Patent Description:
Cast booster explosives are normally cast with a fuse tunnel and a cap well, the tunnel providing a passageway for a fuse connected to a detonator ("cap"), which is received within the cap well.

The prior art shows canisters for cast booster explosives having fuse tunnels and cap wells as part of a canister body into which a flowable explosive is placed to cure or harden. For example, <CIT>, assigned to the assignee of the present application, discloses a plastic canister having a fuse tunnel (<NUM>) formed integrally with the canister body (<NUM>) and a separate cap well (<NUM>) which may be secured within the canister body by means of a cap well mounting fixture (<NUM>). <CIT> discloses an electronic circuit assembly in an encapsulation which may be enclosed within, e.g., a metal sleeve. In addition, see <CIT> for "Primer Cup", <CIT>for "Canister For Cast Primer", <CIT> for "Explosive Booster Casing", <CIT> for "Retainer For Holding A Detonator In A Detonator Receptacle And Explosive Cartridge Container Containing The Same", and <CIT> for "Explosives Booster and Primer".

<CIT> for "Primer Assembly" discloses an explosive primer assembly which utilizes (<FIG>) a primer explosive composition <NUM> which is initiated by a detonator <NUM> disposed within a cap well <NUM>. Initiation is effectuated by a wire conductor <NUM> which passes through a toroid <NUM> which effectuates the initiation by electromagnetic induction. The embodiment illustrated in <FIG> (column <NUM>, line <NUM> et seq. ) encases detonator <NUM> within a tube <NUM> in order to provide a stable attachment of body <NUM> and its cover <NUM> to the cartridge of primer explosive shown in <FIG>. Tube <NUM> has barbs <NUM> to help retain body <NUM> in place when tube element <NUM> is pressed into the cartridge.

<CIT> discloses a shaped explosive charge <NUM> (<FIG>) having a metal barrier plate <NUM> having (<FIG>) a cup-like deflector <NUM> and peripheral annular slots <NUM>. Barrier plate <NUM> serves to deflect to the periphery of explosive charge <NUM> the explosive force of disc-like initiating charge <NUM> (<FIG>). Charge <NUM> is initiated by detonator <NUM>/ explosive pellet <NUM>.

<CIT> is one of several Schlumberger patent publications showing provision of a shock-absorbing barrier to shield the explosives of well-perforating guns from the shock waves generated by adjacent explosives. Two basic shielding techniques are disclosed. One, as illustrated in <FIG>, is to enclose the perforating gun explosive charges <NUM> with a shock-impeding powder material <NUM> emplaced within sleeve <NUM>. The shock-impeding material may be a porous cement comprising a mixture of cement and hollow microspheres (see the paragraph bridging pages <NUM> and <NUM> of this patent). Another shock-impeding technique is illustrated in Figure <NUM> of the patent where barriers <NUM> are interposed between shaped charges <NUM>. This GB patent and some other Schlumberger references, including UK Patent <CIT> and <CIT>, show the broad concept of shielding explosives from shock waves generated by adjacent explosives. The pertinent portions of these three patents are substantially similar to <CIT> and thus are merely cumulative to each other.

<CIT> discloses a rock cracker cartridge having an ignition assembly sleeve, a cracking powder charge and an ignition capsule with an ignition powder charge in an ignition unit sleeve, wherein the ignition assembly sleeve surrounds the ignition unit sleeve when the rock cracker cartridge is primed. <CIT> discloses a canister assembly for a cast booster explosive, the canister assembly comprising a canister body defining a canister interior, and having a canister base, a cap well of generally tubular configuration disposed within the canister interior, the cap well having a length, an outside diameter and an active section, and a proximal open end, the cap well being configured to receive therewithin a detonator; and a protective sleeve surmounting the cap well, whereby the protective sleeve being configured to enclose a major portion of the length of the cap well.

