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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Patent Application No. 61/273,380 entitled “JARRING TOOL WITH MICRO ADJUSTMENT,” filed Aug. 4, 2009, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This disclosure relates to downhole tools in general and, more specifically, to impact jars for freeing stuck tools. 
     BACKGROUND OF THE INVENTION 
     Drilling operations have become increasingly expensive as the need to drill in harsher environments, through more difficult materials, and deeper than ever before have become reality. Additionally, more testing and evaluation of completed and partially finished well bores has become a reality in order to make sure the well produces an acceptable return on investment. 
     In working with more complex and deeper well bores, a greater danger arises that work strings and tools will be stuck within the bore. In addition to the potential to damage equipment in trying to retrieve it, the operation of the well must generally stop while tools are fished from the bore. Moreover, with some fishing techniques, it is possible to damage the well bore itself. 
     Any tool designed for use in a downhole environment may be subject to heat, pressure, and unclean operating conditions. Internal components may be subject to repeated stresses that must be overcome in order to function reliably, and for a suitable length of time, to warrant inclusion in the work string. Additionally, economies may be realized by constructing a tool that is wear resistant enough to be used for a lengthy periods of time before breakdowns or rebuilds. 
     What is needed is a device for addressing the above and related concerns. 
     SUMMARY OF THE INVENTION 
     The invention of the present disclosure, in one aspect thereof, comprises a jarring tool. The tool has a segment of sub housing containing an upper stop proximate a first end thereof. An outer latch piece is connected to an upper end of the lower shaft inside the sub housing, and an upper shaft passes through the stop and connects on a first end thereof to an inner latch piece. A cap is on a second end of the upper shaft retaining a washer stack against the upper stop. A retainer is inside the sub housing and has an first piece providing a restraining region and an open region and a second piece attached to the sub housing. The retainer retains the inner and outer latch pieces in a latched position in the restraining region until the latch pieces are displaced toward the second end of the sub housing through the retainer to the open region as a result of a tensile force on the outer latch piece. 
     The tensile force required to unlatch the first and second latch pieces may be controlled by moving the first retainer piece relative to the second retainer piece by a threading engagement. The outer latch piece may be a collet that grasps the inner latch piece when latched. 
     Some embodiments may have a lower shaft attached to the outer latch piece on a first end thereof and providing a sub end on a second end thereof. A lower stop may be provided proximate the second end of the sub housing, the lower stop having a passage sized to permit extension of the lower shaft therethrough and away from the sub housing and to stop the extension of the shaft by contact with a shoulder of the shaft at a predetermined extension. An intermediate shaft may be provided and arranged in a sliding and concentric relationship to the inner and outer latch pieces to define a protected passageway. 
     A second segment of sub housing may be attached to the upper stop and provide an indicator that displaces and remains displaced in response to contact from the upper shaft cap resulting from unlatching of the inner and outer latch pieces. An access port may be defined in the sub housing and have an attached cover plate defining an opening of a predetermined size to control fluid flow into and out of the sub housing. 
     The invention of the present disclosure, in another aspect thereof comprises latch. An retainer defines a restrictive region and an open region therein, the retainer having cooperating first and second pieces whereby the restrictive region is axially adjustable by moving the first and second pieces relative to one another. The latch has an outer latch piece and an inner latch piece that is biased against movement through the retainer. The retainer retains the inner and outer latch pieces in a latched configuration in the restrictive region and allows the inner and outer latch pieces to unlatch when tensile forces applied to the outer latch piece move the latch pieces into the open region. In some embodiments, the cooperating first and second pieces each define cooperatively threaded cylinders. 
     The retainer may be anchored by a sub segment. The inner latch piece may be biased against movement through the retainer by a shaft attached to a spring washer stack. The outer latch piece may be connected to a sub end. In some embodiments, an intermediate shaft passes through the inner and outer latch pieces both in the latched and unlatched configuration. 
     In some embodiments, a substantially cylindrical housing circumscribes the retainer and has a stop in one end thereof. A shaft passes through the stop and connects to the outer latch piece inside the housing. The shaft provides a shoulder for impacting the stop to create a jarring impact in response to a tensile force applied to the shaft that unlatches the first and second latch piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1F  taken together provide a side cutaway view of one embodiment of the jarring tool of the present disclosure. 
