BACKGROUND The cutting components or drilling means of the drilling tool are designed to withstand forces which are in the main parallel to the longitudinal axis of the drilling tool. However, in the case of horizontal drilling, the weight of the tool and other components attached to the tool also results in loads being applied which are transverse of the longitudinal axis of the tool. Many cutting elements are not designed to withstand this side loading and it results in inaccuracies in the hole being drilled. Therefore it is an object of this invention to reduce side loads applied to a cutting element in drilling tools used for horizontal drilling applications.

Horizontal drilling operations normally involve the commencement of a bore hole at one location which travels in a shallow arc to emerge at another location distant from the entry point. Such horizontal drilling techniques are commonly used to provide a conduit beneath a riverbed, canal or harbour. The conduits are used for laying services such as water and sewage or electrical supply.

It should be understood that such horizontal drilling also includes drilling bore holes which are at an incline from the horizontal. The invention will be useful in those applications where the angle of inclination produces side loadings being applied to the cutting tools which would otherwise result in damage to the cutting elements.

The invention is particularly directed to devices used to enlarge a previously drilled bore hole using a reaming action. That is the bore hole is enlarged from the diameter of the previously drilled bore hole to a second larger diameter bore hole.

It is common practise when drilling a substantially horizontal bore hole to progressively enlarge the diameter of the bore hole with second and subsequent passes.

A disadvantage with previously known horizontal bore hole drilling tools is that the drilling means or cutting tools support the drilling tool in the second enlarged bore hole. Typically, roller cones are used as the cutting tools. The roller cones are normally supported by a bearing assembly on a journal or leg. The periphery of the roller cone is normally slightly proud of the surface of the journal or leg which results in any side loading being applied to the periphery of the roller cone.

The bearing supports for the roller cone and the inserts on the roller cones are not well suited to withstanding these side loads which results in failure of the bearings and inserts after a relatively short period of time. Also, the side loads result in a non-round hole being drilled. This unstabilised movement is called"walking"in the hole where the centre axis of the drilling tool oscillates resulting in an oversize hole that is not round. The penetration rate of the drill is reduced and the cutting tools do not work efficiently.

These problems have previously been tolerated and it is accepted that existing tools have somewhat reduced life expectancy as a result of side loading.

It is the object of the invention to provide a device which has improved efficiency.

BRIEF DESCRIPTION OF THE INVENTION In its broadest form the invention is said to reside in a drilling tool used in a horizontal drilling application for enlarging the diameter of a previously drilled bore hole, the drilling tool comprising; a body, drilling means on said body for enlarging said previously drilled bore hole to a second larger diameter bore hole, a guide extending forwardly of said body that locates in said previously drilled hole to guide the movement of said drilling tool along said previously drilled bore hole, and at least one stabilising surface located rearwardly of said drilling means so that said stabilising surfaces abut the wall of said second bore hole thereby reducing those loads that are applied to said drilling means which are transverse to the longitudinal axis of said drilling tool.

According to the invention, the drilling tool uses a guide which is of the same or slightly smaller diameter of the previously drilled bore hole. The guide supports the forward end of the drilling tool and ensures that the drilling tool progresses along the same path as the previously drilled bore hole.

The improvement in this invention is to provide at least one stabilising surface positioned rearwardly of the drilling means so that the drilling tool is supported horizontally within the bore holes being drilled by the guide and the stabilising surface. Side loads applied to the drilling means are reduced due to it being supported between the guide and the stabilising surface and the tool is maintained substantially on the centre line of the previously drilled hole. This improves both the efficiency and rate of drilling.

The stabilising surface may comprise the outer surface a plurality of ribs that each extend along the longitudinal axis of the drilling tool and that are spaced around the periphery of the drilling tool. Alternatively, the stabilising surface may comprise the outer surface of a ring that is positioned rearwardly of the drilling means or may comprise the outer surface of a plurality of pads that are spaced around the periphery of the drilling tool and located rearwardly of the drilling means.

The stability surfaces may comprise a combination of all or any of the longitudinal ribs, a ring or rings or pads.

Further, the stability surface may comprise a plurality of radially spaced rollers that have an axis of rotation that is parallel to the longitudinal axis of the drilling tool.

The surface of the guide may be cylindrical to fit into the previously drilled bore hole, but may also have a plurality of longitudinal ribs or spaced rollers around the periphery of the guide. In addition, helical longitudinal ribs may be used on the surface of the guide or as the supporting surface behind the drilling means.

The combination of the guide and stability surface according to the invention assists the drilling tool in supporting the load of the drilling tool and the load applied by other components which may be connected to it. These components would include the means used to rotate the drilling tool such as a mud motor, connections and fittings between the mud motor and the drilling tool and further stabilising elements. Typically, there is clearance between the outer diameter of the mud motor and bore hole. The weight of the motor causes a significant load that is transverse to the axis of the drilling tool during horizontal drilling. The drill tube which conveys fluid under pressure to the mud motor also can apply significant side loads to the drilling tool. This is the case even though additional stabilisers may be used uphole of the mud motor along the drill tube.

