WO2011094359A2 - Drilling assembly with underreaming bit and method of use - Google Patents

Abstract

Embodiments provide apparatuses and methods for drilling and underreaming, particularly to underreamer expansion bits. Underreamer drill assemblies and underreaming bits manufactured in accordance with various embodiments may help to resist over-excavating or undermining, which may result in a more stable bore hole.

Description

DRILL ASSEMBLY WITH UNDERREAMING BIT AND METHOD OF USE

Cross Reference to Related Application

[0001] The present application claims priority to U.S. Patent Application No. 61/298,490, filed January 26, 2010, entitled "DRILL ASSEMBLY WITH

UNDERREAMING BIT AND METHOD OF USE," the entire disclosure of which is hereby incorporated by reference in its entirety.

Technical Field

[0001] Embodiments herein relate to the field of excavation, and, more specifically, to apparatus and methods for drilling and underreaming.

Background

[0002] Underreaming is an excavation technique used in boring and in installing piles or steel casings. For instance, underreaming may be used to enlarge or ream a borehole beneath a string of casing or drivepipe. Expansion bits are useful for underreaming, but when different strata are encountered, particularly unconsolidated formations such as sand, gravel, clay, and water, conventional expansion underreamers can cause undermining or over-excavating of loose materials. This can undermine the stability of the hole.

Brief Description of the Drawings

[0003] Embodiments will be readily understood by the following detailed description in conjunction with the accompanying drawings. Embodiments are illustrated by way of example and not by way of limitation in the figures of the

accompanying drawings.

[0004] Figure 1 illustrates a side view of an underreamer assembly with arms in the extended position, in accordance with various embodiments;

[0005] Figure 2 illustrates a side view of an underreamer assembly with arms in the retracted position, in accordance with various embodiments; [0006] Figure 3 illustrates an exploded view of an underreamer assembly, in accordance with various embodiments;

[0007] Figure 4 illustrates a side view of a center shaft, in accordance with various embodiments;

[0008] Figure 5 illustrates a cross-sectional view of a center shaft, in accordance with various embodiments;

[0009] Figure 6 illustrates a perspective view of a center shaft, in accordance with various embodiments;

[0010] Figures 7A, 7B, and 7C illustrate a side view of a bit body (Figure 7A), a side view of a spring retainer (Figure 7B) and cross sectional view of a bit body (Figure 7C), in accordance with various embodiments;

[0011] Figure 8 illustrates a perspective view of a bit body, in accordance with various embodiments;

[0012] Figures 9A, 9B, and 9C illustrate a perspective view (Figure 9A), side view (Figure 9B), and back view (Figure 9C) of an arm, in accordance with various embodiments;

[0013] Figure 10 illustrates a cross- sectional view of a center shaft with the air flow pathway indicated, in accordance with various embodiments;

[0014] Figure 11 illustrates a face view of the underreamer assembly in the extended position, in accordance with various embodiments;

[0015] Figure 12 illustrates a perspective view of the assembly of various interchangeable components, in accordance with various embodiments;

[0016] Figure 13 illustrates a perspective view of the flow of flush media through an exemplary underreamer assembly, in accordance with various embodiments;

[0017] Figure 14 illustrates a cross-sectional view of the flow of flush media through an exemplary underreamer assembly, in accordance with various

embodiments;

[0018] Figure 15 illustrates a side view of an alternate embodiment of the underreamer assembly configured to be driven by a hex drive mechanism, in

accordance with various embodiments;

[0019] Figures 16A, 16B, 16C, and 16D illustrate a side view of an embodiments of a bit body (Figure 16A), a side view of a spring retainer (Figure 16B), a cross sectional view of a bit body (Figure 16C), and a transverse cross sectional view of a bit assembly (Figure 16D), all components of a bit assembly configured for use with a hex drive mechanism, in accordance with various embodiments;

[0020] Figure 17 illustrates a side view of another alternate embodiment of the underreamer assembly configured to be driven by a hex drive mechanism, in

accordance with various embodiments; and

[0021] Figure 18 illustrates a cross sectional view of components of a bit assembly configured for use with a hex drive mechanism, in accordance with various embodiments.

