FIELD

This invention relates to a modular downhole apparatus. BACKGROUND

In the oil and gas exploration and production industry, a number of different tools and techniques have been developed in order to facilitate the drilling of large diameter borehole sections. In some instances, drill bit or hole opener tools have been developed which employ rolling cutter elements in the form of toothed roller cones or cutters which are mounted on bearings to allow for rotation. Typically, in roller cone drill bits the roller cones with their integral bearings are mounted on a plurality of forged legs which are then welded together to form a single assembly. The toothed roller cones face down to create a cutting mechanism and a threaded pin connection faces up for attachment means to form what is generally referred to as a tricone drill bit.

In other instances, drill bit or hole opener tools have been developed which employ fixed drag cutter cutting elements in the form of polycrystalline (PDC) inserts. Such drill bits typically have fixed cutters in the form of PDC inserts brazed in place to form a fixed drag type cutting structure which faces downhole. Again, a threaded pin connection faces uphole for attachment means to form what is generally referred to as a PDC fixed cutter bit.

Both rolling cutter elements and fixed drag cutter element assemblies are in common use for drilling large diameter bore hole sections. Nevertheless, there are a number of drawbacks with conventional tools and techniques.

For example, in conventional rolling cutter elements and fixed drag cutter element assemblies, once built there is no scope to change the configuration of the drill bit. PDC fixed cutter element hole openers are typically manufactured with the PDC cutters brazed into the body of the tool or onto replaceable blades which are then attached to the hole opener body. These PDC fixed cutter hole opener tools are generally used in the smaller sizes below 17 ½" diameter; the fixed cutter PDC shearing mechanism lending itself more successfully to drilling the types of formations encountered in the smaller size deeper hole sections. With roller cutter hole openers, there is also some scope to change out the roller cutters.

While interchangeable roller cutter and fixed cutter elements or assemblies for drill bits and hole openers have been proposed, these suffer problems such as loss of components in the borehole and weakness in the assembled structure. For example, bolted structures can become loose. The tolerance clearance required for manufacture and assembly can lead to vibration and possible fatigue. The assembly may suffer from a lack of space for structural assembly strength, and the requirements for hydraulics and flow bypass. Furthermore, machinability can be difficult and the manufacturing costs can be high. As a result, the vast majority of drill bit and hole openers currently used are welded fabrications, castings or machined from solid.

SUMMARY

Aspects of the present invention relate to a modular downhole apparatus comprising a body configured to receive a tool element for performing a downhole operation, a docking arrangement for coupling the tool element to the body, and a lock arrangement configured to maintain the tool element on the body, and to a method for constructing a modular downhole apparatus.

According to a first aspect, there is provided a modular downhole apparatus comprising:

a body configured to receive a tool element for performing a downhole operation;

a docking arrangement for coupling the tool element to the body, the docking arrangement comprising a catch arrangement provided on at least one of the body and the tool element, the catch arrangement configured to engage a latch arrangement of the other of the body and the tool element to couple the tool element to the body; and a lock arrangement configured to maintain the tool element on the body, the lock arrangement comprising a tapered lock member which on engagement is energised to securely maintain the tool element on the body.

In use, the apparatus may be constructed by coupling the tool element to the body using the docking arrangement and locking the tool element to the body using the lock arrangement. In particular embodiments, the catch arrangement is provided on the body and the latch arrangement is provided on the tool element. However, it will be recognised that in other embodiments, the catch arrangement may be alternatively or additionally be provided on the tool element and the latch arrangement may be provided on the body.

Embodiments of the present invention provide a number of benefits. For example, as the downhole apparatus is modular in nature the downhole apparatus can be quickly and securely assembled prior to use downhole; the apparatus being configurable in a number of different tool element configurations while utilising a common body. Embodiments of the invention thus give an operator the ability to change the apparatus structure or configuration at short notice by adapting the selected tool element or elements, permitting amongst other things the ability to mix tool mechanisms and dimensions to suit borehole characteristics and/or dimensions in the same apparatus, reducing inventory of single unit tools, reducing transport costs and allow servicing in the field to be achieved quickly and easily. Moreover, the provision of an apparatus combining a docking arrangement and the lock arrangement having a tapered lock member which on engagement is energised to securely maintain the tool element on the body provides a modular downhole apparatus that provides a secure and robust connection which prevents or mitigates the risk of component loss in the borehole and which does not suffer from drawbacks associated with vibration. Since the requirement for welded connections is obviated or reduced, an apparatus according to embodiments of the invention also permits the use of materials which are otherwise not available due to their unsuitability for use with welded connections; permitting an apparatus to be constructed which is robust and significantly stronger than conventional tools constructed using welded construction methods.

