Note: Descriptions are shown in the official language in which they were submitted.
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"EXPLOSIVE DETONATION APPARATUS"
Background Of The Invention
Field of the Invention
The present invention relates to a thermally stable,
impact and electrostatic discharge resistant explosive detonator.
More specifically, the present invention relates to an explosive
detonator including an rf attenuator, a semiconductor bridge, a
spark gap, cyclotetramethylene tetranitramine or other explosive
and titanium subhydride potassium perchlorate or 2-(5-cyanotetra-
zolato)pentaamine-cobalt (III) perchlorate and, in some instances,
a bleeder resistor and a capacitor.
Description Of The Prior Art
It is well known in the art to initiate secondary explo-
sine compositions by means of primary explosives. This method,
however, involves the use of materials which are subject to acci-
dental initiation by extraneous sources such as, for example, heat
impact, friction, electrostatic discharge or the like.
The advent of the exploding bridge wire provided a method
of introducing a large amount of energy into a detonator.
Presently available exploding bridge wire detonators usually con-
taro lead azide or pentaerythritol tetranitrate (PETN) as the
explosive material. The use of PETN, however, has limited the use
of such detonators to relatively low temperature environments.
Some detonators have used cyclotrimethylene trinitramine (RDX) or
hexanitrostilbene (HNS) as the explosive material. The detonators
have still required the introduction of a relatively large
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electrical charge into the bridgewire to heat the wire to a
temperature at which it will explode.
Recently, a device referred to as a semiconductor bridge
has been developed for ignition of pyrotechnics and explosives.
The semiconductor bridge consists of an "H" shaped, doped silicon
material 2 sandwiched between a substrate 4 and an aluminum land 6.
The bridge area provides electrical connection between the lands
and the electrical circuit is schematically illustrated in Figure
1. The semiconductor bridge is actuated by a short, low energy
pulse which may be in the range of from about 3 to 5 mJ that vapor-
izes the bridge material creating a hot plasma that ignites a small
quantity of an explosive or pyrotechnic that is placed in intimate
contact with the bridge. The assembly of the electrical circuit
and small quantity of explosive and/or pyrotechnic in a metal or
plastic shell is referred to as an SCB. SCB's operate at much
lower input energies than conventional exploding bridgewire
devices. A study of the mechanism of SCB's was conducted by Sandia
National Laboratories and reported in 1989 in report number SAND
89-2033. The model study was directed to the initiation of the
granular explosive 2-(5-cyanotetrazolato)pentaaminecobalt (III)
perchlorate (CP).
It would be desirable to produce a detonator having
increased temperature stability, shock resistance, and electro-
static discharge resistance.
In one aspect of the present invention, there is provided
an electrical detonator comprising a casing, an explosive charge
A
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contained within the casing, a semiconductor bridge positioned
adjacent the explosive charge and in intimate contact with a
pyrotechnic, a spark gap electrically connected to the
semiconductor bridge, and a pair of electrically conductive wires
which penetrate the casing and connect to the spark gap and
semiconductor bridge to provide a means of introducing an
electrical charge into the semiconductor bridge.
In another aspect of the present invention, there is
provided an electrical detonator comprising a casing having a
plurality of chambers, an explosive charge, the explosive charge
being contained within a first chamber of the plurality of chambers
of the casing, an electrical circuit comprising a pair of
electrically conductive wires which are inserted into the casing to
provide a means of introducing an electrical charge, a
semiconductor bridge positioned adjacent the explosive charge, a
spark gap electrically connected to the semiconductor bridge, a
capacitor electrically connected to the spark gap, and a bleeder
resistor.
A
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure la and Figure lb are schematic illustrations of
the electrical circuit of an SCB, respectively in top plan view and
in cross-section.
Figure 2 is a schematic illustration of one embodiment of
the detonator of the present invention.
Figure 3A and Figure 3B are schematic illustrations of
another embodiment of the present invention including a capacitor,
respectively with and without an attenuator.
Figure 4 is yet another schematic illustration of the
present invention utilizing a flying plate.
Figure 5 is a schematic illustration of a fluid disable
device for use with the detonator of the present invention.
