Abstract:
A radial-linear shaped charge pipe cutter is constructed with the booster explosive packed intimately into a booster aperture that is bored axially through the charge upper end plate. The cutter explosive is initiated at the interface between the upper margin of the cutter explosive and the contiguous inside surface of the upper end plate. This interface is within a critical initiation distance from the half charge juncture plane. In one embodiment, a half charge liner is configured as the assembly of two, coaxial, frusto-cones with the smaller cone diverging from the half charge juncture plane at a smaller angle than the outer cone. In another embodiment, the liner thickness increases from the juncture plane out to the liner perimeter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 10/961,350, filed Oct. 8, 2004 incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to shaped charge tools for explosively severing tubular goods including, but not limited to, pipe, tubing, production/casing liners and/or casing. 
     2. Description of Related Art 
     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. 
     Typical explosive pipe cutting devices comprise a consolidated wheel of explosive material having a V-groove perimeter. 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. 
     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. 
     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. This high velocity collision of the V-groove liner material generates a localized impingement pressure within the material of about 2 to 4×10 6  psi. Under pressure of this magnitude, the liner material is essentially fluidized. 
     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. Resultant impingement pressures against the surrounding pipe wall may be as high as 6 to 7×10 6  psi thereby locally fluidizing the pipe wall material. 
     Traditional fabrication procedures for shaped charge pipe cutters have included an independent fabrication 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. 
     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. 
     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. 
     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 
     The present invention pipe cutter comprises several design and fabrication advantages that include a half cutter fabrication procedure that compresses the booster explosive material intimately into an axially centered aperture that is bored through the upper charge end plate. In this embodiment of the invention, there is no independently prepared booster that is an article separate from the end plate. The booster initiates the cutter explosive charge at a plane common with inner surface plane of the end plate. Although the initiation point is lateral of the half cutter junction plane, the point of explosive initiation is within a critical initiation distance from the juncture plane and nevertheless produces a symmetric shock wave impact on the opposing liner faces. 
     Another, similar embodiment of the invention has a tapered wall for the upper backing plate booster aperture. The taper converges from the exterior surface of the upper backing plate toward the cutter explosive at about 5°. The small, terminus end of the aperture coincides with the upper surface plane of the cutter explosive. 
     A bi-axial liner embodiment of the invention configures the liner of a half charge as a pair of coaxial cone frustums of different conical angles. The base edge of the inner cone is joined to the apex edge of the outer cone. The inner cone frustum that diverges from the half charge juncture plane is formed to a greater conical angle than the outer cone frustum. 
     Another embodiment of the invention is a charge liner having a tapered thickness. The liner thickness increases from the half charge juncture plane out to charge perimeter by a surface angle divergence of about 0.50° to about 1.50°. 
     A further embodiment of the invention comprises a thin wall tube for the booster explosive that is inserted into an axial aperture in the upper backing plate. The length of the booster tube is terminated at or above the half charge juncture plane. The inside face of the upper backing plate is configured to provide a boss extension around the booster aperture. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       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: 
         FIG. 1  is a cross-section of a first embodiment of the invention in assembly with the housing, centralizer and connecting sub. 
         FIG. 2  is a cross-section of a second embodiment of a SC cutter unit 
         FIG. 3  is a cross-section of a third embodiment of a SC cutter unit. 
         FIG. 4  is a cross-section of a fourth embodiment of a SC cutter unit. 
         FIG. 5  is a cross-section of a fifth embodiment of a SC cutter unit. 
         FIG. 6  is an exploded view pictorial of a cooperative pair of liners. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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 a booster detonator  31 . 
     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 may be axially delineated above and below by exterior “break-up grooves”  26  and  28 . The break-up grooves are lines of weakness in the housing  20  cross-section and may be formed within the housing interior as well as exterior as illustrated. The jet window  24  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 . 
     Below the lower break-up groove  28 , 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  38  is secured to the end boss by an assembly bolt  39 . A spacer  37  may be placed between the centralizer and the face of the end boss  36  as required by the specific task. Preferably, the shaped charge housing  20  is a frangible steel material of approximately 55-60 Rockwell “C” hardness. 
     The shaped charge assembly  40  is preferably spaced between the top sub end face  15  and the internal end-face  33  of the end wall  32  by a resilient, electrically non-conductive, ring spacer  56 . An air space of at least 0.100″ between the top sub end face  15  and the adjacent face of the cutter assembly thrust disc  44  is preferred. Similarly, a resilient, non-conductive lower ring spacer  56  provides an air space of at least 0.100″ between the internal end-face  33  and the adjacent cutter assembly lower end plate  48 . 
     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 end plate  46  or  48  and a steel housing  20 . To minimize such ignition opportunities, the upper end plate  46 , for the present invention, are preferably fabricated of non-sparking brass. 
