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FIELD OF THE INVENTION 
     The present invention relates generally to hydraulic jarring tools. 
     BACKGROUND OF THE INVENTION 
     Bidirectional hydraulic jars have provided much needed versatility in the removal of objects lodged in the casings of oil and gas wells. However, there remains a need for a bidirectional jar that is easily adjustable. There is a need for a jarring tool in which the impact surfaces are enclosed and protected from accumulation of debris and the resulting dampening of the blows. There is a need for a tool in which the exposed shoulders on the tool are champfered to facilitate removal from the well. Still further there is a need for a bidirectional jarring tool that can be re-cocked easily, without firing in the reverse direction, for unidirectional jarring operations. The preferred embodiment of the present invention provides these and other advantages as will be apparent from the following description. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a bidirectional hydraulic jar. The jar comprises an outer assembly and an inner assembly. The outer assembly comprises a tubular body with first and second ends. The tubular body defines an inner wall, and the first end comprises a connecting portion. A first impact surface is provided on the tubular body to transmit force in a first direction, and a second impact surface is provided on the tubular body longitudinally spaced from the first impact surface to transmit force in a second direction opposite the first direction. 
     The inner assembly comprises an elongate body having a portion telescopically receivable within the outer assembly. The elongate body defines an outer wall and has first and second ends. The second end comprises a connecting portion. A first impact surface is provided on the elongate body and is adapted to engage the first impact surface on the tubular body of the outer assembly. A second impact surface is provided on the elongate body longitudinally spaced a distance from the second impact surface on the elongate body and adapted to engage the second impact surface on the tubular body of the outer assembly. 
     An annular elongate fluid chamber is formed between the outer wall of the elongate body of the inner assembly and the inner wall of the tubular body of the outer assembly. The fluid chamber comprises first and second portions and a restrictor portion therebetween. The restrictor portion has a smaller radial dimension than the first and second portions. 
     A valve assembly is supported in the fluid chamber and fixed to one of the outer wall of the elongate body of the inner assembly and the inner wall of the tubular body of the outer assembly. The valve assembly is sized for reciprocal movement in the restrictor, and is adapted to obstruct fluid flow through the restrictor portion except for a bleed passage therethrough. This arrangement creates a delay as the valve assembly moves through the restrictor portion and accelerated movement as the valve assembly exits the restrictor portion into the first and second portions of the fluid chamber. 
     The valve assembly comprises a first valve ring comprising first and second ends with a body therebetween. The first end defines a metering face. A second valve ring comprises first and second ends with a body therebetween. The first end defines a metering face adjacent the metering face of the first ring and forms therewith a metering space between the first and second valve rings. The metering space forms part of the bleed passage. 
     At least one of the first and second valve rings is sized for sealing engagement with the restrictor passage and comprises a pass-though opening continuous with the metering space and forming part of the bleed passage. The size of the metering space is adjustable by moving at least one of the first and second valve rings relative to the other to vary the size of the bleed passage. 
     In this way, movement of a selected one of the outer assembly and inner assembly relative to the other one in a first direction causes jarring impacts between the first impact surfaces of the outer and inner assemblies to thrust the jar in a first direction. Movement of the selected one of the outer assembly and inner assembly relative to the other one in a second direction causes jarring impacts between the second impact surfaces of the outer and inner assemblies to thrust the jar in a second direction. Adjusting the size of the metering space between the first and second valve rings varies the force and speed of the jarring impacts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B,  1 C and  1 D are sequential longitudinal sections of a hydraulic jar made in accordance with the present invention and shown in the open position. 
     FIGS. 2A,  2 B,  2 C and  2 D are sequential longitudinal sections of the hydraulic jar shown in the closed position. 
     FIG. 3 is an enlarged longitudinal sectional view of the portion of the jar containing the valve assembly. 
     FIG. 4 is an enlarged longitudinal sectional view of the first valve ring. 
     FIG. 5 is an enlarged cross sectional view of the first valve ring taken along the line  5 — 5  in FIG.  4 . 
     FIG. 6 is an enlarged longitudinal sectional view of the second valve ring. 
