CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. provisional patent application Ser. No. 61/242,251, filed on Sep. 14, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a system and method for landing/positioning a device at a known depth within a tubing string of wellbore without the use of dine, wireline, slickline or similar vehicle lowered from the surface. The present invention is preferably utilized with explosive devices to position a charge in a tubing string for purposes of perforating, cutting, pipe recovery, plugging or similar exercises. More particularly, the invention relates to placement of explosive charges within a tubing string by utilizing restrictions formed in the drill string at known depths to engage explosive devices dropped or otherwise pumped down the drill string. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention provides a series of profile subs which distributed within a pipe string to form a plurality of spaced apart flanges internally disposed around the perimeter of the pipe string along its length. Each flange is characterized by a cross-sectional profile of varying shape with an aperture of a predetermined diameter formed therein. The profile subs are arranged so that the flanges form a series of ever decreasing inner diameter restrictions as the pipe string extends deeper into a wellbore. Utilized in conjunction with these profile subs are a plurality of housings of varying external diameter. These housings are configured to be secured to the exterior of a down hole tool or device, such as a firing head, to permit the device to be landed on a flange at a desired depth. The known distance from the flange to precisely where the device functions in the drillstring is critical to the ability to predict what device is best suited to achieving the desired result. 
         [0004]    More specifically, the intent is to install these profile subs strategically throughout a pipe string, such as a drill string, drill pipe, drill collars, tubing, tubulars or casing, from largest diameter restriction profile to smallest diameter restriction profile so that a device of known designed outer diameter, when dropped or pumped from the surface through the pipe string, will pass through the pipe string until the device strikes a profile beyond which it cannot pass, namely a profile with an inner diameter smaller than the outer diameter of the device. A metal to metal (or other) seal will enable pressure to be applied to the device for various purposes, such as, for example, triggering of a firing head. The type of device utilized in the system can be any tool utilized in down hole applications. 
         [0005]    Although not intended to be limited for use with any particular device, the system is particularly useful in pipe recovery operations that may use tools such as a perforating gun, a jet cutter, a severing tool, torch cutter or a chemical cutter. 
         [0006]    An additional embodiment of the invention incorporates a restriction or internal profile as described above with a specially designed heavyweight drill collar or sub. The heavyweight sub includes a section with a reduced external diameter, relative to the upper and lower portions of the sub. Utilizing an internal profile positioned above the reduced external diameter section of the heavyweight sub, a device to be activated (such as a jet cutter or torch cutter) can be positioned so that the explosive charge is located adjacent the section of the sub with the external diameter, thereby permitting the heavyweight sub to be easily cut. Significantly, once the cut is made and the upper section of drillstring is withdrawn, the severed end of the reduced section at the top of the drillstring still remaining in the borehole is easily accessed by conventional “fishing” grappling technology because the severed end is not excessively flared. This reduced external diameter pipe section also facilitates perforating operations previously made very difficult if not impossible by the thickness of the drill collar. The tensile strength of the sub is designed such that it exceeds the weakest link in the remaining drillstring so that the sub is not the weak point in the drillstring. 
         [0007]    A sleeve or bushing may be installed over the reduced external diameter section of the heavyweight sub to ensure that the sub buckling threshold of the sub is maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates the cross-section of a pipe string with a series of flanges disposed therein to form decreasing restrictions along the length of the pipe string. 
           [0009]      FIG. 2  illustrates one possible profile of a housing that can be attached to device, such as a firing head. 
           [0010]      FIG. 3  illustrates a housing mounted on an object and seated on a flange. 
           [0011]      FIG. 4  illustrates a heavy weight sub that can be utilized with the system of the invention to more easily cut thick walled subs. 
           [0012]      FIG. 5  illustrates a sleeve that can be utilized in conjunction with the heavy weight sub of  FIG. 4  to add strength to the heavy weight sub prior to cutting operations. 
