Abstract:
A beacon assembly located at a downhole end of a drill string proximate a boring tool. The beacon assembly transmits data to an above-ground receiver. The beacon has a housing with a housing wall located between its sensors, such as gradiometers, accelerometers, and other orientation sensors, and an antenna assembly. The antenna assembly has a protective covering made of electromagnetically transparent material.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of provisional patent application Ser. No. 62/008,544, filed on Jun. 6, 2014, the entire contents of which are incorporated herein by reference, 
     
    
     FIELD 
       [0002]    The present invention relates generally to beacons and antennas for use with downhole tools drilling operations. 
       SUMMARY 
       [0003]    The present invention is directed to a downhole tool coupled to a drill string comprising a sensor, an antenna electromagnetically coupled to the sensor, and a wall disposed between the antenna and the sensor. The wall comprises a connection point for connection to the drill string, 
         [0004]    In another embodiment, the present invention is directed to a beacon assembly for attachment to a downhole end of a drill string. The drill string comprises a substantially constant first diameter. The beacon assembly comprises a housing wall, an antenna, and a sensor. The housing wall comprises a first portion and a second portion. The first portion has substantially the first diameter. The second portion has a second diameter which is less than the first diameter. The antenna is located about the second portion of the housing wall. The sensor is located within the housing wall electronic communication with the antenna. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a cross-sectional side view of a downhole tool having an external antenna. 
           [0006]      FIG. 2  is a perspective view of a beacon assembly of the downhole tool of  FIG. 1   
           [0007]      FIG. 3  is a perspective sectional view of the antenna assembly of the downhole tool of  FIG. 1 . 
           [0008]      FIG. 4  is a partial sectional end view of the downhole tool, showing the antenna assembly of the downhole tool. 
           [0009]      FIG. 5  is a cross-sectional side view of an alternative embodiment of the antenna assembly of the downhole tool with the antenna coil shown un-sectioned for clarity. 
       
    
    
     DESCRIPTION 
       [0010]    Horizontal Directional Drilling (HDD) applications typically employ a subsurface tracking beacon and a walk-over tracking receiver to follow the progress of a horizontal borehole. An example of a walkover receiver and method for use thereof is shown in U.S. Pat. No. 8,497,684 issued to Cole, et, al., the contents of which are incorporated herein by reference. The tracking beacon contains devices to measure pitch, roll (bit angle), beacon battery voltage, beacon temperature, and a variety of other physical parameters. Measured information is transmitted by the beacon using a modulated electromagnetic signal. Transmission of the beacon&#39;s signal typically involves an internal antenna consisting of multiple wire turns wrapped around a ferrite rod. The surface tracking receiver contains electronic elements which receive and decode the modulated signal. The surface tracking receiver also detects the signal&#39;s field characteristics and measures the beacon&#39;s emitted signal amplitude to estimate the beacon&#39;s depth and location. 
         [0011]    In some cases, the beacon measurements of interest are magnetic field measurements. Certain applications require the use of magnetic field gradiometers, which are instruments used to determine a magnetic field&#39;s rate of change along a certain path. Magnetic field gradiometers essentially involve magnetic field measurements separated by a known distance along some axis. Construction of a magnetic field gradiometer in the HDD industry is complicated, not only by the limited axial and radial space available for sensor placement, but also by the need to communicate measurements to the surface receiver by a magnetic field transmission. The lack of space makes it desirable to package beacon electronics elements as densely as possible, but the presence of the antenna&#39;s ferrite rod near a gradiometer&#39;s magnetic field sensors is known to be capable of disturbing the gradiometer&#39;s measurement capability. In the case of the most sensitive sensors, the proximity of a ferrite rod to any of the sensing elements can produce undesirable measurement degradation. 
         [0012]    Further, conventional beacon antennas will be inside a beacon housing that attenuates the magnetic field because the beacon housing is conductive and magnetically permeable. To reduce this effect, slots are often provided in the beacon housing. However, limitations include differences in the strength based upon the orientation of the housing, attenuation, and may require specifically clocked housings for accurate measurements. 
         [0013]    The present invention packages the antenna away from sensors and outside of the beacon housing. The invention may also be used with a downhole generator that may be integral with the beacon for power, which could be housed in a common housing. The beacon may be used with a single or dual-member drill string. The beacon could also be used with a drive shaft going through the beacon to drive a downhole tool such as in a coiled tubing application. 
