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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to devices and methods for electrically insulating and isolating an electromagnetic telemetry system within a drill string. In particular aspects, the invention relates to improved systems for providing an insulative gap assembly within a drill string. 
   2. Description of the Related Art 
   Electromagnetic transmitters are used within petrochemical wellbores for the transmission of borehole data and other information to the surface. Such transmitters are often used in measurement-while-drilling (MWD) arrangements wherein downhole conditions are sensed and transmitted to the surface for operators to make adjustments to the drilling operation. An electromagnetic transmitter is formed by electrically separating two metallic drill collars, or subs, by an insulated, tubular “gap sub.” An electrical conductor is disposed through the axial center of the gap sub to permit electrical signals to be alternately provided to the separated drill collars. The separated collars then function as the two poles of a dipole antenna within the earth for sending information wirelessly to a receiver located at the surface of the well. 
   The use of conventional gap subs has been problematic. Conventional gap sub assemblies have been provided by insulated or non-conductive members that are disposed between two conductive portions in a drill string. The gap sub provides electrical isolation of the drill string portions. In this type of arrangement, a longitudinal conductor must be mechanically supported within and along the length of the gap sub. Depending upon the length of the gap sub, the conductor may have to be more than ten feet in length. Such an arrangement is prone to failure, particularly during drilling when abrasive mud is flowed down through the drill string. Additionally, there are times when the length of the gap sub must be changed in order to alter the characteristic of the transmitter antenna. As a result, the conductor must be exchanged for one of different length as well. This is time consuming and allows for installation errors. 
   The use of gap subs for electrical isolation is also known. U.S. Pat. No. 5,138,313 issued to Barrington, for example, discloses an electrically insulative gap sub assembly wherein the outer surface of a drill pipe joint is covered by several molded “gap blocks” of insulative material. This technique is expensive and can be complex in construction. In addition, it is prone to damage within the wellbore. 
   U.S. Pat. No. 4,348,672 issued to Givler describes an insulated drill collar gap sub assembly that is used with a particular toroidal-coupled telemetry system. An insulated gap is formed between a pair of annular sub members by forming a gap between them and filling the gap with a dielectric material. To interconnect the gap sub within the drill string, adjoining sub members are essentially keyed to one another using hexagonal keying. In an alternative version of the device, subs are connected using an axially extending member that resides within an axially extending recess. Pins are used to lock the two subs together, and a dielectric material is disposed in a gap between them. In each case, axial bearing assemblies are necessary to help transmit force through the gap sub. A significant disadvantage to this type of arrangement is the requirement for special tooling to form the various keys or extensions and recesses to mechanically lock the components together. Further, such components would be incompatible with standard drill pipe threaded connections. 
   There is a need to provide improved methods and devices for integrating a telemetry system into a drill string. It would be an improvement over the prior art to provide simpler construction and cost savings over previous insulative sub constructions. The present invention addresses the problems of the prior art. 
   SUMMARY OF THE INVENTION 
   The invention provides devices and methods for incorporating a gap sub assembly into a drill string to electrically isolate portions of a transmitter assembly within, for example, an MWD tool located within the drill string. The gap sub assembly incorporates upper and lower tubular members having an insulated interconnection. In a preferred embodiment, the gap sub assembly incorporates standard threaded end connections having a non-conductive coating thereupon. 
   A central conductor assembly is incorporated into the insulated interconnection and used to receive electrical signals from an MWD device and transmit the signals alternately between the upper and lower poles of the antenna transmitter. The central conductor assembly is retained largely within the lower sub and does not extend along the length of the insulated gap sub. During operation of the MWD device, signals are alternately transmitted to each of the poles of the antenna transmitter so that information may be transmitted to a surface receiver. 
   The methods and devices of the present invention include simplicity and lower cost. The methods and devices of the present invention eliminate the need for a conductive element to be disposed within the gap sub between the two dipole elements. The present invention instead transmits electrical signals to the upper drill string elements through the body of the gap sub itself. If a gap sub of different length is subsequently required, this may be accomplished by merely replacing the gap sub itself without the need to replace the central conductor assembly with one of a different length. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side, cross-sectional view of a portion of an exemplary drill string that incorporates an insulative gap sub assembly constructed in accordance with the present invention. 
       FIG. 2  is a side, cross-sectional view of a lower portion of the drill string shown in FIG.  1 . 
       FIGS. 3 and 4  present a closer, side cross-sectional view of portions of the gap sub assembly constructed in accordance with the present invention. 
