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: 
FIELD OF THE INVENTION  
         [0001]    The invention relates to well bore explosive detonation tools and, in particular, to a hydraulic firing head for a downhole tool.  
         BACKGROUND OF THE INVENTION  
         [0002]    In subterranean well bores, firing heads are used to detonate downhole explosives. Explosives are used downhole in various tools including packers and perforating gun assemblies. In these tools, the firing head is driven to actuate an initiator to detonate explosive charges in the tool.  
           [0003]    One type of firing head is driven hydraulically. These hydraulic firing heads are generally conveyed on a tubing string and controlled by fluid pressure applied through a fluid column in the tubing string. The fluid can be a liquid or gas for example, compressed nitrogen or water. Pressure is applied from surface through the fluid column in the tubing string above the firing head, acting on a piston and attached assembly, which is secured by a number of shear pins. The shear pins are selected and built to shear at a known load. The number of shear pins used to secure the piston determines the pressure at which the head fires. Shearing the pins by applied pressure, drives a firing pin attached to the piston to strike the initiator, which transfers an explosive charge to the detonator in association with the main explosives of the tool.  
           [0004]    Once the explosives are detonated, it is sometimes useful to drain the hydraulic fluid from the tubing string, prior to tripping the tubing to surface. For this purpose, vents closed by sliding sleeves have been installed in the tubing string and in the firing head. However when vents have been included in the firing head, the pressures generated by evacuating fluid or gun detonation pressure tend to drive the piston to close the vents prior to complete draining of the tubing string. Snap rings have been used to lock the piston in a position away from vents. However, hydraulic firing heads often fail to allow complete draining of the tubing string.  
         SUMMARY OF THE INVENTION  
         [0005]    A downhole tool hydraulic firing head has been invented that includes an automatic drain including vents, the opening of which is controlled by movement of the piston. A locking collet in the firing head holds the piston down after firing, ensuring that venturi action or gun detonation pressure acting on the piston does not close off the vents.  
           [0006]    In accordance with a broad aspect of the present invention, there is provided a downhole tool hydraulic firing head comprising: a housing connectable into a tubing string and having a bore extending therethrough from its upper end to its lower end; a vent opening extending through a side wall of the housing, the vent opening being open to the bore; a piston in the bore, the piston being drivable by fluid pressure applied though the bore; a firing pin for activating detonation of an explosive charge, the firing pin connected to the piston to move with the piston; a sleeve on the piston and moveable therewith from a position covering the vent opening to a position clear of the vent opening; and a locking collet including a plurality of collet fingers with engaging lugs acting between the sleeve and the housing to lock the sleeve into the position where it is clear of the vent opening.  
           [0007]    The housing can be formed of one part or multiple interconnected parts, as desired. Manufacture, assembly and repair can be facilitated by forming the housing of multiple interconnected parts. The piston, sleeve, firing pin can be formed integral with each other or of separate parts secured to move together. Again, the use of separate secured parts can facilitate manufacture, assembly and repair.  
           [0008]    The sleeve can be the sidewall of the piston or a cylindrical extension of the piston. Preferably sealing means, such as O-rings are provided on the sleeve to seal against fluid passage through the vent opening when the sleeve is in position covering the vent opening.  
           [0009]    The locking collet is disposed to act between the sleeve and the housing and engages a shoulder. The locking collet and shoulder can be disposed directly on these parts or can be disposed on other parts secured to the housing and the sleeve. The collet can be connected to move with the sleeve, while the shoulder is formed in the housing or, alternately, the collet can be secured to the housing, while the shoulder is in association with the sleeve.  
           [0010]    In one embodiment, the locking collet is secured to the sleeve. Alternately, the locking collet can be secured to the piston or the firing pin, since both the sleeve and the firing pin move with the piston. Alternately the locking collet can be secured to another part, connected to at least one of the firing pin, the piston or the sleeve. Likewise, the shoulder can be formed directly on the housing or on parts secured within the housing.  
           [0011]    The piston can be releasably secured against movement unless a selected amount of fluid pressure is applied to the piston. In one embodiment, a shear pin is used to releasably secure the piston in this way. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will now be described, by way of example only, reference being made to the accompanying drawings in which:  
         [0013]    [0013]FIG. 1 is a vertical section of a hydraulic firing head according to the present invention in the run in position;  
         [0014]    [0014]FIG. 2 is a vertical section of the firing head of FIG. 1 with the collet locked under the collet shoulder;  
         [0015]    [0015]FIG. 3 is a vertical quarter section of a hydraulic firing head according to the present invention through which it is possible to circulate hydraulic fluid prior to driving the piston. The firing head is shown with the piston in the run in position, but with a ball seated therein in preparation for driving the piston to detonate the initiator; and  
         [0016]    [0016]FIG. 4 is vertical quarter section of a hydraulic firing head according to the present invention with an initiator sub attached therebelow. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIGS. 1 and 2, there is shown a downhole tool hydraulic firing head according to one aspect of the present invention. The firing head can be used in any downhole tool where it is desired to initiate an explosive charge by applying pressure to the firing head. The downhole tool can be, for example, a perforating gun assembly or a packer assembly. The firing head includes a tubular housing  10  including upper threads  12  for connection to a tubing string (not shown). Lower threads  14  provide for connection to the remainder of the downhole tool or the tubing string, such as the initiator sub  16  shown in FIG. 4, which will be described in greater detail hereinbelow.  
