EXPLOSIVE CHARGES

Abstract: An explosive device, is described, the device comprising a casing (102) containing an explosive charge (104) having a concave shaped portion facing a blast direction and a line covering at least a part of the explosive charge, the device being characterised in that, in use, in addition to a primary detonation pressure wave, there is a secondary reflected pressure wave following the primary wave, said secondary reflected pressure wave impinging on a rear surface of the liner (106) and generating a stress in the liner greater than the dynamic strength of the liner.

Description: EXPLOSIVE CHARGES The present invention relates to explosive charges and particularly, though not exclusively, to explosive charges for geological applications. Conventional explosive charges of the type known generally as shaped charges have many uses, both civil and military, and include applications such as ordnance, demolition charges, oil well perforating charges or, if linear, an explosive cutting charge. Such charges are described in detail in the book "Fundamentals of Shaped Charges" by Walters and Zukas published by John Wiley & Sons. Shaped charges can be divided into two types, those producing relatively slow moving projectiles, the so- called "explosively formed penetrator or projectile" (EFP) shown in US-A-4 976 203, GB-A-1 342 093 and EP-A-1 0 252 036 and those producing fast moving stretching jets, "jetting shaped charges". These two types can be distinguished by fundamental differences. The liner of an EFP is a shallow dish with the depth of the dish typically being less than 0.25 of its diameter whereas the liner of a jetting shaped charge is typically a cone or hemisphere in which the depth of the liner cavity is greater than 0.4 of its diameter. An EFP is designed to produce a non-stretching projectile moving at a velocity in the range from about 1.5 to about 3 km/s whereas a jetting shaped charge produces a stretching jet with a tip velocity in the range from about 3 to 10 km/s and a rear end velocity less than the tip velocity such that there is a velocity gradient along the jet which causes the jet to stretch. Pressures generated in the liner material on detonation of an EFP are of the order of one fifth of the pressures generated in the liner material on detonation of a jetting shaped charge. The pressures generated in the EFP liner are sufficient to deform the liner by conventional mechanisms including bending, folding and inversion. The pressures generated in the jetting shaped charge liner cause the liner to deform hydrodynamically and collapse onto the axis of symmetry of the charge as if the liner were a fluid. It is this phenomenon that results in the formation of a stretching jet. EFP liners are typically thicker than jetting shaped charge liners of a similar diameter. Because EFPs are not intended to produce very fast moving jets, they require less explosive. Jetting shaped charges in the prior art are therefore typically longer than EFPs of a similar diameter. This invention relates to jetting shaped charges and is not relevant to EFPs. Application of this invention to an EFP could be detrimental rather than advantageous to its performance. Figures 1A to ID show a cross section through a prior art jetting shaped charge indicated generally at 10. The charge 10 comprises an explosive fill 12 within a case 14 surrounding a hollow cavity 16 lined with a liner 18. In Figure 1A, the liner 18 is shown as a hemispherical insert having a thin wall. However, any arcuate geometry may be used depending upon the desired result. As shown, case 14 is a regular cylinder but may take other forms such as tapered or so-called "boat-tailed" cylindrical casings. The shaped charge may be cylindrically symmetric or be linear, in which case the liner 16 would be in the form of a channel with a semi-circular or wedge shaped cross section. The jetting shaped charge of Figure 1A may typically be point initiated by a booster or detonator 20 located along the axis of revolution 22 of the charge. Once the explosive fill 12 of the charge shown in Figure 1A has been detonated by the detonator 20, a detonation front indicated by dotted lines 30 propagates through the explosive fill 12. Successive positions of the detonation front 30 are shown in Figure IB by dotted curved lines. The detonation front 30 strikes the liner 18 causing it to collapse into a high speed stretching jet 34 as indicated schematically in Figures IC and ID. The detonation front 30 also strikes the inside wall of the case 14 causing it to initially deform and ultimately expand and disintegrate, however, this latter effect has been omitted from the drawings in the interests of clarity to provide a clearer representation of the positions of the detonation front 30 and stretching jet 34 relative to the original case 14. As is shown in Figures IC and ID the shape of the deformed liner 18 is collapsed into the stretching jet 34. In conventional jetting shaped charges, the case was believed to fulfil relatively limited functions mainly concerned with maintaining the explosive fill and liner in a preferred