Patent Publication Number: US-2015068765-A1

Title: Explosive well tool firing head

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of presently pending application Ser. No. 13/573,530 filed Sep. 24, 2012. Said application Ser. No. 13/573,530 is a Division of U.S. patent application Ser. No. 13/317,657, filed Oct. 25, 2011, now U.S. Pat. No. 8,302,523 Issued Nov. 6, 2012. Said U.S. Pat. No. 8,302,523 is a Continuation of U.S. patent application Ser. No. 12/798,269, filed Apr. 1, 2010, now U.S. Pat. No. 8,136,439, which is a Continuation-In-Part of U.S. patent application Ser. No. 11/442,807, filed May 30, 2006, now U.S. Pat. No. 7,698,982, which is Division of U.S. patent application Ser. No. 10/762,182, filed Jan. 21, 2004, now U.S. Pat. No. 7,530,397, which is Continuation of U.S. patent application Ser. No. 09/949,990, filed Sep. 10, 2001 and now abandoned, and which all are incorporated herein in their entireties by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the earthboring arts. More particularly, the invention relates to methods and devices for severing drill pipe, casing and other massive tubular structures by the remote detonation of an explosive cutting charge. 
     2. Description of Related Art 
     Deep well earthboring for gas, crude petroleum, minerals and even water or steam requires tubes of massive size and wall thickness. Tubular drill strings may be suspended into a borehole that penetrates the earth&#39;s crust several miles beneath the drilling platform at the earth&#39;s surface. To further complicate matters, the borehole may be turned to a more horizontal course to follow a stratification plane. 
     The operational circumstances of such industrial enterprise occasionally presents a driller with a catastrophe that requires him to sever his pipe string at a point deep within the wellbore. For example, a great length of wellbore sidewall may collapse against the drill string causing it to wedge tightly in the well bore. The drill string cannot be pulled from the well bore and in many cases, cannot even be rotated. A typical response for salvaging the borehole investment is to sever the drill string above the obstruction, withdraw the freed drill string above the obstruction and return with a “fishing” tool to free and remove the wedged portion of drill string. 
     When an operational event such as a “stuck” drill string occurs, the driller may use wireline suspended instrumentation that is lowered within the central, drill pipe flow bore to locate and measure the depth position of the obstruction. This information may be used to thereafter position an explosive severing tool within the drill pipe flow bore. 
     Typically, an explosive drill pipe severing tool comprises a significant quantity, 800 to 1,500 grams for example, of high order explosive such as RDX, HMX or HNS. The explosive powder is compacted into high density “pellets” of about 22.7 to about 38 grams each. The pellet density is compacted to about 1.6 to about 1.65 gms/cm 3  to achieve a shock wave velocity greater than about 30,000 ft/sec, for example. A shock wave of such magnitude provides a pulse of pressure in the order of 4×10 6  psi. It is the pressure pulse that severs the pipe. 
     In one form, the pellets are compacted at a production facility into a cylindrical shape for serial, juxtaposed loading at the jobsite as a column in a cylindrical barrel of a tool cartridge. Due to weight variations within an acceptable range of tolerance between individual pellets, the axial length of explosive pellets fluctuates within a known tolerance range. Furthermore, the diameter-to-axial length ratio of the pellets is such that allows some pellets to wedge in the tool cartridge barrel when loaded. For this reason, a go-no-go type of plug gauge is used by the prior art at the end of a barrel to verify the number of pellets in the tool barrel. In the frequent event that the tool must be disarmed, the pellets may also wedge in the barrel upon removal. A non-sparking depth-rod is inserted down the tool barrel to verify removal of all pellets. 
     Extreme well depth is often accompanied by extreme hydrostatic pressure. Hence, the drill string severing operation may need to be executed at 10,000 to 20,000 psi. Such high hydrostatic pressures tend to attenuate and suppress the pressure of an explosive pulse to such degree as to prevent separation. 
     One prior effort by the industry to enhance the pipe severing pressure pulse and overcome high hydrostatic pressure suppression has been to detonate the explosive pellet column at both ends simultaneously. Theoretically, simultaneous detonations at opposite ends of the pellet column will provide a shock front from one end colliding with the shock front from the opposite end within the pellet column at the center of the column length. On collision, the pressure is multiplied, at the point of collision, by about 4 to 5 times the normal pressure cited above. To achieve this result, however, the detonation process, particularly the simultaneous firing of the detonators, must be timed precisely in order to assure collision within the explosive column at the center. 