         FIG. 2  is an exterior view of a lower end of the jarring tool of  FIG. 1   
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIGS. 1A-1F , a side cutaway view of one embodiment of a downhole jarring tool according to aspects of the present disclosure is shown. These drawings are meant to be understood sequentially as adjoining segments of a jarring tool  100 .  FIG. 1A  illustrates the uppermost end of the tool  100 , which is to be followed by  FIG. 1B ,  FIG. 1C , etc. In the present embodiment,  FIG. 1F  illustrates the bottom most portion of the jarring tool  100 . 
     In the present embodiment, the jarring tool  100  includes three sub housings: an upper sub housing  10  having a distal end  12  and a proximal end  14 ; a center sub housing  102  having a distal end  104  and a proximal end  108 ; and a lower sub housing  112  having a proximal end  114  and a distal end  116 . The proximal end  14  of the upper sub housing  10  connects to the distal end  104  of the center sub housing  102  via an upper connector  16 . The upper connector  16  may be a sub connector with threaded fittings. In the present embodiment, the upper connector  16  also provides an activation indicator  20  that provides a visual indication of whether the tool  100  has been activated. This may be useful when the tool  100  is withdrawn from a well bore. 
     It will be appreciated that the tool of the present disclosure may be adapted for use on slick line or e-line tool strings. In the present embodiment, the various components of the tool  100  provide a central passage way  21  that proceeds the entire length of the tool  100 . Additionally, even though portions of the tool telescope with respect to one another, the central passageway  21  remains relatively protected from movements that can pinch or cut lines. 
     The functionality of the jarring operation of the tool  100  is described in greater detail below. However, the functionality of the upper connector  16  as an activation indicator may best be described here with particular reference to  FIG. 1B . Before the tool  100  has been activated to produce a jarring effect on a work string, an activation rod  22  extends slightly beyond upper connector  16  toward an upper shaft cap  135 . A rod spring  24  urges the rod  22  toward the cap  135 . A set screw  25  prevents the rod  22  from falling out of the upper connector  16 . 
     When in the unactivated position as shown, a recess  26  in the rod  22  is displaced toward the cap  135 . The recess  26  provides clearance for the indicator  20  to pop out from its depressed position when the rod  22  is moved against the force of spring  24 . A spring  28  provides the force to move the indicator  20 . During activation of the tool  100 , the cap  135  contacts the rod  22  which displaces it to allow the indicator  20  to pop up. The activator  20  is at least partially captive to the rod  22  to prevent it from coming completely free form the tool  100 . For example, two halves of the indictor  20  may be threaded through the rod  22 , or a 90 degree turn or series of turns may be required to completely free it. When the indicator  20  is pressed down, the rod spring  24  will urge the rod  22  back toward the cap  135 , resetting the indicator. Various holes or ports, such as port  29  may be provided near or in upper connector  16  to prevent activation or resetting of the indicator  20  by bore pressure. 
     It will be appreciated that in embodiments where the activation indicator mechanism is not needed, the entire upper sub housing  10  and upper connector  16  could be removed from the tool. Similarly, the upper sub housing  10  could be removed leaving the upper connector  16  as an attachment point for the tool  100  in the work string. 
     Referring again now to the complete set of figures, center sub housing  102  attaches on the distal end  104  to the upper connector  16 . The proximal end  108  of the upper sub housing  102  interconnects with a lower connector  110 . The lower connector  110  joins the center sub housing  102  with a lower sub housing  112 . The proximal end  114  of the lower housing  112  connects to the lower connector  110 . 
     A distal end  116  of the lower housing  112  is connected to a lower stop  118 . In the present embodiment, the lower stop  118  provides for sliding engagement and limited passage of a lower shaft  120 . The lower shaft  120  may be interconnected to a lower sub end  122 . The range of motion of the lower shaft  120  relative to the lower housing  112  may be limited by both the lower sub end  122  and by an inner shoulder  124  of the lower stop  118 . The lower shaft  120  provides a shoulder  126 , which will be too wide to pass through the lower stop  118 . As will be described in greater detail below, when the jarring tool  100  is activated, the upper sub housing  10  will extend away from the lower sub end  122  to the point where inner shoulder  124  of the lower stop  118  contacts the lower shaft shoulder  126 . 
     The lower shaft  120  connects to an outer latch piece  130  which cooperates with an inner latch piece latch piece  128 . In the present embodiment, the outer latch piece  130  is a collet device that selectively grasps the inner latch piece  128 , which functions as a stub for the collet. The interfitting inner and outer latch pieces  128 ,  130  are subjected to tensile forces of many thousands of pounds in operation. In order to secure adequate transmission of tensile forces between the inner latch piece  128  and the outer latch piece  130 , the inner latch piece  128  may have a lip  129  extending substantially around a proximal end of the latch piece  128 . Similarly, outer latch piece  130  may have a lip  131  on one or more collet fingers. 
