Abstract:
A shaped charge pipe cutter is constructed with the cutter explosive material packed intimately around an axially elongated void space that is continued through a heavy wall boss portion of the upper thrust disc. The boss wall is continued to within a critical initiation distance of a half-cuter junction plane. An explosive detonator is positioned along the void space axis proximate of the outer plane of the upper thrust disc. Geometric configurations of the charge thrust disc and end-plate concentrate the detonation energy at the critical initiation zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to co-pending U.S. patent application Ser. No. 13/506,691, titled “Shaped Charge Tubing Cutter,” filed on May 10, 2012, the disclosure of which is herein incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to shaped charge tools for explosively severing tubular goods including, but not limited to, pipe, tube, casing and/or casing liner. 
         [0005]    2. Description of Related Art 
         [0006]    The capacity to quickly, reliably and cleanly sever a joint of tubing or casing deeply within a wellbore is an essential maintenance and salvage operation in the petroleum drilling and exploration industry. Generally, the industry relies upon mechanical, chemical or pyrotechnic devices for such cutting. Among the available options, shaped charge (SC) explosive cutters are often the simplest, fastest and least expensive tools for cutting pipe in a well. The devices are typically conveyed into a well for detonation on a wireline or length of coiled tubing. 
         [0007]    Typical explosive pipe cutting devices comprise a consolidated wheel of explosive material having a V-groove perimeter such as a V-belt drive sheave. The circular side faces of the explosive wheel are intimately formed against circular metallic end plates. The external surface of the circular V-groove is clad with a thin metal liner. An aperture along the wheel axis provides a receptacle path for a detonation booster. 
         [0008]    This V-grooved wheel of shaped explosive is aligned coaxially within a housing sub and the sub is disposed internally of the pipe cutting subject. Accordingly, the plane that includes the circular perimeter of the V-groove apex is substantially perpendicular to the pipe axis. 
         [0009]    When detonated at the axial center, the explosive shock wave advances radially along the apex plane against the V-groove liner to drive the opposing liner surfaces together at an extremely high velocity of about 30,000 ft/sec (9,000 m/sec). This high velocity collision of the V-groove liner material generates a localized impingement pressure within the material of about 2 to 4×106 psi (731 kPa). Under pressure of this magnitude, the liner material is essentially fluidized. 
         [0010]    Due to the V-groove geometry of the liner material, the collision reaction includes a lineal dynamic vector component along the apex plane. Under the propellant influence of the high impingement pressure, the fluidized mass of liner material flows lineally and radially along this apex plane at velocities in the order of 15,000 ft/sec (4,600 m/sec). Resultant impingement pressures against the surrounding pipe wall may be as high as 6 to 7×106 psi (731 kPa) thereby locally fluidizing the pipe wall material. 
         [0011]    Traditional fabrication procedures for shaped charge pipe cutters have included an independent formation of the liner as a truncated cone of metallic foil. The transverse sections of the cone are open. In a forming mold with the liner serving as a bottom wall portion of the mold, the explosive is formed or molded against the concave conical face of the liner. At the open center of the truncated apex of the liner, the explosive is formed against the mold bottom surface and around a cylindrical core. 
         [0012]    With the precisely desired explosive material in place, an end plate is aligned over the cylindrical core and pressed against the upper surface of the explosive material at a controlled rate and pressure in the manner of a press platen. When removed from the forming mold, the unified liner-explosive-backing plate comprises half of a shaped charge pipe cutter. 
         [0013]    To complete a full cutter unit, two of the shaped charge half sections, separated from the cylindrical core mold, are joined along a common axis at a contiguous juncture plane of exposed explosive at the truncated apex face planes. A detonation booster is inserted along the open axial bore of the unit left by the molding core. This detonation booster traverses the half charge juncture plane to bridge the explosive charges respective to the two half sections between the opposing end plates. The charged cutter is inserted into a cutter housing that is secured to a cutter sub. 
