Abstract:
A shear coupling assembly utilizes a novel pin coupling member having radial slots formed about a top portion of the insert member, the top portion being radially expandable once engaged in the box coupling member to lockingly engage the threaded connection. Locking of the insert member in the box coupling member prevents backing-off of the connection and maintains a pretension in the neck of the pin coupling member, during use. The top portion of the insert can be radially expanded by forcing a ball bearing into a counterbore formed in the insert member.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the invention relate to a shear coupling assembly used for connecting a downhole pump to a terminal end of an actuating rod string in reciprocating pumped wells, and more particularly to a shear coupling having means for substantially preventing back-off of a threaded connection therebetween. 
       BACKGROUND OF THE INVENTION 
       [0002]    Downhole reciprocating pumps are positioned and actuated in a wellbore by a rod string extending from surface. The rod string is typically either one continuous member or a plurality of sucker rods, connected end-to-end through standard threaded couplings. 
         [0003]    It is known that downhole pumps may become lodged or stuck in a wellbore, often by sand deposited and packed around the pump, either at the downhole pumping location or as the pump is being tripped out of the wellbore. Conventionally, the rod string is removed from the pump by applying a pulling force on the rod string to sever the rod string from the pump. 
         [0004]    A shear coupling assembly is typically used to connect between the pump and a downhole end of the rod string. The shear coupling primarily functions to provide a means for separating the rod string from the pump so as to release and remove the rod string from the wellbore and permit specialized equipment to be inserted into the well annulus to free the pump. Use of the shear coupling at the interface between the rod string and the pump provides a specified location at which the pump and rod string are separated and the shear coupling can be constructed to actuate under a desired design load which is highly predictable. Without the shear coupling, the rod string would sever at a location along the rod string that is unknown and largely unpredictable and which can be problematic for retrieval of the pump. 
         [0005]    It is known to use a shear coupling comprising transversely extending shear pins for joining male and female coupling members between the pump and the rod string. The shear pins are known to be prone to premature fatigue which arises from cyclic compressive stress induced in the shear pins if the rod string “taps down” at the base of each reciprocating stroke. Further, as the shear pins break, fragments fall downhole into the pump, resulting in further problems in freeing the pump. 
         [0006]    In an effort to solve the problems associated with previous shear coupling designs, shear couplings, such as taught in Canadian Patent 1298715 to Mann et al, are known to utilize a threaded connection between a pin coupling member, having an externally threaded head, and an internally threaded box coupling member. Either of the pin coupling member or the internally threaded box coupling member is connected to the pump and the other is connected to the downhole end of the rod string. The threaded head of the pin coupling member threadedly engages the internal bore of the internally threaded coupling member for operatively connecting therebetween. The pin coupling member further comprised a shear neck of reduced diameter between the head and a body of the pin coupling member which is designed to shear under design load to free the pump from the rod string. 
         [0007]    In operation, a pretension is applied to the neck of the pin coupling member during threaded connection to the box coupling. The box coupling seats on a shoulder of the pin coupling so as to maintain the neck in tension during normal operation of the pump for preventing premature fatigue of the shear neck. 
         [0008]    One major problem associated with such threaded connections is “backing off” of the threads due to operation and vibration of the well equipment resulting in subsequent loosening of the threaded connection and a loss of the pretension in the shear neck. Once the pretension is lost, premature failure of the shear coupling is highly imminent as the reduced diameter neck becomes exposed to cyclic compressive stresses associated with the rod string “tapping down” at the bottom of each downstroke, impact loading associated with the “fluid pound” phenomenon and bending stresses associated with flexure of the rod string. Due to the coarse nature of the threads used in the connection, the pretension can be lost with a turn in the connection of less than 180 degrees. 
         [0009]    Conventionally, thread-locking adhesive or epoxy such as LOCTITE® (available from Henkel Technologies, USA) has been used to strengthen the threaded connection between the coupling members. However, degradation of the thread-locking epoxy often results from prolonged exposure to elevated temperatures and chemical compounds, such as pentanes and hexanes, found in the well environment. Further, machining oil residues on the connecting threads may prevent proper adhesion of the epoxy to the metal threads. Additionally, if the epoxy does not evenly fill the crevices in the thread pattern, an inconsistent bonding may develop between the threaded surfaces resulting in a weaker bond. 
         [0010]    Therefore, there is interest in the industry for a shear coupling assembly and a method of connection therein which provides a reliable locking mechanism for permanently locking the connecting threads in place to prevent backing-off and ensuring maintenance of the pretension which is applied to the neck of the shear coupling during assembly. 