         [0018]    Tubular housing  10  includes an inner bore  18  extending from the housing upper end  10   a  to the housing lower end  10   b . Vent openings  20  extend from inner bore  18  to the housing outer surface. While three vent openings are shown, one or more vent openings can be provided about the circumference of the housing. In one embodiment, four vent openings are spaced about the circumference of the housing.  
         [0019]    A piston  22  is slidably disposed in bore  18  and is mounted to allow for axial movement in the bore in response to fluid pressure applied from the tubing string connected at the upper end  10   a  of the firing head. In particular, piston  22  is axially slidable between a run in position, shown in FIG. 1 and a firing position shown in FIG. 2. Piston  22  includes a face  24  against which the fluid pressure acts and a cylindrical sidewall  26  that closely fits within the bore. Sealing members  28  such as O-rings are mounted in glands on the cylindrical sidewall, creating a seal between bore  18  and piston  22  and ensuring that fluid pressure acts on the face  24  of the piston rather than bypassing the piston. As will be appreciated, the piston can assume forms other than the specific embodiment shown such as, for example, the piston body can be solid and/or the piston face can be disposed on the piston closer to its upper end.  
         [0020]    The piston acts as a sleeve within the bore to control the opening of vent openings  20 . In particular, cylindrical sidewall  26  is sized and configured to cover vent openings  20  when the piston is in the run in position (FIG. 1). Another arrangement of sealing members  30  seals between the bore and the piston below the vent openings, when the piston is disposed over the vent openings. As shown in FIG. 2, the piston can be moved such that the cylindrical sidewall is clear of the vent openings, thereby permitting fluid flow therethrough.  
         [0021]    In some embodiments, as illustrated in FIG. 3, it is desirable to circulate fluid through the tubing string above piston  22   a  and the annulus about the tubing string, without driving the piston to move within bore  18 . In such an embodiment, apertures  29  are formed through cylindrical sidewall  26   a  of the piston and positioned to be in fluid communication with vent openings  20  when the piston is in the run in position covering the vent openings. A circulating amount of fluid can pass through apertures  29  and out through the vent openings, without driving the piston to move within the bore. However, when it is desired to drive piston, a ball  31  is dropped from surface, which is sized to seat on piston  22   a  and create a seal therebetween. This seals against fluid flow through apertures  29  and when the pressure of hydraulic fluid the piston is increased, piston  22   a  with ball  31  seated thereon is driven down.  
         [0022]    A firing pin  32  is rigidly connected to piston  22  for movement therewith. Firing pin  32  can be connected in any way, for example by forming integral with, welding to etc., the piston. In the illustrated embodiment, firing pin  32  is secured in a bore  33  in an insert  34  that threads via threaded connection  36  into the rod side of the piston. This arrangement facilitates assembly and repair of the firing head and replacement of the firing pin. Firing pin  32  can be secured in numerous ways to insert  34  such as, for example, by a pin  38  secured between insert  34  and firing pin  32 , by weldments or threaded engagement. The pointed tip  40  of the firing pin extends out below the insert and into initiator sub  16 . In the illustrated embodiment, firing pin  32  is collapsible (as shown in FIG. 2), wherein when the firing pin strikes the initiator, pin  38  shears and the firing pin moves up into the bore. A port  41  between bore  33  and the outer surface of the insert permits equalization of pressure and collapsing of the firing pin.  
         [0023]    The piston is secured against axial movement in bore  18  by shear pins  42 . As will be appreciated, the shear pins are selected to shear at a known load, thereby permitting the piston to move axially within the bore. While shear pins  42  act between bore  18  of the housing and the piston, in the illustrated embodiment, the shear pins are connected between a ring  47  on insert  34  and a shear pin collar  44  mounted in the bore. The shear pins at one end engage in an annular groove  48  of ring  47 , which is secured by pin  38  to insert  34 . Of course, insert  34  could be formed to accept the shear pins, but provision of a separate ring facilitates repair and reuse of the assembly. At their other end, pins  42  are located in holes in the shear pin collar. As best seen in FIG. 4, shear pin collar  44  is retained against axial movement by a shoulder  48  that abuts against lower end  10   b  of the housing and by abutting at its end against a shoulder  50  on the initiator sub.  