     Such precise timing is typically provided by means of mild detonating fuse and special boosters. However, if fuse length is not accurate or problems exist in the booster/detonator connections, the collision may not be realized at all and the device will operate as a “non-colliding” tool with substantially reduced severing pressures. 
     The reliability of prior art severing tools is further compromised by complex assembly and arming procedures required at the well site. Laws and regulations require that explosive components (detonator, pellets, etc.) must be transported separately from the tool body. Complete assembly must take place at the well site. Unfortunately, such final assembly is often undertaken in unfavorable working conditions. 
     Finally, the electric detonators utilized by prior art severing tools are susceptible to premature detonation due to stray electric currents and RF energy fields. 
     An alternative embodiment of the invention that is particularly well suited for single point ignition provides a unitized firing head that is severable from an explosive housing for separate and independent transport to a well site. 
     SUMMARY OF THE INVENTION 
     The pipe severing tool of the present invention comprises an outer housing that is a thin wall metallic tube of such outside diameter that is compatible with the drill pipe flow bore diameter intended for use. The upper end of the housing tube is sealed with a threaded plug having insulated electrical connectors along an axial aperture. The housing upper end plug is externally prepared to receive the intended suspension string such as an electrically conductive wireline bail or a continuous tubing connecting sub. 
     The lower end of the outer housing tube is closed with a tubular assembly that includes a stab fit nose plug. The nose plug assembly includes a relatively short length of heavy wall tube extending axially out from an internal bore plug. The bore plug penetrates the barrel of the housing tube end whereas the tubular portion of the nose plug extends from the lower end of the housing tube. The bore plug is perimeter sealed by high pressure O-rings and secured by a plurality of set screws around the outside diameter of the outer housing tube. 
     The tubular portion of the nose plug provides a closed chamber space for enclosing electrical conductors. The bore plug includes a tubular aperture along the nose plug axis that is a load rod alignment guide. Laterally of the load rod alignment guide is a socket for an exploding bridge wire (EBW) detonator or an exploding foil initiator (EFI). 
     Within the upper end of the outer housing barrel is an inner tubular housing for an electronic detonation cartridge having a relatively high discharge voltage, 5,000 v or more, for example. Below the inner tubular housing is a cylindrical, upper detonator housing. The upper detonator housing is resiliently separated from the lower end of the inner tubular housing by a suitable spring. The upper detonator housing includes a receptacle socket  31  for an exploding bridge wire (EBW) detonator. The axis for the upper detonator receptacle socket is laterally offset from the outer housing barrel axis. 
     Preferably, the severing tool structure is transported to a working location in a primed condition with upper and lower EBW detonators connected for firing but having no high explosive pellets placed between the EBW detonators. At the appropriate moment, the nose plug assembly is removed from the bottom end of the outer housing and a load rod therein removed. The upper distal end of the load rod includes a circumferential collar such as a snap ring. The opposite end of the load rod is visually marked to designate maximum and minimum quantities of explosive aligned along the load rod. 
     Explosive pellets for the invention are formed as solid cylinder sections having an axial aperture. The individual pellets are stacked along the load rod with the load rod penetrating the axial aperture. The upper distal end collar serves as a stop limit for the pellets which are serially aligned along the rod until the lower face of the lowermost pellet coincides with the max/min indicia marking. A restriction collar such as a resilient O-ring is placed around the loading rod and tightly against the bottom face of the lowermost explosive pellet. 
     The rod and pellet assembly are inserted into the outer housing barrel until the uppermost pellet face contiguously engages the upper detonator housing. The rod guide aperture in the nose plug is then assembled over the lower distal end of the load rod and the lower detonator brought into contiguous engagement with the lowermost pellet face. The assembly is then further compressed against the loading spring between the inner tubular housing and the upper detonator housing until abutment between the nose plug shoulder and the lower distal end of the outer housing tube. 
     In the event that the invention severing tool must be disarmed, all pellets may be removed from the housing barrel as a singular unit about the load rod. This is accomplished by removing the lower nose plug which exposes the lower end of the load rod. By grasping and pulling the load rod from the housing barrel, all pellets that are pinned along the load rod below the upper distal end collar are drawn out of the housing tube with the rod. 