         [0014]    With reference now to the figures in general and  FIG. 1  in particular, shown therein is a downhole tool  10 . The downhole tool  10  is connected on a first end  12  to a drill bit (not shown) and a second end  14  to a drill string  11 . As shown, the tool  10  is adapted to connect to a dual member drill string  11  comprising an inner member  11   a  and an outer member  11   b,  though a single member drill string may be utilized with the proposed invention without departing from its spirit. The tool  10  may connect to the drill string  11  at a threaded connection or other known connection at its second end  14 . The tool  10  comprises a front tool body  16 , a beacon assembly  18 , and an antenna assembly  20 . The tool  10  comprises a housing wall  21  which is preferably located about a periphery of the beacon assembly  18  but inside the antenna assembly  20 . The beacon assembly  18  may allow fluid to pass through the center portion of the tool  10  forming an internal passage  13  of the drill string  11  or with an annulus between the inner member  11   a  and outer member  11   b  of a dual member drill string. 
         [0015]    The housing wall  21  preferably has a varying diameter creating a first portion  21   a  and second portion  21   b,  such that the diameter of the housing wall  21  when encasing the beacon assembly  18  (first portion  21   a ) is greater than the diameter of the housing wall when within the antenna assembly  20  (second portion  21   b ). A shoulder may be created between the first portion  21   a  and the second portion  21   b,  or the transition may be tapered or gradual. The housing wall  21  may comprise an opening, or feedthrough  104  ( FIG. 5 ) for the antenna coil  100  ( FIG. 5 ), to traverse between the antenna assembly  20  and the beacon assembly  18 . 
         [0016]    The front toot body  16  allows fluid flow from within the drill string  11  to a drill bit or other tool as well as transmission of rotation from the inner member  11   a  to the drill bit. The beacon assembly  18  comprises a magnet motor  22  and a generator assembly  24 . As relative rotation occurs between the inner member  11   a  and outer member  11   b  of the drill string  11 , components of the downhole tool  10  also rotate relative to one another due to connection made at stem weldment. An exemplar generator assembly  24  utilizing a dual-member drill string  11  may be found in U.S. Pat. No. 6,739,413, issued to Sharp, et. al., the contents of which are incorporated herein by reference. 
         [0017]    The antenna assembly  20  comprises an antenna  26  and a protective casing  29 . The antenna  26  transmits signals generated by the beacon assembly  18  as will be described in further detail with reference to  FIGS. 3-5 . The protective casing  29  is preferably a magnetically transparent sleeve, a material that has a relative permeability of substantially unity. The casing  29  may comprise cast urethane, plastics, ceramics, or other materials that provide structural protection but create little or no interference with the signal of the antenna  26 . 
         [0018]    With reference now to  FIG. 2  the beacon assembly  18  is shown in greater detail, The beacon assembly  18  may be rotationally locked to the inner member  11   a  (not shown). The generator assembly  24  comprises stator poles  30 , bobbins  32 , and a back plate  34 . The stator poles  30 , when rotated relative to magnet motor  22  ( FIG. 1 ) through fluid flow or relative rotation of the inner  11   a  and outer  11   b  drill members, generate a current to power the tool  10 . Alternatively, power for the tool  10  may also be provided by wireline or batteries. 
         [0019]    The beacon assembly  18  further comprises a sensor assembly  40 . The back plate  34  helps to isolate the generator assembly  24  from the sensor assembly  40 . The sensor assembly  40  comprises aboard  42 , a sensor  44 , and a program port  46 . The board  42  provides structural and electrical connectivity for the sensor  44  and program port  46 . The board  42  may be curved to match the shape of the beacon assembly  18 . The sensor  44  comprises one or more sensors for determining an orientation of the downhole tool  10 . Such sensors  44  may comprise one or more yaw, pitch, roll, tension, force, conductivity, or other sensors. For example, an accelerometer may be utilized. The program port  46  allows a user to access data and configure the sensors  44 . Further, while the use of sensors  44  is one advantageous use of the antenna assembly  20  ( FIG. 3 ), another transmission source could be utilized with the antenna assembly disclosed below. 
         [0020]    The antenna assembly ( FIG. 3 ) may also connect to the beacon sensors  44  through port  46 , A locating key  48  may be utilized to lock the clock position of the beacon assembly  18  to the antenna assembly  20  ( FIG. 3 ). In this way, a feedthrough  104  ( FIG. 5 ) may be placed between the sensor assembly  40  and the antenna assembly  20  through the housing wall  21  ( FIG. 3 ). As shown, a center tube  49  passes through the beacon assembly  18  to provide fluid flow and optionally provide rotational torque from the drill string  11  ( FIG. 1 ). 
