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CROSS-REFERENCE TO RELATED APPLICATION 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to equipment and processes for petroleum well completion and production. In particular, the invention pertains to well perforation equipment and processes for mineral production enhancement. 
     SUMMARY OF THE INVENTION 
     Well perforation is a term that is used in the well drilling arts to describe a process of perforating the walls of a fluid mineral production pipe with multiple apertures. Such apertures are clustered in a predetermined pattern within the geologic zone of a fluid mineral bearing strata to facilitate flow of the mineral into the production pipe and up to the surface. The term is also applied to the process of perforating a well casing and cement collar for the same purpose. The well perforation process may also be used to fracture or open a geologic formation along the production face of the well bore wall. 
     Perforation equipment traditionally includes a length of tube or pipe, called a “perforating gun”, having shaped charge explosives set in a helical pattern along the length and around to perimeter of the tube. These shaped charges are designed and aligned to emit a lineally concentrated stream of explosion gas radially from the gun axis. A gun may comprise a multiplicity of such tubes mounted with shaped charges connected in series, end-to-end. 
     Perforating guns are suspended within a well for placement at the desired downhole perforation position by suspension structure such as a string of tubing or wireline. The gun is secured to the end of the tubing string or wireline. 
     Traditional perforating gun assembly usually provides a firing head at the upper end. The firing head is a subsection of tube that contains those devices necessary to convert an action or impulse initiated at the well surface into an explosive detonation. Included among such actions is dropping a weight such as a rod for example, along the bore of the suspension pipe or tubing to impact a percussion primer in the firing head. The primer initiates a detonation cord which carries a detonation wave to all of the shaped charges, sequentially. In another case, the firing head may comprise an exploding bridge wire (EBW) or exploding foil initiator (EFI) detonator as described by. U.S. Pat. No. 5,347,429 for example. The EBW or EFI is initiated by an electric pulse carried on an electric conductor integrated with the wireline support structure to initiate the detonation cord. 
     There are certain circumstances and conditions in the art of deep well drilling for petroleum, under which a perforating gun is disconnected from the supporting wireline or tubing string upon discharge of the perforation charges. When disconnected, the gun is completely severed from any surface linked support and allowed to fall further along the wellbore; usually below the mineral production zone. Often, the depleted gun is abandoned in an extended portion of the wellbore. 
     The sequence of such a well perforation gun disconnect procedure has, traditionally been perceived in the order of a gun discharge followed by the gun disconnect. However, this sequence has been found to be less than satisfactory due to the consequences of detonation shock when the gun discharges. Prior art disconnect devices have been mechanical latching assemblies subject to structural distortion or destruction caused by detonation shock. Hence, a planned disconnect procedure may not be possible after the gun has discharged. 
     It is an object of the present invention, therefore, to provide equipment and procedures to disconnect a perforating gun from surface support structure prior to discharge of the explosive perforating charges. 
     It is also and object of the present invention to provide a disconnect sub between a firing head and gun tube having no mechanical latches or links subject to operational binding or warping. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and further features of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout. 
         FIG. 1  is a section of perforating gun firing assembly relevant to the present invention. 
         FIG. 2  is a cross-sectional detail of the present drop sub. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and downwardly”, “upstream” and “downstream”; “above” and “below”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or other relationship as appropriate. Moreover, in the specification and appended claims, the terms “pipe”, “tube”, “tubular”, “casing”, “liner” and/or “other tubular goods” are to be interpreted and defined generically to mean any and all of such elements without limitation of industry usage. 
       FIG. 1  represents the essential invention environment showing a partial perforating gun assembly including a firing head  10 , a shaped charge pipe cutting sub often interchangeably characterized in the art as a “drop sub”  12  and a perforating gun tube  14 . These are independently severable units that may be unitized by integrally threaded box and pin joints. Such an assembly may be suspended into a well bore by wireline, coiled tubing or pipe joints. 
     In traditional practice, the perforating gun  14  is a tube having a multiplicity of explosive perforating charges  16  set within the tube shell to discharge radially against a production pipe, well casing or raw bore wall. The objective of such perforations is to facilitate the flow of in situ fluids, gas or crude petroleum, from a fluid bearing earth stratum into an inner flow chamber. These explosive charges  16  are often set in a helical pattern around and along the length of the gun tube. While numerous methods have been used to detonate such perforating charges, a continuous detonating cord  28  ( FIG. 2 ) running the length of the gun tube is representative. The cord  28  is connected in proximity to detonation boosters respective to each charge  16  for detonating each of the charges sequentially as the detonation wave carried by the cord progresses along the gun length. 
     Detonating cord  28  ignition may be initiated from the surface by a percussion primer or by an exploding bridge wire (EBW) exploding foil initiator (EFI), for example. In the case of the percussion detonator, a percussion weight is dropped into a gun supporting coiled tube or pipe string. The percussion weight falls along the tubing bore to impact a percussion detonator. In the case of an EBW or EFI, an electric pulse is transmitted by wireline conduit to initiate the detonation. The conduit is often integrated into the windings of a wireline support string. 
     With respect to  FIG. 2 , the drop sub  12  comprises a tubular housing having box threads  24  at one distal end to receive meshing assembly pin threads on a firing head connector  20 . O-rings  23  seal the threaded connector joint from moisture invasion. Pin threads  21  secure the connector  20  to the firing head  10 . Firing head O-rings (not shown) seal against the sealing groove  22 . A bore  25  along the connector  20  axis accommodates detonating cord  28 . 
     A gun connector  30  is secured to the opposite end of the drop sub housing by assembly pin threads  33  meshing with box threads in the drop sub housing. O-rings  34  seal this joint from moisture invasion. Threads  31  secure the gun  14  to the connector  30  with an O-ring moisture seal  32 . A bore  35  along the connector  30  axis facilitates continuity of the detonation cord  28  through the gun connector  30  into the gun  14 . 
     A chamber  42  in the drop sub  12  receives a shaped charge tubing cutter  40  in alignment with a cutting window  44  to provide an explosive cutting plane that is substantially transverse of the housing. The cutting window  44  is a reduced thickness section of pipe wall annulus. 
     The tubing cutter explosive  40  is compressed between top and bottom end plates having an axial aperture to accommodate the detonating cord  28 . Between the end plate bosses  48 , the cutter explosive  40  is in direct face exposure to the detonating cord  28 . 
     When the gun  14  is located at a desired position along the wellbore, an initiating event is performed to begin the detonation sequence by activating the firing head  10 . The firing head  10  ignites the detonation cord. 
     As a detonation wave travels the cord  28  length, the tube cutting charge  40  is discharged first thereby severing the gun  14  from the firing head  10  and any connection with the surface. However, the detonation wave continues along its length through the gun, detonating the perforation charges  16  along the way. 
     The detonation wave traverses the length of cord  28  in a few microseconds. However, the gravity driven acceleration of the gun  14 , comparatively, is much slower. Hence, although the structural link  12  between the gun and any up-hole support from the firing head has been severed, the perforation charges will have discharged and performed their intended function before any significant gun movement. 
     Although the invention disclosed herein has been described in terms of specified and presently preferred embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.

Summary:
A perforating gun assembly comprises a drop sub linking the firing head to the gun tube or tubes. The drop sub includes a 360° perimeter shaped charge explosive for severing pipe having an axial bore to accommodate a pass-through of the perforation charge detonation chord. Consequently, the severing tool is detonated by the traverse of a detonation wave along the chord length to disconnect the perforating gun prior to detonation of the perforation charges.