Rotary steerable tool and control system for a rotary steerable tool

Abstract

This invention relates to a rotary steerable tool (10, 110; 210) and to a control system for a rotary steerable tool. The rotary steerable tool is in use connected to surface equipment by a rotatable drill string. The tool has a first end (12), a second end (14), a longitudinal axis (A-A) between the first end and the second end, and a conduit (16) from the first end to the second end through which drilling fluid can flow through the tool. The tool has at least one piston (20; 120; 220) which is movable in a direction lateral to the longitudinal axis, a chamber (152; 252) for drilling fluid, and a valve (48; 148; 248) between the conduit and the chamber. A controller controls the valve to actuate the piston to steer the tool. The controller actuates the same valve to create pressure pulses in the drilling fluid to communicate downhole information to the surface equipment.

Description

[0001] ROTARY STEERABLE TOOL AND CONTROL SYSTEM FOR A ROTARY STEERABLE TOOL

[0002] FIELD OF THE INVENTION

[0003] This invention relates to a rotary steerable tool and to a control system for a rotary steerable tool. The tool and control system are expected to have their greatest utility in a downhole drilling assembly as might be used when drilling for oil, gas or geothermal energy for example, and the following description therefore refers primarily to such applications. The use of the tool and control system in other applications is nevertheless not excluded.

[0004] BACKGROUND TO THE INVENTION

[0005] During the drilling of a borehole it is desirable to be able to steer the drill bit, i.e. to move the drill bit in a chosen direction as the borehole is being drilled, so that the drill bit does not have to follow a path determined only by gravity and / or by the drilling conditions.

[0006] One method of steering a drill bit is to use a downhole assembly with a bent housing connected to the drill bit. The bent housing results in the leading end (or toolface) of the drill bit being offset from the longitudinal axis of the drill string. The downhole assembly includes a downhole motor connected to the drill bit by way of a flexible driveshaft which rotates within the bent housing, the motor driving the drill bit to rotate whilst the remainder of the drill string, including the bent housing, does not rotate. One such downhole motor is a mud motor which uses the flow of drilling fluid or mud to drive the drill bit. The bent housing allows the drill bit to follow a nonlinear or curved path, the drill bit moving in the direction of the offset.

[0007] If the drill string and bent housing do not rotate the orientation of the offset does not change and the borehole will continue to deviate in the same direction. A linear, or more linear, section of borehole can be drilled by rotating the bent housing (typically by rotating the drill string). This continuously changes the direction of the offset of the drill bit and cancels out the tendency of the drill bit to deviate the borehole in one direction.

[0008] Accordingly, the use of such a method requires the drill string to be non-rotating whilst a curved borehole is being drilled. It is widely recognised that a non-rotating drill string experiences greater friction upon the borehole wall than a rotating drill string, i.e. the resistance to drill bit advance will comprise the resistance of the rock through which the drill bit is moving, plus the resistance to movement of the drill string along the borehole. A drill string which is rotating experiences less resistance to movement along the borehole and therefore enables the drilling of deeper and longer boreholes. Very deep and / or long boreholes are commonly required to reach the remaining reserves of oil and gas for example and those reserves cannot all be reached with a non-rotating drill string.

[0009] In addition, a rotating drill string is less likely to buckle under the applied axial load than a nonrotating drill string. Furthermore, borehole cleaning is improved with a rotating drill string, i.e. drill cuttings in the drilling fluid returning to the surface are less likely to sink and settle at the low side of a (non-vertical) borehole. British patent applications 2 435 060 and 2 440 024 each disclose a steering assembly for a drill bit connected to a rotating drill string. The downhole assembly comprises a mud motor connected to the drill string, a bent housing connected to the mud motor and a drill bit connected to the bent housing. The mud motor is connected to the drill string by way of a slipping clutch mechanism whereby the mud motor and bent housing can be stationary whilst the drill string rotates. The torque which is transmitted by the clutch is necessarily variable so as to match the counter-rotation torque experienced by the mud motor as the drill bit rotates.

[0010] Other steering apparatuses and methods are known, for example the rotary steering tool described in European patent application EP 1 024 245. This steering tool has a set of pistons which are arranged circumferentially around the tool and which can move radially relative to the tool, i.e. towards and away from the borehole wall. It is a feature of rotary steerable tools that they rotate with the drill string and the pistons are therefore actuated periodically and sequentially during each rotation of the tool so that the tool is forced away from the centre of the borehole in a chosen direction. The tool is located relatively close to the drill bit and forcing the tool away from the centre of the borehole causes the drill bit to deviate from a linear path. Such a steering tool allows the operator to determine the direction and degree of curvature of the borehole during the drilling operation.

[0011] In EP 1 024 245 the steering pistons are actuated hydraulically. In the alternative arrangement of US 2017 / 0107762 the steering pistons are actuated mechanically, and specifically by components with inclined actuating surfaces which engage the pistons. The components with inclined actuating surfaces are identified as mandrels in this prior art document and the same terminology will be used in this specification. The mandrels in US 2017 / 01077562 are moved longitudinally and their inclined actuating surfaces engage a correspondingly inclined surface of the respective pistons, the inclined surfaces converting the longitudinal movement of the mandrel into radial movement of the piston. A single mandrel can be configured to engage pistons at opposing sides of the tool and to allow the pistons at one side of the tool to retract as the pistons at the other side of the tool extend.

[0012] Mechanically-driven pistons have the advantage over hydraulically-driven pistons that they can be more mechanically robust. In particular, during rotation the pistons can experience impacts and other significant side forces from the borehole wall and such forces act to retract the pistons. In a hydraulic system robust and reliable check valves are required to hold the pistons in their extended positions, whereas in a mechanical system only a small component of these forces will act to move the mandrel.

[0013] The tool of EP 1 024 245 has six pistons arranged around the circumference of the tool and it is therefore necessary to provide a control system which can actuate six pistons (or six banks of pistons) in each cycle of rotation of the tool. The tool may be rotating at 150 rpm for example and so the control system must be able to actuate each piston (or bank of pistons) once every 400 milliseconds. The tool of US 2017 / 0107762 has four banks of pistons arranged around the circumference, i.e. two pairs of opposing banks of pistons, and the control system of that disclosure therefore has more time to actuate the mandrels.