     An alternative embodiment of the invention consolidates all of the explosive ignition components into a closed cylinder that is independently packaged and transported. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Relative to the drawings wherein like reference characters designate like or similar elements or steps through the several figures of the drawings: 
         FIG. 1  is a sectional view of the invention as assembled without an explosive charge for transport; 
         FIG. 2  is a sectional view of the invention with the bottom nose piece detached from the main assembly housing; 
         FIG. 3  is a sectional view of an assembled, explosive pellet unit; 
         FIG. 4  is a sectional view of the invention with the explosive pellet unit combined with the main assembly housing but the bottom nose piece detached therefrom; 
         FIG. 5  is a sectional view of the invention in operative assembly with an explosive pellet unit. 
         FIG. 6  is an alternative embodiment of the invention illustrating an independently transported firing head. 
         FIG. 7  illustrates a state of arrival for the firing head in a tool arming sequence. 
         FIG. 8  illustrates a first step in a tool arming sequence. 
         FIG. 9  illustrates attachment of a wireline signal sub to the firing head in the tool arming sequence. 
         FIG. 10  illustrates removal of the detonator cover cap in the tool arming sequence. 
         FIG. 11  illustrates alignment of an explosive tube cutting tool with the detonator end of the firing head in the tool arming sequence. 
         FIG. 12  illustrates the final state of armed tool assembly. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the  FIG. 1  cross-sectional view of the invention  10 , a tubular outer housing  12  having an internal bore  14  is sealed at an upper end by a plug  16 . The plug  16  includes an axial bore  18  and an electrical connector  20  for routing detonation signal leads  22 . A boss  17 , projecting from the base of the plug, is externally threaded for the attachment of the desired suspension string such as an electrical wireline or service tubing. 
     An inner housing tube  24  is secured to and extends from the upper end plug  16  into the internal bore  14  of the outer housing  12 . The inner housing tube  24  encloses a capacitive firing cartridge  26 . Below the inner housing  24  is an upper detonator housing  28 . A coil spring  30  links the upper detonator housing  28  to the inner housing tube  24 . An exploding bridge wire (EBW) detonator or exploding foil initiator (EFI)  32  is seated within a receptacle socket formed in the upper detonator housing  28  laterally of the housing axis. Electrical conduits  34  connect the capacitive firing cartridge  26  to the EBW detonator or EFI  32 . 
     An exploding bridge wire (EBW) detonator comprises a small quantity of moderate to high order explosive that is detonated by the explosive vaporization of a metal filament or foil (EFI) due to a high voltage surge imposed upon the filament. A capacitive firing cartridge is basically an electrical capacitor discharge circuit that functions to abruptly discharge with a high threshold voltage. Significantly, the EBW detonator or EFI is relatively insensitive to static or RF frequency voltages. Consequently, the capacitive firing circuit and EBW or EFI function cooperatively to provide a substantial safety advantage. An unusually high voltage surge is required to detonate the EBW detonator (or EFI) and the capacitive firing cartridge delivers the high voltage surge in a precisely controlled manner. The system is relatively impervious to static discharges, stray electrical fields and radio frequency emissions. Since the EBW and EFI detonation systems are, functionally, the same, hereafter and in the attached invention claims, reference to an EBW detonator is intended to include and encompass an EFI. 
     The lower end of the outer housing tube  12  is operatively opened and closed by a nose plug  40 . The nose plug  40  comprises a plug base  42  having an O-ring fitting within the lower end of the outer housing bore  14 . The plug base  42  may be secured to the outer housing tube  12  by shear pins or screws  44  to accommodate a straight push assembly. Projecting from the interior end of the plug base is a guide tube boss  46  having an axial throughbore  48  and a receptacle socket  50  for a detonator cap  66 . 
     Projecting from the exterior end of the plug base  42  is a heavy wall nose tube  52  having a nose cap  54 . The nose cap  54  may be disassembled from the nose tube  52  for manual access into the interior bore  56  of the nose tube  52 . Detonation signal conductor leads  58  are routed from the firing cartridge  26 , through the upper detonator housing and along the wall of housing bore  14 . A conductor channel  60  routes the leads  58  through the nose plug base  42  into the nose tube interior  56 . This nose tube interior provides environmental protection for electrical connections  62  with conductor leads  64  from the lower EBW detonator  66 . 
     Although the electrical connections of both EBW detonators  32  and  66  are field accessible, it is a design intent for the invention to obviate the need for field connections. Without explosive pellet material in the outer housing bore  14 , EBW detonators  32  and  66  are the only explosive material in the assembly. Moreover, the separation distance between the EBW detonators  32  and  66  essentially eliminates the possibility of a sympathetic detonation of the two detonators. Consequently, without explosive material in the tubing bore  14 , the assembly as illustrated by  FIG. 1  is safe for transport with the EBW detonators  32  and  66  connected in place. 
