Abstract:
A string of drill pipe, for example, is assembled with a severing sub in anticipation of a possible need to cut the string at some point in the operation. The severing sub includes a thin wall tube that links opposite end tool joint bosses. The tin wall tube is easily cut by a shaped charge cutter. Rotary drive torque is transmitted between the sub tool join bosses by a concentric external torque tube having a torque transmitting assembly at each boss. The torque tube connection to the upper boss has an inseparable circumferential shoulder engagement with the boss. The lower boss engagement of the torque tube is axially separable. When the thin wall tube is cut, the upper boss and torque tube is withdrawn from the well with the upper pipe string.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Division of U.S. application Ser. No. 13/135,996 filed Jul. 19, 2011. Said application Ser. No. 13/135,996 is a Continuation-In-Part of application Ser. No. 12/579,900 filed Oct. 15, 2009 and claims the priority date for subject matter common therewith. Said application Ser. No. 12/579,900 claims the priority date of Provisional Application No. 61/242,251 filed 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 pipe string suspended within a wellbore without the use of e-line, wireline, slickline or similar tether lowered from the surface. The present invention is preferably utilized to position a downhole tool such as, for example, a jet cutter, a shaped charge, a perforating gun, an explosive charge, a perforating gun or well logging sensor in a tubing string for purposes of pipe cutting, pipe perforation, formation perforation, pipe recovery, well plugging, well logging or similar exercises. In one embodiment, the invention relates to placement of explosive charges or a jet cutter within a short section of easily and confidently severed pipe that may be inserted at numerous locations in a pipe string at numerous predetermined locations for separating an upper portion of a pipe string from a lower portion at a precisely predetermined location. In another embodiment, the invention relates to a well logging method that requires no surface linkage during the survey. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention system provides a series of internally profiled seating subs which are distributed within a pipe string to form a plurality of spaced apart pipe bore apertures immovably disposed along the pipe string length. Each seating sub aperture is characterized by a cross-sectional profile of varying shape with an aperture of a predetermined diameter formed therein. The internally profiled seating subs are arranged so that the aperture diameters decrease in regressive increments as the pipe string extends deeper in a well bore. Utilized in conjunction with these internally profiled seating subs is a sealing plug of an external diameter selected to sealingly engage a specific one of said profiled seating subs. The select diameter sealing plug is configured to be secured to the exterior of a down hole tool assembly that includes a service tool such as a firing head, shaped charge cutter, perforating gun or stand alone well logging instrument to permit the tool assembly to be landed on a seating aperture at a desired depth. The known distance from the seating aperture to precisely where the service tool functions in the pipe string is critical to the ability to predict what service tool is best suited to achieving the desired result. 
         [0004]    More specifically, an invention intent is to install these seating subs at strategically determined points along the length of a pipe string such as a drill string, drill pipe, drill collars, tubing, tubulars or casing in a sequence that progresses from the largest diameter aperture restriction to the smallest diameter aperture restriction. An independent device carrying a plug profile of predetermined diametric dimension, when dropped freely or pumped from the surface through the pipe string, will pass through the pipe string until the device strikes a seating aperture beyond which it cannot pass; e.g. a seating aperture diameter that is smaller than the outer diameter of the plug. A metal-to-metal (or other) seal will enable fluid pressure to be applied to the to the pipe string bore above the seal for various purposes such as, for example, triggering an explosive tool firing head and/or opening a by-pass valve and or revealing the location of a logging tool. The type of device utilized in the system can be any service tool utilized in downhole 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 service tools such as a jet cutter, severing tool, torch cutter or chemical cutter. Other uses for the invention may also include specific placement of perforating guns and well logging sensors. 
