Abstract:
A shear coupling assembly utilizes a second coupling member, a first coupling member and a locking member which is inserted into a bore in the first coupling member to connect between the second coupling member and the first coupling member. In embodiments where a pretension is applied to the shear neck, the locking member maintains the pretension in the shear neck. The bore of the first coupling member has a profiled section which corresponds to a profiled section on the second coupling member which when mated together permit torque to be transferred for co-rotating the second coupling member with the first coupling member for use in rotary pump operations. Advantageously, the profiled portions and the shear neck are encased in the bore of the shear coupling assembly and are protected from corrosive wellbore fluids.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Canadian Application 2,688,801 filed on 17 Dec. 2009, the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     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 pumped wells, and more particularly to a shear coupling for use with rotationally driven rod pumps. 
     BACKGROUND OF THE INVENTION 
     Downhole reciprocating and rotary 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. 
     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. 
     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 a stuck 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. 
     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 in a reciprocating pump if the rod string “taps down” at the base of each reciprocating stroke. Further, in either a reciprocating or rotary pump, as the shear pins break, fragments fall downhole into the pump, resulting in further problems in freeing the pump. 
     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 axial bore of the internally threaded coupling member for operatively connecting therebetween. The pin coupling member further comprises 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. 
     During assembly, a pretension is typically applied to the shear 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 shear neck in tension during normal operation of the pump for preventing premature fatigue of the shear neck. Shear couplings of this design are particularly suited for use in reciprocating pumps but are not useful for rotary pumps as the shear element would take virtually 100% of the torsional load. 
     Canadian Patent 2,425,091 to Bostik teaches a one-piece shear coupling for use with both rotary and reciprocating pumps. The shear coupling comprises a cylindrical body which is adapted at one end for connection to a rod string and at the other end to the pump. A weakness, such as a groove, is formed in the body therebetween so as to provide a stress-concentration point for shearing upon being subjected to a predetermined amount of stress. At least the groove must be treated with an anti-corrosive material as the entirety of the cylindrical body is exposed to potentially corrosive wellbore fluids. One of skill in the art would understand that if designed for tensile loading, the addition of torsional loading as a result of rotary operation would result in premature failure. Further, in the case of axial operations, the groove acts as a stress concentration when subjected to bending forces, such as in a deviated wellbore. 
     There is continued interest in the industry for shear coupling assemblies which provide reliable shearing under design conditions while avoiding the problems associated with parts which may fall into the wellbore or pump after shearing, avoid the effects of corrosive wellbore fluids, are relatively simple and inexpensive to manufacture, and can be used in both reciprocating and rotary pump applications. 
     SUMMARY OF THE INVENTION 
     A shear coupling assembly for drivably connecting between a downhole end of a rod string and a pump for use with both rotary and reciprocating pumps. The shear coupling assembly comprises a first coupling member and a second coupling member having a co-operating substantially irregular male profile and a substantially irregular female profile for mating therebetween, so as to permit transfer of torque and co-rotation during rotary pump operations. A locking member inserted into an axial bore of the first coupling member engages between the first coupling member and the second coupling member to prevent separation therebetween during rotary and reciprocating operations. A shear neck, formed on the second coupling member for permitting separation of the assembly under design loading, is positioned in an axial bore in the first coupling member when assembled and is isolated therein from corrosive fluids in the wellbore. A pretension may be applied to the shear neck to enhance fatigue resistance particularly in reciprocating pump operations or where there are deviations in the wellbore causing rotational bending in rotary pump operations. The locking member further acts to maintain the pretension, if applied. 
     Advantageously, in embodiments of the invention, torsional loading is applied through the mated profiles and not through the shear neck. Further, the mated profiles are located within the axial bore of the shear coupling assembly and are also protected from corrosive fluids in the wellbore, further improving fatigue resistance. 
