Pulsating Rotary Steerable System

Abstract

A drilling assembly disposed on a drillstring can deviate a borehole advanced by a drill bit. A housing coupled between the drillstring and the bit transfers rotation of the drillstring to the bit. A mandrel in the housing's axial bore has a least one inclined surface on at least one side. The mandrel is subject to pressure in the housing and can be displaced longitudinally. A biasing element biases the mandrel toward one position. At least one piston disposed on the housing's side can be moved by the at least one inclined surface and can be displaced transversely in response to the longitudinal displacement of the mandrel. To vary the transverse displacement of the piston and change a trajectory of the borehole, changes to the differential pressure across the mandrel longitudinally displaces the mandrel and moves the piston.

Claims

1. A drilling assembly disposed on a drillstring for deviating a borehole advanced by a drill bit, the assembly comprising: a housing coupled between the drillstring and the drill bit and transferring rotation of the drillstring to rotation of the drill bit, the housing having an axial bore along a longitudinal axis communicating fluid flow from the drillstring to the drill bit; a mandrel disposed in the axial bore of the housing and having a least one inclined surface disposed on at least one side thereof, the mandrel being subject to first pressure in the housing and second pressure in the borehole and being displaceable in the axial bore along the longitudinal axis; and at least one piston disposed on a side of the housing relative to the at least one inclined surface of the mandrel, the at least one piston being displaceable on the housing transverse to the longitudinal axis in response to the longitudinal displacement of the mandrel, wherein the longitudinal displacement of the mandrel responds to a pressure differential between the first and second pressures across the mandrel and varies the transverse displacement of the at least one piston.

2. The assembly of claim 1, further comprising a flow control controlling the fluid flow through the housing, the flow control creating a change in the pressure differential across the mandrel and periodically varying the transverse displacement of the at least one piston with the longitudinal displacement of the mandrel.

3. The assembly of claim 2, wherein the flow control bypasses at least a portion of the fluid flow away from the housing.

4. The assembly of claim 2, wherein the flow control comprises a valve disposed within the fluid flow to the housing.

5. The assembly of claim 2, wherein the flow control in a first condition produces a first of the pressure differential across the mandrel; and wherein the flow control in a second condition produces a second of the pressure differential across the mandrel, the second pressure differential being greater than the first differential pressure.

6. The assembly of claim 1, wherein the transverse displacement of the at least one piston displaces the longitudinal axis of the housing relative to the advancing borehole.

7. The assembly of claim 1, wherein the at least one piston disposed on the side of the housing comprises an external portion exposed externally on the housing and comprises an internal portion disposed in the axial bore of the housing, the internal portion of the at least one piston engageable with the at least one inclined surface of the mandrel.

8. The assembly of claim 1, further comprising a biasing element disposed in the housing and biasing the mandrel toward a first position in the housing.

9. The assembly of claim 8, wherein the biasing element comprises one or more springs having one portion fixed in the axial bore of the housing and having another portion engaged with the mandrel.

10. The assembly of claim 8, wherein the biasing element is loaded with an axial force equating to a transverse force of the at least one piston displaced transversely on the housing when the mandrel is longitudinally displaced toward the first position.

11. The assembly of claim 1, wherein the mandrel defines an internal passage communicating the fluid flow therethrough from adjacent the drillstring to adjacent the drill bit.

12. The assembly of claim 1, wherein the mandrel comprises a first portion sealably engaged in the axial bore of the housing and defining a first piston area subject to the first pressure.

13. The assembly of claim 12, wherein the mandrel comprises a second portion sealably engaged in the axial bore of the housing and defining a second piston area subject to the second pressure.

14. The assembly of claim 13, wherein the housing comprises at least one port communicating the second pressure in the borehole with the second piston area of the mandrel.

15. The assembly of claim 14, wherein the at least one port comprises a nozzle restricting communication between the borehole and a chamber in the housing to which the second piston area is exposed.

16. The assembly of claim 1, wherein the mandrel rotates with the housing.

17. The assembly of claim 1, wherein the mandrel comprises first and second seals sealably engaging the axial bore of the housing and sealing a space between the mandrel and the housing adjacent the at least one inclined surface.

