Abstract:
A block molded from a polymeric material has a through passageway to contain a percussive shock wave. Angularly related channels communicate with the containment passageway for receiving arcuate segments of detonation cord. Alignment of a plurality of blocks provides a convenient packaging setup for the cord. A percussive signal traveling along the cord can be short-circuited in the packaging setup. A diode version of the block can react to the signal in a given direction of travel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a Continuation-in-Part to U.S. patent application Ser. No. 10/645,369 filed on Aug. 21, 2003 and titled “Diode Cutoff and Safe Packaging Method for Textile Detonating Cord”. 

   FIELD OF THE INVENTION 
   The invention relates to the transporting of textile sheathed detonating cord and more particularly to methods used in the packaging of textile detonating cord to achieve a shipping classification allowing shipment of the detonating cord by commercial aircraft. Also the invention relates to the design of an explosive diode to restrict detonation transfer to one direction only. 
   BACKGROUND OF THE INVENTION 
   Detonating cords typically contain a secondary high explosive core encased in an outer textile sheath and plastic jacket. Typical explosive materials used are PETN, RDX, HMX, HNS, and PYX. These textile wrapped detonating cords are used extensively in the petroleum exploration and production industry to initiate other explosive components used in various downhole tools. The textile wrapping provides a highly flexible structure that can be easily threaded through perforating guns. Some examples of components that textile detonating cords are used with are perforating shaped charges, setting tools, and similar items. The well locations where these components are used are widely scattered around the world sometimes in very remote locations. It is highly desirable to be able to ship detonating cord by air from a central store location to the remote field location needing the material. However the regulations governing the shipment of explosives by air are quite stringent. 
   Basically the regulations require that detonating cord explosive materials be packaged in such a manner that an ignition or detonation in one container shall be confined to that container and will not propagate to another container. In practical terms, this means that the maximum amount of detonating cord allowed to detonate in a package is twelve inches to thirty-six inches. 
   The prior art has several examples of packaging methods that have been used to meet the air shipping regulations for explosive materials. U.S. Pat. No. 4,586,602 discloses a detonating cord transport system where the detonating cord is wound on a plurality of separator support members that provide crossover locations at frequent intervals. At these crossover points, a severing means is wrapped around the cord so that the detonation of one cord portion will sever the continuing cord length at the crossover point without initiating the cord. The maximum length of cord that can detonate without encountering a crossover point is approximately one foot. Packaging of detonating cord using this method is quite laborious and involves inserting severing means around the cord and cable ties to anchor the cord in position. 
   U.S. Pat. No. 4,817,787 discloses a different packaging method where a mounting board of insulating material, such as expanded polystyrene, is used to hold the cord. Walled paths are molded into the mounting board through which the cord can be threaded. The cord path has a series of loop regions and adjoining parallel regions through which the parallel cord is separated by the wall. The purpose of the wall is to provide a safety distance where the detonation of a length of cord will cause the adjacent parallel length of cord to be severed without initiation. The minimum wall thickness required for the expanded polystyrene is about 0.205″ minimum. 
   U.S. Pat. No. 4,895,249 discusses a packaging method that is claimed to be an improvement over the detonating cord transport system disclosed in the &#39;602 patent. This patent discloses a method that increases the labor efficiency of packaging detonating cord and efficiencies in the quantity of detonating cord per package. In this patent, the detonating cord is also wound on a plurality of separator support members. The cord is wound in loops that cross over itself at frequent locations. At the crossover points a severing means is inserted which serves as a means of stopping the detonation at the crossover point. A preferred example of a severing means is a nylon-reinforced rubber hose that is slit and placed around one cord section at the crossover point. Each separator support layer can accommodate about 25 feet of detonating cord. Twenty stacked layers will therefore allow a total of 500 feet of detonating cord to be shipped in one package. 
   The prior art disclosed in both the &#39;602 patent and the &#39;787 patent rely on a separate severing means to actually cut the detonating cord. For this system to work, the detonating cord must follow a path very close to the adjacent strand being actually severed. The detonation of the cord will accelerate the independent severing means at high velocity. The material being accelerated actually causes the severing of the detonating cord. For the &#39;249 patent, the severing means is a metal foil sleeve placed over the detonating cord at an actual crossover point. 
