Abstract:
A drilling system, including: a drill bit; and a thruster to apply a force to the drill bit. The thruster may include: an inner tubular member disposed within and configured to axially move within an outer tubular member; a thrust piston to transmit a hydraulic force to the inner tubular member, the thrust piston separating an upstream fluid chamber and a downstream fluid chamber between the inner and outer tubular members; at least one pressure switch fluidly connected to the downstream fluid chamber to control flow of a fluid to and from the downstream fluid chamber via at least one fluid inlet and at least one fluid outlet.

Description:
BACKGROUND OF DISCLOSURE 
       [0001]    1. Field of the Disclosure 
         [0002]    Embodiments disclosed herein relate generally to thrusters that apply a force to a drill bit during the drilling of an underground formation. In another aspect, embodiments disclosed herein relate to control of a thrust force applied to a drill bit by a thruster. More specifically, embodiments disclosed herein relate to controlling a pressure or differential pressure across a thrust piston, thereby limiting the maximum applied thrust force. 
         [0003]    2. Background 
         [0004]    Hydraulic thrusters are used for applying a force to an earth boring drill bit, independent of the drill string weight. Although thrusters may be used during vertical or inclined drilling, hydraulic thrusters are generally advantageous in horizontal or near-horizontal drilling. During horizontal drilling, especially in long horizontal sections, a significant portion of the weight of the drill stem is directed toward the low side of the hole, detracting from the weight available for bit thrust. Hydraulic thrusters allow for extended reach drilling, the drilling of multiple horizontal wells from a single platform, decreasing the drilling costs associated with producing reservoirs that are offshore, in arctic regions, mountains, or near large cities. 
         [0005]    The thruster is a telescoping tube arrangement that allows the drill bit to advance while the tubing string is supported in a rather stationary position at the surface. When the thruster has advanced its full stroke, or a notable portion thereof, the drill string is lowered from the surface, causing the upper end of the thruster to slide down and reset the thruster for the next stroke. When the drilling kelly or the stand being drilled down by the top drive reaches the drill rig floor, circulation is interrupted and another piece of tubing is added to the string at the surface or the coiled tubing is further unspooled into the wellbore. This drilling procedure also causes the thruster to reset. 
         [0006]    Hydraulic thrusters are described in, for example, U.S. Pat. No. 4,615,401 and patents referenced therein (U.S. Pat. Nos. 3,298,449, 3,399,738, 3,797,589, 3,799,277, 4,040,494, and 4,040,495), each of which is assigned to the assignee of the present invention, and each of which is incorporated herein by reference. In the &#39;401 patent, the hydraulic thruster includes a mandrel and sleeve forming two expandable chambers with wall anchors annularly disposed about the sleeve responsive to a pressure differential between a chamber and the bore hole pressure. Valves and actuators are provided to extend and retract a piston between two extremes of relative axial motion between the mandrel and sleeve. 
         [0007]    Hydraulic thrusters are also described in U.S. Pat. No. 5,205,364. In the &#39;364 patent, the hydraulic thruster includes a telescoping assembly for transmitting hydraulic force to the drill bit at the bottom of the tool. The internal hydraulic characteristics of the tool may be varied to vary the force exerted during extension and retraction of the telescoping assembly. The hydraulic characteristics are varied by varying the surface area within the drill tool on which the flow of drilling mud may act when producing the hydraulic force. 
         [0008]    Other patents describing thrusters or equipment for controlling force or weight on the bit, for example, may include U.S. Pat. Nos. 5,316,094, 6,601,652, 7,156,181, 5,476,148, 5,884,716, 5,806,611, 6003,606, 6,230,813, and 6,286,592, and U.S. Patent Application Publication No. 20010045300. 
