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CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of 35 U.S.C. 111(b) provisional application Serial No. 60/106,252, filed Oct. 30, 1998, and entitled Remotely Operable Hydraulic Underreamer, hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to underreamers used to open a hole below a restriction so that the opened hole is larger than the restriction itself. More particularly, this invention relates to a hydraulically actuated underreamer in cooperation with an actuator associated therewith to remotely actuate the underreamer when fluid is circulating in a borehole. The cutter arms of the underreamer retract when fluid circulation ceases through a cutter arm retractor contained within the underreamer housing. 
     2. Description of the Related Art 
     Underreamers are used in the petroleum industry to enlarge boreholes. Such tools can be used in drilling oil, gas or water wells and in mining operations. An underreamer has basically two operative states, a closed or collapsed state where the diameter of the tool is sufficiently small to allow movement of the tool in the narrowest part of the borehole, and an open or partly expanded state where one or more arms with cutters on the ends thereof, pivot out from the body of the tool. In this latter position the borehole is enlarged as the tool is rotated and lowered in the borehole. 
     A drilling type underreamer is typically used in conjunction with a drill bit positioned below or downstream of the underreamer. The drill bit can drill the borehole to be underreamed at the same time as the underreamer enlarges the borehole formed by the bit. Circulation of drilling fluid to the drill bit is required to remove detritus from the bottom of the borehole and to cool and clean the bit as it drills the borehole. 
     Underreamers of this type usually have hinged arms with cutters attached thereto. These pivoted arms tend to break during the drilling operation and must be removed or “fished” out of the borehole before the drilling operation can continue. The tool typically has rotary cutter pocket recesses formed in the body where the arms are retracted when the tool is in a closed state. These pockets have a tendency to fill with debris from the drilling operation which makes collapsing of the arms difficult. If the arms do not fully collapse, the drill string may easily hang up in the borehole when an attempt is made to remove the string from the borehole. Most of the prior art underreamers utilize swing out cutter arms that are pivoted at an end opposite the cutting end of the reamer and are actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Typical examples of these types of underreamers are found in U.S. Pat. Nos. 3,224,507; 3,425,500 and 4,055,226. 
     An early example of a mechanically actuated expandable drill bit that does not use pivoting cutter arms to ream a borehole is taught in U.S. Pat. No. 3,365,010. This drill bit utilizes a pair of blade type cutters that ride in opposed, axially oriented channels. The channels are angled with respect to the axis of the bit such that when the blades impact the bottom of the borehole, shear pins retaining the blades are broken allowing the blades to move up the channels thereby expanding out against the borehole wall for subsequent borehole enlargement. A large pin for each blade retains the expanded blades in a desired position thus controlling the gage of the borehole. When the expandable drill bit is tripped out of the borehole, the blades fall down the angled tracks through frictional and gravitational forces thus diminishing the gage diameter of the expandable drill bit so that the drill string may be removed from the borehole. Once the shear pins are sheared, the expandable drill bit can only be used as a hole opener and only when the expandable drill bit is in contact with the borehole bottom. 
     U.S. Pat. No. 3,433,313 teaches an under-reaming tool having a tubular body with a sleeve movably positioned therein and adapted, when moved in one direction responsive to the pressure of drilling fluid, to move cutters to their opposite direction. Also responsive to the pressure of drilling fluid, the cutters are allowed to retract from their cutting position. A drilling fluid passage is formed in the tubular body to increase the flow area for drilling fluid when the cutters are in their cutting position and a means of positively moving the sleeve in the opposite direction with a wireline tool. The wireline tool is used to stop the flow of drilling fluid thus allowing the cutters to retract. This patent is disadvantaged in that a wireline device must be used to retract the cutters so that the tool may be tripped out of the borehole or to render the under-reamer inoperative downhole. 
     U.S. Pat. No. 5,368,114 teaches an under-reaming tool for use in preferably horizontal and extended reach boreholes. The tool includes a succession of stabilizers, underreamers and expandable stabilizers. A drilling mud activator is provided for the expandable elements of the underreamers and expandable stabilizers. The underreaming device comprises a main body with a number of guiding surfaces distributed over the circumference thereof which have a pitch angle that increases radially in an axial direction. A ring collar formed as a piston in a surrounding cylinder housing forms a small and a large radial annular surface. The piston further forms reaming pads/wings and/or stabilizer pads/wings in sliding contact with a respective guiding surface, the pads being taken up in ports in a jacket surrounding the main body in such a way that the pads can only be moved radially relative to the jacket, the jacket being attached to or formed as a part of the cylindrical housing. This patent is disadvantaged in that, in order to position the pad/wings associated with the reamer and stabilizer function, the pressure differential across the manipulating piston must be controlled and monitored by a microprocessor device positioned downhole; the information being electronically relayed to the rig platform. 
