GB2522062A

GB2522062A - Drill Bit Assembly

Info

Publication number: GB2522062A
Application number: GB1400550.8A
Authority: GB
Inventors: Simon Vincent; John Whitaker; Peter Richards
Original assignee: Fugro Seacore Ltd
Current assignee: Fugro Seacore Ltd
Priority date: 2014-01-14
Filing date: 2014-01-14
Publication date: 2015-07-15
Also published as: GB201400550D0

Abstract

The invention relates to an under reaming drill bit tool 50 comprising a central drill bit assembly 52 having a central housing 54. The central housing has an outer surface and the central drill bit assembly has a central longitudinal axis A. The drill bit assembly comprises at least one under reamer assembly 2 with a cutting assembly (30 figure 1) and is moveable between retracted and extended positions. The or each under reamer assembly has an under reamer housing (4 figure 1) having an outer surface (12 figure 1), at least a portion of the outer surface of the or each reamer housing extends from the cutting assembly and is contiguous with the outer surface of the central housing in both the retracted and the extended positions. A later embodiment relates to a drill bit tool having means to vary the position of the cutting assembly of the central drill bit assembly.

Description

DRILL BIT ASSEMBLY

The present invention relates to a drill bit assembly. The present invention relates more particularly to a drill bit assembly for use in drilling bores for piles and the like, in which a downhole portion of the bore is greater in diameter than the opening of the pile at the surface.

Off shore installations, in particular wind turbines, are being installed at the sea bed in ever deeper water. One particular arrangement finding increasing use for offshore wind turbines is a monopile installed in a bore in the sea bed. In many cases, the monopile has a non-uniform diameter and, in particular has a subterranean portion that is larger in diameter than the portion at the sea bed. The shape of such a monopile requires a drill bit that can expand in diameter within the pile and/or once within the bore below the pile.

Drill bits for forming an underground bore with an underground portion of enlarged diameter are known in the art. The technique for forming such enlarged diameter portions of the bore is known as under reaming. Drill bit assemblies with under reamers are known in the art and used commercially.

GB 2434387 discloses an under reaming drill bit assembly having a drill tool and one or more digging arms pivotably mounted to the tool, so that a cutting means on the arm can perform a cutting action on the wall of the bore being drilled. To increase the diameter of the bore being formed, the arms of the assembly are moved radially outwards, thereby increasing the effective cutting diameter of the assembly. Retraction of the arms radially inwards reduces the effective cutting diameter of the assembly. Retraction of the arms is generally required when it is desired to remove the assembly from the bore.

A known drill bit comprises a plurality of moveable under reamer assemblies arranged circumferentially around the drill bit and moveable

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radially inwards and outwards. Each reamer comprises one or more rotatable bits disposed on the distal end of an arm assembly. The arm assembly of each reamer assembly is pivotably mounted to the drill bit assembly, such that the position of the reamer is moveable between a retracted position, in which the cutting portions of the reamer have a first diameter, and an extended position, in which the cutting portion of the reamer have a second diameter, greater than the first. In this way, a non-uniform bore may be drilled by appropriate movement and positioning of the reamer assemblies during the drilling operation. Such a drill bit assembly has been developed and is in use by Large Diameter Drilling (LDD) Limited.

It has been found that the known drill bit assemblies suffer a number of problems. In particular, it has been found that the arm assemblies, once extended within the bore, can become blocked by cuttings from the drilling operation, preventing the under reamer assemblies from being retracted. This in turn prevents the drill bit assembly from being withdrawn from the bore.

Accordingly, there is a need for an improved drill bit assembly of the aforementioned type.

In a first aspect, the present invention provides an under reaming drill bit tool, the drill bit tool comprising: a central drill bit assembly having a central housing, the central housing having an outer surface, the central drill bit assembly having a central longitudinal axis; at least one under reamer assembly having a cutting assembly and being moveable between a retracted position, in which the cutting assembly is at a first distance from the central longitudinal axis of the central drill bit assembly, and an extended position, in which the cutting assembly is at a second distance from the central longitudinal axis of the central drill bit assembly, the second distance being greater than the first distance; the or each under reamer assembly comprising an under reamer housing having an outer surface, at least a portion of the outer surface of the or each reamer housing extending from the cutting assembly and being contiguous with the outer surface of the central housing in both the retracted position and the extended position.

