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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application is in the field of devices used in the oil well drilling industry, for releasably connecting one tubular element to another tubular element or piece of downhole equipment. 
     2. Background Art 
     In the drilling, completion, production, servicing, and workover of oil and gas wells, it is often necessary to disconnect the work string from a downhole tool, or from a lower section of work string. The downhole tool might include a fluid production device, a drill motor, or a drill bit, or any other bottom hole assembly which might be lowered into the well bore on a work string. Regardless of the type of downhole tool, selective disconnection from the work string may become necessary. 
     For instance, the bottom hole assembly in use may become stuck in the well bore to such an extent that it is impossible to remove from the well bore. In that case, the operator usually must selectively part the work string from the bottom hole assembly, and remove the work string from the well. Then, other tools can be run into the well bore for removal of the stuck bottom hole assembly. These other tools might be devices for grappling and pulling on the bottom hole assembly, or for jarring the bottom hole assembly loose, or even for milling the bottom hole assembly away. 
     It is helpful to have a tubular disconnect device in the work string at the desired disconnect location, to allow a positive and predictable release of the bottom hole assembly from the work string. The disconnect device should be impervious to the stresses and strains generated by the bottom hole assembly, and it should not be subject to inadvertent separation or loosening. The well bore environment also often includes the presence of varying amounts of debris, which is usually borne by the fluid being pumped down through the work string or up through the annulus surrounding the work string. A disconnect device should operate reliably despite the presences of such debris. 
     Various tubular disconnect devices have been developed over the years to achieve this disconnection of the work string from a downhole tool. Some such tools use locking dogs to lock the work string and the downhole tool together. Others may use a grappling device. In either case, the locking dogs or the grappling device are often held in the lock position by a movable piston, with the piston being held in place by a shear pin. After a ball is dropped through the work string, the piston can be displaced by the buildup of working fluid pressure, shearing the shear pin, with the piston being subsequently moved to a position where the locking dogs or the grappling device are no longer held in the lock position. This allows release of the bottom hole tool from the work string, usually by pulling on the work string. Often, in these tools, the bottom hole tool may generate, or be subject to, significant vibrations. These vibrations are often transmitted through the tool into the shear pin, causing it to fail prematurely, thereby inadvertently releasing the downhole tool from the work string. 
     Another tool which has been used as a tubular disconnect device utilizes a collet finger, or a plurality of fingers, to hold the tool to the work string. An upper tool body is locked to a lower tool body by a sliding collet, with the collet finger being held in a locking groove on the lower tool body by a contour on a lower extension of the upper tool body. The work string is pulled upwardly, raising the upper tool body against the force of a spring between the upper tool body and the collet. When the upper tool body has raised sufficiently, the collet finger is allowed to spring free of the groove in the lower tool body, thereby releasing the lower tool body from the upper tool body. This tool can be released only when the downhole tool is held in place with sufficient force to allow the necessary overpulling of the upper tool body to compress the spring. 
     Another known tool has a first set of collet fingers which lock the upper tool body to the lower tool body, with the collet fingers being held in the locked position by the lower skirt of an inner piston. The inner piston is held in attachment to the upper tool body by a spring and a second set of collet fingers. After a ball is dropped through the work string, pressure builds up above the inner piston until an outer piston is displaced upwardly by the same fluid pressure to further compress the spring. Upward displacement of the outer piston allows the second set of collet fingers to release the inner piston from the upper tool body, after which the inner piston is driven downwardly by fluid pressure to release the first set of collet fingers, thereby releasing the lower tool body from the upper tool body. The construction of this tool is complicated and expensive, and its proper function depends upon the spring to withstand the jarring load, to prevent the outer piston from displacing sufficiently to inadvertently release the lower body. 
     Still another known tool has a main piston which holds a set of locking dogs in place, to lock the upper tool body to the lower tool body, with the main piston being held in place by a ball and detent mechanism. A pilot piston holds the ball in the detent, preventing movement of the main piston, with the pilot piston being held in place by shear pins. Dropping of a ball through the work string and application of fluid pressure above the pilot piston shears the shear pins, allowing the pilot piston to release the ball from the detent, resulting in downward movement of the main piston to release the locking dogs. The jarring and impact of high frequency devices on the work string can impart repetitive impact to the shear pins, ultimately resulting in failure of the shear pins and inadvertent release of the tool. Further, the locking dogs of this tool are positioned in cavities that are open to drilling fluids; the particulates carried by the drilling fluid can pack the locking dog cavities sufficiently to immobilize the locking dogs. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a tubular disconnect device which has a collet held in place, relative to an upper body, by a set of outwardly biased collet fingers, or a shear pin and a set of collet fingers. The collet fingers are held in engagement with a groove by a movable piston, thereby preventing the application of any impact or force to the shear pin, where present. The collet holds a set of locking dogs in place, locking the upper body and a lower body together. The piston is biased upwardly away from the collet by a spring. Dropping of a ball through the work string allows application of fluid pressure above the piston, thereby compressing the spring. After sufficient compression of the spring and downward movement of the piston, the collet fingers are released, and the piston abuts the collet. Continued application of fluid pressure pulls the collet fingers out of their groove, and forces the collet downwardly, thereby freeing the locking dogs from engagement with the lower body, releasing the tool. 