Detailed Description of Disclosed Embodiments

[0022] In the following detailed description, reference is made to the

accompanying drawings which form a part hereof, and in which are shown by way of illustration embodiments that may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of embodiments is defined by the appended claims and their equivalents.

[0023] Various operations may be described as multiple discrete operations in turn, in a manner that may be helpful in understanding embodiments; however, the order of description should not be construed to imply that these operations are order dependent.

[0024] The description may use perspective-based descriptions such as up/down, back/front, and top/bottom. Such descriptions are merely used to facilitate the discussion and are not intended to restrict the application of disclosed embodiments.

[0025] The terms "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, "connected" may be used to indicate that two or more elements are in direct physical or electrical contact with each other. "Coupled" may mean that two or more elements are in direct physical or electrical contact.

However, "coupled" may also mean that two or more elements are not in direct contact with each other, but yet still cooperate or interact with each other. [0026] For the purposes of the description, a phrase in the form "A B" or in the form "A and/or B" means (A), (B), or (A and B). For the purposes of the description, a phrase in the form "at least one of A, B, and C" means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C). For the purposes of the description, a phrase in the form "(A)B" means (B) or (AB) that is, A is an optional element.

[0027] The description may use the terms "embodiment" or "embodiments," which may each refer to one or more of the same or different embodiments.

Furthermore, the terms "comprising," "including," "having," and the like, as used with respect to embodiments, are synonymous.

[0028] In various embodiments, methods, apparatuses, and systems for underreaming are provided. Embodiments herein provide underreaming devices, such as bits and bit assemblies, and methods for drilling and underreaming that overcome the shortcomings of conventional underreaming devices. Among other things, underreamers may be useful for a variety of excavation tasks, such as expanding well bores, for instance to increase the yield of the well; for straightening a bend in a hole, which may prevent the advancement of a pile or casing; and/or for excavating tie-back or anchor holes in any type of dirt, rock, or concrete formations.

[0029] Underreamers typically have cutting members that are designed to be moved or extended against a well bore wall after the tool is positioned within the well bore. However, such underreamers typically have an air or fluid media flow pattern that forces air or fluid media to exit the bit assembly in a lateral direction, e.g., against the sides of the hole. This may be problematic when excavating different strata, particularly unconsolidated formations such as sand, gravel, clay, and water, as the pressure and turbulence of the media can undesirably enlarge the hole in an uneven fashion depending on the solidity of the strata. These unconsolidated soil conditions may occur deep in the hole, for instance, beneath rock strata, and conventional expansion underreamers may therefore cause undermining or over-excavating of the loose materials without the knowledge of the operator. This may undermine the stability of the hole and/or any casing, footing, or pile contained therein. By contrast, the

underreamers provided herein in various embodiments may provide a substantially vertical downward media flow pattern through the bottom of the bit, which prevents over-excavation from lateral flow and/or turbulence, and which may increase the stability of the resulting hole.

[0030] Another advantage of the disclosed underreamer devices is that it may have a short vertical distance between the pilot area of the cutting face (e.g., the distal- most portion of the bit) and the proximal edges of the underreaming arms. On a conventional underreaming bit, this distance is about 8-10 inches, requiring the pilot area of the bit to advance that distance in front of the underreaming zone. On the disclosed underreaming bits, this distance may be much shorter, for example, about four, three, or two inches, or even less. In various embodiments, reducing this distance may reduce the mass of the bit, which increases the energy transformation into the drill, making for a more effective boring mechanism.

[0031] In various embodiments, the shorter working distance between the pilot area of the cutting face and the underreaming arms also may increase the efficacy of the underreaming action. When a pile is being driven into the ground and becomes obstructed by a boulder, an underreamer may be used to remove the obstruction and allow the pile to continue to advance. However, if there is a long working distance between the pilot area and the underreaming arms, it may not be possible to advance the underreaming assembly far enough for the arms to be effective at clearing the obstruction. Thus, the short working distance between the pilot area and the

underreaming arms in the bits of the present disclosure may allow the disclosed underreamer devices to be used in such a situation.

