Patent Publication Number: US-6659198-B2

Title: Back reamer assembly

Description:
This non-provisional patent application claims the benefit of U.S. provisional patent application 60/299615, filed Jun. 20, 2001. 
    
    
     FIELD 
     The present invention pertains to apparatus and method for enlarging subterranean holes using a substantially conical bit mounted at the end of a string of segmented drill pipe or flexible tubing. 
     BACKGROUND 
     In recent years the placement of utility service lines in shallow boreholes drilled substantially parallel to the earth&#39;s surface has become more common. Initially, these lines were buried by digging a shallow trench, placing the utility service line in the trench, and then re-filling the trench. Later, it was realized that the horizontal and directional drilling techniques used in deep well drilling could be adapted for the drilling of shallow boreholes for utility lines. Now, it is common to see shallow borehole drilling machines, which include a rack of segmented pipe sections, drilling shallow boreholes for utility lines. 
     The drilling of shallow boreholes for utility lines is typically a two step process. First, a pilot hole is drilled outwardly from a location proximal to the drilling machine. When the shallow borehole reaches its predetermined destination or distal location, the drilling tool is steered upwardly towards the earth&#39;s surface or caused to enter an exit pit. It is at this time that the drilling tool, which was used to drill the initial pilot hole is removed and replaced with a backreaming tool. The backreaming tool is then pulled back through the hole to enlarge the borehole to a desired size. 
     Typical to the subterranean hole boring industry are enlarged drill bits that are utilized in a back drilling, or “backreaming” operation. These enlarged drill bits are known as backreamers. These backreamers are made in various sizes and shapes but are all based on a configuration wherein the drill face is pointed back toward the drilling machine located at the proximal point of the borehole. This configuration enables the bit face to cut while the drill pipe or tubing is pulled back towards the drilling equipment from the distal end of the borehole. 
     Prior art backreaming tools have been clumsy to use and inefficient. Accordingly, a need remains in the art for a backreaming tool which is easy to use and is efficient. 
     SUMMARY 
     The apparatus and method of the present invention includes a backreaming tool assembly for use in enlarging shallow boreholes drilled in an earthen environment which is easy to use and efficient. 
     The backreamer assembly  10  of the present invention differs from conventional backreamers by the placement of a drive mechanism or transmission inside the backreamer cutting tool. A downhole motor on the end of the segmented drill pipe flexible tubing applies the required rotational force to rotate the cutting tool. The drive mechanism within the backreamer cutting tool is a transmission or gearbox. This allows the input power to the backreamer cutting tool to be increased, thereby increasing the output torque at the face of the backreamer cutting tool which encounters the earthen matter surrounding the hole as the backreamer cutting tool moves through and enlarges the borehole. These transmission gearboxes are of a standard helical or planetary design that provide a reduction in input speed and an increase in output torque. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     A better understanding of the backreamer assembly of the present invention may be had by reference to the drawing figures, wherein: 
     FIG. 1 is a schematic side elevational view of the backreamer assembly of the present invention; 
     FIG. 2A is a schematic side elevational view of the backreamer assembly with cutting blades; 
     FIG. 2B is a schematic side elevational view of the backreamer assembly with rock bits; 
     FIG. 3 is an exploded schematic side elevational view of an alternate embodiment of the backreamer assembly; and 
     FIG. 4 is a schematic side elevational view of yet another alternate embodiment of the backreamer assembly. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     As shown in FIG. 1, a substantially conical backreamer assembly  10  according to the present invention is attached to the end of coiled tubing or a string of segmented drill pipe  102 . A connector  104  is used to mechanically adapt the end of the coiled tubing or string of segmented pipe to either a drive motor  106  or to an optimal tracker  108 . Such trackers  108  are well known to those of ordinary skill in the art and provide an indication of location by one of a variety of methods including, for example, GPS, sonic, or RF signals. Attached to the drive motor  106  is a connector or bit drive  110  which mates with a threaded extension  12  on the backreamer assembly. The connector or bit drive  110  provides rotational torque from the drive motor  106  to a gearbox  14  substantially contained within the cutting face of the substantially conical backreamer cutting tool  15 . 
