Embodiments disclosed herein relate to a jar including the following: a mandrel; an outer housing slidably disposed about the mandrel; a ball stop housing disposed below the outer housing; a lower sub disposed below the ball stop housing; and a ball stop assembly disposed in the ball stop housing. The ball stop assembly includes a ball stop pivotally disposed in the ball stop assembly.

[0001]    This application claims the benefit of the following application under 35 U.S.C. 119(e); U.S. Provisional Application Ser. No. 61/531,868 filed on Sep. 7, 2012, the disclosure of which is incorporated by reference in its entirety herein. 
     
    
     BACKGROUND 
       [0002]    In the art of drilling wells for recovery of hydrocarbons, the process incorporates a drill string which has a plurality of threaded tubular members such as drill pipe being approximately 30 foot each in length, the drill pipe threaded end to end which is then used to rotate the drill bit either from the surface or through the use of a drill motor which would rotate the bit without the rotation of the drill pipe itself. Often times during that process, the drill string will become lodged at a certain point along its length within the borehole. 
         [0003]    In the efforts to dislodge the drill pipe or other tools lodged downhole, a type of tool known as a jarring tool would be used in such an attempt. In the current state of the art, jarring tools may be utilized to either jar the stuck or the lodged portion of pipe either in the up or down direction, depending on the makeup of the tool. In most cases, it would be more desirable to jar down on the pipe than to jar up. The reason for this is that drill pipe will usually get lodged when it is being pulled up as opposed to being moved downward, so jarring downward will more likely free the pipe. In such a case, the pipe is probably wedged against an obstruction caused by the upper movement of the pipe, and jarring upward may tend to wedge the debris around the section of pipe even tighter. 
         [0004]    Methods of downward jarring which are currently used in the art include applying compression on the drill string to which a down jar has been attached, whereby the jar releases at a pre-set load, allowing the hammer of the jar to freely travel a short distance impacting the anvil of the tool, delivering a downward blow. The effectiveness of this method has limitations, due to compressional buckling of the drill string, as well as drag. Therefore, it is often difficult to achieve a large downhole jarring force in a vertical well, and the problem is exacerbated in the horizontal portion of a directional drilling operation. A jar in the upward direction can be attached to the top of the stuck pipe or tool, and the jar can be pulled upward until it is tripped. While this type of jarring can produce more force than downward jarring, it is typically in the wrong direction for most instances of stuck pipe. Typically, in oilfield drilling operations, when a drill bit and/or drill string becomes stuck, a jar that is coupled to the drill string may be used to free the drill bit and/or the drill string. The jar is a device used downhole to deliver an impact load to another downhole component, especially when that component is stuck. There are two primary types of jars, hydraulic and mechanical. While their respective designs are different, their operation is similar. Energy is stored in the drillstring and suddenly released by the jar when it fires, thereby imparting an impact load to a downhole component. Jars may also be used to recover stuck drill string components during drilling or workover operations 
         [0005]    Drilling jars typically have a sliding mandrel in a sleeve. In use, the mandrel is driven up or down by some form of stored energy, a hammer on the mandrel striking an anvil on the sleeve so as to impart a shock and (it is hoped) free the stuck pipe. One common form of drilling jar is a hydraulic jar. A hydraulic jar includes two reservoirs of hydraulic fluid separated by a valve. When tension or compression is applied to the tool in a cocked position, fluid from one chamber is compressed and passes through the valve at high flow resistance into the second chamber. This allows the tool to extend or contract. When the stroke reaches a certain point, the compressed fluid is allowed to suddenly bypass the valve. The jar trips as the fluid rushes into the second chamber, instantly equalizing pressure between the two chambers and allowing the hammer to strike the anvil. The greater the force on the jar, the sooner and more forceful the release. 