     In one broad aspect of the invention, a shear coupling assembly for connecting between a downhole pump and a downhole end of a rod string, comprises a shear coupling assembly for driveably connecting between a downhole pump and a downhole end of a rod string comprising: a first coupling member, adapted for connection to one or other of the pump or rod string, having a tubular body and an axial bore formed therethrough, at least a portion of the axial bore having a substantially irregular female profile formed therein; a second coupling member, adapted for connection to one or other of the pump or rod string, having a cylindrical body, a cylindrical insert member extending axially upward therefrom, a shear neck connecting between the cylindrical body and the cylindrical insert member and a substantially irregular male profile formed on the cylindrical body below the shear neck for mating with the substantially irregular female profile of the first coupling member, for driveably engaging the first and second coupling members for co-rotation; and a locking member for insertion into the axial bore of the first coupling member for engagement between the axial bore of the first coupling member and the cylindrical insert member of the second coupling member for axial coupling therebetween. 
     In another broad aspect of the invention, a method of assembling a shear coupling assembly comprises: a method of assembling the shear coupling assembly of claim  1  wherein the cylindrical insert member comprises external threads, comprising: inserting the cylindrical insert member of the second coupling member upwardly into a lower end of the axial bore of the first coupling member, the cylindrical insert member passing the upper shoulder in the axial bore; mating the substantially irregular male profile of the second coupling member with the substantially irregular female profile of the first coupling member for co-rotation therebetween; inserting the locking member into an upper end of the axial bore of the first coupling member; threading the locking member onto the cylindrical insert member until a lower end of the locking member engages the upper shoulder in the axial bore of the first coupling member for maintaining axial coupling therebetween. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional side view of a prior art shear coupling assembly; 
         FIG. 2  is a partial sectional side view of an alternate embodiment of the prior art shear coupling assembly of  FIG. 1 ; 
         FIG. 3  is an exploded sectional view of a shear coupling assembly according to an embodiment of the invention, illustrating a first coupling member, a second coupling member and a locking member; 
         FIG. 4  is a longitudinal sectional view of an assembled shear coupling assembly according to  FIG. 3 , the first and second coupling members being box couplings; 
         FIG. 5  is a longitudinal sectional view of an assembled shear coupling assembly according to an embodiment of the invention wherein the first coupling member is a box coupling member and the second coupling member is a pin coupling member; 
         FIG. 6  is a cross-sectional view of the shear coupling assembly of  FIG. 4  along lines A-A, substantially irregular mated male and female profiles being serrated profiles; 
         FIG. 7  is a cross-sectional view of alternate, substantially irregular, mated male and female profiles being a flat projection of a spherical triangle; 
         FIG. 8  is a cross-sectional view of alternate substantially irregular mated male and female profiles being polygons having four sides; 
         FIG. 9  is a cross-sectional view of alternate substantially irregular mated male and female profiles, being polygons having six sides; 
         FIG. 10  is a cross-sectional view of alternate substantially irregular mated male and female profiles, being polygons having eight sides; 
         FIG. 11  is a cross-sectional view of alternate substantially irregular mated male and female profiles, being polygons having ten sides; 
         FIG. 12  is a cross-sectional view of alternate substantially irregular mated male and female profiles, being polygons having twelve sides; 
         FIG. 13  is a plan view of the locking member according to  FIG. 3 , illustrating an opening in a top for accessing a bore therethrough and a discontinuous diametral slot for engaging a tool end; 
         FIG. 14  is an partially exploded longitudinal sectional view of a shear coupling according to an embodiment of the invention and apparatus for assembly being connected thereto; 
         FIG. 15  is a partial longitudinal sectional view of an assembled shear coupling and apparatus for assembly according to  FIG. 14 ; and 
         FIGS. 16A and 16B  are longitudinal sectional views of a second coupling member of the shear coupling assembly of  FIGS. 4 and 5  affixed to an anvil of the assembly apparatus, more particularly 
         FIG. 16A  illustrates an anvil suitable for affixing a pin coupling member; and 
         FIG. 16B  illustrates an anvil suitable for affixing a box coupling member. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the invention relate to improvements to prior art shear coupling assemblies to permit use with both reciprocating and rotary pump applications. A description of a conventional shear coupling assembly and method of assembly is provided to assist in understanding embodiments of the invention and the advantages therein. 