     FIG. 5 is an enlarged cross sectional view of the second valve ring taken along the line  7 - 7  in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings in general and to FIGS. 1A-1D and  2 A- 2 D in particular, there is shown therein a bidirectional hydraulic jar made in accordance with the present invention and designated generally by the reference numeral  10 . The jar  10  generally comprises an outer assembly  12 , an inner assembly  14  and a valve assembly  16 . The jar  10  is shown in the open or extended position in FIGS. 1A-1D, and in the closed or compressed position in FIGS. 2A-2D. 
     The outer assembly  12  comprises a housing or tubular body  20 . In the preferred embodiment, the tubular body  20  is composed of several components threaded together. However, it will be understood that the number and nature of these components can vary widely. Moreover, the tubular body  20  may be integrally formed. 
     The preferred tubular body  20  has first and second ends  22  and  24  and has an inner bore defined by an inner wall  26 . Preferably, the tubular body  20  comprises a top sub  30  with first and second ends  32  and  34 . The first end  32  forms the first end  22  of the tubular body  20  and may comprise a box end with internal threads  36  or other suitable connecting portion. For example, the first end  22  may be adapted for connection to coil tubing or to another suitable elongate conduit or rod to be suspended in the well and manipulated to operate the jar, as will be described in more detail hereafter. In most instances the second end  34  of the top sub  30  will be externally threaded forming a pin end. The top sub  30  may be made of 4140 heat treated steel, 110 MYS. 
     The tubular body  20  preferably also comprises a fluid housing  40  having first and second ends  42  and  44 , which preferably are internally threaded box ends. The first box end  42  threadedly connects to the second pin end.  34  of the top sub  30 . The fluid housing  40  may be made of 4140 heat treated steel, 110 MYS. 
     Also included as part of the outer assembly is a connecting sub  50  having first and second ends  52  and  54 . The first and second ends  52  and  54  preferably are externally threaded pin ends. Thus, the first pin end  52  is connectable to the second box end  44  of the fluid housing  40 . The connecting sub  50  may be made of 4140 heat treated steel, 110 MYS. 
     Moving down the tool from the top sub  30  is a hammer housing  60 . The preferred hammer housing  60  comprises first and second ends  62  and  64 . Preferably the first and second ends  62  and  64  are internally threaded box ends. Thus, the first box end  62  is connectable to the second pin end  54  of the connecting sub. The inside wall of the hammer housing  60  may be provided with longitudinal grooves  66  for a reason which will become apparent. The hammer housing  60  may be made of 4140 heat treated steel, 110 MYS. 
     Forming the end of the preferred outer assembly  20  is a lower mandrel sub  70  with first and second ends  72  and  74 . The first end  72  of the lower mandrel sub  70  preferably is an externally threaded pin end so as to be connectable to the second box end  64  of the hammer housing  60 . As illustrated in the drawings, the exposed end of the lower mandrel sub preferably is champfered at  76  so as to facilitate insertion of the jar into the well and to discourage the collection of debris as the jar is reciprocated. The lower mandrel sub  70  may be made of 4140 heat treated steel, 110 MYS. 
     Having described the main structural components of the outer assembly  12  forming the tubular body  20 , the preferred construction of the inner assembly  14  will be described. The inner assembly  14  preferably comprises a mandrel or elongate body  80 . The elongate body  80  preferably is tubular to provide a fluid conduit  82  therethrough. However, a solid rod or other non tubular member may be substituted. As used herein, “tubular” denotes having a central throughbore, but is not limited to a member having a circular cross sectional configuration. It may be polygonal, having multiple straight sides, or have other shapes. 
     The elongate body  80  may be integrally formed but preferably will be formed of several components, usually threadedly connected. In the preferred embodiment, the elongate body  80  comprises a lower mandrel  84  having first and second ends  86  and  88 . Preferably, both the first and second ends are externally threaded pin ends. The second end  86 , which forms the second end of the elongate body  80 , may be used to connect, directly or indirectly, to the object stuck in the well or to any other downhole tool. The lower mandrel  84  usually will be provided with a large diameter portion  90  and a small diameter portion  92  forming an annular shoulder  94  therebetween. Preferably, this shoulder  94  is champfered as this will discourage the accumulation of debris and will facilitate the removal of the jar  10  from the well. The lower mandrel  84  may be made of any steel alloy of 140 MYS. 