18. The assembly of claim 1, wherein the at least one piston is sealed in an external opening in the side of the housing.

19. The assembly of claim 1, wherein the at least one piston is extended from the side of the housing in response to a first of the pressure differential across the mandrel longitudinally displacing the mandrel toward a first position.

20. The assembly of claim 19, wherein the at least one piston is retracted from the side of the housing in response to a second of the pressure differential across the mandrel longitudinally displacing the mandrel toward a second position.

21. The assembly of claim 20, wherein a first state of the fluid flow through the housing produces the first pressure differential, and wherein a second state of the fluid flow through the housing produces the second pressure differential, the first and second states of the fluid flow being produced by at least one of a pump and a valve.

22. The assembly of claim 1, wherein the at least one piston comprises more than one piston disposed along a same side of the housing.

23. The assembly of claim 1, wherein the at least one piston comprises first and second pistons disposed on first and second opposing sides of the housing, the first and second pistons being oppositely retracted and extended relative to one another on the first and second opposing sides of the housing.

24. The assembly of claim 23, wherein the first and second pistons produce two comparable deflections of the longitudinal axis of the housing relative to the borehole within one revolution of the housing.

25. A drilling system having a drill bit on a drillstring to advance a borehole, the system comprising: a drive rotating the drillstring to rotate the drill bit; a pump pumping fluid flow along the drillstring to the drill bit; a flow valve operable to control the fluid flow; and a steering mechanism disposed on the drillstring upstream of the drill bit and downstream of the flow valve, the steering mechanism having an inner mandrel and at least one piston, the inner mandrel being displaceable along a longitudinal axis of the steering mechanism in response to a pressure differential across the mandrel from the controlled fluid flow, the inner manner longitudinally displaced varying a transverse displacement of the at least one piston relative to a side of the advancing borehole.

26. A drilling method, comprising: advancing a borehole with a drill bit on a drilling assembly coupled to a drillstring by rotating the drilling assembly and the drill bit with rotation of the drillstring; and deviating the drilling assembly in the advancing borehole by: controlling fluid flow through the drilling assembly; varying a pressure differential across a mandrel inside the drilling assembly with the controlled fluid flow; displacing the mandrel along a longitudinal axis in the drilling assembly with the varied pressure differential; and alternating, relative to the rotation of the drilling assembly, transverse displacement of at least one piston on at least one side of the drilling assembly with the longitudinal displacement of the mandrel.

27. The method of 26, wherein controlling the fluid flow through the drilling assembly comprises: determining a steering direction for the drilling assembly; sensing an angular orientation of the drilling assembly; and varying the fluid flow through the drilling assembly based upon the determined steering direction and the sensed angular orientation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 schematically illustrates a downhole assembly incorporating a steering mechanism according to the present disclosure.

[0027] FIG. 2 illustrates a perspective view of the downhole assembly incorporating the steering mechanism according to the present disclosure.

[0028] FIG. 3A is a cross-sectional view of one steering mechanism according to the present disclosure.

[0029] FIG. 3A-1 illustrates an end-sectional view of the steering mechanism of FIG. 3A.

[0030] FIGS. 3B-3C illustrate cross-sectional views of other steering mechanisms according to the present disclosure.

[0031] FIGS. 4A-4B conceptually illustrate components of flow controls for the disclosed steering mechanism during operation.

[0032] FIG. 5A illustrates a cross-sectional view of the steering mechanism in a first state according to the present disclosure.

[0033] FIG. 5B illustrate a cross-sectional view of the steering mechanism in a second state according to the present disclosure.

[0034] FIGS. 6A-6B are end views of the steering mechanism during operation.

[0035] FIGS. 7A-7C are end views of the steering mechanism during operation using a pair of opposing pistons.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0036] FIG. 1 schematically illustrates a drilling system 10 incorporating a steering mechanism 100 according to the present disclosure. As shown, a downhole drilling assembly 20 drills a borehole 12 penetrating an earth formation. The assembly 20 is operationally connected to a drillstring 22 using a suitable connector 21. In turn, the drillstring 22 is operationally connected to a rotary drilling rig 24, which is well known in the art.