[0014] In order to determine the desired direction in which to steer the drill bit, and consequently the point at which the pistons or mandrels must be actuated in each revolution, it is necessary to take measurements downhole. Firstly, it is necessary to know the orientation of the tool and many downhole assemblies will include sensors to measure the position and inclination of the tool, for example by way of the local gravitational and magnetic forces. In addition, many downhole assemblies include “measurement-while-drilling” (MWD) or “logging-while-drilling” (LWD) equipment in order to assist the operator in determining the optimum drilling direction. Such measurement tools can, for example, measure radioactivity, electrical resistivity and electrical capacitance at different longitudinal positions as the tool advances along the borehole and also at different circumferential positions as the tool rotates within the borehole. Measurement data is typically transmitted to the surface and desired changes in drilling direction are usually determined at the surface and communicated to the tool.

[0015] Measurement data can be transmitted to the surface by a radio signal or by wire but in many applications it is preferable to use a mud pulser in a process known as mud pulse telemetry. A mud pulser is a valve located in the downhole assembly, which valve is normally open and through which drilling fluid passes to the drill bit. The valve can, however, be temporarily closed in order to create a short-term increase (or pulse) in the pressure of the drilling fluid above the valve. The pressure pulse propagates within the drilling fluid to the surface where it can be detected by a pressure sensor. The data to be transmitted is encoded downhole into a sequence of pulses, which sequence can be processed by a computer at the surface in order to retrieve the data.

[0016] SUMMARY OF THE INVENTION

[0017] It is an object of the present invention to provide a rotary steerable tool which is relatively mechanically simple and can therefore be manufactured at relatively low cost.

[0018] It is another object of the present invention to provide a rotary steerable tool which can be used by many different operators and in many different applications. In particular, it is intended to make operation of the tool as simple and straightforward as possible so that it can readily be utilised alongside existing drilling equipment, perhaps as a direct replacement for an existing rotary steerable tool.

[0019] It is another object of the invention to provide a rotary steerable tool having radially acting pistons which are mechanically actuated whereby to benefit from the above-stated advantages of such an arrangement.

[0020] According to the first aspect of the invention there is provided a rotary steerable tool for use in a borehole with the tool connected to surface equipment by a rotatable drill string, the tool having: a first end, a second end and a longitudinal axis between the first end and the second end; a conduit from the first end to the second end through which drilling fluid can flow through the tool; at least one piston which is movable in a direction lateral to the longitudinal axis; a chamber for drilling fluid; a valve between the conduit and the chamber, the valve being movable between a first position and a second position, the valve in the first position connecting the conduit and chamber whereby in use the piston is driven to extend from the tool, and the valve in the second position disconnecting the conduit and chamber whereby in use the piston can retract; a controller for controlling the position of the valve, the controller being connected to at least one sensor for detecting a chosen parameter, the controller being configured to receive a signal corresponding to the detected parameter from the sensor and being further configured to encode the signal and to communicate the encoded signal to the surface equipment by way of one or more pressure pulses in the drilling fluid, wherein the pressure pulses are created by actuating the valve.

[0021] Accordingly, in this aspect of the present invention the same valve is used to actuate the piston to move outwardly of the tool and to generate the pressure pulses to communicate data to the surface equipment.

[0022] Notwithstanding that it is stated above that the valve in the second position disconnects the conduit and chamber, it will be understood that complete disconnection of the chamber from the conduit is not required, provided that the pressure in the chamber is reduced sufficiently to allow the piston to retract into the tool. In particular, in common with known rotary steerable tools, a fluid outlet from the chamber is preferably open continuously. It is arranged that when the valve is in its first position the flow of drilling fluid into the chamber exceeds the flow of fluid out of the chamber so that the pressure in the chamber increases and the piston is extended. On the other hand, when the valve is in its second position the flow of drilling fluid out of the chamber exceeds the (zero or minimal) flow into the chamber so that the pressure in the chamber decreases and the piston can retract. References below to the valve disconnecting the conduit from a chamber therefore embrace complete disconnection and also partial disconnection which is sufficient to permit retraction of a piston.

[0023] A resilient biasing means can be provided to urge the piston to retract, i.e. to move towards the longitudinal axis, when the valve is in the second position. Accordingly, the piston can be positively driven to retract by the resilient biasing means. Alternatively, the tool can rely upon the reaction force of the borehole wall to cause the piston to retract.

[0024] In the simplest embodiment the tool has a single or primary piston. In known fashion, the piston is extended, i.e. pushed outwardly relative to the longitudinal axis and into engagement with the borehole wall, causing the tool to be moved away from the centre of the borehole and consequently causing the drill bit to deviate the borehole. Since the tool is rotating with the drill string it is necessary for the piston to cycle (alternate) between its extended and retracted positions, ideally in each revolution of the tool, the cycle being coordinated with the tool rotation so that the tool is repeatedly pushed away from the centre of the borehole in the same direction and the drill bit is urged to steer the borehole in a consistent direction.

[0025] In a preferred embodiment, however, there is more than one piston. Multiple pistons can be aligned along the longitudinal axis and comprise a single bank of (primary) pistons which are all actuated together. Desirably, however, multiple (primary and secondary) pistons can be arranged circumferentially around the tool so that they can be actuated sequentially to push the tool away from the centre of the borehole. It will be understood that the separate actuation of different pistons requires a separate chamber for each piston (or bank of pistons), and a separate valve position.

[0026] In the preferred embodiments having multiple pistons located circumferentially around the tool it is preferably arranged that the extension of one piston will force the tool away from the centreline of the borehole and thereby force an opposing piston to retract. There is therefore no requirement to provide a means to positively retract the pistons. Preferably, there are three pistons (or three banks of pistons) arranged circumferentially, and ideally equally spaced (120° apart) around the tool. Preferably also, two of the pistons (or two banks of pistons) are actuated together. Accordingly, the primary piston (or the primary bank of pistons) is actuated, then the secondary pistons (or secondary banks of pistons) are actuated, sequentially as the tool rotates. It will be understood that a tool having only two independent sets of pistons will provide less control than a tool having three independent sets of pistons. However, the loss of control is expected to be acceptable to most operators, especially in view of the significant reduction in complexity which this provides, and in particular the benefits in enabling a single valve more easily to actuate the pistons and send encoded pressure pulses to the surface.

[0027] In particular, providing just two independent pistons (or banks of pistons) reduces the number of valve movements required for each revolution of the tool. Alternatively stated, the duty cycle of the valve corresponds to 180° of tool rotation as compared to 120° of tool rotation if all three banks of pistons were to be actuated independently. The preferred arrangement simplifies the control of the valve, reduces the likelihood of wear upon the valve, and can allow an increase in tool rotation rate.