geometric relationship with each other up until the point of detonation and maintaining the pressure extended on the liner after detonation until the case disintegrates a short time thereafter. The ability to confine the explosive products was believed to be related to the thickness of the case, not its density. Since maintenance of pressure on the liner after collapse is unimportant, jetting shaped charges in the prior art with heavy confinement or thick cases were not considered weight efficient. Consequently, case materials such as aluminium for example were frequently employed being economic, easy to manufacture and light. In conventional jetting shaped charges, the length of the explosive fill is determined by the amount of explosive required to provide sufficient explosive energy for the liner collapse process calculated on the basis of a single detonation front striking the liner. Typically, the jet velocity, jet kinetic energy and penetrating power increase as the length of the explosive fill increases up to a point. An explosive fill length of about 2 times the explosive fill diameter provides a value beyond which little improvement is achieved. Usually, an explosive fill length of 1.5 times its diameter is sufficient and explosive fill lengths of 1 to 1.25 times its diameter result in a small penalty in penetrating power for point initiated conical or hemispherical lined charges. The prior art considered that the detonation front from a point initiated charge should be allowed to change from its initially highly spherical geometry (in a cylindrical charge) to be as close as possible to uniform and planar when it reaches the liner as shown in Figure IB. this requirement resulted in the explosive fill "head height" (HH) indicated by the distance 40, i.e. from the liner apex or pole to the detonator 20, of about 1 to 1.5 times the explosive fill diameter. Head heights of less than this were considered to result in detrimental effects. Head heights greater than this were not considered weight efficient. Consequently, in many conventional jetting shaped charges the rear of the case 14 follows the dashed lines 46 effectively omitting the rear portions 44 of the explosive fill as it was considered that this part of the charge and case contributed weight but little or nothing to the overall performance of the charge. There are special types of charges having very short head heights e.g. as described in GB 1,518,977 and GB 2,214,619, however such charges utilise a highly conical shaped liner where it is acceptable to have a detonation front which is highly spherical at the point where it strikes the liner. In such charges, the highly spherical detonation front strikes the liner and causes collapse of the liner long before any peripheral or secondary pressure waves are formed. Alternative methods of reducing the HH have been suggested in the prior art. These include waveshaping and peripheral initiation both of which attempt to ensure a near planar detonation front strikes the liner. An example of waveshaping is known from GB 785,155. In that example, the primary detonation pressure wave strikes a barrier or waveshaper positioned between the detonation point and the liner, so that the detonation pressure wave is slowed down and a peripheral pressure wave is formed which passes around the waveshaper and strikes the liner before the delayed primary wave. Both waveshaping and peripheral initiation increase the complexity and cost of the charge. With waveshaping it is the inclusion of the waveshaper which particularly increases the cost of the charge as well as greatly increases manufacturing complexity due to the need to accurately machine the waveshaper, accurately position and align the waveshaper within the charge and uniformly pack explosive in the vicinity of the waveshaper. According to the present invention, there is provided an explosive device for producing a high velocity jet, the device comprising a case containing an explosive charge having a concave shaped portion facing a jetting direction and a liner covering at least a part of the explosive charge, the device being characterised in that the explosive charge and liner are so arranged and dimensioned that in use, in addition to a primary detonation pressure wave there is a secondary reflected pressure wave following the primary wave, the secondary reflected pressure wave impinging on a rear surface of the liner and generating a stress in the liner greater than the dynamic strength of the liner. In contrast to the charge in GB 785,155, the charge according to the invention does not include a waveshaper. Thus, the charge according to the invention is cheaper and less complex to manufacture. The axial cross sectional shape of the liner may be selected from part-spherical or conical. Where a conical liner is used the included angle at the apex may be 110° or less. More preferably, the liner is of hemi-spherical shape and the HH of the explosive charge may be 0.5 or less of the explosive fill diameter. In the case of a hemi-spherical liner, the HH may be less than 0.25 of the explosive fill diameter. Preferably, the charge