     The collet can be sealed against the tool body to prevent contamination of the dog cavities with particulates in the drilling fluid. An adjustment sleeve can be provided to establish a rigid connection holding all the major body components in place, to prevent displacement during jarring operations. Where the shear pin is present, it maintains the collet in position after release of the collet fingers, until a higher fluid pressure shears the pin, giving the operator a positive indication that release of the tool has been achieved. However, the collet is actually held in place by the collet fingers, preventing application of impact to the shear pin. Since the collet fingers can not be released until the piston contacts the collet, the tool is highly resistant to inadvertent release. 
     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a longitudinal section of the apparatus of the present invention, in the run-in configuration; 
     FIG. 2 is a longitudinal section of the apparatus of the present invention, showing initial displacement of the piston to abut the collet, and release of the collet finger; 
     FIG. 3 is a longitudinal section of the apparatus of the present invention, showing initial displacement of the collet, to pull the collet finger out of its groove and to shear the shear pin; and 
     FIG. 4 is a longitudinal section of the apparatus of the present invention, showing further displacement of the collet, to release the locking dog from the dog cavity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a longitudinal section view of the disconnect device  10  of the present invention, in which a generally cylindrical upper tool body  12  has an upper end provided with threads for attaching the disconnect device  10  to a work string (not shown). The upper tool body  12  has a lower end which is threadedly engaged with the upper end of a generally cylindrical dog housing  14 , which in turn has a lower end which is slidably engaged with the upper end of a generally cylindrical lower body  16 . The terms “upper” and “lower” are used herein to mean essentially “uphole” and “downhole”, respectively. The disconnect device  10  can be used in a horizontal well bore, as well as a vertical well bore. The slidable engagement of the dog housing  14  with the lower body  16  can be by means of splines and grooves, as shown, to provide torsional strength. The lower end of the lower body  16  is provided with threads for attaching the disconnect device  10  to a bottom hole assembly or other downhole tool (not shown). 
     A transversely movable locking dog  18  is carried in a dog slot  19  in the dog housing  14 . The locking dog  18  is shown engaged with a dog cavity  20  on the interior surface of the lower body  16 , thereby longitudinally locking the dog housing  14  to the lower body  16 . The locking dog  18  is held in forcible engagement with the dog cavity by abutment with a raised contour  24  on the exterior surface of a generally cylindrical slidable collet  22 . One or more collet fingers  26  extend upwardly from the upper end of the collet  22 . A generally cylindrical collet sleeve  28  surrounds the upper end of the collet  22  and the collet fingers  26 . The upper end of the collet sleeve  28  abuts a shoulder on the upper body  12 , and the lower end of the collet sleeve  28  abuts a shoulder on the dog housing  14 , to hold the collet sleeve  28  longitudinally in place. 
     The upper end of each collet finger  26  has an outward projection which engages a recess  58  on the interior surface of the collet sleeve  28 . The collet fingers  26  can be outwardly biased to ensure that the fingers  26  engage the recess  58 . Forcible engagement of the collet fingers  26  with the recess  58  in the collet sleeve  28  provides the primary means of longitudinally capturing the collet  22  in place relative to the upper body  12 , thereby longitudinally capturing the collet  22  in place relative to the dog housing  14 . One or more shear pins  30  can be provided to pin the collet  22  to the dog housing  14 . Where provided, the shear pins  30  function as means of informing the operator that release of the tool has been achieved, as will be explained further below. Upper collet seals  32  and one or more lower collet seals  34  seal the annular cavity  56  between the collet  22  and the dog housing  14  and lower body  16  against contamination by drilling fluid, which may be laden with particulates. 
     A generally cylindrical slidable piston  36  is positioned within the upper body  12 , generally above the collet  22 . The slidable piston  36  is shown in its initial position, or run-in Q position, in FIG.  1 . The piston  36  has an outward projection  38  which abuts the upper ends of the collet fingers  26 , to hold the collet fingers  26  in forcible engagement with the recess  58  in the collet sleeve  28 , when the piston  36  is in its initial position. In the condition shown in FIG. 1, the internal bore of the piston  36  is open, allowing the flow of fluids through the piston  36 , and on through the internal bore of the remainder of the disconnect device  10 . A spring  40  is positioned between the piston  36  and the upper end of the collet  22 , to bias the piston  36  upwardly. This initial position of the piston  36  is also its uppermost position, since the outward projection  38  abuts an internal shoulder on the upper end of the collet finger  26 . Further, in this initial position, the lower end  44  of the piston  36  is vertically spaced apart from an internal shoulder  46  on the collet  22 . The upper end  42  of the piston  36  has an internal seat  50  for receiving a ball to be dropped through the work string, as will be explained below. 