The drill bit tool of the present invention provides the advantage of maintaining free movement of the or each under reamer assembly. In particular, each under reamer assembly is kept free to move into the fully retracted position, for example to allow the tool to be withdrawn from the subterranean bore either during or upon completion of the drilling operation. It has been found that the known drill bit tools suffer a significant drawback in operation. With the under reamer assemblies or arms in the extended position, a cavity is formed between the under reamer and the central drill bit assembly. Debris from the cutting operation gathers in the cavity and is difficult to remove. When it is desired to retract the under reamer assemblies, for example to remove the assembly from the bore, the accumulation of debris prevents the under reamers from retracting fully. In some cases, this can prevent the assembly from being recovered from the bore. A significant number of off shore wind farms are located in regions in which the sea bed is predominantly boulder clay, that is a formation of stiff clay with rock boulders dispersed therein. The weaknesses of the known drill bit assemblies have been exposed during drilling operations in such regions.

In addition, it has been found that the provision of the moveable under reamers with a housing, in particular when the housing is arranged to enclose the under reamer, imparts greater strength and rigidity to the under reamer assemblies. This, in turn, allows for improved transmission of forces and loads applied to the cutting assemblies during the cutting operation to the central drill bit assembly. This increases the stability of the entire assembly during drilling and improves the accuracy and speed of the cutting operation.

The provision of an under reamer housing also avoids the need for massive arms as employed in the known assemblies.

The drill bit tool of the present invention comprises a central drill bit assembly having a central housing with an outer surface. The central drill bit assembly is provided with means to connect the tool to a drill string. Such connection means are known in the art. In use, the drill bit tool is lowered by means of the drill string onto the sea bed at the location of the bore and the drill bit tool is rotated in known manner. References herein to above', below', upper' and lower' are to the drill bit assembly and its components when oriented in the position to drill a substantially vertical subterranean bore.

In addition, the central drill bit assembly may comprise other components required for the drilling operation, such as one or more conduits for transporting fluid to flush the tool during cutting. Such conduits are preferably disposed within the housing of the central drill bit assembly.

Similarly, the central drill bit assembly may include the requisite system for moving the under reamer assemblies between the retracted and extended positions. One suitable system for moving the under reamers is a hydraulic system, allowing the under reamers to be moved by means of hydraulic rams.

Other means for moving the under reamers include an electric system for driving one or more electric motors to extend and retract the under reamers.

Again, such systems for moving the under reamers are preferably located within the housing of the central drill bit assembly. In such embodiments, the central drill bit assembly further includes suitable means for connection to utilities, such as lines for supplying hydraulic fluid and/or cables for supplying electrical power.

The central drill bit assembly may also comprise other components and/or systems, such as an air lift system having one or more air delivery

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pipes and one or more larger diameter return pipes for water, air and cuttings return to the surface. Similarly, the central drill bit assembly may comprise a water jetting system having one or more conduits for delivering medium to high pressure water to nozzles located on the cutting faces of the drill bit.

The central drill bit assembly preferably comprises one or more cutting assemblies, in particular disposed on a lower surface of the central housing.

Suitable cutting assemblies comprising cutters are known in the art. A preferred cutting assembly comprises a tapered or frusto-conical cutter mounted to rotate in a cutting cradle. The action of the drill bit tool being rotated in the bore causes the cutter to rotate in its cradle, thereby cutting the formation in contact with the cutter.

The drill bit tool of the present invention further comprises one or more moveable under reamer assemblies. Each under reamer assembly comprises a cutting assembly having one or more cutters. Again, suitable cutters are known in the art, for example as discussed above. Each under reamer may comprise a single cutter or, more preferably, a plurality of cutters.

Each under reamer assembly is mounted to the central drill bit assembly and moveable between a retracted position and an extended position. In the retracted position, the cutting assembly of each under reamer assembly is at a first distance from the central longitudinal axis of the central drill bit assembly. In the extended position, the cutting assembly of each under reamer assembly is at a second distance from the central longitudinal axis of the central drill bit assembly, with the second distance being greater than the first. In a preferred embodiment, each under reamer assembly is moveable such that the cutting assembly moves substantially radially with respect to the central longitudinal axis of the central drill bit assembly between the retracted and extended positions. In one preferred arrangement, the cutting assembly of the or each under reamer assembly moves in a shallow arc in a radial direction between the extended and retracted positions.