         [0006]    An additional embodiment of the invention combines a restriction or internally profiled seating sub as described above with a specially designed cutaway sub. The combination of seating sub and cutaway sub may be integrated with a pipe string at numerous, spaced, but carefully measured locations along the pipe string length and especially above or along the drill string weight collars. The cutaway sub includes a sacrificial section having a reduced external diameter (reduced wall thickness), relative the upper and lower coupling portions of the sub. Utilizing an aperture profile positioned above the section of reduced pipe wall annulus that is to be severed, the appropriate severing tool (such as a jet cutter or shaped charge explosive) may be accurately and confidently located to effect a clean cut. Significantly, once the cut is made and the upper section of drill string is withdrawn, the severed end of the reduced pipe wall annulus remaining with the lower end of the drill string is easily accessed by conventional “fishing” technology because the severed end is not excessively flared. This reduced wall annulus section of pipe also facilitates perforating operations previously made very difficult if not impossible by the thickness of the drill collar. The tensile strength of a particular cutaway sub is designed to be sufficient to support the pipe string below the particular sub. This may be a variable value since those cutaway subs near the lower end of a pipe string support less pipe weight below them than those cutaway subs near the surface or top of a pipe string which must support the weight of the entire string below. 
         [0007]    A sleeve or bushing may be installed over the reduced wall annulus section of the severing sub to ensure that the buckling and torsional strength threshold of the sub is maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The advantages and further features of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout. 
           [0009]      FIG. 1A  illustrates a section of pipe string having two sub units of the invention inserted between a upper pipe section and a lower pipe section. 
           [0010]      FIG. 1B  is a sectioned view of  FIG. 1A  showing a drop assembly within the pipe string in pipe cutting position. 
           [0011]      FIG. 1C  is a sectioned view of  FIG. 1A  showing the discharge of a jet cutting tool against a reduced wall annulus section of the sacrificial mandrel. 
           [0012]      FIG. 1D  is a sectioned view of the severed pipe section of  FIG. 1C  showing withdrawal of the upper pipe section from the severed lower pipe section. 
           [0013]      FIG. 1E  is a sectioned view of the severed pipe stub remaining below the cut of  FIG. 1C . 
           [0014]      FIG. 1F  is a full profile view of the severed stub remainder of the pipe section. 
           [0015]      FIG. 2  portrays the cross-section of a pipe string with a series of seating apertures disposed therein to form decreasing restrictions along the length of the pipe string. 
           [0016]      FIG. 3  illustrates the invention drop assembly. 
           [0017]      FIG. 3A  is an enlarged, partially sectioned view of the drop assembly along the top section A of  FIG. 3 . 
           [0018]      FIG. 3B  is an enlarged, partially sectioned view of the drop assembly along the mid-section B of  FIG. 3 . 
           [0019]      FIG. 3C  is an enlarged, partially sectioned view of the drop assembly along the bottom section C of  FIG. 3 . 
           [0020]      FIG. 4  is an enlarged sectioned view of the present invention firing head. 
           [0021]      FIG. 5  is an exploded view of a preferred cutaway sub embodiment. 
           [0022]      FIG. 5A-A  is a cross-section view of the seating sub at cutting plane A-A of  FIG. 5   
           [0023]      FIG. 6  is a sectioned view of the preferred cutaway sub embodiment. 
           [0024]      FIG. 7  is an exploded view of an alternative cutaway sub embodiment. 
           [0025]      FIG. 8  is a sectioned view of the  FIG. 7  cutaway sub embodiment. 
           [0026]      FIG. 9  is a sectioned view of an alternative sacrificial mandrel embodiment. 
           [0027]      FIG. 10  is a sectioned view of a second alternative cutaway sub embodiment. 
           [0028]      FIG. 11  is a sectioned view of an alternative invention application. 
           [0029]      FIG. 12  is a partially sectioned view of a well logging application of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    As used herein, the terms “up” and “down”, “upper” and “lower”, “above” and “below” and other like terms indicating relative positions above or below a given point of element are used in the description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left or other relationship as appropriate. Moreover, in the specification and appended claims, the terms “pipe”, “tube”, “tubular”, “casing”, “liner” and/or other tubular goods are to be interpreted and defined generically to mean any and all of such elements without limitation of industry usage. 
         [0031]    The basic sequence of the present invention, as practiced, for example, upon a drill string cutting operation, is represented by the six view, A-F of  FIG. 1 . The  FIG. 1A  view shows an assembly of the basic invention components in a downhole pipe string between an upper section  10  and a lower section  16 . An expanded description of each of these constituent components will follow hereafter. 
