between the annular space and drivehead, the top seal assembly including a seal mounting ring and top ring for retaining a seal; 
     a gimbal assembly operatively connected to the top seal assembly and rotatable with the top seal assembly, the gimbal assembly having first and second portions enabling pivotal displacement of the first and second portions with respect to one another during rotation of the polished rod and a gimbal seal between the first and second portions wherein the first portion includes an inner spherical member having a convex and spherical outer surface and the second portion includes a base member having a concave semi-spherical inner surface engageable with a retaining ring having a concave semi-spherical inner surface and the gimbal seal includes at least one o-ring seated between the first and second portions; 
     a bearing assembly operatively connected to the gimbal assembly enabling rotation of the polished rod with respect to the drivehead; 
     a bottom seal assembly operatively connected to the bearing assembly for providing a seal between the drivehead and annular space wherein the bottom seal assembly includes an inner sleeve radial to the polished rod and at least one seal selected from any one or a combination of a pressure seal, mechanical seal debris exclusion seal and labyrinth seal between the inner sleeve and drivehead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further described by way of example only and with reference to the attached drawings, wherein 
     FIG. 1 is a schematic side elevational view of a rotary downhole pump arrangement which is operated by a drivehead including a gimbal and seal arrangement in accordance with the invention; and, 
     FIG. 2 is an axial cross-section through a preferred embodiment of a gimbal and seal arrangement in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Leakproof seal arrangements in accordance with the invention are intended for use in a downhole rotary pump assembly as shown in FIG.  1  and especially in the drivehead  10  thereof which is used for the operation of a progressing cavity downhole pump  12  including a stator  14  and a rotor  16 . The stator  14  is connected to the bottom end of a production tubing  18  and the rotor  16  is mounted to the bottom end of a drive string  20  which is suspended from the drivehead  10  and generally consists of a plurality of connected sucker rods or tubes (not illustrated). The drivehead  10  includes a frame  22  which is concentrically screwed onto the top end of a conventional wellhead assembly  24  for downlole rotary pumps. The drive string  20  includes a polished rod  26  which is affixed to a top end of the sucker rod or tube string and extends through a bore in the drivehead frame  22  as will be described below in detail with reference to FIG.  2 . The drive string  20  is suspended from the drivehead  10  by way of a clamp  28  which is shaped to accommodate an end of the polished rod  26  that protrudes upward from a drive spindle  30  of the drivehead  10 . The clamp  28  is fastened to the drive string above the drivehead and rests on a top surface of the drive spindle  30 . A slip shaft  21 , preferably of hexagonal cross-section (available from KUDU Industries, Calgary, Canada) may be attached to the top end of the polished rod  26  to permit adjustment of the axial position of the drive string  20  in the tubing  18  and the drive spindle  30 , while ensuring the reliable, slip-free transmission of torque to the drive string. In that case, the clamp  28  is mounted on the slip shaft. Torque from an electric motor  32  is transmitted to the drivehead  10  and the drive string  20  by way of a conventional V-belt and pulley arrangement  34  well known to persons of skill in the art of rotary downhole pumping arrangements. Alternatively, the drive spindle  30  may be driven by a right-angle gear drive powered by an internal combustion engine (not illustrated), or a comparable power source, in a manner well known in the art. Leakage of the pumped fluid, which is conveyed in the annular space between the production tubing and the drive string, is substantially prevented by a seal arrangement  40  in accordance with the invention which is incorporated into the drivehead  10 . 
     The preferred embodiment of a gimbal and seal arrangement in accordance with the invention as shown in FIG. 2 is intended for use with a drivehead  10  having a threaded pin connection  11 , or another suitable connection known in the art, for attachment to the top end of a wellhead  24  (see FIG.  1 ). The drivehead has a central bore  13  in fluid connection and coaxial with the production tubing  18  (see FIG.  1 ). 
     The gimbal and seal assembly  40  includes a top seal assembly  100 , a gimbal assembly  102 , a bearing assembly  104 , lower seal assembly  106  and stabilizing assembly  108 . Generally, the top seal assembly provides pack-off seal at the top of the polished rod  26 , the gimbal assembly allows wobble motion for the rotating polished rod to be absorbed, the bearing assembly  104  allows the polished rod, gimbal and lower inner sleeve to rotate with respect to the drivehead  10 , the lower seal assembly  106  provides a stationary seal between the drivehead  10  and rotating lower inner seal sleeve  106   a  and the stabilizer assembly  108  provides a stable bushing for the polished rod within the drivehead  10 . 