[0032] In other embodiments, a further advantage of the disclosed underreaming bits and assemblies is that many of the components are replaceable and/or

interchangeable. Thus, when components of the assembly become worn, they may be replaced with new components without having to replace the whole bit or bit assembly. For instance, the buttons on the arms or pilot face may be replaced, as may the outer body, and the individual arms. Furthermore, in some embodiments, the center shaft may be exchanged with a different center shaft, and/or a different outer body may be substituted, allowing the device to be configured to suit the specific conditions of the task at hand.

[0033] Additionally, the design of the bit assembly and center shaft may result in a highly efficient torque transfer through the center shaft. This may increase the efficiency of the system, and may increase the boring and underreaming efficacy of the device as compared to conventional devices.

[0034] Figure 1 shows an exemplary underreaming device 10 with the arms 70 in an extended position, and Figure 2 illustrates the embodiment shown in Figure 1 with the arms 70 in a retracted position. Figure 3 illustrates an exploded view of the underreamer assembly shown in Figure 1 , and Figure 4 illustrates a side view of the exemplary center shaft shown in Figure 1 , in accordance with various embodiments. Figure 5 illustrates a cross-sectional view of a center shaft, in accordance with various embodiments. Figure 6 illustrates a perspective view of a center shaft, in accordance with various embodiments.

[0035] Referring to Figure 1 , in general, the underreaming device 10 may include a drive device coupler 20, which, in some embodiments, may couple the underreaming device to a drive device, and which may form a part of and communicate rotational torque, impact, vibration and/or linear force to the center actuating shaft 30, which may in turn transfer rotational torque, impact, and/or vibration to the bit body 50 and/or to one or more arms 70. In some embodiments, such as the illustrated embodiment, two arms 70 are used. However, any number of arms may be used, depending on the particular excavation conditions, the size of the arms, and the diameter of the bit body 50 being used. For instance, in various embodiments, underreaming device 10 may have one, two, three, four, five, six, seven, or even more arms, depending on the intended use, the diameter of bit body 50, the diameter of the hole being excavated, or the substrate in which underreaming device 10 is used. In some embodiments, bit body 50 may be configured to have a size appropriate for passing inside a standard steel pipe or casing.

[0036] In various embodiments, bit body 50 may transfer torque to and/or serve as a guide for arms 70. In use, in various embodiments, arms 70 may travel from the extended position shown in Figure 1 to the retracted position illustrated in Figure 2 and back again repeatedly in order to effect an underreaming function. Figure 3 illustrates the relationship between arms 70 and bit body 50, and in particular, how they rest in apertures formed by perpendicular surfaces 60 and primary arm pocket surfaces 61 in bit body 50. In some embodiments, bit body 50 may rotate in a clockwise or counterclockwise direction (or alternate directions) during use, and may advance in an axial (e.g., distal) direction as the hole is excavated.

[0037] Referring to Figure 4, which shows center actuating shaft 30 in side view, and Figure 5, which shows center actuating shaft 30 in cross section, center actuating shaft 30 may have a central hole 22 for air, water, or any other flush media to pass though. Center actuating shaft 30 may also have a main section 32 having a specified diameter, and an adjacent (e.g., approximately 90 degree) face that fits within bit body 50, according to various embodiments. Center actuating shaft 30 may, in some embodiments, guide bit body 50 and transfer impact to the adjacent face (e.g., top or proximal edge) of bit body 50. Center actuating shaft 30 also may have an undercut section 34 having a specified diameter with two adjacent (e.g., approximately 90 degree) faces. In one embodiment, undercut section 34 may have a specified diameter, and may have a size sufficient to allow a spring retainer (not shown) to be collapsed for assembly and disassembly, and to be positioned to keep arm 70 engaged in the retracted position. Although in the illustrated embodiment, the spring retainer functions to couple bit body 50 to center actuating shaft 30, any other retention device may be used for this purpose, as will be appreciated by those of skill in the art.