In use, as is well known in the art, cast booster explosives are normally lowered into a borehole, which may be as deep as <NUM>, <NUM> or <NUM> feet (<NUM>, <NUM> and <NUM> meters, respectively) or deeper. More than one booster may be loaded into a given borehole and in such case the two or more boosters are normally positioned at different depths within a given borehole. The booster explosive(s) are employed to initiate a bulk explosive such as an ANFO slurry or emulsion which is poured into the borehole.

As is also well-known, booster explosives are utilized in blasting systems which may comprise numerous boreholes filled with a suitable cap-insensitive explosive such as an ANFO slurry or emulsion. It is important that the sequence of explosions from borehole to borehole be carefully timed so that each borehole is detonated at an appropriate time in order to maximize blasting efficiency. If a "downstream" borehole is intended to be detonated after an adjacent or nearby "upstream" borehole, it is possible that the shock wave from the detonation of the upstream borehole may damage the detonator in the downstream borehole, so as to prevent the downstream borehole from initiating. The circuitry of electronic delay detonators is particularly vulnerable to damage by the shock waves generated by prior upstream or adjacent explosions. Failure of any borehole to detonate is of course highly undesirable. Detonation failures result in uninitiated explosives in muck piles, present severe safety hazards and greatly reduce the efficiency of the blasting system.

Generally, the present invention is defined by claim <NUM> and provides a canister assembly and a booster explosive comprising the canister assembly. The canister assembly has a cap well which is protected by a shock-absorbing barrier to reduce the possibility of damage to the detonator lodged within the cap well by shock waves generated by prior adjacent explosions. The shock wave protection for the detonator is attained by enclosing a substantial portion of the cap well within a shock-absorbing barrier which may comprise a protective sleeve, while leaving unshielded an active section of the cap well, that is, the section of the cap well which encloses the explosive end section of the detonator. Leaving the explosive end section unshielded facilitates planned initiation by the detonator of the cast booster explosive surrounding the cap well. The protective sleeve may be made of any suitable material such as metal or plastic or a closed cell synthetic polymeric foam, or a combination thereof.

Specifically, in accordance with the present invention there is provided a canister assembly for a cast booster explosive, the canister assembly comprising: a canister body defining a canister interior, and having a canister base, a cap well of generally tubular configuration disposed within the canister interior, the cap well having a length, an outside diameter, an active section terminating in a distal closed end, and a proximal open end, the cap well being configured to receive therewithin a detonator comprising a shell having an explosive end section and a firing train section, such detonator to be disposed within the cap well with at least a portion of such explosive end section disposed in the active section of the cap well. A protective sleeve surmounts the cap well and encloses a major portion of the length of the cap well, the protective sleeve being configured to leave exposed the active section of the cap well.

Another aspect of the present invention provides that the protective sleeve has an inside diameter which is greater than the outside diameter of the cap well, which results in an annular cap well space between the inside diameter of the protective sleeve and the outside diameter of the cap well.

Other aspects of the present invention provide one or more of the following features, alone or in any suitable combination: the protective sleeve has a terminal end which terminates adjacent the active section of the cap well, and the canister assembly further comprises a sleeve seal closing the annular cap well space at the terminal end of the protective sleeve; the protective sleeve has a base end opposite the terminal end and the base end of the protective sleeve is mounted on the canister base in order to seal the annular cap well space at the base end of the protective sleeve; and the annular cap well space has a thickness of from about <NUM> inch (<NUM>) to about <NUM> inch (<NUM>).

Still other aspects of the present invention provide that the protective sleeve comprises a tube, e.g., a metal tube, the tube having one or more of the following characteristics: a wall thickness of from about <NUM> inch (<NUM> centimeter) to about <NUM> inch (<NUM> centimeter); a length of from about <NUM> inches (<NUM> centimeters) to about <NUM> inches (<NUM> centimeters); and an outer diameter of from about <NUM> inch (<NUM> centimeters) to about <NUM> inch (<NUM> centimeters).