[0037] The downhole assembly 20 includes a control assembly 30 having a sensor section 32, a power supply section 34, an electronics section 36, and a downhole telemetry section 38. The sensor section 32 has directional sensors, such as accelerometers, magnetometers, and inclinometers, which can be used to indicate the orientation, movement, and other parameters of the assembly 20 within the borehole 12. This information, in turn, can be used to define the borehole's trajectory for steering purposes. The sensor section 32 can also have any other type of sensors used in Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD) operations including, but not limited to, sensors responsive to gamma radiation, neutron radiation, and electromagnetic fields, such as available on Weatherford's HEL system.

[0038] The electronics section 36 has electronic circuitry to operate and control other elements within the assembly 20. For example, the electronics section 46 has a downhole processor (not shown) and downhole memory (not shown). The memory can store directional drilling parameters, measurements made with the sensor section 32, and directional drilling operating systems. The downhole processor can process the measurement data and telemetry data for the various purposes disclosed herein.

[0039] Elements within the assembly 20 communicate with surface equipment 28 using the downhole telemetry section 28. Components of this telemetry section 38 receive and transmit data to an uphole telemetry unit (not shown) within the surface equipment 38. Various types of borehole telemetry systems can be used, including mud pulse systems, mud siren systems, electromagnetic systems, angular velocity encoding, and acoustic systems.

[0040] The power supply section 34 supplies electrical power necessary to operate the other elements within the assembly 20. The power is typically supplied by batteries, but the batteries can be supplemented by power extracted from the drilling fluid by way of a power turbine, for example.

[0041] During operation, a drill bit 40 is rotated, as conceptually illustrated by the arrow R.sub.B. The rotation of the drill bit 40 is imparted by rotation R.sub.D of the drillstring 22 at the rotary rig 24. The speed (RPM) of the drillstring rotation R.sub.D is typically controlled from the surface using the surface equipment 28. Additional rotation to the drill bit 40 could also be imparted by a drilling motor (not shown) on the drilling assembly 20.

[0042] During operation, the drilling fluid system 26 pumps drilling fluid or mud from the surface downward and through the drillstring 22 to the assembly 20. The mud exits through the drill bit 40 and returns to the surface via the borehole annulus. Circulation is illustrated conceptually by the arrows 14.

[0043] To directionally drill the advancing borehole 12 with the downhole assembly 20, a flow control 50 is operated to change pressure of the flow of the fluid (circulated drilling mud) from the drillstring 22, through the drilling assembly 20, and out the bit 40. The pressure change then alters the operation of the steering mechanism 100. The flow control 50 is controlled using orientation information measured by the sensor section 32 cooperating with control information stored in the downhole processor of the electronics section 36 to direct the trajectory of the advancing borehole 12.

[0044] In particular, the flow control 50 can be a fluid flow restriction, a fluid release element, a pulser, a flow bypass, etc., which can be disposed anywhere within the flow of fluid (i.e., within the mud system 26, the drillstring 22, the drilling assembly 20, or elsewhere). For the purposes of discussion, reference is made to the flow control 50 as a pulser. In a particular embodiment as disclosed herein, the pulser 50 is disposed in or associated with the drilling assembly 20 upstream of the steering mechanism 100.

[0045] The steering mechanism 100 steers the assembly 20 using active deflection of the assembly 20. During operation of the assembly 20, for example, the pulser 50 controls the flow of fluid through the assembly 20 and creates a change in differential pressure in the steering mechanism 100. The steering mechanism 100 then uses the varied pressure differential to periodically extend/retract one or more piston(s) 150 relative to the drill bit's rotation R.sub.B to define the trajectory of the advancing borehole 12. In the end, the extension disproportionately engages the drill bit 40 against a certain side in the advancing borehole 12 for directional drilling. (Reference to disproportionate engagement at least means that the engagement in advancing the borehole 14 is periodic, varied, repetitive, selective, modulated, changing over time, etc.) In the end, the extension of the pistons 150 can be coordinated with the orientation of the drilling assembly 20 in the advancing borehole 10 to control the trajectory of drilling.