           [0013]      FIG. 6  illustrates a firing head employing a housing configured to seat in a flange as shown in  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    As shown in  FIG. 1 , the invention consists of a series of subs  10   a ,  10   b ,  10   c ,  10   d , with internal profiles  12  of varying diameter “D” forming restrictions in the interior annulus of the subs. The subs are arranged in a pipe string  11  extending from the surface  13  into a wellbore  15  so that the largest diameter profile or restriction is nearest to the surface, with ever decreasing (in diameter) profiles, such that the deepest/lowest sub in the string has the smallest diameter profile or restriction. For example, in  FIG. 1 , profile  12   a  of sub  10   a , nearest the surface  13 , has the largest diameter D a  restriction, while profile  12   d  of sub  10   d , deepest in wellbore  15 , has the smallest diameter D d  restriction. The consecutive diameters D a , D b , D c  and D d  decrease with depth in wellbore  15 . In any event, the flanges are disposed to engage the outer housing  14  (shown in  FIG. 2 ) of a tool, device or object being pumped, dropped or otherwise lowered or inserted or conveyed along the pipe string. 
         [0015]    In one preferred embodiment, the pipe subs  10  are only approximately two feet long and can be readily threaded into a pipe string during make-up. In one embodiment of the invention, up to five pipe subs  10  are provided and arranged so that the restriction diameter between consecutive subs decreases from the first sub (nearest the surface) to the last sub (deepest in the wellbore) in the pipe string, wherein another embodiment, at least five such pipe subs are provided. In one embodiment of the invention, at least ten pipe subs  10  are provided and arranged so that the restriction diameter between consecutive subs decreases from the first sub (nearest the surface) to the last sub (deepest in the wellbore) in the pipe string. In one embodiment of the invention, at least twenty pipe subs  10  are provided and arranged so that the restriction diameter between consecutive subs decreases from the first sub (nearest the surface) to the last sub (deepest in the wellbore) in the pipe string. In one embodiment of the invention, at least fifty pipe subs  10  are provided and arranged so that the restriction diameter between consecutive subs decreases from the first sub (nearest the surface) to the last sub (deepest in the wellbore) in the pipe string. Of course, the number of pipe subs and restrictions will depend on the length of the overall pipe string and the diameter of the pipe in which restrictions are formed. 
         [0016]    While the restrictions may take any shape, in the preferred embodiment, the restrictions are formed of a lip or flange  16  symmetrically disposed around the interior  18  of a pipe sub  10 , thereby forming an aperture axially aligned relative to the pipe sub. Those skilled in the art will appreciate that the flange  16  need not extend fully around the interior of the pipe sub so long as an aperture functioning as a restriction is formed, thereby creating a seat on which an object can land. Nor does the aperture need to be symmetrical or axially formed relative to the pipe sub, so long at the overall system comprises apertures of varying size arranged in consecutive order as described herein. For example, flange  16  may take the form of one or more tabs, fingers or projections extending into the annulus of a pipe sub so as to form a “restriction” therein. 
         [0017]    In one embodiment, the flange  16  has an upper surface  17  and a lower surface  19 . The upper surface  17  is contoured so as to engage an object provided with similarly contoured profile, thereby permitting a seal to be formed between the object and the flange when the object is seated on upper surface  17 . In the example of  FIG. 1 , upper surface  17  is curved to form a concave profile and disposed to receive an object with a correspondingly rounded or football shape (such as is shown on housing  14  of  FIG. 2 ). Once an object is seated, a seal is formed between the object and the flange  16  as pressure is applied to the object by the fluid column above the object or otherwise by the downwardly pumped fluid to the extent the object is disposed to pass fluid therethrough. In one example, if the object is an explosive device, pressure from the surface applied to the upper end of the explosive device not only maintains the seal as described, but may also be utilized to activate the explosive charge below the seal. 
         [0018]    While the tool, device or object conveyed in a pipe string may be externally shape itself for landing on and engaging a flange  16 , the intent of the invention is to provide a universal locator housing  14  that can be secured to the exterior of a variety of standard down hole devices or tools, thereby providing flexibility in the system for use with whatever tool and for whatever purpose is desired. Thus, in one embodiment of the invention, locator housing  14  may be integrally formed as part of the device with which it is utilized, while in another embodiment of the invention, housing  14  may be secured to the exterior of such device. For example, housing  14  may be provided with internal threads  20 , such as shown in  FIG. 2 , disposed to engage the external threads  22  provided on the external surface of a device  24 . Of course, while threads are described as the attachment mechanism, any fasteners or locking system may be utilized so long as housing  14  is secured to a device  24  to create the external landing profile described herein. 