         [0009]    Referring now to  FIG. 1 , a simplified cross-sectional view of a prior art thruster  1  is illustrated. Thruster  1 , shown in the retracted position, may include an inner mandrel assembly  2 , which may include one or more tubular components. Threads  3  may connect inner mandrel assembly  2  to the lower drill stem (not shown) toward the bit (not shown). Threads  4  may connect inner mandrel assembly  2  to the upper drill stem (not shown). Inner mandrel assembly  2  is disposed in and is axially movable with respect to outer tubular assembly  5 . One or more anchor pistons  6  may be provided to anchor thruster  1  with respect to the hole wall (not shown). Drilling fluid supplied to the bore  2 A of inner mandrel  2  and to the drill bit (not shown) defines a high pressure area, and drilling fluid returning from the bit in the annulus  7  formed between the outer tubular assembly  6  and the hole wall defines a low pressure area. During thrusting, thrust mechanism  8  may be used to allow the high pressure drilling fluid into chamber A, allowing fluid in chamber B to escape to annulus  7 , thus creating a pressure differential across thrust mechanism  8 , causing the inner mandrel  2  to advance in direction  9 , and putting weight on the bit corresponding to the thrust force generated by the pressure differential. 
         [0010]    A cross-sectional view of a simplified thrust mechanism  8 , which may be used in the thruster of  FIG. 1 , is illustrated in  FIG. 2 . Thrust mechanism  8  may include an inner tubular member  12  and an outer tubular member  14 . Drilling mud flowing through the bore  16  of inner tubular member  12  flows to the drill bit (not shown), and returns to the surface via annulus  18 , such as between outer tubular member  14  and a drill casing (not shown). When mud is flowing through thruster  1  ( FIG. 1 ), bore  16  is at a higher pressure than fluid returning through annulus  18 . A piston  20 , separating a first fluid chamber  22  and a second fluid chamber  24 , may transmit an axial force  26  to inner tubular member  12 . During thrusting, high pressure drilling mud flows from the bore  16  of the thruster  1  through inlet  28  into first fluid chamber  22 , displacing fluid in second fluid chamber  24  through outlet  30  and causing the inner tubular member  12  to advance in the direction of axial force  26 . The axial force  26  that is generated, for example, may be a function of the differential pressure between the fluid in bore  16  and annulus  18 . 
         [0011]    Many of the patents cited above use such a differential pressure to control the force applied to the drill bit. Although not shown in  FIG. 2 , thrust mechanism  8  may typically include ball valves, springs, and other mechanisms to control the flow of fluid into and from the high and low pressure chambers, respectively, during thrusting and retraction. One problem associated with this type of thruster technology includes the need to estimate the pressure and required thrust force prior to drilling. The thruster and the associated internal parts are generally selected and fabricated at the surface prior to installation on a drill string, and many of the parts used to control fluid flow, such as springs, check valves, flow orifices, and others, are sized and selected based on an expected downhole pressure. 
         [0012]    Often, an actual downhole pressure differs from the expected downhole pressure. The difference between actual and expected downhole pressure, even by as little as 25 or 50 psi, may result in ineffective control of the force applied to the drill bit by the thruster, often as a result of the thrust mechanism fully opening or fully closing. Additionally, fluctuations in pressure drop across the bit and changes in the weight of the drilling fluid used (and hence bore pressure) may also result in ineffective control of the force applied to the drill bit by the thruster. Ineffective thruster control may lead to stalls, motor wear, stuck bits, and inefficient rate of penetration, among other problems known to those skilled in the art. 
         [0013]    Various methods and apparatus have been proposed to compensate for a change in downhole conditions and to minimize the effect such changes have on the operation of the thruster and the force applied to the drill bit. For example, a pressure-modulation valve assembly is disclosed in U.S. Pat. No. 6,102,138. Such methods and apparatus unnecessarily increase the total number of drilling components of a drill string, where the additional apparatus may be prone to failure or malfunction due to various conditions encountered during drilling. 
         [0014]    Accordingly, there exists a need for a thruster that may control the force applied to a drill bit independent of the downhole pressure or the pressure drop across the motor and bit. Additionally, there exists a need for a thruster that may control the force applied to the bit independent of the pressure of the supplied drilling fluid. 
       SUMMARY OF THE DISCLOSURE 
       [0015]    In one aspect, embodiments disclosed herein relate to a drilling system, including: a drill bit; and a thruster to apply a force to the drill bit. The thruster may include: an inner tubular member disposed within and configured to axially move within an outer tubular member; a thrust piston to transmit a hydraulic force to the inner tubular member, the thrust piston separating an upstream fluid chamber and a downstream fluid chamber between the inner and outer tubular members; at least one pressure switch fluidly connected to the downstream fluid chamber to control flow of a fluid to and from the downstream fluid chamber via at least one fluid inlet and at least one fluid outlet. 