[0028] The chamber for drilling fluid can be a cylinder for the piston whereby the piston is actuated hydraulically. Preferably, however, the piston is actuated mechanically by way of a mandrel, the mandrel having an inclined surface in contact with a corresponding inclined surface of the piston. The pressure of the drilling fluid in the chamber is used to move the mandrel which in turn moves the piston. As above explained, with a shallow incline only a small proportion of any reaction force from the borehole wall upon the piston passes to the mandrel and thereby to the fluid in the chamber, reducing the requirement for robust check valves. Preferably, the mandrel moves in a longitudinal direction relative to the remainder of the tool.

[0029] The contacting inclined surfaces between the mandrel and the piston can if desired be lubricated by oil or by the drilling fluid. Alternatively or additionally roller bearings, ball bearings or the like can be used to reduce the friction between the surfaces of the mandrel and the piston.

[0030] Preferably, the mandrel is driven in a first (piston extending) direction by pressure within the chamber, and is driven in a second (piston retracting) direction also by fluid pressure. Alternatively, a resilient biasing means engages the mandrel and acts to move the mandrel in the second direction corresponding to retraction of the piston. In these alternative embodiments, the mandrel is moved in the first (piston extending) direction when the pressure of drilling fluid in the chamber is sufficient to overcome the resilient biasing means (plus the frictional resistance to mandrel movement) and the mandrel is moved in the opposing second (piston retracting) direction when the pressure in the chamber is reduced. In all cases, movement of the mandrel in the second direction can positively retract the piston. Alternatively, movement of the mandrel in the second direction can allow the piston to retract, with the force to retract the piston being provided by the borehole wall.

[0031] As above stated, the preferred arrangement has three pistons (or three banks of pistons) arranged circumferentially, specifically a primary piston and two secondary pistons (or respective banks of pistons), with the secondary pistons all being actuated together and the primary piston(s) being actuated independently. The tool therefore has a second chamber and the valve in its second position can actuate the secondary pistons. Accordingly, the tool in its simplest form can have a valve with just two positions, a first position in which the conduit is connected to the first chamber and disconnected from the second chamber (in which position the primary piston extends and the secondary pistons retract) and a second position in which conduit is connected to the second chamber and disconnected from the first chamber (in which position the secondary pistons extend and the primary piston retracts). In such an arrangement one or other of the primary and secondary pistons are extended at all times and a linear (or more linear) length of borehole can be drilled by appropriate sequencing of the piston extensions as the tool rotates.

[0032] Preferably, however, the valve has a third position for use when a linear length of borehole is to be drilled. It can be arranged that the valve in the third position disconnects the conduit from the first chamber and from the second chamber whereby the primary piston(s) and the secondary pistons can all retract. It is nevertheless preferred that a linear (or more linear) length of borehole can be drilled with all of the pistons extended with a substantially equal force, so that the force exerted on the borehole wall by the primary piston(s) cancels out the forces exerted by the secondary pistons (and vice versa). It is preferred that all of the pistons are extended when it is desired to drill a linear length of borehole as the pistons will act to centralise the tool and can also act to stabilise the tool and drill string.

[0033] It is therefore preferable that the valve in its third position connects the conduit and the first chamber and also connects the conduit and the second chamber, whereby in use the primary and secondary pistons are all driven to extend (together) by the pressure of the drilling fluid in the first and second chambers.

[0034] Preferably, the piston(s) moves in a direction which is radial, i.e. perpendicular to the longitudinal axis. The term “lateral” is nevertheless used above to clarify that the pistons do not need to move along an axis which is precisely perpendicular to the longitudinal axis, as long as they have a component of movement (and preferably a major component of movement) in the perpendicular direction.

[0035] Desirably, the piston is located in a radial extension of the tool. In a tool having multiple pistons there is preferably a corresponding number of multiple radial extensions. In known fashion, the radial extensions help the tool to act as a stabiliser for the downhole assembly. Spaces between the radial extensions are provided to allow drilling fluid and entrained drill cuttings to flow back to the surface. Desirably, the radial extensions are shaped as rhombuses with rounded corners.

[0036] Preferably, the primary pistons are offset from the secondary pistons along the longitudinal axis of the tool. Offsetting the respective pistons longitudinally enables more of the volume of the body of the tool to be utilised for the pistons and their supporting structures and driving componentry without impacting other pistons.

[0037] There is also provided a control system for a rotary steerable tool, the rotary steerable tool having at least one piston which can be actuated to move laterally relative to the remainder of the tool, the tool having a valve to actuate the piston and a controller for the valve, whereby actuation of the piston can provide steering control for the tool, the control system having an operating cycle of a predetermined duration, the controller being configured to operate the valve for steering control during each operating cycle, the controller being further configured to operate the valve for transmitting data during each operating cycle. The control system takes advantage of the fact that for certain periods the drill bit is not required to deviate the borehole by the maximum possible curvature. In other words the drill bit is often required to deviate from a linear path relatively slowly (or not at all) and during those periods the valve is not continuously required to actuate the piston for steering control and can instead be used to send encoded pressure pulses to the surface.

[0038] Preferably, the operating cycle is split into two parts, with a first part during which the valve can be operated for steering control (only) and a second part during which the valve can be operated for data transmission (only). With such a system the second part of each operating cycle is dedicated to data transmission and pressure pulses can be sent to the surface regardless of the extent of valve operation during the first part of the operating cycle.

[0039] For example, the control system can have a predetermined operating cycle of five seconds and can operate the valve for steering control during the first four seconds of that operating cycle and can operate the valve for transmitting data to surface equipment during the final one second. If maximum borehole curvature is required the piston is actuated (cyclically) to steer the drill bit for all four seconds of the first part whereas if less borehole curvature is required the piston is actuated in the first part for less than four seconds. In both cases the final one second of each operating cycle is used to transmit data to the surface (as and when required).

[0040] The first and second parts of the cycle can therefore be fixed proportions of the operating cycle. Alternatively, they can be variable proportions. If the first and second parts are variable proportions of the operating cycle they are preferably varied by the controller depending upon the borehole curvature required, so that the first part of the operating cycle is increased if greater borehole curvature is required, and vice versa.

[0041] The operating cycle is not necessarily related to the rotation of the tool in the borehole, but preferably is long enough to contain multiple revolutions of the tool. In a typical drilling operation the drill string and tool may rotate at 150 rpm, i.e. once every 400 milliseconds. An exemplary operating cycle of five seconds therefore includes multiple revolutions of the tool. Providing an operating cycle which includes multiple revolutions of the tool is preferred since it ensures that the valve can actuate the piston(s) for steering for one or more complete revolutions before switching to data transmission.

