GB2588962A

GB2588962A - Self-regulating brake

Info

Publication number: GB2588962A
Application number: GB1916699.0A
Authority: GB
Inventors: Peter Wallis Nicholas; Baker Nicholas
Original assignee: Gyro Tech Ltd
Current assignee: Gyro Tech Ltd
Priority date: 2019-11-15
Filing date: 2019-11-15
Publication date: 2021-05-19
Anticipated expiration: 2039-11-15
Also published as: GB2588962B; GB201916699D0

Abstract

A borehole tool 100 for inserting into a borehole such that the borehole tool descends within the borehole under gravity. The borehole tool comprises at least one sensor 102 configured to collect data indicative of a property of the borehole and a braking assembly 103. The braking assembly comprises at least one wheel 104a - d configured to contact a surface within the borehole, and rotate as the borehole tool descends within the borehole, and a brake 122a - d configured to apply a braking force to the at least one wheel on rotation thereof to resist rotation of the at least one wheel. The braking force applied to the at least one wheel is in dependence on a rotation rate of the at least one wheel. The braking assembly may comprise an eddy current braking assembly and the brake may comprise an electrically conductive element and at least one magnet.

Description

Self-regulating brake

Technical field

This invention relates to borehole tools and braking assemblies for use with borehole tools.

Background

In some applications it is desired to accurately track the path of a drilled borehole. This typically involves using a borehole tool to take measurements, along the length of a borehole, that are indicative of azimuth (the angle of the borehole with respect to north), inclination and/or the depth of (or distance along) the borehole. These measurements enable a complete three-dimensional map of the borehole to be constructed. The collection of data indicative of some, or all, of these measurements may be referred to as a "survey".

In order to collect these measurements along the borehole, it is common to deploy the borehole tool into and along the length of the borehole using a conducting wireline cable or a slickline. The borehole tool may be a borehole survey tool. This allows the borehole tool to be conveyed into and out of the borehole in a controlled manner, however the time expended in lowering and recovering the borehole tool can interrupt the drilling process.

A more rapid deployment into the borehole can be achieved by dropping the borehole tool into the borehole and allowing it to descend within the borehole under gravity (or freefall within the borehole). In order to limit the shock and vibration experienced by the borehole tool, particularly on impact at the bottom of the borehole, it may be necessary to reduce the transit speed of the borehole tool.

In oil and gas applications, boreholes are typically filled with a drilling fluid and so the transit speed of the borehole tool is reduced by fluid resistance. The extent to which the borehole tool is slowed down depends on the viscosity of the drilling fluid, and when low viscosity drilling fluids are utilised, it may be necessary to utilise additional means to reduce the transit speed of the borehole survey instrument. For example, US6209391 discloses the use of buoyancy aids.

In some situations, a borehole survey may be desired for a borehole in which little or no drilling fluid is present for some or all of the borehole length. This may be because circulation of the drilling fluid is lost due to high permeability, cavernous or fractured rock formations. In the absence of the drilling fluid, the borehole tool may attain a high terminal velocity and be subjected to high levels of shock on impact at the bottom of the borehole. In some cases, the shock experienced on impact may catastrophically damage the borehole tool such that no further survey measurements can be taken.

Summary

According to a first aspect of the invention, there is provided a borehole tool for inserting into a borehole such that the borehole tool descends within the borehole under gravity, the borehole tool comprising at least one sensor configured to collect data indicative of a property of the borehole, and a braking assembly comprising at least one wheel configured to contact a surface within the borehole, and rotate as the borehole tool descends within the borehole, and a brake configured to apply a braking force to the at least one wheel on rotation thereof to resist rotation of the at least one wheel, wherein the braking force applied to the at least one wheel is in dependence on a rotation rate of the at least one wheel.

Optionally, a greater rotation rate of the at least one wheel causes a greater braking force to be applied to the at least one wheel.

Optionally, the brake comprises a self-regulating brake.

Optionally, the braking assembly further comprises a biasing member configured to bias the at least one wheel outwardly with respect to the borehole tool.

Optionally, the at least one wheel is rotatably coupled to the borehole tool by a lever, and wherein the biasing member is configured to rotate the lever outwardly with respect to the borehole tool.

Optionally, the braking assembly comprises a plurality of wheels. Optionally, the braking assembly comprises a pair of wheels.

Optionally, the wheels are coupled to opposed sides of the borehole tool.

Optionally, the braking assembly comprises a further pair of wheels coupled to opposed sides of the borehole tool, wherein the pair of wheels and the further pair of wheels are angularly displaced relative to each other about a longitudinal axis of the borehole tool.

Optionally, the pair of wheels and the further pair of wheels are angularly displaced relative to each other about the longitudinal axis by 90 degrees.

Optionally, the pair of wheels and the further pair of wheels are offset along the longitudinal axis of the borehole tool.

Optionally, the borehole tool further comprises a further braking assembly coupled to the borehole tool.

Optionally, each braking assembly is coupled proximal to a rear and a forward end of the borehole tool respectively.