     The significance of having a severing tool that requires no detonator connections at the well site for arming cannot be minimized. Severing tools are loaded with high explosive at the well site of use. Often, this is not an environment that contributes to the focused, intellectual concentration that the hazardous task requires. Exacerbating the physical discomfort is the emotional distraction arising from the apprehension of intimately manipulating a deadly quantity of highly explosive material. Hence, the well site arming procedure should be as simple and error-proof as possible. Complete elimination of all electrical connection steps is most desirable. 
     The load rod  70 , best illustrated by  FIGS. 2 ,  3  and  4 , is preferably a stiff, slender shaft having an end retainer  72  such as a “C” clip or snap ring. Preferably, the shaft is fabricated from a non-sparking material such as wood, glass composite or non-ferrous metal. Individual high explosive “pellets”  74  are cylindrically formed with a substantially uniform outer perimeter OD and a substantially uniform ID center bore. The term “pellets” as used herein is intended to encompass all appropriate forms of explosive material regardless of the descriptive label applied such as “cookies”, “wafers”, or “charges”. The axial length of the pellets may vary within known limits, depending on the exact weight quantity allocated to a specific pellet. The pellets are assembled as a serial column over the rod  70  which penetrates the pellet center bore. A prior calculation has determined the maximum and minimum cumulative column length depending on the known weight variations. This maximum and minimum column length is translated onto the rod  70  as an indicia band  76 . The maximum and minimum length dimensions are measured from the rod end retainer  72 . The OD of the end retainer  72  is selected to be substantially greater than the ID of the pellet center bore. Hence the pellets cannot pass over the end retainer and can slide along the rod  70  length no further than the end retainer. When loading the tool with explosive in the field, the correct quantity of explosive  74  will terminate with a lower end plane that coincides within the indicia band  76 . An elastomer O-ring  78  constricted about the shaft of rod  70  compactly confines the pellet assembly along the rod length. 
     A lower distal end portion  79  of the rod extends beyond the indicia band  76  to penetrate the guide bore  48  of the bore plug base  42  when the bottom nose plug  40  is replaced after an explosive charge has been positioned. This rod extension allows the high explosive to be manually manipulated as a singular, integrated unit. In full visual field, the explosive charge is assembled by a columned alignment of the pellets over the penetrating length of the rod. When the outside surface plane of the last pellet in the column aligns within the indicia band  76 , the lower end retainer  78  is positioned over the rod and against the last pellet surface plane to hold the column in tight, serial assembly. Using the rod extension  79  as a handle, the explosive assembly is axially inserted into the housing bore  14  until contiguous contact is made with the lower face of the upper detonator housing  28 . 
     One of the synergistic advantages to the unitary rod loading system of the invention is use of lighter, axially shorter pellets, i.e. 22.7 gms. These lighter weight pellets enjoy a more favorable shipping classification (UN 1.4S) than that imposed on heavier, 38 gm pellets (UN 1.4D). In a prior art severing tool, the lighter weight pellets would be avoided due to “cocking” in the tool barrel  14  during loading. The loading rod system of the present invention substantially eliminates the “cocking” problem, regardless of how thin the pellet may be. 
     With the explosive assembly in place, the lower end of the housing is closed by placement of the nose plug  40  into the open end of the housing. The rod end projection  79  penetrates the guide bore  48  as the plug base  42  is pushed to an internal seal with the housing bore  14 . To assure intimate contact of the opposite end EBW detonators  32  and  66  with the respective adjacent ends of the explosive assembly, the upper detonator housing  28  is displaced against the spring  30  to accommodate the specified length of the explosive column. Accordingly, when the nose plug  40  is seated against the end of the outer housing tube  12 , both EBW detonators are in oppositely mutual compression as is illustrated by  FIG. 5 . The severing tool is now prepared for lowering into a well for the pipe cutting objective 
     Presently applied Explosive Safety Recommendations require the severing tool  10  to be electrically connected to the suspension string i.e. wireline, etc., before arming ballistically. Ballistic arming with respect to the present invention means the insertion of the explosive Pellets  24  into the housing bore  14 . 
     On those occasions when the severing tool must be disarmed without discharge, it is only necessary to remove the nose plug  40  and by grasping the rod extension  79 , draw the pellets  74  from the tube bore  14  as a single, integrated item. 