[0046] The operation of the pulser 50 not only extends/retracts the piston(s) 150, but also advantageously alters the thrust load of the drilling assembly 20 along its longitudinal axis, which alters the weight-on-bit/rate of penetration of the assembly 20. In this way, the steering mechanism 100 provides active deflection of the downhole assembly 20 and can operate the deflection in conjunction with the thrust load along the assembly's longitudinal axis. For instance, deflection in one direction or another can be timed toward or away from a desired trajectory and can be timed in conjunction with an increase or a decrease in thrust load of the assembly 20.

[0047] Moreover, the resultant rotational speed R.sub.B of the drill bit 40 can be periodically varied by periodically varying the rotational speed of a mud motor (not shown) and/or by periodically varying the rotational speed R.sub.D of the drillstring 22. Such periodic bit speed rotation R.sub.B (referred to herein as a bit speed effect) results in preferential cutting of material from a predetermined arc of the borehole's wall, which in turn results in deviation of the borehole 10. Further details of the bit speed effect are disclosed in incorporated U.S. Pat. No. 7,766,098.

[0048] Given the above description of the drilling system 10, discussion now turns to embodiments of the drilling assembly 20 having the steering mechanism 100 to achieve directional drilling.

[0049] FIG. 2 illustrates a perspective view of the drilling assembly 20 incorporating the steering mechanism 100 according to the present disclosure. As already noted, the steering mechanism 100 of the drilling assembly 20 is disposed on a drillstring (not shown) for deviating a borehole advanced by the drill bit 40. The control assembly 30 of the drilling assembly 20 extends from the drillstring (not shown). As shown, a stabilized direction sonde tool 35, such as Weatherford's Integrated Direction Sonde tool, can be part of the control assembly 30. The steering mechanism 100 extends from this tool 35 and has the drill bit 40 connected to its distal end.

[0050] One or more of the pistons 150 are disposed on the steering mechanism 100 near the drill bit 40. The pistons 150 can be arranged on one or more sides of the mechanism 100 and can be disposed at stabilizer ribs or other features 115. The overall length of the drilling assembly 20 can vary. In one arrangement, the steering mechanism 100 extends about 20-ft or so from the stabilizer tool 35 to the drill bit 40. Other configurations are possible.

[0051] Further details of the steering mechanism 100 are provided in the cross-sectional views of FIGS. 3A-3C. FIG. 3A is a cross-sectional view of one embodiment of the steering mechanism 100 according to the present disclosure, and FIG. 3A-1 illustrates an end view of the steering mechanism 100 of FIG. 3A. As alternatives, FIGS. 3B-3C illustrate cross-sectional views of other embodiments of the steering mechanism 100 according to the present disclosure.

[0052] As shown in each of the embodiments, the steering mechanism 100 includes a housing 110, an inner mandrel 120, and a biasing element 130. The housing 110 couples at one end 114 toward the drilling assembly's other components and the drillstring (not shown). The other end of the housing 116 couples toward the drill bit (not shown), such as by coupling to a bit sub 106 for the drill bit. As the drillstring rotates, the housing 100 thereby transfers the drillstring's rotation to the drill bit 40.

[0053] The housing 110 has an axial bore 112 along the housing's longitudinal axis (L) communicating the drillstring with the drill bit. The mandrel 120 is disposed in the axial bore 112 of the housing 110 and is displaceable therein along the longitudinal axis (L) between a first position (FIGS. 3A-3C) and a second position (discussed below).

[0054] The mandrel 120 has an internal passage 122 communicating therethrough from adjacent the drillstring at the housing's proximal end 114 to adjacent the drill bit at the housing's distal end 116. The mandrel 120 is generally tubular and includes a first seal 125 disposed thereon. This first seal 125 is sealably engaged with the housing's bore 112 and defines a first piston area A.sub.1 exposed to flow of fluid through the housing 110 from the drillstring to the drill bit.