[0038] In various embodiments, center actuating shaft 30 also may have air holes 36 that communicate with the main central hole 22. Air holes 36 may be used for cleaning debris from the mechanism, according to various embodiments. Embodiments of center actuating shaft 30 also may have an extended step surface 38 with adjacent sides parallel to center actuating shaft 30. A ramp angle surface 40 may serve as a transfer between the extended step surface 38 and a retract step surface 42, which may have adjacent sides that are substantially parallel to the shaft. According to various embodiments, ramp angle surface 40 may have one or more air holes 44 that communicate with central hole 22. In embodiments, air holes 44 may be used for clearing debris or other media from the bit assembly. In addition, center actuation shaft 30 may have a radial air groove 46. Some embodiments may provide a face 48

perpendicular to center actuating shaft 30, and in some embodiments face 48 may include one or more carbide (or other rock cutting material) compact buttons or inserts 90. [0039] Figures 7A, 7B, and 7C illustrate a side view of a bit body (Figure 7A), a side view of a spring retainer (Figure 7B) and cross sectional view of a bit body (Figure 7C), in accordance with various embodiments. Referring to Figures 7A-7C, bit body 50 may have a main diameter bore 52 with an adjacent face that may guide bit body 50 on center shaft 30 and may transfer impact force through the adjacent face to the face of center actuating shaft 30. Also included in some embodiments is a radial groove 54 that has two adjacent faces that fit a spring retainer and/or spring retainer ring 55. In some embodiments, holes 56 run perpendicularly through bit body 50 and center actuation shaft 30. In use, keys, pins, or other elongated objects may be inserted through holes 56 to collapse spring retainer ring 55 and uncouple bit body 50 from center actuating shaft 30. Thus, bit body 50 may be easily uncoupled from center actuating shaft 30, for instance when bit body 50 requires replacing due to wear, when a different diameter bit body 50 is desired, or when arms 70 need to be accessed or replaced.

[0040] In various embodiments is a radial pocket surface 58 that may act as a stop to prevent over extension of the arms 70 in the extended position. In some embodiments, perpendicular surfaces 60 are included and disposed to mate with arms 70. Additional surfaces may include primary arm pocket surfaces 61 and secondary arm surfaces 62, both of which may be generally perpendicular to center actuation shaft 30. Some embodiments may also include a radial angle surface 63 that may include compact inserts 90. Additionally, bit body 50 may include one or more face flushing slots 64 running perpendicular to center actuating shaft 30, and main return flushing slots 66 parallel to center actuating shaft 30 that may intersect with one or more face flushing slots 64. Some embodiments also may include a face 68 generally

perpendicular to center actuating shaft 30 that may have additional carbide (or other cutting material) compact buttons or inserts 90 coupled thereto.

[0041] Figure 8 illustrates a perspective view of an example of bit body 50, in accordance with various embodiments, and Figures 9A, 9B, and 9C illustrate a perspective view (Figure 9A), side view (Figure 9B), and back view (Figure 9C) of an example of arm 70, in accordance with various embodiments. Referring to Figure 9, arm 70 may be a removable and replaceable component that has a radial diameter surface 72 that may define a ream diameter, and a radial plot surface 74 having a specified diameter. Also included in some embodiments is a radial angle surface 76 that has inserts 90 coupled thereto. A perpendicular face 78 also may have compact inserts 90, and may act as part of the pilot surface in some embodiments. Another perpendicular surface 80 may be configured to contact and mate with the bit body arm pocket 60. Also included in some embodiments is a plurality of major arm surfaces 81 that may contact and mate with the major arm pocket surfaces 61. Also contemplated are minor arm surfaces 82 that may contact and mate with minor arm surfaces 62.