         [0014]    A notable characteristic of secondary order explosives of the type used in shaped charge cutters such as RDX and HMX is that the detonation velocity roughly corresponds to the compression density of the charge. A greater charge density generally increases the detonation velocity. Hence, more densely compressed charges, generally, are more energetic, emit greater velocity jets and generate greater cutting pressure. However, another characteristic of densely compressed high explosives is a greater difficulty to detonate. It has been a general rule of practice, therefore, that more densely compressed, energetic charges require larger, more intimately positioned ignition boosters. 
         [0015]    Larger boosters, for more densely compressed explosives, introduce other complications to the downhole tubing cutter design. Larger boosters require larger diameter axial apertures in the cutter explosive geometry thereby reducing the available volume within the explosive material envelope for high explosive material. 
         [0016]    It must be recognized that for a given nominal pipe size, there is a corresponding inside diameter. A cutter housing, meaning the housing outside diameter, must fit loosely within the inside diameter of the pipe that is to be cut. The outside diameter of the cutter explosive wheel must fit within the housing and the outside diameter of the explosive wheel substantially dictates the depth of the liner V-groove. 
         [0017]    As the dimensional restrictions progress radially inward, a final distance absolute arises between the inside diameter wall of the booster aperture and the V-groove apex. The radial depth of this annular plane between the V-groove apex and the aperture wall is characterized as the “induction” distance. If insufficient, the explosive detonation will not decompose the liner material into a lineal cutting jet. There is advantage, therefore, for using the smallest diameter booster (and, hence, aperture diameter) that will reliably detonate the cutter charge. 
         [0018]    International standards of transportation safety (UN Recommendations on the Transport of Dangerous Goods, Section 16) require that high order explosives such as HMX and RDX are packaged in a manner to promote deflagration rather than explosion upon uncontrolled heating as in an accidental fire. In general, compliance with this regulation precludes any sealed enclosure or confinement of the cutter explosive. If heated, an unconfined explosive will simply out-gas and burn. If the explosive is confined, however, the gas may develop sufficient pressure to initiate a detonation. Hence, in the interest of safety, there should be a gas venting route in any transport packaging. 
         [0019]    To comply with these safety requirements, shape charge cutter equipment is therefore transported to a job site in various degrees of disassembly. 
         [0020]    Unfortunately, the environmental circumstances of a drilling rig floor, which is where final cutter assembly must occur, are often hostile and usually not conducive to the attentive care required for final assembly of a high explosive tool. Hence, there are strong incentives to transport a cutter unit to the job site in the greatest degree of assembly that safety, prudence and regulation allow. 
         [0021]    A representative cutter assembly usually requires the shaped charge explosive to be positioned within an environmental housing which is atmospherically open and unsealed for transport. When finally assembled for downhole placement and detonation, an explosive booster charge is positioned in the axial aperture through the explosive cones. The cutter housing is secured to a top sub which seals the housing enclosure. The housing and top sub are secured to a firing head having an electrically initiated detonator and a capacitive discharge circuit. Upon final assembly for downhole placement and detonation, the housing, top sub and firing head are secured together as a firing unit. When assembled, the detonator is physically positioned in ignition proximity to the booster and the combination of housing, top sub and firing head is totally sealed from the environment outside the housing wall. In process sequence, surface signals prompt a capacitive discharge circuit to electrically discharge into the detonator. The detonator discharge initiates the booster within the axial aperture proximate of the explosive cone interface. The booster ignition detonates the explosive cutter cones. 
         [0022]    Each of these firing unit assembly joints is hydraulically sealed by an O-ring. As normally fabricated, however, there is an open channel space along the axis of the assembled unit. Consequently, the opportunity exists at each of the assembly joints for external pipe bore fluid to enter the open channel space and corrupt the shaped charge explosive in the event of O-ring seal failure. This opportunity is exacerbated by rough or poorly machined seal surfaces. 