     In the preferred design, the elongate body  80  comprises an impact transfer member  100  having first and second ends  102  and  104 , which preferably both have threaded box ends. The second box end  104  of the impact transfer member  100  is threadedly connected to the first pin end  86  of the lower mandrel  86 . The outer wall of the impact transfer member  100  preferably is provided with longitudinal ribs  106 , configured to ride in the longitudinal grooves  66  of the hammer housing  60 . This prevents rotation of the inner assembly  14  relative to the outer assembly  12 , without hindering telescopic movement. The impact transfer member  100  may be made of any steel alloy of 140 MYS. 
     Still further, the inner assembly  14  comprises an inner mandrel  108  having an outer wall  110  and first and second ends  112  and  114 . The first end  112  forms the first end of the elongate body  80 . The second end  114  of the inner mandrel  108  preferably is externally threaded for connection to the first box end  102  of the impact transfer member  100 . The outer wall  100  of the inner mandrel  108  comprises an intermediate section  118  and a reduced diameter section  120  extending from the first end  112  to the intermediate section and forming an annular shoulder  122  therebetween. The inner mandrel  108  may be made of alloy steel with high tensile and yield strength, such as EDT  150 . 
     Now it will be appreciated that the various components of the elongate body  80  collectively form an outer wall  126 , and that a portion of the elongate body is telescopically received in the tubular body  20  of the outer assembly  12 . In this embodiment, the inner mandrel  108 , the impact transfer member  100  and the small diameter portion  92  of the lower mandrel  84  are slidably received in the outer assembly  12 . 
     The jar  10  comprises a hammer assembly for creating jarring impacts in the tool in opposite directions. To this end, the tubular body  20  is provided with a first impact surface  130  to transmit force in a first direction, in this case toward the first end of the tubular body. In this embodiment, the first impact surface  130  (FIG. 1C) is found on the second end  54  of the connecting sub  50 ; however, it will be appreciated that a suitable surface could be provided in other locations. A second impact surface  132  (FIG. 2A) is also provided on the tubular body  20 . The second impact surface  132  is spaced longitudinally from the first impact surface  130  and is positioned to transmit force received in a second direction opposite the first direction. In this case, the second direction is toward the second end  24  of the tubular body  20 . Preferably, the second impact surface  132  is provided on the first end  72  of the lower mandrel sub  70 . 
     As mentioned previously, in this embodiment, the elongate body  80  of the inner assembly  14  includes an impact transfer member  100 , best seen in FIGS. 1B and  2 B. The first and second ends  102  and  104  of the impact transfer member  100  provide first and second impact surfaces  134  (FIG. 1B) and  136  (FIG.  2 B), positioned to contact the first and second impact surfaces  130  and  132  on the outer assembly. 
     Where the inner assembly  14  is fixed to the stuck object (not shown) at the second end  88  of the lower mandrel  84  and the outer assembly  12  is supported on coil tubing at the first end  32  of the top sub  30 , axial movement of the outer assembly by manipulation of the coil tubing causes the outer assembly to move back and forth on the inner assembly. Thus, when the outer assembly  12  is pulled in a first direction (to the right in FIGS.  1 A- 1 D), the second impact surface  132  on the first end  86  of the lower mandrel sub  84  impacts the second impact surface  136  on the impact transfer member  100 , as seen in FIG.  1 B. This impact thrusts the jar  10  in the first or upward direction. When the outer assembly  12  is pushed is pushed in the opposite direction (to the left in FIGS.  2 A- 2 B), the first impact surface  130  on the first end  52  of the connecting sub  50  impacts the first impact surface  134  on the impact transfer member  100  to thrust the jar  10  in a second or downward direction. 