         [0032]    The  FIG. 1A  illustration is usually most relevant to that heavyweight section of drill pipe at the bottom end of a drill string having joints of pipe with extremely thick wall annuli. To the well driller&#39;s art, these pipe joints with exceptionally thick walls are known as “drill collars”. The invention seating sub  12  and cutaway sub  14  may be positioned at the upper end of the collar section or at any intermediate point or at numerous points below the upper end. However, those of ordinary skill will understand that the principles described herein with respect to drill collars are applicable to any form or application of pipe or tube. 
         [0033]    Referring to the sectioned view of  FIG. 1B , an independent drop assembly  22  is released at the surface to be driven by pump pressure or to descend in free-fall along the pipe bore to terminate upon a plug seating aperture  24  in the seating sub  12 . A drop assembly extension  26 , usually extending below the seating aperture  24  is shown to support a jet cutting pyrotechnic tool such as a thermite or shaped charge explosive  28 . The extension  26  length is selected to place the jet cutter  28  within the pipe bore opposite a thin wall section  30  of a sacrificial mandrel  20  portion of the cutaway sub  14 . 
         [0034]      FIG. 1B  illustrates the drop assembly  22  as firmly resting upon seating aperture  24 . Fluid pressure within the upper pipe string bore is increased to open a firing head valve disposed within the drop assembly  22 . Opening the firing head valve initiates the jet cutter  28  ignition sequence to discharge a high temperature cutting jet along cutting plane  29  against the thin wall section  30  of the sacrificial mandrel  20  as represented by  FIG. 1C . 
         [0035]    With the thin wall section  30  of the sacrificial mandrel  20  severed,  FIG. 1D  shows the seating sub  12  and torque sleeve portions of the upper pipe string  10  as free to separate from the sacrificial mandrel stub  32  which remains fixed to the well bottom.  FIG. 1E  shows the sacrificial mandrel stub  32  portion of the cutaway sub  14  in section as remaining with the well bottom pending further, independent action of recovery or well abandonment.  FIG. 1F  shows the mandrel stub  32  in full profile. 
         [0036]    Seating Sub 
         [0037]    While  FIG. 1  illustrates the invention in one particular application and embodiment,  FIG. 2  illustrates a greater and more generic application wherein a series of seating subs  12  are distributed along the length of the supported pipe string. The seating subs  12   a,    12   b,    12   c,  and  12   d  are internally profiled by plug seating apertures  24  of graduated diameter “D” forming restrictions in the interior diameter of the subs. The subs, positioned at measured locations in a pipe string  10  extending from the surface  11  into a well bore  19 , are arranged 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. 2 , seating aperture  24   a  of sub  12   a , nearest the surface  11 , has the largest diameter D a  restriction, while aperture  24   d  of sub  12   d,  deepest in wellbore  19 , has the smallest diameter D d  restriction. The consecutive diameters D a , D b , D c , and D d  decrease with depth along wellbore  19 . In any event, the seating apertures  24  are disposed to engage the sealing plug  34  (shown in  FIG. 3 ) of the drop assembly  22 . 
         [0038]    In one preferred embodiment, the seating subs  12  are only approximately two feet long and can be readily threaded or inserted into a pipe string during make-up. In one embodiment of the invention, up to five seating subs  12  are provided and arranged so that the effective 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 other embodiments of the invention, at least fifty seating subs  12  may be provided and arranged so that the effective 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 the course of such pipe string make-up, records will be made of the number of standard pipe joints or drill collars between each seating sub  12 . Hence, the distance from the top end of the pipe string to each seating aperture is a measured value. Of course, the number of seating subs and restrictions will depend on the length of the overall pipe string and the diameter of the pipe in which restriction are formed. 