[0042] It will be understood that since the same valve is used for steering and for data transmission the opening and closing of the valve in the second part of the operating cycle will inevitably change the pressure in the chamber(s) and generate some steering force; it is therefore preferred that several complete revolutions of the tool are undertaken with complete steering control before each revolution (or partial-revolution) without steering control, so that any negative impact upon steering which is caused by the valve movements for data transmission is minimal.

[0043] Desirably, the frequency of valve movements for data transmission is higher than the frequency for steering control. This has two benefits. Firstly, cycling the valve more rapidly during data transmission will reduce any adverse impact upon steering. Secondly, the surface equipment where the pressure pulses are detected will more readily be able to identify the data transmission pulses, or alternatively stated will more readily be able to distinguish between the pressure pulses caused by valve movements for steering the tool from the pressure pulses generated for data transmission. Preferably, the controller actuates the valve for steering the tool at the start of each operating cycle. Since the controller will actuate each piston (or bank of pistons) sequentially during each rotation of the tool, the pressure profile which is created will be identifiable by the surface equipment and will enable the surface equipment to synchronise with the tool during each operating cycle. Also, since the pressure profile for steering pulses is dependent upon the rate of rotation of the tool the surface equipment will be able to determine the rate of rotation.

[0044] According to the second aspect of the invention there is provided a rotary steerable tool for use in a borehole, the tool having: a first end, a second end and a longitudinal axis between the first end and the second end; a conduit from the first end to the second end through which drilling fluid can flow through the tool; a primary piston which is movable in a direction lateral to the longitudinal axis; a secondary piston which is movable in a direction lateral to the longitudinal axis, the primary and secondary pistons being circumferentially spaced apart around the tool; a first chamber for drilling fluid; a second chamber for drilling fluid; a valve between the conduit and the first and second chambers, the valve being movable between a first position and a second position, the valve in the first position connecting the conduit and the first chamber whereby in use the primary piston is driven to extend from the tool, and the valve in the second position connecting the conduit and the second chamber whereby in use the secondary piston is driven to extend from the tool; wherein the first chamber and the second chamber are respective parts of the conduit whereby drilling fluid flows from the first end to the second end through the first chamber and / or through the second chamber.

[0045] According to this second aspect of the invention the drilling fluid flows from the first end of the tool to the second end of the tool through one or both of the first chamber and the second chamber (only) and there is no separate part of the conduit which bypasses the first and second chambers. Avoiding the use of a separate part of the conduit bypassing the first and second chambers maximises the space available for the valve and allows the structure of the valve to be simplified. Simplifying the structure of the valve, and increasing the size of the valve in accordance with the space available, are expected to increase the reliability and longevity of the valve, both of which are important criteria for a downhole tool.

[0046] Arrangements according to the second aspect also maximise the flow of drilling fluid into the respective chambers whereby to facilitate rapid extension of the pistons. Alternatively stated, drilling fluid can flow into a chamber at the full pumping rate (i.e. the rate at which it is pumped by the surface equipment).

[0047] In the simplest embodiments the valve has only two positions and all of the drilling fluid flows either into the first chamber or into the second chamber depending upon the position of the valve. Preferably, however, the valve has a third position connecting the conduit with both of the first and second chambers whereby in use the primary and secondary pistons are driven to extend from the tool together.

[0048] It will be understood that since there is no separate part of the conduit bypassing the first and second chambers it is necessary for drilling fluid to flow through one or both of the chambers in order to pass to the drill bit and it is preferable that the flow of drilling fluid to the drill bit is continuous (or substantially continuous). As above stated, when it is desired to force the tool away from the centreline of the borehole and to drill a curved length of borehole, the first and second pistons are extended cyclically, and specifically once for each revolution of the tool. Providing a third valve position allows drilling fluid to flow to the drill bit by way of the first and second chambers together, the first and second pistons being extended together and providing a balanced force upon the tool.

[0049] Embodiments utilising both aspects of the invention are particularly beneficial because the opening and closing movements of the valve act upon the drilling fluid at the full pumping rate. The pressure pulses are thereby maximised.

[0050] The optional features which are set out for each aspect of the invention can be utilised with the other aspect of the invention where compatible; unnecessary repetition of the optional features is thereby avoided.

[0051] BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] The invention will now be described in more detail, by way of example, with reference to the accompanying schematic drawings, in which:

[0053] Fig .1 shows a perspective view of a rotary steerable tool according to the first aspect of the present invention;

[0054] Fig.2 shows a longitudinal cross-sectional view of the tool of Fig.1 with the primary piston extended;

[0055] Fig .3 shows a view as Fig.2 but with the secondary pistons extended;

[0056] Fig .4 shows a longitudinal cross-section through the tool of Fig.1 ;

[0057] Fig.5 shows the structure of the pistons and mandrels of the tool of Fig.1 ;

[0058] Fig.6 shows a perspective view of the two parts of the valve of the tool according to the first aspect of the invention;

[0059] Fig.7 shows a plan view of the two parts of the valve of Fig.6;

[0060] Fig.8 shows a plan view of the valve of Fig.6 in its first position;

[0061] Fig.9 shows a plan view of the valve of Fig.6 in its second position;

[0062] Fig.10 shows a plan view of the valve of Fig.6 in its third position;

[0063] Fig .11 shows a graph representing operating cycles of a first embodiment of control system of the invention according to the first aspect, with maximum borehole curvature;

[0064] Fig .12 shows a graph as Fig. 11 but with less than maximum borehole curvature;

[0065] Fig.13 shows a graph representing operating cycles of a second embodiment of the control system of the invention with less than maximum borehole curvature;

[0066] Fig.14 shows a cross-sectional representation of the tool according to the second aspect of the invention;

[0067] Fig.15 shows a more representative graph of an operating cycle of a control system of the invention according to the first aspect; Fig.16 shows a graph representing the pressure pulses for data transmission; and Fig .17 shows a cross-sectional representation of a tool according to the invention.

[0068] DETAILED DESCRIPTION

[0069] Fig.1 shows a rotary steerable tool 10 according to the first aspect of the invention. The tool has a first end 12 and a second end 14 which in known fashion are configured for connection to other parts of a downhole assembly. In one practical application the first end 12 may be connected to a drill string (not shown) and the second end 14 may be connected to a drill bit (also not shown). In other practical applications one or more other downhole tools may be located between the tool 10 and the drill string and / or between the tool 10 and the drill bit.

[0070] In known fashion the tool 10 is connected to the drill string and rotates with the drill string. The drill string is in turn connected to surface equipment.

