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RELATED APPLICATIONS 
     This nonprovisional application is a divisional of United States nonprovisional Patent application: application Ser. No. 09/036,886 filed Mar. 5, 1998 by Paul D. Ringgenberg et al. for “Electrically Insulating Gap Subassembly for Downhole Electromagnetic Transmission,” now U.S. Pat. No. 6,098,727. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to downhole telemetry and, in particular to, an electrically insulating gap subassembly for electrically insulating sections of a pipe string such that electromagnetic waves may be developed thereacross for carrying information between surface equipment and downhole equipment. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background is described in connection with transmitting downhole data to the surface during measurements while drilling (MWD), as an example. It should be noted that the principles of the present invention are applicable not only during drilling, but throughout the life of a wellbore including, but not limited to, during logging, testing, completing and producing the well. 
     Heretofore, in this field, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The utilization of MWD with real time data transmission provides substantial benefits during a drilling operation. For example, continuous monitoring of downhole conditions allows for an immediate response to potential well control problems and improves mud programs. 
     Measurement of parameters such as bit weight, torque, wear and bearing condition in real time provides for a more efficient drilling operation. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques. 
     At present, there are four major categories of telemetry systems that have been used in an attempt to provide real time data from the vicinity of the drill bit to the surface, namely mud pressure pulses, insulated conductors, acoustics and electromagnetic waves. 
     In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar near the bit. This type of system typically transmits at 1 bit per second as the pressure pulse travels up the mud column at or near the velocity of sound in the mud. It has been found, however, that the rate of transmission of measurements is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of mud flow. 
     Insulated conductors, or hard wire connection from the bit to the surface, is an alternative method for establishing downhole communications. This type of system is capable of a high data rate and two way communications are possible. It has been found, however, that this type of system requires a special drill pipe and special tool joint connectors which substantially increase the cost of a drilling operation. Also, these systems are prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string. 
     Acoustic systems have provided a third alternative. Typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission. 
     The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. A current carrying downhole data is input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string. An electromagnetic receiver is inserted into the ground at the surface where the electromagnetic data is picked up and recorded. It has been found, however, that it is necessary to have an electrically insulated subassembly in the drill string in order to generate the electromagnetic waves. Conventional electromagnetic systems have used dielectric materials such as plastic resins between the threads of drill pipe joints or within sections of drill pipe. It has been found, however, that these dielectric materials may be unable to withstand the extreme tensile, compressive and torsional loading that occurs during a drilling operation. 
     Therefore, a need has arisen for a gap subassembly that electrically isolates portions of a drill string and that is capable of being used for telemetering real time data from the vicinity of the drill bit in a deep or noisy well using electromagnetic waves to carry the information. A need has also arisen for a gap subassembly that is capable of withstanding the extreme tensile, compressive and torsional loading that occurs during a drilling operation. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises an electrically insulating gap subassembly that electrically isolates portions of a drill string that is capable of being used for telemetering real time data from the vicinity of the drill bit in a deep or noisy well using electromagnetic waves to carry the information. The apparatus of the present invention is capable of withstanding the extreme tensile, compressive and torsional loading that occurs during a downhole operation such as drilling a wellbore that traverses a hydrocarbon formation and production of hydrocarbons from the formation. 
     The electrically insulating gap subassembly of the present invention comprises first and second tubular members each having a threaded end connector. An isolation subassembly having first and second threaded end connectors is disposed therebetween and respectively coupled to the threaded end connectors of the first and second tubular members. The isolation subassembly may be made of aluminum and have anodized surfaces. 
     The electrically insulating gap subassembly may include an outer sleeve disposed exteriorly about the isolation subassembly. The outer sleeve may extend exteriorly about a portion of the first and second tubular members. The electrically insulating gap subassembly may also include an inner sleeve disposed interiorly within the isolation subassembly. The inner sleeve may extend interiorly within a portion of the first and second tubular members. The inner sleeve and the outer sleeve are composed of an insulating material such as fiberglass. A glue may be used to attach the inner sleeve and the outer sleeve to the isolation subassembly. 
     The electrically insulating gap subassembly may have an insulating coating between the threaded end connectors of the first and second tubular members and the isolation subassembly. The insulating coating may be, for example, a ceramic or aluminum oxide. 