   Both of these packaging methods require that the detonating cord be bent back to either cross over itself or pass dose by in a parallel orientation to insure severing of the detonating cord. Placing severe bends in the detonating cord is detrimental if the cord remains in this packaging orientation for an extended period of time. 
   In the preferred embodiment of this invention, the detonating cord can be space apart at a greater distance that allows the radius of the loop to be increased to avoid damaging the detonating cord. Also the cord sections pass each other at the severing location in an arcuate configuration that avoids any sharp bends in the detonating cord. This packaging method allows the detonating cord to be stored in this configuration for an extended length of time. Also since no separate severing means is required, there will be a resultant material and labor costs. 
   It is the objective of the present invention to provide an improved method for packaging detonating cord that will meet the requirements for shipment by commercial air carriers in the United States and internationally. It is another objective of the present invention to obviate the need for a severing means and instead rely on the detonation properties of the cord to sever itself. It is another object of the present invention to provide an explosive diode whereby the propagation of detonation is restricted to one direction only. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a diode cutoff block with a loop of detonating cord exiting the block. 
       FIG. 2  shows a hinged diode cutoff block with a loop of detonating cord threaded through the base section of the block. 
       FIG. 3  shows detonating cord assembled in a diode configuration such that a detonation is only allowed to propagate in one direction. 
       FIG. 4  shows a transport packaging segment where detonating cord and diode blocks are aligned on a foamed polystyrene sheet in a series of loops to allow safe packaging of the detonating cord. 
       FIG. 5  shows a transport package comprising a stack of sheets such as that shown in  FIG. 4 . 
       FIG. 6  is a sectional view of the  FIG. 5  package showing cardboard separators between the sheets. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   A diode cutoff block is shown in  FIG. 1 . In this drawing, the diode cutoff block ( 10 ) is shown. The block has two through holes  16 ,  16  for detonating cord and a channel through the block for focusing the air blast of the detonating cord. Detonating cord ( 12 ) passes through the diode block ( 10 ) and then forms a loop ( 14 ) before the cord passes through the diode block in the opposite direction. The diode block functions by focusing the air blast of the detonating cord through the block and severing the adjacent detonating cord section. The length of the loop ( 14 ) must be selected to allow the air blast to sever the adjacent detonating cord before the detonation wave can travel around the loop and pass through the block again. 
   The block can be made out of a variety of materials such as metal, wood, or plastic. From a cost and weight standpoint, the preferred material is usually plastic. The dimensions of the block are determined by the quantity of explosive loading in the detonating cord. Typically textile detonating cords have an explosive loading ranging from 4 grains per foot to 400 grains per foot. A typical textile detonating cord for the oil well servicing industry has a coreload of about 80 grains per foot. With a higher coreload detonating cord, the distance between the thru holes must be increased slightly and the block made thicker. The actual dimensions are determined by evaluating the severing capabilities of various samples of detonating cord in different block dimensions. For an 80 gr/ft detonating cord, the distance between thru holes ranges from 0.250″ to about 1.250″. The loop needs to have a minimum detonating cord length of about 6 inches to allow adequate severing of the detonating cord. 
   The diode cutoff block illustrated in  FIG. 1  is shown as a solid block. While a solid block performs well, it is difficult to attach and remove the block from the detonating cord. An improved diode block is shown in  FIG. 2 . A hinged plastic block ( 20 ) is shown in the open position. A length of detonating cord ( 12 ) is shown passing through the block. A slot for the detonating cord ( 28 ) is molded into both the base of the hinged block ( 34 ) and the lid of the hinged block ( 22 ). The lid and the base of the block are joined by a living plastic hinge ( 30 ) that allows the block portions to be flexed. Locking tabs ( 32 ) hold the block fixed in the dosed position when the block is shut. By pressing the locking tabs ( 32 ) together, the block can be opened easily. 
   When the hinged block is dosed, an air blast channel  26  is formed similar to that in the one-piece block. The functioning of the block  20  is identical to the cutoff block  10  described earlier. When a length of detonating cord detonates, the air blast from the cord will sever the adjacent length prior to the detonation front passing around the loop and back through the block. 