The tapered lock member may be configured to be insertable into the body to fit into a space between the tool element and the body to lock the tool element to the body.

The tapered lock member may be configured to apply a force on the tool element which urges the tool element towards a locked position relative to the body. The action of inserting the tapered lock member may apply the force on the tool element which urges the tool element towards the locked position. The tapered lock member may be biased so as to urge the tool element towards a locked position relative to the body, for example, by one or more spring element operatively associated with the tool element. Beneficially, the lock arrangement of embodiments of the present invention urges the tool element towards the locked position in use, such that differences in tolerance or fit between the tool element and the body - which may otherwise result in damaging wear or vibration effects - may be obviated or mitigated, since the tool element is energised or biased to the most secure position available.

The lock member may be configured to provide an interference fit with the slot and the tool element. In use, the lock member may be configured to provide an interference fit with the slot and the tool element such that it fills the space entirely. The space may be formed by one or both of the tool element or the slot having a varied cross section depth across its length. The space may be tapered. The space may be formed between the tool element and the body, within a slot, when the tool element is in a latched configuration.

In particular embodiments, the lock member comprises a tapered lock bar.

The downhole apparatus may comprise a single lock member. In particular embodiments, the downhole apparatus comprises a plurality of lock members.

The number of lock members may be equal to the number of tool elements, for example there may be one locking member per tool element. Alternatively, the apparatus may comprise a plurality of lock members per tool element, for example two lock members per tool element.

As described above, the modular downhole apparatus comprises a body which is configured to function as a common hub which can be readily re-dressed in the field with one or more tool element of the same or different tool configuration, the docking arrangement and/or the lock arrangement being common for each tool configuration.

In some embodiments, the downhole apparatus may be configurable to define a hole opener.

In some embodiments, the downhole apparatus may be configurable to define a drilling tool.

In some embodiments, the downhole apparatus may be configurable to define a stabiliser.

In some embodiments, the downhole apparatus may comprise a multi-function tool. For example, the downhole apparatus may be configured to perform a plurality of hole opening, stabilisation, and drilling operations.

While the apparatus may comprise a single tool element, in particular embodiments, the apparatus comprises a plurality of tool elements. In use, the tool elements may be disposed around the body. The tool elements may be circumferentially disposed around the body. The tool elements may be circumferentially spaced around the body.

The tool element may comprise a cutting element.

The tool element may comprise a roller cutter.

The tool element may comprise a fixed cutter.

The tool element may comprise a fixed drag cutter.

The tool element may comprise a stabiliser element. In embodiments where the apparatus comprises at least two tool elements, the tool elements may be the same type of tool or may be different types of tools. For example, the downhole apparatus may comprise a plurality of roller cutters, or a plurality of fixed drag cutters, or a plurality of stabilisers, or any combination of roller cutters, fixed drag cutters, and stabilisers. In some embodiments, the apparatus may comprise three tool elements, such as three cutting elements. In some embodiments, the apparatus may comprise six tool elements.

The tool elements may be positioned at different operating diameters. Beneficially, embodiments of the present invention may use a common body with a variety of tool elements of different outer dimension or diameter, one or a plurality of the tool elements being coupled to the body to define an apparatus with a given dimension or outer diameter.

The downhole apparatus may further comprise, or may be operatively associated with, a pilot drill bit. For example, the downhole apparatus may comprise a large diameter drill bit and the pilot drill bit may be coupled to the large diameter drill bit to effect drilling operations.

It will be recognised that any suitable number or combination of tool elements may be utilised, as required to perform a particular operation or to respond to particular downhole conditions, borehole characteristics or the like, while utilising a common docking arrangement provided by the apparatus.