[0055] The biasing element 130 is disposed in the housing 110 and biases the mandrel 120 toward the first position (FIG. 3A-3C). For example, the biasing element 130 can include one or more coil springs 132 or the like disposed in the annular space between the mandrel 120 and the housing's bore 112. Several of these springs 132 can be stacked along a length of the mandrel 120 and can be separated by spacer rings 133. One portion 136 of the biasing element 130 is fixed in the axial bore 112 of the housing 110, and another portion 134 of the biasing element 130 is engaged with the mandrel 120. In this way, longitudinal displacement of the mandrel 120 is biased in the housing 110 by the biasing element 130.

[0056] An intermediate section 140 of the mandrel 120 extends through the housing's bore 112. This section 140 can be a separate component with a passage 142 and can be attached to the rest of the mandrel 120, which can facilitate manufacture, although an integral construction can be used. The intermediate section 140 has a least one inclined surface 145 disposed on at least one side of the section 140. As shown here in FIGS. 3A-3C, the section 140 includes a plurality of inclined surfaces 145, although more or less could be used depending on the implementation. As shown in FIG. 3A, the inclined surfaces 145 can be disposed along more than one side of the mandrel 120, such as on opposing sides. Alternatively as shown in FIGS. 3B-3C, the inclined surfaces 145 can be disposed on one side or the other of the mandrel 120.

[0057] The mandrel 120 also has a distal section 160 extending through the housing's bore 112. This distal section 160 can be a separate component with a passage 162 and can be attached to the rest of the mandrel 120 (i.e., to the intermediate section 140), which can facilitate manufacture, although an integral construction can be used. The distal section 160 is generally tubular and includes a second seal 165 disposed thereon. The second seal 165 is sealably engaged with the housing's bore 112 and defines a second piston area A.sub.2 exposed to a fluid chamber 118 of the housing 110.

[0058] The housing 110 has at least one port 107 communicating with the fluid chamber 118. In the present example, the bit sub 106 has the port 107, although other configurations are possible. In one embodiment, the fluid chamber 118 can be exposed to the borehole annulus so that the chamber 118 holds the annular pressure, which acts against the second piston area A.sub.2 of the mandrel 120. This pressure is in contrast to the bore pressure in the housing 110 that acts against the first piston area A.sub.2 of the mandrel 120.

[0059] In further embodiments, the port 107 for the chamber 118 may include a nozzle or restriction 108 to control the flow of fluid through the port 107 between the chamber 118 and the surrounding annulus. Such a nozzle 108 can have an orifice or the like configured in size to provide a dampening effect to the mechanism's operation. In an alternative embodiment, the chamber 118 can be sealed with a cap 108 and can be pre-charged to a desired pressure to match an expected annular pressure or some other pressure.

[0060] At least one piston 150 is disposed on at least one side of the housing 110 relative to the at least one inclined surface 145 of the mandrel 120. As shown in FIG. 3A, sets of pistons 150a-c/150a-c can be disposed along more than one side of the housing 110, such as on opposing sides. Alternatively as shown in FIGS. 3B-3C, one set of pistons 150a-c or 150a-c can be disposed only along one side of the housing 110. These and other configurations can be used.

[0061] The at least one piston 150 disposed on the side of the housing 110 has an external portion exposed externally on the housing 100 and has an internal portion disposed in the axial bore 112 of the housing 110. The internal portion of at least one piston 150 is inclined and is engageable with the at least one inclined surface 145 of the mandrel 120. In this way, the at least one piston 150 is alternatingly displaceable on the housing 110 between extended and retracted conditions transverse to the longitudinal axis (L) of the mandrel 120 in response to longitudinal displacement of the mandrel 120.

[0062] As shown in the end view of FIG. 3A-1, the at least one piston 150 can be disposed at a raised pad or stabilizer rib 115 on the circumference of the housing 110. In fact, several of such raised stabilizer ribs 115 can be provided.