Optionally, the borehole tool further comprises an odometer configured to measure the distance the at least one wheel rotates through.

Optionally, the braking assembly comprises an eddy current braking assembly.

Optionally, the brake comprises an electrically conductive element and at least one magnet.

Optionally, one of the electrically conductive element and the at least one magnet is coupled to the at least one wheel and configured to rotate therewith, and the other of the electrically conductive element and the at least one magnet is fixed relative to the borehole tool, the at least one wheel configured to rotate with respect to the borehole tool, and the relative rotation causes the braking force to be applied to the at least one wheel.

Optionally, the brake comprises a plurality of magnets angularly spaced about the circumference of the at least one wheel.

Optionally, the odometer comprises a sensor configured to detect a position of the at least one magnet to determine the distance the at least one wheel rotates through.

Optionally, the brake comprises a fluid-filled cavity, and wherein rotation of the at least one wheel is resisted by a fluid within the fluid-filled cavity.

Optionally, a surface of the at least one wheel forms at least part of a wall of the fluid-filled cavity.

Optionally, the fluid of the fluid-filled cavity applies the braking force to the surface of the at least one wheel on rotation thereof.

Optionally, the brake further comprises at least one fin configured to extend within the fluid filled cavity.

Optionally, the at least one fin is coupled to the wheel such that rotation of the at least one wheel causes rotation of the at least one fin within the fluid-filled cavity.

Optionally, the rotation of the at least one fin is resisted by the fluid within the fluid-filled cavity to cause the braking force to be applied to the at least one wheel.

According to the invention in a further aspect, there is provided a braking assembly for a borehole tool for inserting into a borehole such that the borehole tool descends within the borehole under gravity, the borehole tool comprising at least one sensor configured to collect data indicative of a property of the borehole, the braking assembly comprising at least one wheel configured to contact a surface within the borehole, and rotate as the borehole tool descends within the borehole, and a brake configured to apply a braking force to the at least one wheel on rotation thereof to resist rotation of the at least one wheel, wherein the braking force applied to the at least one wheel is in dependence on a rotation rate of the at least one wheel.

Brief description of drawings

Figure 1 shows an exemplary borehole tool; Figure 2 shows a section view of an exemplary wheel and brake; Figure 3 shows a plan view of an exemplary wheel; Figure 4 shows a section view of an exemplary wheel and brake; Figure 5 shows a section view of an exemplary wheel and brake; and Figure 6 shows a plan view of an exemplary wheel.

Detailed description

Generally disclosed herein are borehole tools for inserting into a borehole such that the borehole tool descends within the borehole under gravity. The borehole tool may comprise at least one sensor configured to collect data indicative of a property of the borehole. The at least one sensor may be utilised to obtain a borehole survey, as described herein. For example, the at least one sensor may comprise a gyro sensor configured to collect data indicative of azimuth and/or an accelerometer configured to collect data indicative of inclination. The borehole tool may further comprise a braking assembly comprising at least one wheel and a brake configured to apply a braking force to the at least one wheel on rotation thereof. The braking force applied by the brake may be in dependence on a rotation rate of the at least one wheel. That is, the brake may comprise a self-regulating brake. This allows the speed that the borehole tool travels through the borehole to be regulated dynamically without any input from the operator/user once the borehole tool is inserted into the borehole. For example, the brake may be configured such that a greater rotation rate of the at least one wheel causes a greater braking force to be applied to the at least one wheel.

Figure 1 shows an exemplary borehole tool 100. The borehole tool 100 comprises at least one sensor 102 and a braking assembly 103. The exemplary borehole tool 100 shown in Figure 1 comprises one braking assembly 103, however the skilled person will appreciate that in alternative arrangements, a plurality of braking assemblies may be provided. For example, exemplary borehole tools may comprise two or more braking assemblies. One braking assembly may be proximal to a forward end and one braking assembly may be proximal to a rearward end of the borehole tool.

The at least one sensor 102 may comprise a rate sensor, such as a gyro sensor, configured collect data indicative of the earth's rotation. The data collected by the rate sensor may be utilised to calculate an azimuth of the borehole. The skilled person will appreciate that data indicative of the earth's rotation may be collected using gyrocompassing, which provides an absolute measurement of azimuth at each survey point, or continuous methods in which change of azimuth is measured as the borehole tool is conveyed along the borehole. In such continuous methods, the data indicative of the earth's rotation may comprise data collected relating to rotation of the gyro sensor and/or borehole tool. The at least one sensor may further, or alternatively, comprise an inclination sensor, such as an accelerometer, configured to collect data indicative of inclination of the borehole.

The braking assembly 103 may comprise at least one wheel 104. The exemplary braking assembly 103 shown in Figure 1 comprises four wheels 104a-d, however the skilled person will appreciate that alternative braking assemblies may comprise any number of wheels. The wheels 104a-d are configured to rotate with respect to the borehole tool 100. The wheels 104a-d are configured to rotate about an axis transverse to a longitudinal axis 106 of the borehole tool 100.