         [0016]    In another aspect, embodiments disclosed herein relate to a thruster, including: an inner tubular member disposed within and configured to axially move within an outer tubular member; a thrust piston to transmit a hydraulic force to the inner tubular member, the thrust piston separating an upstream fluid chamber and a downstream fluid chamber between the inner and outer tubular members; a downstream valve member mechanically coupled to a downstream magneto-actuator and disposed in the downstream fluid chamber; and at least one pressure switch fluidly coupled to the downstream fluid chamber to control a position of the downstream valve member via the magneto-actuator; wherein the position of the downstream valve member affects a flow of a fluid to and from the downstream fluid chamber via at least one fluid inlet and at least one fluid outlet. 
         [0017]    In another aspect, embodiments disclosed herein relate to a process to drill an underground formation. The process may include: supplying a fluid to a thruster, wherein the thruster includes: an inner tubular member disposed within and configured to axially move within an outer tubular member; a thrust piston to transmit a hydraulic force to the inner tubular member, the piston separating an upstream fluid chamber and a downstream fluid chamber between the inner tubular member and the outer tubular member; at least one pressure switch fluidly connected to the downstream fluid chamber; and regulating a flow of the fluid to and from the downstream fluid chamber in response to a signal from the at least one downstream pressure switch to maintain the hydraulic force applied to the inner tubular member proximate a hydraulic force set point. 
         [0018]    Other aspects and advantages will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a simplified schematic drawing of a prior art thruster. 
           [0020]      FIG. 2  is a schematic drawing of a simplified thrust mechanism that may be used in the prior art thruster of  FIG. 1 . 
           [0021]      FIG. 3  is a simplified schematic drawing of a thruster according to embodiments disclosed herein. 
           [0022]      FIG. 3A  is a simplified schematic drawing of an actuator useful in embodiments of the thrusters described herein. 
           [0023]      FIG. 3B  is a simplified schematic drawing of an actuator useful in embodiments of the thrusters described herein. 
           [0024]      FIG. 3C  is a simplified schematic drawing of an actuator useful in embodiments of the thrusters described herein. 
           [0025]      FIG. 4  is a simplified schematic drawing of a thruster according to embodiments disclosed herein. 
           [0026]      FIG. 5  is a simplified schematic drawing of a thruster according to embodiments disclosed herein. 
           [0027]      FIG. 6  is a simplified schematic drawing of a thruster according to embodiments disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    In one aspect, embodiments disclosed herein relate to control of a thrust force applied to a drill bit by a thruster. More specifically, embodiments disclosed herein relate to controlling a pressure or differential pressure across a thrust piston, thereby limiting the maximum applied thrust force. Other embodiments disclosed herein relate to a method of drilling a formation using a thruster that may limit the thrust force applied to the bit independent of bore and annulus fluid pressures. 
         [0029]    As described above, prior art thrusters generate an axial force based upon a difference in bore and annulus pressures. In contrast, thrusters disclosed herein include mechanisms to regulate the pressure in one or both of the upstream and downstream fluid chambers. The axial force generated according to embodiments disclosed herein, for example, may be a function of the differential pressure between the fluid in the upstream and downstream fluid chambers. 
         [0030]    Referring now to  FIG. 3 , a simplified schematic drawing of a thruster  50  according to embodiments disclosed herein is illustrated. Thruster  50  may include an inner tubular member  52  and an outer tubular member  54 . Drilling mud flowing through the bore  56  of inner tubular member  52  flows to the drill bit (not shown), and returns to the surface via annulus  58 , such as between outer tubular member  54  and a drill casing (not shown). When mud is flowing through thruster  50 , bore  56  is at a higher pressure than fluid returning through annulus  58 . A thrust piston  60 , separating an upstream fluid chamber  62  and a downstream fluid chamber  64 , may transmit an axial force  66  to inner tubular member  52 . During thrusting, high pressure drilling mud flows from the bore  56  of the thruster  50  through inlet  68  into upstream fluid chamber  62 , displacing low pressure fluid in downstream fluid chamber  64  through outlet  70  and causing the inner tubular member  52  to advance in the direction of axial force  66 . 