         [0039]    While the seating aperture  24  may take any shape, in the preferred embodiment, the apertures are formed of a lip or flange symmetrically disposed around the interior  42  of a seating sub  12 , thereby forming an immovable opening that is axially fixed and aligned relative to the internal bore of the seating sub. Preferably, this seating aperture is formed with a continuous, fluid sealing face  44 . However, those skilled in the art will appreciate that for certain applications that do not require a fluid tight seal, the seating aperture  24  need not extend fully around the interior of the seating sub  12  so long as a resulting aperture is formed to function as a restriction, thereby creating a seat on which an object can land. Nor does the aperture need to be symmetrical or axially aligned relative to the pipe sub, so long as the overall system comprises apertures of varying size arranged in consecutive order as described herein. For example, the seating aperture  24  may take the form of one or more tabs, fingers or projections extending into the bore of a pipe sub so as to form a “restriction” therein. 
         [0040]    In one preferred embodiment, the seating aperture  24  has an upper sealing surface  44  and lower surface  46 . The upper surface  44  is contoured so as to engage an object provided with a similarly contoured profile, thereby permitting a seal to be formed between the object and the sealing surface when the object is seated on the upper surface  44 . In the example of  FIG. 2 , upper surface  44  is curved to form a concave profile and disposed to receive an object with a correspondingly rounded or tapered shape (such as is shown on drop assembly  22  of  FIG. 3 ). Once an object is seated, a seal is formed between the object and the sealing surface  44  as pressure is applied to the object by the fluid column above the object or otherwise by downwardly pumped fluid to the extent the object is disposed to pass fluid therethrough. In one example, if the object is connected to a 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. 
         [0041]    Drop Assembly 
         [0042]    The drop assembly  22  illustrated by  FIG. 3  is a preferred configuration for a tool, device or object that may be conveyed in a pipe string and externally shaped for landing on and engaging the seating aperture  24 . One intent of the invention is to provide a universal tool body adapted to receive a specifically sized sealing plug element  34  secured to the exterior of the tool body. A variety of standard downhole devices or service tools attached to the tool body, usually below the sealing plug, provide flexibility in the system for use with whatever tool and for whatever purpose is desired. Thus, in one embodiment of the invention, sealing plug  34  may be integrally formed as part of the device with which it is utilized, while in another embodiment of the invention, sealing plug  34  may be secured to the exterior of such device as an independent attachment 
         [0043]    The basic elements of the drop assembly  22  are shown by the enlarged sections of  FIGS. 3A ,  3 B, and  3 C which correspond to segments A, B and C of  FIG. 3 . With respect to  FIG. 3A , a fishing head  50  may be provided at the upper end of the assembly  22  for independent tool descent or removal from the pipe string when desired. The anticipated normal use of the drop assembly  22  is a free release of the assembly at the surface  11  into the pipe string bore for pumped displacement or free-fall until the sealing plug  34  engages the seating aperture  24 . To control the rate of assembly descent, one or more units of swab cups  52  are provided to restrict the flow rate of standing bore fluid past the assembly as it descends. If pumped down the pipe bore, the swab cups  52  provide a ring seal between the assembly  22  and the pipe bore wall to increase the operational area of the upper pressurized fluid upon the assembly  22 . Additional to the swab cups  52  are one or more resilient centralizers  54  to keep the assembly aligned with the pipe string axis during the descent. Although there are many pipe centralizer configurations. the present embodiment provides three spring blades  56  secured to a carrier tube  58 . Apertures  59  in the carrier tube wall allow pressure equalization between the carrier tube interior and the surrounding pipe string bore. 
         [0044]      FIGS. 3B and 3C  collectively illustrate the drop assembly firing head  60  which is also shown in enlarged section by  FIG. 4 . Central to the firing head  60  is a release valve mechanism comprising a differential area piston  62  that is initially held against an annular ledge as a bottom seat  64  in a bore sleeve by shear pins  65 . The piston  62  upper diameter  67  is greater than the diameter  68  below the fluid port  66 . Displacement of the piston  62  from an initial, port  66  closing position may only occur in an upward direction into a blind bore  70  by pressure differentially shearing pins  65 . Accordingly, the piston  62  is positively caged from accidental or shock release as it descends along the pipe string bore. 
         [0045]    The sleeve  63  is threaded onto a tube extension  70  below the swab cup  52 . Tube extension  70  includes a blind bore  71  of substantially the same inside diameter as the large diameter  67  of the piston  62 . 