The wheels 104a-d may be arranged along the longitudinal axis 106 of the borehole tool 100. In the exemplary borehole tool 100, the wheels 104a-d may be arranged in pairs. That is, the braking assembly may comprise a pair of wheels 108a comprising wheels 104a and 104b, and a pair of wheels 108b comprising wheels 104c and 104d. Again, the skilled person will appreciate that the borehole tool may comprise any number of pairs of wheels. In alternative arrangements, the wheels 104a-d may not be arranged in pairs. For example, the skilled person will appreciate that the borehole tool may comprises a plurality of wheels that may be offset from one another (either along the longitudinal axis 106 of the borehole tool 100 and/or angularly about the longitudinal axis 106 of the borehole tool 100). In alternative arrangements, at least one of the wheels 104a-d may be arranged such that it is opposed to a brake, such as a static frictional pad located on or coupled to the borehole tool 100.

The wheels 104a, 104b and 104c, 104d of each pair 108a, 108b may be located at substantially the same length along the longitudinal axis 106. The skilled person will appreciate however that different arrangements of the wheels 104a-d are possible. In alternative borehole tools, all of the wheels 104a-d may be longitudinally offset from each other along the longitudinal axis 106.

In the example of Figure 1, the wheels 104a,b and 104c,d of each pair 108a, 108b are coupled to opposed sides of the borehole tool 100. The pair of wheels 108a are offset from the pair of wheels 108b along the longitudinal axis 106 of the borehole tool 100.

The pair of wheels 108a may be angularly displaced relative to the pair of wheels 108b about the longitudinal axis 106. In the exemplary borehole tool 100, the pair of wheels 108a are angularly displaced relative to the pair of wheels 108b about the longitudinal axis 106 by 90 degrees. As such, the wheels 104a-d of the exemplary borehole tool are arranged in 90-degree intervals around the longitudinal axis. This is advantageous as the wheels can be used to centralise the borehole tool 100 within the borehole.

Exemplary borehole tools may further comprise at least one biasing member 110 configured to bias at least one of the wheels 104a-d outwardly with respect to the borehole tool 100. In exemplary borehole tools, the at least one biasing member 110 may be configured to bias the at least one wheel 104 outwardly such that the at least one wheel 104 contacts an inner surface of the borehole. In exemplary borehole tools, the biasing member 110 may be configured to bias the at least one wheel against the inner surface of the borehole (that is, a force may be applied to the wheel to push it into contact with the inner surface of the borehole). The borehole tool 100 shown in Figure 1 comprises a biasing member 110a-d for each wheel 104a-d such that all of the wheels 104a-d are biased outwardly with respect to the borehole tool 100 (only the biasing members 110a and 110b are visible in Figure 1). The biasing members 110a-d may comprise compression springs, however the skilled person will appreciate that alternative biasing members may be utilised, such as leaf springs, torsion springs, any other form of spring and/or any element manufactured from a resiliently deformable material and biasing a wheel 104a-d outwards.

At least one of the wheels 104a-d may be coupled to the borehole tool 100 by a lever 112. The lever 112 may be rotatably coupled to the borehole tool 100 such that the at least one wheel 104a-d is moveable outwardly from the borehole tool 100. The at least one wheel may be rotatably coupled to the lever 112, for example using a bushing or bearing. In the exemplary borehole tool 100 of Figure 1, each of the wheels 104a-d are rotatably coupled to the borehole tool 100 by respective levers 112a-d. The levers 112a-d are rotatably coupled to the borehole tool 100 at pivot points 114 (only pivot point 114a is visible in Figure 1) proximal to an end of the respective lever 112a-d.

In the exemplary borehole tool 100, the levers 112a-d are biased outwardly from the borehole tool 100 by the respective biasing members 110a-d. Since the wheels 104a-d are coupled to respective levers 112a-d, the biasing members 110a-d therefore bias the wheels 104a-d outwardly with respect to the borehole tool 100. In the exemplary borehole tool 100 of Figure 1, the biasing members 110a-d are held in a primed state between the respective lever 112a-d and a surface of the borehole tool 100. The biasing members 110a-d are configured to exert a force on the respective levers that acts to rotate the levers 112a-d outwardly from the borehole tool 100 about respective pivot points 114a-d (only pivot point 114a is visible in Figure 1). Since the wheels 104a-d are coupled to respective levers 112a-d, the wheels 104a-d are also biased outwardly from the borehole tool 100. That is, the axis of rotation of the wheels 104a-d is biased outwardly from the longitudinal axis 106 of the borehole tool 100.