A further aspect of the invention comprises a use of the above described drilling tool for back reaming. In conventional drilling, lengths of drill string are gradually added as the drilling tool progresses along the previously drilled bore hole. In the case of back reaming, the drilling tool is pulled through a previously drilled hole so that the drill rod is gradually disconnected as the drilling tool advances along the previously drilled bore hole.

In this aspect of the invention, the end of the guide of the drilling tool is attached to a series of connected drill pipes. The drill pipe conveys pressurised fluid which is transferred through conduits within the drilling tool to a mud motor which is located behind the drilling tool. The mud motor is used to rotate the drilling tool while tension is applied to the string of drill tubes to pull the drilling tool along the previously drilled bore hole.

This aspect of the invention has the advantage of placing the drill string under tension to achieve drilling of the hole rather than using compressive forces within the drill string in the case of conventional drilling.

In this embodiment the guide and the stabilising surface still support the load on the drill bit thereby increasing the accuracy of drilling.

BRIEF DESCRIPTION OF THE DRAWINGS In order for the invention to be more fully understood, preferred embodiments will now be described. However, it should be realised that the invention is not to be restricted to the precise features described in the embodiments.

Embodiments are illustrated in the accompanying diagrams in which; Fig 1 shows a cross sectional view of a horizontal drilling operation, Fig 2 shows a side view of a drilling tool according to a first embodiment, Fig 3 shows a perspective view of a drilling tool shown in Fig 2, Fig 4 shows a perspective view of a drilling tool according to a second embodiment of the invention, Fig 5 shows an assembly of a drilling tool, mud motor and spiral stabiliser, and Fig 6 shows the assembly of a drilling tool using roller stabilisers in combination with a roller stabiliser.

DESCRIPTION OF A PREFERRED EMBODIMENT Fig 1 shows a typical horizontal boring process under a waterway in which a horizontal bore hole generally shown as 10 is being formed. The horizontal bore hole 10 starts at one point and exits the surface at another point. The bore hole 10 has an arcuate path which enables it to pass under certain features such as roadways and a canal as shown in Fig 1. As seen in Fig 1, the bore hole 10 is generally always at an angle with respect to the horizontal. This angle is generally fairly slight which results in significant loads being applied transversely to the longitudinal axis of drilling equipment used to form the bore hole 10.

Also seen in Fig 1 is a drill rig 11 which is positioned at the commencement of the bore hole and is used to apply a longitudinal force to the drill string to advance the drilling tool along a bore hole 10 and to rotate the drill string.

As shown in more detail in the enlargement in Fig 1, there is a first bore hole 9 of a first diameter which has already been drilled and then the drill rig 11 is used to drill out the hole 9 to a larger diameter hole 8 using the drilling tool 7 operated by a drill string 6 from drilling rig 11.

Fig 2 shows a side view and Fig 3 shows a perspective view of a drilling tool 12 in accordance with this invention. The drilling tool 12 comprises a central body 13 to which drilling means are attached. In this embodiment the drilling means comprises a plurality of cone rollers 14. Cone rollers 14 are well know in the industry and have hardened inserts positioned within the surface of each cone roller. These hardened inserts are used to provide a compressive impact load to the surface of the rock which results in chipping or shattering of the rock. This progressive chipping or shattering enables advancement of drilling tool 12.

The drilling tool 12 also comprises a guide 15 which is located at the forward end of the drilling tool 12. Guide 15 locates within the previously drilled hole 16 which is also referred to as a pilot hole. The guide 15 is used to support the forward end of the drilling tool 12 and to also ensure that the drilling tool 12 progresses along the required path.

The previously drilled hole 16 is enlarged by the cone rollers 14 to a second diameter bore hole 17. Stabiliser surfaces 18 between and behind the cone rollers 14 are used to support the drilling tool 12 between the previously drilled bore hole 16 and the second larger diameter bore hole 17. The stabiliser surfaces 18 abut against the wall of the second diameter bore hole 17.

In the embodiment shown in Fig 2, the stabiliser surfaces 18 comprise a plurality of radially spaced ribs 20 where the outer surface of each rib 20 comprises the stabiliser surface 18. In addition, a ring 21 is placed at the rear of the drilling tool 12 at the ends of each rib 20. The outer surface of the ring comprises the stabiliser surface 18.

As can be seen from the invention illustrated in Figs 2 and 3, any side loading which may be applied to the drilling tool 12 will be supported by the guide 15 and the stabiliser surfaces 18. This will reduce the side loading which is applied to the cone rollers 14 which are positioned between these two elements.

The guide 15 comprises supporting surfaces 23 which are formed on either extensions 24 of the ribs 20 or additional ribs 25 that extend along the guide 15.