A slot may be provided in the body of the downhole apparatus. In particular embodiments, the apparatus may comprise a plurality of the slots. In use, the slot may form part of the docking arrangement of the apparatus for coupling the tool element or elements to the body. The slot may be provided on an outer surface of the body. The slot may be configured to receive the tool element.

The slot may comprise a first end. The first end of the slot may comprise an upper, or uphole, end. The first end of the slot may correspond to a first end of the body. The slot may comprise a second end. The second end of the slot may correspond to a lower end of the body. The lower end of the body and slot corresponds to the position of the tool further downhole than the upper end of the slot and body.

The at least one slot may be open at the lower end and blind ended at the upper end. The slot may have sidewalls extending from the outer surface of the body to a base formed in the body.

The slot may have a T-shaped cross section, in that the base and a lower part of the sidewalls of the slot form the wider part of the T-shaped section and an upper part of the sidewalls extend to the outer surface of the body as the narrower part of the T-shape.

The catch arrangement may be positioned within the slot.

In particular embodiments, the catch arrangement may comprise a latch recess formed into a base of the slot. Alternatively or additionally, the catch arrangement may comprise at least one recess formed in the sidewalls of the slot, for example in the upper part of the sidewalls of the slot. Alternatively or additionally, the catch arrangement may comprise at least one latch recess formed in the sidewalls of the slot.

In particular embodiments, the catch arrangement may comprise a plurality of latch recesses.

The tool element may be configured to be insertable into the slot prior to location of the downhole apparatus downhole.

The tool element may be configured to be slidable into the slot from the lower end of the body. Alternatively or additionally, the tool element may be pushed radially into the slot.

The tool element may be configured for clearance fit with the slot.

The tool element may be configured such that a portion of the tool element's cross section is the same shape as the cross section of the slot.

The tool element may comprise an upper portion and a base portion.

The upper portion may protrude from the upper surface of the body.

The upper portion may further comprise a tool assembly, for example when the tool element is cutting tool, the tool assembly may be blades.

The base portion may have a cross section corresponding to the slot, such that the tool element is insertable into the slot.

The latch arrangement may be formed on the base portion of the tool element.

The latch arrangement may be configured as a latch block which forms the base portion into the cross-sectional shape required for the tool element to fit into the slot.

The latch arrangement may be configured as a latch block formed on the upper portion of the tool element.

The latch block may be sized to fit into the corresponding latch recess in the slot.

The latch arrangement may comprise a plurality of latch blocks formed on the tool element. The apparatus may have an unlatched configuration and a latched configuration. When in the unlatched configuration, the latch arrangement may be aligned with the corresponding catch arrangement on the body. When in the latched configuration, the latch arrangement may be engaged with the catch arrangement. It will be recognised that elements of the catch arrangement and latch arrangement may be provided on one or both of the tool element and the body.

The tool element may be configured to be pushed into the latched configuration.

In use an operator may insert the tool element into the slot into the unlatched configuration where the latch and catch arrangements are aligned. The operator may then push the tool element down into the slot, to engage the latch with the catch and thus latching the tool element within the slot.

In the unlatched configuration, the tool element may be easily inserted and removed from the body.

The docking arrangement according to embodiments of the present invention may provide a number of benefits. For example, the engagement between the catch arrangement and the latch arrangement and/or the engagement between the tool element and the slot facilitates the transfer of forces. In particular, but not exclusively the shape and interlocking configuration of the docking arrangement provides a downhole apparatus which is capable of withstanding the significant tensile and compressive forces applied to the downhole apparatus in use.

The downhole apparatus may be configured to form part of a downhole assembly. The downhole tool apparatus may form part of a downhole tubing string, work string or the like.

The downhole apparatus may comprise, or may be configured for attachment to, a top sub. In use, the top sub may permit connection of the apparatus to an uphole component of the downhole tubing string.

The lock member may be secured in the body by attachment of the body to the top sub.

The lock member may further comprise a means to secure the locking bar in position within the body, for example a fastener. In particular embodiments, the means to secure the locking bar in position within the body may comprise a screw member, cap or the like.

The body may further comprise a least one flow path within the main body. The body may further comprise one or more gallery. The body may further comprise at least one jetting nozzle. In use, the jetting nozzle may be configured for cleaning and cooling the tool element or elements.