     The explosive material  60  traditionally used in the composition of shaped charge tubing cutters comprises a precisely measured quantity of powdered 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″, 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  62  and base  64  as illustrated by  FIG. 6 . 
     This frusto-conical liner  50  is placed in a press mold fixture with a portion of the fixture wall bridging the liner apex opening  62 . 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 apex opening  62 . The lower end plate  48  is place 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 . The upper half section is identically formed except for the booster aperture  70  along the central axis  13  of the upper end plate  46 . A complete cutter assembly comprises the contiguous union of the apex zones  62  respective to the lower and upper half sections along the juncture plane  72 . 
     Distinctively, the end plates  46  and  48  of the  FIG. 1  embodiment each include an axial aperture  70  and  74  of about 0.125″ diameter. These apertures  70  and  74  are charged with an initiation booster explosive  78  such as Primer HMX. There is no independently loaded booster case for the  FIG. 1  embodiment. The booster charge  78  in the apertures  70  and  74  is terminated at the respective aperture/cutting charge interface  66  and  76 . Although the original explosive initiation point of the cutting charge  60  only occurs at the interface  66  with the upper end plate aperture  70 , that is because only the upper booster charge  78  is in proximity with the detonator  31 . To prevent orientation error in the field while loading a cutter housing, therefore, both end plates  46  and  48  are charged with booster explosive  78 . Consequently, there is no oriented up or down to the charge. Regardless of which orientation the shaped charge assembly is given when inserted in the housing  20 , the detonator  31  will engage a booster charge  78 . 
     Loading the booster charge  78  directly into the end plates  46  and  48  provides certain manufacturing and field assembly advantages. The field assembly steps of inserting a booster cartridge after placing the shaped charge assembly  40  in the housing are eliminated. The material logistics of separately packaged booster cartridges is also eliminated. However, to assure a symmetric application of explosive forces on the opposing faces of the V-grooved liner, the cutting charge initiation point  66  should be within a critical initiation distance of about 0.050″ to about 0.100″ from the juncture plane  72  for a 2.50″ cutter. The critical initiation distance may be increased or decreased proportionally for other sizes. The velocity or intensity of the booster explosion as influenced by the charge properties or the shape of the booster vent  82  as explained relative to  FIG. 2  may also influence the critical initiation distance. 
     A modification of the  FIG. 1  embodiment is represented by  FIG. 2  showing the end plates  80  and  89  as having a tapered booster vents  82 . Typical of this embodiment, the end plate booster vents may have a taper angle of about 10° between an approximately 0.080″ inner orifice diameter  86  to an approximately 0.125″ diameter outer orifice diameter  84 . 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. 
     The tapered booster vent is intimately charged with booster explosive. Original initiation of the tapered booster charge occurs at the plane of the outer orifice  84  having initiation proximity with a detonator  31 . The initiation shock wave propagates inwardly toward the inner orifice plane  86 . As the shock wave progresses along the tapered booster vents  82 , the concentration of shock wave energy intensifies due to the progressive increase in confinement of the explosive energy. Consequently, the tapered booster charge shock wave strikes the cutter charge  60  at the inner orifice plane  86  with an amplified impact. 
     The  FIG. 3  embodiment of the invention comprises a shaped charge having upper and lower end plates  46  and  48  corresponding to the  FIG. 1  embodiment. The liner  90  of each shaped charge cutter half section  92  and  94 , however, is a composite of two frusto-cones  96  and  98 . The innermost frusto-cone  96  may diverge from the juncture plane  72  by an angle θ of about 25° to about 32°. The outermost frusto-cone  98  may diverge from the juncture plane  72  by an angle ρ of about 40° to about 70°. 
       FIG. 4  of the invention illustrates an embodiment having upper and lower end plates  80  and  82  corresponding to those of  FIG. 2  but differing with a tapered thickness section of the cutter liner  100 . The liner thickness increases progressively from the apex opening  62  to the base opening  64 . For example, the inner cone surface  102  may extend from the juncture plane  72  at an angle α of about 30°. The outer conical surface  104  of the liner  100  may diverge from the juncture plane  72  at an angle β that is about 0.50° to about 1.50° greater than the angle α. 
     The  FIG. 5  embodiment of the invention differs significantly from the foregoing embodiments, first with the interior configuration of the respective end plates  110  and  112 . Each have substantially cylindrical bosses  114  and  116  projecting inwardly from the substantially planar inside surfaces  115  and  117 . Neither boss  114  nor boss  116  projects to the juncture plan 
     Distinctively, the upper end plate  110  is axially bored for an aperture  120  of about 0.080″ to about 0.125″ diameter. The aperture  120  receives a booster cartridge  122  having a brass tube wall, for example, wall of about 0.010″ to about 0.030″. The booster cartridge  122  projects from the inner end of the aperture  120  to the juncture plane  72  of the cutter explosive  60 . 
     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.