         [0019]    Likewise, while the locator housing  14  shown in  FIG. 2  had an elongated, convex shaped external surface having the profile of a football, the housing  14  may be of any desired shape so long as it is capable of engaging the corresponding flange with which the housing is intended to land. It is the outer most diameter or perimeter dimension that is the key since it is this dimension that determines if the housing will engage a particular restriction or pass through the restriction. In another preferred embodiment, for example, housing  14  could simply be a ring or collar or external flange of a predetermined outer diameter disposed to seat against a corresponding internal flange within a pipe section. In this regard, while it is desirable that a seal be formed between the object and the flange, it is not a necessity with respect to positioning of the object at a specific, known location. 
         [0020]    In one preferred embodiment, locator housing  14  is secured on (or otherwise integrally formed with) a flow-through tube  26 , shown in  FIG. 2 , which allows fluid to flow by the location at which the housing  14  has landed. Thus, fluid may continue to be used downstream of the flange for various purposes. Preferably, flow-through tube  26  is universally configured to attach to a variety of tools. In the embodiment of  FIG. 2 , for example, flow-through tube  26  is disposed to thread onto the end of device  24 . Those skilled in the art will appreciate that in embodiments employing a flow-through tube or similar vehicle, the length of tube  26  can readily be altered to place the device  24  at a location a predetermined distance away from the flange  16  on which housing  14  is seated. More specifically, flow-through tube  26  has an interior annulus  28 . The upsteam end of tube  26  is provide with pressure/flow ports  30  which permit flow to continue through tube  26  once housing  14  is seated on flange  16 . Moreover, ports  30  permit flow through tube  36  once an operation is complete. As such, upon retrieval of tube  26 , the formation of a fluid column above tube  26  is avoided. In this same vein, tube  26  may also be provided with a fishing head  32  or similar structure to permit retrieval of the tube. Threads  22  are disposed on the exterior of tube  26  thereby permitting housing  14  to be readily secured thereto. As mentioned above, the length of tube  26  can be altered as desired to adjust the positioning of device  24  relative to housing  14  (and the flange  16  on which housing  14  is seated.) Of course, while a flow-through tube  26  is most desirable for this placement method, any type of hollow or solid body vehicle can be used for placement of a tool using this method. In one embodiment, it is the ability to select a vehicle of a varied length that forms one novel feature of the invention. 
         [0021]    Of course, while an embodiment having flow-though capabilities is desirable, it is not a necessity. Thus, flow-through tube  26  could simply be an elongated, solid or hollow tube, rod, shaft or other placement vehicle capable of attachment to a device  24  so as to position the device  24  at a desired distance away from (generally below) the flange on which housing  14  has landed. By adjusting the length of such vehicle, the specific placement of device  24  within a wellbore can be achieved. Notably, while the invention generally contemplates that the device  24  will be positioned below a landing flange, the system may also be utilized to position a device above a landing flange. In such case, the housing  14  would be downstream or below the device  24  to which it is attached. 
         [0022]    With more specific reference to  FIG. 2 , in this illustration of the invention, device  24  is a firing head  34 . As shown, tube  26  is secured to the upper end of firing head  34  so that the annulus  28  of tube  26  is in fluid communication with the interior of firing head  34 . In this embodiment, firing head  34  is a direct pressure firing head that can be activated using pressure provided by the fluid passing through tube  26 . Firing head  26  is provided with flow access ports  38  which are exposed during the firing process. 
         [0023]    With reference to  FIG. 3 , housing  14  is shown mounted on a tube  26  and seated against a flange  16   c . As can be seen, the outer diameter OD of housing  14  is slightly larger than the inner diameter D c  of the restriction formed by flange  16   c , such that housing  14  seats against flange  16   c  after having passed through the larger diameter restrictions D a  and D b  of flanges  16   a  and  16   b , respectively. In seating against flange  16   c , a metal to metal seal is formed between housing  14  and flange  16   c . Tubing  26  may also be provided with an additional set of flow ports  38  to permit flow before, during or after an operation, as desired. 
         [0024]    In any event, the housing  14  generally seats in the restriction above where the desired operation is to take place, thereby sealing off the upper end of the tubing string from the location of the operation. The downward fluid pressure against housing  14  maintains the seal during such operation. Fluid passing through apertures  30  into the interior of tube  26  can be utilized to control the desired operation. 