         [0023]    The mechanics of O-ring sealing includes a pressure differential induced distortion of the polymer material from which the O-ring is made. Under a high pressure differential, these principles are extremely reliable. Under a low pressure differential, a fluid tight seal is much more problematic if the seal surfaces are roughly machined or corrupted by deposits. If the pressure differential upon the O-ring is insufficient to force-flow the O-ring polymer material into intimate sealing contact withal of the sealing surface, fluid will by-pass the seal and enter the forbidden zone. For explosive tools such as shaped charge cutters and perforators, such low pressure leakage may be disabling. 
         [0024]    Curiously, in a deep well environment, a tool with a low pressure leak may ultimately acquire a complete seal as the tool descends into realms of greater pressure. 
         [0025]    To further simplify the job site assembly task, it would be helpful, therefore, to eliminate the need for an explosive booster thereby initiating the cutter explosion only by the firing head detonator. It would also be helpful to provide an internal fluid seal means along the internal channel of the assembly firing unit. 
         [0026]    Over years of experience, use and experimentation, the explosion dynamics of shaped charge cutters has evolved dramatically. Some prior notions of critical relationships have been revealed as not so critical. Other notions of insignificance have been discovered to be of great importance. The summation of numerous small departures from the prior art traditions has produced significant performance improvements or significant reductions in fabrication expense. 
       BRIEF SUMMARY OF THE INVENTION 
       [0027]    The present invention pipe cutter comprises several design and fabrication advantages that include a half cutter fabrication procedure that compresses the explosive material intimately around an axially centered core mandrel to form an axial aperture that is continued through an end plate characterized herein as an upper thrust disc and a lower end plate. In this embodiment of the invention, a charge detonator is positioned along the tool axis adjacent the outer surface plane of the thrust disc. There is no need for an independently prepared booster or booster material that is an article separate from the thrust disc. Although the axial aperture remains as an essential cutter element, the diameter of the aperture may be significantly reduced. The charge detonator initiates the cutter explosive charge from a plane substantially common with outer surface plane of the thrust disc. While the detonation initiation point is axially displaced from the half cutter junction plane, the detonation energy wave is propagated along an aperture within a heavy wall boss to a critical initiation distance adjacent the junction plane. The heavy wall of the boss protects the cutter explosive from asymmetric detonation as the energy wave travels along the aperture to the juncture plane. 
         [0028]    Another, similar embodiment of the invention provides a dense material plug in the lower end plate aperture to reflect the detonation wave back upon itself at the juncture plane. As before, the heavy wall of the boss protects the cutter explosive from asymmetric detonation as the energy wave travels along the aperture. 
         [0029]    Another invention embodiment provides a tapered wall for the upper thrust disc aperture. The taper angle of the aperture converges from the exterior surface of the upper backing plate (thrust disc) toward the cutter explosive at about 5° from the tool axis. The small, terminus end of the aperture coincides with the upper plane of the critical ignition space above the half-cutter junction plane. 
         [0030]    Also featured by the present invention is a fluid seal element between the open channel along the firing unit and the interior volume of the cutter housing to reliably prevent the migration of moisture into the cutter housing due to leaks into the open channel from faulty seals above the cutter housing. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0031]    The invention is hereafter described in detail and with reference to the drawings wherein like reference characters designate like or similar elements throughout the several figures and views that collectively comprise the drawings. Respective to each drawing figure: 
           [0032]      FIG. 1  is a cross-section of a first embodiment of the invention in assembly with the housing, centralizer and connecting sub. 
           [0033]      FIG. 2  is a cross-section of a second embodiment of the invention in assembly with the housing, centralizer and connecting sub. 
           [0034]      FIG. 3  is a cross-section of a third embodiment of the invention in assembly with the housing, centralizer and connecting sub. 
           [0035]      FIG. 4  is a cross-section of a fourth embodiment of the invention in assembly with the housing, centralizer and connecting sub. 
           [0036]      FIG. 5  is a plan view of an end plate showing marker pocket borings. 
           [0037]      FIG. 6  is a cross-section view of an end plate along cutting plane  6 - 6  of  FIG. 5 . 
           [0038]      FIG. 7  is a bottom plan view of a top sub after detonation of the cutter. 