According to a second aspect, there is provided a method of assembling a modular downhole apparatus, the method comprising:

coupling a tool element of a modular downhole apparatus to a body of a downhole apparatus using a docking arrangement, the docking arrangement comprising a catch arrangement provided on at least one of the body and the tool element, the catch arrangement configured to engage a latch arrangement of the other of the body and the tool element to couple the tool element to the body; and

inserting a lock member of a locking arrangement of the modular downhole apparatus into the body to securely the tool element in position, the lock member comprising a tapered lock member which on engagement is energised to securely maintain the tool element on the body.

The method may comprise arranging a plurality of tool elements around the body. The tool elements may be the same or different. For example, in some embodiments, the plurality of tool elements may comprise roller cutters. In some embodiments, the plurality of tool elements may comprise fixed cutters, such as fixed drag cutters. In other embodiments, the plurality of tool elements may comprise a combination of different tool elements, such as roller cutters, fixed drag cutters, stabilisation members.

The body may comprise at least one slot configured to receive the at least one tool element, wherein the method further comprises inserting the tool element into the slot.

The tool element may have an unlatched configuration and a latched configuration, wherein in the unlatched configuration the latch mechanism is aligned with the corresponding catch mechanism on the body and wherein in the latched configuration, the latch mechanism is engaged with the catch mechanism.

The method may comprise inserting the tool element into the slot into the unlatched configuration.

The method may further comprise moving the tool element from the unlatched configuration into the latched configuration. This moving step may involve applying a force to the tool element to push the latch mechanism into engagement with the catch mechanism.

The method may further comprise inserting the locking member into the body to lock the tool element around the body. The method may further comprise attaching the downhole apparatus with the locking member in the body to a top sub, and securing the downhole apparatus to the top sub such that the locking member is secured in the body by the top sub.

The method may comprise securing the locking member in position within the body with a securing means, for example a screw arrangement. The body may then be attached to a top sub.

The method may further comprise securing a pilot drill bit to the body such that the downhole apparatus is a large diameter drill bit.

According to a third aspect, there is provided a kit for a modular downhole apparatus, the kit comprising:

a body configured to receive at least one tool element for performing a downhole operation; and

at least one tool element,

a docking arrangement for coupling the tool element to the body, the docking arrangement comprising a catch arrangement provided on at least one of the body and the tool element, the catch arrangement configured to engage a latch arrangement of the other of the body and the tool element to couple the tool element to the body; and a lock arrangement configured to maintain the tool element on the body, the lock arrangement comprising a tapered lock member which on engagement is energised to securely maintain the tool element on the body.

The kit may further comprise a plurality of tool elements. The tool element secured to the body may comprise a selected tool element or tool elements from the plurality of tool elements of the kit.

The tool element may be for example cutters such as roller cutters or fixed drag cutters.

The tool element may be a stabiliser.

The kit may comprise a variety of different tool elements for use with the body. The tool elements may be configured to have different operating diameters. The kit may further comprise a top sub.

The kit may further comprise a pilot drill bit.

It will recognised that embodiments of this invention provide a modular downhole apparatus and a method of assembling a modular downhole apparatus, such as a modular hole opener, a modular drill bit or the like, in such a way that the need for welded assemblies is eliminated. The apparatus may be constructed with a standardized docking system which enables a variety of cutting structures to be easily attached to a common body in such a way that one apparatus can accommodate a variety of cutting structures and hole sizes. For example, a 20" bodied tool may cover a range of hole sizes from 22" to 36" using a standard roller cone, roller cutter or PDC cutter assembly mounted to a common body with the standardized docking system and with the cutting structures set at different operating diameters. Embodiments of this apparatus and method of construction may also allow for hybrid cutting structures to be accommodated where, for example, both rolling element and PDC cutting structure may be mounted on the same common body while cutting on the same or different planes. The apparatus and method may also permit the possibility of having a pilot hole drilled by configuring the rolling element or PDC fixed cutter cutting structures to rotate at the same speed as the body of the downhole apparatus or at a higher speed than the body of the apparatus by means of a downhole mud motor.