Figure 6 is a schematic illustration of another embodi-
ment of the detonator of the present invention wherein the
detonator is configured into a small compact block.
Figure 7 is a schematic illustration of the assembly of
the embodiment of the detonator of the present invention shown in
Figure 6 assembled within a perforating gun.
Summary Of The Invention
The discovery now has been made that a detonator may be
prepared having improved impact resistance, electrostatic discharge
resistance and thermal stability utilizing 2-(5-cyanotetra-
zolato)pentaaminecobalt (III) perchlorate (CP) or titanium
subhydride potassium perchlorate (THKP) in combination with a
semiconductor bridge to form an ignition source (SCB). The
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detonator comprises a case or shell having an open end into which
is inserted in sequence, a quantity of granular cyclotetramethylene
tetranitramine (HIHX) or other explosive, an SCB positioned adjacent
one end of the HI~ZX which is electrically connected to a spark gap
and, in some embodiments, a capacitor and bleeder resistor, and
finally an rf attenuator. The electrical connections extend out-
ward from the rf attenuator through the end of the case. The
components are sealed or
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otherwise bonded within the casing to form the detonator. In an optional
embodiment,
the detonator may include a flying plate initiator. In a another optional
embodiment, the
detonator may be configured into a small block where the safety circuitry is
contained in
the circuitry tube, which is positioned parallel to the detonation tube which
houses the
explosive.
Description Of The Preferred Embodiment
Figures 2-4 provide a schematic illustration of variations of the detonator 10
of the
present invention comprising a casing or shell 12, having a varying diameter
bore
therethrough containing a quantity of HMX or other explosive identified as 13,
an SCB 16
and a spark gap 18 which are connected by a pair of electrically conductive
wires 20 and
22 to a means for introducing an electrical charge into the SCB 16, and means
for sealing
the casing and various optional constituents such as an rf attenuator 24, a
bleeder resistor
and a capacitor in the electrical circuit.
The casing or shell 12 of the detonator 10 of the present invention comprises
a
cylindrical tube having a bore 14 of varying diameter therethrough, the
diameter being
sufficient to permit inclusion of an SCB within the bore 14. Typically the
wall thickness
of the case will be in the range of from about 0.075 to about 0.125 inches.
The casing
12 may be comprised of substantially any material of high acoustic impedance
such as,
for example, aluminum, steel and particularly stainless steel, brass, rigid
plastics and the
like capable of withstanding exposure to a temperature of about 400°F
for a period of at
least about one hour without structural failure.
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The SCB 16 is positioned within the casing 12 such that it will be in intimate
contact or at least close proximity to the explosive to be placed within the
bore 14.
Preferably, the SCB 16 is positioned such that it will be in contact with the
surface of the
explosive exposed in bore 14. The SCB 16 may be substantially any of those
which are
commercially available in a size capable of insertion within the casing.
Suitable SCB's are
available, for example, from Thikol Corporation, Elkton, Maryland, and SCB
Technologies,
Inc., Albuquerque, New Mexico. The SCB 16 preferably is of the type activated
by an
electrical charge of from about 18 to about 24 volts at an amperage of from
about 3 to
about 4 amps. It is to be understood however, that other SCB's also may be
suitable if
they result in initiation of the deflagration reaction with the explosive
composition in the
detonator. The design of the SCB 16 is generally of the type having the
electrical circuit
illustrated in Figures 1 A and 1 B, and which was previously described. If
desired, the SCB
may be prepared by compressing a quantity of from about 50 to about 100
milligrams of
a pyrotechnic in a suitable size metal or plastic container into intimate
contact with a
semiconductor bridge. It is contemplated that the pyrotechnic would be either
CP, BNCP
or THKP, or any other pyrotechnic or explosive having the characteristic of
being reliably
initiated by the SCB. Typically the CP or THKP is compressed by application of
from
about 10,000 to 12,000 psi of force to the material. The open end of the
container
having the electrical connections extending therefrom may be sealed by, for
example,
epoxy or the like.