         [0031]    To regulate thrust force, or differential pressure between the upstream chamber  62  and the downstream chamber  64 , for example, thruster  50  may include a pressure switch  72 , which may be in fluid communication with the downstream fluid chamber  64 . Pressure switch  72 , in some embodiments, may be a pressure limit switch, activating at a pressure set point. When the fluid in chamber  64  reaches a predetermined set point pressure, the pressure switch  72  may actuate. Upon actuation, pressure switch  72  may send an electronic signal to a control mechanism (not shown) for regulating the flow of fluid into or out of downstream fluid chamber  64  through downstream inlet  74  and outlet  70 . 
         [0032]    By sending a signal to regulate the flow of fluid into and out of downstream fluid chamber  64 , pressure switch  72  may limit the thrust force applied to the drill bit, thus avoiding the full on or full off scenarios often encountered with prior art thrusters. For example, by limiting the flow of fluid through outlet  70 , pressure will build in downstream fluid chamber  64 , limiting the applied thrust force. As another example, by allowing fluid to flow in through inlet  74 , pressure will also increase in downstream fluid chamber  64 , due to high pressure fluid in bore  56 , limiting the applied thrust force. 
         [0033]    The control mechanism may in turn send a signal or a current to a valve member  76  to regulate the flow of fluid into and out of downstream fluid chamber  64 . Valve member  76  may include, for example, an actuator  78 , a drive rod  80 , and a gate member  82 . The signal or current transmitted to valve member  76  may cause actuator  78  to extend or contract, as illustrated by the arrows, causing a similar displacement in drive rod  80 , causing gate  82  to open and/or close fluid inlet  74  and/or fluid outlet  70 . Other means of regulating fluid flow using a signal from a pressure switch are also contemplated herein. 
         [0034]    Actuator  78  may include any one of several types of actuators responsive to electronic signals or currents. For example, actuator  78  may include magnetostrictive actuators, shape memory alloy actuators, and linear motor actuators. Examples of each of these are illustrated in  FIGS. 3A-3C . 
         [0035]    As illustrated in  FIG. 3A , actuator  78  may include a magnetostrictive actuator, including permanent magnets  84 , drive rod  85 , coil  86 , preload springs  87 , and output rod  88 . Upon application of a current through coil  86 , drive rod  85  may expand or contract in response to the magnetic field generated, thereby displacing output rod  88  to control the position of the gate member  82  and thus control the flow of fluid to and from the downstream cavity  64 . 
         [0036]    As illustrated in  FIG. 3B , actuator  78  may include a shape memory alloy actuator, including shape memory alloy spring  90 , piston  92 , and drive rod  94 . Upon application of an electrical current, shape memory alloy spring may expand or contract, thereby displacing piston  92  and drive rod  94  to control the position of the gate member  82 , and thus control the flow of fluid to and from the downstream cavity  64 . 
         [0037]    As illustrated in  FIG. 3C , actuator  78  may include a linear motor actuator, including a stationary member  96 , a motive member  97 , and a drive rod  98 . Linear motor actuators may include flat linear motor actuators and, as illustrated, tubular linear motor actuators. In some embodiments, a signal sent from the control mechanism to the linear motor actuator may control the position of the motive member  97 , and thus drive rod  98 , with respect to stationary member  96 . In other embodiments, a signal sent from the control mechanism to the linear motor actuator may control an output force exerted on drive rod  98 . In this manner, the linear motor actuator may control the position of gate member  82 , and thus control the flow of fluid to and from the downstream cavity  64 . 
         [0038]    Referring now to  FIG. 4 , a simplified schematic drawing of a thruster  100  according to embodiments disclosed herein is illustrated. Thruster  100  may include an inner tubular member  102  and an outer tubular member  104 . Drilling mud flowing through the bore  106  of inner tubular member  102  flows to the drill bit (not shown), and returns to the surface via annulus  108 , such as between outer tubular member  104  and a drill casing (not shown). When mud is flowing through thruster  100 , bore  106  is at a higher pressure than fluid returning through annulus  108 . A thrust piston  110 , separating an upstream fluid chamber  112  and a downstream fluid chamber  114 , may transmit an axial force  116  to inner tubular member  102 . During thrusting, high pressure drilling mud flows from the bore  106  of the thruster  100  through inlet  118  into upstream fluid chamber  112 , displacing low pressure fluid in downstream fluid chamber  114  through outlet  120  and causing the inner tubular member  102  to advance in the direction of axial force  116 . 