     As shown in FIGS. 2A and 2B, the exterior of the substantially conical backreamer cutting tool  15  includes means for cutting into an earthen environment. If the earthen environment is primarily soils, blades  16 , as shown in FIG. 2A, are used. If the earthen environment is primarily rock, one or more rock bits  18 , as shown in FIG. 2B, are used. To adapt to the need to drill holes for varying diameters, the geometry of the outer surface of the cutting tool  15  may be constructed to be adjustable. Such adjustable diameter earthen drills are well known to those of ordinary skill in the art. 
     As shown in FIGS. 3 and 4, when the cutting tool  15  is connected directly to a drive motor  106  which may be an electric motor mounted on the downhole end of a string of segmented drill pipe or a coiled tubing string  102 , the backreamer assembly  10  of the present invention achieves greater cutting torque than that that can be supplied by the electric motor itself. Specifically, the electric motor may only be capable, after a built in gear reduction, to provide 350 ft. lbs. of torque at 900 rpm. Drilling may not typically occur at an rpm this high, but once the cutting tool  15 , including its own gearbox  14 , is attached, the applied torque, assuming a 13:1 gear ratio, becomes 4550 ft. lbs. at 70 rpm. This level of torque enables effective backreaming in a variety of different bore hole applications. 
     The cutting blades  16  on the exterior of the backreamer assembly  10  may be constructed and arranged to surround the gearbox  14 . Similarly, the attachment of the backreamer to the drill string or motor may be accomplished in a variety of ways well known to those of ordinary skill in the art. For example, shown in FIG.  3  and FIG. 4 is a hollow sleeve  20  which extends back from the cutting tool  15  and threadably engages the housing  107  surrounding the drive motor  106 . Contained within the hollow sleeve  20  is a crossover piece  22  and bit drive  24  which transmits rotational torque from the drive motor  106  to the gearbox  14 . 
     As previously indicated, a still further advancement of the backreamer assembly  10  of the present invention is the ability to provide a variable bit diameter. A variable bit diameter can be achieved by using an adjustable face that either increases or decreases the bit diameter by adjusting the bit face angle. Alternatively, the variability could be achieved using modular additions attached to a coupling  26  on the distal end of the cutting tool  15 . Such modular additions include attachment pieces (not shown) that would increase the length of the backreamer, thereby increasing its diameter, or vice versa for decreasing the diameter of the cutting tool  15 . 
     While the present invention shows the drive motor  106  at the end of the segmented drill string or coiled tubing  102 , those or ordinary skill in the art will understand that the advancements in motor design may produce an electric motor or a hydraulic motor small enough to be contained within the cutting tool  15 . 
     Alternatively, when a hydraulic motor such as a mud motor which is typically used in downhole drilling operations is used, the mud motor may include an external thread located near the bit drive on the end of the motor housing. This external thread occasionally is utilized to mount various types of collars for stabilization or mud flow around the external surface of the mud motor. In the drilling of shallow boreholes, the mud motor utilizes a blank collar during the initial drilling of the borehole. When it is time to attach the cutting tool  15 , the collar is removed and a housing is threaded to the mud motor starting at the bit drive of the motor and moving toward the trailing end. This housing is connected to the body of the cutting tool  15  so that the load is transferred from the face of the cutting tool  15  to the housing through a bearing pack. This housing provides the means to allow a physical connection to the mud motor that can be pulled in tension, so that the gearbox within the cutting tool  15  is not subjected to drilling forces. It is also possible to use a gearbox that would be able to take the direct drilling load, thereby eliminating the need for the housing. Inside of the housing is located the crossover shaft  22  that is threaded into the bit drive  24  of the motor, and the opposing end  23  is inserted into the drive of the gearbox  14 , thereby creating a driveshaft transfer. This power transfer insures that all of the energy is directed into the gearbox  14 . 
     Because a mud motor is a hydraulic motor, there is a fluid port through the crossover shaft  22  that allows drilling fluids to pass through to the face of the cutting tool  15 . From the shaft, the fluid is ported through the housing to the face of the cutting tool  15  and is prevented via seals from entering into the gearbox chamber. Located on the back side of the cutting tool  15 , it would be possible and is intended to provide a coupling  26  that may include a threadable connection to allow for the attachment of other components such as swivels, tracking tools, pipe, etc. 
     While the system, apparatus, and method have been described according to the preferred and alternate embodiments, those of ordinary skill in the art will understand that numerous other embodiments of the disclosed invention may be made. Such other embodiments shall be included within the scope and meaning of the appended claims.