         [0006]    As jars are returned to the surface after use and/or placed in a derrick, jars may accidentally fire. Such accidental firing can result in significant safety hazards at a drilling location. Traditionally, in an attempt to prevent accidental firing, an external jar clamp is manually placed on a shaft of the jar located between the internal mandrel assembly and the external cylinder assembly. The clamp acts as an external stop that would prevent axial movement of the tool. However, in the event the external clamp was not properly fastened to the jar, the clamp could fall off of the jar during storage, thereby creating a falling object hazard at the drilling location. 
         [0007]    In certain situations, internal mechanical latches have also been used in an attempt to prevent accidental firing of the jar. However, internal mechanical latches result in additional steps prior to firing a jar, increasing operational complexity and may unlatch if a load is accidentally exceeded on the rig floor. 
         [0008]    Accordingly, safety mechanisms for jars to prevent accidental firing may be desired. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    In one aspect, embodiments disclosed herein relate to a jar including the following: a mandrel; an outer housing slidably disposed about the mandrel; a ball stop housing disposed below the outer housing; a lower sub disposed below the ball stop housing; and a ball stop assembly disposed in the ball stop housing. The ball stop assembly includes a ball stop pivotally disposed in the ball stop assembly. 
         [0010]    In another aspect, embodiments disclosed herein relate to a jar including the following: a mandrel; an outer housing slidably disposed about the mandrel; a low pressure chamber having a first port and formed between the mandrel and the outer housing; a high pressure chamber having a second port and formed between the mandrel and the outer housing; a fluid passage between the first and second port; and a valve disposed in the fluid passage. The valve may be a needle valve or a seal rod. 
         [0011]    In another aspect, embodiments disclosed herein relate to a jar including the following: a mandrel; an outer housing slidably disposed about the mandrel; a low pressure chamber formed between the mandrel and the outer housing; a high pressure chamber formed between the mandrel and the outer housing; and a separator. The separator may be a spring which controls fluid communication between an annulus and the jar or a pressure activated valve disposed between the low pressure chamber and the high pressure chamber. 
         [0012]    This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  illustrates a partial cross-sectional view of a drilling jar in accordance with one or more embodiments. 
           [0014]      FIGS. 2 and 3  illustrate side schematic representations of drilling jars in accordance with one or more embodiments. 
           [0015]      FIG. 4  illustrates a break-away side view of a ball stop assembly in accordance with one or more embodiments. 
           [0016]      FIG. 5  illustrates a break-away view of a ball stop assembly in accordance with one or more embodiments. 
           [0017]      FIGS. 6A-6E  illustrate operational views of a ball stop assembly transitioning between closed and open positions in accordance with one or more embodiments. 
           [0018]      FIGS. 7A and 7B  illustrate cross-sectional views of a ball stop assembly in accordance with one or more embodiments. 
           [0019]      FIG. 8A  illustrates a side view of a drilling jar in accordance with one or more embodiments. 
           [0020]      FIG. 8B  illustrates a cross-sectional view of a drilling jar in accordance with one or more embodiments. 
           [0021]      FIG. 8C  illustrates a cross-sectional view of portion  200  of  FIG. 8B  in accordance with one or more embodiments. 
           [0022]      FIGS. 9A and 9B  illustrate partial cross-sections of a safety bypass for a drilling jar in accordance with one or more embodiments. 
           [0023]      FIGS. 10A and 10B  illustrate partial cross-sections of a safety bypass for a drilling jar in accordance with one or more embodiments. 
           [0024]      FIGS. 11A and 11B  illustrate partial cross-sections of a safety bypass for a drilling jar in accordance with one or more embodiments. 
           [0025]      FIGS. 12A and 12B  illustrate partial cross-sections of a safety bypass for a drilling jar in accordance with one or more embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Drilling jars are used to free stuck drill strings or to recover stuck drill string components during drilling or workover operations. The jars provide an impact blow either in the up or down directions. The driller can control the jarring direction, impact intensity and jarring times from the rig floor. The magnitude and direction of the load used to initiate the impact blow (jar) achieve this control. Examples of hydraulic jars are disclosed in U.S. Pat. Nos. 5,431,221, 5,174,393, 5,595,244, 5,447,196, 5,503,228, 5,595,253 and such patents are hereby incorporated by reference herein. 