Other aspects of the present invention provide for a canister assembly further comprising one or both of a cast booster explosive disposed within the canister interior and a fused detonator disposed within the cap well.

Referring now to <FIG> and <FIG>, there is shown an embodiment of the present invention wherein a canister assembly (un-numbered) comprises part of a booster explosive <NUM>. The canister assembly comprises a canister body <NUM> having a canister base <NUM> in which an open base passage 14a and a cap well mounting fixture 14b are formed. A cap well <NUM> is mounted on cap well mounting fixture 14b and is surmounted by a protective sleeve <NUM> along a portion of its length.

Contained within the body <NUM> of the canister assembly is a cast explosive <NUM>, which, as is well known in the art, may be Pentolite or the like. Cast explosive <NUM> has formed therein a cavity (un-numbered) which is configured to receive cap well <NUM> and the protective sleeve <NUM>, as more fully described below. Cast explosive <NUM> has also formed therein a fuse tunnel <NUM>. A detonator <NUM> has connected to it a fuse <NUM>, which may be shock tube or any other suitable fuse extending from the fuse end (unnumbered) of detonator <NUM> through base passage 14a, thence through fuse tunnel <NUM> and outwardly of fuse tunnel <NUM> at the top 16a of cast explosive <NUM>, in the usual manner. Cap well <NUM> has a distal closed end 20a and a proximal open end 20b, the latter of which is securely mounted onto cap well mounting fixture 14b. The cap well itself may comprise a synthetic polymeric material (plastic) closed at one end and open at its opposite end to receive the detonator therein, as shown in the canister described in the above-mentioned <CIT>. However, any suitable canister and cap well configuration is useable in the present invention. The canister body <NUM> and canister base <NUM> and, if present, an optional canister top (not shown) may be made of molded plastic or any suitable material such as waxed or coated cardboard, plastic sheeting, or the like. Detonator <NUM> is disposed within cap well <NUM> with the explosive end section 24a of detonator <NUM> (<FIG>) at or adjacent to the closed end 20a of cap well <NUM>. As seen in <FIG>, detonator <NUM> is of conventional construction and is comprised of a shell 24b having the usual crimp 24c closing the open end of shell 24b, and a detonator tip 24d. Detonator <NUM> further has a firing train section 24e which contains, as is well known in the art, an electronic or pyrotechnic firing train. Firing train section 24e is the portion of the detonator <NUM> protected by protective sleeve <NUM>.

Detonator <NUM>, which may be of conventional construction, is positioned within cap well <NUM> with detonator explosive charge <NUM> positioned at or immediately adjacent to the distal closed end 20a of cap well <NUM>. A small air head space (not shown) may optionally be left between the tip 24d of detonator <NUM> and the distal closed end 20a of cap well <NUM>.

Those skilled in the art will understand that the canister assembly and cast booster explosive of the present invention may be made by any suitable manufacturing process. An efficient process is to mold from a suitable synthetic polymeric material canister body <NUM> integrally with fuse tunnel <NUM> and to separately mold cap well <NUM> from the same or a different synthetic polymeric material, as disclosed in the above-mentioned <CIT>. Cap well <NUM>, protective sleeve <NUM> and sleeve seal <NUM> are then mounted within canister body <NUM>. Thereafter, a flowable explosive is introduced into canister body <NUM> and hardens into cast explosive <NUM>. Normally, the detonator <NUM> and its fuse <NUM> are not inserted until the point of use, for obvious safety reasons.

In <FIG> and <FIG>, the shell 24b of detonator <NUM> is broken away at the explosive end section 24a thereof in order to show detonator explosive charge <NUM>. A shock-absorbing barrier is comprised, in the illustrated embodiment, of a protective sleeve <NUM>, which extends from the proximal open end 20b of cap well <NUM> and stops short of the portion of cap well <NUM> which encloses explosive end section 24a of detonator <NUM>. Thus, the terminal end 28a (<FIG>) of protective sleeve <NUM> stops short of detonator explosive charge <NUM> contained within explosive end section 24a. As seen in <FIG>, protective sleeve <NUM> extends to the proximal open end 20b of cap well <NUM>.