     Other related underreamer type patents include U.S. Pat. Nos. 4,141,421 and 4,889,197. 
     In general, prior art underreamers are disadvantaged in that those having cutting structures located at the end or near the end of a pivotable arm are inherently vulnerable to breakage of the pivot pins which retain the arms on the mandrel. Also, the mandrel recesses into which the cutters retract when they are collapsed tend to become easily fouled with debris during the cutting operation when the arms are extended. As a result, the arms are difficult to retract into the mandrel recesses. 
     Moreover, if the prior art tools are capable of transmitting fluid therethrough, it is typically accomplished using a centrally located hole or cylindrical tube. Most often, three cutters are used to dress each tool. A majority of these type tools do not positively collapse. Rather, a cutter retraction spring forces a piston to retract. The cutters however are not generally connected to the piston retraction spring and must retract through release of engagement with the borehole wall or retract through gravitational forces. The piston is usually in such a position that it is not possible to isolate fluid pressure from acting on it without preventing circulation of the drilling fluid. In other words, the tool cannot be enabled or disabled without stopping fluid circulation. 
     Another disadvantage of the prior art is that the cutter size and positioning are not optimized for the full range of hole opening sizes. In order to adjust the expanded diameter of a conventional underreamer, it is necessary to replace the cutting arms with larger or smaller arms or to adjust the location of their pivot points inwardly or outwardly with respect to the axis of the tool. It may even be necessary to replace the underreamer altogether with one which will provide a different expanded diameter. 
     Yet another disadvantage of the prior art is that the hydraulic capability is not optimized for the high fluid flow rates required. 
     The present invention overcomes the deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     An expandable underreamer is disclosed which consists of an underreamer body forming at least a pair of opposed downwardly and inwardly angled slots. A means is provided to circulate fluid through the underreamer body and a means is also provided for connection to a drill string. At least a pair of cutter assemblies housed within the underreamer body is adapted to engage in the opposed angled slots formed by the underreamer body. Each cutter assembly consists of a cutter support body forming track engaging means at a first end, a piston drive means at a second end of the support body and underreaming cutter means formed therebetween. The piston is slidably engaged with a sleeve formed in the underreamer body; the sleeve being parallel with the angled slots formed in the underreamer body. The sleeve further is in fluid communication with a control port formed in the underreamer body. Fluid under pressure, when admitted to the piston sleeve below the piston drives the cutter assembly upwardly and outwardly along the angled slots to commence an underreaming operation. Spring means is additionally provided in the underreamer body to retract the cutter assemblies when fluid is shut off at the control port. 
     The hydraulically operated underreamer opens a borehole below a restriction that is larger than the restriction itself. The underreamer has a cutter system with a pair of cutters that engage the formation by traversing upward and outward along a track that is angled with respect to an axis of the underreamer body. The force pushing the cutters to the extended position is supplied by a piston built into each cutter support. The cutters may be actuated by a single piston acting on both the cutter support assemblies. Pressure acting on these pistons/piston comes from the pressure differential between the annulus and the drill string during circulation of the drilling fluid. 
     The cutters are supported on both sides of their cutting structure and are maintained in sliding contact with the underreamer body. The support arrangements on each side of the cutting structure also serve as guides for movement of the cutter assemblies with respect to the underreamer body. A spring opposes the upward and outward motion of the cutter systems and returns the cutters to the collapsed position in the absence of differential pressure (hydraulic pumps off). 
     An adjustment mechanism is used to set the stroke of the cutter system thereby determining the hole opening diameter. 
     The body of the underreamer tool incorporates by-pass ports for transmitting drilling fluid through to the remainder of the of the bottom hole drilling assembly. The bottom hole assembly could consist of mud motors, drill bits, MWD, etc. 
     The underreamer also contains a flow passage between the mud flow and the piston chamber that, when closed will disable the tool (it will not extend the cutters and no fluid will go through the underreamer nozzles). This feature enables the user to drill and underream simultaneously or to drill only if a hydraulic control device is incorporated with the underreamer. 
     The underreamer tool of the present invention utilizes by-pass ports to transmit drilling fluid through the housing of the tool. The fluid does not travel through the actuating piston to flow through the tool. 
     Further the underreamer uses only two cutters that enables the structure to be very strong and provides larger, more robust cutters than comparable prior art tools of approximately the same size. The two cutter configuration also allows for maximum support of the cutting structure within the body of the tool. 