     Where the inner assembly  14  is attached to the coil tubing and the outer assembly  12  is fixed to the stuck object (not shown), manipulation of the coil tubing causes the inner assembly to move back and forth in the outer assembly. Thus, when the inner assembly  14  is pulled in a first direction (to the left in FIGS.  1 A- 1 D), the second impact surface  136  on the impact transfer member  100  impacts the second impact surface  132  on the first end  86  of the lower mandrel sub  84  to thrust the jar  10  in the first or upward direction (to the left in FIGS.  1 A- 1 D). When the inner assembly  14  is. pushed is pushed in the opposite direction (to the right in FIGS.  2 A- 2 B), the first impact surface  134  on the impact transfer member  100  impacts the first impact surface  130  on the first end  52  of the connecting sub  50  to thrust the jar  10  in a second or downward direction (to the right in FIGS.  2 A- 2 D). 
     Most jarring tools comprise a hammer assembly in which one element is deemed the hammer, or striking member, and one element is deemed the anvil, or the impact receiving member. Now it will be seen that in this invention, the impact transfer member  100  of the inner assembly functions alternately as a hammer and an anvil, depending on whether the outer assembly  12  or the inner assembly  14  is attached to the coil tubing. Likewise, the first and second impact surfaces  130  and  132  on the outer assembly  12 , may act as hammer or anvil surfaces, again depending on whether the inner or outer assembly is attached to the coil tubing. 
     The jar  10  includes a hydraulic chamber enclosing the valve assembly  16  for creating the jarring impacts. To this end, an annular elongate fluid chamber  140  is formed between the outer wall  126  of the elongate body  80  of the inner assembly  14  and the inner wall  26  of the tubular body  20  of the outer assembly  12 . As best seen in FIG. 3, the fluid chamber  140  comprises first and second portions  142  and  144 . A restrictor portion  146  therebetween is formed by a smaller inner diameter section  148  of the fluid housing  40 . The restrictor portion  146  has a smaller radial dimension than the first and second portions  142  and  144 . In the illustrated embodiment, the fluid chamber  140  is formed by the inner bore  150  of the fluid housing  40  and the outer wall  152  of the inner mandrel  108 . 
     The valve assembly  16  is supported in the fluid chamber  140  and fixed to either the outer wall  126  of the elongate body  80  of the inner assembly  14  or to the inner wall  26  of the tubular body  20  of the outer assembly  10 . In the preferred embodiment, the valve assembly  16  is fixed to the reduced diameter portion  120  of the inner mandrel  108  adjacent the shoulder  122 . 
     The valve assembly  16  is sized and positioned for reciprocal movement in the restrictor portion  146  as the outer assembly  12  is moved axially relative to the inner assembly  14 . The valve assembly  16  is adapted to obstruct flow through the restrictor portion  146  except for a bleed passage, described in more detail below. This will create a delay as the valve assembly  16  moves through the restrictor portion  146  and then accelerated movement as the valve assembly exits the restrictor portion into either the first portion  142  or the second portion  144  of the fluid chamber  140 . It is this sudden accelerated movement that generates the jarring impact when the first and second impact surfaces  132  and  134  of the outer assembly  12  engage the first and second impact surfaces  134  and  136  of the inner assembly  14 . 
     With continuing reference to FIG. 3, the valve assembly  16  will be described in more detail. In its preferred form, the valve assembly  16  comprises first and second valve rings  162  and  164 . The first valve ring  162 , shown also in FIGS. 4 and 5, has first and second ends  166  and  168  and a body  170  therebetween. The second end  168  defines a metering face  172 , which preferably is frusto-conical in shape. The first valve ring  162  preferably is made of 4140 heat treated alloy steel. 
     The second valve ring  164 , shown in FIGS. 6 and 7, comprises first and second ends  176  and  178  and a body  180  therebetween. The first end  176  of the second ring  164  defines a metering face  182  adjacent the metering face  172  of the first valve ring  162 . The second valve ring  164  preferably is made of ductile iron alloy. 
     In this embodiment, where the valve assembly  16  is fixed to the inner assembly  14 , the first and second rings  162  and  164  are provided with threads  180  and  182  receivable on threads  192  formed on the reduced diameter portion  120  of the inner mandrel  108 . When both rings  162  and  164  are positioned on the reduced diameter portion  120 , with the adjacent ends  168  and  176  spaced a distance apart, the space therebetween forms a metering space  194 . 