         [0024]    A locking collet  52  is connected to insert  34  to move axially with piston  22 . In particular, locking collet  52  is engaged on a reduced diameter section of insert  34  and retained against axial movement on the insert by abutment between piston  22  and an enlarged lower section  34   a  of the insert. Locking collet  52  includes a plurality of collet fingers  54  which terminate in collet lugs  56 . Collet lugs  56  extend outwardly to be catchable under shoulder  58  on shear pin retaining collar  44 . Insert  34  includes an annular tapered section  60  adjacent fingers  54  which permit the fingers to flex inwardly to pass retaining collar  44 .  
         [0025]    Piston  22 , insert  34 , ring  47 , firing pin  32 , locking collet  52  and shear pin collar  44  can be assembled with pin  38  and shear pins  42  outside of housing and inserted into the bore in assembled form. In the bore, the assembly is held in place by threading initiator sub  16  onto lower threads  14 . This facilitates manufacture, assembly and repair of the firing head.  
         [0026]    As noted hereinbefore, housing  10  is threaded to initiator sub  16 . The initiator sub includes an initiator  70 , which is detonated when firing pin  32  strikes thereagainst.  
         [0027]    Numerous seals, for example, O-rings  72 ,  74  are provided to effect a fluid tight seal below the piston. Threads  78  on the lower end of the initiator sub are connectable to the remainder of the downhole tool such as, for example, the perforating guns.  
         [0028]    Operation  
         [0029]    The downhole tool hydraulic firing head of the present invention is assembled by connecting firing pin  32 , ring  47  and locking collet  52  to, insert  34 . The insert is then threaded into piston  22  and sealing members  28  and  30  are installed into the glands on the piston. Shear pin collar  44  is slid onto the ring and shear pins  42  are inserted through holes in the collar to extend into groove  46  about the ring. The number of shear pins is selected depending on the shear load of the shear pins used and the hydraulic pressure at which it is desired to drive the piston.  
         [0030]    The assembly of the piston, firing pin and collar  44  is then inserted into bore  18  of housing  10 . The assembly is introduced to the lower end of the housing until shoulder  48  butts against the housing. Initiator sub  16  is then threaded onto lower end  10   b  so that collar  44  is held against axial movement in the tool.  
         [0031]    The firing head and initiator sub are then connected through threads  12  to a tubing string having a bore in fluid communication with the upper portion of bore  18 . A lower string including the explosive charges to be detonated (not shown) is connected to threads  78  of initiator sub  16 . The explosive charges can be, for example, contained in a perforating gun.  
         [0032]    The hydraulic firing head and attached strings are then run in to a selected position wherein it is desired to detonate the explosive charges. When the hydraulic firing head is incorporated into a perforating gun assembly, the strings and perforating gun assembly are run in until the guns are adjacent the position where it is desirable to perforate the casing.  
         [0033]    In the run in position, shear pins  42  secure piston such that cylindrical sidewall  26  covers vent openings  20  and seals against fluid flow therethrough. In addition, firing pin  32  is spaced above but aligned for entry into initiator  70 .  
         [0034]    When the downhole tool is in position, fluid is introduced from surface to the tubing string and, thereby to bore  18 , until the pressure against face  24  exceeds the holding capacity of shear pins  42 .  
         [0035]    In the embodiment of FIG. 3 wherein the sleeve has apertures  29  therein for circulation of fluid therethrough, when the downhole tool is in position, ball  31  is dropped to seat on piston  22   a . Then fluid is introduced to the tubing string until the pressure against the ball exceeds the holding capacity of the shear pins.  
         [0036]    When pins  42  shear, piston  22  is driven down such that firing pin  32  is driven to detonate the initiator and, thereby, detonate the explosives. The firing pin can collapse into bore  33  of the insert, if it is of the collapsible-type.  
         [0037]    At the same time, the cylindrical sidewall moves clear of the vent openings, permitting fluid from the tubing string to drain therethrough. As piston moves down, collet fingers ride over shear pin collar  44  and flex inwardly into tapered section  60 . Once lugs  56  pass shoulder  58  of collar  44 , they flex out and catch under the shoulder. The engagement between lugs  56  and shoulder  58  is such that venturi action, caused by fluid evacuation through the vent openings, or detonation pressure from below piston is unable to disengage the collet lugs from under shoulder  58  and thus, the piston remains down clear of the vent openings.  
         [0038]    It will be apparent that many other changes may be made to the illustrative embodiments, while falling within the scope of the invention and it is intended that all such changes be covered by the claims appended hereto.

Summary:
A downhole tool hydraulic firing head includes an automatic drain that is opened when the piston is driven to fire. The automatic drain includes a vent opening from the bore and a sleeve disposed within the bore to move axially with the piston. The sleeve is moveable from a position covering the vent opening to a position clear of the vent opening. A locking collet is disposed between the sleeve and the housing, which locks the sleeve into the position, clear of the vent opening, when the sleeve is moved into that position. This locks the sleeve against returning to the position covering the vent opening when it is urged to move in that direction such as, for example, by venturi forces of the evacuating fluid or by pressure generated from the explosive charges. The firing head is useful to detonate downhole explosive charges, such as those in a perforating gun.