The borehole tool 100 may further comprise at least one retaining pin 118a. The retaining pin 118a may be configured to limit the extent of rotation of the lever 112a under the force of the biasing member 110a. In the exemplary borehole tool 100 of Figure 1, the retaining pin 118a may be configured to limit the extent of the rotation of the lever 112a to substantially 5 degrees from the longitudinal axis 106 of the borehole tool 100. In alternative arrangements, the retaining pin 118a may be configured to limit the rotation of the lever 112a to substantially 3 degrees, 4 degrees, 6 degrees, 8 degrees and 10 degrees from the longitudinal axis 106. In some arrangements, the position of the retaining pin 118a may be adjustable, such that the maximum extent of rotation of the lever 112 may be adjusted. The retaining pin may be configured to engage with a corresponding recess 120a on the lever. The retaining pin 114a is positioned such that the point at which the retaining pin 114a engages the recess 120a defines the maximum rotation of the lever 112a outwardly with respect to the borehole tool 100. In alternative arrangements, the lever 112a may not comprise a recess and a substantially planar surface of the lever may engage the retaining pin to limit rotation. The exemplary borehole tool 100 comprises four retaining pins 118a-d corresponding to each wheel 104a-d and lever 112a-d.

The maximum rotation of the levers 112a-d may be set such that the diameter of the borehole tool (defined by the outer extent of the wheels 104a-d) is greater than the diameter of the borehole into which the borehole tool 100 is to be inserted. For example, in exemplary arrangements, the maximum rotation of the levers 112a-d may be set such that the diameter of the borehole tool (defined by the outer extent of the wheels 104a-d) is substantially 10 mm larger than the internal dimensions of the borehole into which the borehole tool is to be inserted. In alternative arrangements, the maximum rotation of the levers 112a-d may be set such that the diameter of the borehole tool (defined by the outer extent of the wheels 104a-d) is substantially 5 mm, substantially 8 mm, substantially 9 mm, substantially 11 mm or substantially 12 mm larger than the internal dimensions of the borehole into which the borehole tool is to be inserted. This means that when the borehole tool is inserted into the borehole, the wheels are pushed inwardly. The levers 112a-d are therefore rotated inwardly and the biasing members 110a-d are further compressed. As such, the recesses 120a-d of the levers are disengaged from the retaining pins 114a-d, and the force of the biasing member acts to push the wheel against an inner surface of the borehole, rather than push the lever 120a-d against the retaining pin 114a-d. The borehole may comprise a drill string or pipe, and the borehole tool 100 may be inserted into the drill string or pipe. Typically the internal diameter of the drill string/pipe may be 70 mm or 80 mm, although the skilled person will appreciate the internal diameter may vary along the length of the borehole, and the borehole tool 100 may be dimensioned to correspond to the internal dimensions of the drill string/pipe.

The braking assembly 103 may further comprise at least one brake 122 configured to apply a braking force to a corresponding wheel 104a-d on rotation thereof to resist rotation of the corresponding wheel 104a-d. The brake 122 may be configured to apply a braking force in dependence on the rate of rotation of the corresponding wheel 104a-d. In some arrangements, the brake 122 may be configured to apply an increased braking force to the corresponding wheel 104a-d as the rate of rotation of the corresponding wheel 104a-d increases. In the exemplary borehole tool 100 shown in Figure 1, all of the wheels 104a-d have corresponding brakes 122a-d. The skilled person will appreciate however that in alternative arrangements, any number of the wheels 104a-d may have corresponding brakes, and as such, some wheels may not have an associated brake.

Figure 2 shows a section view of an exemplary wheel and a brake. Figure 2 shows the wheel 104a and the brake 122a, however the skilled person will appreciate that the below description may equally apply to any of the wheels 104b-d and the brakes 122b-d. Broadly, the brake 122a is configured to apply a force restricting rotation of the wheel 104a based on a rate of rotation of the wheel 104a. One example brake is described herein, but others are possible within that scope.

The exemplary brake 122 may comprise an eddy current brake. The exemplary brake 122 may comprise an electrically conductive element 124 and at least one magnet 126.

The electrically conductive element may comprise copper or aluminium, for example.

In some arrangements, copper may be preferred due to its higher electrical conductivity, which may provide a larger braking force (as explained below). However, the skilled person will appreciate that any electrically conductive material may be utilised for the electrically conductive element 124. The exemplary brake 122 shown in Figure 2 comprises a plurality of magnets 126a-n, specifically eight magnets, however the skilled person will appreciate that substantially any number of magnets could be utilised.

In the exemplary brake 122 shown in Figure 2, the electrically conductive element 124 may fixed relative to the borehole tool 100. Since the wheel 104a is configured to rotate relative to the borehole tool 100, the electrically conductive element 124 may also be fixed relative to the wheel 104a. The plurality of magnets 126a-n may be coupled to the wheel 104a and configured to rotate therewith. As such, relative movement between the electrically conductive element 124 and the plurality of magnets 126a-n may occur when the wheel 104a rotates. In alternative arrangements, the electrically conductive element 124 may be moveable with respect to the borehole tool 100, so long as relative movement between the electrically conductive element 124 and the plurality of magnets 126a-n occurs on rotation of the wheel 104a.