         [0027]      FIG. 1  shows a cross section through a lower detent area  11  of prior art jar  10 . Downward force arrow  13  is shown and represents the force applied to mandrel  12  of jar  10 . This force applied to mandrel  12  is transmitted to outer cylindrical housing  14  via detent piston  19  and results in an increase in pressure in the hydraulic fluid that is contained in lower chamber  16  between outer cylindrical housing  14  and mandrel  12 . 
         [0028]    The magnitude of the pressure in lower chamber  16  is directly proportional to the magnitude of the force applied to mandrel  12 . This high-pressure fluid is allowed to flow through orifice (not shown) to an upper chamber  20 . The result of this fluid flow is a relative axial movement between outer housing  14  and mandrel  12 . When this relative axial movement is sufficient to place the orifice in juxtaposition to relief area  17  of outer housing  14 , a sudden release of high pressure fluid occurs which results in an impact blow being delivered to the “knocker” part of the jar (not shown). The “knocker” is usually located at the upper most end portion of the drilling jar. 
         [0029]    As explained above, during the removal of one or more jars from a wellbore, they are stored on the derrick floor in the open position with two or more drill collars above it. The weight of the drill collars and the jar itself may close the jar causing accidental firing/unintentional impact blows of the jar. Unintentional impact blows result in safety concerns for rig operators. Safety clamps are typically used to prevent this occurrence, but they present a significant falling hazard as they can be 30 to 90 ft above the floor. 
         [0030]    Referring to  FIGS. 2 and 3 , a schematic representation of a jar connected to a ball stop assembly according to one or more embodiments of the present disclosure is shown. As illustrated in  FIG. 2 , jar  100  is connected to a ball stop assembly  105 , which is connected to a lower sub  110 .  FIG. 2  illustrates jar  100  fully compressed without the Kelly mandrel shaft exposed.  FIG. 3  also illustrates jar  100  connected to a ball stop assembly  105 , which is connected to a lower sub  110 . However, in  FIG. 3 , jar  100  is extended with an exposed portion of Kelly mandrel shaft  115  exposed. Ball stop assembly  105  prevents unintentional impact blows, as ball stop assembly  105  acts as an internal stop that prevents axial movement of jar  100 . The ball stop assembly  105  will be described in detail below. 
         [0031]    Referring to  FIG. 4 , a break-away schematic illustration of a ball stop assembly according to one or more embodiments of the present disclosure is shown. As illustrated, a lower jar assembly  120 , having a lower mandrel  125  is disposed below a ball stop housing  130 . When the tool is assembled, ball stop housing  130  slides over lower mandrel  125  into contact with lower jar assembly  120 . In this embodiment, ball stop housing  130  contacts lower jar assembly  120  at a lower jar assembly shoulder  135 . Depending on the specific design, ball stop housing  130  may be coupled to lower jar assembly  120  through a screw-type connection, or alternatively with bolts, rivets, or through other connections known in the art. 
         [0032]    During assembly, a ball stop assembly  105  is disposed in ball stop housing  130 . Lower sub  110  may then be coupled to ball stop housing  130  through a screw-type connection, or alternatively with bolts, rivets, or through other connections known in the art. When ball stop housing  130  is made-up with lower sub  110 , a top extension  140  of lower sub  110  may contact a ball retainer  145  of ball stop assembly  105 . Thus, when assembled, lower jar assembly  120  is coupled to ball stop housing  130 , which is coupled to lower sub  110 , such that lower mandrel  125  may communicate axially through ball stop housing  130  and ball stop assembly  105 . 