The or each under reamer assembly may be moveably mounted to the central drill bit assembly in any suitable way. It is particularly preferred that each under reamer assembly is pivotably mounted to the central drill bit assembly, the under reamer assembly moving about the pivot between the retracted and extended positions. More preferably, the pivot is arranged above the cutting assembly of the under reamer assembly, whereby the cutting assembly of the under reamer assembly moves in an arc below the pivot mounting. In this way, the action of withdrawing the drill bit tool upwards within the bore urges the under reamer assembly into the retracted position.

As noted above, the or each under reamer assembly comprises an under reamer housing. The housing at least comprises a portion extending from the cutting assembly of the under reamer assembly, the said portion having an outer surface that is contiguous with the outer surface of the central housing of the central drill bit assembly. In this respect, references to the surfaces being contiguous is to the arrangement in which the outer surface of the housing of the under reamer is continuous with or sufficiently close to the outer surface of the central housing as to prevent the ingress of debris from the cutting operation.

As mentioned, at least the portion of the outer surface of the under reamer housing extending from the cutting assembly is contiguous with the central housing in both the retracted and extended positions. This portion of the under reamer housing has a generally a lower or downwards facing outer surface, from which the cutters of the cutting assembly project. In this respect, the housing of the under reamer assembly is such that, when viewed from below, there is no void or space visible between the cutting assembly and the central housing of the central drill bit assembly in either the retracted or extended position. In this way, debris generated by the action of the cutting assembly is prevented from moving directly in between the under reamer assembly and the central drill bit assembly.

Preferably, the under reamer housing further comprises one or more side portions having an outer surface contiguous with the outer surface of the central housing. The side portions extend upwards from the said portion of the housing, such that when the under reamer assembly is viewed from the side no void or space is visible between the cutting assembly and the central housing in either the retracted or extended position.

In a preferred embodiment, the under reamer housing comprises an upper portion extending from the cutting assembly to the central housing, such that when the under assembly is viewed from above, no void or space is visible between the cutting assembly and the central housing in either the retracted or extended position.

More preferably still, the housing of the under reamer assembly encloses the interior of the under reamer assembly, such that substantially the entire outer surface of the under reamer assembly is contiguous with the outer surface of the central housing in both the retracted and extended positions.

In one preferred embodiment, the under reamer housing comprises a lower housing portion, from which the cutting assembly projects, opposing, generally triangular side portions extending upwards from a respective edge of the lower housing portion, and a generally rectangular upper housing portion extending between the lower housing portion and the opposing side portions.

To maintain the contiguous relationship between the outer surface of the under reamer housing and the central housing, the under reamer housing

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may be formed from portions that are flexible and/or fold. However, to impart strength and rigidity to the under reamer housing, thereby to allow loads to be transferred from the cutting assembly of the under reamer assembly to the central drill bit assembly, the under reamer housing is preferably rigid, in particular with the portions of the under reamer housing being formed generally as plates of suitable gauge. In one preferred embodiment, one or more portions of the under reamer housing are arranged telescopically with respect to the central housing, that is that in the retracted position, at least part of the under reamer housing portion extends into the central housing of the central drill bit assembly. To ensure contiguity between the outer surface of the under reamer housing and the outer surface of the central housing, part of the under reamer housing portion remains within the central housing with the under reamer in the extended position.

In a preferred embodiment, the under reamer housing comprises a lower portion and opposing side portions that are arranged telescopically with respect to the central housing.

The drill bit tool may comprise a single moveable under reamer assembly. More preferably, the drill bit tool comprises a plurality of moveable under reamer assemblies. It is especially preferred that the plurality of rnoveable under reamer assemblies are distributed circumferentially around the central drill bit assembly, most preferably distributed in a regular circumferential pattern. For example, the tool may comprise two, three, tour or more moveable under reamer assemblies.

In one preferred embodiment, the drill bit tool comprises three moveable under reamer assemblies arranged in a regular pattern around the central drill bit assembly such that the angle between adjacent moveable under reamer assemblies is 1200. Other patterns may also be envisaged, for example with a plurality of moveable under reamer assemblies arranged in a plurality of groups.