     The electrically insulating gap subassembly of the present invention may include a dielectric material disposed between the isolation subassembly and the first and second tubular members. In this embodiment, an electrically conductive isolation subassembly constructed from, for example steel, may be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of an offshore oil or gas drilling platform operating isolation subassemblies of the present invention; and 
     FIGS. 2A-2B are quarter-sectional views of a downhole electromagnetic transmitter and receiver utilizing an isolation subassembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     Referring to FIG. 1, a downhole electromagnetic signal transmitter and a downhole electromagnetic signal repeater in use in conjunction with an offshore oil and gas drilling operation are schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22  including blowout preventers  24 . Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering drill string  30 , including drill bit  32 , electromagnetic transmitter  34  and downhole electromagnetic signal repeater  36 . 
     In a typical drilling operation, drill bit  32  is rotated by drill string  30 , such that drill bit  32  penetrates through the various earth strata, forming wellbore  38 . Measurement of parameters such as bit weight, torque, wear and bearing conditions may be obtained by sensors  40  located in the vicinity of drill bit  32 . Additionally, parameters such as pressure and temperature as well as a variety of other environmental and formation information may be obtained by sensors  40 . The signal generated by sensors  40  may typically be analog, which must be converted to digital data before electromagnetic transmission in the present system. The signal generated by sensors  40  is passed into an electronics package  42  including an analog to digital converter which converts the analog signal to a digital code utilizing “ones” and “zeros” for information transmission. 
     Electronics package  42  may also include electronic devices such as an on/off control, a modulator, a microprocessor, memory and amplifiers. Electronics package  42  is powered by a battery pack which may include a plurality of batteries, such as nickel cadmium or lithium batteries, which are configured to provide proper operating voltage and current. 
     Once the electronics package  42  establishes the frequency, power and phase output of the information, electronics package  42  feeds the information to electromagnetic transmitter  34 . Electromagnetic transmitter  34  may be a direct connect to drill string  30  or may electrically approximate a large transformer. The information is then carried uphole in the form of electromagnetic wave fronts  46  which propagate through the earth. These electromagnetic wave fronts  46  are picked up by receiver  48  of electromagnetic repeater  36  located uphole from electromagnetic transmitter  34 . 
     Electromagnetic repeater  36  is spaced along drill string  30  to receive electromagnetic wave fronts  46  while electromagnetic wave fronts  46  remain strong enough to be readily detected. Receiver  48  of electromagnetic repeater  36  may electrically approximate a large transformer. As electromagnetic wave fronts  46  reach receiver  48 , a current is induced in receiver  48  that carries the information originally obtained by sensors  40 . 
     The current from receiver  48  is fed to an electronics package  50  that may include a variety of electronic devices such as amplifiers, limiters, filters, a phase lock loop, shift registers and comparators. Electronics package  50  processes the signal and amplifies the signal to reconstruct the original waveform, compensating for losses and distortion occurring during the transmission of electromagnetic wave fronts  46  through the earth. Electronics package  50  forwards the signal to a transmitter  52  that generates and radiates electromagnetic wave fronts  54  into the earth in the manner described with reference to transmitter  44  and electromagnetic wave fronts  46 . 
     Electromagnetic wave fronts  54  are received by electromagnetic pickup device  64  located on sea floor  16 . Electromagnetic pickup device  64  may sense either the electric field or the magnetic field of electromagnetic wave front  54  using electric field sensors  66  or a magnetic field sensor  68  or both. 
     Electromagnetic pickup device  64  then transmits the information received in electromagnetic wave fronts  54  to the surface via wire  70  that is connected to buoy  72  and wire  74  that is connected to a processor on platform  12 . Upon reaching platform  12 , the information originally obtained by sensors  40  is further processed making any necessary calculations and error corrections such that the information may be displayed in a usable format. 
     Even though FIG. 1 depicts a single repeater  36 , it should be noted by one skilled in the art that the number of repeaters, if any, located within drill string  30  will be determined by the depth of wellbore  38 , the noise level in wellbore  38  and the characteristics of the earth&#39;s strata adjacent to wellbore  38  in that electromagnetic waves suffer from attenuation with increasing distance from their source at a rate that is dependent upon the composition characteristics of the transmission medium and the frequency of transmission. For example, repeaters, such as repeater  36 , may be positioned between 2,000 and 5,000 feet apart. Thus, if wellbore  38  is 15,000 feet deep, between two and seven repeaters would be desirable. 