         [0033]    Referring to  FIG. 5 , a break-away schematic illustration of ball stop assembly  105  according to one or more embodiments of the present disclosure is shown. In this embodiment, ball stop assembly  105  includes a spring slide  150  having yoke pins  155  extending from a lower axial portion thereof. Ball stop assembly  105  further includes a ball retainer  145  having a plurality of pivot pins  160  extending internally therein. Pivot pins  160  are configured to hold a ball stop  165 , while allowing the ball stop  165  to rotate when motion applied by slide assembly  150  axially translates yoke pins  155 . The axial movement of spring slide  150 , and thus yoke pins  160  may thereby cause ball stop  165  to rotate about pivot pins  160 . Ball stop  165 , as illustrated is hollow through the center, so as to allow the lower mandrel (not shown) to move axially therethrough when the ball stop  165  is rotated into an open position. The positions of ball stop  165  will be explained in detail below. 
         [0034]    A spring  170  is disposed around spring slide  150  and held in place with a seal  175 . Seal  175  is fixed relative to spring slide  150 . When assembled, the ball stop assembly  105  is disposed in the ball stop housing  130  ( FIG. 4 ), such that an area between spring slide shoulder  180  and seal  175  (and between spring slide  150  and ball stop housing  130 ) is a sealed chamber filled with air. 
         [0035]    Referring to  FIGS. 6A-6E , schematic representations of ball stop assembly  105  during actuation according to one or more embodiments of the present disclosure are shown. As illustrated,  FIG. 6A  is representative of ball stop assembly  105  in a closed, non-actuated position, while  FIG. 6E  is representative of ball stop assembly  105  in an open, actuated position. All of  FIGS. 6A-6E  show ball stop assembly  105  having a slide assembly  150  with a spring  170  disposed therearound, and sealed to form an air chamber (as disclosed above) via seal  175 . Ball stop  165  is held in ball retainer  145  with pivot pins  160  and ball stop  165  is connected to yoke pins  155 . Spring  170  is biased such that ball stop assembly  105  is in a closed position (as illustrated in  FIG. 6A ). In the closed position, ball stop  165  is oriented so that there is no internal passage through ball stop assembly  105  to allow the lower mandrel  125  ( FIG. 4 ) of the jar to translate therethrough. However, when ball stop  165  is oriented in an open position (as illustrated in  FIG. 6E ), the lower mandrel  125  of the jar can freely move axially through a passage (not shown) in ball stop  165 . 
         [0036]    The ball stop  165  is rotated by converting axial movement of slide assembly  150  to rotate ball stop  165 . As illustrated herein, actuation occurs as a result of a pressure differential created by the difference between the pressure of the drilling fluid and the sealed chamber of air, which is created by sealing the spring  170  via seal  175 . As internal drilling fluid pressure increases, the spring assembly  150  translates axially and rotates ball stop  165  into the open position. This process is illustrated through the progression of  FIGS. 6A to 6E . When drilling fluid pressure decreases, the spring  170  acts on slide assembly  150 , moving slide assembly  150  in the opposite direction to rotate ball stop  165  into a closed position. This process is illustrated through the progression of  FIGS. 6E to 6A . Thus, by varying the drilling fluid pressure, the ball stop assembly  105  may be rotated into open and closed positions through the drilling/jarring process. When drilling fluid pressure is ultimately decreased as the jar is removed from the wellbore, the ball stop assembly  105  will be in a closed position, such that lower mandrel (not shown) cannot pass through ball stop  165 . Because lower mandrel (not shown) cannot pass through ball stop  165 , the jar cannot unintentionally fire, thereby preventing safety hazards at the drilling rig. 
         [0037]    Referring now to  FIGS. 7A and 7B , a cross-sectional illustration of an embodiment of the present disclosure is shown. As illustrated in  FIGS. 7A and 7B , in the event of a failure of seal  175  or another component of ball stop assembly  105 , fluid may still pass through ball stop assembly  105 , thereby allowing drilling to continue. As illustrated in  FIG. 7A , while in the closed position, lower mandrel  125  is in contact with ball stop  165 , however, as the opening through ball stop  165  is smaller than the external diameter of lower mandrel  125 , lower mandrel  125  cannot translate therethrough. However, because ball stop  165  includes a narrow fluid passage  180 , fluid may still pass from lower mandrel  125  to lower sub  110  and on to other components of the drilling tool assembly, such as a drill bit (not shown). 