An annular air space is provided between the outside diameter of the cap well and the inside diameter of the protective sleeve. Although the protective sleeve snugly fitted about the exterior of the cap well may serve as the sole shock absorbing barrier, improved shock resistance is attained by a combination of a protective sleeve and an annular air space between the exterior of the cap well and the interior of the protective sleeve. As seen in <FIG>, the inside diameter D of protective sleeve <NUM> is greater than the outside diameter d of cap well <NUM> so that the width w of the annular air space is (D-d)/<NUM>. As is conventional practice, cap well <NUM> is tapered along its length to be of slightly smaller diameter at distal closed end 20a (<FIG>) than at proximal open end 20b. The tapered inside diameter facilitates insertion of detonator <NUM> into cap well <NUM> from open end 20b of cap well <NUM>. The identically tapered outside diameter d (<FIG>) of cap well <NUM> results in the width w of the annular air space <NUM> gradually increasing in size from adjacent open end 20b to adjacent closed end 20a. The size range of width w may of course differ from one to another embodiment of the invention. In some embodiments, width w ranges from about <NUM> inch (<NUM>) to about <NUM> inch (<NUM>), or from about <NUM> inch (<NUM>) to about <NUM> inch (<NUM>), or from about <NUM> inch (<NUM>) to about <NUM> inch (<NUM>). Water, soil and other foreign materials, as well as the bulk explosive slurry or emulsion, or cast explosive pentolite or the like, must be kept out of the annular air space if the air space is to provide effective shock wave attenuation. The annular air space is therefore sealed at both ends of the protective sleeve as described below. These expedients serve to reduce the transmission of shock waves from prior adjacent explosions to a detonator within the cap well, thereby reducing the chance of damage to an adjacent, e.g., "downstream" detonator, and subsequent borehole failure. "Downstream" and "upstream" are relative terms used in the sense that the sequence of explosions travels from upstream to downstream. Thus, an upstream booster explosive is intended to be detonated before a downstream booster explosive.

The annular air space surrounding the cap well containing the detonator enhances the shock wave protection as compared to the protective sleeve snugly fitted around the cap well. Either arrangement, a snugly-fitted protective sleeve or a protective sleeve which provides an annular air space, is a much poorer transmission medium for explosive shock waves than would be a solid cast explosive such as Pentolite disposed in direct contact with the cap well.

The annular air space <NUM> should be protected against infiltration by ground water, soil particles, particles of ammonium nitrate from the ANFO, etc., especially if the cast booster explosive is positioned within a borehole for a significant length of time before detonation. Such infiltration will greatly reduce or eliminate the shock-absorbing ability of the annular air space. In order to prevent such infiltration into the annular air space <NUM>, which is formed between the outer wall 20c (<FIG>) of cap well <NUM> and the inner wall 28c of protective sleeve <NUM>, the terminal end 28a of protective sleeve <NUM> is closed by a sleeve seal <NUM>. Sleeve seal <NUM> is of generally annular configuration and may be formed of any material suitable for sealing against liquid or particulate infiltration, for example, sleeve seal <NUM> may be made of a suitable rubber or other natural or synthetic polymeric material. As seen in <FIG> and <FIG>, sleeve seal <NUM> comprises a tubular body 34a from which an annular crown 34b extends longitudinally, and a ring 34c extends radially of tubular body 34a and rests on the terminal end 28a of protective sleeve <NUM>. A portion of ring 34c at the right-hand side of <FIG> is broken away to better show a portion of terminal end 28a of protective sleeve <NUM>. Sleeve seal <NUM> is sufficiently compressible to form a tight force-fit seal to close the terminal end 28a of protective sleeve <NUM> to prevent infiltration of ground water, etc., into annular air space <NUM>. Generally, sleeve seal <NUM> may comprise any suitable elastomeric material such as a water-resistant rubber such as that sold under the trademark NEOPRENE®. As seen in <FIG>, the bottom of protective sleeve <NUM> is sealed by being firmly seated on canister base <NUM>.

