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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of and claims priority to pending U.S. patent application Ser. No. 14/120,528, entitled “Shaped Charge Casing Cutter,” filed May 29, 2014. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     FIELD OF THE INVENTION 
     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. 
     BACKGROUND OF THE INVENTION 
     The capacity to quickly, reliably and cleanly sever a pipe or well casing deeply within a wellbore is an essential maintenance and salvage operation in the petroleum drilling and exploration industry. Cutting large, 7 inch to 20 inch nominal diameter casing and casing liner is particularly challenging. 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 similar to a V-belt drive sheave. The surfaces of the circular V-groove are clad with a thin metal liner. Pressed contiguously against the metal liner is a highly explosive material such as HMX, RDX or HNS. 
     This V-grooved wheel of shaped explosive is aligned coaxially within a housing sub and the sub is disposed internally of the pipe that is to be cut. Accordingly, the plane that includes the circular perimeter of the V-groove apex is substantially perpendicular to the pipe axis. 
     Upon ignition of the explosive, the explosion shock wave reflects off the opposing V surfaces of the grooved wheel to focus onto the respective metal liners. The opposing liners are driven together into a collision that produces a fluidized mass of liner material. Under the propellant influence of the high impingement pressure, this fluidized mass of liner material flows lineally and radially along the apex plane at velocities in the order of 22,000 ft/sec, for example. 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. 
     This principle may be applied to large diameter pipe such as well casing which may be cut while positioned within a wellbore with a toroidal circle of explosive having an outside face formed in the signatory V-groove cross-section. This toroidal circle of explosive is placed and detonated within a toroidal cavity of a housing. However, formation of an explosive torroid of sufficient size to sever a large diameter casing requires relatively large quantities of explosive. As an integral unit, such quantities of explosive exceed prudent transportation limitations. For practical reasons of transport and safety, therefore, the mass of the toroidal explosive circle is divided into multiple, small quantity modules of cross-sectional increments which are transported to a well site in separate, isolated packages. 
     Explosively cutting a 20 inch casing may require a shaped charge of as much as 1000 gms. (35.27 ounces) of high explosive (ex. HMX). However, international standards of transportation safety (United Nations Recommendations on the Transport of Dangerous Goods, Edition 17, Vol. I, Chapter 2.1, Division 1.4) limit the public transport of a single unit of hazard class or high explosive to 45 gm (1.59 ounces). Consequently, to transport a shaped charge cutter of size sufficient to cut a 20 inch casing, it is essential for the explosive elements of the cutter to be designed for shipment as a multiplicity of small, less than 38 gm./unit, modules configured for operational assembly at the point of use. 
     Unfortunately, the environmental circumstances of a drilling rig floor, which is where final cutter assembly must occur, are often severe and usually not conducive to the attentive care required for final assembly of a high explosive tool. Hence, there are strong incentives to design the individual explosive modules with the greatest degree of assembly ease and tolerance. But large module assembly tolerance often results in collective space between modules. In the case of modular assembly for shaped charges, such assembly space can severely diminish the cutter capability. 
     Other issues for large diameter casing cutters arise with deep wells under considerable hydrostatic pressure. Large surface areas for prior art casing cutter housings may be distorted under deep well fluid pressure, also resulting in reduced cutting capacity or a malfunction of the tool. 
     BRIEF SUMMARY OF THE INVENTION 
     The present casing cutter invention comprises several design and fabrication advantages including a substantially solid structural interior that is substantially impervious to high well pressure. Shaped charge explosive material is distributed in modules around the full circle of an approximate toroidal cavity that is held open against well pressure by a full-circle belting structure. 
     Preferably, the modules are further divided into smaller units corresponding to upper and lower half sections of the approximate toroid. The shaped charge metal liner is independently fabricated as a pair of matching cone frustums. 
     Collective tolerance space between the modules and modular units of explosive material is closed around the toroid circumference by paper card stock shims between adjacent explosive modules. 
     The back-side surfaces of the shaped charge assembly may be resiliently biased into intimate contact against the liner cone surfaces by an O-ring spring bearing upon the explosive module back-sides. A gap between the adjacent apex surfaces of the modules accommodates module fabrication tolerances. 
    
    
     
       BRIEF DESCRIPTION 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 preferred embodiment of the invention in assembly with the housing, centralizer and top sub. 
         FIG. 2  is a plan view of the initiation spool. 
         FIG. 3  is an elevation view of the initiation spool. 
         FIG. 4  is a plan view of the explosive assembly. 
         FIG. 5  is a plan view of an individual explosive unit. 
         FIG. 6  is an end elevation view of an individual explosive unit. 
         FIG. 7  is a side elevation view of an individual explosive unit. 
         FIG. 8  is a pictorial view of the metallic liners. 
         FIG. 9  is a plan view of an alternate initiation spool. 
         FIG. 10  is a cross-section of the invention provided with buffer chambers and an alternate detonation configuration. 
     