[0042] Some embodiments may include one or more radial stop surfaces 84, which may contact and mate with bit body radial pocket surface 58. Further

embodiments also may include a radial back angle surface 86 for aiding in the retractions of radial arms 70. A parallel actuating surface 88 may contact and mate with center actuating shaft 30 during operation, and in some embodiments it may contact and mate with retract step surface 42, the ramp angle surface 40, and/or extended step surface 38. Embedded in the surface may be buttons or compact inserts 90, according to various embodiments.

[0043] Figure 10 illustrates a cross- sectional view of an example of center actuating shaft 30 with the air or media flow pathway indicated, in accordance with various embodiments, and Figure 11 illustrates a face view of the underreamer assembly 10 with arms 70 in the extended position, in accordance with various embodiments.

[0044] In operation, in various embodiments, drive device coupler 20 (which in some embodiments may be an integral part of center actuating shaft 30) may be the adapter to which a down hole hammer, drill rod for hydraulic hammer, or any other means to impart rotational torque, impact energy, vibration, or linear feed force may be used to move (for instance rotate or advance) center shaft 30. In various embodiments, center actuating shaft 30 (and thus coupler 20) may be interchanged with other shafts/couplers in order to change the assembly to be, for instance, mated with a down hole hammer or a drill rod for a hydraulic hammer.

[0045] In some embodiments, the rotational torque from center actuating shaft 30 may be imparted to arms 70 by means of contact with surfaces 42, 44, and 38. The rotational torque may then be transferred from arms 70 into bit body 50 in some embodiments by means of contact with surfaces 81 and 82 of arms 70 and their mated surfaces 61 and 62 bit body 50.

[0046] Figure 15 illustrates a side view of an alternate embodiment of the underreamer assembly configured to be driven by a hex drive mechanism; and Figures 16A, 16B, 16C, and 16D illustrate a side view of an embodiments of a bit body (Figure 16A), a side view of a spring retainer (Figure 16B), a cross sectional view of a bit body (Figure 16C), and a transverse cross sectional view of a bit assembly (Figure 16D), all components of a bit assembly configured for use with a hex drive mechanism, in accordance with various embodiments. In these embodiments, instead of imparting torque directly to arms 70, which then impart torque to bit body 50, rotational torque from center actuating shaft 30 may be imparted to bit body 50 via drive members 31 , 51. Although a hex drive interface is illustrated, any torque transfer interface may be substituted, for instance a spline drive mechanism or any interface having any number of flat surfaces, facets, recesses in, or projections from center shaft 30 that are configured to mate with and/or engage corresponding features in bit body 50. In various embodiments, use of the hex/spline drive mechanism (for instance, drive members 31 , 51 ) may help to avoid excessive loading and wear on surfaces 38. 40, and 42 on center actuating shaft 30, and mated surfaces 61 and 62 on bit body 50, while maintaining alignment of the components and ensuring smooth operation of arm 70 deployment and retraction. Thus, the illustrated hex drive mechanism may help avoid causing excessive wear and tear on arms 70 and bit body 50.

[0047] Whereas Figures 15 and 16 illustrate an example of a center shaft 30/bit body 50 drive members 31 , 51 being located proximal to spring retainer 55, one of skill in the art will appreciate that such drive members 31 , 51 may alternately or additionally be located distal to spring retainer 55, as shown in Figures 17 and 18. Figure 17 illustrates a side view of another alternate embodiment of the underreamer assembly configured to be driven by a hex drive mechanism, in accordance with various embodiments; and Figure 18 illustrates a cross sectional view of components of a bit assembly configured for use with a hex drive mechanism, in accordance with various embodiments. In some embodiments, such as the embodiment illustrated in Figures 17 and 18, a hex drive, spline drive, or other drive member 31 , 51 may be located at any point between the distal tip of center shaft 30 and spring retainer 55. In some embodiments, positioning drive members 31 , 51 distal to (e.g., below) spring retainer 55 may increase the area of contact between opposing drive members 31 , 51 , which may reduce wear and may decrease the height of bit body 50 in some embodiments.

