Patent Publication Number: US-6901839-B2

Title: Blast attenuation device and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. application Ser. No. 10/313,834, filed Dec. 6, 2002 now U.S. Pat. No. 6,805,035, which is hereby incorporated herein in its entirety by reference. 

   BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to the attenuation of blasts and, in particular, to apparatuses and methods for attenuating blasts with a shield formed of attenuation, or absorptive, material. 
   2) Description of Related Art 
   An explosion is typically characterized by a blast or sharp increase in pressure that propagates in a wavelike manner outward from a point or area of origination. Whether intentionally or unintentionally initiated, such blasts can result in severe damage to buildings, vehicles, and personnel. For example, a blast from a bomb that is detonated in a car parked near a building can cause structural damage to the building, damage components therein, and/or injure people within the building. Similarly, ballistic and aerial explosive devices can cause costly damage to buildings and other types of structures. An explosion originating in a cargo container can rupture the container and propagate therefrom. Explosive blasts can also travel through media other than air, for example, an underwater blast that propagates to a boat, submarine, or other vessel and inflicts damage. 
   The use of barriers for attenuating the blasts associated with explosions is well known. For example, buildings at risk of blast damage during battle conditions are sometimes protected by walls formed of concrete, sand bags, and the like. Such dense barriers provide a protective effect to an area by deflecting and/or attenuating the blast and thereby preventing the blast from reaching the protected area or at least reducing the momentum or overpressure of the blast that does propagate to the area. In some cases, however, the blast may refract over or around the barrier and propagate into the protected area. Additionally, the construction of barrier devices can be prohibitively expensive, and such barriers can be impractical for protecting high structures, structures in densely populated regions, mobile structures, or underwater structures. Further, barriers can detract from the aesthetic appeal of a structure or area. 
   Thus, there exists a need for a blast attenuation device that provides an effective and space efficient shield for a protected area, including an area that includes a tall structure, a structure in a densely populated region, a mobile structure, or an underwater structure. The shield should be cost effective for construction, operation, and maintenance. Further, the shield should be adaptable to minimize the aesthetic impact of the shield or to render the shield aesthetically appealing. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a system and method for producing a shield for protecting an area. The shield provides an attenuation of a pressure blast, and can be used with tall, mobile, and underwater structures, including structures in densely populated areas. 
   According to one embodiment, the present invention provides a shielding system for attenuating a pressure blast to shield a protected area. The system includes a source for providing an attenuation material, i.e., an absorbing material, and a delivery system with a plurality of nozzles fluidly connected to the source by one or more passages. A valve device is configured to control the delivery of the attenuation material through the nozzles. The valve device can be actuated by a detector in response to a perceived blast threat, for example, an approach of a blast originator toward the protected area. In one embodiment, pipes are disposed at a peripheral area of a building, and the nozzles can be configured to direct the shield to extend substantially vertically and proximate to walls of the building. 
   The source can provide solid attenuation particulates, water or other liquids that the nozzles deliver as droplets, or a gas delivered as bubbles in a liquid medium. The attenuation material can be delivered as particulates having an average size of between about 0.01 mm and 1.0 mm, and the shield can have a three dimensional, or volumetric, packing factor of between about 0.001 and 0.01. According to one aspect, the packing factor is non-uniform across its thickness, for example, to generally increase in a direction from the origination toward the protected area. 
   According to another embodiment, the present invention provides a pressure attenuation shield for attenuating a pressure blast and shielding a structure. The shield is formed of one or more sprays of attenuation material that are disposed proximate a periphery of the structure and between an origination of the pressure blast and the structure so that the shield attenuates the pressure blast by at least about 14.7 psi within a thickness of less than about 1 meter of the spray. According to one aspect, the shield includes first and second generally parallel walls disposed between an origination of the pressure blast and a protected area. A flexible host material such as a gelatinous fluid is disposed in the space between the walls, and an attenuation material is disposed as particulates suspended in the host material. The attenuation material is configured to attenuate the pressure blast and thereby reduce the pressure blast to below a damage threshold of a protected article in the protected area. The shield can be configured to form a cargo container. 