     Prior Art Shear Coupling Assembly 
     Having reference to  FIGS. 1 and 2 , a conventional prior art shear coupling assembly  1 , applicable to reciprocating pump applications only, 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. 
     The pin coupling member  10  comprises a cylindrical body  14  having an insert member  16  with external threads 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 shear neck  18  during assembly, as described below. The pretension in the shear 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. 
     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 axial 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 
     In an embodiment of a prior art method of assembly, 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 . 
     A tension 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 tension rod. With the tension 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 shear neck  18 . The tensile load on the tension rod is then released and the tension 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 
     As shown in  FIGS. 3-5 , a shear coupling assembly  100  according to embodiments of the invention, for use in both reciprocating and rotary pumping applications, driveably connects a first coupling member  102  and a second coupling member  104 . The first and second coupling members  102 ,  104  are driveably connected both axially and rotationally as is described herein. The rotational connection is by means other than at a shear neck  116  to prevent rotational loads from affecting the design purpose of the shear neck  116 . A locking member  130  connects axially between the first and second coupling members  102 , 104 . 
     Having reference to  FIGS. 4 and 5 , and as one of skill in the art would appreciate, the second coupling member  104  can have either pin or box coupling ends for connection between the pump and the rod string, resulting conventionally in a box/box ( FIG. 4 ) or a box/pin ( FIG. 5 ) combination. Thread sizes for the box and pin couplings within a shear coupling assembly  100  can be the same or can be different, resulting in a crossover arrangement suitable in some applications, as is understood in the art. 
     Having reference again to  FIGS. 3 and 4 , the first coupling member  102  comprises a tubular body  106  having an internal, axial bore  108  formed therethrough for receiving at least a portion of the second coupling member  104  therein. The internal axial bore  108  comprises a lower portion  110  having a substantially irregular female profile P f  formed therein. 
     The second coupling member  104 , as in the prior art, comprises a cylindrical body  112  having an externally threaded insert member  114  extending axially outwardly therefrom and connected to the body  112  by a shear neck  116  designed to part under design loads. In the embodiment shown herein, the shear neck  116  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  114  is bored with an internally threaded axial counterbore  118  used to pretension the shear neck  116  during assembly, as described for the prior art shear coupling assembly  1 . 
     Unlike the prior art, the cylindrical body  112 , below the shear neck  116 , further comprises a substantially irregular male profile portion P m  which acts to engage the substantially irregular female profile P f  formed in the axial bore  108  of the first coupling member  102 , when inserted therein. Thus, when the male and female substantially irregular profiles P m , P f  are mated, the first and second coupling members  102 , 104  are rotationally, driveably connected, permitting the transfer of torque therebetween for co-rotation of the first coupling member  102  with the second coupling member  104  during rotary pumping operations. 
     As in the prior art shear coupling shown in  FIG. 1 , the cylindrical body  112  of the second coupling member  104  further comprises a radial contact shoulder  120 . The radial contact shoulder  120  extends radially outwardly below the shear neck  116  and the substantially irregular male profile P m . 
     As shown in  FIGS. 6-12 , the substantially irregular male profile P m  and substantially irregular female profile P f  are radially variable about the circumference of the profile P f ,P m  so that the profile P f ,P m  of one of the coupling members  102 , 104  interferes with the profile P f ,P m  of the other of the coupling members  102 , 104 . 