       SUMMARY OF THE INVENTION 
       [0011]    An improved pin coupling member for a shear coupling assembly comprises discontinuities formed in a top portion of an insert member of the pin coupling member which permit the top portion of the insert member to be expanded radially outward after operative engagement with a box coupling member so as to lockingly engage the threaded connection therebetween to prevent backing-off of the connection. Thus, pretension applied to a neck of the pin coupling member during operative engagement with the box coupling member is maintained during use. 
         [0012]    In one broad aspect of the invention, a pin coupling member for a shear coupling assembly adapted for connecting a downhole pump to a downhole end of a rod string, the pin coupling member comprises: a cylindrical body; a cylindrical insert member extending axially outwardly from the cylindrical body and having an externally threaded surface adapted to engage an internally threaded axial bore of a box coupling member; a shear neck connecting between the cylindrical body and the insert member; a counterbore formed in the insert member; and two or more radial discontinuities formed in a top portion of the insert member being of sufficient axial depth for permitting the top of the insert member to flex radially outwardly in response to a force applied to the counterbore for lockingly engaging the threaded connection between the pin coupling member and the box coupling member. 
         [0013]    In another broad aspect of the invention, a shear coupling assembly adapted for connecting between a downhole pump and a downhole end of a rod string, comprises: a box coupling member having a tubular body; and an internally threaded axial bore formed therethrough; and a pin coupling member having a cylindrical body; a cylindrical insert member extending axially outwardly from the cylindrical body and having an externally threaded surface for engaging the internally threaded axial bore of the box coupling member; a shear neck connecting between the cylindrical body and the insert member; a counterbore formed in the insert member; and two or more radial discontinuities formed in a top portion of the insert member and being of sufficient depth for permitting radially outward flexing of the top portion in response to a force applied to the counterbore for lockingly engaging the threaded connection between the pin coupling member and the box coupling member. 
         [0014]    In another broad aspect of the invention, a method of assembling a shear coupling assembly comprises: threading an insert member of a pin coupling member into a threaded axial bore of a box coupling member, the insert member having a counterbore and two or more radial discontinuities formed therein at a top portion for permitting outward radial expansion of the top portion; applying pretension to a neck of the pin coupling member during the threading of the box coupling member to seat on the pin coupling member and extending axially between a cylindrical body of the pin coupling assembly and the insert member; and thereafter inserting an expansion member into the counterbore for expanding the top portion of the insert member radially outward within the axial bore of the box coupling member for lockingly engaging the threaded connection therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a sectional side view of a prior art shear coupling assembly; 
           [0016]      FIG. 2  is a partial sectional side view of an alternate embodiment of the prior art shear coupling assembly of  FIG. 1 ; 
           [0017]      FIG. 3A  is an exploded perspective view of a shear coupling assembly according to an embodiment of the invention; 
           [0018]      FIG. 3B  is a side view of an assembled shear coupling assembly according to  FIG. 3A ; 
           [0019]      FIG. 4  is a plan view of the pin coupling member according to  FIG. 3A ; 
           [0020]      FIG. 5  is a side view of the pin coupling member according to  FIG. 3A ; 
           [0021]      FIG. 6  is a 90 degree rotated side view of the pin coupling member according to  FIG. 5 ; 
           [0022]      FIG. 7  is a longitudinal sectional view of the assembled shear coupling according to  FIG. 3A , along section lines A-A of  FIG. 4 , after insertion of an expansion member; 
           [0023]      FIG. 8  is a longitudinal sectional view of the assembled shear coupling according to  FIG. 3A , along section lines B-B of  FIG. 4 , after insertion of an expansion member; 
           [0024]      FIGS. 9A-9E  are plan views of the pin coupling member illustrating alternate slot patterns in a top of an insert portion; and 
           [0025]      FIGS. 10A-10C  are side views of expansion members according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    Embodiments of the invention relate to improvements to prior art shear coupling assemblies to substantially prevent backing-off of a threaded connection within the shear coupling assembly when in use. The improvements are described in the context of a conventional shear coupling assembly having a male and a female member threadedly connected therebetween. A description of the conventional shear coupling assembly and method of assembly is provided to assist in understanding the embodiments of the invention. 
       Prior Art Shear Coupling Assembly 
       [0027]    Having reference to  FIGS. 1 and 2 , a conventional prior art shear coupling assembly  1  comprises two members, a pin coupling member  10  and a box coupling member  12 . Either of the pin or box coupling member  10 ,  12  can be connected to either of a pump or a rod string (not shown) for permitting connection therebetween. 