     The lower shaft  120  is slidingly engaged with an intermediate shaft  121 . The intermediate shaft  121  provides a circumferential stop  123  that selectively engages with the lip  129  of the inner latch piece  128 , as further described below. The intermediate shaft  121  is also slidingly engaged through the inner latch piece  128  and connects to an upper shaft  134 . The intermediate shaft  121  and upper shaft  134  function as a single connected unit, and in some cases may be a single shaft. 
     In the present embodiment, the upper shaft  134  is slidingly engaged through a connector stop  149  that interfits into the connector  110 . It can be seen that a bias spring  140  surrounds a portion of the upper shaft  134  and the intermediate shaft  121  and presses against the inner latch piece  128 . The lip  129  of the inner latch piece  129  engages the stop  129  on the intermediate shaft  121 . Thus the inner latch piece  128 , the intermediate shaft  121 , and the upper shaft  134  are urged away from the stop  149  insofar as the other components will allow. 
     The upper shaft  134  partially proceeds through the stop  149  toward the distal end  104  of center sub housing  102 . Within center sub housing  102  and surrounding upper shaft  134  is a washer stack  142 . The washers of the washer stack  142  may be spring washers, such as Belleville washers. The cap  135  may retain the washer stack  142  on the upper shaft  134 . 
     Referring back particularly to  FIG. 1D , it can be seen that the inner latch piece  128  is shown nested within the outer latch piece  130 , with the intermediate shaft  121  passing through both. This is the unactivated position. Under tensile force on the tool  100  (e.g., the distal end  12  of the upper sub housing  10  and the lower sub end  122 ), the lip  131  of the outer latch piece will move into contact with the stop  129  of the inner latch piece, which will be moved into contact with the stop  123  of the inner shaft if it is not already. This will cause the upper shaft  134  to begin compressing the washer stack  142 . 
     A retainer  141  is fitted into the lower sub housing  112  in a position proximate the latch pieces  128 ,  130 . The outer latch piece  130  will be restrained from pulling away from the inner latch piece  128  because of limited clearance inside the retainer  141 . In the present embodiment, the retainer  141  is anchored in place to the lower subhousing by screws  150 ,  152 . The retainer  141  comprises two pieces: an outer piece  148  into which screws  150 ,  152  may be threaded; and an inner piece  143  that is threaded into the outer piece  148 . 
     The outer retainer piece  148  generally provides enough clearance to allow the inner and outer latch pieces  128 ,  130  to separate, but at least a portion of the inner retainer piece  143  does not. In the embodiment shown, a restrictive region  144  prevents the latch pieces  128 ,  130  from separating while an open region  146  allows the components to separate until tensile force. Because the inner and outer pieces  143 ,  148  are threaded together, the restrictive region  144  can be moved further from the washer stack  142 . This will cause the washer stack  142  to undergo a greater degree of compression before the latch pieces  128 ,  130  can separate. In the present embodiment, the inner and outer pieces  143 ,  148  can be adjusted even after the tool  100  is assembled by a slot (not shown) on the housing  112 . However, it is understood that, in operation or deployment of the tool, the inner and outer pieces  143 ,  148  remain fixed relative to one another and to the housing  112 . 
     When the latch pieces  128 ,  130  disengage, the lower shaft  120  will no longer be retrained from moving away from the upper shaft  134 . Under tensile force, the lower shaft will slide through the stop  118  until the stop shoulders  124  abut the shaft shoulders  126 . Depending upon the tensile force applied, a large jarring force along the length of the tool  100  may be produced. This impact or upward jarring motion can be utilized to free stuck tools in a drilling well. 
     It will be appreciated that the greater the tensile strength applied to the tool  100  the greater the jarring force produced. The tool  100  relies upon acceleration of the upper sub housing  10  away from the sub end  122  to produce its jarring impact. It will be appreciated that the greater the tensile force required to activate the tool  100 , the greater the impact jar will be. Coarse adjustments can be made by varying the spring rater and number of washers in the stack  142 . However, finer adjustments can be made by moving the open region  146  closer to or further from the washer stack  142 . This results in lesser or greater amounts of compression of the stack  142  that are required to pull the latch pieces  128 ,  130  through the restrictive region  144  and into the open region  146  where they can disconnect. The threading between the inner retainer piece  143  and the outer retainer piece  148  may be made relatively fine to allow for micro adjustments to be made to the release point of the tool  100 . In this way, it can be tailored to the application at hand. A sufficient jar can be produced to free stuck tools, while keeping the impact small enough not to unnecessarily damage any part of the rig or work string. As discussed more fully below, ports or openings can be provided in the sub housings to allow for adjustment of the tool  100  even after assembly. 
