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BACKGROUND OF THE INVENTION 
     This invention relates to devices for anchoring a tubing string to a surrounding well casing. Tubing anchors have a variety of uses in oilfield operations. For example, when an oil well is produced with a downhole rod pump, during the pumping cycle the weight of the fluid in the tubing string shifts between a load on the rod string on the upstroke to a load on the tubing on the downstroke. For installations which do not have an anchor, during the downstroke the weight of the fluid in the tubing causes the tubing to stretch, but on the upstroke the load is transferred to the rod string so the tubing contracts. In an unanchored installation, this cycle causes wear in the rods and tubing because of the rubbing of the rods and rod boxes against the inner wall of the tubing. In unanchored installations the pump efficiency is decreased by the decrease in the effective stroke length. 
     A tubing anchor maintains the tubing in tension, thereby lengthening the effective stroke length of the pump and preventing the stretching/contracting cycle of the tubing string. Tubing anchors generally operate by the urging of slips against the interior wall of the production casing of the well by the application of mechanical or hydraulic force. Anchors which are activated hydraulically typically have a “live slip” or “floating slip” (hereinafter “live slip”) which is urged outwardly against the interior of the casing wall upon the application of hydraulic pressure to a piston which drives the live slip radially outward. The piston may have a relatively large diameter and is capable of providing a substantial applied force to the live slip thereby maintaining the position of the anchor in the casing and keeping the tubing string in tension. 
     When tubing needs to be pulled from the well, the tubing anchor needs to be released. The releasing procedures usually involve pulling up on the tubing string or rotating the tubing string. However, during this process, it is not uncommon for the live slip or other anchor components to separate from the tubing anchor and fall downhole, either to the well bottom or to be stopped by casing restrictions or by a smaller diameter casing/liner below the set point of the anchor. The loss of the live slip downhole is problematic. Anchor slips are typically manufactured from hardened steel and can form an obstruction requiring an expensive fishing job or, alternatively, can be left downhole with the potential for causing problems in the future. 
     SUMMARY OF THE INVENTION 
     Embodiments of the method and apparatus disclosed herein provide a solution to the disadvantages described above. For purposes of this disclosure, the terms “lower,” “bottom,” “downward,” etc., refer to a direction facing toward the bottom of a well and the terms “upper,” “top,” “up,” etc., refer to a direction facing toward the surface. The terms “inward” and “inwardly” refer to a direction facing toward the central axis of the disclosed hydraulic anchor and the terms “outward” and “outwardly” refer to a direction facing towards the inside wall of the casing string. 
     An embodiment of the apparatus is utilized in hydrocarbon producing wells for anchoring a tubing string within a length of well casing, where the apparatus restrains axial (i.e., upward and downward) motion of the tubing string. Embodiments of the apparatus have a mandrel which is made up into the tubing string, typically with the mandrel having threaded ends on each end which are made up into tubing couplings. The mandrel has an upper end, a lower end, and a plurality of axially oriented (i.e., aligned along the long axis of the mandrel from end-to-end) slip recesses spaced circumferentially about the mandrel. There are at least two different types of slip recess. One type will have a fixed slip mounted within the recess. The second type of slip recess is a live slip recess, which has disposed within it, or extending from it, a live slip. It is to be understood from the disclosure and claims below that statements that the live slip is “disposed within” the live slip recess refer to the live slip being contained within the live slip recess when the anchor is an uninstalled configuration, but once the live slip is set by application of hydraulic force, a portion of the live slip will radially extend outside of the live slip recess and grip against the interior wall of the casing, placing the anchor in an installed configuration. Unless stated otherwise, descriptions and claims which provide that the live slip is “disposed within the live slip recess” apply to both the installed and uninstalled configuration as described above. 
     The live slip recess has an upper end and a lower end (relative to the upper end and lower end of the mandrel), a first groove in the upper end and a second groove in the lower end. The first groove has a first width, which refers to the dimension from an inward wall, with respect to the central axis of the mandrel, to an opposing outside wall. Likewise, the second groove has a second width, again referring to the dimension from an inward wall to an opposing second wall. The first width and second width may have the same dimension. The first wall may be defined by a flat portion at the innermost portion of the live slip recess. Alternatively, the first wall may be defined by a shoulder member outward of the flat portion. 
     The live slip has a top end and a bottom end, which respectively correspond to the upper end and lower end of the live slip recess. The top end of the live slip has an axially tab. Likewise, the lower end also has an axially extending tab. When the live slip is disposed within the live slip recess, the tab at the top end is disposed within the first groove and the tab at the lower end is disposed within the second groove. The tabs at the top end and bottom end have a thickness, which may be the same. It is to be appreciated that the range of travel of the live slip in an outward direction (i.e., away from the central axis of the mandrel) is defined by the differences between the widths of the grooves and the thicknesses of the tabs. That is, once the outward facing surfaces of each tab engage the outer (second) wall of the groove, the live slip is restricted from any further outward travel. However, the engagement face of the live slip will usually engage the inner surface of the casing before the live slip travels the full range of travel. 