     With continued reference to FIG. 3, it will be appreciated that there must be a bleed space  200  through the restrictor portion  146  when the valve assembly  16  passes through it. In accordance with this invention, this bleed space  200  may go through one or both of the valve rings  162  and  164 , or alternately may go through one of the valve rings and around the circumference of the other. In the embodiment shown herein, the first valve ring  162  is imperforate and is sized to permit passage of fluid around the circumference of its body  170  in the annular space  196  between the body and inner wall of the restrictor portion  146 . 
     The circumference of the body  180  of the second valve ring  164  is sized for sealing engagement with the restrictor portion  146  and at least one pass-through opening is provided through the body. This pass-through opening is continuous with the metering space  194  and forms a part of the bleed passage  200 . Preferably, the pass-through opening comprises three pass-through bores  202   a ,  202   b  and  202   c  extending end to end through the body  180  of the ring  164 . 
     Where one of the rings is perforated by the pass-through bores, as is the ring  164 , and one is imperforate, as the ring  164 , it is advantageous to make both the metering faces  172  and  182  substantially frusto-conical so that the metering space  194  also will be frusto-conical, one complementing the other. This streamlines fluid flow between the pass-through bores  202   a ,  202   b , and  202   c  and the annular space  196 . Where both rings have pass-through bores, the metering space may or may not be frusto-conical. 
     Now it will be appreciated that the size of the metering space  194  is adjustable by moving at least one of the rings  162  and  164  axially relative to the other to vary the size of the space and thus the size of the bleed passage  200 . Adjusting the size of the bleed passage varies the force and speed of the jarring impacts delivered by the jar  10 . A larger metering space, and thus a larger bleed passage, creates faster movement of the valve assembly and a lesser impact. Conversely, a smaller metering space and a smaller bleed passage, provides a slower passage of the valve and greater impact. 
     In this embodiment, it is preferred to thread the first ring  162  axially toward or away from the second ring  164 . Since both rings  162  and  164  are threadedly attached to the inner mandrel  108 , set screws (not shown) may be used to secure the selected positions of the valve rings against unintended axial movement during operation of the jar  10 . Thus, the rings  162  and  164  may be provided with one or more transverse openings, all designated collectively by the reference numeral  204 . 
     In the preferred embodiment described herein, the valve assembly is mounted on the inner assembly for reciprocal movement inside a fluid chamber with a restrictor portion formed on the inside wall of the outer assembly. It will be appreciated, though, that this arrangement can be reversed as well. That is, the valve assembly can be fixed to the outer assembly, with the restrictor portion formed on the inner assembly. 
     Shown through the drawings, and not indicated by reference numerals, are various circumferential grooves provided at numerous locations in the jar  10  for O-rings to provided fluid seals as needed. The O-rings are omitted to simplify the illustrations. 
     Now it will be apparent that the bidirectional hydraulic jar of the present invention offers many features and advantages. The speed and force of impact can be easily adjusted. The jar can be re-cocked without a reverse jar by manipulating the jar so that the valve assembly is about centered in the restricting portion of the fluid chamber, and then reversing direction. This allows the jar to be used when jarring impacts are desired in one direction only. The impact points, both upper and lower, in this jar are both enclosed in the tubular body or housing. This prevents debris from accumulating at an exposed impact and causing dampening of the blow. 
     Changes can be made in the combination and arrangement of the various parts and elements described herein without departing from the spirit and scope of the invention as defined in the following claims.

Summary:
An improved bidirectional hydraulic jar. The jar comprises a two-ring valve assembly supported preferably on the inner mandrel for reciprocal movement inside a fluid chamber. The chamber comprises two larger portions joined by a narrowed restrictor portion. The valve obstructs passage of fluid through the restrictor portion except for a bleed passage, which comprises an adjustable metering space between the two valve rings. The jarring force can be adjusted by varying the size of the metering space, which is done simply by axially repositioning one of the valve rings. All impact surfaces are enclosed in the housing to prevent downhole debris from dampening the blows. The shoulder on the exposed end of the mandrel is champfered to prevent build-up of debris and to facilitate removal of the tool from the well. This jar can be re-cocked easily, without firing in the reverse direction, for unidirectional jarring operations.