         [0028]    The pin coupling member  10  comprises a cylindrical body  14  having an externally threaded insert member  16  extending axially outwardly therefrom and connected to the body  14  by a shear neck  18  typically having a relatively reduced section. The insert member  16  is cylindrical and smaller in diameter than the cylindrical body  14 . The reduced section of the shear neck  18  has a known cross-sectional area and acts as a preferential point of parting in the connection between the rod string and the pump under design loading. A top  20  of the insert member  16  is bored with an internally threaded axial counterbore  22  adapted for use for pretensioning the neck  18  during assembly, as described below. The pretension in the neck  18  is maintained through a seating interface between the pin and box coupling members  10 ,  12  along a radial contact shoulder  24  formed at a top of the cylindrical body  14 . The body  14  further comprises a threaded connection  26  at an end opposite the insert member  16  for threaded connection to either the rod string or the pump and can be either a female connection ( FIG. 1 ) or a male connection ( FIG. 2 ) as shown. 
         [0029]    The box coupling member  12  comprises a tubular body  28  having an internal threaded axial bore  30  that co-operates with a thread profile of the externally threaded insert member  16  and which extends substantially a full length of the bore  30 . A first end  32  of the tubular body  28  is connected to the pin coupling member  10  at the externally threaded insert member  16 . A second end  34  of the body tubular body  28  is threadedly connected to either the rod string or the pump. 
       Prior Art Method of Assembly 
       [0030]    Prior to assembling the pin and box coupling members  10 , 12 , a thread-locking epoxy or adhesive is typically applied to the externally threaded insert  16  and the radial contact shoulder  24  of the pin coupling member  10 . The externally threaded insert  16  is inserted into either the first or second end  32 ,  34  of the box coupling member  12  and is advanced along the internally threaded axial bore  30  until the radial contact shoulder  24  of the pin coupling member  10  approaches, but does not yet fully contact, the first or second end  32 , 34  of the box coupling&#39;s tubular body  28 . 
         [0031]    A tensioning rod commonly called a ready rod or bolt (not shown), having an external thread at one end matching the profile of the internally threaded counterbore  22 , is inserted through the axial bore  30  of the box coupling member  12  and is threaded into the internally threaded counterbore  22 . The shear neck  18  is placed into tension by pulling upwardly on the tensioning rod. With the tensioning rod and shear neck  18  under tension, the box coupling member body  12  is further advanced along the externally threaded insert  16  until the tubular body  28  firmly contacts the radial contact shoulder  24  of the pin coupling member  10 . Contact between the tubular body  28  of the box coupling member  12  and the radial contact shoulder  24  of the pin coupling member  10  acts to maintain the pretension in the neck  18 . The tensile load on the tensioning rod is then released and the tensioning rod is unthreaded and removed from the assembly  1 . The assembled shear coupling  1  is unused for sufficient time to permit the thread-locking epoxy to dry and harden. 
       Embodiments of the Invention 
       [0032]    Having reference to  FIGS. 3A-6  and in an embodiment of the invention, a shear coupling assembly  100  utilizes an improved pin coupling member  102 . 
         [0033]    The pin coupling member  102 , as in the prior art, comprises a cylindrical body  114  having an externally threaded insert member  116  extending axially outwardly therefrom and connected to the body  114  by a shear neck  118  designed to part under design loads. In the embodiment shown herein, the shear neck  118  has a reduced section. While referred to in the industry as a shear neck, it is believed the parting is a tensile failure. The insert member  116  is bored with an internally threaded axial counterbore  122  used to pretension the neck  118  during assembly, as described for the prior art shear coupling assembly  1 . 
         [0034]    In embodiments of the invention, a top portion  120  of the insert member  116  comprises two or more radial discontinuities  124 , such as slots or grooves, formed radially across a radius of the top portion  120  of the insert member  116 , from an outer threaded surface  126  of the insert member  116  to join the internally threaded axial counterbore  122 . The radial discontinuities  124  extend axially into the insert member  116  sufficiently to permit radially outward flexing of at least the top portion  120  thereof. 
         [0035]    As shown in  FIGS. 7 and 8 , the improved pin coupling member  100  is threadedly connected to the inner threaded surface  129  of the box coupling member  12  as described for the prior art shear coupling assembly  1 . 
         [0036]    After release and removal of the tensioning rod from the insert after pretensioning of the neck  118 , the insert member  116  is locked to the box coupling member  12 . An expansion member  130 , such as a ball or a wedge or the like, is positioned at an opening  132  of the internally threaded axial counterbore  122  of the insert member  116 . Pressure is applied downwardly to the expansion member  130  to move the expansion member  130  into the counterbore  122  sufficient to flex the top portion  120  of the insert  116  radially outwardly. The outer threaded surface  126  of the insert member  116  is caused to more strongly engage or lock with the inner threaded surface  129  of the box coupling member  12 , substantially preventing backing-off of the threaded connection during use. Applicant believes that local deformation of the co-operating threads of the insert member  116  and threaded inner bore  129  of the box coupling member  12  may assist to ensure that the threaded connection cannot loosen during use and further acts to maintain the pretension in the shear neck  118  substantially preventing premature failure of the shear neck  118 . 