   The present invention also provides a method of attenuating a pressure blast to shield a protected area. The method includes detecting a threat of a pressure blast and, in response to the threat, spraying particulates to form the shield between an origination of the pressure blast and the protected area so that the shield attenuates the pressure blast from the origination. 
   Further, the present invention provides a method of constructing the system for attenuating a pressure blast and mitigating blast damage to a structure. The method includes determining a maximum initial pressure against which the structure is to be protected, determining an acceptable pressure to which the structure may be subjected, and selecting an attenuation material comprised of particles having a desired radius, mass density, and three-dimensional packing factor. A minimum thickness is determined, for example, according to a mathematical expression, for a particle mist of the attenuation material required to reduce the initial pressure to the acceptable pressure. A delivery system is mounted to the exterior surface of the structure such that the system is capable of providing the particle mist at least as thick as the determined minimum thickness. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
       FIG. 1  is perspective view of a blast attenuation system adapted to mitigate damage to a building according to one embodiment of the present invention; 
       FIG. 2  is a chart illustrating the thicknesses of blast attenuation shields of different particulate materials that are required for attenuating blast pressures to a final pressure of 0.25 psi; 
       FIG. 3  is a plan view of a blast shield with a non-uniform packing factor that partially reflects, partially attenuates, and partially transmits a blast shield according to one embodiment of the present invention; 
       FIG. 4  is a perspective view of a blast attenuation system adapted to mitigate damage to an underwater structure according to another embodiment of the present invention; and 
       FIG. 5  is a perspective view of a shield that is configured to form a cargo container according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
   Referring now to the figures, and in particular  FIG. 1 , there is shown a blast attenuation system  10  according to one embodiment of the present invention, which is configured to provide an attenuation shield  70  around a protected area  80 . The blast attenuation system  10  can similarly be used to protect other areas of any size and shape. Each protected area  80  can also include one or more structures such as buildings  82  or vehicles. The blast attenuation system  10  includes a delivery system  12  that includes a network of passages, such as pipes  14 , disposed at an outer periphery  84  of the protected area  80 . The pipes  14  can be formed of metal or plastic, and can be conventional pipes that are used in water distribution systems. The pipes  14  can be made an integral part of the building  82 , for example, by locating the pipes  14  partially within the exterior walls of the building  82 . Alternatively, the pipes  14  can be mounted on the exterior of the building  82  as shown in  FIG. 1 , for example, by adding the attenuation system  10  to the exterior of an existing building to thereby improve the protection of the building from blast damage. In any case, the attenuation system  10  can be designed to be visually unobtrusive or appealing, for example, by decorating the pipes  14  in a color or style that complements the exterior walls of the building  82 . 
   The pipes  14  are fluidly connected to a source that provides an attenuation material for delivery through the pipes  14 . The attenuation material can be a solid, liquid, or gas, as further described below. The source can be a water pipe that delivers water from a ground water supply  16  such as a public water supply system. Preferably, the source includes a reservoir that holds a volume of the attenuation material sufficient to provide the protective shield for at least a predetermined duration. For example, a water reservoir  18  can be located at the top of the building  82  and fluidly connected to the ground water supply  16  so that the attenuation system  10  remains operational even if a connection  20  to the ground water supply  16  is interrupted. The reservoir can also provide the attenuation material to other systems of the building  82 , for example, a sprinkler system or other fire extinguishing system. 