         [0025]    Turning to  FIG. 4 , an additional component of the system can include a sub  40  specifically configured to be easily cut when a charge is positioned adjacent thereto utilizing the flange  16  and housing  14  described herein. Specifically, heavyweight or thick-walled subs are commonly found on the lower end of a pipe strings in wellbores. Those skilled in the art can appreciate that such subs are more difficult to cut, sever or perforate than standard subs due to the thicker walls characteristic of heavyweight subs. Such subs typically require much greater amounts of explosive for these operations. Naturally, larger charges tend to result in greater damage in the area of the detonation, which could frustrate the purpose of the operation, such as recovery efforts. For example, severing of a heavyweight sub utilizing a standard severing tool typically results in a large flare at the point of the cut. This large flare can inhibit access to and recovery of the pipe below the flare. 
         [0026]    In the invention, sub  40  is generally provided with an out diameter “d” to correspond with the outer diameter of the drill string with which sub  40  is utilized. A portion “b” of sub  40 , however, is provided with an outer diameter “c” that is less than outer diameter “d”. Since the interior annulus  42  of sub  40  is preferably of a constant inner diameter along the length of sub  40 , the result is that the wall thickness along portion “b” is less than the wall thickness along the rest of sub  40 , permitting sub  40  to be more easily cut, perforated or severed along portion “b” relative to the thicker walled portions of sub  40 . In the preferred embodiment, sub  40  includes a flange  16 , positioned upstream of portion “b” a distance of “a”. Thus, knowing distance “a” and utilizing a firing head such as is shown in  FIG. 2  and a placement vehicle such as flow-through tube  26 , an explosive device can be precisely positioned within sub  40  adjacent portion “b.” Those skilled in the art will appreciate that sub  40  can be integrally formed or can be formed in two or more attachable sections. For example, portion “b” may be externally threaded to secure to internal threads provided in the ends of sub  40  with larger diameters “d.” 
         [0027]    An optional blowout plug  60  may also be provide in the wall of sub  40 . Plug  60  can be utilized when circulation through sub  40  is impeded, which could prevent housing  14  from properly seating on flange  16 . By opening plug  60 , circulation through sub  40  can be enhanced, thereby allowing housing  14  (attached to a device  24 ) to be pumped down the pipe string until it is seated as desired. 
         [0028]    Those skilled in the art will appreciate that the reduced portion “b” of sub  40  not only permits the use of less explosive during cutting operations, it results in less damage to the downstream portion of the sub  40 . In other words, the system leaves very little flare or destruction of the reduced portion “b” of sub  40 , thereby permitting the severed portion to be more easily fished than prior art subs, which were typically lost below the cut. 
         [0029]    With reference to  FIG. 5 , sub  40  may be provided with a bushing or sleeve  46  disposed to strengthen portion “b” of sub  40  during run-in and operations prior to severing. As shown sleeve  46  if formed of an elongated tube  48  approximately the length of portion “b” of sub  40 . At a first end, tube  48  is provide with interior threads  50  which are disposed to engage with exterior threads  52  provided on sub  40  at the approximate upstream transition point between the thin walled diameter “c” and the thick walled diameter “d” of sub  40 . A lag screw may be provided at the first end to further secure tube  48  to the upper end of sub  40 . At the second end of tube  48 , it is seen that no such threads are provided. Rather, the end and interior surface of tube  48  are disposed to seat in a notch  50  disposed around the perimeter of sub  40 , thereby forming a metal to metal seal therebetween. In one embodiment of the invention, tube  48  may include apertures  53  to allow fluid flow into the annular space  54  formed between the exterior of portion “b” and the interior of tube  48 . While sleeve  46  provides anti-buckling support during normal operations, those skilled in the art will appreciate that upon severing, sleeve  46 , being threadingly engaged with sub  40 , remains attached to the portion of sub  40  that is withdrawn or pulled from the wellbore following a cutting operation. Moreover, sleeve  46  is desirable because it maintains a uniform diameter for sub  40 , and hence, keeps annulus  54  free of debris prior to severing of portion “b” of sub  40 . 
         [0030]    As further illustrated in  FIG. 5 , in another embodiment of the invention, a ring  56  can be provided at the top of sub  40  in the pin box  58  during make-up, thereby providing a flange  14  as described above. Ring  56  can simply be inserted into box  58  and positioned as shown or ring  56  may be secured therein, such as, for example, by providing ring  56  with external threads disposed to engage a portion of the threads in pin box  58  prior to stabbing in another pipe joint. 