[0063] The mandrel 120 has first and second seals 144a-b sealably engaging the axial bore 112 of the housing 110 on both ends of the intermediate section 140. These seals 144a-b seal the annular space between the mandrel 120 and the housing 110 around the inclined surface(s) 145 and piston(s) 150 to prevent fluid leakage therebetween. Each piston(s) 150 is likewise sealed in an external opening 114 in the side of the housing 100 to prevent fluid leakage.

[0064] For further isolation, an intermediate chamber 164 can be sealed in the housing 110 around the mandrel 120 between the distal seal 144b and the second mandrel seal 165. This intermediate chamber 164 can be filled with oil or the like.

[0065] As discussed in more detail below, the flow control (i.e., pulser 50) of the drilling assembly 20 controls the flow of fluid through the housing 110 and creates a change in the differential pressure across the mandrel 120. In turn, the differential pressure across the mandrel 120 longitudinally displaces the mandrel 120, which then either extends or retracts the piston(s) 150 on the housing 100 to change the trajectory of the assembly 20. In this way, periodically varying the longitudinal displacement of the mandrel 120 can periodically vary the transverse displacement of the piston(s) 150 to control the drilling trajectory.

[0066] FIGS. 4A-4B conceptually illustrate components of the flow control 50 for the steering mechanism 100. As already noted, the flow control 50 can be disposed anywhere in the fluid flow from the surface to the steering mechanism 100, and the flow control 50 can be a pulser disposed in the fluid flow upstream of the steering mechanism 100the actual placement can vary depending on the implementation. The pulser 50 is preferably housed in proximity to the control components in the drilling assembly 20. As indicated here, for example, the pulser 50 may be disposed in a tubular member 25, which can be part of the drilling assembly 20.

[0067] As shown in the embodiment of FIG. 4A, the pulser 50 can control communication of the fluid flow through drilling assembly (20) and eventually to the steering mechanism (100) downstream by selectively moving a plunger or valve element 51 relative to one or more bypass ports 27 in the tubular member 25. An actuator 58 (under control of the control components at the surface or downhole) moves the plunger 51 relative to the bypass ports 27 to vary fluid communication to the steering mechanism (100).

[0068] When the pulser 50 is in a closed condition as shown in FIG. 4A, downhole flowing drilling fluid may be prevented from flowing out the bypass ports 27 so that the fluid passes further downstream to the steering mechanism (100). When the pulser 50 is in an opened condition, the plunger 51 is extended further in the tubular member 25. At least some of the flow of fluid exits through the bypass ports 27, which starves flow downstream to the steering mechanism (100). In this way, the pulser 50 can bypass at least a portion of the flow of fluid away from the steering mechanism (100). Accordingly, opening and closing of the pulser 50 affects or changes the differential pressure in the steering mechanism (100) and affects the longitudinal displacement of the mandrel (120) and extension of the piston(s) (150).

[0069] As shown in the embodiment of FIG. 4B, the pulser 50 can control fluid communication through the tubular member 25 by selectively moving a valve element 54 relative to a seat 56 in the tubular member 25. An actuator 52 (under control of a control components at the surface or downhole) moves the valve element 54 relative to the seat 56 to vary fluid communication to the steering mechanism (100).

[0070] The pulser's actuator 52 controls the valve element 54 and can be connected to other components within the assembly (20), which may or may not be in the control section (40). During operation, the actuator 58 can control the valve element 54 to move between an open condition as shown (in which drilling fluid can pass to the mechanism 100) and a closed condition (not shown) (in which drilling fluid cannot pass downhole to the mechanism 100).