     Still further the underreamer utilizes a pair of cutters without the conventional hinge pin associated therewith. Thus, the port area through the tool can be increased. Hence, the flow rate through the tool is substantially doubled. 
     An advantage of the present invention over the prior art is that the underreamer mechanism is built such that the cutters are forced to the collapsed position by a spring when there is insufficient pressure to overcome the spring force (pumps off). 
     Another advantage of the present invention over the prior art is that the tool utilizes fluid bypass ports to transmit fluid there through. The fluid does not travel through the piston to pass through the tool. 
     Still another advantage of the present invention over the prior art is the use of two cutters which enables the structure to be very strong (larger than comparable tools of the same size). The two cutter configuration of the present invention allows for maximum support of the cutting structure within the body of the underreamer tool. 
     Yet another advantage of the present invention over the prior art is by utilizing the two cutter design and eliminating the conventional hinge pin designs, the port area may be increased through the tool body. The allowable fluid flow rate through the present design is substantially doubled. 
     The above noted objects and advantages of the present invention will be more fully understood upon a study of the following description in conjunction with the detailed drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-section of a remotely operable hydraulic underreamer apparatus; 
     FIG. 2 is a partial cross-section of the apparatus rotated ninety degrees from FIG. 1; 
     FIG. 3 is a perspective view of the cutter assembly return plunger; 
     FIG. 4 is a perspective view of the cutter assembly support structure mounted to the underreamer body illustrating the angled tracks formed by the support structure that engage track runners formed by the moveable body of the cutter assembly; 
     FIG. 5 is a view taken through plane  5 — 5  of FIG. 2; 
     FIG. 6 is a partial cross-section of the apparatus illustrating the cutter assembly in the fully extended position, the actuating piston being forced upstream by the fluid under pressure being directed toward the moveable cutter actuation pistons connected to each of the cutter assemblies; 
     FIG. 7 is a view taken through  7 — 7  of FIG. 6; and 
     FIG. 8 is a view taken through plane  8 — 8  of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIGS. 1,  2 ,  3  and  4 , the remotely operable hydraulic underreamer  10  includes a longitudinal underreamer body  12  that typically is connected to a drill string or drill string apparatus (not shown) at threaded connection  15  at upstream end  14  of body  12 . The lower end  16  is affixed by a threaded connection  17  to a fluid control assembly (not shown) or other suitable components. One fluid control assembly is shown and described in U.S. patent application Ser. No. 09/183,692 filed Oct. 30, 1998, hereby incorporated herein by reference. 
     The body  12  contains a cutter return assembly, generally designated as  18 , which includes an elongated pin  9  having a radially enlarged portion  24  and a threaded portion  27  at its upper end. The return assembly  18  is screwed into a threaded retainer  11  disposed within a conduit  13  formed in body  12 . Lock nuts  8  are also supplied on the threaded end  27 . The return assembly  18  includes a cutter return plunger  20  which is shown in greater detail in FIG.  3 . The plunger  20  comprises a shaft  21  forming an upper end  23  and a base end  22  which is divided by a slot  25 . The return assembly  18  also comprises a compressible spring  19  that is disposed within the conduit  13  between the enlarged portion  24  of pin  9  and the upper end  23  of the plunger  20 . As a result, the plunger  20  is biased downward within the conduit  13 . 
     Below the conduit  13 , a cutter assembly slot  39  houses a pair of moveable cutter assemblies  40 . The cutter assemblies  40  are moveable between a collapsed position, depicted in FIGS. 1 and 2, and a radially extended position, shown in FIGS. 6,  7  and  8 , which is used for underreaming. 
     The base ends  22  of the return plunger  20  each present a translation surface  22 A which contacts a complimentary translation surface  49  on the body  41  of each moveable cutter assembly  40 . The plunger  20  is urged against the moveable cutter assembly  40  by the spring  19  positioned between flange  24  formed by pin  9  of return assembly  18  and end  23  of plunger body  21 . 
     A cutter assembly support structure  30 , shown in detail in FIG. 4, is affixed within the underreamer body  12  to reside within the slot  25  of the plunger  20 . The cutter assembly support structure  30  has a pair of angled cutter assembly tracks  31 ,  31   a  formed on opposite sides of the support structure  30 . The support structure  30  is bolted through bolt holes  32  formed in the upper end of the body  34  to the underreamer body  12 . 
     Located beneath the plunger  20  are a pair of moveable cutter assemblies  40 . Each of the cutter assemblies  40  comprises an elongated, sleeve-like body  41  that supports a cutter  43  which is rotatably secured to a journal bearing  44 . 