head height may be less than 0.35 of the explosive fill diameter. The minimum head height may be 0.1 of the explosive fill diameter. Preferably, the head height lies in the range from 0.15 to 0.35 of the explosive fill diameter. The present invention is concerned with the class of charges known as "jetting shaped charges". The explosive charge of the present invention provides a charge where, in use, in addition to a primary detonation pressure wave, there is a secondary reflected pressure wave following the primary wave, the secondary reflected pressure wave impinging on a rear surface of the liner and generating a stress in the liner greater than the dynamic strength of the liner. For the avoidance of doubt, the "dynamic strength" of the liner has been specified since the strength of many materials is dependent upon the strain rate. Due to the strain rates in explosive shaped charges being extremely rapid it is not possible to specify commonly reported strength parameters such as proof strengths for example. The important point is that in the explosive devices of the present invention, the magnitude of the secondary reflected pressure wave is sufficiently high to cause additional deformation of the liner over and above that caused by the initial primary detonation wave and which causes deformation of the liner in prior art charges. As stated above the invention does not include a waveshaper as in GB 785,155. In fact, in the invention, a waveshaper would interfere with the propagation of the primary detonation pressure wave to the inner walls of the case which is necessary for the formation of the secondary pressure wave. In the invention, the secondary wave is created by a reflection from the inner walls of the primary wave which has propagated outwardly from the point of initial detonation without the obstruction of any barrier. In the explosive device according to the present invention, the size and geometry of the explosive filling, concave portion or cavity, liner and case are such that a secondary reflected pressure wave following on from the primary detonation pressure wave is formed. In the present invention, the magnitude of the secondary reflected pressure wave is itself greater than the dynamic strength of the liner and, in effect, provides a booster force imparting additional force and velocity to the formed jet as will be explained in greater detail hereinbelow. Prior art explosive devices are configured such that any secondary reflected pressure wave formed is either small and/or virtually extinguishes itself or becomes of an insignificant magnitude before reaching the liner. The secondary reflected pressure wave is created by a reflection of the detonation front formed during the initial detonation of the explosive charge from the interior wall of the charge case. This secondary reflected wave impinges upon the rear face of the liner after the primary pressure wave has reached it, the secondary wave forming a collapsing toroid about the charge axis (in a cylindrical charge) behind the collapsing liner and finally forms a pressure spike of extremely high pressure on the charge/liner axis further accelerating the collapse of the liner to form the jet. This is opposite to the charge in GB 785,155 in which the peripheral pressure waves strike the liner well before the delayed central pressure wave. The magnitude of the final pressure spike may be a factor of two or three times greater than that of the primary detonation wave pressure which, for example, in one embodiment of the present invention may reach 300 kbars. However, in prior art explosive devices, the large HH of 1 to 2 explosive fill diameters results in any secondary pressure wave formed petering out before reaching the liner. Furthermore, in addition to the large HH of prior art charges, the case is frequently made of a low density material such as an aluminium alloy for example and which further diminishes the potential magnitude of any secondary pressure wave. The density of common aluminium alloys is about 2.7 to about 2.9 g/cc. We have found that to utilise the secondary pressure wave to its maximum, the density of the material from which the casing is constructed should be maximised. Preferably, the minimum density at which a large effect is observed is obtained with a ferrous metal such as steel, for example, which has a density of about 7.8 g/cc. More preferably, in order to further maximise the secondary reflected wave pressure, the density may be greater and depleted uranium may be employed having a density of about 18.7 g/cc. We have found that maximising the magnitude of the initial secondary pressure wave is a case density dependent effect rather than a material strength consideration. The case will eventually disintegrate and such disintegration occurs largely irrespective of the case material. The case will initially deform on detonation of the explosive filling but disintegration occurs after collapse of the liner to form the jet. A case of aluminium alloy or steel and having an insert of a higher density such as steel, copper or depleted uranium, for example, may be employed giving cost