You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a safety vent valve. Yet more specifically, the present invention relates to a safety vent valve for a perforating gun system. 
   2. Description of Related Art 
   Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore. 
   One typical example of a perforating system  4  is shown in  FIG. 1 . As shown, the perforating system  4  comprises one or more perforating guns  6  strung together to form a perforating gun string  3 , these strings of guns can sometimes surpass a thousand feet of perforating length. Connector subs  18  provide connectivity between each adjacent gun  6  of the string  3 . Many gun systems, especially those comprised of long strings of individual guns, are conveyed via tubing  5 . Others may be deployed suspended on wireline or slickline (not shown). 
   Included with the perforating gun  6  are shaped charges  8  that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, quickly expanding explosive gases are formed whose force collapses the liner and ejects it from one end of the charge  8  at very high velocity in a pattern called a “jet”  12 . The jet  12  perforates the casing and the cement and creates a perforation  10  that extends into the surrounding formation  2 . The resulting perforation  10  provides fluid communication between the formation  2  and the inside of the wellbore  1 . In an underbalanced situation (where the formation pressure exceeds the wellbore pressure) formation fluids flow from the formation  2  into the wellbore  1 , thereby increasing the pressure of the wellbore  1 . Moreover, as the explosive gases cool and contract, a large pressure gradient is created between the inside of the perforating gun body  14  and the wellbore  1 . This pressure differential in turn draws wellbore fluid within the perforating gun body  14  through gun apertures  16 . 
     FIGS. 2   a  and  2   b  illustrate a portion of a gun string  3  for providing additional detail of the connector sub  18  disposed between the two perforating guns  6 . As shown, the connector sub  18  has a protruding member  19  on each of its ends formed to mate with a corresponding recess  21  provided on the end of each perforating gun  6 . The guns  6  as shown are secured to the connector sub  18  by a series of threads  23  formed on the inner diameter of the recesses  21  and the outer diameter of the protruding member  19 . 
   Also disposed within the gun string is a detonating cord  20  for providing an initiating/detonating means for the shaped charge  8 . Detonation of the shaped charge  8  is accomplished by activating the detonating cord  20  that in turn produces a percussive shockwave for commencing detonation of the shaped charge explosive  8 . Typically the shockwave is initiated in the detonating cord  20  at its top end (i.e. closest to the surface  9 ) and travels downward through the gun string  3 . To ensure propagation of the shockwave to each individual gun  6  making up the gun string  3 , each connecting sub  18  is also equipped with a section of detonating cord  20 . The section of detonating cord  20  in the connecting sub  18  resides in a cavity  22  formed therein. Transfer charges  24  on the end of each segment of the detonating cord  20  continue travel of the shock wave from the end of one gun body  6 , to the section of detonating cord  20  in the connecting sub  18 , from the connecting sub  18  to the next adjacent gun body  6 , and so on. The shock wave transfer function of the transfer charges  24  produces a passage  26  between the gun bodies  6  and the connecting sub  18 . As shown in  FIG. 2   b,  the shaped charge  8  detonates in response to exposure of the shock wave produced by the detonating cord  20 . Detonation of the shaped charge  8  in turn leaves an aperture  16  that provides fluid flow from the wellbore  1  to inside of the gun body  14 . Similarly, detonation of the transfer charges  24  in response to the detonating cord shock wave, creates the passage  26  provides a fluid flow conduit between the inside of the perforating gun bodies  6  and the connecting sub cavity  22 . Accordingly, the cavity  22  is subject to wellbore pressures subsequent to exposure of the detonating cord shock wave. Often the debris within the wellbore fluid can be carried with the fluid into the cavity  22 . When retrieving the gun system  4  from the wellbore  1 , the cavities  22  will be vertically oriented that in turn can allow the fluid debris to collect within the passages  26  thereby creating a potential clogging situation that can trap the wellbore fluid within the connecting sub  18 . Since the wellbore fluid pressure can often exceed  1000  psi, this trapped pressure can present a personnel hazard during disassembly of the gun string  3 . Therefore, an apparatus and method for eliminating the potential for trapped pressure within the connecting sub  18  is needed. 