         [0038]    As illustrated in  FIG. 7B , while in an open position, lower mandrel  125  translates through ball stop  165 , thereby allowing fluid communication therethrough. Thus, in the event the ball stop assembly  165  fails, fluid communication through ball stop assembly  105  is provided so as to not interfere with the drilling operation. 
         [0039]    During operation of the jar, as explained above, the pressure generated by mud pumps allows the jar to remain in an open position due to the hydrostatic head. Thus, the tool may be operated substantially automatically, as the tool will modulate between open and closed positions as a result of the pressure generated by the mud pumps. In an alternate embodiment, modulation of the tool between open and closed positions may occur through manual actuation of a ball stop. 
         [0040]    Referring to  FIGS. 8A-8C , a manual drilling jar locking assembly according to embodiments of the present disclosure is shown. Referring specifically to  FIG. 8A , an external side view of a jar according to embodiments of the present disclosure is shown. In this embodiment, an operating stem  190  is shown extending externally from the jar  195 . In order to modulate jar between a closed and open position, an operator may manually manipulate operating stem  190  to turn an internal component of jar  195 . 
         [0041]    Referring to  FIGS. 8B and 8C , a cross-sectional view of  FIG. 8A  and a close perspective of section  200  of  FIG. 8B , respectively, are shown. As illustrated, operating stem  190  is connected to a ball stop  165 , such that rotation of operating stem  190  rotates ball stop  165  between an open and closed position, similar to the rotation of ball stop  165  discussed above. In this embodiment, operating stem  190  may include, for example, a screw that when turned imparts rotation to ball stop  165 , thereby changing the orientation of ball stop  165  within jar  195 . Those of ordinary skill in the art will appreciate that the jar may thus be modulated between open and closed positions as the jar is placed in or removed from the wellbore. Thus, the jar may be stored in a closed position, such and accidental firing cannot occur and be modulated into an open position before the jar is disposed in the wellbore. 
         [0042]    Referring to  FIGS. 9A and 9B , a partial cross-section of a safety bypass for a drilling jar according to one or more embodiments of the present disclosure is shown. Specifically,  FIG. 9A  illustrates a jar in a closed or firing condition, while  FIG. 9B  illustrates the jar in an open or non-firing condition. In this embodiment, a detent section  300  (as explained above with respect to  FIG. 1 ) of a drilling jar is shown. Detent section  300  includes a high pressure chamber  305  and a low pressure chamber  310 . A fluid passage  315  provides fluid communication between high pressure chamber  305  and low pressure chamber  310 . Fluid communication is provided through a first port  320  in low pressure chamber  310  and a second port  322  in high pressure chamber  305 . Detent section  300  further includes a needle valve  323  disposed in fluid passage  315  and configured to translate axially within fluid passage  315 . 
         [0043]    As a drilling jar having detent section  300  is run into a wellbore, annular pressure acts on needle valve  323 , causing needle valve  323  to translate axially downwardly. The axial translation of needle valve  323  within fluid passage  315  blocks second port  322 , thereby preventing fluid from flowing from high pressure chamber  305  to low pressure chamber  310 . Because fluid is prevented from flowing between high pressure chamber  305  and low pressure chamber  310 , pressure is allowed to build within high pressure chamber  305  by the downward force of the mandrel  12  ( FIG. 1 ) via detent piston  319 , thereby allowing the jar to fire. 
         [0044]    As the jar is removed from the wellbore, the annulus pressure decreases, thereby causing needle valve  323  to translate axially upwardly, as the spring  325  of needle valve biases the needle valve into an open condition. In an open condition, fluid is allowed to flow from high pressure chamber  305  through second port  322 , into fluid passage  315 , through first port  320 , and into low pressure chamber  310 . When the jar is in an open condition, and fluid is allowed to flow between high pressure chamber  305  and low pressure chamber  310 , pressure cannot build in high pressure chamber  305 , thereby preventing the jar from firing. 