In addition to the one or more moveable under reamer assemblies, the drill bit tool may comprise one or more fixed wing assemblies extending laterally outwards from the central drill bit assembly. Each fixed wing assembly comprises a cutting assembly as hereinbefore described. In one preferred embodiment, the drill bit tool comprises a plurality of fixed wing assemblies. Preferably, the fixed wing assemblies are arranged circumferentially around the central drill bit assembly in an alternating pattern with the moveable under reamer assemblies.

Any suitable means may be employed to move the or each moveable under reamer assembly between the retracted and extended positions. In one embodiment, one or more hydraulic rams are employed to move the under reamer assemblies.

In one preferred embodiment, the drill bit tool comprises an actuation assembly for moving the or each moveable under reamer assembly. A preferred actuation assembly comprises an actuation member moveable along the longitudinal axis of the central drill bit assembly. A preferred actuation member is an actuation sleeve moveable along an inner housing of the central drill bit assembly. The actuation member may be moved by any suitable means, with hydraulic rams being particularly suitable. Each moveable under reamer assembly is connected to the actuation member by a connecting member, such as a rod or arm, whereby movement of the actuation member along the longitudinal axis of the inner drill bit assembly causes the under reamer assembly to move between the retracted and extended positions.

As noted above, the central drill bit assembly comprises a cutting assembly, in particular on a lower face or surface thereof. It has been found that the form of the bore being drilled depends upon the position of the cutting assembly of the central drill bit assembly, in particular the position of the cutting assembly in relation to the cutting assemblies of the one or more moveable under reamer assemblies and/or the one or more laterally extending fixed wing assemblies. More specifically, it has been found that the profile of the bore being drilled may be varied by adjusting the vertical position of the cutting assembly of the central drill bit assembly relative to the one or more moveable under reamer assemblies and/or the one or more fixed wing assemblies.

Accordingly, in a further aspect, the present invention provides a drill bit tool, the drill bit tool comprising: a central drill bit assembly having a central housing, the central housing having an outer surface, the central drill bit assembly having a central longitudinal axis; at least one moveable under reamer assembly having a cutting assembly and being moveable between a retracted position, in which the cutting assembly is at a first distance from the central longitudinal axis of the central drill bit assembly, and an extended position, in which the cutting assembly is at a second distance from the central longitudinal axis of the central drill bit assembly, the second distance being greater than the first distance; and/or at least one fixed wing assembly having a cutting assembly and extending laterally from the central drill bit assembly; wherein the central drill bit assembly comprises a cutting assembly on a lower portion thereof, the position of the cutting assembly being variable along the longitudinal axis of the central drill bit assembly, such that the position of the cutting assembly of the central drill bit assembly is variable with respect to the position of the cutting assembly of the or each moveable under reamer assembly and/or the or each fixed wing assembly.

The cutting assembly of the central drill bit assembly is variable with respect to the position of the cutting assemblies of the or each fixed wing assembly and/or the or each moveable under reamer assembly. Most preferably, the position of the cutting assembly of the central drill bit assembly along the longitudinal axis of the central drill bit assembly may be changed.

In one preferred embodiment, the cutting assembly is connected to the central drill bit assembly by way of a mount of variable height, such that the position of the cutting assembly along the central longitudinal axis may be varied by the mounting. Preferably, the mount is able to comprise a spacing insert disposed between the lower end of the central drill bit assembly and the cutting assembly, whereby changing the height of the insert varies the position of the cutting assembly. The insert is preferably of a fixed height, such that exchanging the insert for one of a different height or omitting the insert varies the position of the cutting assembly in the drill bit tool. The drill bit tool may be provided with one or more inserts of different heights, such that selection and installation of a given insert will determine the position of the cutting assembly in the tool and, hence, the profile of the bore to be drilled.

The central drill bit assembly, the moveable under reamer assembly and the fixed wing assembly are preferably as hereinbefore described.

It has been found that, depending upon the nature of the formation being drilled, having the cutting assembly of the central drill bit assembly disposed below the cutting assemblies of the moveable under reamer assemblies and/or the fixed wing assemblies assists in maintaining the verticality of the bore.