The drilling tool 12 has a spigot 26 with a threaded connection 27 to enable the drilling tool 12 to be attached to other drilling equipment such as a mud motor or drill string. The drilling tool 12 has a central bore (not illustrated) which extends through the threaded connection 27, spigot 26, body 13 and guide 15. The central bore opens to the end of the guide 15 and there is a female threaded coupling within the bore opening at the end guide 15. A drill string (not shown) may extend from this threaded coupling to the end of the drill hole furthest from the drill rig and enable the drilling toll to be pulled through the drilled hole to provide back reaming.

Angled ribs 19 extend from the ring 21 to the spigot 26. The ribs 19 provide back reaming as the drilling tool 12 is withdrawn and also assist the re-entry of the drilling tool 12 into tubular casing that may partially extend into the hole being drilled from its entry point. Normally such casing terminates in solid or consolidated ground. Re- entry of a drilling tool into the casing is not normally a problem. However, if the ground is soft or the hole at the end of the casing is otherwise enlarged at the end of the casing, a conventional drilling tool can catch and prevent its further withdrawal.

The ribs 19 ensure that the drilling tool 12 is guided into the casing so that it does not catch. Similar ribs can be used on other components such as stabilisers.

Fig 4 illustrates a second embodiment of the invention. This embodiment also comprises a guide 15, a body 13 and stabilising surfaces 18. However, in this embodiment, the stabilising surfaces 18 comprise external surfaces of longitudinal ribs 20 and pads 29 that are attached to a ring 21. This results in a gap between the surfaces 18 on the pads 29 and the surfaces 18 on the ribs 20. This assists in the withdrawal of the drilling tool 12 and allows drilling debri to pass across the drilling tool 12 rather than accumulating at the rear of the drilling tool 12 which may result in the drilling tool 12 jamming within the bore hole.

Fig 5 shows an assembly of a drilling tool 12 with a mud motor 30. The diameter of the mud motor 30 is less than the second bore hole 17 so it is supported by the drilling tool 12 and a spiral stabiliser 31 which has stabilising surfaces 33 on their external surfaces. The drill tube 32 extends out of the hole from the stabiliser 31.

The mud motor, which is suspended within the second bore hole 17 obviously applies a significant load to the drill tool 12. However, minimal side loading is applied to the roller cones 14 as a result of the drilling tool 12 being supported by the guide 15 and the stabilising surfaces 18 on the drilling tool 12 and stabilising surfaces 33 on the spiral stabiliser 31.

An alternative arrangement for the drilling tool is illustrated in Fig 6. In this embodiment, a plurality of rollers 34 are fitted to the guide 15. Preferably, the rollers 34 are installed within the extensions 24 of the ribs 20 and the additional ribs 25.

Rollers may also be installed in the drilling tool 12 so that they comprise the stabiliser surfaces 18. In this case, the rollers 34 would also be installed within the ribs 20.

Rollers 34 are also mounted in the stabiliser 31. The stabiliser 31 shown in Figs 5 and 6 may also be provided with angled ribs 19 which facilitate entry into the casing as the tool and stabiliser 31 are being withdrawn.

In addition to longitudinal ribs 20 and additional ribs 25, both ribs 24 and 25 may be helically shaped to ensure that a stabilising surface 18 or a support surface 23 is always in contact with either the second bore hole 17 or the bore hole 16 as the drill tool 12 is rotated. In addition, the twists of the helical flutes can be such that in combination with the rotating of the drill tool, drilling debris is drawn rearwardly along the guide 15 and the drill tool 12.

Another aspect of the invention comprises the use of the drilling tool 12 in a back reaming assembly. In this embodiment, the drilling tools shown in Figs 2 to 4 can have a drill tube connected to the forward end of the guide 15. A mud motor can be connected to the rear of the drilling tool 12 with pressurised drilling fluid flowing through the drilling tool 12 to power the mud motor. Tension can then be applied to the drill tube to apply the drilling force to the roller cones 14 while the necessary rotation of the drilling tool 12 is caused by the mud motor. In this embodiment the drill tube is operated in tension rather than in compression which is the case with conventional drilling. Tensile force does not cause buckling of the drill rod which occurs when compressive forces are applied. Accordingly, much smaller diameter drill tube can be used to withdraw a comparatively large diameter drill tool 12.

With this embodiment, a pilot hole is first drilled. Rather than withdrawing the drill string, the first drilling tool and other components are removed from the drill string as they emerge so that the back reaming drilling tool 12 and mud motor may be attached. The same drill string is then used to draw the drilling tool 12 back through the pilot hole to produce a second larger diameter bore hole.

As will be seen from the above description, the invention provides an improved drilling tool which is better suited to withstanding side loads that normally result from horizontal drilling operations. Accordingly, it is expected that drilling tools according to the invention will have an increased operating life which will significantly improve the efficiencies of drilling operations.