The skilled person will appreciate that in alternative arrangements, the plurality of magnets 126a-n may be fixed relative to the borehole tool 100 and the electrically conductive element 124 may be coupled to the wheel and configured to rotate therewith.

The plurality of magnets 126a-n may be angularly spaced around the circumference of the wheel 104a-d, as shown in Figure 3. In the exemplary arrangement of Figure 3, the plurality of magnets may be equally spaced. The skilled person will appreciate that alternative arrangements may also be utilised. The exemplary wheel 104a of Figure 3 comprises eight magnets, however the skilled person will appreciate that substantially any number of magnets could be used.

As shown in Figure 2, the plurality of magnets 126a-n may be located in corresponding recesses 128a-n of the wheel 104a. In alternative arrangements, the wheel 104a may not comprise recesses, and the plurality of magnets 126a-n may be adhered to a surface of the wheel 104a.

The wheel 104a may be rotatably coupled to the electrically conductive element 124. In the exemplary arrangement shown in Figure 2, the wheel 104a is rotatably coupled to the electrically conductive element 124 by a bearing or bushing 130.

The wheel 104a may be coupled to the electrically conductive element 124 such that a separation is defined between the plurality of magnets 126a-n and the electrically conductive element 124. The separation between the electrically conductive element 124 and the plurality of magnets 126a-n may be one of substantially 5 mm, substantially 4 mm, substantially 3 mm, substantially 2 mm and substantially 1 mm. As will be apparent to the skilled person, the plurality of magnets 126a-n and the electrically conductive element could be separated by substantially any distance provided that that magnetic field produced by the plurality of magnets 126a-n is able to extend into the electrically conductive element 124. In the exemplary arrangement shown in Figure 2, faces 132a-n of each of the plurality of magnets 126a-n may be substantially parallel with a face 134 of the electrically conductive element 124.

The operation of the borehole tool 100 will now be described with reference to Figures 1-3.

The borehole tool 100 is inserted into a borehole. As described above, the borehole may comprise a drill string or pipe, and as such inserting the borehole tool 100 into the borehole encompasses inserting the borehole tool 100 into the drill string or pipe, or other structures within the borehole.

The borehole tool 100 may be inserted into the borehole in any orientation. For example, in exemplary borehole tools 100 which comprise a single braking assembly 103 located proximal to an end of the borehole tool 100, either the braking assembly end or the non-braking assembly end may be inserted into the borehole first. In exemplary arrangements, the borehole tool 100 may comprise two braking assemblies 103 located proximal to either end of the borehole tool 100. In such arrangements, the borehole tool may be inserted into the borehole in either orientation.

As discussed above, in the exemplary arrangement in the drawings, the wheels 104a-d are biased outwardly with respect to the borehole tool 100 by biasing members 110a-d. Inserting the borehole tool 100 into the borehole compresses the biasing members 110a-d such that the wheels 104a-d move inwards towards the borehole tool 100. In the exemplary arrangement shown in Figure 1-3, inserting the borehole tool 100 into the borehole causes the levers 112a-d to rotate inwardly to further compress the biasing members 110a-d. Once inserted into the borehole, the biasing members 110a-d exert a force on the wheels 104a-d to push them outwardly with respect to the borehole tool. This force may be resisted by a surface within the borehole tool. As such, the biasing members 110a-d act to push the wheels 104a-d against a surface within the borehole.

The borehole tool 100 is released such that it descends within the borehole under gravity. As the borehole tool 100 descends within the borehole, the wheels 104a-d which are in contact with a surface within the borehole begin to rotate. The wheels 104a-d rotate with respect to the borehole tool 100. Rotation of the wheels 104a-d causes relative movement between the electrically conductive member 124 and the plurality of magnets 126a-n. The relative movement causes a braking force to be applied to resist rotation of the corresponding wheel 104a-d. The braking force applied to the wheel 104a-d is in dependence on the rate of rotation of the wheel 104a-d. This is because the relative movement between the plurality of magnets 126a-n and the electrically conductive member 124 causes eddy currents to be induced in the electrically conductive member 124. This is a result of the change in magnetic field experienced by any given point of the electrically conductive member 124 as the wheel 104a-d and the magnets 126a-n rotate. The eddy currents produce a magnetic field that resists the rotation of the wheel 104a-d. As such, the faster that the wheel is rotating, the larger the magnetic field produced by the eddy currents, and therefore the greater the braking force applied to the wheel 104a-d.

The braking force applied to the wheel 104a-d will vary as: rwheel = mrr2 ad B2 R2 Where n is the number of magnets; r the magnet radius; a is the conductivity of the material of the wheel 104a-d; d is the thickness of the wheel 104a-d; B is the magnetic field strength produced by the magnet; R is the radius of the wheel 104a-d and 6 is the angular velocity of the wheel 104a-d.

The skilled person will appreciate that the braking force, T -wheet. may therefore be maximised by increasing the number and/or radius of the plurality of magnets, and/or utilising a high strength magnet material. In exemplary arrangements, the magnets may have a diameter of substantially 15 mm. Exemplary arrangements may utilise eight magnets.