         [0039]    To regulate thrust force, or differential pressure between the upstream chamber  112  and the downstream chamber  114 , for example, thruster  100  may include a pressure switch  122 , which may be in fluid communication with each of the upstream fluid chamber  112  and the downstream fluid chamber  114 . Pressure switch  122 , in some embodiments, may be a differential pressure limit switch, activating at a differential pressure set point. When the differential pressure of the fluid in upstream and downstream chambers  112 ,  114  reaches a pre-determined differential pressure set point, the pressure switch  122  may actuate. Upon actuation, pressure switch  122  may send an electronic signal to a control mechanism (not shown) for regulating the flow of fluid into or out of downstream fluid chamber  114  through downstream inlet  124  and outlet  120 . 
         [0040]    By sending a signal to regulate the flow of fluid into and out of downstream fluid chamber  114 , pressure switch  122  may regulate the thrust force applied to the drill bit, as described above. 
         [0041]    The control mechanism may in turn send a signal or a current to a valve member  126  to regulate the flow of fluid into and out of downstream fluid chamber  114 . Valve member  126  may include, for example, an actuator  128 , a drive rod  130 , and a gate member  132 . The signal or current transmitted to valve member  126  may cause actuator  128  to extend or contract, as illustrated by the arrows, causing a similar displacement in drive rod  130 , causing gate  132  to open and/or close fluid inlet  124  and/or fluid outlet  120 . 
         [0042]    Referring now to  FIG. 5 , a simplified schematic drawing of a thruster  150  according to embodiments disclosed herein is illustrated. Thruster  150  may include an inner tubular member  152  and an outer tubular member  154 . Drilling mud flowing through the bore  156  of inner tubular member  152  flows to the drill bit (not shown), and returns to the surface via annulus  158 , such as between outer tubular member  154  and a drill casing (not shown). When mud is flowing through thruster  150 , bore  156  is at a higher pressure than fluid returning through annulus  158 . A thrust piston  160 , separating an upstream fluid chamber  162  and a downstream fluid chamber  164 , may transmit an axial force  166  to inner tubular member  152 . During thrusting, high pressure drilling mud flows from the bore  156  of the thruster  150  through inlet  168  into upstream fluid chamber  162 , displacing low pressure fluid in downstream fluid chamber  164  through outlet  170  and causing the inner tubular member  152  to advance in the direction of axial force  166 . 
         [0043]    To regulate thrust force, or differential pressure between the upstream chamber  162  and the downstream chamber  164 , for example, thruster  150  may include a pressure switch  172 , which may be in fluid communication with the downstream fluid chamber  164 . Pressure switch  172 , in some embodiments, may be a pressure limit switch, activating at a pressure set point. When the fluid in chamber  164  reaches a pre-determined set point pressure, the pressure switch  172  may actuate. Upon actuation, pressure switch  172  may send an electronic signal to a control mechanism (not shown) for regulating the flow of fluid into or out of downstream fluid chamber  164  through downstream inlet  174  and outlet  170 . Thruster  150  may also include a pressure switch  173 , which may be in fluid communication with the upstream fluid chamber  162 . When the fluid in chamber  162  reaches a pre-determined set point pressure, the pressure switch  173  may actuate, sending an electronic signal to a control mechanism (not shown) for regulating the flow of fluid into or out of upstream fluid chamber  162  through upstream inlet  168  and upstream outlet  175 . By sending a signal to regulate the flow of fluid into and out of upstream fluid chamber  162  and downstream fluid chamber  164 , pressure switches  173 ,  172  may each, separately or collectively, limit the thrust force applied to the drill bit. 