[0071] Having an understanding of the steering mechanism 100 and the flow control or pulser 50, discussion now turns to operation of the drilling assembly 20. FIGS. 5A-5B illustrate cross-sectional views of the steering mechanism 100 in two states of operation. Because the steering mechanism 100 can have one or more piston(s) 150 disposed along one or more sides of the housing 100, FIGS. 5A-5B are visually divided so that all of the various arrangements, such as discussed in FIGS. 3A-3C, can be shown and described together. For instance, for the embodiment in FIG. 3A having pistons 150a-c and 150a-c on opposing sides of the housing 110, the entire steering mechanism 100 in FIGS. 5A-5B would be representative of the operation of the opposing sets of the pistons 150a-c and 150a-c. By contrast, for the embodiment in FIG. 3B having the one set of pistons 150a-c on one side of the housing 110, the upper half of the steering mechanism 100 in FIGS. 5A-5B would be representative of the operation of the pistons 150a-c, while the other sides of the housing 110 would lack pistons. Finally, for the embodiment in FIG. 3C having the one set of pistons 150a-c on the opposing side of the housing 110, the lower half of the steering mechanism 100 in FIGS. 5A-5B would be representative of the operation of the pistons 150a-c, while the other sides of the housing 110 would lack pistons.

[0072] As shown in FIG. 5A, for example, a first state of operation occurs when a first level of the pressure differential across the mandrel 120 longitudinally displaces the mandrel 150 toward the first position (i.e., uphole to the left away from the end 116 to which the drill bit couples). In response to the displacement of the mandrel 120, first of the pistons 150a-c on a first side of the housing 110 are extended (i.e., on the upper half of FIG. 5A). This would be the case for the arrangement of the mechanism 100 in FIGS. 3A-3B. Meanwhile, the pistons 150a-c on the second, opposing side of the housing 110 are retracted (i.e., on the lower half of FIG. 5A). This would be the case for the arrangement of the mechanism 100 in FIGS. 3A and 3C.

[0073] The first level of the pressure differential across the mandrel 120 is generally given by the bore pressure acting against the first piston area A.sub.1 minus the annular pressure acting against the second piston area A.sub.2 in conjunction with the spring force of the biasing element 130. For example, the first level of the pressure differential in the first state of FIG. 5A can be produced by an unenergized flow state. In other words, the pulser 50 can be (fully or at least partially) closed, or the pump(s) used in pumping the fluid through the assembly 20 can be off or at a low pump rate.

[0074] The biasing element 130 may be preloaded with a certain axial force that produces a particular transverse force of the extended pistons 150a-c on the housing 110. This may prevent at least some of the counterforce from the borehole wall against the extended pistons 150a-c from displacing the mandrel 120.

[0075] As shown in FIG. 5B, by contrast, a second state of operation occurs when a second level of the pressure differential across the mandrel 120 longitudinally displaces the mandrel 120 toward the second position (i.e., downhole to the right toward the end 116 to which the drill bit couples). In response to the displacement of the mandrel 150, first of the pistons 150a-c on the first side of the housing 110 are retracted (i.e., on the upper half of FIG. 5A). This would be the case for the arrangement of the mechanism 100 in FIGS. 3A-3B. Meanwhile, the pistons 150a-c on the second, opposing side of the housing 110 are extended (i.e., on the lower half of FIG. 5A). This would be the case for the arrangement of the mechanism 100 in FIGS. 3A and 3C.

[0076] The second level of the pressure differential across the mandrel 120 is generally given by the bore pressure acting against the first piston area A.sub.1 minus the annular pressure acting against the second piston area A.sub.2 in conjunction with the spring force of the biasing element 130. The second level of the pressure differential in the second state of FIG. 5B can be produced by an energized flow state. In other words, the pulser 50 can be (fully or mostly) opened, and the pump(s) can be actively pumping.

[0077] As expressed herein, the mandrel 120 rotates with the housing 110, and the housing 110 rotate with the drillstring. As the drill bit rotates with the housing 110 and the drillstring, the transverse displacement of the piston(s) 150 can then displace the longitudinal axis of the housing 110 relative to the advancing borehole. This, in turn, tends to change the trajectory of the advancing borehole. To do this, the extension/retraction of the piston(s) 150 is timed relative to a desired direction to deviate the drilling assembly 20 during drilling. For example, FIG. 6A-6B show end views of the steering mechanism 100 with one movable piston 150 extended therefrom. Because the steering mechanism 100 is rotated along with the drillstring (22), the operation of the steering mechanism 100 is cyclical to substantially match the period of rotation of the drillstring (22).