The brakes 122a-d may therefore be considered self-regulating, since there is no user input required in order to adjust the braking force during the descent of the borehole tool 100 within the borehole. This ensures that the borehole tool moves at an acceptable speed throughout the descent. For example, in substantially vertical portions, the borehole tool 100 may approach or travel at terminal velocity. At this point the wheels are rotating at greater rate than when, for example, the borehole tool 100 is travelling through inclined portions of the borehole. As such, a greater braking force is applied in vertical portions of the borehole tool than in inclined portions than when the borehole is descending through inclined portions. Further, if any of the wheels "slip" at any point during the travel of the borehole tool through the borehole (for example, if they come out of contact with the inner surface of the borehole, or slide along the inner surface of the borehole without rotating) then the braking force applied to the borehole tool reduces, since the wheels are not rotating. Once the wheels gain traction again and begin to rotate once more (for example when the wheels come back into contact with the inner surface of the borehole, or when the wheels are once again pushed against the inner surface of the borehole with sufficient force) the braking force will once more be applied in dependence on the rate of rotation of the wheels.

The skilled person will appreciate that alternative brakes may be utilised in order to achieve the self-regulating effect described above.

In exemplary arrangements, at least one wheel may form part of a power generation system. For example, the at least one wheel may form part of a dynamo. Rotation of the at least one wheel may generate an electrical current which may be used to apply a braking force to the at least one wheel. The current may be generated using electromagnetic induction as a result of the rotation of the at least one wheel. The current generated may be an AC current. In exemplary arrangements the brake corresponding to the at least one wheel may comprise a brake controller configured to control the brake to apply a braking force to the at least one wheel in dependence on the electrical current generated.

The current generated may be in dependence on the rate of rotation of the at least one wheel. For example, in some arrangements, a greater rate of rotation may induce a greater electric current, and as such a greater braking force may be applied to the at least one wheel.

Figure 4 shows a further exemplary brake 422, which may be utilised with the borehole tool 100 of Figure 1 as an alternative or in addition to the brakes 122.

The brake 422 comprises a cavity 440. The cavity 440 may be a fluid-filled cavity and may comprise a fluid configured to resist rotation of the wheel 404a. The fluid may be, for example, a high viscosity fluid such as a high viscosity oil. A surface of the wheel 404a may form at least a part of a wall of the fluid-filled cavity 440. As such, rotation of the wheel 404a may be resisted by the fluid within the fluid-filled cavity 440.

The wheel 404a may comprise a lip 442. The lip may extend radially inwardly such that the cavity 440 is formed between the lip 442 and an outer wall 443 of the wheel 404a.

As described above, the wheel 404a may be rotatably coupled to the borehole tool 100, optionally via the lever 112a. In the exemplary arrangement of Figure 4, the wheel 404a is rotatably coupled to the borehole tool 100 or lever 112a via a stationary portion 446. The wheel 404a may be rotatably coupled to the borehole tool 100, for example via the stationary portion 446, by a bearing or bushing 430. The stationary portion 446 may be fixed relative to the borehole tool 100 and the wheel 404a. The wheel 404a may be rotatable with respect to the stationary portion 446.

At least part of the stationary portion 446 may be received in the cavity 440. In the exemplary arrangement shown in Figure 4, the stationary portion may comprise an annular plate 448 configured to be received in the cavity 440.

The fluid-filled cavity 440 may be liquid-tight. In the exemplary arrangement of Figure 4, the brake 422 comprises a seal 450. The seal 450 may comprise a hydraulic o-ring seal or a compression seal. In alternative arrangements, the fluid-filled cavity may be made liquid-tight using welding. The seal 450 may be located between the wheel 404a and the stationary portion 446.

When the borehole tool 100 is inserted into the borehole, as described above, and begins descending within the borehole under gravity, the wheel 404a begins to rotate. 25 As the wheel 404a rotates, a film layer of the fluid within the cavity 440 adjacent to the surfaces of the wheel 404a (e.g. adjacent to the lip 442 and the outer wall 443) adheres to the wheel 404a and moves at the same velocity as the wheel 404a. Conversely, the film layer of the fluid within the cavity 440 adjacent to the stationary portion (e.g. adjacent to the annular plate 448) will remain stationary. A laminar flow is established in the fluid between the surfaces of the wheel 404a that form the cavity 440 and the stationary portion 446. The shearing of the fluid layers dissipates energy within the fluid and applies a braking force to the wheel 404a. The magnitude of the braking force applied will vary as: F= where /.2 is the fluid viscosity, V is the velocity of the wheel 404a relative to the stationary portion 446, A is the area of the stationary and moving plates (i.e. the surfaces of the wheel that form the cavity and the part of the stationary portion within the cavity), and h is the separation between the plates (i.e. the separation between the surfaces of the wheel that form the cavity and the part of the stationary portion within the cavity).

As such, the braking force applied varies linearly with the angular velocity of the wheel 404a. This results in an increased braking force being applied to the wheel 404a as the rate of rotation of the wheel 404a increases.

Figure 5 shows a further exemplary brake 522, which may be utilised with the borehole tool 100 of Figure 1 as an alternative or in addition to the brakes 122 and/or the brakes 422. Many of the features of the brake 522 depicted in Figure 5 are similar to those described above in respect of the brake 422 of Figure 4. As such, a description of these features is not given again here and corresponding reference numerals are used to identify them.

Similarly to the brake 422, the exemplary brake 522 may comprise a cavity 540. The cavity 540 may be a fluid-filled cavity and may comprise a fluid configured to resist rotation of the wheel 504a. The fluid may be, for example, a high viscosity fluid such as a high viscosity oil. A surface of the wheel 504a may form at least a part of a wall of the fluid-filled cavity 540. As such, rotation of the wheel 504a may be resisted by the fluid within the fluid-filled cavity 440.

In the exemplary arrangement, the cavity 540 is formed between a wall 543 of the wheel 505a and a wall of the stationary portion 546. The skilled person will appreciate that alternative structural arrangements to form the cavity 540 may be utilised (for example, the lip arrangement of Figure 4, or further arrangements).

The brake 522 may comprise at least one fin 552. The at least one fin 552 may be configured to extend within the cavity 540. The at least one fin 552 may be coupled to the wheel 505a such that rotation of the wheel 505a causes rotation of the fin 522 h within the cavity 540. The exemplary brake 522 may comprise a plurality of fins 522a-n. The plurality of fins 522a-n may be angularly spaced about a circumference of the wheel 505a. The plurality of fins 522 may be equally spaced about the circumference of the wheel 505a. The skilled person will appreciate that substantially any number of fins may be used.

When the borehole tool 100 is inserted into the borehole, as described above, and begins descending within the borehole under gravity, the wheel 504a begins to rotate.

As the wheel 504a rotates, the plurality of fins 522a-n coupled to the wheel 504a begin to rotate within the cavity 540. This causes the fluid within the cavity 540 to be pumped around the cavity 540 as it is forced away from the advancing surface of the fins 522a-n. As such, kinetic energy of the fins 522a-n (and the wheel 505a) is converted into hydrodynamic energy of the fluid flow around the cavity. This causes retardation of the rotation of the fins 522a-n and therefore causes a braking force to be applied to the wheel 505a.

The skilled person will appreciate that the greater the rate of rotation of the wheel 505a (and therefore the greater the rate of rotation of the fins 522-n), the larger the braking force applied.

The skilled person will appreciate that there may be further ways of controlling a brake of a borehole tool. For example, the borehole tool may comprise an accelerometer configured to measure the acceleration of the borehole tool within the borehole. A braking force may be applied to the borehole tool in dependence on the acceleration measured by the accelerometer (e.g. a larger braking force may be applied for increased acceleration).

Exemplary borehole tools may further comprise an odometer configured to measure the distance that at least one of the wheels of the braking assembly rotates through.

This measurement may be utilised to provide an indication of depth of the borehole. As mentioned above, depth data may be utilise when performing borehole surveys.

An odometer may be utilised with any of the wheels 104a-d, 404a and 504a described above. The skilled person will appreciate that measurement of the rotation of the wheel could be achieved in many ways, for example, by utilising optical, inductive, mechanical or rotary encoders, or other proximity detectors.

Advantageously, when the odometer is utilised with the brake 122 as shown in Figures 1 and 2, the magnets 126a-n may be utilised to provide a measurement of the rotation of the wheel 104a-d.

In such arrangements, the odometer may comprise a sensor. The sensor may comprise a hall effect sensor or a sense coil, or any other sensor capable of detecting a magnetic field of a magnet. The sensor may be arranged to detect the position of the plurality of magnets 126a-n as they rotate with the wheel 104a. For example, the sensor may be positioned such that it is able to detect the magnets passing by as the wheel 104a rotates. The magnetic field detected by the sensor as a magnet passes may be considered a pulse or trigger event. The number of magnets utilised within the brake 122 will determine how many pulses or trigger events are detected by the sensor during a single rotation of the wheel. For example, the exemplary brake 122 as shown in Figure 3 comprises eight magnets and so the sensor would detect eight pulses or trigger events per rotation of the wheel 104a. The skilled person will appreciate that the larger the number of magnets utilised, the greater the measurement resolution, since each pulse or trigger will represent a rotatory increment of the wheel 104a.

The circumference of the wheel 104a and the number of pulses detected by the sensor can be utilised to provide a depth measurement.

In exemplary arrangements, odometers may be utilised with more than one of the wheels 104a-d. Having multiple odometer readings can account for one of the wheels 104a-d "slipping" as the borehole tool travels through the borehole. That is, if one of the wheels comes out of contact with the inner surface of the borehole tool such that it does not rotate, or else slides along the inner surface of the borehole tool without rotating.