     An alternative embodiment of the invention, illustrated by  FIG. 6 , represents an independent firing head tool section  80  wherein all of the explosive initiation components are integrated as a transportable unit separate from the major tool explosive. The independent firing head  80  externally comprises a housing tube  82  that is fitted with removable end caps  84  and  90  that protect and environmentally seal the internal components. 
     The upper end cap  84  may be secured by an assembly mechanism such as screw threads  85  internally of the housing tube bore  100  that begin axially from an O-ring seal face  86 . The end cap  84  may be a closed plug having corresponding external screw threads  85  leading an O-ring channel  87 . Preferably, the internal threads  85  are compatible with external screw threads of a wireline signal sub or other means by which the assembled downhole tool is suspended and actuated. 
     The lower end cap  90  also is a closed plug having a deep internal bore  92 . The internal bore opening may be provided with an O-ring seal surface  96  followed axially by assembly means such as internal threads  95 . 
     In a presently preferred design of the firing head  80 , the main housing tube includes a primary bore  100  of a first internal diameter extending from the upper end threads  85  to an annular abutment end  102 . A secondary bore  104  extends from the abutment  102  to the lower end of the tube  82 . The lower distal end  104  of the housing  82  forms a socket boss  104  that is externally seized to receive the internal bore of detonator retainer  106 . A cylindrical projection from the base of the detonation retainer  106  provides a detonator socket  107  for securing the position of a detonator element  108  such as a Pacific Scientific EBW Part No. 2-300180. 
     External threads  95  for the lower end cap  90  extend from the base of the socket boss  104  to an O-ring  96  channel. 
     The axial space within the housing  82  for secure confinement of electronic components is preferably defined between the annular abutment  102  and an internal snap ring  105 . Spacing cylinders  110  and  112  of nonconductive materials such as plastic or elastomer isolate and axially confine a capacitor firing cartridge  114  such as the PX-1 fireset by Ecoss, Inc. of Houston, Tex. within the primary bore  100 . 
     At the upper end of the electronic assembly within the primary bore  100  between the snap ring  105  and the upper end of spacer  110  is an electrical contact plug  116  of non-conductive material. Embedded within the plug  116  is an electrically conductive ground surface  117  electrically connected to a ground terminal pin  118 . A resilient contact pin  119 , preferably positioned along the bore axis, passes axially through the plug  116 . Electrically conductive leads  120  and  122  connect the ground surface  117  and resilient contact  119  to the capacitor firing cartridge  114 . Electrically conductive discharge leads  124  and  126  connect the firing cartridge  114  to the detonator  108 . 
     In application, the firing head  80  is delivered to a well head in independent crating or packaging with the end caps  84  and  90  secured in place by meshing threads, for example, as represented by  FIG. 7 . Also, the firing cartridge  114  is electrically connected to the terminal pin  118  and resilient contact  119 . Additionally, the firing cartridge discharge leads  124  and  126  are connected to a socket mounted detonator  108 . 
     Upon removal from the transport crating, the upper end cap  84  is removed to expose the internal upper threads  85  as shown by  FIG. 8 . With the end cap  85  removed, a wireline signal sub  130  is attached with a connection adapter  131 . This assembly of signal sub  130  engages the wireline carried signal conductors with the ground surface  117  and resilient contact  119  for electrical continuity with the firing cartridge  114 . Notably, the end cap  90  has remained in place throughout the wireline connection procedures as shown by  FIG. 9  to safely confine any accidental or unintended discharge of the detonator  108 . To this end and objective, those of ordinary skill will understand that the end cap  90  should be constructed with sufficient structural integrity to confine an unintended discharge of the detonator  108 . Obviously, the type and quantity of explosive used to compound the detonator  108  will determine the parameters of structural sufficiency for the end cap  90 . 
     At this point, the lower end cap  90  is removed to expose the external screw threads  95  and detonator  108  as illustrated by  FIG. 10 . Next, an explosive well tool such as a tubing cutter  133  illustrated by  FIG. 11  is inserted over the detonator  108  and turned over the threads  95  to the final operational position shown by  FIG. 12  with the detonator  108  in ignition proximity with the explosive elements of the tubing cutter  133 . The completed assembly is now ready for well placement and discharge. 
     Numerous other modifications and variations may be made of the structures and methods described and illustrated herein without departing from the scope and spirit of the invention disclosed. Accordingly, it should be understood that the embodiments described and illustrated herein are only representative of the invention and are not to be considered as limitations upon the invention as hereafter claimed.