     Following activation of the tool  100  producing the desired jar, the tool  100  may be reset while still in the bore. When compressive forces are applied to the activated tool  100 , the outer latch piece will push against the stop  123  of the intermediate shaft  121  and/or the lip  129  of the inner latch piece  128  forcing them back through the restrictive region  144  where there is sufficient clearance for the pieces to relatch. The bias spring  140  will then act to push the inner latch lip  129  and stop  123  back into the restrictive region  144  as is shown in  FIG. 1D . At this point the tool  100  has been reset and can be used again to produce additional jarring. As described above, indicator  20  on the upper connector  16  can be examined to determine that the tool  100  has been deployed at least once while down hole. 
     It will be appreciated that the configuration of the latching mechanism operates to maintain the central passageway  21  through the nested and sliding arrangement of the components. This allows for safe passage of a communications or power line through the tool  100  in instances where it is needed. The sub housings can be fitted with electrical connections as needed to facilitate the user of the tool  100  as an e-line tool. 
     As the tool  100  may be utilized as part of an active work string, the tool  100  may connect to other tools further down the string and function as an ordinary segment of drill pipe until activated. In order to prevent the lower shaft  120  and lower sub connector  122  from rotating relative to the upper sub housing  10 , the lower stop  118  may be provided with rigid inserts  160  that are slidably engaged with slots  162  on the lower shaft  120 . This configuration allows for telescoping extension of the tool  100  to produce the desired jarring effect as described, but also allows the tool  100  to transmit rotational movement that may be needed in the work string to rotate a drill bit or other tool. 
     Referring now to  FIG. 2 , an exterior view of a lower end of the jarring tool of  FIG. 1  is shown. Here additional features can be seen that affect the performance of the tool  100 . As described above, the tensile force required to activate the tool can be adjusted by varying the washer stack  142  and the retainer  140 . These affect both the release point and the jarring force produced by the tool. 
     In addition to these adjustments, the behavior of the tool  100  following release can be fine tuned. It will be appreciate that well fluids may enter the tool  100 . The speed at which these fluids can be displaced, as well as the force required to move them, will have an effect on the jarring force and speed of the tool  100 . It can be seen in  FIG. 2  that various access ports or covers may be placed at locations along internal moving components of the tool  100 . For example, access port  202  is proximate the washer stack  142 . Access ports  204 ,  206  are provided proximate either side of the retainer  141 . Access port  210  is near stop  118 . By controlling how quickly fluid can displace from the washer stack  142 , the latch pieces  128 ,  130 , and the lower shaft  120 , impact force can be increased or decreased. In the embodiment shown, access ports  208 ,  210  are provided with slots  208 ,  212 , respectively. This allows for more rapid fluid flow into and out of the tool  100 , which is tend to increase impact force and the speed with which the tool  100  releases or deploys. The size and number of openings in the access ports can be varied according to the desired function of the tool  100 . 
     Access ports may also be useful for servicing or adjusting internal components. For example port  202  may be removed to provide access to the washer stack  142  and the stop  149 . 
     It will be appreciated that various embodiments of the tool of the present disclosure can be utilized with a wide variety of drilling and downhole technology. Non-limiting examples include drill pipe, e-line, and slick line strings. Sub ends and housings may be chosen according to the work string. Similarly, the overall size of the tools  100  may be chosen based on well bore size and other requirements. Once located in the down hole work string, the tool  100  functions as ordinary drill pipe or other string segment until called upon to create an upward jarring force on the work string. In this respect, the tool  100  may be considered as a segment of sub housing with an extensible, jar producing joint in the middle. 
     Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.

Summary:
A jarring tool is disclosed. The tool has a segment of sub housing containing an upper stop proximate a first end thereof. An outer latch piece is connected to an upper end of the lower shaft inside the sub housing, and an upper shaft passes through the stop and connects on a first end thereof to an inner latch piece. A cap is on a second end of the upper shaft retaining a washer stack against the upper stop. A retainer is inside the sub housing and has an first piece providing a restraining region and an open region and a second piece attached to the sub housing. The retainer retains the inner and outer latch pieces in a latched position in the restraining region until the latch pieces are displaced toward the second end of the sub housing through the retainer to the open region as a result of a tensile force on the outer latch piece.