     Having reference to  FIG. 6 , and in an embodiment of the invention, the substantially irregular male and female profiles P m , P f  are substantially circular, serrated or splined profiles. One suitable spline profile is a 30° pressure angle, fillet root side fit, 32/64 pitch with 48 teeth, according to ANSI standard ANSI 92.1. As one of skill would understand, other serrated or splined profiles applied to the torque transmitting profiles P m , P f  of the shear coupling assembly  100  would also be suitable. 
     In embodiments of the invention, shown in  FIG. 7 , the substantially irregular male profile P m  and the substantially irregular female profile P f  have cross-sectional areas enclosed by curves having three sides; in other words, substantially spherical triangles. 
     As shown in  FIGS. 8-12 , the substantially irregular male profile P m  and substantially irregular female profile P f  are polygons having four or more sides. 
     Having reference again to  FIG. 4 , the substantially irregular female profile P f  is greater in axial length than that of the substantially irregular male profile P m  to ensure that a lower end  122  of the first coupling member&#39;s body  106  fully engages the radial contact shoulder  120  of the second coupling member  104  when assembled. 
     The axial bore  108  of the first coupling member  102  further comprises a restricted portion  124  above the substantially irregular female profile P f . The restricted portion  124  is sized to permit passage of the insert member  114  of the second coupling member  104  therethrough, but is smaller in diameter than the substantially irregular male profile P m  of the second coupling member  104 . The axial bore  128  above the restricted portion  124  is substantially circular, the restricted portion  124  forming a radially inwardly extending, upper shoulder  126  between the substantially circular portion  128  and the substantially irregular female profile P f . As a result of the greater relative length of the substantially irregular female profile P f  over the substantially irregular male profile P m  does not contact the restricted portion  124 . 
     Having reference to  FIGS. 3 and 4 , in order to ensure the first and second coupling members  102 ,  104  remain axially engaged when assembled, the locking member  130  is used. The locking member  130  engages between the first coupling member  102  and the second coupling member  104  for preventing axial separation of the second coupling member  104  from the first coupling member  102 , when assembled. 
     In an embodiment of the invention, as shown in  FIGS. 3 ,  4  and  13 , the locking member  130  comprises a cylindrical body  132  having a threaded axial bore  134  formed therein. The cylindrical body  132  has a diameter sized so as to be insertable downward into the circular portion  128  of the axial bore  108  of the first coupling member  102  for threading onto the externally threaded insert member  114  of the second coupling member  104  which extends above the upper shoulder  126  when inserted therein. The locking member  130  is threaded onto the externally threaded insert member  114  until a lower end  136  of the locking member  130  engages the upper shoulder  126  in the bore  108 . Thus, the second coupling member  104  cannot separate from the first coupling member  102  under normal axial loading. 
     Advantageously, when assembled, the torque transferring profiles P m , P f  and the shear neck  116  are protected within the assembled shear coupling assembly  100  from exposure to potentially corrosive wellbore fluids for improving fatigue resistance. 
     As shown in  FIG. 4 , the cylindrical body  132  of the locking member  130  is longer than the insert member  114  on the second coupling member  104  to ensure that the lower end  136  of the locking member  130  engages the upper shoulder  126  in the first coupling&#39;s axial bore  108  before a top  138  of the locking member&#39;s bore  132  engages the insert member  114 . 
     Having reference to  FIG. 13 , as an aid for assembly, the locking member  130  further comprises engagement means  140  formed in an upper surface  142  of the cylindrical body  132  for engaging a tool end for threading the locking member  130  onto the insert member  114 . In an embodiment, the engagement means  140  is a profile, such as a diametral slot, which is compatible with the tool end. 