   BRIEF SUMMARY OF THE INVENTION 
   An embodiment of the present invention involves a connecting sub comprising a housing, a pressure producing element within the housing, and a vent valve in operable communication with the pressure producing element, wherein the vent valve is selectively opened in response to activation of the pressure producing element. The connecting sub may further comprise a cavity formed within the housing. When the vent valve is in the opened position it provides fluid communication between the cavity and the outside of the housing. A frangible member may be included within the vent valve. The pressure producing element may comprise a detonating cord. The pressure producing element may include a shock wave producing member, such as a detonating cord, or a combustible material, such as a propellant. 
   One embodiment of the connecting sub may comprise a first end, a second end, a perforating gun attachable to the first end, a shock wave producing member disposed within the perforating gun, a first transfer charge combinable with the connecting sub shock wave producing member and a second transfer charge combinable with the perforating gun shock wave producing member. A second perforating gun may be included with the connecting sub attachable to the second end, a shock wave producing member disposed within the second perforating gun, a third transfer charge combinable with the connecting sub shock wave producing member and a fourth transfer charge combinable with the second perforating gun shock wave producing member. A retaining ring coupled to the housing and to the vent valve can also be included with the connecting sub. 
   The connecting sub can further comprise a coupling member coupled to the shock wave producing member. The coupling member can be an opening formed to receive the shockwave producing member therethrough, a hook shaped member, or opposing elements formed to receive the shockwave producing member therebetween. 
   A method of safely venting a downhole tool is included herein. The method includes providing a frangible element on the downhole tool, activating a pressure producing substance, wherein activating the pressure producing substance ruptures the frangible element thereby creating apertures through the wall of the downhole tool to create fluid communication between the inner and outer surfaces of the downhole tool. The pressure producing substance can include a detonating cord, a propellant, as well as combinations thereof. Fluid communication between the inside and outside of the downhole tool. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a partial cutaway side view of a perforating system. 
       FIG. 2   a  illustrates a partial cutaway of a portion of a perforating string. 
       FIG. 2   b  depicts a partial cutaway of a portion of a perforating string. 
       FIG. 3  is a cutaway side view of a segment of a perforating string in accordance with an embodiment of the present disclosure. 
       FIG. 4  is a perspective view of a cutaway of a vent valve. 
       FIG. 5  is a cutaway side view of a segment of a perforating string in accordance with an embodiment of the present disclosure. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The device of the present disclosure comprises a safety vent valve useful for relieving fluid pressure within a downhole tool. With reference now to  FIG. 3  one example of a downhole tool with a vent valve is illustrated. More specifically, the embodiment shown is a segment of a perforating string  31  that comprises a connector sub  28  and gun bodies  32 , where the gun bodies  32  are disposed on both ends of the connector sub  28 , the connector sub  28  is shown without shaped charge explosives. The embodiment of the connector sub  34  of  FIG. 3  comprises a housing  39  having a cavity  48  formed therein and configured on both of its ends for coupling with a perforating gun  32 . One example of a coupling means comprises threads  41  disposed on the outer surface of the ends of the housing  39  formed to mate with corresponding threads on the inner circumference of the end of the gun bodies  32 . A recess  35  is provided within the wall of the connector sub  28  extending from the outer surface of the connector sub  28  into a cavity  48  residing within the body of the cavity  48 . While the recess  35  is shown in an orientation substantially perpendicular to the axis of the connector sub  28 , it is not limited to this configuration but instead can be formed at any other angle between the outer surface of the connector sub  28  and the cavity  48 . In the embodiment of the connector sub  28  of  FIG. 3 , the cavity  48  is sealed and thus not in fluid communication with either the gun bodies  32  or its outer surface. Bulkheads  55 ,  56  at the mating edges of both the connector sub  28  and the gun bodies  32  are formed of rigid non-porous material, thereby creating a fluid flow barrier. Additionally, as discussed in more detail below, the presence of a vent valve  34  in the recess  34  prevents fluid flow therethrough when the vent valve  34  is in the closed configuration. 