           [0039]      FIG. 8  is a plan view of a backing plate showing an alternative marker pocket pattern of slots. 
           [0040]      FIG. 9  is a bottom plan view of the cutter assembly with the invention centralizer. 
           [0041]      FIG. 10  is a side view of the invention centralizer. 
           [0042]      FIG. 11  is an operational plan view of the invention centralizer. 
           [0043]      FIG. 12  is a cross-section of a fifth embodiment of the invention showing all essential elements of the firing head assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    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. 
         [0045]    Referring initially to the invention embodiment of  FIG. 1 , the cutter assembly  10  comprises a top sub  12  having a threaded internal socket  14  that axially penetrates the “upper” end of the top sub. The socket thread  14  provides a secure mechanism for attaching the cutter assembly with an appropriate wire line or tubing suspension string not shown. In general, the cutter assembly has a substantially circular cross-section. Consequentially, the outer configuration of the cutter assembly is substantially cylindrical. The “lower” end of the top sub includes a substantially flat end face  15 . The end face perimeter is delineated by a housing assembly thread  16  and an O-ring seal  18 . The axial center  13  of the top sub is bored between the assembly socket  14  and the end face  15  to provide a socket  30  for an explosive detonator  31 . 
         [0046]    The cutter housing  20  is secured to the top sub  12  by an internally threaded sleeve  22 . The O-ring  18  seals the interface from fluid invasion of the interior housing volume. A jet window section  24  of the housing interior is that inside wall portion of the housing  20  that bounds the jet cavity  25  around the shaped charge between the outer or base perimeters  52  and  54  of the liners  50 . Preferably, the upper and lower limits of the jet window  25  are coordinated with the shaped charge dimensions to place the window “sills” at the approximate mid-line between the inner and outer surfaces of the liner  50 . Representatively, the shaped charge housing  20  may be a frangible steel material of approximately 55-60 Rockwell “C” hardness. 
         [0047]    Below the jet window  25 , the cutter housing cavity is internally terminated by an integral end wall  32  having a substantially flat internal end-face  33 . The external end-face  34  of the end wall may be frusto-conical about a central end boss  36 . A hardened steel centralizer assembly  38  may be secured to the end boss by an assembly bolt  39 . 
         [0048]    With respect of  FIGS. 9 ,  10  and  11 , a preferred centralizer assembly comprises a plurality of blade plates  82 ,  83 , and  84 . For example, a set of three blade plates may be used in a 1.50 inch tubing bore. Typically, the blades may be fabricated of Rockwell C60 hardness spring steel of approximately 0.004 inch (0.101 mm) thickness, having, for example, four, 0.250 inch (6.35 mm) wide blades with a 0.765 inch (19.431 mm) radius length. These blades are loosely stacked, serially, along the cylindrical, axially centralized, journal surface of shoulder screw  86 . An axially centralized aperture through each of the blade plates is dimensioned to allow substantially free rotation of the plates about the shoulder screw journal surface. In the presently preferred embodiment, the shoulder screw head confines the several blade plates to the length of the shoulder screw journal surface. 
         [0049]    Relative to prior art centralizer blade plates of about 0.015 inch (0.381 mm) thickness, approximately 0.765 inch (19.431 mm) radius length and approximately 0.250 inch (6.35 mm) width for a 1.50 inch (38.1 mm) tubing bore, the present invention provides a much lower bending strength for each blade and freedom to angularly reorient about the tool axis as it traverses the tubing bore length as represented by  FIG. 11 . The blade plate  82  is shown as rotated from angular symmetry by the internal tube seam weld  88  without compromise of a central radial alignment with the tube bore axis. 