In a further aspect, the present invention provides a drill string comprising a drill bit tool as hereinbefore described.

Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which: Figure 1 is a perspective view of a moveable under reamer assembly of one embodiment of the present invention; Figure 2 is a view from below of a drill bit tool of one embodiment of the present invention comprising a plurality of moveable under reamer assemblies of Figure 1 with the moveable under reamer assemblies in a retracted position; Figure 3 is a vertical cross-sectional view of the drill bit tool of Figure 2 along the line Ill -Ill; Figure 4 is a perspective view from below of the drill bit tool of Figure 2; Figure 5 is a is a view from below of the drill bit tool of Figure 2 with the moveable under reamer assemblies in an extended position; Figure 6 is a vertical cross-sectional view of the drill bit tool of Figure 4 along the line VI -VI; Figure 7 is a perspective view from below of the drill bit tool of Figure 5; and Figure 8 is a diagrammatical representation of different bore profiles obtainable with the drill bit tool of Figures 2 to 7.

Referring to Figure 1, there is shown a perspective view of an under reamer assembly for use in a drill bit tool of the present invention. The under reamer assembly, generally referred to as 2, is shown oriented in the general position it would have during an operation to drill a substantially vertical subterranean bore.

The under reamer assembly 2 comprises an under reamer housing 4 having a generally rectangular lower housing portion 6 with a lower outer surface 6a, opposing side edges S and an outer edge 10. Generally triangular side housing portions 12 having an outer surface 1 2a extend upwards from the side edges 8 of the lower housing portion 6 and comprise inner side housing portions 1 4a and outer side housing portions 1 4b. A generally rectangular upper housing portion 16 having an outer surface 16a extends from the outer edge 10 of the lower housing portion 6 and between the outer side housing portions 14b.

The under reamer housing 4 is provided with a pivot mounting 18 in the form of a tube 20 extending between the upper portions of the side housing portions 12 and for receiving a pivot shaft to pivotably mount the under reamer housing to the drill bit tool, as described hereinafter.

The under reamer assembly 2 further comprises a cutting assembly, generally indicated as 30 extending from the lower surface 6a of the lower housing portion 6. The cutting assembly 30 comprises a plurality of cutters 32, each cutter comprising a cradle 34 mounted to the lower housing portion 4 and a frusto conical cutting wheel 36 mounted in bearings in the cradle 34.

The cutting wheels 36 are formed with teeth or buttons 38 in a known manner and in a known pattern, depending upon the ground conditions and the underground formation being drilled.

The under reamer housing 4 encloses the interior of the under reamer assembly 2 and provides a connection for transferring loads from the cutting assembly 30 to the central drill bit assembly, as described hereinbelow.

Turning to Figures 2 to 4, there is shown a drill bit tool of the present invention comprising a plurality of moveable under reamer assemblies of Figure 1 in a retracted position. The drill bit tool incorporates a plurality of under reamer assemblies as shown in Figure 1 and described above. The central drill bit tool has a central longitudinal axis A extending vertically as viewed in Figure 3.

The drill bit tool, generally indicated as 50, comprises a central drill bit assembly 52 having a generally cylindrical outer housing 54 arranged concentrically around the longitudinal axis A and provided with an upper flange 56 and a lower flange 58. The outer housing 54 has housing portions 54a that abut the respective side housing portions 12 of the moveable under reamer housing 4. In this way, the outer surface of the side housing portions 12 of each under reamer housing 4 is contiguous with the outer surface of the respective housing portion 54a of the outer housing 54.

A connection assembly 60 is mounted to the upper flange 56 and provides a means for connecting the drill bit tool 50 to a drill string or the like by a bolted connection, in known manner.

A cutting assembly 60 is mounted to the lower flange 58 of the central drill bit assembly 52 by way of a generally cylindrical insert 52a and comprises a plurality of cutters 62 arranged to extend from the lower surface of the cutting assembly 60. The cutters 62 each have the general arrangement described above in respect of the cutters 32 of the under reamer assembly and shown in Figure 1. The pattern in which the plurality of cutters 62 is arranged is shown in Figure 2.