         [0046]    A reduced diameter pintle  72  projects from the lower face of piston  62  into the bore  74  of a fluid transfer tube  73 . the upper end of the transfer tube is perforated by a plurality of biased angle apertures  75 . Each of the apertures  75  contains a latching ball  76  which has substantially the same diameter as the annulus thickness that is the differential between the pintle  72  radius and radius of the counterbore  77  in the bore sleeve  63 . 
         [0047]    For the preferred embodiment, the transfer tube  73  extends through an axial bore  77  in the sealing plug  34  into a release sleeve  78 . A fluid flow annulus is provided between the outer perimeter of the transfer tube  73  and the inside wall of the sealing plug bore  77 . 
         [0048]    At the release sleeve end of the transfer tube  73 , the transfer tube  73  is given an enlarged outside diameter  79  for a sliding, O-ring seal fit within a release sleeve bore restriction  122  between annular chambers  123  and  124 . The lower chamber  124  is ported by apertures  126  into the surrounding pipe string annulus 
         [0049]    A firing pin housing tube  128  is threaded into the release sleeve  78  ( FIG. 4 ). The upper end of firing pin  130  is seated within the lower end of the transfer tube bore  74  with an O-ring fluid seal. The lower distal end  131  of the transfer tube engages a perimeter shoulder on the pin  130  to limit penetration of the pin  130  into the transfer tube bore  74 . The outside perimeter of the transfer tube  74  lower end is given and O-ring fluid seal fit within the housing tube bore. The up end  137  ( FIG. 3C ) of a linking tube  138  between a tool coupling  134  and the lower end of the housing tube  128  provides a travel limit shoulder for the transfer tube  73  and hence, the firing pin  130 . For the purpose of a pyrotechnic tool such as a jet or shaped charge tubing cutter, a percussion activated explosive initiator  135  will be secured in the tool coupling  134 . The stroke of the transfer tube  73  along the housing tube bore  132  is designed to bring the firing pin  130  striker point  139  into physical contact with the percussive initiator  135 . 
         [0050]    In most applications, plug  34  engagement of a predetermined seating aperture  24  will isolate the pipe string bore into an upper fluid pressure zone above the seating aperture  24  and a lower pressure zone below the seating aperture  24 . The pressure in the upper zone at the seating aperture  24  is determined by the fluid head standing above the seating aperture  24  and any externally applied pump pressure. Pressure in the pipe string bore below the seating aperture  24  is usually determined by multiple factors such as the standing fluid head in the wellbore annulus, the presence of well packers, and the in situ bottom hole well pressure. 
         [0051]    To trigger the firing pin against the explosive initiator  135 , fluid pressure in the upstream pipe bore is raised by pump pressure to exceed that of below the seating aperture by a sufficient differential to shear the pins  65 . Upper pipe bore fluid pressure enters the drop assembly through ports  66  to bear against the differential area piston  62 . Due to the dimensional difference between the large diameter  67  end of the piston and smaller diameter end  68 , a net shear force on the piston  62  is borne by the shear pins  65 . When the pins  65  fail under this differential area force, the piston  62  is driven upward into the blind bore  71  of extension tube  70 . When the piston  62  enters the blind bore  71 , the pintle  72  is extracted from the upper bore end of transfer tube  73 . Resultantly, the latching balls  76  are released into the bore  74  of transfer tube  73 . 
         [0052]    When the differential area piston  62  shifts upward into the blind bore  71 , pressurized fluid in the upper pipe string bore also enters the inner chamber of the bore sleeve  63  to bear against the transfer tube  73  cross-section. The force of such cross-sectionally applied fluid pressure drives the transfer tube  73  downward along the sealing plug bore  77  and firing pin striker point  139  against the explosive initiator  135 . Simultaneously, the enlarged diameter section  79  of the transfer tube  73  is shifted downwardly from sealing contact with the release sleeve bore restriction  122 . The latter shift permits fluid flow from the upper pipe string segment to pass through the port  66  into the flow annulus between the transfer tube  73  and sealing plug bore  77  and out the release sleeve aperture  126  thereby bypassing the pipe string bore seal at the plug seating aperture  24 . 