[0048] In some embodiments, arms 70 may be deployed by the linear (e.g. axial) force applied to center actuating shaft 30 and transferred to bit body 50. As bit body 50 moves, arms 70 may be engaged by means of contact with surfaces 88, 42, 44, and 38. At the point of full deployment, arms 70 may be kept from deploying further by means of surfaces 84 and 58, which collectively may be referred to as the "stop pocket." Impact energy may be imparted from center actuating shaft 30 by means of surface 32 and its adjacent surface 42 in bit body 50, and into arms 70 by means of surfaces 60 and 80, in accordance with various embodiments.

[0049] Figure 12 illustrates a perspective view of the assembly of various interchangeable components. As described above, one advantage of the disclosed underreaming bits and assemblies is that many of the components are replaceable and/or interchangeable. Thus, when components of the assembly become worn, they may be replaced with new components without having to replace the whole bit or bit assembly. For instance, the buttons on the arms or pilot face may be replaced, as may bit body 50, and individual arms 70. Furthermore, in some embodiments, center shaft 30 may be exchanged with a different center shaft, and/or a different bit body 50 may be substituted, allowing worn components to be replaced, and/or the device to be configured to suit the specific conditions of the task at hand.

[0050] As described above, in use, retention device 55 may be collapsed or otherwise disengaged, for instance by inserting keys, pins, or other elongated objects through holes 56, thus uncoupling bit body 50 from center actuating shaft 30. Once bit body 50 has been removed from center actuating shaft 30, arms 70 may be removed from the interior of bit body 50 by retracting them into bit body 50 and lifting them out of the recesses in which they rest. Replacement arms may then be inserted into bit body 50 if desired, and/or a replacement bit body 50 may be coupled to center shaft 30, for instance one having a larger or smaller diameter, a different number of arms, or other desirable features, such as a different drive mechanism.

[0051] Figure 13 illustrates a perspective view of the flow of air, water, and/or flush media through an exemplary underreamer assembly; and Figure 14 illustrates a cross-sectional view of the flow of air, water, and/or flush media through an exemplary underreamer assembly; all in accordance with various embpodiments. As illustrated in in Figures 13 and 14, in various embodiments, substantially all of the air, water, and/or flush media may pass through the underreamer assembly and exit through the distal cutting face in a downward direction, substantially parallel to the longitudinal axis of the device and perpendicular to the distal cutting face. In various embodiments, such downward airflow may direct substantially all of the air, water, and/or flush media away from the side (lateral) walls of the bore hole, thus preventing over excavation when loose strata are encountered during a boring operation. In particular embodiments, unlike conventional underreamers, essentially no flush media pass laterally against the sides of the bore hole, as any air, water, and/or flush media traveling laterally towards the bore hole walls will be diverted by arms 70 and/or the outer edge of bit body 50, particularly when arms 70 are in the extended position. Thus, in various embodiments, at least 80%, 85%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% of the air, water, and/or flush media may be directed away and along the central axis and perpendicular to the distal cutting face. Similarly, in some embodiments, less than 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1 % of the air, water, and/or flush media may be directed laterally towards the side wall of the bore hole. Thus, the device may avoid the over-excavation problems associated with conventional underreamers.

[0052] Debris removal may be accomplished in some embodiments by flush media of air, water, or any combination of fluid or gas, through central hole 22, into air holes 44, and deflected by surface 88 of arms 70. Following that, the flush media may move around center actuating shaft 30 and radial air groove 46 in some embodiments, e.g., to exit the assembly in a direction parallel to the drilled hole, without impacting the sides of the hole. The media may then be returned via face flushing slot 64 into return flushing slot 66 in some embodiments. A small amount of flush media may be diverted to holes 36 to keep the mechanism clean of debris in certain embodiments.

[0053] Although certain embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent embodiments or implementations calculated to achieve the same purposes may be substituted for the embodiments shown and described without departing from the scope. Those with skill in the art will readily appreciate that embodiments may be implemented in a very wide variety of ways. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Therefore, it is manifestly intended that embodiments be limited only by the claims and the equivalents thereof.