         [0031]    While those skilled in the art will understand that the device can be any type of tool or equipment that might otherwise be conveyed into a pipe string, in one preferred embodiment illustrated in  FIG. 6 , the device is a firing head  34  that employs a housing  14  configured to seat on a flange  16  (see  FIG. 1 ) is shown. In this embodiment, housing  14  is integrated with firing head  34  and includes a fishing neck  32  and ports  30  and  38  as previously described. Firing head  34  is designed to be pumped or dropped down a pipe string as described above without the aid of a conveyance vehicle, such as coiled tubing. One of the unique features of the this embodiment is that firing head  34  is disposed to permit fluid flow through it. As such, following firing, the firing head can be fished from the wellbore without having a fluid column built up above the firing head during retrieval. Specifically, in this embodiment of the invention, a piston  60  is slidingly mounted in annulus  28 . Piston  60  has an upper pressure surface  62  in fluid communication with annulus  28 . As such, fluid within annulus  28  can be utilized to activate piston  60 . Piston  60  is attached to firing pin  64 . Firing pin  64  is secured in place by one or more shearing pins  66  designed to shear under a predetermined pressure applied to pressure surface  62 . In a first “unfired” position (shown in  FIG. 6 ), piston  60  is adjacent ports  38  in the outer wall of the tubular member. O-rings  68  maintain the seal between piston  60  and the tubular member. When sufficient fluid pressure is applied to the upper pressure surface  62  of piston  60 , pins  66  shear and piston  60  is axially displaced within annulus  28  to a second “fired” position, thereby causing firing pin  64  to strike percussion detonator  70 , resulting in activation of firing head  34 . 
         [0032]    As piston  60  is displaced in the manner described above, ports  38  are exposed to annulus  28 , permitting fluid communication therebetween. Thus, as firing head  34  is fished out of the wellbore, fluid passes through port  30  into annulus  28  and back out of ports  38 , preventing the formation of a fluid column over the firing head. In this regard, the pressure drop within firing head  34  when ports  38  are exposed or open provides confirmation that firing head  34  has been activated as desired, i.e., fired. The dual port arrangement is particularly desirable since the system is retrieved via a fishing neck and is not incorporated into a pipe string, where the aforementioned retrieval problems relating to a water column would not be experienced. 
         [0033]    Piston  60  and firing pin  64  need not be separate components. Rather, firing pin  64  can simply be provided with an upper pressure surface exposed to annulus  28  for actuation of firing pin  64 . Likewise, rather than providing a piston  60  to inhibit flow through ports  38 , a sleeve can be provided over ports  38 , that upon actuation of firing pin  64 , is axially shifted so as to expose ports  38 . 
         [0034]    Those skilled in the art will appreciate that the system described herein provides certainty as to the depth of a tool in a pipe string. Once a housing has landed on a seat, the exact distance to a desired area of operation is known. 
         [0035]    An additional benefit of the system is that a symmetrically disposed flange within an annulus allows tools positioned with the system to be centralized in a pipe string resulting in substantially improved performance of the explosives relating to the pipe recovery system. 
         [0036]    Furthermore, by creating a seal between a housing and a flange  16 , uphole pressure can be utilized to induce a desired operation, such as actuation of firing head. 
         [0037]    While the system of the invention is best utilized in the context of a vertical wellbore, those skilled in the art will understand that the invention may also be utilized in other elongated tubing sections where a fluid is pumped through the tube and an operation at a precise distance into the tube is required, including without limitation, horizontal wellbores, sewer lines, pipe lines and the like. 
         [0038]    Likewise, while the system preferably eliminates the need for eline, wireline, slickline or similar vehicles as a method for placement of a device, the system may still be utilized in conjunction with such vehicles to control the travel of such devices through the pipe string.
A system for positioning a wireless pipe recovery tool in a wellbore, the system having a series of locator pipe subs inserted along the length of a pipe string in a wellbore, wherein each locator pipe sub has a seat disposed around its internal periphery so as to form an aperture of s smaller diameter than the diameter of the pipe sub. The pipe subs have apertures of varied diameter and are consecutively arranged along the pipe string so that the aperture diameters decrease with increasing depth. A locator housing selected from a plurality of locator housings of varied outer diameter is attached to the wireless pipe recovery tool, the outer diameter of the locator housing selected to be slightly larger than the aperture in which the tool is intended to seat.