You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to jarring tools used in coiled tubing operations. 
         [0003]    2. Description of the Related Art 
         [0004]    Hydraulic jars are used to help jar loose or unstick an object in a wellbore. In milling operations, a jar is often incorporated into the running string above the milling bottom hole assembly. Jars are tools which generally include an inner central mandrel and an outer housing. These components are telescopically moveable with respect to each other. A spring is used to store energy within the jar device which can be released by actuation of a valve within the jar device. Bi-directional jars are capable of causing jarring impacts in two axial directions. A bi-directional jar device is described in U.S. Pat. No. 6,712,134 (“Modular Bi-Directional Hydraulic Jar with Rotating Capability”) issued to Stoetz. U.S. Pat. No. 6,712,134 is owned by the assignee of the present invention and is hereby incorporated by reference. 
         [0005]    In conventional use, a jar device is run into a wellbore in a neutral position. Roughly half of the length of the mandrel is exposed when the jar is in its neutral position. The inventor has recognized that an exposed length of mandrel subjects the jar device to becoming hung up on obstacles within the wellbore. In addition, the inventor has recognized that jar devices occasionally inadvertently create jarring forces during run-in, which can cause undesirable damages to well components. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides hydraulic jar devices which are releasably locked in an initial configuration to prevent inadvertent operation during run-in and which will eliminate exposed lengths of mandrel. The releasable lock feature may be incorporated into either bi-directional or single-directional jar devices. The releasable lock functions to isolate the jar actuation mechanism against inadvertent operation, or firing. The first and second telescopic portions of the jar device are releasably secured against axial movement with respect to each other by a releasable locking mechanism. In preferred embodiments, a mandrel sleeve radially surrounds the exposed mandrel portion of the jar device. The mandrel sleeve is also affixed to each of the first and second telescopic portions of the jar device to preclude axial movement of the first and second portions. 
         [0007]    At least one of the first and second telescopic portions of the jar device is releasably secured to the mandrel sleeve. In a described embodiment, the mandrel sleeve is fixedly secured to the first telescopic portion of the jar device and releasably secured to the second telescopic portion of the jar device. 
         [0008]    In a described embodiment, frangible members, such as shear screws, are used to secure the mandrel sleeve to the second telescopic portion until the jar device is ready to be actively used. Preferably, the jar device is secured in a fully contracted position. When it is desired to make the jar device active for use in creating and applying jarring forces, a tensional force of predetermined level, or overpull, is applied to the running string to shear the frangible members. 
         [0009]    In an exemplary method of operation, the jar device is run into a wellbore on a work string. If the work string becomes stuck within the wellbore below the jar device, an operator will exert a pulling force upon the work string which is sufficient to release the releasable locking mechanism. Thereafter, the jar device can be operated normally to create jarring impacts within the wellbore. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, wherein like reference numerals designate like or similar elements throughout the several figures of the drawings and wherein: 
           [0011]      FIG. 1  is a side, cross-sectional view of an exemplary wellbore which contains a milling arrangement having a jar device in accordance with the present invention. 
           [0012]      FIG. 2  is a side, partial cross-sectional view of an exemplary jar device constructed in accordance with the present invention, the jar device being in an initial run-in position. 
           [0013]      FIG. 3  is an enlarged side, partial cross-sectional view of the releasable lock portion of the jar device. 
           [0014]      FIG. 4  is an enlarged side, partial cross-sectional view of the releasable lock portion of the jar device with its telescoping portions released. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]      FIG. 1  depicts a wellbore  10  which has been drilled from the surface  12  down through the earth  14 . A plug  16  is shown which was previously set within the wellbore  10 . It is desired to remove the plug  16  by milling. 
         [0016]    A milling arrangement, generally indicated at  18 , is disposed within the wellbore  10  to remove the plug  16  by milling. The milling arrangement  18  includes a running string  20  which is preferably formed of coiled tubing of a type known in the art. A milling bottom hole assembly  22  is located at the distal end of the milling arrangement  18 . The milling bottom hole assembly  22  typically includes a mud motor which rotationally drives a milling bit. 
         [0017]    At surface  12  are located a number of devices which are used to operate the milling arrangement  18 . A coiled tubing injection mechanism (not shown) of a type known in the art is used to inject the coiled tubing into the wellbore  10 . Draw works are means to manipulate and pull on the coiled tubing running string  20 . A fluid pump  24  is also located at the surface  12  to flow fluid through the coiled tubing running string  20  to operate the milling bottom hole assembly  22 . 
         [0018]    A jar device  26  is also incorporated within the milling arrangement  18  above the milling bottom hole assembly  18 . The jar device  26  is functional to create and apply jarring impact forces to the milling bottom hole assembly  22 . As a result, the jar device  26  is useful in situations where the milling bottom hole assembly  22  becomes stuck or hung up in the wellbore  10 . 