It should be understood that the individual features defined above in accordance with any aspect of the present disclosure or below in relation to any specific embodiment of the disclosure, or in any of the appended claims may be utilised, either alone or in combination with any other defined feature, in any other aspect or embodiment of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an isometric perspective view of a downhole apparatus according to a first embodiment of the invention;

Figure 2 shows an exploded perspective view of the downhole apparatus of Figure 1 , in a partially assembled configuration;

Figure 3 shows an isometric perspective view of a main body of the downhole apparatus shown in Figures 1 and 2;

Figure 4 shows an exploded perspective view of part of the downhole apparatus shown in Figures 1 to 3, in a partially assembled condition;

Figure 5 shows an exploded perspective view of the downhole apparatus shown in Figures 1 to 4, in a partially assembled condition;

Figure 6 shows an isometric perspective view of the main body of the downhole apparatus shown in Figures 1 to 5;

Figure 7 shows an exploded perspective view of a downhole apparatus according to a second embodiment of the invention; Figure 8 shows another exploded perspective view of the downhole apparatus shown in Figure 7;

Figure 9 shows another exploded perspective view of the downhole apparatus shown in Figure 7;

Figure 10 shows an exploded side view of the downhole apparatus shown in

Figure 7;

Figures 1 1A-1 1 D show end views and cross sectional views of the downhole apparatus shown in Figure 7;

Figure 12 shows an enlarged perspective view of part of the downhole apparatus shown in Figure 7;

Figure 13 shows an enlarged side view of part of the downhole apparatus shown in Figure 7;

Figure 14 shows an exploded perspective view of a downhole apparatus according to a third embodiment of the present invention

Figure 15 shows another exploded perspective view of a downhole apparatus shown in Figure 14;

Figure 16 shows an exploded side view of the downhole apparatus shown in Figure 14;

Figure 17A shows a left side view of the downhole apparatus shown in Figure 14;

Figure 17B shows longitudinal section A-A of Figure 17A;

Figure 17C shows longitudinal section B-B of Figure 17A;

Figure 17D shows a right side view of the downhole apparatus shown in Figure

14;

Figure 17E shows a perspective view of the downhole apparatus shown in

Figure 14;

Figure 17F shows another perspective view of the downhole apparatus shown in Figure 14;

Figure 18A-18C show steps in assembly of the downhole apparatus shown in Figure 14;

Figure 19 shows a perspective cut-away view of the downhole apparatus shown in Figure 14;

Figure 20 shows an exploded perspective view of a downhole apparatus according to a fourth embodiment of the invention; Figure 21 shows another exploded perspective view of the downhole apparatus shown in Figure 20;

Figure 22 shows an exploded side view of the downhole apparatus shown in Figure 20;

Figure 23A shows a left side view of the downhole apparatus shown in Figure

20;

Figure 23B shows a left side view of the downhole apparatus shown in Figure

20;

Figure 23C shows section A-A of Figure 23B;

Figure 23D shows section B-B of Figure 23B;

Figure 23E shows a right side view of the downhole apparatus shown in Figure

20;

Figure 23F shows a perspective view of the downhole apparatus shown in Figure 20;

Figure 23G shows another perspective view of the downhole apparatus shown in Figure 20;

Figures 24 and 25 show step in assembly of the downhole apparatus shown in Figure 20; and

Figures 26A-26D show downhole apparatus according to various embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to Figures 1 to 6 of the accompanying drawings, there is shown a downhole apparatus according to a first embodiment of the invention. In this illustrated embodiment, the downhole apparatus takes the form of a modular hole opener apparatus having a top sub 1 , a main body 2, tapered locking bars 3, roller cone cutters 4 and PDC cutter arms 5.

Figure 1 shows the modular hole opener in an assembled configuration with all the modular roller cone and PDC cutter arms 4 and 5 in their fully located docked position with all tapered locking bars 3 installed and the top sub 1 connected while Figure 2 shows an exploded view for ease of reference.

Figure 3 shows the main body 2 in isolation from the rest of the apparatus. As shown, the body 2 comprises a plurality of circumferentially arranged and spaced docking slots 6 for receiving the cutter arms 4, 5. Figure 3 shows the main body 2 with docking slots 6 empty. As shown in Figure 3, main body 2 has a throughbore 8 passing along the central axis of the main body 2 which extends from a threaded portion 9 of a lower extended section 7 to an upper threaded portion 10 which connects to a threaded pin 1 1 of the top sub 1. This throughbore 8 communicates with a throughbore 8a which passes though the central axis of the top sub 1 to facilitate the transmission of drilling fluids to jets or nozzles (not shown) in the main body 2. The jets or nozzles are for the provision of cooling and cleaning to the modular rolling element 4 and PDC cutting structures 5.