In another embodiment of the present invention, the protective sleeve <NUM> may be a close fit about the outside wall of cap well <NUM> so as to substantially eliminate the annular air space <NUM>. This embodiment is obtained by a force-fit of protective sleeve <NUM> about most of the exterior wall of cap well <NUM> stopping short of at least the portion of cap well <NUM> which encloses the explosive end section 24a of detonator <NUM>. In this embodiment the protective sleeve alone is relied upon to provide attenuation of shock waves from prior adjacent explosions.

As noted above, protective sleeve <NUM> may be made of any suitable material, for example, any suitable metal such as brass or any suitable synthetic polymer (plastic) material, or wood, cardboard, etc., or combinations thereof. Protective sleeve <NUM>, when made of brass, may be a seamless tube having a wall thickness of at least about <NUM> inch (<NUM> centimeter), for example, from about <NUM> inch to about <NUM> inch (<NUM> centimeter) or from about <NUM>,<NUM> inch to about <NUM> inch (<NUM> centimeter). The length of protective sleeve <NUM> and the other exemplary dimensions given herein may of course vary depending on the specific dimensions of the cap well, the degree of desired shock wave protection, etc. For example, the length of protective sleeve <NUM> may vary from about <NUM> inches (<NUM> centimeters) to about <NUM> inches (<NUM> centimeters) in length, and the outer diameter of protective sleeve <NUM> may be from about <NUM> inch (<NUM> centimeters) to about <NUM> inch (<NUM> centimeters). While a tube as described above is simple to manufacture, obviously the protective sleeve, whether dimensioned to be a snug, close fit around the cap well or dimensioned to provide an annular air gap, may be of more complex design, e.g., it may comprise a multi-layer tube with layers of different materials, a coated tube, etc..

In other embodiments of the present invention, which may be referred to as "central cap well" embodiments, the cap well may be positioned to extend along the central longitudinal axis of the canister so that the cap well and the detonator contained therein are equidistant from the canister wall in all directions. This results in the same degree of protection from shock waves by the surrounding body of cast explosive regardless of the orientation of the booster explosive to the source of the shock wave, i.e., to the location of a nearby explosive which is to be detonated before the booster charge of the invention.

<FIG> shows a central cap well booster explosive <NUM>' with a cardboard canister body <NUM>' broken away and cast booster explosive and detonator omitted for clarity of illustration. Cap well <NUM>' is seen to be disposed along the central longitudinal axis of booster explosive <NUM>' and the fuse tunnel (not shown in <FIG>) will of necessity be offset from the central longitudinal axis. A protective sleeve <NUM>' extends from the bottom of the cap well <NUM>' and stops short of the closed end 20a' of cap well <NUM>' in order to leave unshielded by sleeve <NUM>' the active section 20d' of cap well <NUM>, that is, the portion of cap well <NUM>' which houses the explosive end section of the detonator (not shown in <FIG>). This facilitates initiation of the cast booster (not shown in <FIG>) surrounding and enclosing cap well <NUM>' and protective sleeve <NUM>'. Sleeve seal <NUM>' seals the top of the protective sleeve <NUM>' to prevent entry of foreign objects into the annular air space formed between the exterior of cap well <NUM>' and protective sleeve <NUM>'.