     The live slip recess may have a cylindrical piston housing which extends into the interior of the mandrel. The live recess may also have a pair of opposite facing arcuate slip shoulders with the piston housing disposed between the opposite facing slip shoulders. Each slip shoulder may have an arcuate groove facing the piston housing, where each arcuate groove defines a bend having a curvature which is concentric to the curvature of the cylindrical piston housing. The live slip is disposed within the arcuate grooves of the slip shoulders as generally described above, where the tabs of the live slip may have an arcuate end which generally corresponds to the curvature of the arcuate groove in which the tab is disposed. A piston is disposed within the cylindrical piston housing, where the piston is retained within the cylindrical piston housing by a piston engagement surface on the inward facing side of the live slip. 
     Also disclosed herein is a method of restraining axial motion of a string of tubing within the length of well casing. In the method, a first part of a tubing string is run into the well, a hydraulic anchor as described above is made up into the tubing string by making the lower end of the anchor into the upper most end of the first part of the tubing string and making up the lower most end of a second part of the tubing string into upper end of the anchor and filling the tubing with liquid, causing the piston to drive the live slip outwardly into the inside wall of the casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic view of a production system for a hydrocarbon producing well which might utilize embodiments of the present invention. 
         FIG. 2  shows a perspective view of an embodiment of the present invention. 
         FIG. 3  shows a sectional view of an embodiment of the present invention. 
         FIG. 4  shows an exploded view of an embodiment of the present invention. 
         FIG. 5  shows a perspective view of an embodiment of a mandrel utilized in the present invention. 
         FIG. 6  shows a side view of the embodiment of the mandrel depicted in  FIG. 5 . 
         FIG. 7  shows a top view of the embodiment of the mandrel depicted in  FIG. 5 . 
         FIG. 8  shows a bottom view of the embodiment of the mandrel depicted in  FIG. 5 . 
         FIG. 9  shows a front/rear view of the embodiment of the mandrel depicted in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring specifically to the figures,  FIG. 1  shows a schematic view of a production system for a hydrocarbon producing well, of the type in which embodiments of the present apparatus and method might be utilized. A hydraulic anchor  10  is schematically shown placed in a tubing string  14 . A rod string  12  operates inside the tubing string  14  which reciprocates a plunger of a downhole pump  16 . As the plunger is lifted upwards, the load of the fluid column contained in the tubing string  14  above the plunger is carried by the rod string  12 . On the downstroke, the traveling valve inside the plunger opens and the load of the fluid column contained in the tubing is shifted to the tubing string  14 . Unless the tubing string is anchored, the tubing string will stretch and contract in response to this cyclical loading. The hydraulic anchor  16  maintains the tubing string  14  in tension by the expansion of slips into the inside wall  18  of casing string  20 . 
       FIG. 2  depicts an embodiment of the disclosed tubing anchor  10 . The general components of the tubing anchor  10  are the mandrel  22 , the live slip  24 , fixed slip  26 , and end connections  28 . 
       FIG. 3  depicts a sectioned view of the tubing anchor  10  taken along its mid-line. As shown in this figure, the internal diameter D of the tubing anchor  10  may be full opening to allow the passage of any tool or device which will pass through the tubing. End connections  28  are depicted as threaded ends for making up into couplings of the tubing string  14 . However, the mandrel  22  might be fabricated with female ends with internal threads as well, depending upon the requirements of the particular application. 
       FIG. 4  shows an exploded view of the tubing anchor  10 . As shown in the figure, this embodiment comprises the mandrel  22 , the live slip  24 , fixed slips  26 , and end connections  28 . Live slip  24  fits into a live slip recess  30 . Fixed slips  26  may be attached within fixed slip recesses  32  with fasteners  34 . A cylindrical piston housing  36  is fashioned into the mid-section of the live slip recess  30 . The cylindrical piston housing  36  extends into the interior of the mandrel  22  via aperture  74  which is defined by mandrel wall  76 . A piston  38  is disposed within the cylindrical piston housing. Piston  38  is utilized to translate hydraulic pressure to a force which acts to urge live slip  24  outwardly to grip the inside diameter  18  of casing string  20 . Although not shown in the figure, the outside facing surface  62  of live slip  24  which grips the inside diameter  18  of casing string  20  will typically have serrations or teeth which allow a secure bite into the inside surface of the casing. Piston  38  operates as a component of a unit which acts to utilize hydraulic pressure from the inside of the hydraulic  10  to exert an outward force on live slip  24 , the unit comprising the piston  38 , piston o-ring  40 , seal cup  42 , seal cup washer  44 , cap screw  46 , anchor sleeve  48  and anchor sleeve o-ring  50 , the unit maintained within the cylindrical piston housing by the live slip  24  on the outside and the mandrel wall  76  on the inside. 