The slots 6 are machined such that they are open at their lower end, being T- shaped in cross section and blind ended at their upper end. Latch recesses 12 are machined into the base of these docking slots 6 in the wider part of the T-section of the docking slots 6. These machined recesses are proportioned to match latch blocks 13 which form the T-shaped base part of the modular roller cone and PDC cutter arms assemblies 4 and 5. The width of the narrow section of the docking slots 6 is proportioned to provide a clearance fit to the narrow section of the modular roller cone and PDC cutter arm assemblies 4 and 5 such that they can be easily slid into the T- shaped open end of the module docking slots 6 to the point where they bottom out at the blind end of the module docking slot 6 with the top face 14 of the modular roller cone and PDC cutter arm assemblies 4 and 5 abutting the stop face 15 of the module docking slot 6. In this unlatched position, the latch blocks 13 align with the latch recesses 12 and the roller cone and PDC cutter arm assemblies 4 and 5 can be pushed down into their latched position.

To fill the space left in the T-slot created when pushing the modular roller cone and PDC cutter arms 4, 5 down into their locked position and to prevent the modular roller cone and PDC cutter arm assemblies from moving back up into their disassembly (unlatched) position, a pair of tapered locking bars 3 are installed through entry holes at the upper end of the main body 2 down either side of the modular roller cone and PDC cutter arms above the T-section and within the docking slots 6. These entry holes being located within a pitch circle diameter (PCD) that is less than the outside diameter of the upset 16 on the top sub 1. The action of making up the top sub 1 with the main body 2 loads the tapered locking bars 3 into place, forcing and locking the latch blocks 13 into the latch recess 12 in such a way that all assembly clearances are taken up and the latches 13 and thereby, the modular roller cone and PDC cutter arm assemblies 4 and 5 are locked into position. The tapered locking bars 3 which lock the latches 13 down into the latch recesses 12 machined into the base of the docking slots 6 are inserted through holes (not shown) within a PCD smaller than the upset 16 on the top sub 1. They are then loaded and locked into place by the action of screwing and torqueing together the threaded pin connection 1 1 to the threaded box connection 10 of the main body 2.

In this state the loaded and locked tapered locking bars 3 can only escape if the pin connection 1 1 fails or unscrews. The pin connection is designed to exceed the torsional tensile and compressive strength of any other connection within the drilling tubulars in which these tools may be run then the loss of these pins and consequently loss of a modular roller cone or PDC cutter arm is effectively impossible within all reasonable downhole conditions.

Furthermore, the forces required in this locked condition to shear or break the modular roller cone or PDC cutter arms 4 and 5 out of the main body 2 are the forces required to shear the latch blocks 13 from the modular roller cone or PDC cutter arm 4 and 5 or to shear through the arms themselves. These modular roller cone or PDC cutter arms 4 and 5 are designed to withstand several times the torsional, tensile and compressive loads that could be applied by the drilling tubulars on which the modular hybrid hole opener or drill bit is run. The top sub 1 pin connection 1 1 is designed such that its torsional, tensile and compressive strength substantially exceeds any other connection in the drilling assembly to which this tool may be attached.

Referring to Figure 6, the roller cone or PDC drill bit may be driven at higher speed than the modular hole opener by means of a motor (not shown) located in or above the top sub 1 driving by means of hollow drive shaft 18 connected to the lower extended sub 19 which would be mounted in bearings within the main body 2.

Not shown in this embodiment for clarity are jetting nozzles within the main body 2 for cleaning and cooling the modular roller cone or PDC cutter arms 4 and 5. These nozzles may be placed in a variety of locations on the main body 2 to be directed at the roller cone and or PDC cutting structures. Flow or debris bypass slots 17 shown in the Figures 1 to 6 can be machined in the main body 2 between the module docking slots 6.