    
    
     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 to  FIG. 1 , a top sub  10  is formed with an axial cavity  12  for receipt of a detonator sub-assembly not shown. Internal threads  14  proximate of the sub body upper end provide a convenient mechanism for securing the top sub  10  to a tubing string, for example. External threads  16 , as shown in  FIGS. 1 and 10 , at the lower end of the top sub  10  secure the top sub to the upper housing plate  22  of the shaped charge housing  20 . 
     The shaped charge housing  20  assembly basically comprises four major components. Upper and lower housing plates  22  and  24  are separated by initiation spool  26 . The housing plates and initiation spool are all of substantially circular perimeter. The upper and lower housing plates  22  and  24  are secured into a belting ring  28  with a plurality of threaded fasteners  29 . Notably, the belting ring fit with the housing plate perimeters is designed to oppose distortions and closure of the toroidal cavity  21  between the plate perimeters due to high external fluid pressure. O-ring seals  25  environmentally secure the toroidal cavity  21  around the housing perimeter inside of the belting ring. The belting ring outside diameter is only slightly less than the inside diameter of the casing that is to be severed. Centering springs  27  may be secured to the housing to project radially outward by a predetermined distance determined by the internal diameter of the casing to be severed. 
     The belting ring  28  thickness is notched about its internal perimeter to provide a narrow penetration band  23  in the radial expansion plane of the shaped charge cutting jet. 
     Referring to  FIGS. 2 and 3 , the initiation spool  26  may be a substantially solid disc having parallel face planes and at least one transverse detonator cord boring  30  between the face planes that is intersected at the disc center by a detonator aperture  32 . The perimeter of the disc is channeled by a detonator cord confining groove  34 . Preferably, the transverse detonator cord  36  is continuous between opposite outer perimeters of the initiation spool  26  for termination at close adjacency against adjacent detonator cord in the confining groove  34 , while confining groove  34  is in close adjacency against explosive units  54 . The two arcuate cord portions  38  that form a detonating circle have respective opposite distal ends that terminate against side elements of the transverse cord. 
     With further reference to  FIG. 1 , the upper and lower housing plates  22  and  24  are formed to substantially the same profile. In an embodiment, the annular edges  40  and  41  of the respective housing plates  22  and  24  are substantially concentric with corresponding center sections  42  and  44 . The annular edge  40  of the upper plate  22  is in parallel alignment with the plane of the circular plate center section  42 . As a mirror reflection, the annular edge  41  of the lower housing plate  24  is in parallel with the plane of the circular plate center section  44 . 
     An approximately toroidal cavity  21  is formed within the interior surfaces of the plate rims and the belting ring to confine a circular assembly of explosive modules  50 . Each module  50  is a radial increment of a shaped charge circle. The plan view of  FIG. 4  illustrates the circular alignment of the modules  50  with juxtaposed radial joint planes  52 . Each module  50  comprises a matching pair of explosive units  54 , with no unit exceeding 45 gms. of explosive, for example. The three orthographic views of  FIGS. 5, 6 and 7  show a single unit  54  having a body  56  of compressed, high explosive material. 
     As the individual units are positioned against a respective housing plate interior surface  49  in the circle illustrated by  FIG. 4 , it will be understood that each unit must be formed to a small undersize tolerance for assembly convenience. When all of the units are positioned and pressed together, collectively, this necessary tolerance is accumulated as an intolerable space between the first and last units that may be 0.254 mm (0.010 inches) or more. Leaving such a space may severely influence the shaped charge performance. An unfilled inter-unit space of 1.588 mm (0.0625 inches) has been measured to reduce cutting penetration by half. Of course, this space may be packed with loose explosive but such a solution is not only time consuming but hazardous. 
     Filling the spaces with metallic shims has also been found to be unsatisfactory. Cutting performance is nevertheless reduced. Surprisingly, it has been found that the spaces may be filled with “card stock” paper shims  53  without measurable loss of cutting penetration. Typical specifications for card stock paper include a paper sheet that is calendared to an approximate density range of 135 to 300 g/m 2  (3.982 oz./yd. to 8.848 oz./yd.) and thickness range of 0.254 mm to 0.381 mm (0.01 in. to 0.015 in). In practice, the card stock shim is cut into the section shape of an explosive unit as shown by  FIG. 7  and inserted in the space between adjacent explosive units  54 . Preferably, only one card stock shim is positioned between an adjacent pair of explosive units  54 . Collective spaces greater than a single card stock thickness may be closed by inserts between multiple pairs of explosive units and/or modules. 
     It has long been believed that intimate contact of the shaped charge explosive material with the interior surface  49  of the housing structure enhanced the cutting energy release. U.S. Pat. No. 6,505,559 to J. Joslin et al. assumed this relationship by their disclosed use of “glue” to secure segmented explosive units to a backing plate. However, when practiced in the environment of a drilling rig floor, the difficulties of gluing explosive units in place are numerous. Moreover, Applicants have discovered the intimate relationship to be less critical than originally believed. 
     Of far greater importance is the intimate relationship of the explosive with the contiguous liner. In the prior art fabrication process, the independently formed metallic liner is placed in a molding receptacle and powdered explosive distributed over the liner. Subsequently, a forming die is forced against the powdered explosive to compact it against the liner surface and adhere it intimately thereto. 