   The recess  35  provided in the connector sub  28  is formed to receive the vent valve  34 . The vent valve  34  as illustrated comprises a body  38  formed into a generally annular configuration. An embodiment of the vent valve  34  is provided in a cross sectional view in  FIG. 4 . However the vent valve  34  of the present disclosure is not limited to the embodiment of  FIG. 4 , but can instead include any suitable cross sections such as rectangular, oval, a multi-sided configuration (hexagonal, octagonal, etc), or any other suitable form. The vent valve  34  shown also includes a membrane  40  disposed within its body  38  that lies in a plane substantially perpendicular to the axis of the vent valve  34 . The vent valve  34  can be a uni-body construction machined from a single piece of stock material, or can be comprised of two separate segments joined together proximate to the location of the membrane  40 . 
   The membrane  40  of the embodiment of  FIG. 3  and  FIG. 4  fully encompasses the annular region within the body  38  thereby preventing fluid flow through the vent valve  34 —when in this configuration. However the membrane  40  is frangible and thus when ruptured, can allow fluid through the vent valve  34 . One example of a suitable membrane for use with the present device is a rupture disk. An example of a suitable material for the vent valve  34  and sub is any alloy steel capable of withstanding the expected downhole conditions. Other alternatives include glass, ceramic, aluminum, cast iron, plastics, and articles formed from NYLON®. Proper choice of material is well within the scope of those skilled in the art. 
   The body  38  further comprises a skirt section  44  extending downward from the membrane  40 ; optionally included within the skirt  44  is an opening  46  that provides a passageway through the skirt  44 . The opening  46  is aligned generally perpendicular to the axis of the housing  38 . The opening  46  should have dimensions sufficient to accommodate the detonating cord  36  to pass therethrough. One embodiment of the vent valve  34  may include a shoulder stop  45  formed on the outer circumference of the body  38  in an orientation generally coaxial to the body  38 . In the embodiment including the shoulder stop  45 , the recess  35  will have an increased diameter proximate to its opening to receive the shoulder stop  45  therein. A ridge  47  formed by a reduction in the recess diameter should be included in cooperation with the shoulder stop  45 , proper placement of the shoulder stop  45  in conjunction with the ridge  47  can situate the opening  46  within the cavity  48  for proper placement of the detonating cord  36  therethrough. Once spatially aligned, the vent valve  34  can be rotated (if needed) for alignment with the detonating cord  36 . 
   The vent valve  34  can be retained within the recess  35  with a retaining ring  50 . The ring  50  can be disposed within the recess in any number of ways, such as threaded, press fit, snap ring, welded, or any other suitable manner. 
   It should be pointed out that the vent valve  34  of the present device is not limited to those having a frangible member such as the membrane, but instead can include any device or apparatus responsive to shock waves. One additional example could be that of a sliding manifold having strategically placed ports such that the member when pushed upward in response to a shock wave, the ports could be situated to allow fluid communication from the cavity  48  of the connector sub  28  to the outer surroundings of the connector sub  28 . Another alternative embodiment includes a spring-loaded relief valve that is responsive to a pressure differential between the cavity and ambient conditions, and opens when the cavity pressure exceeds ambient pressure by some set amount. The spring loading could then reseat the valve for repeated uses and or repeated pressure loadings. 
   A portion of a detonating system  33  is shown within the connector sub  28  and gun bodies  32 . The portion of the detonating system  33  shown comprises, detonating cords  36  and transfer charges  37  and extends through the gun bodies  32  as well as into the connector sub  28 . As previously discussed, initiation of detonation systems typically occurs on the section of the detonating system closest to the surface  9 . Initiation of the detonating system  33  produces a shock wave within the detonating cord  36  that propagates downward through the detonating system  33  (and cord  36 ). Moreover, the shockwave is transferred between successive segments of the gun string (i.e. adjacent gun bodies  32  and the connector sub  28 ) by virtue of the transfer charges  37  provided at the terminating point of each end of the detonating cord  36  within segment. The detonating cord  36  can be of any shape (i.e. round, flat, smaller, larger diameter, and varying diameter), the chemical composition of the detonating cord is also not limited to a single composition. The detonating cord for use with the device and apparatus herein described can include any cord useful in transferring a shock wave along a string wherein the shock wave can activate a vent device. Additionally, electrical detonators may be used as a means for producing the aforementioned shock wave. 