     Having reference to  FIGS. 13-15 , to set design tensile loading limits and to increase fatigue resistance, it is generally desirable to apply a pretension to the shear neck  116  during assembly. Best seen in  FIG. 13 , and in an embodiment, the upper surface  142  of the locking member&#39;s cylindrical body  132  further comprises an opening  144  through which a tension rod  208  can extend for applying pretension to the shear neck  116 . After insertion of the locking member  130  downwardly into the axial bore  108  and prior to threading the locking member  130  onto the insert member  114 , the tension rod  208  is inserted through the opening  144  for engagement with the counterbore  118  in the insert member  114 . An upward pulling force is applied to the tension rod  208  and while maintaining the upward pulling force, the locking member  130  is rotated for fully engaging with the insert member  114  for axially driveably connecting the first and second coupling members  102 ,  104  and for maintaining the pretension in the shear neck  116 . 
     Method of Assembly of Shear Coupling 
     As one of skill in the art will appreciate there are a number of ways in which embodiments of the invention may be assembled. Applicant has described herein one such method of assembly. 
     Having reference to  FIGS. 14-16B  and in an embodiment of the invention, the second coupling member  104  is affixed to a base  200 , such as by an anvil  202  configured for either a pin coupling ( FIG. 16A ) or a box coupling ( FIG. 16B ), connected to the base  200 . Typically, the base  200  forms part of a frame  204  to which a hydraulic cylinder  206  is connected for applying pretension to the shear neck  116  during assembly. 
     The first coupling member  102  is lowered onto the second coupling member  104 , inserting the insert member  114  into the lower end  110  of the axial bore  108  of the first coupling member  102 . The insert member  114  passes through the restricted portion  124  of the axial bore  108 . The first coupling member  102  is lowered until the lower end  122  of the first coupling member  102  contacts the radial contact shoulder  120  of the second coupling member  104 . The substantially irregular male and female profiles P m , P f  of the first and second coupling members  102 , 104  are mated so as to permit transfer of torque therebetween. The locking member  130  is inserted downwardly into the upper portion  128  of the axial bore  108  of the first coupling member  102  for threaded connection to the insert member  114  of the second coupling member  104 . 
     As shown in  FIGS. 14 and 15 , where a pretension is to be applied to the shear neck  116 , a tension rod  208  is inserted into the axial bore  108  of the first coupling member  102  after insertion of the locking member  130 . The tension rod  208  passes through the opening  144  in the upper surface  142  of the locking member  130  and engages with the threaded counterbore  118  in the insert member  114 . As in the prior art, an upward pulling force is applied to the tension rod  208  at a predetermined force, such as by the hydraulic cylinder  206 . In this embodiment, while the upward pulling force is maintained, the locking member  130  is threaded onto the insert member  114  until the lower end  136  of the locking member  130  engages the upper shoulder  126  in the first coupling member&#39;s axial bore  108 . Thereafter, the tension rod  208  is released from the counterbore  118  and the tension rod  208  is removed from the axial bore  108  of the first coupling member  102 . 
     As shown in  FIG. 15 , prior to connecting the tension rod  208  to the hydraulic cylinder  206 , an engagement tool  210  having a tool end  212  is inserted into the axial bore  108  of the first coupling member  102  for engaging the engagement means  140  on the upper surface  142  of the locking member  130  for threading the locking member  130  therein. In the embodiment shown, the engagement tool  210  is integral with the tension rod  208  and is independently rotatable thereabout. Alternatively, the engagement tool  210  can be positioned for rotation about the tension rod  208  after the tension rod  208  is engaged with the counterbore  118  of the insert member  114 . 
     The engagement tool  210  is thereafter rotated with the tool end  212  engaged in the diametral slot  140  to thread the locking member  130  onto the insert member  114 . The locking member  130  is threaded until the lower end  136  of the locking member  130  engages the upper shoulder  126  in the first coupling member&#39;s axial bore  108 . The engagement tool  210  is thereafter released from the locking member  30  for retraction from the axial bore  108 . 
     As with the prior art shear coupling assembly, a thread-locking epoxy or adhesive is typically applied to the radial contact shoulder  120  of the second coupling member  104  and to the threads of the insert member  114  and locking member  130 , prior to assembly.