     The present invention procedure calls for a partial assembly of the shaped charge housing  20  by attaching the belting ring  28  to the lower housing plate  24  by means of fasteners  29 . Additionally, the initiation spool  26  is centered upon the lower plate center section  44 . This provides an open but walled circular channel within the belting ring interior perimeter. Within this circular channel, the appropriate number of explosive units  54  are positioned with the outer end face  55  of each explosive unit placed contiguously against the inner face  60  of the belting ring  28  while the inner end face of the explosive units  54  is positioned adjacent to the center section  44  outer perimeter. The outer face  58  of each explosive unit  54  is supported by two or more O-rings  46 ,  48 . Contiguous continuity between the several units  54  about the module  50  circle is completed by inserting a required number of shims  53  between one or more pairs of units  54 . Upon this assembly of explosive units  54 , the conical frustum  57  of a first liner half is placed against the inner face  59  of the explosive units. 
     Alignment of the upper half of the cutter ring onto the previously assembled lower half begins with positioning the minor diameter edge  62  of the upper frustum  57  against the minor diameter edge  62  of the lower frustum  57 . See  FIG. 8 . If correctly dimensioned, the major diameter edge  63  of the upper frustum will be contiguously confined against the upper inside face  60  of the belting ring  28 . The upper layer of explosive units  54  are placed upon the upper liner frustum with contiguous fits against the belting ring and initiation spool  26  outer perimeter. A sufficient number of shims  53  are positioned between adjacent pairs of explosive units  54  to complete the contiguous continuity. 
     When the shaped charge housing assembly  20  is completed by securing the upper housing plate  22  to the belting ring  28 , the upper and lower plate O-rings  46 ,  48  exert a mutually opposed bias upon the explosive units  54  and the respective frustums  57 . 
     It is important to note that the explosive unit  54  dimensions described above provide an open space  65  between the proximate explosive units  54  to accommodate other dimensional tolerance variations. In view of the tightly confined environment of Applicant&#39;s explosive cutter assembly and the consequential fluctuations of manufacturing tolerances, a free movement space for the units  54  is essential to assure intimate contact with the liner frustums  57 . Although paper shims  53  successfully fill the circumferential tolerance space between adjacent explosive units  54 , it is the resilient bias of the O-rings  46  that press the units  54  into necessary intimate contact with the liner material  57 . 
       FIG. 9  illustrates an alternative embodiment of the invention ignition system in which two concentric layers of HMX comprising a center pellet  31  and an outer initiation pellet  37  are separated by a single initiation spool  33 . In an embodiment, center pellet  31  is a single piece while outer initiation pellet  37  comprises a plurality of increments  39 , none of which exceed regulatory and safety limits. In the depicted figure, the outer initiation pellet  37  is divided into six increments  39 , although it can be appreciated that the segmentation can be greater or lesser depending on the shockwave profile and the regulatory transport requirements. Initiation spool  33  comprises a plurality of grooves  35  which focus and amplify the shock wave created by center pellet  31 , allowing the invention to achieve higher working pressures and lessening the amount of explosive required to achieve equal detonation output to a solid explosive spool. As with the outer initiation pellet  37 , while initiation spool  33  is depicted as having eight grooves  35 , the configuration may vary. 
       FIG. 10  illustrates the above configuration in cross-section, showing center pellet  31 , initiation spool  33 , and outer pellet  37  within boring  30  created between the face planes of housing plates  22 ,  24 . 
     As a further invention enhancement,  FIG. 10  illustrates the invention housing as including buffer chambers  74  and  76  within annular channels  75  and  77 . O-rings  80  seal the respective chamber volumes from the downhole fluid environment. The function of these annular channels  75  and  77  and buffer chambers  74  and  76  is to absorb and suppress energy reflections from the housing plates  22  and  24 . Unbuffered, such reflected energy tends to disrupt the planar uniformity of the cutting disc as it erupts from the liner apex. A disturbed cutting disc results in a flared wall cut and an enlarged perimeter of “flash” on the pipe wall about the cutting plane. In can of course be appreciated that while these two improvements are illustrated together in  FIG. 10 , these buffer chambers could be used independently of the concentric nested ignition configuration of  FIG. 9 , and vice versa. 
     While a preferred embodiment of our invention has been illustrated in the accompanying drawings and described 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.

Summary:
A shaped charge casing cutter is constructed with the cutter explosive formed into radial section modules aligned in a toroidal cavity between a pair of housing plates. The center sections of the housing plates are contiguously aligned with opposite parallel surfaces of a center disc. The housing plates comprise annular edges or rims, and the rims can be offset from respective center disc planes in opposite directions from each other to form a toroidal cavity. The toroidal cavity is enclosed by a circumferential belt secured to said housing plate rims. V-grooved shaped charge explosive in the form of multiple pi sections is distributed about the cavity to intimately contact a pair of frusto-conical liners. Assembly tolerance space between the pi sections is filled by dense paper card stock.