[0071] The tool 10 has a longitudinal axis A-A between the first end and the second end. The longitudinal axis A-A is substantially aligned with the centreline of the borehole at the tool location. In the absence of a steering force the longitudinal axis A-A is typically coincident with the centreline of the borehole.

[0072] The tool 10 has a conduit 16 which runs from the first end 12 to the second end 14 and through which drilling fluid which is pumped from the surface can flow through the tool from the drill string to the drill bit. Only a part of the conduit 16 is visible in the drawings, specifically in Fig.4.

[0073] The tool 10 has eight pistons 20, each of which is movably mounted in the tool and can move in a radial direction (i.e. perpendicular to the longitudinal axis A-A). The pistons 20 are arranged in three banks of pistons, the pistons in each bank being aligned along the longitudinal axis A- A, and each bank of pistons being circumferentially spaced apart around the tool 10. As better seen in the cross-sectional views of Figs. 2 and 3, in this embodiment the banks of pistons 20 are spaced apart by 120° around the circumference of the tool 10. Two pistons 20a comprise primary pistons and are actuated together. Six pistons 20b comprise secondary pistons and are actuated together.

[0074] A particular benefit of this arrangement of pistons is that none of the pistons is diametrically opposing another piston. This allows the pistons to occupy a greater proportion of the crosssection of the tool without fouling another piston. Also, as is seen in Figs. 4 and 5, the primary pistons 20a are not longitudinally aligned with the secondary pistons 20b. This again allows more space in which to accommodate the respective pistons.

[0075] As seen in Fig.1 , each piston 20 is mounted in a radial extension 22 of the tool 10, and each radial extension 22 is shaped as a rhombus with rounded corners. The spaces between the radial extensions provide pathways for drilling fluid and entrained drill cuttings to pass the tool on the way to the surface. Such radial extensions can maximise the radial support for the tool and can also maximise hole cleaning (i.e. they can reduce the likelihood that solids will settle out of the drilling fluid adjacent to the tool).

[0076] It will be understood that the pistons 20 could be hydraulically actuated, i.e. each piston could lie within a cylinder into which drilling fluid can flow to move the piston. In this preferred embodiment, however, the pistons 20 are mechanically actuated, specifically by respective mandrels 24 (Fig.5) which are mounted to move longitudinally relative to the remainder of the tool 10.

[0077] The mandrels 24 and pistons 20 have cooperating inclined surfaces which convert longitudinal movement of the mandrels 24 into radial movement of the respective pistons 20.

[0078] It will be seen in particular from Fig.5 that in this embodiment each of the pistons 20 is mounted to a respective piston carrier 26. The piston carriers 26 partially wrap around the mandrels 24 and thereby engage upper and lower surfaces of the respective mandrels 24. Accordingly, movement of the mandrels 24 in one longitudinal direction causes the pistons 20 to extend and movement of the mandrels in the opposing longitudinal direction causes the pistons to retract. It will be appreciated that such (positive) retraction of the pistons is optional and the pistons could in other embodiments be allowed to retract passively due to reaction forces from the borehole wall (see for example the embodiment of Fig.14).

[0079] Each set of aligned mandrels 24 is rigidly connected to a respective push rod 28. The push rods 28 terminate in a housing 30, and specifically within respective chambers (not seen) inside the housing 30. The ends of the push rods 28 are formed into pistons and the chambers act as cylinders for those pistons, whereby drilling fluid can be ported into a chamber to force the push rod 28 to move (towards the right as drawn in Figs.4 and 5), and in turn to cause the connected pistons 20 to extend.

[0080] As above stated, in this embodiment the secondary six pistons 20b are arranged to move together, i.e. the two push rods 28 are connected to a common piston inside the housing 30 and move together so that the two banks of pistons 20b all extend and retract at the same time. The two primary pistons 20a also move together, but independently of the secondary pistons 20b.

[0081] The housing 30 also contains a valve 32 (Figs.6-10) which in this embodiment can be switched between first, second and third operational positions. The valve 32 is in two parts, a fixed part 34 and a rotary part 36. In particular, the fixed part 34 is rigidly mounted in the housing 30 and the rotary part 36 is rotatably mounted in the housing 30. The fixed part 34 has two first openings 40 which are in communication with a first chamber, and two second openings 42 which are in communication with a second chamber.

[0082] The rotary part 36 of the valve 32 has a boss 44 and a central bore 46. The central bore 46 is connected to (or is a part of) the conduit 16 whereby drilling fluid can flow through the valve 32 to the drill bit. In this embodiment therefore, only a proportion of the drilling fluid is diverted inside the housing 30 to pass to the outside of the boss 44 from where it can pass through the openings 40 and 42 to the respective chambers. The remainder of the drilling fluid passes through the central bore 46 and conduit 16 and bypasses the first and second chambers.

[0083] The rotary part 36 has two flanges 48 which act as valve members. In the assembled valve 32 the flanges 48 lie against (or at least very close to) the fixed part 34 of the valve 32.

[0084] As seen in Figs. 8-10, the rotary part 36 of the valve 32 can move (rotate) between its operational positions in order to open and close the openings 40, 42 and thereby to permit or restrict the flow of drilling fluid to the chambers controlling the push rods 28 and the mandrels 24. Fig.8 shows the valve 32 in its first operational position in which the openings 40 are open and the openings 42 are closed. In that position drilling fluid can flow into the first chamber and can move the push rod 28a and the mandrels 24a to the right as drawn in Figs. 4 and 5, which movement causes the bank of (two) aligned primary pistons 20a to extend. Little or no drilling fluid flows to the second chamber, however, so that the (six) secondary pistons 20b retract. This is the situation represented in Fig.2.

[0085] Fig.9 shows the valve 32 in its second operational position in which the openings 42 are open and the openings 40 are closed. In that position drilling fluid can flow into the second chamber and can move the two push rods 28b and the mandrels 24b to the right as drawn in Figs. 4 and 5, which movement causes the two banks of (three) secondary pistons 20b to extend. Little or no drilling fluid flows to the first chamber, however, so that the (two) primary pistons 20a retract. This is the situation represented in Fig.3.

[0086] Fig.10 shows the valve 32 in its third operational position in which one of the openings 40 is partially open and both of the openings 42 are fully open. In that position drilling fluid can flow into the first and second chambers causing all of the pistons 20 to extend. Notwithstanding that the cross-sectional area through which drilling fluid can flow through the openings 40 and 42 differs in the third operational position shown in Fig.10, it is arranged that the resulting force at the pistons 20 is substantially equal whereby the forces on the borehole cancel out and the tool is substantially centred in the borehole. Clearly, the relative cross-sectional areas of the openings which are required to provide balanced piston forces will depend upon the detailed structure of the tool and the required valve position can be determined accordingly.