         [0044]    The control mechanism may in turn send a signal(s) or a current(s) to valve members  176 ,  177  to regulate the flow of fluid into and out of one or both of upstream and downstream fluid chambers  162 ,  164 . Valve members  176 ,  177  may include, respectively, for example, actuators  178 ,  179 , drive rods  180 ,  181 , and gate members  182 ,  183 . The signal(s) or current(s) transmitted to valve members  176 ,  177  may cause actuators  178 ,  179  to extend or contract, as illustrated by the arrows, causing a similar displacement in drive rods  180 ,  181 , causing gates  182 ,  183  to open and/or close fluid inlets  174 ,  175  and/or fluid outlets  170 ,  171 . In some embodiments, valve action on both sides of the thrust piston  160  is required in order to have hydraulic volume flow in the upstream and downstream chambers  162 ,  164 . 
         [0045]    Referring now to  FIG. 6 , a simplified schematic drawing of a thruster  200  according to embodiments disclosed herein is illustrated. Thruster  200  may include an inner tubular member  202  and an outer tubular member  204 . Drilling mud flowing through the bore  206  of inner tubular member  202  flows to the drill bit (not shown), and returns to the surface via annulus  208 , such as between outer tubular member  204  and a drill casing (not shown). When mud is flowing through thruster  200 , bore  206  is at a higher pressure than fluid returning through annulus  208 . A thrust piston  210 , separating an upstream fluid chamber  212  and a downstream fluid chamber  214 , may transmit an axial force  216  to inner tubular member  202 . During thrusting, high pressure drilling mud flows from the bore  206  of the thruster  200  through inlet  218  into upstream fluid chamber  212 , displacing low pressure fluid in downstream fluid chamber  214  through outlet  220  and causing the inner tubular member  202  to advance in the direction of axial force  216 . 
         [0046]    To regulate thrust force, or differential pressure between the upstream chamber  212  and the downstream chamber  214 , for example, thruster  200  may include a differential pressure switch  222 , which may be in fluid communication with each of the upstream fluid chamber  212  and the downstream fluid chamber  214 . When the differential pressure of the fluid in upstream and downstream chambers  212 ,  214  reaches a pre-determined differential pressure set point, the pressure switch  222  may actuate, sending an electronic signal to a control mechanism (not shown) for regulating the flow of fluid into or out of one or both of upstream and downstream fluid chambers  212 ,  214 , thereby limiting the thrust force applied to the drill bit. 
         [0047]    The control mechanism may in turn send a signal(s) or a current(s) to valve members  226 ,  227  to regulate the flow of fluid into and out of one or both of upstream and downstream fluid chambers  212 ,  214 . Valve members  226 ,  227  may include, respectively, for example, actuators  228 ,  229 , drive rods  230 ,  231 , and gate members  222 ,  223 . The signal(s) or current(s) transmitted to valve members  226 ,  227  may cause actuators  228 ,  229  to extend or contract, as illustrated by the arrows, causing a similar displacement in drive rods  230 ,  231 , causing gates  232 ,  233  to open and/or close fluid inlets  224 ,  225  and/or fluid outlets  220 ,  221 . In some embodiments, valve action on both sides of the thrust piston  210  is required in order to have hydraulic volume flow in the upstream and downstream chambers  212 ,  214 . 
         [0048]    As described above, operation and control of the thrusters described herein may be affected by remote signals, such as by actuating valves and other thruster components. In some embodiments, the control settings for the valves, actuators, and pressure switches may be adjusted using remote signals. 
         [0049]    In other embodiments, the operation and control of the thrusters described herein may be affected by down-linking a signal from the surface. For example, a signal from the surface may be used to communicate with the thruster control mechanism, such as to influence the forward movement of the thruster to initiate a change in drilling rate, a change in drilling direction, or other drilling parameters, for example. Down-linking signals, in some embodiments, may include a change in pump pressure at the surface held for a given period of time. In other embodiments, down-linking signals may include a positive and/or negative pressure pulses, such as may be actuated by a change in standpipe pressure, for example. In this manner, down-linking may be used to accurately position a well and improve drilling performance. 
         [0050]    Embodiments disclosed herein may include one or more pressure switches and/or differential pressure switches to result in the desired thrust control. In some embodiments, the pressure switches may actuate upon increasing pressure or pressure differential. In other embodiments, the pressure switches may actuate upon decreasing pressure or pressure differential. In yet other embodiments, combinations of pressure switches actuating upon increasing and decreasing pressure differential may be used, such as where a valve member opens upon increasing pressure differential in response to a signal from a first pressure switch, and the valve member closes upon decreasing pressure differential in response to a signal from a second pressure switch. Additionally, embodiments may include pressure switches and differential pressure switches in fluid communication with one or more of the upstream chamber, the downstream chamber, the inner tubular member bore, and the annulus between the outer tubular member and the hole wall, with the pressure switch actuating upon a give pressure or pressure differential so as to regulate thrust force. 