       FIG. 5  is an isometric view of portions of an exemplary conductor assembly shown apart from other portions of the gap sub assembly. 
       FIG. 6  is an isometric view of an exemplary insulative ring assembly shown apart from other portions of the gap sub assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring first to  FIG. 1 , there is shown an exemplary hydrocarbon well  10  that is being drilled from the surface  12  downward through an earth formation  14 . A drill string  16  is shown disposed within the wellbore  18  of the well  10  and is composed of a number of interconnected drill pipe sections  20 ,  22 ,  24 ,  26  that support a bottom hole assembly (BHA)  28 . As is well known, the BHA  28  includes a drill bit that cuts through the earth formation  14  during drilling operations. Although only a few drill pipe sections are shown in  FIG. 1 , it should be understood that in practice there are often a large number of such sections. Each of the drill pipe sections  20 ,  22 ,  24 ,  26  is a conductive tubular member, and they are interconnected to one another using standard threaded connections of a type well known in the art. 
   In the drill string  16  depicted in  FIG. 1 , an exemplary MWD tool  30  is shown that is located just above the BHA  28 . The MWD tool  30  is operable to transmit downhole data to a receiver, schematically shown at  32 , that is located on the surface  12 . 
     FIG. 2  provides a more detailed view of lower portions of the drill string  16 , including the upper portion of the MWD tool  30 . The lower portion (not shown) of the MWD tool  30  includes a transmitter (not shown), of a type known in the art, which is used to transmit received data to the receiver  32 . Suitable MWD tools for use as the tool  30  include the NaviTrak® I and Navitrak® II, which are available commercially from Baker Hughes INTEQ. As both  FIGS. 1 and 2  illustrate, a gap sub assembly  33  that includes upper and lower subs  34  and  36 , respectively, which separate the MWD tool  30  from the lowest drill pipe section  26 . The upper sub  34  is also referred to as a “gap sub.” The upper sub  34  is a metallic, conductive member with an insulated coating upon its entire inner and outer radial surfaces and axial ends except upon the upper threads  37  (see FIG.  2 ), by which the sub  34  is connected to drill string section  26 , and as otherwise noted herein. An external stabilizing collar  35  radially surrounds portions of the upper and lower subs  34 ,  36  and serves to protect the insulated coating on the outer radial surface of the gap sub  34  from being damaged or rubbed off by contact with the wellbore  18 . The lower sub  36  defines a borespace  38  within. It is noted that the lower sub  36  may be formed integrally with the outer housing of the MWD tool  30 . 
   A longitudinal conductor assembly  40  extends centrally through the borespace  38  in a spaced relation from the walls of the lower gap sub  36 . The structure of the conductor assembly  40  may be better appreciated by reference as well to  FIG. 5 , which depicts portions of the conductor assembly  40  apart from other components of the gap sub assembly  33 . The conductor assembly  40  includes an upper retaining portion  42  and a lower conducting portion  44  that is affixed to the retaining portion  42  by a threaded connection  43 . The upper retaining portion  42  features an annular plug member  46  and a central tubular portion  48  that is interconnected to the annular plug member  46  by a plurality of radially-extending spokes  50 . The spokes  50  define spaces  51  (visible in  FIG. 5 ) therebetween so as to permit fluids to pass through. The plug member  46  includes an outer radial surface  52  that contains a number of recesses  54  within which O-ring seals  56  reside. 
   The central tubular portion  48  and the lower conducting portion  44  retain a conductive element  58  that is disposed longitudinally therethrough. The conductive element  58  connects at it lower end (not shown) to signal components housed within the MWD tool  30 . Because the details of such connections are well-known, and differ depending upon the specific MWD tool used, these connections will not be described in any detail here. The lower conducting portion  44  of the conductor assembly  40  includes an outer housing  60  that encloses the conductive element  58 . Members  42  and  60  are generally insulated from the gap sub  34  by the insulative coating on the interior surface of the gap sub  34  and an insulative coating covering the radially interior surface of the lower sub  36 . However, it is noted that the MWD components that are retained within the lower sub  36  are electrically and mechanically connected with the lower sub  36  and, thus the lower sub  36  provides a ground for MWD components. 