         [0021]    With reference to  FIG. 3 , the antenna  26  comprises an end support  50 , a support tube  52 , at least one ferrite rod  54 , a nonconductive tube  56  and a shield  58 . The end support  50  provides an insulating support for the antenna  26  within the tool  10  so that electromagnetic interference of the housing wall  21  at the ends of the antenna  26  is minimized. Further, any electromagnetic interference between the antenna  26  and sensors  44  is also minimized. The support tube  52  is disposed about the housing wall  21  and locates the ferrite rods  54  within the antenna assembly  20 . The shield  58  is preferably highly conductive, non-magnetic. Aluminum may be used in the shield  58 , as could other materials such as copper. Preferably, the shield covers the end support  50 . There may be a further insulator between the shield  58  and the housing wall  21 . The nonconductive magnetic field layer, or tube  56  is located between the shield  58  and ferrite rods  54  and insulates them from each other. Further, the tube  56  may be a non-magnetic material such as plastic. Without the nonconductive tube  56  or similar structure, the magnetic field would be pushed outward but some eddy currents would flow within the housing wall  21 . The tube  56  may be a hollow cylinder, or may be comprised of multiple pieces with nonconductive, non-magnetic properties. 
         [0022]    The ferrite rods  54  are located between the plastic tube  56  and protective casing  29  and magnify signal strength of the beacon signals corresponding to readings of the beacon assembly  18 . A coiled antenna wire  100  ( FIG. 5 ) may be provided about the ferrite rods  54  to transmit the beacon signals. Further, as shown in  FIG. 5 , an antenna wire  100  may be utilized without ferrite rods. The coiled antenna wire  100  is preferably a single layer to minimize its profile, but a multi-layer antenna may be used. 
         [0023]    With reference now to  FIG. 4 , the antenna assembly  20  is shown in cross section. The housing wall  21  is removed for clarity. As shown, the antenna assembly  20  comprises twenty five ferrite rods  54 , though other numbers of rods may be used. Additionally, the ferrite rods  54  themselves may be removed and elements of the housing wall  21  may be used with an antenna coil. The antenna coil  100  may be also utilized about the ferrite rods. In general, the arrangement of the antenna assembly  20  from inside to outside is housing wall  21  ( FIG. 3 ), shield  58 , tube  56 , ferrite rods  54 , antenna coil  100  ( FIG. 5 ), protective casing  29 . An insulating gap or material (not shown) may be utilized between the housing wall  21  and shield  58 . Further, the plastic tube  56  may be replaced with a layer of any non-conductive material, such as air. 
         [0024]    In operation, the antenna assembly  20  of  FIG. 4  operates when current passes through the antenna windings  100  to generate a magnetic field corresponding to beacon readings. The field passes through the tube  56  and permeates the shield  58  according to skin depth rules. The eddy current induced in the shield  58  will “push” the magnetic field out away from the tool  10 , minimizing power loss. The insulating gap (not shown) prevents eddy currents from reaching the housing wall  21 . 
         [0025]    In  FIG. 1 , the antenna assembly  20  and beacon assembly  18  are shown with linear displacement for clarity. One of skill in the art will appreciate that these assemblies may be placed at any location longitudinally relative to one another without critically impairing the spirit of this invention. In fact, the antenna assembly  20  may be disposed about a portion of the housing wall  21  that is disposed about the beacon assembly  18 . 
         [0026]    With reference now to  FIG. 5 , an alternative embodiment of the antenna assembly  20  is shown. The antenna assembly  20  comprises a housing wall  21  with a first, large diameter portion  21   a  and a recessed, second portion  21   b.  The recessed portion  21   b  is covered, or filled, with a protective casing  29 . The antenna coil  100  is wrapped around the housing wall  21  and within the protective casing  29 . The protective casing  29  may comprise a urethane material or other magnetically transparent material. The antenna coil  100  is connected to the beacon assembly  18  ( FIG. 1 ) through the feedthrough  104 . The feedthrough  104  may comprise small radial holes made in the housing wall  21 . 
         [0027]    One skilled in the art will appreciate that the embodiments contained herein may be modified without departing from the spirit of the invention contained herein. For example, alternative sensors or antenna arrangements, and materials may be utilized.