     Each cutter assembly  40  includes an angular arm  53  which is affixed to the journal bearing  44  supporting one of the rotatable cutters  43 . Each angular arm  53  forms on its upper end, angled grooves  42  that are shown in FIGS. 7 and 8 and which slidably interfit with alignment tracks  31 ,  31   a  formed on opposite sides of body  34  of support structure  30 . The journal bearing  44  is also affixed to the cutter assembly body  41  at its opposite end. The cutter  43 , then, is advantageously and securely affixed within the cutter assembly  40  at two points. 
     Each cutter assembly  40  also includes a cutter assembly drive piston  45  that is slidably received within a sleeve  46  formed within body  12 . The angle of the axis of the piston sleeve  46  is generally parallel to that formed by the corresponding grooves  42  and tracks  31  of support body  34 . The slidable interface of the angled grooves  42  on angled arm  53  with the tracks  31  provides a guide for the cutter assembly  40  which helps to ensure that the assembly  40  extends upwardly and outwardly toward its radially extended position in the intended manner without becoming misaligned. This guide mechanism also helps to ensure that the cutter assembly  40  does not become misaligned as it is moved from the radially extended position to the collapsed position. Disposal of the drive piston  45  within the piston sleeve  46  provides another mechanism for guiding the cutter assembly  40  as it is extended and retracted so that the cutter assembly does not become misaligned. As a result, the cutter assembly  40  is provided with both an upper and a lower guide. Of course, the same is true with respect to the cutter assembly  40  positioned adjacent to track  31   a  on the opposite side of support body  34 . 
     Each piston  45  defines a concentric fluid passage  47  therewithin. Fluid flow therethrough is partially blocked by a fluid restricting nozzle  48  positioned at the upper exit end of the passage  47 . In effect, then, each piston  45  presents a fluid pressure receiving area upon which fluid pressure may be applied to move the piston  45 , and hence the cutter assembly  40  in response thereto. A fluid inlet  50  formed in the body  12  of the underreamer  10  directs fluid to the sleeves  46  when the valve  52  (shown in phantom in a closed position) is opened by a fluid manipulating means (not shown) connected to end  16  of the underreamer. The fluid manipulating means may comprise a control device which is described in U.S. patent application Ser. No. 09/183,692, filed Oct. 30, 1998 and entitled “Fluid Flow Control Devices and Methods for Selective Actuation of Valves and Hydraulic Drilling Tools”, hereby incorporated herein by reference, which has been assigned to the assignee of the present invention. In an alternative embodiment, the fluid manipulating means may be eliminated completely and the valve  52  removed or secured in an open position. In this embodiment, drilling fluid is free to enter the fluid inlet  50 , and the underreamer  10  is simply controlled by selective operation of the pump located at the surface of the well. 
     In operation, when the valve  52  is opened (see FIG. 6) fluid under pressure is directed to the fluid pressure receiving areas formed by each of the pistons  45  affixed to the cutter assemblies  40 . The restricted nozzle  48  creates sufficient back pressure to overcome the force of the spring  19  thereby forcing the cutter assembly to slide upwardly and outwardly along the parallel paths of the sleeves  46  and grooves  42  thus positioning each of the cutters  43  against the formation to enlarge or ream the borehole. 
     The base  22  of the plunger  20  contacts the translation surface  49  of the body  41  of the cutter assembly  40 . As the cutter assemblies  40  extend radially and upwardly, the plunger  20  is pushed upwardly within the conduit  13 . As the plunger  20  is pushed upwardly, its upper end  23  eventually stops against stop face  26  at the lower end of the pin  9  of return assembly  18  (see FIG. 6) thereby determining the extent of the diameter of the borehole to be reamed. 
     FIG. 5 further illustrates disposition of the drive pistons  45  within their sleeves  46  as well as the location of bypass passages  55  formed in the body  12 . The four fluid bypass ports  55  are clearly shown in FIGS. 5,  7  and  8  to remain clear of the internal working mechanism of the underreamer  10 . The bypass passages  55  allow for the operation of other drilling tools such as drill bits and the like by directing fluid through body  12  of the underreamer  10  without interference from cutter assemblies  40 . Because the fluid passages  55  are not required to be disposed around a central piston, they can be made larger than with conventional underreamers to essentially double fluid flow through body  12 . 
     FIGS. 7 and 8 more clearly illustrate the position of each of the cutter assemblies  40  on opposite sides of the support structure  30 . The grooves  42  are formed on the body  41  of the cutter assemblies  40  and engage alignment tracks  31 ,  31   a  formed within the body  34  of the support structure  30 . 