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The THKP utilized in the present invention is defined by the formula
TiHX/KC104
wherein x is greater than 0.6 and less than 1.9. The THKP is available from,
for example,
SCB Technologies, Inc., Albuquerque, New Mexico. THKP may be produced by a
number
of commercially known methods. One successful method of synthesis involves a
very
carefully controlled vacuum heating cycle followed by a controlled air
oxidation step to
thermally dehydride commercially available titanium hydride. This product is
then blended
with potassium perchlorate to yield the THKP. Generally, the subhydride is
blended with
the perchlorate in a ratio of about 1:2 by weight, however, it is to be
understood that
other ratios known to those skilled in the art may be employed.
An explosive charge 13 is positioned within an end 26 of the case 12 to
improve
initiation of a detonating cord such as a "PRIMACORD°" detonating cord
manufactured
by Ensign-Bickford Company or other secondary explosive. The explosive charge
is
introduced into the casing as a powder and thereafter is compressed by
application of, for
example, a ram to the explosive at end 26 of casing 12. The explosive charge
may
comprise HMX, hexanitrostilbene (HNS) bis(picrylamino) trinitropyridine (PYX),
trinitrotrimethylenetriamine (RDX) and mixtures thereof, or the like. It is
contemplated
that the explosive charge would have a velocity of detonation of 5,000 meters
per second
or greater. The end 26 of casing 12 may be sealed by a thin metal or plastic
disk which
is pressed into place or by a thin layer of epoxy to provide a seal on the
exposed end of
the explosive in the bore 14 in detonator 10.
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The HMX or other explosive is compressed to a density in the range of from
about
1.4 to about 1.6 grams per cubic centimeter at the exposed end. This results
in a
variation in the density of the HMX or other explosive in the bore from
approximately the
bulk density of the explosive at one end to the full compressed density at the
other end.
The length of the bore is such that the quantity of HMX present will, upon
initiation,
effect a transition from deflagration-to-detonation prior to passage of the
combustion front
through the mass of compressed HMX present within the bore. Typically, the
bore 14 will
have a length of at least about 1 inch for a bore diameter of about 0.1
inches. The bore
within the casing 12 generally is flared in a frustoconical manner at the end
at which
initiation is to occur to provide a larger surface area upon which to initiate
deflagration.
The SCB 16 is connected by an electrically conductive wire 17 to a spark gap.
The
spark gap 18 is utilized to protect the detonator against accidental
initiation by an
electrostatic discharge. Suitable spark gaps are available from, for example,
Reynolds
Industries and Lumex Opto. Typically the spark gap will have a voltage
threshold of from
about 80 to about 200 volts before passage of an electrical charge to the SCB
16 occurs.
Spark gaps are available with various ratings and detonators can be prepared
having
different known spark gaps to permit controlled initiation of individual or
multiple explosive
charges in response to different electrical charges transmitted from an
electrical source.
To facilitate placement of the SCB 16 and spark gap 18 within the casing 12,
the
components are preferably potted in a plastic resin such as epoxy or other
material, or
affixed to a substrate to permit maintenance of a fixed position within the
casing 12.
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While not required or essential, potting of the electrical components assists
in reducing
detonator failures.
The SCB 16 and spark gap 18 are provided with electrically conductive wires 20
and 22 which provide an electrical connection which extends outside of the
casing 12.
The casing 12 can be sealed by insertion of, for example, an rf attenuator,
comprising a
ferrite bead having passageways therethrough for the wires passing from the
end of the
casing 12. The casing 12 then may be crimped to retain the bead in position.
The rf
attenuator reduces the strength of any radio signal present to a level whereby
the signal
is incapable of accidental initiation of the detonator. Suitable devices
include the MN 68
ferrite device available from Attenuation Technologies, La Plata, Maryland.
The casing 12
also may be sealed with plastic resins or the like which bond to the casing to
seal the
various components within the casing.
In an alternate embodiment of the present invention, illustrated schematically
in
Figure 3, a capacitor 32 and a bleeder resistor 33 may be included within the
electrical
circuit created by the SCB 16 and spark gap 18 within the detonator casing 12.
The
bleeder resistor 33 and the capacitor 32 are placed in parallel to one
another, and then in
series with the spark gap 18. The capacitor 32 is utilized to store electrical
energy
sufficient to pass the spark gap and initiate the SCB and the resistor is used
to slowly
drain the capacitor in the event the capacitor is partly charged during an
interrupted firing
of the detonator. Typically, the capacitor is selected to provide a
capacitance of 3.5 mF
and the resistor is chosen to have a 1,000,000 to 2,000,000 ohm resistance.