   The attenuation system  10  can be operated continuously, but preferably a valve device  22  is configured to control the flow of the attenuation material from the reservoir  18  to the delivery system  12  so that the attenuation system  10  can be turned on and off by adjusting the valve device  22  between open and closed positions. The valve device  22  can be manually operable so that an operator can initiate the system  10 , for example, to deploy the attenuation shield in response to a perceived blast threat. The valve device  22  can also be automatically operable by one or more detectors  24  configured to detect the perceived blast threat. For example, each detector  24  can be an optical or electromagnetic device adapted for detecting motion or heat and thereby detecting an unauthorized entry or approach to the protected area  80 , such as an entry through a barricade, fence, or restricted area. The detector  24  can also be configured to receive a signal transmitted from a communication device or input by an operator. In one advantageous embodiment of the invention, the valve device  22  and detector  24  are configured to react quickly to the perceived blast threat so that the valve device  22  can be repositioned in response to a possible blast originator, such as a vehicle, entering the detection zone outside the protected area  80 , and the shield  70  can be deployed before the possible originator reaches an outer periphery of the shield  70 . The valve device  22  can be a fast-acting solenoid or pyrotechnic valve, for example, with a response time of 0.10 milliseconds or less. 
   The pipes  14  or other passages of the delivery system  12  are configured to deliver the attenuation matter to a plurality of nozzles  26 . Preferably, the nozzles  26  are configured to deliver the attenuation material proximate to the periphery  84  of the protected area  80  and at least partially and, more commonly, completely surrounding the protected area  80 . For example, the pipes  14  can extend horizontally around the protected area  80  so that the protected area  80  is entirely enclosed horizontally, and the nozzles  26  can be configured to spray the attenuation material to form the shield  70  vertically. The pipes  14  can also be disposed at multiple elevations, thereby providing a uniform shield, which can be deployed more quickly and more uniformly than a shield sprayed from a single pipe. For example, as illustrated in  FIG. 1 , the protected area  80  includes the building  82 , and the pipes  14  are disposed at the top of the building  82  and at incrementally lower levels. Upon initiation of the system  10  depicted in  FIG. 1 , each of the nozzles  26  can begin spraying the attenuation material to form the shield  70  vertically. The shield  70  horizontally surrounds the building  82  such that a pressure blast originating outside the protected area  80  must propagate through the shield  70  to horizontally enter the protected area  80 . The delivery system  12  can also extend over or under parts of the enclosed area  80 , such as over a roof of the building  82 , so that the shield  70  extends horizontally to protect the protected area  80  from vertical propagation of the pressure blast. 
   The shield  70  can be formed of any type of material or combination of materials. In addition to liquids such as water, the attenuation material can comprise any solid materials, for example, sand, grains, or polystyrene foam in particulate form, such as Styrofoam® pellets. By the term “solid” it is not meant that the attenuation particles must be solid throughout. For example, the attenuation material can comprise shelled objects such as hollow balls similar to the type commonly used for table tennis, which are formed of celluloid or other polymer materials. Solid attenuation particulates can be delivered through the delivery system  12  described above, for example, by blowing air through the delivery system  12  to propel the solid particulates to the nozzles  26 , which can be adapted for delivering the solid particulates. The particulates can be collected in bins or drains located at the lower periphery of the protected area  80  below the nozzles  26 , and the particulates can be reclaimed for re-use in the attenuation system  10  or for other uses. Further, the delivery system  12  can be configured to deliver the attenuation material in any direction. For example, the delivery system  12  can be disposed at the peripheral base of the protected area and configured to deliver the attenuation material upwards to form a vertically extending shield. The delivery system  12  can comprise pipes, as described above, or the attenuation material can be delivered from a tray or channel, which can also be used to reclaim the attenuation material. 
   The effective attenuation of the shield is influenced by the pressure blast, a thickness D of the shield  70 , a radius r and density ρ p  of the individual particles of the attenuation material, a three-dimensional packing factor F of the attenuation material, and a density ρ a  of the ambient medium. The packing factor F is the ratio of the number of particles in a specific volume of the shield  70  relative to the maximum number of particles that can be disposed in the same volume. In one advantageous embodiment of the invention, the packing factor F is between about 0.001 and 0.01. 
   For cases where the density ρ p  of the particles of the attenuation material is much greater than the density ρ a  of the ambient medium, the required thickness D of the shield  70  for attenuating an initial pressure P i  due to the pressure blast to a final pressure P f  can be approximated by assuming that the attenuation material behaves according to a Brownian motion model. For example, the required thickness D can be determined according to the following equation: 
       D   =     1.24   ⁢     r     F     11   12         ⁢           (       ρ   p       ρ   a       )       1   4       ⁡     [     ln   ⁡     (       P   i       P   f       )       ]         1   2             
 
where the initial and final pressures P i , P f  are measured as overpressures or gauge pressures, i.e., pressures measured above the ambient pressure. Thus, if water is used as the attenuation material in an atmosphere of air at 100 kPa, the density ρ p  of the particles is about 1 grams/cubic centimeter and, the density ρ a  of the air is about 1.3 kilogram/cubic meter, and the thickness D of the shield  70  is given by: 
       D   =     6.53   ⁢           r     F     11   12         ⁡     [     ln   ⁡     (       P   i       P   f       )       ]         1   2       .           
 
   The thickness D of the shield  70  can be designed and adjusted according to the pressure blast threat and the necessary protection. For example, a bomb detonated outside the building  82  could cause a pressure blast to propagate to the building  82  and cause an initial overpressure pressure P i  of about 100 kPa (14.7 psi) to occur temporarily outside the shield  70 . Conventional windows, such as windows  83  on the building  82  of  FIG. 1 , typically break when subjected to an overpressure of about 0.5 psi, i.e., when the pressure outside the building  82  is 0.5 psi higher than the pressure within the building  82 .  FIG. 2  illustrates the attenuation effect of shields formed of sand, water, and polystyrene foam pellets with particles of radius r of 0.1 mm and a packing factor F of 0.001. As shown, the required thickness D for attenuating the blast to a final overpressure of 0.25 psi, i.e., so that the final pressure P f  is only 0.25 psi higher than the ambient pressure, varies according to the attenuation material and the initial overpressure P i . By reducing the final overpressure to only 0.25 psi, a safety factor of two is provided for preventing breakage of the windows  83  that are able to withstand an overpressure of 0.5 psi. 
   A variety of materials can be used for attenuation, and the thickness D can be adjusted according to the desired protection and the attenuation material. For example, an attenuation shield of water droplets with a radius r of 0.1 mm, a packing factor F of 0.001, and a thickness D of about 75 cm would reduce the initial pressure P i  of 100 kPa (14.7 psi) to a final pressure P f  of 0.25 psi, thus significantly reducing the probability that the windows  83  at the exterior of the building  82  will break. If the shield  70  is formed of droplets that are larger, for example, about 1 mm, the packing factor F can be increased to provide a similar attenuation effect. Similarly, if the shield is formed of a particles that are more or less dense than water, the thickness D or the packing factor F can be increased to provide a similar attenuation effect. Preferably, the attenuation material, radius r, and packing factor F, are selected so that the shield  70  attenuates an expected blast with an initial pressure P i  greater than 100 kPa by at least about 0.1 psi per cm of thickness D. For example, the shield  70  can be configured to attenuate such a blast by least about 14.7 psi within a thickness of less than about 1 meter of the shield  70 . 
   Further, the shield  70  can partially reflect the pressure blast away from the protected area  80  and thereby provide an additional protective effect to mitigate damage due to the blast. For example, upon impinging on the shield  70 , a pressure blast is partially reflected and partially transmitted due to the variation in impedance characteristics between the shield  70  and the ambient medium that results from the mismatched densities ρ p ,ρ a . Transmission into the shield  70  is enhanced if the densities ρ p ,ρ a  and, hence, the impedances of the shield  70  and the ambient medium are closely matched, and reflectance is increased if the impedances are mismatched. In one embodiment, the nozzles  26  are configured to deliver the attenuation matter so that the shield  70  is non-uniform, or stratified, throughout its thickness so that the shield  70  defines a packing factor F that is higher in some portions of the shield  70  and lower in other portions. The shield  70  can be configured so that the non-uniformities affect the reflectance and absorption characteristics of the shield  70 . For example, as shown in  FIG. 3 , the packing factor F can be made to increase in a direction extending from an origination  86  of a pressure blast toward the protected area  80  so that the pressure blast first impinges on the portion of the shield  70  where the packing factor F is lowest and then propagates through shield portions with increasingly higher packing factors F. Thus, the impedance of the shield  70  at an outer periphery of the shield  70  is closely matched to the ambient medium, and the reflection of the blast is minimized so that the pressure blast is transmitted into the shield  70  and attenuated therein. Further, the nozzles  26  are configured to deliver the attenuation material such that the packing factor F is highest at an inner periphery of the shield  70  so that the impedance of the shield  70  is mismatched with the ambient medium. Thus, after the pressure blast propagates to the inner periphery of the shield  70 , the impedance mismatch causes the blast to be partially reflected away from the protected area  80  and transmitted again through the shield  70  for further attenuation therein. Alternatively, the nozzles  26  can be configured to deliver the attenuation material such that the shield  70  has a high packing factor F at its outer periphery so that initial reflectance of the pressure blast is increased. In some cases, absorption of the pressure blast may be preferable to reflectance. For example, if the building  82  is located among other structures, reflectance of the pressure blast therefrom may increase the damage to the other nearby structures. Further, subsequent reflections of the blast may impinge on other portions of the building  82  that are not protected by the shield  70 , such as the roof of the building  82 . 
   According to another advantageous embodiment of the present invention, the attenuation material can comprise a gas such as air disposed as bubbles in a liquid medium. For example,  FIG. 4  illustrates a delivery system  12  that comprises a network of pipes  14  configured at the periphery  84  of the protected area  80  that includes an underwater structure  88  such as a submarine. The nozzles  26  are configured to deliver the air to form bubbles in the ambient medium, which is water in this embodiment. The air bubbles, which rise in the water, provide a shield  70   a  for protecting the protected area  80  from pressure blasts that propagate through the water, for example, originating from an underwater explosive such as a depth charge. The shield  70   a  can provide an attenuating effect similar to the effect described above. Additionally, the impedance mismatch between the shield  70   a  and the water can result in significant reflectance of the pressure blast away from the protected area thereby decreasing the final pressure P f  of the blast that propagates to the protected area  80  and mitigating the damage of the blast. 
   Although the shields  70 ,  70   a  are described above as a spray of the attenuation material, the particulates of the attenuation material can alternatively be configured as a static shield. For example, solid particulates can be embedded in a solid or liquid medium such as a flexible host material, such as sponge, feathers, foam, or gel, which is positioned between the protected area and the possible location of a blast origination. In one embodiment, illustrated in  FIG. 5 , a shield  70   b  is configured to form a double-hulled cargo container  100 . The container  100  defines a space between an inner wall  102  and an outer wall  104 . Particulates  72  of the attenuation material are disposed between the inner and outer walls  102 ,  104 , in the flexible host material that fills space. For example, particulates formed of sand, foam, or other materials can be disposed in any a gelatinous fluid or any other flexible host material. The shield  70   b  can be used to mitigate damage outside the container  100 , that results from a blast originating within the container  100  or to mitigate damage within the container  100  from a blast outside the container  100 . For example, if a bomb that is transported within the container  100  explodes, the shield  70   b  would mitigate damage to the vehicle transporting the container  100  as well as other cargo being transported by the vehicle. Preferably, the shield  70   b  provides sufficient attenuation to reduce an expected pressure blast to below a damage threshold of articles in the protected area. The protected articles can include cargo in the container  100 , other cargo near the container  100 , a vehicle used to transport the container  100 , and the like. The appropriate thickness D of the shield  70   b  can be determined according to the foregoing discussion. 
   Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.