   Unlike a conventional detonating cord detonator, such as those shown in U.S. Pat. No. 4,771,694 to Bartholomew and U.S. Pat. No. 4,998,478 to Beck, the block of the present invention has a blast channel  26  of width w and height d such that d corresponds to the diameter of the detonating cord. The blast channel extends through from one side to an opposed side of said block. With conventional detonating cord, of outside diameter on the order of ¼″ or less, the width w of the blast channel  26  is preferably on the order of one inch. These proportions, coupled with the choice of material for the block, provide a blast focusing effect that assures that a percussion signal in one loop end portion will sever the adjacent end portion of the detonating cord loop. The plastic block is preferably molded from P4G4 polypropylene for this purpose. 
   The detonating cord loop end portions extend across the blast channel and form a loop of uninterrupted detonating cord, quite unlike the aligned ends of separate detonating cords in a conventional detonator. Slots are defined by the mating base and lid segments of the block, and are oriented transversely of the blast channel, but communicate with it, so that a percussion signal in one end portion of the detonating cord loop in one slot will lead to an air blast in the channel that will sever the detonating cord loop end portion in the adjacent slot. 
   It is also possible to use a diode block as a directional cutoff device.  FIG. 3  shows an arrangement of a diode cutoff block that only permits one-way detonation transfer. The main detonating cord transmission line ( 40 ) goes from the top of the  FIG. 3  drawing to the bottom of the drawing. A diode block ( 10 ) is attached to webbing material ( 48 ) to hold the block in the correct position. A jumper detonating cord ( 42 ) is secured to the main detonating cord ( 40 ) with hog rings ( 44 ). The other end of the jumper detonating cord passes through the thru hole  50  in the diode block. The main detonating cord ( 40 ) passes through the other thru hole  52  in the block. Silicone rubber tubing ( 46 ) is placed over the jumper detonating cord to prevent the detonation of the jumper cord from damaging the main detonating cord length ( 40 ). 
   The diode cutoff direction is illustrated by the arrow ( 56 ). If the main detonating cord lead  40  is initiated at the top of the figure, the detonation wave will progress towards the bottom of the figure. The main detonating cord will initiate the jumper detonating cord at the hog ring connection ( 44 ). The jumper detonating cord is much shorter than the main detonating cord lead that has a series of loops  40   a ,  40   b ,  40   c  in it. Thus the detonation front from the jumper cord  44  will arrive at the diode block  10  and sever the main detonating cord lead at  50 / 52  before the detonation front from the main lead  40  arrives at the block. Thus the block  10  will function as an explosive diode, permitting the detonation front to pass through the block only in the direction opposite that of the cutoff direction  56 . 
     FIG. 4  illustrates a packaging transport section for safe packaging of detonating cord. An expanded polystyrene tray ( 70 ) is formed with a series of detonating cord loops ( 72 ) molded into the surface of the plastic. The channel for the detonating cord is slightly wider than the diameter of the detonating cord and slightly deeper than the diameter of the detonating cord. This allows accurate positioning of the detonating cord and makes the packaging easy to assemble. Cavities are molded into the tray and sized and dimensioned to receive diode cutoff blocks  74 ,  74 . These blocks  74 ,  74  may be similar to those described above with reference to  FIGS. 1-3 . If the detonating cord were to be accidentally initiated in some fashion, the detonation would only propagate until it encounters the first cutoff block. 
   The length of each loop is about twelve inches. In this design the maximum length of detonating cord that may be detonated is about eighteen inches before the detonating cord will be severed in a diode cutoff block. Each detonating cord transport section holds about eighteen feet of detonating cord. By stacking multiple transport sections, larger quantities of detonating cord can be packaged. For example, stacking twenty-eight transport sections will allow packaging 500 feet of detonating cord in an outer 4G fiberboard box. 
     FIGS. 5 and 6  show a complete packaging for a stack of sheets  70 ,  70  such as that shown in  FIG. 4 .  FIG. 6  shows separator pads  75  between the foam sheets or trays  70 ,  70 . 
   Thus, block  20  in  FIG. 2  provides a slightly arcuate shape for the detonating cord logs segments contained in the slot and this configuration avoids any necessity for a separate severing means to act on the detonating cord when an unwanted percussive signal passes through the adjacent cord end portions. The geometry of the containment or blast passageway and of the slots verified in proprietary tests conducted by an independent testing laboratory, yielded good results. The recommendation of that laboratory was to classify the packaging described herein as meeting federal requirements for transporting by highway, by rail, and by civil aircraft in the USA. 
   The foregoing disclosure and the embodiments shown in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.