         [0053]    This fluid by-pass opening between ports  66  and  126  allows the drop assembly and any attached tool to be withdrawn from the pipe string by a wireline connected to the drop assembly fishing neck  50 . As the drop assembly  22  is lifted, the by-pass opening allows fluid in the pipe string bore to drain past the drop assembly into the pipe string bore below the drop assembly. 
         [0054]    Cutaway Sub 
         [0055]    The foregoing description has been of a system for precisely placing a specialty tool along the length of a pipe string bore. Among the numerous downhole operations receiving advantage from such positioning accuracy is that of pipe cutting. There are occasions when it is advantageous to sever a pipe string downhole and withdraw the upstring portion. The severed lower portion of the pipe string may be either abandoned in place or, as the usual case, recovered by one of numerous “fishing” techniques. When the objective is to sever a drill pipe, care is taken to place the cutting tool at a point along the pipe length between the pipe coupling joints. Pipe coupling joints normally have a considerably greater wall thickness than the nominal wall of the pipe. The thinner wall thickness of the nominal pipe wall is more easily severed with a ‘clean’ cut face without flash, burrs or flare which may interfere with extraction of either the severed, uphole string or of the downhole string. 
         [0056]    Drill collars, however, are a special case wherein the outside diameter of a pipe joint is the same as the coupling diameter along the entire joint length. The functional purpose of such a configuration is for ballast weight at the bottom end of the drill string. Moreover, when a pipe string becomes ‘stuck” in a borehole in progress, it is frequently due to bore wall sloughing into the bore annulus around the drill collars. Hence arises the occasional necessity to sever the drill collar string mid-length. It is for this task, that the combination of the seating sub  12  as described above with a cutaway sub  14  is particularly useful. With respect to  FIG. 1 , for example, the seating sub  12  and cutaway sub  14  are positioned between upper and lower drill collars  10  and  16 , respectively. Depending on the length of the drill collar assembly there may be a plurality of seating sub and cutaway sub combinations distributed along the drill collar segment of the pipe string. 
         [0057]    Turning to the exploded view of  FIG. 5  and cross-sectional views of  FIGS. 5A-A  and  6 , one preferred embodiment of a cutaway sub  14  is shown to include a sacrificial mandrel  20  having male threaded end-pins  140  at both ends. Axially adjacent the end-pins are stepped bosses  142  and  144 . between the two stepped bosses  142  and  144  is a relatively thin wall tube section  30  having an outside diameter that is substantially less than the nominal drill pipe or collar diameter. The upper (smaller) stepped portion  146  of boss  142  adjacent the threads  140  is formed with chordal wrench flats corresponding to the wrench flats  149  in the torque sleeve collar  147  shown by  FIG. 5A-A . The number of wrench flats  149  is shown on the inside perimeter of the sleeve collar  147  are only a representative example. Those of ordinary skill will understand the collar  147  and boss step  146  may be given as many flats as required to transfer the forces necessary for rotatively driving the drill string below the seating sub  12 . 
         [0058]    The greater outside diameter section of stepped boss  142  is dimensioned to receive the inside diameter of torque sleeve  18  with a slip-fit overlay. 
         [0059]    The smaller, outside diameter section  150  of lower boss  144  also is preferably given a value corresponding to a slip fit overlay of the torque sleeve  18 . The larger diameter section  152  of the lower boss  144  may be essentially the same diameter as the drill collars  10  or  16 . The shoulder  153  between the two sections is cut with an undulating profile such as the lug socket profile  154  for meshing with a corresponding lug socket profile  156  in the end of torque sleeve  18 . 