A series of tests was conducted by suspending prototype test embodiments of booster explosives of the present invention in water spaced apart at different selected distances from a donor explosive charge. The donor explosive charges were suspended in the water at the same depth as the test embodiments, at about <NUM> feet (<NUM> meters) below the surface. The donor charges comprised two <NUM> gram Pentolite charges, to provide a donor charge of <NUM>,<NUM> grams of Pentolite. Each of the test embodiments was configured so that the distance between the cap well (<NUM>, <FIG>) and the wall of the canister body (<NUM>, <FIG>) varied from <NUM> inch (<NUM> centimeters) to <NUM> inches (<NUM> centimeters). The test embodiments were not oriented rotationally to the donor charges during the tests. Consequently, the amount of cast explosive between the cap well and the wall of the canister body which directly faced the oncoming shock wave randomly varied between <NUM> and <NUM> inches (<NUM> to <NUM> centimeters). Both the donor charges and the test embodiments utilized electronic delay detonators. The tests were conducted by initiating both the donor and test embodiment detonators at the same time, with the donor charge delay detonator programmed to detonate while the test embodiment delay detonator was still counting down towards its detonation time. Those of the test embodiment detonators which were significantly damaged by the shock wave generated by the donor charge failed to initiate their associated explosive charges. The test results are graphically shown in <FIG>, which shows on the left-hand vertical axis the percentage of test embodiments which were not damaged by the shock wave engendered by detonation of the donor charges. (A single donor charge was used for a single test embodiment in each test. ) The graph thus shows the percentage of test embodiment booster explosives which functioned properly, that is, which were not damaged by the donor explosive shock wave. The horizontal axis of the chart shows the distance in feet between the test embodiments and the donor charges. The indicated distances are center to center distances between the donor charge and the test embodiment.

The test embodiments identified as "SR" in the graph of <FIG> are embodiments as described herein where there is no annular air space, i.e., the protective sleeve <NUM> fits snugly about the exterior of the cap well <NUM>. The test embodiments identified in the graph of <FIG> as "SR2" are in accordance with the illustrated embodiments of the present invention wherein the annular air space <NUM> (<FIG>) is about <NUM> inch (<NUM> centimeter) wide. That is, there is about <NUM> inch (<NUM> centimeter) distance between the exterior wall of cap well <NUM> and the interior wall of protective sleeve <NUM>. The protective sleeve <NUM> used in all test embodiments was a brass sleeve <NUM> inch (<NUM> centimeters) in outer diameter with a wall thickness of <NUM> inch (<NUM> centimeter). The brass sleeve was <NUM> inches (<NUM> centimeters) in length, leaving the portion of the cap well which enclosed the explosive end section 24a of detonator <NUM> exposed, i.e., uncovered by protective sleeve <NUM>.

Each test embodiment utilized an electronic delay detonator sold under the trademark DigiShot® by Dyno Nobel Inc. , and programmed for a <NUM>,<NUM> millisecond delay. The delay detonator was <NUM> inches (<NUM> centimeters) in length and had an explosive end about <NUM> inch (<NUM> centimeters) in length which contained about <NUM> gram of lead azide initiator enclosed by a base charge comprised of about <NUM> gram of PETN.

Each test embodiment comprised a booster explosive containing <NUM> grams of Pentolite in a plastic cylinder measuring about <NUM> inches (<NUM> centimeters) in length and about <NUM> inches (<NUM> centimeters) in diameter.

The results plotted in the graph of <FIG> may be tabulated as follows.

Claim 1:
A canister assembly for a cast booster explosive, the canister assembly comprising:
a canister body (<NUM>) defining a canister interior, and having a canister base (<NUM>),
a cap well (<NUM>) of generally tubular configuration disposed within the canister interior, the cap well (<NUM>) having a length, an outside diameter, an active section terminating in a distal closed end (20a), and a proximal open end (20b), the cap well (<NUM>) being configured to receive therewithin a detonator (<NUM>); and
a protective sleeve (<NUM>) surmounting the cap well (<NUM>), the protective sleeve (<NUM>) being configured to leave exposed the active section of the cap well (<NUM>) and to enclose a major portion of the length of the cap well (<NUM>).