Claims

Claims What is claimed is:

1 . An underreamer drill assembly comprising:

a center shaft having a central axis, a proximal end, and a distal end, wherein the center shaft is configured to receive drive energy from a driver at the proximal end and adapted to couple to a bit body at the distal end;

a bit body adapted to couple to the center shaft and configured to couple to at least one underreamer arm; and

at least one underreamer arm configured to underream a bore hole having a side wall, wherein the center shaft, bit body, and underreamer arm form a distal cutting face that is substantially perpendicular to the central axis, and wherein the underreamer drill assembly is configured to direct air, water, or flush media substantially away from the side wall of the bore hole.

2. The underreamer drill assembly of claim 1 , wherein the center shaft and bit body are adapted to direct air, water, or flush media substantially along the central axis and perpendicular to the distal cutting face.

3. The underreamer drill assembly of claim 1 , wherein the center shaft and bit body are adapted to direct air, water, and flush media substantially along the central axis and perpendicular to the distal cutting face.

4. The underreamer drill assembly of claim 1 , wherein the at least one underreamer arm is configured to be extended against the side wall of the bore hole.

5. The underreamer drill assembly of claim 1 , wherein the at least one underreamer arm comprises a distal pilot surface, a proximal upper surface, and an axial height spanning the distance between the distal pilot surface and the proximal upper surface.

6. The underreamer drill assembly of claim 5, wherein the axial height is less than 8 inches.

7. The underreamer drill assembly of claim 5, wherein the axial height is less than 6 inches.

8. The underreamer drill assembly of claim 5, wherein the axial height is between about 2 and about 4 inches.

9. The underreamer drill assembly of claim 1 , wherein the distal cutting face comprises a plurality of compact cutting inserts.

10. The underreamer drill assembly of claim 9, wherein the compact cutting inserts are replaceable.

1 1 . The underreamer drill assembly of claim 9, wherein the compact cutting inserts comprise carbide buttons.

12. The underreamer drill assembly of claim 1 , wherein the center shaft, bit body, and underreamer arms are modular.

13. The underreamer drill assembly of claim 12, wherein the bit body is adapted to be removed and replaced with a second bit body having a different diameter.

14. The underreamer drill assembly of claim 12, wherein the center shaft is adapted to be removed and replaced with a second center shaft that is adapted to couple to a different drive mechanism.

15. The underreamer drill assembly of claim 1 , wherein the center shaft comprises a drive adapter.

16. The underreamer drill assembly of claim 15, wherein the drive adapter is configured to receive rotational torque, impact energy, vibration, or linear feed force from a drive mechanism.

17. The underreamer drill assembly of claim 16, wherein the adapter is configured to move the center shaft in response to the rotational torque, impact energy, vibration, or linear feed force.

18. The underreamer drill assembly of claim 16, wherein the drive mechanism is a down hole hammer or a drill rod for hydraulic hammer.

19. The underreamer drill assembly of claim 1 , wherein the center shaft is adapted at or near the distal end to couple to the bit body via a hex or spline drive mechanism.

20. An underreamer drill assembly comprising:

a center shaft having a central axis, a proximal end, and a distal end, wherein the center shaft is configured to receive drive energy from a driver at the proximal end and adapted to couple to a bit body via a hex drive mechanism at the distal end;

a bit body adapted to couple to the center shaft via the hex drive mechanism and configured to couple to at least one underreamer arm; and

at least one underreamer arm configured to underream a bore hole having a side wall; wherein the center shaft, bit body, and underreamer arm form a distal cutting face that is substantially perpendicular to the central axis; wherein the underreamer drill assembly is configured to direct air, water, or flush media substantially along the central axis and perpendicular to the distal cutting face; and wherein the center shaft is configured to transmit drive energy from the driver to the at least one arm and move the at least one arm from a retracted position to an extended position.