         [0051]    As described above, use of pressure switches and actuators may provide for passive thrust force control. For example, a pressure switch may actuate at a minimum or maximum desired thrust force, sensing a fully opened or fully closed condition, and thereafter adjusting the pressures in the upstream and downstream chambers. 
         [0052]    Embodiments disclosed herein may include one or more actuators to result in active thrust force control. In some embodiments, two or more actuators, of the same or different type, may be used in parallel, such as operating two or more gate members, or in series, such as to achieve a longer stroke length. Additionally, intermediate components may be used intermediate drive rod and gate member, such as lever arms and bell cranks, among others, so as to result in the desired valve action or stroke length. 
         [0053]    Embodiments disclosed herein may include two or more actuators and pressure switches in parallel to control fluid flow into and from a fluid chamber. In some embodiments, the two or more pressure switches may include different pressure set points, such that a valve member may reset prior to a subsequent cycle, for example. Pressure set points may be varied minimally so as to maintain a similar maximum thrust force upon actuation of the various switch/actuator/valve combinations. 
         [0054]    As described above, use of pressure switches and actuators in parallel or series may provide for active thrust force control. For example, when approaching a fully opened or fully closed condition, the pressure switches may actuate, adjusting the pressures in the upstream and downstream chambers and thereby operating the thruster within a desired range of thrust force. 
         [0055]    In other embodiments, two or more actuators and pressure switches may be used in series to control fluid flow into and from a fluid chamber. For example, two or more pressure switches may include different set points, such that actuators extend or contract at different pressure set points. Upon actuation of a first pressure switch/actuator pair, a minimal flow opening may be provided to limit thrust force. If differential pressure continues to increase following actuation of the first pressure switch/actuator pair, a second and subsequent pressure switch/actuator pairs may provide additional flow area to limit the thrust force applied to the bit. In this manner, thrust force may vary less significantly than an on/off type actuator/valve member. 
         [0056]    Although described with reference to the pressure chambers, one skilled in the art will recognize that embodiments of thrusters disclosed herein may include components that may be typically included in thrusters, such as the thrusters described in U.S. Pat. No. 4,615,401 and others mentioned above. For example, thrusters disclosed herein may include anchor assemblies, ball valves, seals, springs and spring assemblies, threaded connections, spacers, snap rings, bearings, pins, valve seats, and rods, among others. Components used to regulate fluid flow during resetting of the thruster may also be included. 
         [0057]    In some embodiments, additional measurement and control devices may also be used to limit or control the thrust force. For example, a sensor measuring rate of penetration may be used to actuate the valve members, thereby controlling the flow of fluid into and from the upstream and downstream fluid chambers. In this manner, rate of penetration may be maintained within a desired range, such as within an optimal range for a particular drill bit. Stroke measurement devices or position sensors may also be used to indicate the thruster position, thereby allowing an operator to slow the rate of thrust toward the end of a stroke. 
         [0058]    In some embodiments, power and currents supplied to the control mechanisms, pressure switches, and actuators may include electrical currents supplied from batteries. In other embodiments, power and currents may be supplied to the control mechanisms, pressure switches, and actuators may include electrical currents supplied from downhole generators, such as turbine generators and the like. 
         [0059]    Advantageously, embodiments disclosed herein may provide for improved thrust force control, or improved control of the weight on bit. Actuators, pressure switches and valve members described herein may advantageously limit the pressure differential between upstream and downstream chambers, thus limiting the thrust force transmitted by the thrust piston to the inner tubular member. Additionally, for embodiments limiting the pressure or pressure differential within each fluid chamber, the maximum thrust force applied may be controlled independent of fluid pressure in the inner bore and the annulus. Embodiments disclosed herein, through limiting applied thrust force, may advantageously maintain weight on bit within a desired range, improving rates of penetration, and decreasing motor wear and the occurrence of stuck bits and stalls, among other common problems known in the art. 
         [0060]    While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims.