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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION  
       [0001]     The field of this invention is trigger devices that are actuated downhole to operate a tool and more particularly triggers that are held immobilized until properly positioned and then released preferably with a spark, however initiated.  
       BACKGROUND OF THE INVENTION  
       [0002]     Space is always a concern in downhole tool design. Some tools need to be retained in an unset position until properly placed in the well. It is only when they are properly located that it is desired to set the tool. Such tools in the past have had trigger mechanisms that are retained in an immovable position for run in until proper placement of the associated downhole tool is achieved. One technique for holding things immobile until the tool is properly placed has involved disabling the trigger with a mechanical device that is held against movement by a Kevlar® high strength fiber and an associated electrically powered heat source generally powered by stored batteries in the downhole tool. The generation of sufficient heat burns the fibers and releases the trigger so that the tool can set. Such a system is described in U.S. Pat. No. 5,558,153. The problem is that to generate enough heat downhole to burn the fibers and not damage adjacent components proves to be challenging for several reasons. One issue is the physical size of the battery pack to get a heater hot enough for long enough to compromise the fibers. Another issue is the very high temperatures needed to undermine the fibers and the effect on the overall design of the tool from having to keep heat sensitive components away from the heated area.  
         [0003]     Another design featured a battery operated heater coil in a downhole tool to release the trigger by applying heat and melting a plug to start the setting sequence. This design is reflected in U.S. Pat. No. 6,382,234. Here again the same problems described above are encountered. The battery size to have the required electrical capacity to create enough heat to melt the fusible plug presents a very real space concern in a downhole tool where space for a large power supply is at a premium. The cost and the reliability of a large battery pack is an issue. On many occasions, safety is another issue since some batteries need special shipping and handling requirements.  
         [0004]     Other alternatives still involved the large battery pack to accomplish a release of the trigger. For example, U.S. Pat. No. 5,558,153 also suggests using solder wire that melts at relatively low temperatures to be the trigger material or using the stored power in the battery to advance a knife to physically cut the fiber as opposed to undermining it with a battery operated heat source.  
         [0005]     The present invention seeks to address the issues described above with the prior designs. One way it approached the problem is to choose a material that will readily go away by being combustible. The power then required to release the trigger is greatly reduced since it takes significantly less energy to create a spark that will create a burning of the trigger retainer to result in a release of the tool. What is proposed in one embodiment is the use of fuse material that is designed to be readily ignited. While no explosives need be set off, the fuse material that serves to hold the trigger against the setting of the tool simply readily ignites with a brief spark and burns to the point where it releases its hold on the setting mechanism and the tool simply sets. In another embodiment the wire burns and removes a barrier to flow that in turn set the tool. These and other advantages of the present invention will become more apparent to those skilled in the art from a review of the preferred embodiment that is described below along with the associated drawings while recognizing that the claims further below indicate the scope of the invention.  
       SUMMARY OF THE INVENTION  
       [0006]     A trigger device keeps a downhole tool from setting until it is properly positioned. Regardless of the type of tool or the type of associated trigger, the retainer is combusted which results in setting the tool. This can happen by freeing a piston to move, or allowing flow through a port or by other mechanisms. The material is preferably one that combusts readily such as a material used in fuses for explosives. The battery requirements are to simply create the brief spark that starts the combustion, making the battery size significantly smaller than what would be needed to power a heater to melt materials that were used in the past. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows one type of trigger using opposed sleeves held together by wire;  
         [0008]      FIG. 2  is the rotated view of  FIG. 1  showing the wire;  
         [0009]      FIG. 3  is an alternative embodiment of a trigger that uses a piston retained by a wire going through it;  
         [0010]      FIG. 4  shows an alternative embodiment to hold a piston with the wire until the tool is ready to be actuated when the wire is compromised and the piston moves;  
         [0011]      FIG. 5  shows another embodiment where an opening in a plug is created that sets the tool with the flow through the opening where the opening is created by compromising the wire.  
         [0012]      FIG. 6  is another embodiment showing a piston held by a shearing member with chambers on opposed sides so that when the material in the opening is compromised well hydrostatic reaches one side of the piston to break the shear device and move the piston against the opposite chamber to set the tool. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     Referring to  FIGS. 1 and 2 , one type of retainer for a trigger mechanism or device for a downhole tool is illustrated. It shows a piston  10  that has some applied force  12  acting on it that would otherwise make it move but for a restraint. In this case the restraint is a pair of sleeves  14  and  16  that are longitudinally split and held together by a spring or split ring  18 , for example. What completes the assembly to keep the piston  10  immovable despite the applied force  12  is a wire  20  shown in  FIG. 2 . In this application, the term “wire” refers to the shape as being elongate without limitation to composition or cross-sectional shape or area and without limitation to any specific function such as conducting power. The force  12  can come from a variety of sources such as hydrostatic pressure, various springs or other energy storage devices or equivalents. In general, movement of the piston  10  sets an associated tool that is not shown. While a piston is shown, any type of trigger for the downhole tool is envisioned regardless of shape or the nature of its movement or whether the movement directly or indirectly sets the underlying tool. The design of  FIGS. 1 and 2  contemplates variations such as retaining the piston  10  with a c-ring whose open end is held fast against the piston  10  to keep it from moving by the wire  20 . When the wire or ignitable material  20  is compromised the c-ring is pushed apart by the force  12  and the piston  10  advances to set the tool.  
         [0014]     The wire  20  is such that initial energy input to it, preferably in the form of a brief spark that can be actuated electrically, mechanically or by other equivalent methods, sets in motion an event that continues without need for further energy input. Because of this feature, the battery or other energy source  22  and the ultimate recipient of the energy or power that creates a spark  24  collectively can be significantly smaller than prior designs that required continuous power input to disable the wire  20 . Mechanical spark devices that employ relative movement to create a spark can also be used as well as other devices that will transmit the initial burst of energy necessary to disable the wire  20 . For example the wire  20  can be made from PYRO FUZE® which consists of two metallic elements in intimate contact with each other. When these two elements are brought to the initiating temperature, they alloy rapidly resulting in instant deflagration without support of oxygen. Initiation is by heat and heat alone. All that is required is the exposure of the composite to the proper minimum temperature. The trigger reaction will reach temperatures in excess of the boiling point of the constituents. Once started, the reaction will not stop until alloying is completed or the unalloyed composite is subjected to some form of massive cooling that overwhelms the composite so that it cannot reach minimum operating temperature. The reaction end products consist normally of tiny discreet particles of the alloy of the participating materials. The present PYROFUZE composition was chosen for a number of desirable characteristics; it is a composite of alloys of Palladium and Aluminum. It is available in the following physical forms: Wire; and, Ribbon rolled from wire.  
         [0015]     The PYROFUZE® reaction is not of an explosive or pyrotechnic nature. The only energy released is thermal (approximate minimum reaction temperature 2800° C./5000° F., 325 calories per gram, 2890 calories per cubic centimeter). The minimum initiation temperature is 650° C./1200° F.  
         [0016]     Wire:  
                                                                 Outer Jacket   Inner Core                                    Chemical composition:   Balance Palladium   #5056 Aluminum           5% Ruthenium       Resistivity:   62 ohms per cmf                  
 
         [0017]     Other materials are contemplated that also have explosive materials incorporated into the wire or result in a pyrotechnic response with the distinguishing characteristic being that the initial energy input that leads to the weakening of the wire  20  that ultimately lets the tool set continues after it is started without continuous energy input of the prior designs. Regardless of composition of the wire or the mechanism of the response to the energy input the defining difference is that the process continues without further energy input. Accordingly, fuses that are used to set explosives could function as wire  20 .  
         [0018]      FIG. 3  shows another way to run a wire  20  through a piston  10  to hold it against a force  12 . Here the wire  20  passes through the piston  10  while being held near opposite ends by anchors  26  and  28 .  
         [0019]      FIG. 4  illustrates a piston  40  held in a body  42  by use of the restraining material  44  and optionally further retained by an adhesive such as epoxy  46 . When the material  44  is ignited it burns to undermine itself and the surrounding adhesive  46 . At that point the tool (not shown) can be set from movement of the piston  40  or alternatively from fluid flow around it where the material  44  and adhesive  46  used to be. This can occur with or without piston movement.  
         [0020]      FIG. 5  does not use any piston. Instead a body  48  has an aperture  50  that is initially plugged by the ignitable material  52  optionally secured in a sealing manned with an adhesive  54 . Upon ignition of material  52  such as from a spark, however generated, the assembly that blocks the aperture  50 , and in so doing restrains an actuating member from operating, no longer resists differential pressure and flow through aperture  50  results in actuating the member that sets the tool.  
         [0021]      FIG. 6  illustrates a piston  60  held by a shear device  62 . Piston  60  separates atmospheric or low pressure chamber  64  from chamber  66 . Chamber  66  is also initially at atmospheric or low pressure that is well below the surrounding hydrostatic pressure at the anticipated depth for setting the tool. Plug  68  is in place in port  70  of chamber  66  to restrain the piston  60  stationary with the aid of shear device  62  which is optional if piston  60  is in pressure balance from chambers  64  and  66 . Plug  68  is made from a material that will be compromised with a brief spark and will continue to be compromised without additional energy input. This opens port  70  and puts a sufficient differential pressure on the piston  60  to break the shear device  62  and set the tool, not shown. In this example, well hydrostatic is used to move the piston after the spark.  
         [0022]     In a different alternative a pressurized chamber can be isolated from one side of the piston by a plug as illustrated in  FIG. 5 . When that plug disappears the pressurized chamber, that is higher than hydrostatic is allowed to cat on one side of the piston when the opposite side of that piston is exposed to well hydrostatic. As a result the piston moves and the tool sets.  
         [0023]     In yet other embodiments the heat given off from the spark igniting the material that continues to combust can also be harnessed to trigger the tool to set. In such embodiments the heat given off can cut a cord or compromise the actual retaining device to allow the tool to set.  
         [0024]     While the retaining member has been illustrated in the preferred embodiment to be a wire, other shapes are contemplated as it is the mechanism of what happens after initial energy input that sets the present invention apart and a variety of shapes for the retaining member are contemplated to be within the scope of the invention apart from a wire shape.  
         [0025]     While the preferred embodiment has been set forth above, those skilled in art will appreciate that the scope of the invention is significantly broader and as outlined in the claims which appear below.

Summary:
A trigger device keeps a downhole tool from setting until it is properly positioned. Regardless of the type of tool or the type of associated trigger, the retainer of the present invention is combusted. The material is preferably one that combusts readily such as a material used in fuses for explosives. The battery requirements are to simply create the brief spark that starts the combustion, making the battery size significantly smaller than what would be needed to power a heater to melt materials that were used in the past.