[0087] Figs. 6-10 show a valve 32 with a rotary part 26 having two extending flanges 48 to control the flow of fluid through the openings 40 and 42. It will be understood that alternative valve structures could be used, including for example a valve with a rotary part having a continuous flange with ports to selectively communicate with the openings 40, 42. A continuous flange will provide a better balanced rotary part which is perhaps less prone to tilting (and consequently leakage) during use.

[0088] In common with known rotary steerable tools, in this embodiment the chambers in the housing 30 have respective leakage conduits which are permanently open, and which leakage conduits open to the outside of the tool, and therefore into the annulus containing drilling fluid which is flowing back to the surface. The pressure of the drilling fluid in the annulus is significantly lower than the pressure of the drilling fluid in the conduit 16. It is arranged that when the openings 40, 42 are open drilling fluid flows into the respective chamber faster than it flows out along the leakage conduit, whereas when the openings 40, 42 are closed drilling fluid flows out of the respective chamber. When the volume of fluid in the chambers increases the respective pistons 20 are extended by way of their respective push rod(s) 28 and mandrels 24, whereas when the volume of fluid in the chambers decrease a respective return spring (not shown) in the housing 30 causes the pistons 20 to retract by way of their respective push rod(s) 28 and mandrels 24.

[0089] It will be understood that the drilling fluid surrounding the boss 44 is in direct communication with the drilling fluid in the drill string and therefore the drilling fluid at the surface. Accordingly, as the valve 32 is moved between its open and closed positions the pressure in the drilling fluid surrounding the boss 40 changes and that change in pressure creates a pressure pulse which propagates to the surface. It will also be understood that the combined cross-sectional area of the openings 42 is larger than the combined cross-sectional area of the openings 40. Accordingly, the pressure in the drilling fluid surrounding the boss 40 also differs depending upon the valve position.

[0090] The tool 10 has a controller (not shown) which can control the operational position of the valve 32. When it desired to deviate the borehole the controller causes the valve 32 to move between its first and second positions once for each revolution of the tool so that the primary pistons 20a are extended once each revolution (Fig.2) and the secondary pistons 20b are extended once each revolution (Fig.3). It will be understood that when the pistons 20a, b are extended in the angular positions represented in Figs.2 and 3 the tool 10 is driven towards the left-hand side of the borehole as drawn, the cyclical alternating actuation of the pistons 20a, b causing the longitudinal axis A-A of the tool 10 to be driven in a consistent direction away from the centre of the borehole, and consequently causing a steering force at the drill bit in a consistent direction.

[0091] In addition, according to the first aspect of the present invention the controller is also connected to at least one sensor for detecting a chosen parameter. The chosen parameter may be the local force of gravity for example, which enables the controller to determine the orientation (toolface) of the tool continuously or repeatedly whereby to actuate the pistons 20a, b at the appropriate point as the tool 10 rotates.

[0092] In addition, the controller can be connected to a sensor for detecting a parameter to be transmitted to the surface. For example, the sensor may detect the electrical resistance and / or capacitance of the earth surrounding the borehole as the tool 10 rotates and advances. The controller encodes the signal according to a predetermined routine and actuates the valve 32 to open and close rapidly in order to generate pressure pulses in the drilling fluid and thereby to send the encoded signal to the surface by mud pulse telemetry. Figs. 11 -13 represent the pressure in the drilling fluid caused by steering operations and by telemetry operations of the valve 32.

[0093] Figs .1 1 -13 all represent graphs of pressure adjacent to the boss 44 (on the vertical axis) against time (on the horizontal axis). It will be understood that the pressure adjacent to the boss 44 will not drop to zero and so these figures represent the changes in pressure over time rather that the absolute pressures.

[0094] In Fig.11 the valve 32 is used for steering only and there is no telemetry signal. A predetermined time period T is chosen for an operating cycle of the tool, which time period is sufficiently long to include multiple revolutions of the tool 10. In this example the tool 10 is rotating at 150 rpm, i.e. once every 400 milliseconds. The time period T is predetermined to be 2 seconds, so that the operating cycle is equivalent to five complete revolutions of the tool 10.

[0095] Each operating cycle is split into two predetermined time periods Tsand TT. During the first part Tsof each time period T the valve 32 is used for steering and during the second part TT of each time period T the valve 32 is used for telemetry.

[0096] In this embodiment the first part Tsis 1 .6 seconds and the second part TT is 0.4 seconds. The first part Tstherefore spans four complete revolutions of the tool 10. Fig.11 represents a situation in which maximum borehole curvature is required, so that the valve 32 cycles between its first and second operational positions, and the pistons 20a and 20b are sequentially extended, four times during each period Ts. The pressure P is shown to be cycling between two different pressure values during the period Tsbecause of the differing pressures depending upon which of the openings 40 or 42 are open.

[0097] Fig.12 represents a situation in which less borehole curvature is required, so that the valve 32 is cycled between its first and second operational positions only for two out of every four revolutions in each time period T.

[0098] Fig .15 provides another graph representing the pressure changes during one sequence of valve movements during the time period Ts. The graph of Fig .15 is perhaps more representative of a practical tool than the graphs of Figs. 11 -13 because the pressure does not change instantaneously but instead increases and decreases gradually as the valve moves. Fig.15 is still a representation, however, as the rate of increase and decrease in the pressure will also not be consistent in practice, nor likely match each other. In a practical tool it is also not necessary that the maximum pressure is the same for each of the two valve positions.

[0099] Fig .15 also represents the pressure profile against the rotation angle, in order to clarify that the valve’s opening and closing movements for steering the tool are directly related to the rotation of the tool and are only indirectly related to time.

[0100] It will be understood that the pressure profile shown in Fig.15 will be transmitted to the surface. The frequency of the pulses in the profile is completely dependent upon the rate of rotation of the tool. The surface equipment can identify this pressure profile and can use it to determine the rotation rate.

[0101] The surface equipment can also use this pressure profile to determine the start of each operating cycle and thereby remain synchronised with the tool. Thus, notwithstanding that the two valve cycles shown in Fig.12 could be actuated at any time within the period Tsthe controller actuates the valve at the start of each operating cycle to enable the surface equipment to synchronise with the tool.