         [0050]    Substantially free rotation of the centralizer blade plates about the cutter assembly axis  13  has additional advantages in a wireline operation. Wirelines for downhole tool control and tethering typically comprise a double helix winding of high tensile strength wire with the outer layer winding turned in the opposite hand direction from the first, inner layer. These steel wire windings are laid around one or more insulated signal or electrical power conduits. Although the radial difference between the inner and outer windings is minute, this small difference imposes substantial torsional force over several miles of wireline length. To relieve the wireline of this internal torsional stress as the suspended tool descends into a well, the tool must be allowed to rotate about the tool/wireline axis. However, the frictional bearing of traditional centralizers on the internal bore wall of well tubing and the internal standing tube assembly seam of the well tubing inhibit any rotation of the tool as it descends into the well. Consequently, the wireline is restrained from relieving internal torsional stress. Resultantly, the two wound wire strength layers of the wireline may separate, forming a bulbous “bird cage” as it is known in the art. By permitting the centralizer blades to freely rotate about the tool axis, the wireline is allowed to rotate about its own axis to relieve this internal torsional stress. 
         [0051]    The shaped charge assembly  40  is preferably spaced between the top sub end face  15  and the internal end-face  33  of the cutter housing  20  by a pair of resilient, electrically non-conductive, ring spacers  56  and  58 . An air space of at least 0.100″ (2.54 mm) is preferred between the top sub end face  15  and the adjacent face of the cutter assembly thrust disc  46 . Similarly, a resilient, non-conductive lower ring spacer  58  provides an air space that is preferably at least 0.100″ (2.54 mm) between the internal end-face  33  and the adjacent cutter assembly lower end plate  48 . 
         [0052]    Loose explosive particles can be ignited by impact or friction in handling, bumping or dropping the assembly. Ignition that is capable of propagating a premature explosion may occur at contact points between a steel, shaped charge thrust disc  46  or end plate  48  and a steel housing  20 . To minimize such ignition opportunities, the thrust disc  46  and lower end plate  48 , for the present invention, are preferably fabricated of non-sparking brass. 
         [0053]    The outer faces  91  and  93  of end plates  46  (upper thrust disc) and  48 , as respectively shown by  FIG. 1 , are blind bored with marker pockets  95  in a prescribed pattern such as a circle with uniform arcuate spacing between adjacent pockets as illustrated by  FIGS. 5 and 6 . These pockets  95  in the outer face  91 ,  93  are selectively weakened areas of the end plates. When the explosive material  60  detonates, the marker pocket walls are converted to jet material in a development similar to a V-shaped charge cutting liner. These cutting jets of fluidized end plate material scar the lower end face  15  of the top sub  12  with impression marks  99  in a pattern corresponding to the original pockets as shown by  FIG. 7 . When the top sub  12  is retrieved after detonation, the uniformity and distribution of these impression marks  99  reveal the quality and uniformity of the detonation and hence, the quality of the cut. For example, if the top sub face  15  is marked with only a half section the end plate pocket pattern, it may be reliability concluded that only half of the cutter explosive correctly detonated. 
         [0054]      FIG. 8  illustrates an alternative pattern of marker pockets shown as radial slots  97  distributed about the plate axis in substantially uniform arcuate segments. 
         [0055]    The explosive material  60  traditionally used in the composition of shaped charge tubing cutters comprises a precisely measured quantity of powdered, high explosive material such as RDX or HMX. The  FIG. 1  invention embodiment includes a liner  50  that is formed into a truncated cone. The liner  50  substance may be an alloy of copper and lead, for example. In some cases, a thin sheet, 0.050″ (1.27 mm), for example, of the alloy is mechanically formed to the frusto-conical configuration. Other methods of liner fabrication may provide a mixture of metal powders that is pressed or sintered to the frusto-conical form. In either case, the frusto-conical liner  50  is formed with open circular zones for the apex and base. 
         [0056]    This frusto-conical liner  50  is placed in a press mold fixture with a portion of the fixture wall bridging the liner apex opening as an annulus around a central core post. A precisely measured quantity of powdered explosive material such as RDX or HMX is distributed within the internal cavity of the mold intimately against the interior liner surface and the fixture wall bridging the liner apex opening around the core post. Using a central core post as a guide mandrel through an axial aperture  47  in the upper thrust disc  46 , the thrust disc is placed over the explosive powder and the assembly subjected to a specified compression pressure. This pressed lamination comprises a half section of the cutter assembly  40 . 