benefits . As noted above, the HH of prior art explosive devices are generally in the range from 1 to 2 times the explosive fill diameter. However, we have found that to optimise secondary reflected pressure wave utilisation, the HH should be much less than is common in prior art devices. In the present invention, it has been found that a HH of 0.25 explosive fill diameter has achieved jet characteristics similar to those of a conventional device with a HH of 1.5 explosive fill diameter. This is due to the fact that a shorter HH is necessary to utilise the secondary reflected pressure wave to its full effect and to avoid the secondary reflected wave energy from being largely dissipated before reaching the liner. Therefore, explosive devices having much smaller quantities of explosive and also of smaller physical dimensions may be used to the same effect as prior art charges or, alternatively, devices of the same weight or physical dimensions but with greatly increased power may be produced. It should be noted that a HH of 0.25 explosive fill diameter relates to an explosive device having a hemi-spherical shaped liner. The optimum HH may vary for charges having different shaped liners such as segmental, conical or flat conical for example. However, the principles relating to case material density and utilisation of the secondary pressure wave will remain substantially the same regardless of liner and cavity geometry. A liner of a ductile material such as copper or aluminium is frequently used. However, a liner of depleted uranium may be employed and which gives benefits in terms of increased kinetic energy in the jet due to its greater density. The outer diameter of the liner may be substantially equal to the bore diameter of the case. The explosive device of the present invention may also comprise a linear device for cutting, for example, in which case the charge "diameter" will be the charge dimension across the width, i.e. where the liner is channel shaped, in the direction across the width of the channel . In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figures 1A to ID show a cross section through a schematic representation of a prior art jetting shaped charge and its manner of operation; Figures 2A to 2E show a cross section through a schematic jetting shaped charge according to the present invention and its manner of operation; Figure 3 shows a cross section of a first embodiment of a charge according to the present invention; and Figure 4 which shows a schematic cross section of a short section of an oil well in oil-bearing rock strata; Referring now to Figure 2 and where the same features are denoted by common reference numerals. Figures 2A to 2E show a cross section of a jetting shaped charge 50 according to the present invention. In this embodiment, the charge 50 comprises a steel body or case 52 filled with explosive material 54, a cavity 56 having a hemispherical liner 58, the charge being detonated by a detonator 60. In the present invention, when the explosive filling 54 is detonated by the detonator 60, a hemispherical detonation front indicated by the dashed line 62 propagates and strikes the liner 58 causing the liner to begin to collapse as shown in Figure 2B. Simultaneously, secondary pressure waves indicated by dashed lines 64 are reflected by the inner surface 66 of the case 52 as indicated in Figure 2C. These secondary pressure waves 64 travel towards the axis of symmetry 68 of the shaped charged sweeping across the rear face 70 of the liner 58, the direction of the travelling secondary wave 64 being indicated by the arrows 72. At the axis 68, the pressure waves 64 constructively interfere to produce a very high pressure region 76, as shown in Figure 2, which strikes the rear face of the liner 58. The combined effect of the initial detonation front 62, the reflected secondary waves 64 and the high pressure region 76 is to modify the collapse of the liner 58 to form a jet 80 with improved characteristics. In the present invention, it is important that the explosive filling 54 and case 52 exist in the region which is substantially radially outward of the point of detonation caused by the detonator 60. It is also important that the case in this region is substantially parallel to the device axis 68 in order for the reflected waves 64 to be maximised. If the case 52 is boat tailed, for example, in this particular region (as shown by the dotted line 46 in Figure IC for example) the reflective secondary waves 64 do not converge towards the charge axis 68 to a sufficient extent, consequently dissipating much of the energy in the reflected secondary pressure waves . The Table below shows the peak constructive pressures of the secondary reflected wave 76 within a charge of the type shown in Figure 2 having cases of aluminium alloy, steel and depleted uranium. TABLE In the above Table, the peak pressures generated by the secondary wave 76 are shown in kbars . The figures in brackets show the peak constructive pressure as a factor of the primary detonation pressure wave 62. As may be seen from the Table, for a steel case the peak constructive pressure of