         [0060]    It will be understood that the rotary torque transfer function accomplished by the meshed wrench flats  149  in the torque sleeve collar  147  and the mandrel boss  146  may also be served by a multiplicity of meshing splines. In either case, the sleeve  18  is assembled with the mandrel  20  by an axially sliding fit to mesh the sleeve lug profiles  156  with the corresponding profiles  154  in the mandrel boss  144 . Simultaneously, the wrench flats  149  mesh with corresponding flats on the mandrel boss  142 . When the mandrel threads  140  are meshed with corresponding threads in the seating sub  12 , the torque sleeve  18  is firmly secured against the upper mandrel boss shoulder  146  and the dominance of all torsional stress transferred by the seating sub  12  to the sacrificial mandrel  20  is carried by the torque sleeve.  18 . 
         [0061]    As previously described, numerous sub-sets of seating subs  12  and cutaway subs  14  may be distributed along the pipe string additional to those among the drill collars. When an occasion arises to sever the pipe string at a specific point, the drop assembly  22  is equipped with the sealing plug  34  corresponding to the assigned seating aperture  24  that is most proximate above the point of desired string separation. The pipe cutting tool, also secured to the drop assembly, is positioned below the sealing plug  34  at the same, precisely known distance as is the center of the thinwall section If sacrificial mandrel  20  below the seating aperture  24 . Hence, when the drop assembly  22  settles upon the seating aperture  24 , it is known with confidence, that cutting tool is correctly positioned relative to the sacrificial mandrel  20 . 
         [0062]    It is also known, with confidence, that the drop assembly  22  has, in fact, settled against the designated seating aperture  24  by the fluid pressure rise within the pipe string bore against a surface pump supply. As the drop assembly descends the pipe string. The pipe bore pressure remains at circulation pressure. When the sealing plug  34  settles against the seating aperture  24 , circulation is terminated and bore pressure abruptly rises against the firing head  60 . This pressure rise will continue until the shear pin  65  rupture pressure is achieved to shift the differential area piston  62  upwardly off the bottom seat  64  and release the latching balls  76 . When the latching balls fall into the transfer tube bore  74 , the transfer tube  73  shifts downwardly to open the upstream fluid port  66  to flow communication with downstream fluid flow port  126 . When flow communication is established between fluid ports  66  and  126 , the bore pressure abruptly drops to the circulation pressure. Consequently, when the pipe string pressure abruptly spikes and then falls, it may be known that the drop assembly  22  has settled on the seating aperture  24 , the firing head has opened, the firing pin as fallen and the pipe cutter  28  or perforating gun has discharged. 
         [0063]    In the usual course of operations, after discharge of the cutter  28 , the upper pipe string is withdrawn from the wellbore along with the seating sub  12 , the torque sleeve  18  and the upper portion of the sacrificial mandrel  20  including the upper boss  142 . Of the original cutaway sub  14 , only the lower boss  144  and lower pipe string remain in the wellbore subject to abandonment or further retrieval operations. 
         [0064]    An alternative embodiment  80  of the cutaway sub with increased buckling strength is represented by  FIGS. 7 and 8  as having a reduced wall thickness tube  81  between stepped bosses  84  and  85 . The upper end of the reduced wall tube  81  is terminated by an interior portion of the upper stepped boss  84 . The lower end of the tube  81  is terminated by the interior portion of the lower stepped boss  85 . Both interior boss portions are of greater outside diameter than the reduced wall tube  81 . At an axial set-back in opposite directions are an intermediate pair of stepped bosses  86  and  87  having a greater OD than the interior bosses  84  and  85 . The abutment transition between the interior and intermediate bosses is profiled with lug detents  92 . Meshing with the lug detents  92  are the lug projections  91  at opposite distal ends of a split sleeve  90 . There may be a plurality of such meshing lug projection  91  and detents  92 . 
         [0065]    The internal bore  101  of torque sleeve  100  is sized to pass freely but closely with a slip fit over the intermediate bosses  86  and  87 . Lug  102  on the lower end of sleeve  100  are sized and configured to mesh with the lug detents  94  in the lower pin collar  88 . Referring to  FIG. 8 , an inside abutment face  104  of end collar  103  is positioned at the distal end of sleeve bore  101  to engage a mating abutment face on the intermediate stepped boss  86  as the sleeve lugs  102  mesh with the collar detents  94 . Internal wrench flats on the upper stepped boss  96  as described for  FIG. 5A-A  are sized and configured to mesh with mating wrench flats (not shown) on the interior perimeter of the sleeve  100  end collar  103 . 