     Even though FIG. 1 depicts transmitter  34 , repeater  36  and electromagnetic pickup device  64  in an offshore environment, it should be understood by one skilled in the art that transmitter  34 , repeater  36  and electromagnetic pickup device  64  are equally well-suited for operation in an onshore environment. In fact, in an onshore environment, electromagnetic pickup device  64  would be placed directly on the land. Alternatively, a receiver such as receiver  48  could be used at the surface to pick up the electromagnetic wave fronts for processing at the surface. 
     Additionally, while FIG. 1 has been described with reference to transmitting information uphole during a measurement while drilling operation, it should be understood by one skilled in the art that repeater  36  and electromagnetic pickup device  64  may be used in conjunction with the transmission of information downhole from surface equipment to downhole tools to perform a variety of functions such as opening and closing a downhole tester valve or controlling a downhole choke. In this example, transmitter  34  would also serve as an electromagnetic receiver. 
     Further, even though FIG. 1 has been described with reference to one way communication from the vicinity of drill bit  32  to platform  12 , it should be understood by one skilled in the art that the principles of the present invention are applicable to two way communications. For example, a surface installation may be used to request downhole pressure, temperature, or flow rate information from formation  14  by sending electromagnetic wave fronts downhole using electromagnetic pickup device  64  as an electromagnetic transmitter and retransmitting the request using repeater  36  as described above. Electromagnetic transmitter  34 , serving as an electromagnetic receiver, would receive the electromagnetic wave fronts and pass the request to sensors, such as sensors  40 , located near formation  14 . Sensors  40  then obtain the appropriate information which would be returned to the surface via electromagnetic wave fronts  46  which would again be retransmitted by repeater  36 . As such, the phrase “between surface equipment and downhole equipment” as used herein encompasses the transmission of information from surface equipment downhole, from downhole equipment uphole or for two way communications. 
     Representatively illustrated in FIGS. 2A-2B is one embodiment of an electromagnetic transmitter and receiver, such as electromagnetic transmitter  34 , or a downhole electromagnetic signal repeater, such as repeater  36 , which is generally designated  76  and which will hereinafter be referred to as repeater  76 . For convenience of illustration, FIGS. 2A-2B depict repeater  76  in a quarter sectional view. Repeater  76  has a box end  78  and a pin end  80  such that repeater  76  is threadably adaptable to drill string  30 . Repeater  76  has an outer housing  82  and a mandrel  84  having a full bore so that when repeater  76  is interconnected with drill string  30 , fluids may be circulated therethrough and therearound. Specifically, during a drilling operation, drilling mud is circulated through drill string  30  inside mandrel  84  of repeater  76  to ports formed through drill bit  32  and up the annulus formed between drill string  30  and wellbore  38  exteriorly of housing  82  of repeater  76 . Housing  82  and mandrel  84  thereby protect the operable components of repeater  76  from drilling mud or other fluids disposed within wellbore  38  and within drill string  30 . 
     Housing  82  of repeater  76  includes an axially extending generally tubular upper connecter  86  which has box end  78  formed therein. Upper connecter  86  may be threadably and sealably connected to drill string  30  for conveyance into wellbore  38 . 
     An axially extending generally tubular intermediate housing member  88  is threadably and sealably connected to upper connecter  86 . An axially extending generally tubular lower housing member  90  is threadably and sealably connected to intermediate housing member  88 . Collectively, upper connector  86 , intermediate housing member  88  and lower housing member  90  form upper subassembly  92 . Upper subassembly  92  is electrically connected to the section of drill string  30  above repeater  76 . 
     An axially extending generally tubular isolation subassembly  94  is securably and sealably coupled to lower housing member  90  by outer threads  96  and inner threads  97 . An axially extending generally tubular lower connector  98  is securably and sealably coupled to isolation subassembly  94  by outer threads  100  and inner threads  101 . 