As shown in the figures, the drill bit tool comprises the insert 52a mounted to the lower flange 58 of the central drill bit assembly 52, which serves to position the cutting assembly 60 below the outer housing 54. The inclusion of the insert 52a and the height of the insert (that is in the vertical orientation shown in Figures 3 and 6) can be used to adjust the position of the cutting assembly 60 relative to the outer housing 54, in turn changing the profile of the bore being drilled. In light of this, the drill bit assembly 52 may be provided with a plurality of inserts 52a having different heights, the selection of which will determine the profile of the bore being drilled.

As shown in Figure 3, the central drill bit assembly 52 comprises a generally cylindrical inner housing 64 arranged concentrically around the longitudinal axis A and extending between the upper flange 56 and the lower flange 58 of the outer housing 54. A flushing conduit 66 extends from the upper end of the inner housing 64 within the inner housing. The flushing conduit 66 projects beyond the lower end of the inner housing 64 and has an opening 68 adjacent the cutting assembly 60. In use fluid within the flooded bore hole is carried away from the cutting assembly 60 through the flushing conduit 66 using the air lift principle known in the art, with air injected through lines 66a in the upper portion of the central drill bit assembly 52 to generate an upflow of fluid within the flushing conduit 66, to be produced to the surface.

The drill bit tool 50 further comprises a plurality of fixed wing assemblies 80. Each fixed wing assembly 80 comprises a generally rectangular housing 82 extending radially outwards from the outer housing 54 of the central drill bit assembly 52. Each fixed wing assembly 80 has a lower end 84 carrying a cutting assembly 86 comprising a plurality of cutters 88 having the general arrangement described above in respect of the cutters 32 of the under reamer assembly and shown in Figure 1. The pattern in which the plurality of cutters 88 is arranged on the underside of each wing reamer assembly 80 is shown in Figure 2.

The drill bit tool 50 further comprises a plurality of moveable under reamer assemblies 2, having the arrangement shown in Figure 1 and described above. Each moveable under reamer assembly 2 is arranged to extend and be moveable in a radial direction with respect to the central drill bit assembly 52. The fixed wing assemblies 80 and the moveable under reamer assemblies 2 are arranged in an alternating circumferential pattern around the outer housing 54 of the central drill bit assembly 52, such that each fixed wing assembly 80 is adjacent two moveable under reamer assemblies 2. and vice versa.

In the embodiment shown in the figures, the drill bit tool comprises a total of six lateral cutting assemblies, formed from three moveable under reamer assemblies 2, arranged in an equidistant pattern around the central drill bit assembly, such that the angle between adjacent moveable under reamer assemblies is 120°, and three fixed wing assemblies 80 arranged in a similar pattern. It is to be understood that the total number of lateral cutting assemblies may be greater or fewer, in particular that the drill bit assembly may comprise fewer than three moveable under reamer assemblies or greater than three under reamer assemblies, for example four, five or six moveable under reamer assemblies.

Each moveable under reamer assembly 2 is pivotally mounted to the outer housing 54 of the central drill bit assembly 52 by means of a pivot rod 90 extending through the tube 20 in the upper portion of the under reamer housing 4.

Each moveable under reamer assembly 2 is moveable about its respective pivot rod 90. An actuation assembly, generally indicated as 102, is provided to move each moveable under reamer assembly 2. The actuation assembly 102 comprises an actuation sleeve 104 arranged concentrically around the inner housing 64 of the central drill bit assembly 52 and moveable up and down the exterior of the inner housing 64. The actuation sleeve 104 is shown in the raised position, with the moveable under reamers 2 retracted, in Figure 3 and in the lowered position, with the moveable under reamers 2 extended, in Figure 5. Hydraulic rams 106 extend between a first mount 108 on the underside of the upper flange 56 of the central drill bit assembly 52 and a second mount 110 on the lower portion of the actuation sleeve 104. The hydraulic rams 106 are operable by a supply of pressurised hydraulic fluid to raise and lower the actuation sleeve 104. The actuation assembly 102 further comprises actuation arms 112, each extending between and pivotally connected at a respective end to a first mount 114 on the upper portion of the actuation sleeve 104 and a second mount 116 in the interior of the housing 4 of a moveable under reamer assembly 2. As shown in the figures, movement of the actuation sleeve 104 up and down the inner housing 54 causes each actuation arm 112 to move the respective moveable under reamer assembly 2 inwards and outwards.