the secondary reflected wave is almost as great as the pressure of the primary detonation wave 62 and a factor of 2.9 greater in the case of a depleted uranium case thus, showing the effect of case density in the explosive charge of the present invention. Figure 3 shows a first embodiment 100 of a charge according to the present invention. The charge is a so- called "perforator" for use in the oil industry for example for making holes in an oil pipe buried in rock strata to allow access of oil into the pipe. The charge comprises a cup-shaped steel body or case 102 having an explosive charge 104 contained therein and a hemispherical liner 106. In this case the liner 106 is a so- called sub-calibre liner being of a smaller diameter than that of the inner bore 108 of the body 102. The overall diameter of the device 100 is about 45mm. The charge as shown has a spigot 110 for fixing a detonating cord. As may be seen from the drawing, the head height 114 of this embodiment is less than 0.25 of the diameter of the explosive fill 104. Figure 4 shows a small schematic section of an oil well. A rough hole 154 is drilled in the ground and passes through oil-bearing rock strata 152. A steel pipe or "casing" 150 is located in the hole 154. Concrete grout 156 is pumped around the casing 150 and allowed to set to prevent the hole from collapsing and to fix the casing 150 into the rock 152. A carrier strip 158 holding a plurality of perforator charges 160 as described above with reference to Figure 3 for example is passed down the casing 150. Associated with the carrier strip 158 is a detonating cord (not shown) for initiating the charges 160. On firing the charges 160, directional jets are created which perforate the casing 150, the grout 154 and the oil bearing rock strata 152, as indicated by the dashed lines 162, to allow access of the oil (not shown) contained in the rock 152 into the casing 150 from where it is conducted to a suitable valve (not shown) at the surface to control the flow thereof. As an alternative arrangement the carrier strip 158 of Figure 4 can comprise a tube surrounding the perforator charges 160 to protect them from the oil well environment. In this arrangement, the perforator charges 160 would on detonation produce jets which perforate the carrier tube wall by means of sections of reduced thickness opposite each perforator charge as well as the casing, grout and oil-bearing rock strata.

Claims: CLAIMS 1. An explosive device producing a high velocity jet, the device comprising a case containing an explosive charge having a concave shaped portion facing a jetting direction and a liner covering at least a part of the explosive charge, the device being characterised in that the explosive charge and liner are so arranged and dimensioned that in use, in addition to a primary detonation pressure wave there is a secondary reflected pressure wave following the primary wave, the secondary reflected pressure wave impinging on a rear surface of the liner and generating a stress in the liner greater than the dynamic strength of the liner. 2. An explosive device according to claim 1 wherein the axial cross sectional shape of the liner is selected from the group comprising part-spherical and conical, conical liners having an included angle of 110° or less. 3. An explosive device according to either claim 1 or claim 2 wherein head height of the explosive charge is less than 0.5 of the explosive fill diameter diameter . 4. An explosive device according to claim 3 wherein the head height lies in the range from 0.1 to 0.5 of the explosive fill diameter. 5. An explosive device according to claim 4 wherein the head height lies in the range from 0.15 to 0.35 of the explosive fill diameter. 6. An explosive device according to any one preceding claim wherein the case has a minimum material density of about 7g/cc. 7. An explosive device according to claim 4 wherein the case is made of a ferrous material. 8. An explosive device according to any one preceding claim wherein the case includes an insert of a material having a density greater than 7g/cc. 9. An explosive device according to any one preceding claim wherein the case includes an insert of depleted uranium. 10. An explosive device according to any one preceding claim wherein the liner is hemi-spherical. 11. An explosive device according to any one preceding claim wherein the liner is made of depleted uranium. 12. An explosive device according to any one preceding claim wherein the liner diameter is less than the internal diameter of the case. 13. An explosive device wherein the internal shape of the case is in the form of a cylinder along substantially the whole of its internal length. 14. An explosive device substantially as hereinbefore described with reference to the accompanying description and Figure 2; or Figure 3 of the drawings .

Inventors: GREGSON PAUL RAYMOND, ROGERS MICHAEL CLIVE, GREGSON, PAUL, RAYMOND, ROGERS, MICHAEL, CLIVE
Applicants: BRITISH NUCLEAR FUELS PLC, GREGSON PAUL RAYMOND, ROGERS MICHAEL CLIVE, GREGSON, PAUL, RAYMOND, ROGERS, MICHAEL, CLIVE
Application year: 2000
Main classification: F42B
IPC codes: F42B1