     Dielectric member  102  is disposed between the isolation subassembly  94  and lower housing number  90 . Dielectric material  104  is disposed between outer threads  97  of isolation subassembly  94  and inner threads  96  of lower housing member  90 . Dielectric member  102  and dielectric material  104  are electrically insulating materials that provide substantial load bearing capabilities such as a ceramic, anodized aluminum or a resin such as mycarta. Similarly, dielectric member  106  is disposed between isolation subassembly  94  and the lower connector  98  while dielectric material  108  is disposed between outer threads  100  of isolation subassembly  94  and inner threads  101  of lower connector  98 . 
     Isolation subassembly  94  may be made of aluminum having a strength of, for example, a 60,000 psi. Isolation subassembly  94  may be anodized to confers an electrically insulating coating on the surface of isolation subassembly  94 . 
     An outer sleeve  110  is disposed exteriorly of isolation subassembly  94 , lower housing member  90  and lower connector  98  between shoulder  112  of lower housing member  90  and shoulder  114  of lower connector  98 . Outer sleeve  110  is formed from an electrically insulating material, such as pre-formed or built-up fiberglass. Outer sleeve  110  has the same outer diameter as the lower housing member  90  and lower connector  98 . Outer sleeve  110  provides insulation to isolation subassembly  94  and protects isolation subassembly  94  from corrosion and contact with the sides of wellbore  38  and rig tongs when isolation subassembly  94  is joined with other sections of drill string  30 . 
     An inner sleeve  116  is disposed on the inner surface of isolation subassembly  94 , and extends into lower housing member  90  and lower connector  98  between shoulder  118  of lower housing member  90  and shoulder  120  of lower connector  98 . Inner sleeve  116  is an electrical insulator that helps protect the inner surface of isolation subassembly  94  from, e.g., drilling mud and other corrosive materials. 
     The contact points between the isolation subassembly  94  and lower housing member  90  and lower connector  98 , respectively, are electrically insulated in several ways. Specifically, the outer surface of isolation subassembly  94  may be anodized aluminum and dielectric members  102 ,  106  along with dielectric material  104 ,  108  provide electric isolation between isolation subassembly  94 , lower housing member  90  and lower connector  98 . In addition, inner threads  97  of lower housing member  90  and inner threads  101  of lower connector  98 , which are made of steel, may be coated with an insulating material. For example, insulating materials such as ceramic, Polytetrafluoroethylene or an aluminum oxide coating are suitable. 
     Outer sleeve  110  and inner sleeve  116  also provide electrical insulation between isolation subassembly  94 , lower housing member  90  and lower connector  98 . In addition to protecting isolation subassembly  94  from potential damage during handling and use such as scratching, outer sleeve  110  and inner sleeve  194 , also provide for corrosion protection for the anodized aluminum isolation subassembly  94 . 
     Alternatively, with the use of dielectric members  102 ,  106  along with dielectric material  104 ,  108 , sufficient electrical isolation may be obtained using an electrically conductive isolation subassembly  94  constructed from, for example, steel, that is disposed between lower housing member  90  and lower connector  98 . In this embodiment, a suitable insulating material such as ceramic, Polytetrafluoroethylene or an aluminum oxide coating may be placed between inner threads  97  of lower housing member  90  and outer threads  96  of isolation subassembly  94  as well as between inner threads  101  of lower connector  98  and outer threads  100  of isolation subassembly  94 . Also, in this embodiment, the distance between the dielectric members  102 ,  106  is preferably at least two diameters of isolation subassembly  94 . 
     In the past, when an insulating coating was applied to threads, the contact stress of torquing the joint commonly damaged the coating. Isolation subassembly  94  of the present invention provides a modified shoulder that allows the threads to be made up manually and then permits the threads to be loaded. Specifically, collar  109  may be used to load outer threads  96  of isolation subassembly  94  and inner threads  97  of lower housing member  90 . First, isolation subassembly  94  and lower housing member  90  are mated together without applying full torque. Thereafter, collar  109  is rotated on outer thread  96  of isolation subassembly  94  toward lower housing member  90 , thereby loading outer threads  96  and inner threads  97  without damaging the insulating coating. Likewise, collar  111  may be used to load outer threads  100  of isolation subassembly  94  and inner threads  101  of lower connector  98  in a similar manner. This procedure allows for the loading of outer threads  100  and inner threads  101  without any sliding action to damage the coating. Collars  109 ,  111  may be locked into place using set screws. 