     An adjustment sleeve  48  on the exterior of the disconnect device  10  is threadedly engaged with the exterior surface of the dog housing  14 . The lower end of the adjustment sleeve  48  abuts the upper end of the lower body  16 . When the adjustment sleeve  48  is threaded in the downward direction, it applies downward force against the lower body  16  and upward force against the dog housing  14 . The lower body  16  in turn applies downward pressure against the dog  18 , which then reacts downwardly against the dog slot  19  in the dog housing  14 . Therefore, it can be seen that adjustment of the adjustment sleeve  48  will apply a desired tension to the dog housing  14 , to remove any looseness or slack in the assembled dog housing  14  and lower body  16 . The dog housing  14  is itself threaded to the upper body  12 , so there is no looseness in the major body components of the disconnect device  10 , thereby minimizing the impact which can be imparted to the collet fingers  26  and the shear pin  30 . 
     An overpressure device  52 , such as a rupture disk, is provided between the internal bore of the lower body  16  and the annular space surrounding the lower body  16 , below the collet  22 . This allows the operator to overpressurize the internal bore to establish a flow path to the annulus. A fluid bypass device  54 , such as a weep valve, is provided between the internal bore of the lower body  16  and the annular space surrounding the lower body  16 , above the lower seal  34  in the lower end of the collet  22 . An enlarged internal diameter in the lower body  16  can be provided adjacent the weep valve  54  to place the weep valve  54  in fluid flow communication with the annular space  56 , between the collet  22  and the dog housing  14  and the lower body  16 , as shown. The weep valve  54  prevents the annular space  56  from overpressurizing and locking the tool against release. 
     As seen in FIG. 2, when it is desired to release the disconnect device  10 , a ball B is dropped through the work string to seat on the seat  50  at the upper end  42  of the piston  36 . Fluid being pumped through the work string then builds pressure above the piston  36 , driving the piston  36  downwardly and compressing the spring  40  until the lower end  44  of the piston  36  abuts the shoulder  46  on the collet  22 . It can be seen that, at this position of the piston  36 , the outward projection  38  on the piston  36  has moved downwardly away from its abutment with the upper end of the collet finger  26 , thereby releasing the collet finger  26  to be pulled out of engagement with the recess  58  in the collet sleeve  28 . Therefore, at this point, the collet  22  is no longer captured or locked longitudinally relative to the upper body  12  and the dog housing  14 . 
     As seen in FIG. 3, as the piston  36  is driven further downwardly by hydraulic pressure, abutment of the piston  36  with the collet  22  drives the collet  22  downwardly, pulling the upper ends of the collet fingers  26  out of the recess  58  in the collet sleeve  28 . Recall that the collet fingers  26  were released to be pulled out of the recess  58  by the initial downward movement of the piston  36 , described above. The weep valve  54  allows fluid to escape the annular space  56  as the collet  22  moves downwardly. Simultaneously, the shear pin  30 , when present, is sheared, to give the operator a noticable pressure drop to provide positive indication that the collet  22  has moved downwardly. 
     As the piston  36  and the collet  22  continue to be driven downwardly by hydraulic pressure, as shown in FIG. 4, the raised external contour  24  on the collet  22  moves below the dog  18 . This releases the dog  18  from forcible engagement with the dog cavity  20  in the lower body  16 . Downward movement of the piston  36  and the collet  22  can continue until an external shoulder  60  on the collet  22  abuts an internal shoulder  62  on the lower body  16 , or until the collet  22  abuts the upper end of the dog  18 . Once the dog  18  has been released from forcible engagement with the dog cavity  20  in the lower body  16 , the upper body  12  and the dog housing  14  are free to move longitudinally relative to the lower body  16 . Therefore, the work string, the upper body  12 , the dog housing  14 , the collet  22 , and the piston  36  can be pulled out of the well bore, leaving the lower body  16  and any equipment attached therebelow in the well. The lower body  16  can be provided with a grappling contour, as shown, to facilitate its removal from the well bore with a grapple. 
     While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.

Summary:
An apparatus for disconnecting a bottom hole assembly from a work string, having an upper tool body, a piston biased away from a slidable collet, locking dogs held in engagement with a lower tool body by the slidable collet, and one or more collet fingers holding the collet in place, relative to the upper body. The piston can be hydraulically displaced against the biasing spring, thereby releasing the collet finger, followed by further displacement of the piston to contact the collet, then displacement of the piston and the collet to release the locking dogs from engagement with the lower tool body. A shear pin in the collet can provide a positive pressure indication of release of the disconnect device.