         [0037]    In an embodiment of the invention, best seen in  FIGS. 3A ,  7  and  8 , the expansion member  130  is a sphere, such as a ball bearing, which has a diameter slightly larger than a major diameter of the internal thread of the insert&#39;s counterbore  122 . 
         [0038]    Alternately, as shown in  FIGS. 10A-C , the expansion member  130  is shaped so as to have a greatest extent  131  being slightly larger than the major diameter of the internal thread of the insert&#39;s counterbore  122  formed along a length of the expansion member  130 . 
         [0039]    A chamfer  134  is cut around the opening  132  or upper periphery of the counterbore  122  so as to provide a seat to aid in concentrically positioning the ball bearing  130  prior to the application of downward force thereto and to ease the entry of the ball bearing  130  into the counterbore  122 . In an embodiment, force is applied to the ball bearing  130  until a major diameter of the ball bearing  130  is below a top face  121  of the insert  116 . In an embodiment, the ball bearing  130  is inserted until a top of the ball bearing  130  is flush with the top face  121  of the insert  116 . 
         [0040]    In one embodiment, a hydraulic press (not shown) is used to apply force to the ball bearing  130  to force the ball bearing  130  into the counterbore  122 . 
         [0041]    As shown in  FIGS. 4-6  and  9 A and in one embodiment, four radial, orthogonal discontinuities or slots  124  are formed about a circumference of the top portion  120  of the insert  116 . In this embodiment, the radial slots  124  extend axially to a depth sufficient to permit radial expansion of at least the top portion  120  of the insert  116  upon insertion of the ball bearing  130 . In one embodiment the radial slots  124  extend about one half the depth of the insert member  116 . Alternately, the radial slots  124  may extend further into the top portion  120  to ensure flexure. 
         [0042]    In an embodiment as shown in  FIG. 9B , two radial slots  124  are formed at 180 degrees from each other and effectively extend across the diameter of the insert&#39;s top portion  120 . 
         [0043]    In an embodiment as shown in  FIG. 9C , three radial slots  124  are spaced evenly about the circumference of the top portion  120  of the insert  116 . 
         [0044]    In an embodiment as shown in  FIG. 9D , six radial slots  124  are spaced evenly about the circumference of the top portion  120  of the insert  116 . 
         [0045]    In an embodiment as shown in  FIG. 9E , eight radial slots  124  are spaced evenly about the circumference of the top portion  120  of the insert  116 . Applicant believes that additional slots  124  may be added as required to radially deflect the top portion  120  of the insert member  116 . 
         [0046]    Diametrically opposed slots  124 , formed across the top portion  120  of the insert  116  can aid in the formation of the slots. 
       EXAMPLE  
       [0047]    In an embodiment of the invention as shown in  FIGS. 7 and 8 , a shear coupling assembly  100  sized for ¾″ sucker rod, includes both a box coupling member  12  and a pin coupling member  102 . The shear coupling assembly  100  is typically manufactured using 4140 AISI, HTSR alloy steel or other suitable structural material. The threaded surfaces for connection to the pump and the sucker rod are standard API threaded connections. 
         [0048]    In this embodiment, the total length of the pin coupling member  102  is 4.687″ and the insert member  116  is 0.843″ in length. Four radial, orthogonal slots  124  are cut in the top portion  120  of the insert member  116  of the pin coupling member  102 , forming a cross-shaped discontinuity in the top portion  120  of the insert member  116 . The slots  124  extend axially about 0.375″ into the insert member  116  and are about 1/16″ to 1/32″ in width. The insert member&#39;s counterbore  122  has a diameter of 7/16″ and is threaded using a 7/16″—14 UNC Class 2B thread at an effective depth of ⅝″. The chamfer  134  formed at the opening  132  of the counterbore  122  is at 45°× 1/16″. 
         [0049]    The expansion member  130 , used to radially expand the discontinuous top portion  120  of the insert member  116 , is a ½″ ball bearing  130  made of a high compressive strength material, having a suitable hardness and surface finish. 
         [0050]    In operation, the ball bearing  130  is inserted through the axial bore  30  in the box coupling member  12  after the removal of the tensioning rod. The box coupling member  12  is 4.05 inches in length. At a starting depth, a top of the ball bearing  130  resting on the opening  132  of the counterbore  122  of the insert member  116  engaged therein is at about 2.25 inches from the top of box coupling  12 . Force is applied to the ball bearing  130 , such as using a hydraulic press, to force the ball bearing  130  into the counterbore  122 . The travel of the press is about 0.37+0.03 inches to position the major dimension of the ball bearing  130  flush with the top face  121  of the insert member  116 . The top of the ball bearing  130  after insertion is at about 2.46 inches from the top of box coupling  12  and about 0.16 inches above the top of the insert member  116 .