     Alternatively, isolation subassembly  94  may be coupled with lower housing member  90  and lower connector  98  using thermal torque. Outer threads  96 ,  100  of the isolation subassembly  94  are cooled, while inner threads  97  of lower housing member  90  and inner threads  101  of lower connector  98  are heated. The respective threads are then joined together and torqued to a low value. As outer threads  96 ,  100  of isolation subassembly  94  heat up and while inner threads  97  of lower housing member  90  and inner threads  101  of lower connector  98  cool, a load is created on the threads. By using the thermal torque assembly method, a large load may be placed on outer threads  96 ,  100  of isolation subassembly  94  while eliminating the contact stress associated with high torque that can cause scratching of the anodized aluminum outer threads  96 ,  100  of the isolation subassembly  94  and the coated steel inner threads  97 ,  101  of lower housing member  90  and lower connector  98 , respectively. 
     Additionally, it should be noted by one skilled in the art that the threaded connections of isolation subassembly  94  may be further strengthened by the addition of an epoxy therebetween, such as HalliBurton WELD A. Likewise, dielectric members  102 ,  106  and dielectric material  104 ,  108  as well as outer sleeve  110  and inner sleeve  116  may be secured in place using an epoxy. 
     Thus, isolation subassembly  94  provides a discontinuity in the electrical connection between lower connector  98  and upper subassembly  92  of repeater  76 , thereby providing a discontinuity in the electrical connection between the portion of drill string  30  below repeater  76  and the portion of drill string  30  above repeater  76 . 
     It should be apparent to those skilled in the art that the use of directional terms such as above, below, upper, lower, upward, downward, etc. are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. It is to be understood that repeater  76  may be operated in vertical, horizontal, inverted or inclined orientations without deviating from the principles of the present invention. 
     Mandrel  84  includes axially extending generally tubular upper mandrel section  142  and axially extending generally tubular lower mandrel section  144 . Upper mandrel section  142  is partially disposed and sealing configured within upper connector  86 . A dielectric member  146  electrically isolates upper mandrel section  142  from upper connector  86 . The outer surface of upper mandrel section  142  may have a dielectric layer  148  disposed thereon. Dielectric layer  148  may be, for example, a Polytetrafluoroethylene layer. Together, dielectric layer  148  and dielectric member  146  serve to electrically isolate upper connector  86  from upper mandrel section  142 . 
     Between upper mandrel section  142  and lower mandrel section  144  is a dielectric member  150  that, along with dielectric layer  148 , serves to electrically isolate upper mandrel section  142  from lower mandrel section  144 . Between lower mandrel section  144  and lower housing member  90  is a dielectric member  152 . On the outer surface of lower mandrel section  144  is a dielectric layer  154  which, along with dielectric member  152 , provides for electric isolation of lower mandrel section  144  from lower housing number  90 . Dielectric layer  154  also provides for electric isolation between lower mandrel section  144  and isolation subassembly  94  as well as between lower mandrel section  144  and lower connector  98 . Lower end  156  of lower mandrel section  144  is disposed within lower connector  98  and is in electrical communication with lower connector  98 . 
     Intermediate housing member  88  of outer housing  82  and upper mandrel section  142  of mandrel  84  define annular area  158 . A receiver  160 , an electronics package  162  and a transmitter  164  are disposed within annular area  158 . In operation, receiver  160  receives an electromagnetic input signal carrying information which is transformed into an electrical signal that is passed onto electronics package  162  via electrical conductor  166 . Electronics package  162  processes and amplifies the electrical signal. The electrical signal is then fed to transmitter  164  via electrical conductor  168 . Transmitter  164  transforms the electrical signal into an electromagnetic output signal carrying information that is radiated into the earth utilizing isolation subassembly  94  to provide the electrical discontinuity necessary to generate the electromagnetic output signal. 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
An electrically insulating gap subassembly for inclusion in a pipe string ( 30 ) comprising a pair of tubular members ( 90, 98 ) having an electrically insulating isolation subassembly ( 94 ) threadably disposed therebetween is disclosed. The electrically insulating isolation subassembly ( 94 ) has an anodized aluminum surface that provides electrical isolation to interrupt electrical contact between the two tubular members ( 90, 98 ) such that electromagnetic waves ( 46, 54 ) carrying information may be generated thereacross.