[0102] Fig.12 also shows a sequence of short pressure pulses 50 which are generated in the second part TT of each period T. The magnitude and duration of each of the pressure pulses 50 is sufficient to propagate to the surface where they can be detected and decoded. In its simplest form the tool 10 can be set up only to send encoded signals for a single parameter and the number of pressure pulses sent in each period TTcan be determined according to the value of the parameter. Alternatively, the second period TT can be broken down into separate segments with telemetry signals corresponding to different parameters being allocated to specific segments.

[0103] Fig.16 shows an enlarged representation of the time period TT. AS above stated, the surface equipment can synchronise with the tool in each operating cycle. The controller and the surface equipment are both set up to know the type and content of the data which will be transmitted. For example, in the example of Fig .16 the time period TT is divided into five smaller time periods and a pressure pulse can be triggered in each of those smaller time periods. In this example a short pressure pulse is transmitted in the first three smaller time periods, no pressure pulse is sent in the fourth smaller time period and a long pressure pulse is transmitted in the final time period. It will be understood that in a practical tool the number of smaller time periods can be significantly greater than five, and the options for pulse length or pulse frequency within each of the smaller time periods can significantly greater, whereby more data can be transmitted. It will be understood that the tool controller and the surface equipment are both set up for a defined set of data to be transmitted, in a defined order, in each operating cycle, for example data identifying the toolface, inclination, azimuth, gamma radiation, etc.

[0104] The detailed way in which the period TTis used to transmit encoded signals to the surface can be chosen to suit each particular application. The methodology of mud pulse telemetry is well established and most of the known methodologies can be used with the present invention to communicate encoded data corresponding to a single parameter or multiple parameters, as desired.

[0105] Since the decoding equipment at the surface can be kept in phase with the controller of the tool 10 as above explained it is only necessary for the operator to establish how the period TT is to be segmented and / or utilised.

[0106] Fig.13 represents an alternative control system in which the tool 10 takes advantage of periods of partial (or no) steering operation to send telemetry signals. Specifically, if the controller determines that parts of the time period Tsare not required for steering it can utilise those parts to send additional telemetry signals. Thus, since the valve in this situation is required for steering operation for only around half of the time period Ts, the remainder of that period, plus the time period TT, can be used for telemetry.

[0107] It will be seen that the pressure pulses created for telemetry are significantly shorter in duration than the pressure pulses created for steering, i.e. the valve 32 is switched much more quickly for telemetry than steering. The valve movements for telemetry are therefore intended to have little or no effect upon steering.

[0108] Fig.14 represents part of a tool 110 according to the second aspect of the invention. In this aspect the first chamber 152a and the second chamber 152b are respective parts of the conduit through which drilling fluid passes to the drill bit, i.e. the conduit 1 16 does not bypass the chambers.

[0109] The tool 110 has a primary piston 120a and a secondary piston 120b. Whilst the pistons 120 are shown diametrically opposed in Fig.14 that is for simplicity only and the tool will preferably have three pistons (or banks of pistons) spaced 120° apart around the tool. Also, two of the pistons (or banks of pistons) are preferably connected to move together as in the earlier embodiment.

[0110] Each of the pistons 120 is engaged by a respective mandrel 124 at cooperating inclined surfaces. The mandrels 124 are urged in a first direction (downwardly as drawn) to extend the pistons by fluid pressure in the respective chamber 152. The mandrels are urged in a second direction (upwardly as drawn) by a resilient biasing means which in this embodiment are respective return springs 154.

[0111] It will be recognised that movement of the mandrels 124 in the second direction does not directly retract the pistons, but rather provides the space to allow the pistons to retract. In practice the pistons will retract due to the reaction force of the borehole wall as the mandrels 124 are moved. The mandrel 124a acts as a piston in the cylinder provided by the first chamber 152a. The first chamber 152a has an inlet opening 140 and an outlet 156a. As shown, the cross-sectional area of the inlet opening 140 is significantly larger than that of the outlet 156a. Accordingly, when the inlet opening 140 is open as shown in Fig.14 high pressure drilling fluid passes through the inlet opening 140 and into the first chamber 152a. Notwithstanding that the outlet 156a is permanently open, the pressure of the fluid in the first chamber 152a is sufficient to move the mandrel 124a downwardly as drawn and to drive the primary piston 120a to extend.

[0112] The mandrel 124b acts as a piston in the cylinder provided by the second chamber 152b. The second chamber has an inlet opening 142 and an outlet 156b which are similarly configured to the inlet opening and outlet of the first chamber 152a described above. Fig.14 shows the inlet opening 142 closed by the flange 148 of the valve 132. It is arranged that the pressure of the fluid in the second chamber 152b is not sufficient to move the mandrel 124b downwardly, and on the contrary the return spring 154 causes the mandrel 124b to move upwardly as drawn and to allow the secondary piston 120b to retract.

[0113] When steering is required it is arranged that the valve 132 rotates about the longitudinal axis of the tool, whereby the inlet openings 140 and 142 are opened and closed cyclically, once each revolution of the tool. The number and size of the inlet openings 140 and 142 can be chosen to suit the application, and the valve 132 can have an appropriate number and disposition of flanges 148 to provide the required fluid flow rates into the first and second chambers. In particular, it can be arranged that the valve 132 has only two operational positions (with the inlet openings 140 and 142 open or closed respectively). Preferably, however, the valve 132 has a third operational position in which both of the openings 140,142 are fully or partially open together and the primary and secondary pistons are (all) extended.

[0114] Fig .17 represents a part of another tool 210 according to the invention. In the tool 210 the first chamber 252a is permanently connected to the conduit 216 by a flow channel 258a and the second chamber 252b is permanently connected to the conduit 216 by a flow channel 258b. Drilling fluid can flow to and from each of the first and second chambers by way of the respective flow channel.

[0115] The tool 210 has a primary piston 220a and a secondary piston 220b. As in Fig.14, whilst the pistons 220 are shown diametrically opposed in Fig.17 that is for simplicity only and the tool will preferably have three pistons (or banks of pistons) spaced 120° apart around the tool. Also, two of the pistons (or banks of pistons) are preferably connected to move together as in the earlier embodiment.

[0116] Each of the pistons 220a, b is engaged by a respective mandrel 224a, b at cooperating inclined surfaces.

[0117] The mandrels 224a, b are connected to respective piston members 260a, b and are moved by the fluid pressure of drilling fluid acting upon those piston members. Downwards movement of the mandrels 224a, b in the orientation as drawn will cause the respective pistons 220a, b to extend, and upwards movement of the mandrels will enable the pistons 220a, b to retract, similarly to the embodiment of Fig.14. The piston members 260a, b are urged in a first direction (downwardly as drawn) by fluid pressure in the respective chamber 252a, b, i.e. an increase in fluid pressure above the piston member. The piston members are urged in a second direction (upwardly as drawn) by fluid pressure below the piston member. Specifically, internal porting (not shown) connects the first chamber 252a to a flow channel 262a, and separate internal porting (not shown) connects the second chamber 252b to a flow channel 262b.