         [0019]    The jar device  26  is preferably a bi-directional jar device which is capable of generating and applying jarring forces in opposing axial directions (i.e., “firing” in opposing directions). In other words, a bi-directional jar device can both generate and apply a downward jarring force within the wellbore as well as an upward jarring force with the wellbore. Suitable bi-directional jar devices for use with this invention include the NOV TerraForce bidirectional jar. However, other jar devices may be used as well. 
         [0020]    Although the particular jar device  26  is shown within a milling arrangement  18 , it should be understood that the invention is not limited to situations wherein milling is conducted and that this situation is described for illustrative purposes only. In fact, jar devices such as jar device  26  can be used in other situations, such as fishing operations to contact and apply jarring forces to an object which has become stuck in the wellbore. 
         [0021]    The interior workings of jar device  26  are not described in detail herein. Generally, jar devices convert the strain energy of a pulling force or a pushing force into kinetic energy as a hammer is caused to strike an anvil. Jar devices are typically self-contained in that they do not require a flow of liquid or air from surface in order to operate. It is noted that the jar actuation mechanism  27  may be the same as that described in U.S. Pat. No. 6,712,134 issued to Stoetz. However, other known jar designs might also be used, such as hydraulic devices which incorporate fluid reservoirs that are disposed annularly between the telescoping portions of the jar device. In this type of jar design, hydraulic oil passes through a small orifice to resist movement of the telescoping portions. The passage is formed within a moveable piston which isolates two annular fluid reservoirs as a pulling force is applied to the telescoping portions. Eventually, the moveable piston with orifice clears a narrow passage which allows oil to rush around it and allow the telescoping portions to contact each other to deliver a hammer blow to an anvil. 
         [0022]    Referring now to  FIGS. 2-4 , it can be seen that exemplary jar device  26  generally includes a first telescopic portion  28  having an outer housing  30  and a first threaded end connection  32  in order to connect the jar device to the running string  20 . The outer housing  30  encloses the jar actuation mechanism  27  for the jar device  26 . A second telescopic portion  34  is disposed in a nested manner within the outer housing  30 . The second telescopic portion  34  features a reduced diameter mandrel  36  and an enlarged diameter connection sub  38 . The enlarged diameter connection sub portion  38  has a larger diameter than the mandrel  36  presents a second threaded end connection  40  which is shaped and sized to connect to the milling bottom hole assembly  22 . It is noted that mandrel  36  slides in and out of the outer housing  30  of the first telescopic portion  28  during actuation of the jar device  26 . 
         [0023]    In an initial run-in position, which is illustrated in  FIGS. 2 and 3 , a releasable lock, generally indicated at  42 , which secures the first and second telescopic portions  28 ,  34  against relative movement. This releasable lock isolates the actuation mechanism of the jar device  26  against inadvertent firing. The releasable lock  42  in the depicted embodiment includes a mandrel sleeve  44  which radially surrounds the mandrel  36 . Mandrel sleeve  44  is shown in a transparent manner in  FIG. 2 . The mandrel sleeve  44  is secured to the first telescopic portion  28  by threaded connection  46 . The enlarged diameter connection sub  38  has a reduced diameter neck portion  48 . It is noted that conventional jar devices can have the neck portion  48  created by milling away material of the connection sub portion  38 . Preferably, an annular locking groove  50  is formed in the neck portion  48 . 
         [0024]    One or more frangible shear members  52  are disposed through the mandrel sleeve  44  and into the locking groove  50 . The one or more frangible shear members  52  are preferably shear screws which are designed to rupture at a predetermined shear force. 
         [0025]    According to an exemplary method of operation, the milling arrangement  18  is run into the wellbore  10  to remove plug  16  by milling. If the milling bottom hole assembly  22  becomes stuck in the wellbore  10 , an operator at surface  12  can unlock the jar device  26  to become active by pulling upwardly on the running string  20  with sufficient force to shear or rupture the frangible members  52 . When the frangible members  52  are ruptured, the jar device  26  will be moved to the unlocked position shown in  FIG. 4 . In the unlocked position, the jar device  26  can be actuated normally to create jarring impacts, as needed. 
         [0026]    It is noted that the jar device  26  could be used with any wellbore work string, including a fishing arrangement. The mandrel sleeve  44  also radially surrounds the mandrel  36  to prevent the mandrel  36  from being exposed to debris or obstructions within the wellbore  10 . Preferably, the mandrel sleeve  44  will be shaped and sized to have substantially the same outer diameter as the outer housing  30  of the first telescopic portion  28  and the connection sub  38  of the second telescopic portion  34  so that there are no discontinuities in the outer surface of the jar device  26  which might cause hang ups. 
         [0027]    Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.

Summary:
A jar device is provided with a releasable locking mechanism which secures the telescopic portions of the jar device from relative movement until released. The locking mechanism is released by a tensional force which ruptures one or more frangible members.