       FIG. 5  depicts an embodiment of mandrel  22 , showing the live slip recess  30  and the fixed slip recesses  32 . It is assumed, for purposes of  FIG. 5 , that the upper end of mandrel  22  (i.e., the end facing toward the surface) is depicted at end  28   U  and the lower end of the mandrel is depicted at end  28   L . With respect to the upper end  28   U  and the lower end  28   L , the live slip recess  30  has an upper end which is defined by slip shoulder  52   U  and a lower end defined by slip shoulder  52   L  where the slip shoulders are opposite facing and the cylindrical piston housing  36  is disposed between the slip shoulders on a generally flat section  56 , which forms the bottom of the live slip recess  30 . Slip shoulder  52   U  and slip shoulder  52   L  respectively comprise grooves  54   U  and  54   L . Grooves  54 U and  54 L will generally have the same width, where the grooves define a bend which is concentric to the curvature of the cylindrical piston housing  36 . As shown in  FIG. 6 , the upper slip shoulder  52   U  has a lip section  58   U  which overhangs the generally flat section  56 , where groove  54   U  is defined between the lip section and the generally flat section. Likewise, the lower slip shoulder  52   L  has a lip section  58   L  which also overhangs the generally flat section  56 , with groove  54   L  defined between the lip section and the generally flat section. 
     Live slip  24  is disposed within the live slip recess  30 . The live slip  24  has an inward facing side  60 , an outward facing side  62 , an upper end  64   U  and a lower end  64   L . The inward facing side  60  has a piston engagement surface  66  which is in facing relation with the cylindrical piston housing  36 . The upper end  64   U  and the lower end  64   L  respectively comprise axially extending tabs  68   U  and  68   L . Each of the axially extending tabs  68   U  and  68   L  are disposed within a corresponding arcuate groove  54   U  and  54   L  of the respective slip shoulders  52   U  and  52   L , where each tab has an arcuate end having a curvature generally corresponding to the curvature of the arcuate groove in which the tab is inserted. 
     As best seen in  FIG. 3 , the range of travel of the live slip  24  in a direction normal to the long axis L of the mandrel  22  is defined by the differences between the thickness of the tabs  68   U  and  68   L  with respect to the widths of arcuate groove  54   U  and  54   L , which are determined by the distance between the lip sections  58   U  and  58   L  and the generally flat section  56 . Live slip  24  is generally retained within live slip recess  30  by fasteners  70  which extend through apertures  72  into slots  74  of the live slip, thereby allowing the outward travel of the live slip with respect to the live slip recess. 
     An acceptable geometry of a mandrel  22  is depicted in  FIGS. 7-9 . As indicated in  FIG. 9 , the mandrel  22  may have a fluted body which allows the passage of gas and fluid past the outside of the anchor, in the annulus formed by the body of the anchor and the interior wall  18  of the casing string  20 . As further shown in  FIG. 9 , the exterior of the mandrel  22  may define a circle and may, if necessary, be engaged by an overshot for emergency retrieval. 
     The anchor  10  is made up as a component of the tubing string  14 , with the portion of the tubing comprising the downhole pump  16  placed below the anchor. For liner pumps, the pump liner will be made up as an integral part of the tubing string with the plunger installed on the end of the rod string  12 . Alternatively, for insert pumps, the entire pump is run inside the tubing string  14  on the rod string  12 . In either case, the internal diameter of anchor  10  should be at least as large as the inside diameter of the tubing string to allow passage of either the pump plunger or the insert pump  16 . Once the anchor  10  is installed in the lower portion of the tubing string  14 , the remainder of the tubing string is made up above the anchor. Once the tubing string is installed, the rod string  12  is made up and run inside of the tubing string with either a pump plunger or an insert pump  16  on the end of the rods. Once the rod string  12  and the tubing string  14  are installed, the tubing string is filled with fluid. Once the fluid level inside the tubing string  14  is higher (i.e., closer to the surface) than the fluid level inside the casing string  20 , assuming fluids of equivalent density, piston  38  will start imposing an outward force against live slip  24  until a maximum hydraulic force is reached when the liquid level inside the tubing string  14  reaches the surface. 
     The hydraulic force is removed by draining the tubing string  14  of fluid by either activating a drain in the tubing string  14 , pulling the pump  16  off seat, or cutting a hole in the tubing. Because of the interactive structures of the live slip  24  and the live slip recess  30 , the live slip will be maintained disposed within the live slip recess, avoiding the problems otherwise presented when a live slip parts from the anchor. 
     While the above is a description of various embodiments of the present invention, further modifications may be employed without departing from the spirit and scope of the present invention. Thus the scope of the invention should not be limited according to these factors, but according to the following appended claims.

Summary:
A hydraulic tubing anchor used in hydrocarbon producing wells has structure which positively contains a live slip within the mandrel of the anchor, thereby preventing the live slip from separating from the tubing anchor and dropping deeper into the well. Structure at the ends of the live slip engage within corresponding grooves of the slip recess.