     With respect to the stabilizing assembly  108 , a bushing  108   a  is seated within the drivehead  10  through which the polished rod  26  passes in order to provide radial stability to the rod  26  rotating with respect to the bushing  108   a.  A retaining clip  108   b  holds the bushing  108   a  in place. 
     The bronze bushing  108   a  provides a measure of stabilization for the polished rod  26 . However, this stabilization is not sufficient to protect the upper seals and bearings because a clearance of 10-20 mm on the radius of the bushing  108   a  must be provided to accommodate the variation in diameters of polished rods and to allow polished rods to be easily insertable into the assembly. In addition, the bushing becomes a pivot point for polished rod wobble and thus it may cause premature wear of both the polished rod and bushing. 
     The inner seal sleeve  106   a  and inner bearing sleeve  104   a  must also have sufficient clearance that the polished rod will not come into contact with them due to wobble. This is necessary to prevent the bearings  104   b  from being over-loaded. 
     Polished rod wobble makes it difficult to maintain a seal around the polished rod at the rotating seal in rotating seal assemblies  9  for example, (the seal assembly  100 ) because of the amount of cyclical deformation. The gimbal assembly  102  allows the seals  100   a  to wobble with the polished rod so that there is no cyclic deformation of the seals  100   a.    
     The bearing assembly  104  includes an inner bearing sleeve  104   a  fixed to roller bearings  104   b  seated within an outer bearing sleeve  104   c  seated within drivehead  10 . The roller bearings  104   b  are retained between the inner and outer bearing sleeves by inner bearing screw ring  104   d,  outer bearing screw ring  104   e  and bearing seat  104   f  on outer bearing sleeve  104   c.  A grease seal  104   g  retains bearing grease within the bearing assembly  104 . 
     The lower seal assembly  106  includes a lower inner rotating seal sleeve  106   a  connected to the inner bearing inner sleeve  104   a  and recessed within the drivehead  10 . A plurality of seals including a single lip pressure activated seal  106   b,  double lip pressure activated seals  106   c,  sand exclusion seal  106   d  and labyrinth seal  106   e  are located radially to the rotating lower inner seal sleeve  106   a  and abutting the drivehead  10 . The seals are designed to prevent leakage from the annular space  13  to the bearing assembly  104  from between the rotating lower inner seal sleeve  106   a  and drivehead  10 . 
     The gimbal assembly  102  includes a base  102   a  having a semi-spherical inner surface  102   b,  a spherical gimbal member  102   c  seated against the inner surface  102   b,  a retaining ring  102   d  for connection to the base  102   a  and having a semi-spherical inner surface  102   b ′ and appropriate sealing rings  102   e  seated within the base  102   a  and retaining ring  102   d  allows a pivoting motion of the gimbal member  102   c  with respect to the base  102   a  and retaining ring  102   d.  Sealing rings  102   e  prevent fluid from passing between the gimbal member  102   c  and the exterior of the gimbal assembly  102 . 
     The top seal assembly  100  includes a pack-off seal  100   a  for gripping the polished rod  26  and for sealing the polished rod  26  between the annular space  13  and drivehead  10 . The top seal assembly  100  includes a seal mounting ring  100   b  for connection to the gimbal member  102   c  and for retaining pack-off seal  100   a  between the seal mounting ring  100   b  and polished rod  26 . The pack-off seal  100   a  is retained within the seal mounting ring  100   b  by top ring  100   c,  which may be tightened against seal  100   a  by threads  100   d.    
     In operation, the pack-off seals  100   a  provide the primary seal between the annular space  13  and the drivehead  10 . In addition, the top seal assembly  100  provides clamping between the polished rod  26  and the top seal assembly  100 , the gimbal assembly  102 , the bearing assembly  104  and the lower seal assembly  106 , thereby allowing rotation of the top seal assembly  100 , gimbal assembly  102 , inner bearing sleeve  104   a  and lower inner seal sleeve  106   a  with the polished rod. As a result of the gimbal assembly  102 , any wobble in the rotating polished rod is absorbed with sealing rings  102   e  preventing leakage at the gimbal assembly  102 . Leakage through the bearing assembly  104  is prevented by the lower seal assembly  106 . 
     Reliance upon the seals  100   a  to provide sufficient friction to rotate the gimbal assembly  100  and the inner bearing sleeve  104   a  and the lower inner seal sleeve  106   a  may be avoided by placing a clamp (not shown) on the polished rod  26  which engages the top ring  100   c  with, for example, a mating tongue and groove arrangement or dog clutch arrangement. This will have the additional advantage of keeping the top ring  100   c  in compression upon the seals  100   a.    
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.