It should be clear to anybody practiced in the art that by adding or screwing a pilot drill bit onto the lower connection 9 in the lower extended section 7, the modular hole opener effectively becomes a large diameter drill bit. Refinements can be added to this modular design. For example, the lower extended section 7 may be shortened to bring the pilot bit cutting structure into closer proximity with the cutting structure of the modular roller cone or PDC cutter arms 4 and 5. The modular hole opener may be dressed with all roller cone or all PDC cutter arms. The drill bit may be a roller cone drill bit attached to an "all PDC cutter arm dressed" modular hole opener or a PDC drill bit connected to an "all roller cone cutter arm dressed" modular hole opener.

Figures 7 to 13 show a downhole apparatus according to a second embodiment of the invention. In this embodiment, the downhole apparatus takes the form tri-cone roller cutter, and employs an alternative docking arrangement. For ease of reference, like components between the first embodiment and the second embodiment are represented by like reference numerals.

As shown, main body 20 has a plurality of docking slots 60 around the outer surface with a plurality of flow or debris bypass channels 26 machined into the body between the docking slots 60. The main body 20 has a throughbore 80 and a threaded pin connection 1 15 at one end to allow attachment of the main body 20 to, for example, a top sub. The main body 20 also includes nozzles 29 which, as shown in Figures 8B and 8D are in fluid communication with throughbore 80. The nozzles 29 allow for fluid jets to be directed at cutters 44 when the modular hole opener is in use. The main body 20 in this embodiment has three docking slots 60 configured to receive three roller cutter arms 40.

Docking slots 60 have a T-shape cross section which is open at the lower end of the main body 20 and blind ended at the upper end of the main body 20. In this embodiment, latch recesses 66 are formed in the sidewalls of the slot 60. Figures 7 A - 7C shows three latch recesses 66 per slot 60, although the skilled person will appreciate that any number of latch recesses may be machined into the slot.

The roller cutter arm 40 has a top portion 42 to which the roller cutter 44 is attached. When inserted into the slot 60, the top portion 42 will sit above the surface of the body 20. Latch blocks 46 are machined into the underside of the top portion 42 and are sized to correspond to the latch recesses 66 of the slots 60. The arm 40 has a bottom portion 48 with a T-shaped cross section corresponding to that of the docking slot 60. The widest part 45 of the T-shaped bottom portion 48 has a depth d that increases gradually from one end 41 of the cutter arm 44 to end 43 nearest the cutter 44, this can be seen more clearly in Figure 7C. The docking slots 60 and cutter arm 40 are designed for clearance fit with each other.

This embodiment also utilises a locking bar arrangement whereby tapered locking bars 30 are inserted into holes 28 formed in the main body 20 to lock the cutter arms 40 in place after the latch blocks 46 have latched with latch recesses 66 formed in the docking slots 60. There are two holes 28 and two locking bars 30 per docking slot 60. Therefore, in this particular embodiment six locking bars are used to secure the cutter arms 40 in position. The holes 28 are positioned such that a locking bar 30 may be inserted either side of the cutter arm 40 within the slot 60. Springs 32 are then inserted into the entry holes 28 followed by bolts 34 to secure the locking bars in position within the main body 20.

Figure 11 D illustrates a cross-sectional view of the downhole apparatus with cutter arms 40 and locking bars 30 secured in place. The widest part 45 of the T- shaped bottom portion 48 of the cutter arm 40 has a depth d which increases from end 41 to end 43. Thus, when the cutter arm 40 is latched into position within the docking slots, a tapered space 65 is formed between the widest part 45 of the T- shaped bottom portion 48 and the sidewalls of the T-shaped slot 60. The locking bar 30 is tapered to fit into this space 65.

In use, an operator may insert cutter arm 40 into docking slot 60 by positioning the T-shaped bottom portion 48 into alignment with the slot 60. The operator can then slide the cutter arm 40 into the slot until the arm 40 bottoms out on the blind end of the slot 60. In this position, the latch blocks 46 of the cutter arm 40 will be aligned with the latch recesses 66 formed in the sidewalls of the docking slot 60. In this position, the end 43 of the cutting arm is shaped to abut the open end 63 of the slot 60. The operator can then push the cutter arm 40 down such that the latch blocks 46 latch with the latch recesses 66. Locking bars 30 are then inserted into the main body 20 followed by springs 32 and bolts 34 to secure the locking bars 30 in place. As above, the forces required in this locked condition to shear or break the cutter arms 40 out of the main body 20 are the forces required to shear the latch blocks 46 from the arms 40 or to shear through the arms 40 themselves.