In use, the drill bit tool is employed to drill a subterranean bore in the known manner, with the tool being rotated with the cutting assemblies in contact with the formation to be bored. When it is required to increase the diameter of the bore, the actuation sleeve 104 is lowered by means of the hydraulic rams 106, as a result of which the actuation arms 112 urge each moveable under reamer assembly 2 radially outwards from the retracted position shown in Figures 2 and 3. Similarly, when it is desired to reduce the diameter of the bore being formed, the actuation sleeve 104 is raised, withdrawing the moveable under reamer assemblies 2 inwards.

As shown in the figures, the moveable under reamer housing 4 is arranged such that it forms a contiguous surface with the outer surface of the outer housing 54 of the inner drill bit assembly 52 in all positions between the retracted position to the extended position. In particular, as shown in Figure 4, in the retracted position, the outer surface of the housing side portion 12 of each moveable under reamer assembly 2 is in contact with the outer housing 54 of the inner drill bit assembly 52 and contiguous with the outer surface of the housing portion 54a. Similarly, as shown in Figure 7, in the extended position, the outer surface of the housing side portion 12 of each moveable under reamer assembly 2 remains in contact with the outer housing 54 of the inner drill bit assembly 52 and contiguous with the outer surface of the housing portion 54a.

In the retracted position, shown in Figure 4, and the extended position, shown in Figure 7, a portion of the housing 4 of the moveable under reamer assembly 2 lies within the outer housing 54 of the inner drill bit assembly 52.

In particular, the inner portion 1 4a of the side housing portion 12 of each moveable under reamer assembly 2 remains within the outer housing 54 of the inner drill bit assembly 52.

In operation, debris from the boring operation is prevented from entering the inner drill bit assembly 52 or the housing 4 of each moveable under reamer assembly 2, thereby significantly reducing the possibility of the operation of the components of the drill bit tool being hindered. In particular, each moveable under reamer assembly is not prevented from moving inwards towards the retracted position by an accumulation of debris within the inner drill bit assembly. Should a component fail, such that the moveable under reamers cannot be moved, the action of recovering the drill bit tool upwards within the bore urges each moveable under reamer assembly into the retracted position, thereby allowing the drill bit tool to be recovered.

Turning to Figure 8, there is shown a diagrammatical representation of the bottom portion of a bore drilled using the drill bit tool of Figures 2 to 7.

The bore, generally indicated as 202, has a bottom or floor 204. With the drill bit tool assembled as shown in Figures 2 to 7, that is with the insert 52a in place, the profile of the floor 204 of the bore 202 is as shown in Figure 8 and comprises an outer portion 204a and an inner portion 204b separated by a step 204c, with the inner portion 204b being deeper than the outer portion 204a. Depending upon the formation being drilled, forming the floor of the bore as shown in Figure 8 can improve the verticality of the bore and assist in keeping the bore vertical during the drilling operation.

The depth of the inner floor portion 204b relative to the outer floor portion 204a may be varied by changing the height of the insert 52a included in the central drill bit assembly of the drill bit tool. Accordingly, the inner floor portion 204b may be deeper or shallower relative to the outer floor portion 204a by selection of an insert of appropriate height. Omitting the insert 52a and mounting the cutting assembly directly to the lower end of the central drill bit assembly housing produces a flatter profile to the floor 204, as indicated by the dotted line A in Figure 8.