     The cutter assemblies  40  are moved to a collapsed position, i.e., retracted into the slot  39  within the underreamer body  12 , by force exerted by the spring  19  when the valve  52  shuts off the fluid under pressure to cutter assembly drive pistons  45 . As the cutter assemblies  40  are assisted to their collapsed position by downward urging of the plunger  20  by spring  19 , they are also permitted to move radially inwardly as the translation surfaces  22  of the plunger  20  contact the translation surfaces  49  of each arm  53 . The surfaces  22  and  49  are capable of sliding, or translational, movement with respect to one another. As the plunger  20  and spring  19  urge the cutter assemblies  40  downwardly the arms  53  will thus translate radially inwardly with respect to the plunger  20 . Hence, the cutter assemblies  40  are urged into their respective recesses  39  by the spring  19  and do not rely on gravitational forces to retract the cutters  43  (see FIGS.  1  and  2 ). The use of translation surfaces  22  and  49  avoids the need to rely upon pivot points and pins which are vulnerable to breakage. 
     Although the cutters  43  are depicted as rotary cutters, they could also be conical or cylindrical in shape and retain any number of formation cutting means such as hardened milled teeth, tungsten carbide inserts or diamond inserts. Moreover, the formation cutting means affixed to the support  41  could use non-rotating blades having cuttings means retained in the blades as pointed out with respect to the rotary cutters without departing from the teachings of the present invention. 
     Among the advantages to be realized by the present invention is the use of two separate fluid pressure receiving areas for independent actuation of the two cutter assemblies. Because fluid pressure is applied to the cutter assemblies  40  from below rather than above, the cutter assemblies move upwardly and radially outwardly as they move toward their radially expanded positions. 
     Further, construction in accordance with the present invention provides for a rugged and strong underreamer design. Specifically, the cutter assemblies feature attachment at two points. There are no pivot pins to potentially break off during operation of the underreamer. The use of the alignment tracks  31  ensures that alignment of the cutter assemblies  40  with respect to the underreamer body  12  is maintained so that the cutter assemblies  40  are not bent or twisted about the longitudinal axis of the underreamer body  12 . 
     Adjustment of the threaded end  27  within retainer  11  and lock nuts  8  upon the threaded end  27  of pin  9  can be used to precisely determine the radial distance from the central axis of the body  12  to the cutting face  43   a  of a cutter  43  in its extended position. The threaded end  27  is then turned using a wrench to move the pin  9  upward or downward within the conduit  13 . The upward or downward movement of the pin  9  adjusts the axial position of the stop face  26  within the conduit  13 , thereby adjusting the limit of movement for the upper end  23  of the return plunger  20  as the underreamer  10  is moved to its expanded position. When upward movement of the plunger  20  is limited to a greater extent by downward movement of the pin  9  within the conduit  13 , the distance to which the cutter assemblies  40  are able to be radially extended is correspondingly limited. Conversely, upward movement of the pin  9  within the conduit  13  allows the cutter assemblies  40  to move radially outwardly to a greater extent. Adjustment of the location for pin  9  in this manner thus results in the diameter of the tool with the cutter assemblies  40  to be adjusted to conform to a desired diameter. Precise adjustment of the movement of the cutter assemblies  40  is accomplished by placing a ring gage, of a type known in the art for the measurement of tool diameter, across the diameter of the cutter assemblies  40  when the cutter assemblies  40  are placed in a radially expanded position as is illustrated in FIG.  6 . The axial location of the pin  9  is then adjusted until the diameter of the expanded cutters essentially matches that of a desired hole opening diameter, as measured by the ring gage. In this manner, an underreamer constructed in accordance with the present invention is capable of being easily adjusted to expand to a variety of different radial sizes without the need to remove parts and replace them. 
     It will of course be realized that various modifications can be made in the design and operation of the present invention without departing from the spirit thereof. It would, for example, be obvious to one skilled in the art to design the underreamer with more than a pair of cutters without departing from the scope of this invention. Thus, while the principal preferred construction and mode of operation of the invention have been explained in what is now considered to represent its best embodiments which have been illustrated and described, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically illustrated and described.

Summary:
An underreamer opens a borehole below a restriction that is larger than the restriction itself. The underreamer includes cutters which engage the formation by traversing outward and upward. The force pushing the cutters to the extended position is supplied by a piston built into each cutter support. Pressure acting on these pistons comes from the pressure differential between the annulus and drill string during circulation of drilling fluid. The cutters are supported on both sides of the cutting structure by arms that are supported by and in sliding contact with the body. A spring opposes the upward and outward motion of the cutter mechanism and returns the cutters to their collapsed position in the absence of differential pressure. The body of the tool incorporates by-pass ports for transmitting fluid through the tool to the remainder of the bottom hole assembly.