Suitable
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capacitors and resistors are available from, for example, Carlton-Bates Co.,
Texarkana,
Texas. The electrical circuitry includes the SCB 16, which in turn is
positioned adjacent
the explosive charge 13.
In yet another embodiment of the present invention, illustrated schematically
in
Figure 4, initiation of the explosive charge is effected with a flying plate.
In this
embodiment, the end 26 of bore 14 is divided into segments 34, 35 and 36 and
one
segment contains no explosive. A quantity of granular explosive is positioned
within
segment 34 of bore 14 and compressed adjacent the SCB 16. A disk 38 then is
inserted
into the casing 12. The disk 38 generally has a diameter substantially the
same as the
inner diameter of the casing 12. The disk 38 may be comprised of aluminum,
plastic or
the like in accordance with the known techniques of initiation using flying
plates. The
thickness of the plate may vary, with the specific thickness being dependant
on the
energy necessary to detonate the explosive charge. The flying plate material
selection and
size determinations are considered to be well within the knowledge and
capabilities of
those individuals skilled in the art. In one embodiment a retainer 39 then is
inserted
within segment 35 of the casing 12 adjacent to the disk 38 and a prepressed
pellet of
explosive is positioned within segment 36 with the remainder of the detonator
10 being
as previously described.
The detonator 10 of Figures 3A, and 4 can alternatively be assembled without
an
rf attenuator 24, as illustrated in FIG. 3B, and still operate safely. The rf
attenuator 24
is generally added as a precaution to reduce the strength of any radio signals
that are
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present and, thus, act to prevent accidental initiation of the detonator 10.
Tests have
shown that accidental initiation of the detonator 10 is highly unlikely, even
when the rf
attenuator 24 is not used, because the capacitor 32 and bleeder
resistor 33 together act to attenuate and rf energy that may be encountered
during assembly and use.
The detonator 10 of the present invention may be utilized in environs subject
to
fluid influx in which it is desired to disable an explosive charge in the
presence of such
fluids. One particular application wherein a fluid disable is desirable is in
the operation of
subterranean formation perforating guns. Typically, the guns comprise a number
of
perforating charges contained within a sealed metal housing. If fluids enter
the interior
of the housing the performance of the perforating charges is effected and it
also may
result in misfires of the charges. When used, for example, in a perforating
gun to activate
a detonating cord connected to the perforating charges, the detonator may be
connected
to the detonating cord with a coupler 50 of the type schematically illustrated
in Figure 5.
The coupler 50 comprises a body 52 having a bore 54 therethrough which may
have
differing diameters down its length. The bore 54 is of a diameter at one end
sufficient to
fit over the end, in preferably a compression fit, of the detonator 10. The
other end of
the bore 54 is such as to accept insertion of the end of a detonating cord 55
therein. The
bore 54 is of sufficient length that a void remains between the opposed end
faces of the
detonator and detonating cord 55 when positioned within the coupler. The
coupler also
includes at least one port 56 through the side wall of the coupler 50 in the
region of the
void. The port 56 is of sufficient size that upon exposure of the coupler 50
to a fluid, the
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fluid can flow through the port 56 and into the void. Entry of a fluid into
the void will,
upon detonation of the detonator, result in energy absorption by the fluid so
as to prevent
activation of the detonating cord 55. While the coupler 50 has been described
as a device
separate from said detonator 10, it is to be understood that a detonator
casing could be
produced in which the features of the coupler 50 would be incorporated and
whereby a
direct connection of the detonator could be effected with a detonating cord.