It is noted that many of the features of the exemplary borehole tools described above and shown in the drawings may be included in other exemplary borehole tools. As such, the different drawings are not necessarily to be considered as separate embodiments and features from one drawing may be transferred to a borehole tool in another drawing.

The skilled person will be able to envisage other braking assemblies and borehole tools without departing from the scope of the appended claims.

Claims

CLAIMS: 1. A borehole tool for inserting into a borehole such that the borehole tool descends within the borehole under gravity, the borehole tool comprising: at least one sensor configured to collect data indicative of a property of the borehole; and a braking assembly comprising: at least one wheel configured to contact a surface within the borehole, and rotate as the borehole tool descends within the borehole, and a brake configured to apply a braking force to the at least one wheel on rotation thereof to resist rotation of the at least one wheel, wherein the braking force applied to the at least one wheel is in dependence on a rotation rate of the at least one wheel.
2. A borehole tool according to claim 1, a greater rotation rate of the at least one wheel causes a greater braking force to be applied to the at least one wheel.
3. A borehole tool according to claim 1 or 2, wherein the brake comprises a self-regulating brake.
4. A borehole tool according to any preceding claim, wherein the braking assembly further comprises a biasing member configured to bias the at least one wheel outwardly with respect to the borehole tool.
5. A borehole tool according to claim 4, wherein the at least one wheel is rotatably coupled to the borehole tool by a lever, and wherein the biasing member is configured to rotate the lever outwardly with respect to the borehole tool.
6. A borehole tool according to any preceding claim, wherein the braking assembly comprises a pair of wheels.
7. A borehole tool according to claim 6, wherein the wheels are coupled to opposed sides of the borehole tool.
8. A borehole tool according to claim 6 or 7, wherein the braking assembly comprises a further pair of wheels coupled to opposed sides of the borehole tool, wherein the pair of wheels and the further pair of wheels are angularly displaced relative to each other about a longitudinal axis of the borehole tool.
9. A borehole tool according to claim 8, wherein the pair of wheels and the further pair of wheels are angularly displaced relative to each other about the longitudinal axis by 90 degrees.
10. A borehole tool according to claim 8 or 9, wherein the pair of wheels and the further pair of wheels are offset along the longitudinal axis of the borehole tool.
11. A borehole tool according to any preceding claim comprising a further braking assembly coupled to the borehole tool.
12. A borehole tool according to claim 11, wherein each braking assembly is coupled proximal to a rear and a forward end of the borehole tool respectively.
13. A borehole tool according to any preceding claim, further comprising an odometer configured to measure the distance the at least one wheel rotates through.
14. A borehole tool according to any preceding claim, wherein the braking assembly comprises an eddy current braking assembly.
15. A borehole tool according to claim 14, wherein the brake comprises an electrically conductive element and at least one magnet.
16. A borehole tool according to claim 15, wherein one of the electrically conductive element and the at least one magnet is coupled to the at least one wheel and configured to rotate therewith, and the other of the electrically conductive element and the at least one magnet is fixed relative to the borehole tool, the at least one wheel configured to rotate with respect to the borehole tool, and wherein the relative rotation causes the braking force to be applied to the at least one wheel.
17. A borehole tool according to claims 15 or 16, comprising a plurality of magnets angularly spaced about the circumference of the at least one wheel.
18. A borehole tool according to any of claims 15 to 17, when directly or indirectly dependent on claim 13, wherein the odometer comprises a sensor configured to detect a position of the at least one magnet to determine the distance the at least one wheel rotates through.
19. A borehole tool according to any of claims 1 to 13, wherein the brake comprises a fluid-filled cavity, and wherein rotation of the at least one wheel is resisted by a fluid within the fluid-filled cavity.
20. A borehole tool according to claim 19, wherein a surface of the at least one wheel forms at least part of a wall of the fluid-filled cavity.
21. A borehole tool according to claim 20, wherein the fluid of the fluid-filled cavity applies the braking force to the surface of the at least one wheel on rotation thereof.
22. A borehole tool according to any of claims 19 to 21, wherein the brake further comprises at least one fin configured to extend within the fluid filled cavity.
23. A borehole tool according to claim 22, wherein the at least one fin is coupled to the wheel such that rotation of the at least one wheel causes rotation of the at least one fin within the fluid-filled cavity.
24. An apparatus according to claim 23, wherein the rotation of the at least one fin is resisted by the fluid within the fluid-filled cavity to cause the braking force to be applied to the at least one wheel.
25. A braking assembly for a borehole tool for inserting into a borehole such that the borehole tool descends within the borehole under gravity, the borehole tool comprising at least one sensor configured to collect data indicative of a property of the borehole, the braking assembly comprising: at least one wheel configured to contact a surface within the borehole, and rotate as the borehole tool descends within the borehole, and a brake configured to apply a braking force to the at least one wheel on rotation thereof to resist rotation of the at least one wheel, wherein the braking force applied to the at least one wheel is in dependence on a rotation rate of the at least one wheel.