Referring now to Figures 14 to 19, there is shown a downhole apparatus according to another embodiment. In this embodiment, the downhole apparatus takes the form of a six armed roller cutter. The downhole apparatus is similar to the downhole apparatus shown in Figures 7 to 13 and employs the same docking arrangement as that described above with reference to the second embodiment.

As shown, main body 220 has six docking slots 60 around the outer surface with flow or debris channels 226 machined into the body between the docking slots 260. Nozzles 229 are also present within the body to allow jets of fluid to be directed at the cutting elements attached to the main body, when in use. The main body 220 is connected to a top sub 200 via box connection 215 and is also connected to lower connections 222 and 223, with a throughbore 280 extending throughout.

The six docking slots 260 are configured to receive six roller cutter arms 240. When the cutter arms 240 are latched in position, a tapered space 265 is formed between the slot 260 and the arm 240. Tapered locking bars 230 are inserted into the main body 220 via entry holes 228 to fill this space 265. There are two entry holes 228 and two locking bars 230 per docking slot 260 such that the locking bars extend into the space 265 created on either side of the cutter arm 240 when latched in the docking slot 260. Locking bars 230 are biased in the entry holes 228 using springs 232.

This embodiment further utilises a cap 250 and rim 252 to further secure the locking bars in position. Thus, the locking bars 230 have two separate securing mechanisms. The locking bars 230 can only escape from the holes 228 if both the cap 250 and rim 252 screw connection and spring 232 arrangement fail.

In use, an operator will assemble the main body 220 with the top sub 200 and lower connections 222 and 223 before inserting cutter arm 240 into a docking slot 260 by positioning the T-shaped bottom portion 248 into alignment with the slot 260. The operator can then slide the cutter arm 240 into the slot 260 until the arm 240 bottoms out on the blind end of the slot 260. In this position, the latch blocks 246 of the cutter arm 240 will be aligned with the latch recesses 266 formed in the sidewalls of the docking slot 260. In this position, the end 243 of the cutting arm 240 is shaped to abut the open end 263 of the slot 260. The operator can then push the arm 240 down such that the latch blocks 246 latch with the latch recesses 266. Locking bars 230 are then inserted into the main body 220 followed by spring 232 securing and biasing the locking bars 30 in place. The cap 250 is position over the top sub 200 then screwed onto the main body 220 followed by the rim 252.

Referring now to Figures 20 to 25, there is shown a downhole apparatus according to another embodiment. In this embodiment, the downhole apparatus takes the form of a "hybrid" or mixed cutter modular hole opener incorporating both roller cutter arms 340 and fixed drag cutter arms 370. The downhole apparatus is similar to the downhole apparatus shown in Figures 7 and 14 and employs the same docking arrangement as that described above with reference to the second and third embodiments.

As shown, arms 340, 370 are arranged to alternate between roller cutters 344 and fixed drag cutters 374, although in this particular embodiment rim 252 is positioned onto main body 320 before the cap 350 is screwed onto the body 320. It should be understood that the embodiments described are merely exemplary of the present invention and that various modifications may be made without departing from the scope of the invention.

For example, the main body of any of the previously described embodiments may be configured to receive any number of cutter arms by selecting a body with the appropriate number of docking slots.

Galleries may be provided in the main body of the hole opener, for example to provide fluid lubrication and communication.

The standardised docking and locking arrangements described not only introduce flexibility into the selection of tool elements for use in a hole opener or drill bit but also provide a secure and reliable attachment mechanism which is superior to previously available attachment mechanisms. For example, and referring to Figures 26A-26D, it will be recognised that the modular nature of the described apparatus allows the operator to select the number and type of cutter or tool element as required for a particular operation. It will be also be appreciated that any of the above embodiments may utilised with stabiliser arm attachments rather than cutter arms, wherein the stabiliser arm will have the standardised docking arrangement required to engage with docking slots formed on the main body of the tool.