Figures 6 and 7 show a further embodiment of the subject invention, referred
to as
a "block detonator" 60, whereby the casing 12, instead of being configured as
a single
tube (as shown in FIGS. 2, 3A, 3B, and 4) is configured as a block. In a
preferred
embodiment, the block detonator comprises a plurality of chambers, such as a
first
chamber 61, a second chamber 62, and a third chamber 63. The first chamber 61,
also
referred to as a safety circuit tube, houses the circuitry of the detonator
60, namely a
spark gap 18, a bleeder resistor 33, and a capacitor 32. The bleeder resistor
33 and the
capacitor 32 are placed in parallel to one another, and then in series with
the spark gap
18. The second chamber 62 houses the booster 65 and the deflagration-to-
detonation
(DTD1 tube 67, which in turn houses the semiconductor bridge (SCB) 16 and the
explosive
charge 13. The electrical circuitry of the safety circuit tube is electrically
connected to
the SCB 16, which in turn is affixed to the DTD tube 67 and positioned
adjacent the
explosive charge 13. The explosive charge 13, housed within the DTD tube 67,
is
positioned adjacent the booster 65. The booster is a heavier explosive charge
which
provides side initiation or explosive transfer to the detonating cord (not
shown), which
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passes through the third chamber 63 for alignment of the detonating cord
adjacent to the
booster 65. The block detonator 60 is used with bottom-fired hollow carrier
perforating
guns where a fluid-disabled feature is required. This embodiment of the
present invention
is similar in geometry to conventional block detonators that have been used in
the oilfield
for many years, but is preferred over the prior art block detonators because
it does not
contain primary explosives.
It is contemplated that the block detonator 60 can be configured to have
varying
geometries and different numbers of chambers, and that the embodiment shown in
FIG.
6 is but one such arrangement.
The block detonator 60 will disable itself if the gun 70 has inadvertently
flooded.
Conventional block detonators rely on fluid to create a barrier between the
donor charge
of the detonator and the detonating cord. The block detonator 60 relies on the
wetting
action of fluid migrating into the casing 12 and DTD tube 67 to inert the
explosive booster
65, thus providing fire/no-fire reliability and eliminating the critical
geometry characteristic
found in conventional block detonators.
Also, by configuring the safety circuitry of the block detonator 60 such that
it is
parallel to the DTD tube 67, the overall length of the detonator 60 can be
shortened to
where the detonator is compact enough to fit into the bottom of a standard
perforating
gun without any modification to the gun or the bull plug (See FIG. 7).
Furthermore, this
embodiment of the detonator 60 can side initiate standard 80 grain detonating
cords
commonly used in perforating guns, making for safe and simple arming of the
gun.
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The operation of the block detonator 60 is illustrated in FIG. 7. The negative
lead
22 of the detonator 60 is attached to a scratcher-type grounding spring 76
outside of a
perforating gun carrier 70 known in the art. The grounding spring 76 is then
inserted into
the gun 70 and pushed forward until it abuts the lowermost shaped charge 78.
The
positive lead 20 is then spliced to the lead wire 73 that runs through the gun
70, arming
the detonator 60 electrically. An insulating sleeve 74 is placed over the
splice.
Thereafter, the detonating cord 75 is passed through the block detonator 60 to
provide
the ballistic connection. The detonator 60 is pushed into the lower portion of
the gun 70
and the cord 75 is trimmed flush with the edge of the detonator 60. The wires
20, 22
are carefully tucked inside the gun 70 and a bull plug 72 is threadedly
engaged to the
bottom of the gun 70 to complete the arming sequence.
To further illustrate the present invention and not by way of limitation, the
following
example is provided.
EXAMPLE
A detonator is prepared utilizing a casing comprising 303 Stainless Steel
having a
diameter of 0.312, a length of 4.3 inches, and a wall thickness of about
0.106. A
quantity of HMX is pressed into the bore in the end of the casing having the
cross-section
as illustrated in Figure 2. An SCB is connected to a spark gap and potted in
epoxy within
the casing prior to addition of the HMX. The electrical connections for the
SCB and spark
gap are passed through a rubber washer which is crimped within the end of the
casing,
sealing the detonator. The detonator was secured in a test fixture and
connected to an
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electrical power source. An electrical charge having a voltage of 80-200 volts
and current
of 2.0 amps which was then applied to the detonator. The detonator fired.
While that which is considered to comprise the preferred embodiments of the
invention has been described herein, it is to be understood that changes and
modifications
may be made in the apparatus and chemical compositions by an individual
skilled in the
art without departing from the spirit or scope of the invention as set forth
in the
specification and the drawings.