   Optionally, the rupturing step may be accomplished by pressure formed by combustion of a material, such as the combustion of a propellant. The combustible material could be situated proximate to the frangible portion of the vent valve wherein the high pressure resulting from the ensuing combustion exerts a sufficient force on the frangible portion to cause it to rupture. Optionally, the region housing the combustible material could be sealed thereby allowing the pressure to build in order to cause the rupture of the frangible portion. Thus instead of an instantaneous micro-second event, the device of the present disclosure could be activated with a combusting compound acting on a millisecond time basis. 
   In operation, a perforating string having the segment  31  of  FIG. 3  is disposed in a wellbore  1  for perforating the wellbore  1 . As previously discussed, perforating the wellbore  1  is accomplished by activating a detonation system of the perforating string that in turn detonates the shaped charges  30  associated with the perforating system. Detonation of the shaped charges occurs in response to the shock wave of the detonation system. Activation of the detonation system is accomplished by actuating a firing head. As is known, firing heads are typically included with the perforating string in its uppermost segment and are in electrical or mechanical communication with the detonating cord. Upon activation of the detonating system, the resulting shock wave travels along the length of the detonation system and passes through each segment of the detonating cord  36 . The membrane  40  of  FIG. 3  is frangably configured to burst in response to exposure of the pressure formed due to the shock wave passing through detonating cord  36 . Bursting the membrane  40  removes the fluid flow barrier of the vent valve  34  and in turn provides open fluid communication between the cavity  48  and the topside of the connector sub  28 . Thus the same shock wave that causes detonation of the shock waves also allows venting between the cavity  48  and the region ambient to the connector sub  28 . 
     FIG. 5  illustrates an embodiment of the perforating string segment  31  a after detonation of the detonating system. Here the discharge of the shaped charge causes either fragmentation or disintegration of its individual elements, and is thus no longer present. Similarly, the detonating cord  36  and transfer charges  37  have been expended during use and are also not present. The resulting detonations of the shaped charges provide an aperture  54  through the wall of the gun body  32   a  and the discharge of the transfer charges  37  similarly produce passages  52  between the connector sub  28   a  and the adjacent gun bodies  32   a  thereby allowing fluid flow from the respective gun bodies  32   a  into the cavity  48   a.  This results in a fluid flow path A 1  from outside of the gun bodies  32   a  into the cavity  48   a.  Moreover, the rupture of the membrane  40   a  allows free flow of fluid from the cavity  48   a  to outside of the connector sub  28   a.  Accordingly, if during retrieval of the string segment  31   a  the passages  52  become blocked, the free flow of fluid through the now opened vent valve  34   a  prevents any pressure differential between the cavity  48   a  and ambient to the connector sub  28   a.    
   The membrane thickness can be reduced at strategically selected locations along the surface of the membrane  40  to ensure its rupturing in response to an applied shock wave. Optionally, the membrane  40  can include a scored portion  42  along the surface of one of its sides to facilitate bursting the membrane  40 . Also alternatively, the coupling member for joining the detonating cord  36  with the vent valve is not limited to the opening  46  but may include a coupling member that is a J-shaped member for coupling the vent valve  34  with the detonating cord  36 . Additionally, the coupling member may comprise multiple flexible elements for coupling with the cord  36 . It should be pointed out that the generation of a shock wave is not limited to the use of a detonating cord. 
   The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, the invention described herein is applicable to any shaped charge phasing as well as any density of shaped charge. Moreover, the invention can be utilized with any size of perforating gun. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.

Summary:
A perforating system connection sub comprising a vent valve for providing fluid flow communication through the connection sub wall. The vent valve is selectively opened and may include a frangible member. The frangible member is rupterable by the shock wave produced by ignition of an associated detonation cord.