         [0045]    Those of ordinary skill in the art will appreciate that as the jar is stored in the derrick, the jar is at ambient pressure and needle valve will be biased in an open condition, thereby preventing pressure from building in high pressure chamber  305 . Thus, as long as the jar remains in the derrick and stored, the jar will not unintentionally fire. As such, this embodiment of the present disclosure provides a pressure sensing device that diverts the flow of hydraulic fluid away from the pressure building detent system, thereby serving as a secondary safety mechanism when a jar is returned to the surface and placed in the derrick. 
         [0046]    Referring to  FIGS. 10A and 10B , a partial cross-section of an alternative safety bypass for a drilling jar according to embodiments of the present disclosure is shown. Specifically,  FIG. 10A  illustrates a jar in a closed or firing condition, while  FIG. 10B  illustrates the jar in an open or non-firing condition. In this embodiment a detent section  300  of a drilling jar is shown. Detent section  300  includes a high pressure chamber  305  and a low pressure chamber  310 . A fluid passage  315  provides fluid communication between high pressure chamber  305  and low pressure chamber  310 . Fluid communication is provided through a first port  320  in low pressure chamber  310  and a second port  322  in high pressure chamber  305 . In this embodiment, a plunger  330  is disposed in fluid passage  315  and a seal rod  335  is disposed in fluid passage  315  below plunger  330  proximate second port  322 . 
         [0047]    As the jar is run into the wellbore, annulus pressure acts on plunger  330 , compressing a spring  325 , preventing seal rod  335  from moving axially. As temperature increases, seal rod  335  thermally expands, thereby sealing second port  322  and preventing the flow of fluid from high pressure chamber  305  through fluid passage  315  into low pressure chamber  310 . Because fluid cannot flow from high pressure chamber  305  into low pressure chamber  310 , pressure builds within high pressure chamber  305  by the downward force of the mandrel  12  ( FIG. 1 ) via detent piston  319 , thereby allowing the jar to fire. 
         [0048]    When the jar is removed from the wellbore, annulus pressure decreases and a spring  325  allows plunger  330  to retract into a biased, open position. As the temperature decreases from the downhole temperatures, the seal rod  335  contracts and allows fluid to bypass from high pressure chamber  305  through fluid passage  315  and into low pressure chamber. Because fluid is allowed to flow from high pressure chamber  305  and low pressure chamber  310 , pressure cannot build in high pressure chamber  305 , thereby preventing the jar from unintentionally firing while the jar is stored in the derrick. 
         [0049]    In certain embodiments, seal rod  335  may be mechanically held within fluid passage  315 , thereby not requiring plunger  330 . In such an embodiment, the temperature increase as the jar is run into the wellbore causes seal rod  335  to thermally expand, thereby blocking second port  322 , allowing pressure to build within high pressure chamber  305 , and allowing jar to fire. 
         [0050]    Referring to  FIGS. 11A and 11B  a partial cross-section of an alternate safety bypass for a drilling jar according to one or more embodiments of the present disclosure is shown. Specifically,  FIG. 11A  shows a jar in an open position, allowing free flow of fluids between chambers, while  FIG. 11B  shows a jar in a closed position, thereby not allowing the free flow of fluid between chambers. 
         [0051]    Turning specifically, to  FIG. 11A , a jar  400  is shown having an outer housing  401 , a mandrel  402 , pressure chamber  405  and a pressure chamber  410 . A separator  415  is disposed therebetween, the separator  415  having a plurality of valves. A first valve  420 , a pressure activated valve, allows fluid to flow from the pressure chamber  410  to the pressure chamber  405 , while a second valve  425 , a reverse free flow valve, allows fluid to only flow from pressure chamber  405  to pressure chamber  410 . Jar  400  may further include a plurality of seals  403  configured to seal between separator  415  and outer housing  401 . 
         [0052]    As illustrated, first valve  420  is in the open position, thereby allowing fluid to flow freely from pressure chamber  410  to pressure chamber  405 . This condition occurs as the jar  400  is run into the wellbore as a result of annulus pressure acting on first valve  420 . Due to the annulus pressure, the first valve  420  is forced open, thereby allowing the free flow of fluid from pressure chamber  410  to pressure chamber  405 . Because fluid may flow therebetween, mandrel  402  can move down with respect to outer housing  401  allowing the tool to go from open position (on surface) to firing position (downhole). 