   The outer radial surface  52  of the plug member  46  contains a groove  62 , and an insulative ring member  64  resides therein. The insulative ring member  64  is secured against the outer radial surface  52  by an inwardly-biased C-ring or snap ring  66 , visible in FIG.  3 . The insulative ring member  64  is shown apart from other components in FIG.  6 . The insulative ring member  64  formed of an insulative material such as ceramic or a plastic polymer, such as PEEK (PolyEtherEtherKeytone). The insulative ring member maintains electrical isolation between the upper gap sub  34  and the plug member  46 . However, a portion of the insulative material from the ring member  64  is removed at gap  68 , thereby providing a conductive pathway from the upper gap sub  34  to an electrical contact element  70  that is disposed within the gap  68 . If desired, the contact element  70  may be spring biased radially outwardly to ensure good contact with the upper gap sub  34 . The contact element  70  is electrically interconnected to the conductive element  58  via a conductive pressure plug  72  which prevents wellbore fluids from entering lateral bore  74  and coming into contact with the conductive element  58 . An alignment pin  76  is disposed through the insulative ring member  64  and the spoke  50  to ensure proper alignment of the components. 
   The upper gap sub  34  and sub  36  are provided with a unique insulated interconnection that is preferably formed by disposing a non-conductive material layer between the two components and functions to preclude transmission of electrical signals thereacross. The lower end of the upper gap sub  34  features a box-type threaded connector  78  that is shaped and sized to be complimentary to the pin connector  80  at the upper end of the lower gap sub  36 . As best shown in  FIG. 3 , the upper gap sub  34  also defines a cylindrical recess  82  within its lower end. There is no insulative coating upon the radially interior wall of the recess  82 , thereby allowing electrical transmission between the body of the gap sub  34  and the contact element  70 . The plug member  46  and insulative ring member  64  reside within the recess  82 . When the plug member  46  and ring member  64  are seated within the recess  82  between the two subs  34 ,  36 , an electrical connection is created between the lowest drill pipe section  26  and components housed within the MWD tool  30 . In addition, an electrical connection is present between the components housed within the MWD tool  30  and the lower sub  36  that houses these components. 
   Prior to assembly, one or both of the threaded connectors  78 ,  80  are coated with an insulative material, shown schematically at  84  to provide electrical isolation between the gap sub  34  and MWD sub  36 . Suitable insulative materials for this application include ceramic oxide or a plastic epoxy mix, preferably containing small ceramic particles to transmit compressive forces. Additionally, if the subs  34 ,  36  are formed of a titanium alloy, titanium oxide may be used as the insulative material. It is currently preferred that the insulative material be applied as a spray coating to a thickness suitable for inhibiting transfer of electricity between the subs  34  and  36 . 
   In operation, the gap sub assembly  33  electrically isolates the MWD tool  30  from the upper drill string pipe sections  20 ,  22 ,  24 ,  26 . At the same time, an electrical signal may be passed between the central components housed within the MWD tool  30  and both of the separated poles of the dipole antenna formed within the drill string  16 . One pole of this antenna is provided by the lower sub  36 , via the ground connection of the MWD components with the lower sub  36 . A signal may be transmitted from the MWD components to the upper gap sub  34  and the interconnected remainder of drill string  16 , i.e., the second pole of the antenna, via the electrical pathway established by the conductive element  58 , pressure plug  72  and contact element  70 . The MWD components may be operated to produce a signal that may be transmitted by this antenna and detected by the receiver  32  at the surface  12 . 
   The gap sub assembly  33  of the present invention is advantageous in that it is inexpensive to employ and the components involved are simple to construct. No resins or specialized tools are needed to construct the gap sub assembly  33 . In addition, the components involved are highly resistant to damage from downhole pressures, temperatures and physical hazards. 
   In addition, if it is necessary to change the gap sub  34  out for a gap sub that is of a different length, it is not necessary to replace the conductor assembly  40 . The conductor assembly  40  will transmit signals to both poles of the antennae arrangement regardless of the length of gap sub  34  that is used. Additionally, the characteristics of the antenna signal provided may be altered merely by changing out the gap sub  34  for a gap sub of a different length because signals sent to the gap sub  34  are propagated along the length of the housing rather than along a conductor retained within the sub. 
   Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.

Summary:
Devices and methods for incorporating a gap sub assembly into a drill string to electrically isolate portions of a transmitter assembly within, for example, an MWD tool located within the drill string. The gap sub assembly incorporates upper and lower subs having an insulated interconnection. A central conductor assembly is axially disposed within the lower sub and does not extend through the length of the gap sub. The central conductor assembly is used to transmit electrical power and data across the gap sub assembly between the upper portions of the drill string and transmitter components housed within the MWD tool disposed below the gap sub assembly.