         [0066]    A seating sub  106  may be constructed with tapered box threads  107  and  108  at opposite ends. When the tapered threads  82  and  108  are in full engagement, the inside abutment faces of the sleeve collar  104  and intermediate boss  86  are in compressed juxtaposition. 
         [0067]    Those of skill in the art will appreciate the operative consequence of the  FIGS. 7 and 8  assembly as not only stiffening the cutaway sub  80  but is also capable of transferring drive torque across the cutaway sub  80  through both inner and outer sleeves as well as the thinwall tube  81 . However, when the thinwall tube  81  is severed, the upper pipe string maintains firm assembly with the sleeve  100  and upper stepped boss elements of the sub  80  for withdrawal from the borehole. When the sleeve  100  is withdrawn. The split sleeve  90  halves have no radial confinement and merely fall away form the severed lower portion of the sub. 
         [0068]    In some cases, even the release of the split sleeve halves  90  as borehole debris is intolerable or extremely expensive for a follow-up fishing trip to remove the resulting debris. Responsive to those applications. A third embodiment of the invention as represented by  FIGS. 9 and 10  is suggested wherein the inner step  84  of the upper boss is grooved with a perimeter encircling channel  114 . The substantially cylindrical surfaces of both inner steps  84  and  85  may be cut with wrench flats  110  and  112 . 
         [0069]    A further modification of the  FIGS. 9 and 10  embodiment may include lug and detent engagements of the split sleeve  119  at the lower end as suggested for the  FIGS. 7 and 8  embodiment. In either case, whether by lug and detent or by wrench flats, drive torque is transferred from the top seating sub  106  to the lower pin  83  through the additional structure of inner split sleeve  81  and torque sleeve  100 . 
         [0070]    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 drop assembly has landed on a seating aperture  24  and the pipe string pressure is raised against the shear pins  65  to be abruptly released, the drop assembly is known to be on the designated seating aperture and the exact position of a tool attached to the drop assembly relative to the seating aperture is also known. 
         [0071]      FIGS. 11 and 12  illustrate an alternative embodiment of a drop assembly configured for placement of a non-explosive tool such as a battery powered well logging sensor for detecting certain geologic characteristics of the earth where penetrated by the wellbore. Distinctively, the transfer tube  73  element of the drop assembly needs no firing pin. Consequently, the distal end of the transfer tube  73  is closed with an end plug  157 . The firing head  60  becomes a one-time pressure actuated release valve. The housing tube  128  becomes an extension to which a battery pack  164 , a data recorder  162  and well logging sensor  160  are attached. The seating aperture  24  is positioned within the seating sub  12  to allow at least the sensor  160  end to extend beyond the open end  25  of the seating sub. 
         [0072]    When a free falling drop assembly, for example, carries sensitive instrumentation such as well logging sensors, it may be prudent to finish the internal bore of the seating sub  12  for an extended distance above the seating aperture  24  to more closely interact with the swab cups  52  to slow the drop assembly descent before engaging the seating aperture  24 . 
         [0073]    The total length of the pipe string, including the distal end  25  of the seating sub  12  and the position of the sensor  160  relative to the seating aperture  24  will be known. When pump pressure shears the pins  65  and a pump pressure spike is suddenly released, it is known, with confidence, exactly where the sensor  160  is located within the wellbore  19 . If the data recorder  162  operates continuously, the well may be logged continuously from the known position as the supporting pipe string is withdrawn with the logging tool attached. It will be recalled that the firing head by-pass valve is open therefore permitting standing pipe bore fluid above the seating aperture  24  to by-pass the seal and equalize the fluid pressure as the pipe string rises. 
         [0074]    An additional benefit of the system is that a symmetrically disposed seating aperture within a pipe bore 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. 
         [0075]    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. 
         [0076]    Likewise, while the system preferably eliminates the need for e-line, 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. 
         [0077]    Although the invention disclosed herein has been describe in terms of specified and presently preferred embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modification of the invention are contemplated which may be made without departing from the spirit of the claimed invention.