Claims

CLAIMS1. An under reaming drill bit tool, the drill bit tool comprising: a central drill bit assembly having a central housing, the central housing having an outer surface, the central drill bit assembly having a central longitudinal axis; at least one under reamer assembly having a cutting assembly and being moveable between a retracted position, in which the cutting assembly is at a first distance from the central longitudinal axis of the central drill bit assembly, and an extended position, in which the cutting assembly is at a second distance from the central longitudinal axis of the central drill bit assembly, the second distance being greater than the first distance; the or each under reamer assembly comprising an under reamer housing having an outer surface, at least a portion of the outer surface of the or each reamer housing extending from the cutting assembly and being contiguous with the outer surface of the central housing in both the retracted position and the extended position.
2. The drill bit tool according to claim 1, wherein the central drill bit assembly comprises means for moving the or each under reamer assembly.
3. The drill bit tool according to claim 2, wherein the said means comprises one or more hydraulic rams.
4. The drill bit tool according to any preceding claim, wherein the central drill bit assembly comprises one or more fluid delivery lines and/or one or more return lines.
5. The drill bit tool according to claim 4, wherein the central drill bit assembly comprises one or more nozzles for delivering medium to high pressure water.
6. The drill bit tool according to any preceding claim, wherein the central drill bit assembly comprises a cutting assembly.
7. The drill bit tool according to claim 6, wherein the cutting assembly is disposed on a lower end portion or surface of the central drill bit assembly.
8. The drill bit tool according to any preceding claim, wherein the cutting assembly of the or each under reamer assembly moves substantially radially between the retracted and extended positions.
9. The drill bit tool according to claim 8, wherein the cutting assembly moves in a shallow arc between the retracted and extended positions.
10. The drill bit tool according to any preceding claim, wherein the or each under reamer assembly is pivotably mounted to the central drill bit assembly.
11. The drill bit tool according to claim 10, wherein the or each under reamer assembly is pivotable about a pivot above the assembly, with the under reamer assembly moves in an arc below the pivot mounting.
12. The drill bit tool according to any preceding claim, wherein the or each under reamer housing comprises a housing portion extending from the cutting assembly to the central housing.
13. The drill bit tool according to claim 12, wherein the or each under reamer housing further comprises one or more side housing portions.
14. The drill bit tool according to claim 13, wherein the or each under reamer housing further comprises an upper housing portion.
15. The drill bit tool according to claim 14, wherein the or each under reamer housing encloses the interior of the under reamer assembly.
16. The drill bit tool according to any preceding claim, wherein the or each under reamer housing is rigid.
17. The drill bit tool according to any preceding claim, wherein one or more portions of the or each under reamer housing are arranged telescopically with respect to the central housing.
18. The drill bit tool according to claim 17, wherein a portion of the or each under reamer housing remains within the central housing when the under reamer assembly is in the extended position.
19. The drill bit tool according to any preceding claim, comprising a plurality of under reamer assemblies arranged in a regular circumferential pattern around the central drill bit assembly.
20. The drill bit tool according to any preceding claim, further comprising one or more fixed wing assemblies.
21. The drill bit tool according to any preceding claim, further comprising an actuation assembly for moving the or each under reamer assembly.
22. The drill bit tool according to claim 21, wherein the actuation assembly comprises an actuation member moveable along the longitudinal axis of the central drill bit assembly, the or each under reamer assembly being connected to the actuation member by a connecting member.
23. The drill bit tool according to claim 22, wherein the actuation member is a sleeve moveable along the central drill bit assembly.
24. A drill bit tool, the drill bit tool comprising: a central drill bit assembly having a central housing, the central housing having an outer surface, the central drill bit assembly having a central longitudinal axis; at least one moveable under reamer assembly having a cutting assembly and being moveable between a retracted position, in which the cutting assembly is at a first distance from the central longitudinal axis of the central drill bit assembly, and an extended position, in which the cutting assembly is at a second distance from the central longitudinal axis of the central drill bit assembly, the second distance being greater than the first distance; and/or at least one fixed wing assembly having a cutting assembly and extending laterally from the central drill bit assembly; wherein the central drill bit assembly comprises a cutting assembly on a lower portion thereof, the position of the cutting assembly being variable along the longitudinal axis of the central drill bit assembly, such that the position of the cutting assembly of the central drill bit assembly is variable with respect to the position of the cutting assembly of the or each moveable under reamer assembly and/or the or each fixed wing assembly.
25. The drill bit tool according to claim 24, wherein the position of the cutting assembly of the central drill bit assembly may be varied along the central longitudinal axis.
26. The drill bit tool according to either of claims 24 or 25, wherein the cutting assembly is mounted to the central drill bit assembly by a variable mount.
27. The drill bit tool according to claim 26, wherein the variable mount comprises an insert.
28. The drill bit tool according to claim 27, comprising a plurality of inserts of different heights, the position of the cutting assembly being determined by selection of the appropriate insert.
29. A drill string comprises a drill bit tool according to any preceding claim.
30. A drill bit tool substantially as hereinbefore described having reference to the accompanying figures.