[0078] If it is desired to deviate the drill bit in a direction towards the direction given by arrow D, it is necessary to extend the movable piston 150 as it faces the opposite direction O. The steering mechanism 100 rotates with the assembly (20), which in turn rotates with the drillstring (22). The orientation of the movable piston 150 can therefore be determined by the sensor section (32) of the assembly (20). When it is desired to deviate the borehole in the chosen direction (the direction of the arrow D), the control assembly (30) can calculate the orientation of the diametrically opposed position O and can instruct the pulser (50) to operate accordingly. Specifically, the pulser (50) may produce the displacement of the mandrel (120) so the piston 150 extends at a first angular orientation relative to the desired direction D and then retracts at a second angular orientation for the rotation of the steering mechanism 100.

[0079] Because the movable piston 150 does not move instantaneously to its extended condition, it may be necessary that the active deflection functions before the piston 150 reaches the opposite position O and that the active deflection remains active for a proportion of each rotation. If desired, it can be arranged that the angles and are equally-spaced to either side of the position O, but because it is likely that the movable piston 150 will extend gradually (and in particular more slowly than it will retract) it may be preferable that the angle is closer to the position O than is the angle .

[0080] Of course, the steering mechanism 100 as disclosed herein can have additional piston(s) arranged at different angular orientations about the mechanism's circumference. Extension and retraction of the additional piston(s) can be comparably controlled in conjunction with what has been discussed with reference to FIGS. 6A-6B so that the control assembly (30) can coordinate multiple retractions and extensions of serval pistons 150 during the rotations. Thus, the displacement of the mandrel 120 and piston(s) 150 can be timed with the rotation of the drillstring (22) and the assembly 20 based on the orientation of the steering mechanism 100 in the advancing borehole and can be timed to direct the drill bit (40) in a desired drilling direction. Because the drillstring (22) rotates, the timing of the displacement can be performed with each rotation or any subset of the rotations.

[0081] For example, FIGS. 7A-7C illustrate end views of the steering mechanism having opposing pistons 150a and 150a during operation. As shown in FIG. 7A, an intermediate state of operation can have the opposing pistons 150a and 150a both partially extended from the housing 110. During one revolution of the housing 110 from FIGS. 7B-7C, the opposing pistons 150a and 150a can produce two pushing actions of the steering mechanism 100 toward a desired direction D.

[0082] As shown first in FIG. 7A as the housing 110 rotates, a first of the piston 150a is fully extend to engage the borehole wall starting at some angular orientation opposite the desired direction D. This extension produces a first pushing action that continues further throughout the rotation beyond FIG. 7B. At some point, the first piston 150a is retracted while a second of the pistons 150a then fully extends to engage the borehole wall, as shown in FIG. 7C. This second extension occurs at some angular orientation opposite the desired direction D and produces a second push that continues further throughout the single rotation beyond FIG. 7C.

[0083] A control system, such as disclosed in incorporated U.S. Pat. No. 7,766,098, can be used to extend and retract the pistons 150a-c. When the pulser 50 is open, the pistons 150a-c can attain an active state being extend, retracted, or both, depending on the number and arrangement of the pistons 150a-c about the circumference of the housing 110. When the pulser 50 is closed, the pistons 150a-c can attain a default state as the mandrel 120 returns by the bias of the spring 130. Again, depending on the number and arrangement of the pistons 150a-c about the circumference of the housing 110, the pistons 150 can be extend, retracted, or both in this default state, but in a different configuration from the active state. Any number of intermediate states can also be achieved in which pistons 150 on more than one side of the housing 110 are extended (at partially) at the same time.

[0084] The pistons 150 can have surface treatment, such as Tungsten Carbide hard facing, or other feature to resist wear. The housing 100 can be configured for more than one borehole size. For example, the housing can be used for drilling 8, 8, and 8 in. hole sizes. However, different pistons 150 of different lengths can be used with the housing 110 for the different hole sizes. This gives some versatility and modularity to the assembly.

[0085] In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the disclosed subject matter. Therefore, it is intended that the disclosed subject matter include all modifications and alterations to the full extent that they come within the scope of the disclosed embodiments or the equivalents thereof.