         [0053]    Referring to  FIG. 11B , as the jar  400  is removed from the wellbore, there is no annulus pressure to keep first valve  420  open, thereby resulting in first valve  420  closing, preventing fluid from flowing from pressure chamber  410  to pressure chamber  405 . As first valve  420  closes, the outer diameter of the separator is sealed, thereby preventing axial movement of jar  400  and effectively locking jar  400 . Because jar  400  is locked, the jar cannot unintentionally fire. Those of ordinary skill in the art will appreciate that a plurality of first and/or second valves  420 / 425  may be used to further increase the flow rate of fluids between pressure chamber  405  and pressure chamber  410 . 
         [0054]    Referring to  FIGS. 12A and 12B , a partial cross-section of an alternative safety bypass for a drilling jar according to one or more embodiments of the present disclosure is shown. In this embodiment, a separator  500  prevents fluid from flowing in/out of a jar  505 . Jar  505  includes an outer housing  506  and a mandrel  507 . A plurality of seals  508  may seal between separator  500  and outer housing  506  and between separator  500  and mandrel  507 . Specifically,  FIG. 12A  illustrates jar  505  in an open condition, wherein fluid is allowed to flow into jar  505 , thereby allowing jar  505  to be fired.  FIG. 12B  illustrates jar  505  in a closed condition, wherein fluid is not allowed to flow into jar  505 , and as such, jar  505  cannot fire. 
         [0055]    Referring specifically to  FIG. 12A , as jar  505  is run into a wellbore, pump pressure pushes separator  500  axially downward, compressing spring  510 . The compressing of spring  510  and associated axial translation of separator  500  downward opens annulus pressure communication port  515 , and allows annulus pressure to keep separator  500  down, in an open position. When separator  500  is in an open condition, fluid may freely flow into and out of jar  505  as jar  505  is stroked, which is required in order for jar  505  to operate. 
         [0056]    Referring now to  FIG. 12B , as jar  505  is removed from the wellbore, annulus pressure decreases and returns to atmospheric pressure, at which point the spring  510  biases separator  500  in a closed position. As separator  500  is in a closed position, fluid cannot flow into jar  505 . Because fluid cannot flow into jar  505 , jar  505  is effectively hydraulically locked, thereby preventing axial movement and preventing unintentional firing. Because jar  505  is stored at atmospheric pressure in the derrick, jar  505  stored in derrick between uses cannot unintentionally fire. 
         [0057]    Embodiments of the present disclosure may provide primary and secondary safety mechanisms for drilling jars. In certain embodiments, primary safety mechanisms may prevent axial translation of a mandrel within a jar, thereby preventing the jar from accidentally firing. In other embodiments, secondary safety mechanisms may prevent pressure from building within the detent, thereby passively preventing a jar from firing unless the jar is in the wellbore. Such primary and secondary safety mechanisms may allow drilling jars to be stored in a derrick with less risk of accidentally firing, as the jar may not be capable of building hydraulic pressure or axially translating a lower mandrel. 
         [0058]    Multiple primary and secondary safety mechanisms may be used on a single jar, thereby further increasing the safety of the jar. For example, in certain embodiments, a primary safety mechanism preventing axial movement of the lower mandrel may be used in the same jar as a secondary safety mechanism, such as a mechanism that prevent hydraulic pressure from building in the detent. Additionally, in certain embodiments, both active and passive safety systems may be used. For example, in certain embodiments an operator may be required to manually actuate an operating stem in addition to the jar having a secondary passive safety system, such as a system to prevent hydraulic pressure from building in the detent system. Those of ordinary skill in the art will appreciate that various combinations of the safety systems disclosed herein may be combined without departing from the scope of the present disclosure. 
         [0059]    Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from pressure lock for jars Accordingly, all such modifications are intended to be included within the scope of this disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.