         [0057]    The lower half section of the charge assembly  40  is formed in the same manner as described above, each having a central aperture  62  of about 0.125″ (3.18 mm) diameter in axial alignment with thrust disc aperture  47  and the end plate aperture  49 . A complete cutter assembly comprises the contiguous union of the apex zone half sections respective to the lower and upper half sections along the juncture plane  64 . Notably, the thrust disc  46  and end plate  48  are each fabricated around respective annular boss sections  70  and  72  that provide a protective material mass between the respective apertures  47  and  49  and the explosive material  60 . These bosses are terminated by distal end faces  71  and  73  within a critical initiation distance of about 0.050″ (1.27 mm) to about 0.100″ (2.54 mm) from the assembly juncture plane  64  for a 2.50″ (63.5 mm) cutter. The critical initiation distance may be increased or decreased proportionally for other sizes. Hence, the explosive material  60  is insulated from an ignition wave issued by the detonator  31  until the wave arrives in the proximity of the juncture plane  64 . 
         [0058]    Distinctively, the apertures  47 ,  49  and  62  for the  FIG. 1  embodiment remain open and free of boosters or other explosive materials. Although an original explosive initiation point for the cutting charge  40  only occurs between the boss end faces  71  and  73 , the original detonation event is generated by the detonator  31  outside of the thrust disc aperture  47 . The detonation wave is channeled along the empty thrust disc aperture  47  to the empty central aperture  62  in the cutter explosive material. Typically, an explosive load quantity of 1.36 oz (38.6 gms) of HMX compressed to a loading pressure of 3,000 psi (20,000 kPa) may require a moderately large detonator  31  of 0.015 oz (420 mg) HMX for detonation 
         [0059]    The  FIG. 1  embodiment obviates any possibility of orientation error in the field while loading a cutter housing. A detonation wave may be channeled along either boss aperture  47  or  49  to the explosive  60  around the central aperture  62 . Regardless of which orientation the shaped charge assembly is given when inserted in the housing  20 , the detonator  31  will initiate the cutter explosive  60 . 
         [0060]    A modification of the invention is represented by  FIG. 2  showing the axial aperture  80  in the thrust disc  46  to be tapered with a conically convergent diameter from the disc face proximate of the detonator  31  to the central aperture  62 . Typical of this embodiment, the thrust disc aperture  80  may have a taper angle of about 10° between an approximately 0.080″ (2.03 mm) inner diameter to an approximately 0.125″ (3.18 mm) diameter outer diameter. The taper angle, also characterized as the included angle, is the angle measured between diametrically opposite conical surfaces in a plane that includes the conical axis  13 . 
         [0061]    Original initiation of the  FIG. 2  cutter charge  60  occurs at the outer plane of the tapered aperture  80  having initiation proximity with a detonator  31 . The initiation shock wave propagates inwardly along the tapered aperture  80  toward the explosive junction plane  64 . As the shock wave progresses axially along the aperture  80 , the concentration of shock wave energy intensifies due to the progressively increased confinement and concentration of the explosive energy. Consequently, the detonator shock wave strikes the cutter charge  60  at the inner juncture plane  64  with an amplified impact. 
         [0062]    Comparatively, the same explosive charge  60  as suggested for  FIG. 1  comprising, for example, approximately 1.36 oz (38.6 gms) of HMX compressed under a loading pressure of about 3,000 psi (20,000 kPa), when placed in the  FIG. 2  embodiment may require only a relatively small detonator  31  of HMX for detonation. Significantly, the conically tapered aperture  80  of  FIG. 2  appears to focus the detonator energy to the central aperture  62  thereby igniting a given charge with much less source energy. 