[0118] When steering is required it is arranged that the valve 232 rotates about the longitudinal axis of the tool, whereby the inlet openings 240 and 242 are opened and closed cyclically, once each revolution of the tool. The number and size of the inlet openings 240 and 242 can be chosen to suit the application, and the valve 232 can have an appropriate number and disposition of flanges 248.

[0119] In the position shown in Fig.17 the valve 232 is in its first position with drilling fluid in the conduit 216 in communication with the first chamber 252a and the first flow channel 262a. The first piston member 260a and the first mandrel 224a are therefore together urged downwardly as drawn to extend the first piston 220a. At the same time, the high pressure of the conduit 216 is communicated to the region below the second piston member 260b by way of the flow channel 262a to urge the piston member 260b and the second mandrel 224b upwardly as drawn to enable the second piston 220b to retract. When the valve 232 moves to its second position with drilling fluid in the conduit 216 in communication with the second chamber 252b and the second flow channel 262b the situation is reversed and the mandrel 224b is urged downwardly and the mandrel 224a is urged upwardly as drawn.

[0120] The region below the mandrels 224a, b is connected to the annulus surrounding the tool 210 by conduits 264. In this embodiment therefore the mandrels 224a, b operate in chambers which are filled with drilling fluid and are lubricated by drilling fluid. It will be understood, however, that in an alternative embodiment the chambers containing the mandrels can be isolated from the drilling fluid and filled with oil. For example, the regions below the mandrels can be closed, suitable sliding seals can be provided for the shafts between the piston members 260 and the mandrels 224, and sealed pistons can be provided in each of the conduits 264. The sealed pistons can allow the oil-filled chambers to be compensated to the annulus fluid pressure, in known fashion. Locating the mandrels in oil-filled chambers is expected to benefit the lubrication of the mandrel / piston interface and to reduce the wear of that interface.

Claims

CLAIMS1 . A rotary steerable tool for use in a borehole with the tool connected to surface equipment by a rotatable drill string, the tool having: a first end, a second end and a longitudinal axis between the first end and the second end; a conduit from the first end to the second end through which drilling fluid can flow through the tool; at least one piston which is movable in a direction lateral to the longitudinal axis; a chamber for drilling fluid; a valve between the conduit and the chamber, the valve being movable between a first position and a second position, the valve in the first position connecting the conduit and chamber whereby in use the at least one piston is driven to extend from the tool, and the valve in the second position disconnecting the conduit and chamber whereby in use the at least one piston can retract; a controller for controlling the position of the valve, the controller being connected to at least one sensor for detecting a chosen parameter, the controller being configured to receive a signal corresponding to the detected parameter from the sensor and being further configured to encode the signal and to communicate the encoded signal to the surface equipment by way of one or more pressure pulses in the drilling fluid, wherein the pressure pulses are created by actuating the valve.

2. The rotary steerable tool according to claim 1 having a resilient biasing means configured to bias the piston to retract.

3. The rotary steerable tool according to claim 1 or claim 2 having more than one piston.

4. The rotary steerable tool according to claim 3 in which multiple pistons are aligned along the longitudinal axis.

5. The rotary steerable tool according to claim 3 or claim 4 having three pistons separated around the longitudinal axis.

6. The rotary steerable tool according to claim 5 in which the three pistons comprise a primary piston and two secondary pistons, and in which the two secondary pistons are actuated together.

7. The rotary steerable tool according to claim 6 having a first chamber for drilling fluid and a second chamber for drilling fluid, the valve in the first position connecting the conduit to the first chamber for actuating the primary piston and the valve in the second position connecting the conduit to the second chamber for actuating the secondary pistons.

8. The rotary steerable tool according to claim 7 in which the valve has a third position in which the conduit is connected to the first chamber and to the second chamber for actuating the primary piston and the secondary pistons.

9. The rotary steerable tool according to any one of claims 6-8 in which the primary piston is offset from the secondary pistons along the longitudinal axis.

10. The rotary steerable tool according to any one of claims 1 -9 in which the at least one piston is actuated by a mandrel.1 1 . The rotary steerable tool according to claim 10 in which the mandrel moves in a longitudinal direction relative to the remainder of the tool.

12. The rotary steerable tool according to claim 10 or claim 11 in which the mandrel is driven in a first direction by fluid pressure within the chamber, and is driven in a second direction by fluid pressure in a separate chamber.

13. The rotary steerable tool according to any one of claims 10-12 in which the mandrel has a surface in contact with a surface of the piston, the contact surfaces of the mandrel and piston being inclined relative to the longitudinal axis.

14. The rotary steerable tool according to claim 13 in which the contact surfaces are lubricated by oil.

15. The rotary steerable tool according to any one of claims 1 -14 in which the at least one piston is located in a respective radial extension of the tool.

16. The rotary steerable tool according to any one of claims 1 -15 in which the controller is configured to operate according to a control system, the control system having an operating cycle of a predetermined duration, the controller being configured to actuate the valve for steering of the tool and to actuate the valve for creating pressure pulses for communicating the encoded signal to the surface equipment, during each operating cycle.

17. The rotary steerable tool according to claim 16 in which the operating cycle has a first part during which the valve can be operated for steering of the tool and a second part during which the valve can be operated for creating pressure pulses for communicating the encoded signal to the surface equipment.

18. The rotary steerable tool according to claim 17 in which the first and second parts are fixed proportions of the operating cycle.

19. The rotary steerable tool according to any one of claims 16-18 in which the operating cycle comprises multiple revolutions of the tool in use.

20. The rotary steerable tool according to any one of claims 16-19 in which the controller is configured to actuate the valve for steering the tool at the start of each operating cycle, whereby to provide a pressure pulse which can be recognised by the surface equipment and enable the surface equipment to synchronise with the operating cycle.

21. The rotary steerable tool according to any one of claims 1-20 in which the controller actuates the valve with a first actuation frequency when creating pressure pulses for communicating the encoded signal to the surface equipment and actuates the valve with a second actuation frequency for steering the tool, and in which the first actuation frequency is higher than the second actuation frequency.

22. A rotary steerable tool according to any one of claims 1 -21 in which the chamber(s) is(are) a parts of the conduit whereby drilling fluid flows from the first end to the second end through the chamber(s).

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