         [0063]    Although the  FIG. 3  invention embodiment relies upon an open, substantially cylindrical aperture  47  in the upper thrust disc  46  as shown in the  FIG. 1  embodiment, either no aperture is provided in the end plate boss  72  of  FIG. 3  or the aperture  49  in the lower end plate  48  is filled with a dense, metallic plug  76 . The plug  76  may be inserted in the aperture  49  upon final assembly or pressed into place beforehand. As in the case of the  FIG. 2  embodiment, a  FIG. 3  cutter comprising, for example, approximately 1.36 oz (38.6 gms) of HMX compressed under a loading pressure of about 3,000 psi (20,000 kPa) also may require only a relatively small detonator  31  of HMX for detonation. Apparently, the detonation wave emitted by the detonator  31  is reflected back upon itself in the central aperture  62  by the plug  76  thereby amplifying a focused concentration of detonation energy in the critical zone  62 . 
         [0064]    The  FIG. 4  invention embodiment combines the energy concentrating features of  FIG. 2  and  FIG. 3  but further adds a relatively small, explosive initiation pellet  66  in the central aperture  62 . Of course, the explosive initiation pellet  66  concept may also be applied to the  FIG. 1  embodiment. 
         [0065]    The  FIG. 12  invention embodiment is distinguished by the thin, 0.0097-0.010 in. (0.2464-0.2540 mm), material vessel shaped as a sealing cup  100  that separates the detonator  31  from the outer face of the thrust disc  46 . Sealing cup  100  encloses the detonator  31  as a receptacle and includes a fluid tight rim or sidewall fit to the internal bore wall  35  of the top sub  12 . This fluid tight fit between the cup  100  wall and the top sub bore wall  35  may be, for a few examples, an interference press fit, a threaded fit, a soldered fit or an integrally machined portion of the top sub  12  material. In any case, the distal end face of cup  100  is positioned from the lower end face  15  of the top sub as to assemble within about 0.032 in. (0.812 mm) of juxtaposition with the thrust disc  46  outer face  91  when the top sub shoulder  27  engages the distal edge of the cutter housing thread sleeve  23 . This cup  100  provides an absolute barrier to any moisture that may penetrate any assembly seals  102  above the seal  18 . 
         [0066]    The cutter housing  20  is destroyed upon a single use by detonation of the explosive material  60 . Hence, the interior sealing surfaces of the threaded sleeve  23  are normally new and highly polished to assure a fluid seal of the O-ring  18  across the low pressure transitional zone of a well bore. Also, the top sub  12 , however, is not often reused. However, tubing or pipe string units above the top sub  12  having fluid paths through tool joints into the top sub cavity  108  frequently are subject to corruption, contamination and scarring due to repeated assembly and disassembly. For this reason, the seals  102  between the firing head housing  110  for the capacitance discharge unit  112  and the top sub  12  are more likely to leak as the tool descends the well bore through the low fluid pressure zone. Such leaks allow well bore fluid, mostly water, to migrate past the sub assembly threads  106  into the internal cavity  108 . Once in the cavity  108 , migrating fluid continues past the detonator retainer  114  into the cutter housing  20 . This fluid flow path along the top sub cavity  108  is reliably blocked by the cup  100 . 
         [0067]    Operationally, the assembly is dimensioned to place the distal end of the detonator  31  against the interior bottom of the cup  100  when all assembly joints are tight. Since the detonator  31  is external of the charge aperture  47 , it may be as large as need be to rupture the thin film of the cup  100  bottom and detonate the cutter explosive material  60 . 
         [0068]    Although several preferred embodiments of the invention have been illustrated in the accompanying drawings and describe in the foregoing specification, it will be understood by those of skill in the art that additional embodiments, modifications and alterations may be constructed from the invention principles disclosed herein. These various embodiments have been described herein with respect to cutting a “pipe.” Clearly, other embodiments of the cutter of the present invention may be employed for cutting any tubular good including, but not limited to, pipe, tubing, production/casing liner and/or casing. Accordingly, use of the term “tubular” in the following claims is defined to include and encompass all forms of pipe, tube, tubing, casing, liner, and similar mechanical elements. 
         [0069]    Having thus described the preferred embodiments, the invention is claimed as follows: