Patent Publication Number: US-11396774-B2

Title: Steering actuation mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to U.S. Provisional Application No. 62/905,800 filed Sep. 25, 2019, entitled “Steering Actuation Mechanism,” the disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure relates generally to rotary steerable systems (RSS), e.g., drilling systems employed for directionally drilling wellbores in oil and gas exploration and production. More particularly, embodiments of the disclosure relate to mechanisms for extending a pad of the rotary steerable system to thereby steer the RSS through a geologic formation. 
     Directional drilling operations involve controlling the direction of a wellbore as it is being drilled. Usually the goal of directional drilling is to reach a target subterranean destination with a drill string, and often the drill string will need to be turned through a tight radius to reach the target destination. Generally, an RSS changes direction either by extending a steering pad to push against one side of a wellbore with a steering force to thereby cause the drill bit to push on an opposite side of the wellbore (in a push-the-bit system), or by bending a main shaft running through a non-rotating housing to point the drill bit in a particular direction with respect to the rest of the tool (in a point-the-bit system). In a push-the-bit system, the steering pads may be actuated by hydraulic pistons that extend reciprocate in a piston bore defined in a housing of the RSS. Elastomeric seal members are often provided to establish a seal between the piston and the housing, but these seal members often have a limited service life due to the harsh downhole environment in which these seal members are employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is described in detail hereinafter, by way of example only, on the basis of examples represented in the accompanying figures, in which: 
         FIG. 1  is a partial cross-sectional side view of a directional drilling system including an RSS according to example embodiments of the disclosure; 
         FIGS. 2A and 2B  are partial, cross-sectional views of a steering actuation mechanism of the RSS of  FIG. 1  in extended ( FIG. 2A ) and retracted ( FIG. 2B ) configurations illustrating a pair of seal-less pistons retained to a steering pad with freedom of movement along one axis with respect to the steering pad; 
         FIGS. 2C to 2E  are partial, cross-sectional and perspective views of another embodiment of a steering actuation mechanism in extended ( FIGS. 2C and 2E ) and retracted ( FIG. 2D ) configurations illustrating a pair of seal-less pistons retained to a steering pad with freedom of movement along an axis with respect to the steering pad, as well as with freedom of rotation about an axis through the piston; 
         FIG. 2F  is a perspective view of the piston of  FIGS. 2C to 2E ; 
         FIG. 3  is a partial, perspective view of another embodiment of a steering actuation mechanism including a single elongated piston retained to a steering pad; 
         FIG. 4  is a partial, perspective view of another embodiment of a steering actuation mechanism including an elongated cylindrical piston disconnected from a steering pad; 
         FIGS. 5A and 5B  are partial, cross-sectional views of another embodiment of a steering actuation mechanism in extended ( FIG. 5A ) and retracted ( FIG. 5B ) configurations illustrating a pair of generally cylindrical pistons including a groove that may receive an elastomeric seal therein; 
         FIGS. 6A, 6B and 6C  are cross-sectional views of the cylindrical piston of  FIGS. 5A and 5B  including various seal members received within the groove; 
         FIGS. 7A and 7B  are partial, cross-sectional views of another embodiment of a steering actuation mechanism in extended ( FIG. 7A ) and retracted ( FIG. 7B ) configurations illustrating a keyed piston having an extended skirt on a lateral side of the piston and a ball retained on the piston in rolling contact with a steering pad; 
         FIGS. 8A and 8B  are partial, cross-sectional views of another embodiment of a steering actuation mechanism in extended ( FIG. 8A ) and retracted ( FIG. 8B ) configurations illustrating a piston having an angled skirt and a roller ball retained by an axel on the piston; and 
         FIG. 8C  is a perspective view of the piston of  FIGS. 8A and 8B . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to steering mechanisms for use in RSS systems that do not require an elastomeric piston seal. The steering mechanisms may include pistons having a convex cross-section with respect to an axis of a piston bore. The pistons permit hydraulic pressure to be applied due to a limited gap size between the piston and the bore, e.g., between a widest portion of the convex cross-section of the piston and an adjacent wall of the piston bore. The pistons may be retained to a steering pad, which may reduce impact forces associated with applying and relieving the hydraulic pressure. The pistons may be elongated in a direction orthogonal to the axis of the piston bore, which reduces a leak flow area for a given cross-sectional area of the piston. A groove may be provided around the piston for a receiving a back-up seal therein. The back-up seal may include wear resistant particles or balls embedded in a matrix, and the particles or balls may be preloaded to serve as flow restrictors even when worn. The pistons may include skirt that is elongated on one lateral side thereof, which may discourage tilting of the piston within a piston bore. The pistons may also include a pocket in which a ball or roller is retained to engage the steering pad. 
     Referring to  FIG. 1 , a directional drilling system  10  includes an RSS  100 . The directional drilling system  10  is illustrated including a tower or “derrick”  12  that is buttressed by a derrick floor  13 . The derrick floor  13  supports a rotary table  14  that is driven at a desired rotational speed, for example, via a chain drive system through operation of a prime mover (not shown). The rotary table  14 , in turn, is operable to provide rotational force to a drill string  20 . The drill string  20 , which includes a drill pipe section  22 , extends downwardly from the rotary table  14  into a directional borehole  24 . The borehole  24  may exhibit a multi-dimensional path or “trajectory.” The three-dimensional direction of the bottom  26  of the borehole  24  of  FIG. 1  is represented by arrow  28 . 
     A drill bit  30  is attached to the distal, downhole end of the drill string  20 . When rotated, e.g., via the rotary table  14 , the drill bit  30  operates to break up and generally disintegrate the geological formation  32 . The drill string  20  is coupled to a “drawworks” hoisting apparatus  34 , for example, via a kelly joint  36 , swivel  38 , and line  39  through a pulley system (not shown). During a drilling operation, the drawworks  34  can be operated, in some embodiments, to control the weight on drill bit  30  and the rate of penetration of the drill string  20  into the borehole  24 . 
     During drilling operations, a suitable drilling fluid  41  or “mud” can be circulated, under pressure, out from a mud pit  42  and into the borehole  24  through the drill string  20  by a hydraulic “mud pump”  44 . Drilling fluid  41  passes from the mud pump  44  into the drill string  20  via a fluid conduit (commonly referred to as a “mud line”)  48  and the kelly joint  36 . The mud  31  is discharged at the borehole bottom  26  through an opening or nozzle in the drill bit  30 , and circulates in an “uphole” direction towards the surface through an annular space  50  between the drill string  20  and the side  52  of the borehole  24 . As the drilling fluid  41  approaches the rotary table  14 , it is discharged via a return line  55  into the mud pit  42 . A variety of surface sensors  58 , which are appropriately deployed on the surface of the borehole  24 , operate alone or in conjunction with downhole sensors  60  deployed within the borehole  24 , to provide information about various drilling-related parameters, such as fluid flow rate, weight on bit, hook load, etc. 
     A surface control unit  62  may receive signals from surface sensors  58  and downhole sensors,  60  and other devices via a sensor or transducer  63 , which can be placed on the mud line  48 . The surface control unit  62  can be operable to process such signals according to programmed instructions provided to surface control unit  62 . Surface control unit  62  may present to an operator desired drilling parameters and other information via one or more output devices  64 , such as a display, a computer monitor, speakers, lights, etc., which may be used by the operator to control the drilling operations. Surface control unit  62  may contain a computer, memory for storing data, a data recorder, and other known and hereinafter developed peripherals. Surface control unit  62  may also include models and may process data according to programmed instructions, and respond to user commands entered through a suitable input device  66 , which may be in the nature of a keyboard, touchscreen, microphone, mouse, joystick, etc. 
     In some embodiments of the present disclosure, the rotatable drill bit  30  is attached at a distal end of a bottom hole assembly (BHA)  70  including the rotary steerable system (RSS)  100 . The RSS  100  includes steering pads  102  for steering the drill bit  30  through the formation  32 , and thereby defining the trajectory of the borehole  24 . The steering pads  102  may be extendable in a lateral direction from a longitudinal axis A of the RSS  100  to push against the geologic formation  32 . The extent to which each of a plurality of radially spaced steering pads  102  are extended may be adjustable to assist in controlling the direction of the borehole  24 . In some embodiments, the RSS  100  may include a stabilizer (not shown) at a proximal or uphole end thereof. The BHA  70  and/or RSS  100  can provide some or all of the requisite force for the bit  30  to break through the geologic formation  32 , e.g., “weight on bit” and torque for turning the drill bit  30 , and provide the necessary directional control for drilling the borehole  24 . 
     The BHA  70  and or/the RSS  100  may comprise a Measurement While Drilling (MWD) System and/or a Logging While Drilling (LWD) System, with various sensors to provide information about the formation  32  and downhole drilling parameters. The MWD and or LWD sensors in the BHA  70  may include, but are not limited to, a device for measuring the formation resistivity near the drill bit, a gamma ray device for measuring the devices for determining the inclination and azimuth of the drill string, and pressure sensors for measuring drilling fluid pressure downhole. The MWD System may also include additional/alternative sensing devices for measuring shock, vibration, torque, telemetry, etc. The above-noted devices may transmit data to a downhole communicator  74 , which in turn transmits the data uphole to the surface control unit  62 . 
     The transducer  63  can be placed in the mud line  48  to detect the mud pulses responsive to the data transmitted by the downhole communicator  74 . The transducer  63  in turn generates electrical signals, for example, in response to the mud pressure variations and transmits such signals to the surface control unit  62 . Alternatively, other telemetry techniques such as electromagnetic and/or acoustic techniques or any other suitable techniques known or hereinafter developed may be utilized. By way of example, hard wired drill pipe may be used to communicate between the surface and downhole devices. In another example, combinations of the techniques described may be used. A surface transmitter/receiver  76  communicates with downhole tools using, for example, any of the transmission techniques described, such as a mud pulse telemetry technique. This can enable two-way communication between the surface control unit  62  and the downhole communicator  74  and other downhole tools. 
     Referring to  FIGS. 2A and 2B , the RSS  100  includes a steering pad  102 , which is extendable in a lateral direction by a steering actuation mechanism  104 . The RSS  100  includes a housing  106  defining a longitudinal axis A 1 . The housing  106  includes a pair of piston bores  108 , which may be generally straight extending along respective piston axes A 2 , A 3  in a lateral direction with respect to the longitudinal axis A 1 . The steering pad  102  is pivotally coupled to the housing  106  about a pivot axis A 4 , which may be generally parallel to the longitudinal axis A 1 . A piston  110  is disposed within each of the piston bores  108  and is movable along the respective piston axis A 2 , A 3 . A hydraulic chamber  112  is defined in the housing  106  adjacent each of the pistons  110 , which may be selectively pressurized to extend the pistons  110  radially from the piston bores  108  as illustrated in  FIG. 2A . The pistons  110  push on the steering pad  102  to pivot the steering pad  102  radially outwardly about the pivot axis A 4 . Relieving the hydraulic pressure from the hydraulic chamber  112  permits the pistons  110  and steering pad  102  to return to radially retracted positions with respect to the housing  106  as illustrated in  FIG. 2B . A gap “G” is defined between each of the piston  110  and the piston bore, the gap extending along the piston bore from the hydraulic chamber to an exterior of the housing
         The pistons  110  include each include a T-shaped flange  114  projecting from a radially outward surface of the piston  110 . The flanges  114  provide a broad bearing area across which the pistons  110  press against the steering pad  102  to pivot the steering pad  102  radially outward. The flanges  114  are received in a T-slot  116  defined in the steering pad  102 , which retains the pistons  110  with respect to the steering pad  102 . As the steering pad  102  pivots, the T-slots  116  permit the steering pad  102  to move along the pistons  110  in a direction  118  obliquely arranged with respect to the piston axes A 2  and A 3 . The direction  118  is orthogonal to the pivot axis A 4  of the steering pad. The pistons  110  define a convex cross-section in a plane through the piston axes A 2 , A 3 , which in some embodiments may be arcuate such that the pistons  110  generally define a spherical or ball-shaped portion. A diameter “D” across a widest portion of the pistons  110  may be closely fit with a bearing  120  in the piston bores  108  to retain hydraulic fluid within the hydraulic chambers  112 . The close fit permits hydraulic pressure to accumulate sufficiently without a sealing member closing a gap “G” between the pistons  110  and the bearings  120  such that the pistons  110  may extend with the steering force that provides the necessary directional control for drilling the borehole  24 . For example, in some embodiments, the gap “G” may have a width of about less than 0.003 inches may be provided between the pistons  110  and a wall of the bearings  120 . The gap “G” extends along the piston bores  108  between the hydraulic chamber  112  and an exterior of the housing  106 . The arcuate shape of the pistons  110  permit the pistons  110  to pivot along with the steering pad  102  while maintaining a close fit with the bearing  120 . The close fit restricts fluid flow through the gap “G” such that fluid pressure may accumulate in the hydraulic chamber  112  to extend the pistons  110 . The bearings  120  (or the piston bores  108 ) may be constructed of carbide, metallic or ceramic materials, with or without coatings thereon.       

     In operation, the pistons  110  remain retained to the steering pad  102  such that the pistons  110  do not subject the steering pad  102  to impact forces as the hydraulic chambers  112  are pressurized. Similarly, the hydraulic chambers  112  are not subject to impact loads from the pistons  110  when hydraulic pressure in the hydraulic chambers  112  is relieved. The T-slots  116  also provide a degree of freedom for the pistons  110  to slide along the steering pad  102 . The sliding motion allows the pistons  110  to readily pivot while moving along the pivot axes A 2 , A 3  without jamming.
         Referring to  FIGS. 2C, 2D and 2E , a steering actuation mechanism  154  is illustrated that provides one additional degree of freedom for a piston  156  than the steering actuation mechanism  104  shown in  FIGS. 2A and 2B . The T-slot  116  defines a throat  116   t  extending to an exterior surface  102   e  of the steering pad  102  and a head space  116   h  spaced from the exterior surface  102   e . The throat  116   t  defines a throat width “Wt” less than a head-space width “Wh” defined by the head space  116   h  (see  FIG. 2E ). The piston  156  can be free to rotate around its rotational axis A 3   a  as well as sliding in the direction  118  in the T-slots  116  ( FIG. 2A ). This freedom to rotate about axis A 3  a reduces the abrasive wear between the piston  156  and piston bore  108  and/or bearings  120  a, as well as allow for more uniform erosion wear of the piston  156 . As illustrated in  FIGS. 2E and 2F , the piston  156  includes a generally circular flange  158  at an upper end thereof. The flange  158  may rotate within the T-slot  116  of the steering pad  102  while guiding the relative movement in the direction  118  between the piston  156  and the steering pad  102 . The free rotational movement about the axis A 3  a permits frictional wear to be distributed about a perimeter P 1  ( FIG. 2F ) of the circular flange  158  and a perimeter P 2  ( FIG. 2F ) around an arcuate portion of the piston  156  that engages the piston bore  108 . Similar to the piston  110  described above, the piston  156  defines a convex cross-section permitting the piston  156  to freely pivot within the piston bore  108  about an axis A 10  parallel to the pivot axis  104  of the at least one steering pad  102 .       

     Also shown in  FIGS. 2C and 2D , a bearing  120   a  in the piston bore  108  is constructed of two layers of material. An inner layer  160  may be constructed of a material having high erosion/abrasion resistance such as tungsten carbide, ceramic, polycrystalline diamond, etc. An outer layer  162  may be constructed of a material having high fracture toughness such as stainless steel, titanium alloys, etc. Referring to  FIG. 3 , a steering mechanism  204  includes a single piston  210  extending from an elongated cylindrical piston bore  218 . The piston  210  is elongated and generally spherocylindrical or capsule-shaped, having a generally cylindrical medial portion  220  and spherical ends  222 . The piston  210  is retained to a steering pad  224  by flanges  226  of the piston  210  slidably received in a pair of T-slots  228  defined in the steering pad  224 . The piston  210  may operate substantially similarly to the pair of pistons  110  (see  FIG. 2A ) and may provide a reduced leak flow area for a given piston area and a similar gap distance. For example, a combined perimeter of the two spherical pistons  110  each having a 1.5-inch diameter would be 9.42 inches with a total cross-sectional area of 3.534 in 2  across the piston bores  108 . The perimeter P 1  of a spherocylindrical piston  210  having the same cross-sectional area across the piston bore  218  would be 7.07 inches. Since the perimeter P 1  is about 25% less than the combined perimeter of the two spherical pistons  110 , for an equally sized gap defined between the pistons  110 ,  210 , about a 25% reduction in the leak flow area may be achieved by providing a spherocylindrical piston  210 . In other embodiments (not shown) a single piston or a plurality of pistons retained to a steering pad may be a prolate spheroid and a corresponding piston bore may be an elliptical cylinder. 
     Referring to  FIG. 4 , a steering actuation mechanism  304  includes a piston  310  extending from a piston bore  318 . Similar to the piston  210  ( FIG. 3 ), the piston  310  is elongated and generally spherocylindrical or capsule-shaped, having a generally cylindrical medial portion  320  and spherical ends  322 . Unlike the piston  210  the piston  310  may not be retained to a steering pad  324 . Rather, the piston  310  may engage the steering pad  324  when moved to the radially extended position illustrated by a hydraulic force. The piston  310  may roll in against the steering pad  324  as the steering pad  324  pivots. When the hydraulic force is relieved, the piston  310  may disengage the steering pad  324  and move to a radially retracted position within the piston bore  318 . The piston  310  provides a reduced leak flow area compared to a plurality of ball shaped pistons having a similar cross-sectional area. In other embodiments (not shown) a single piston or a plurality of pistons detached from an associated steering pad may be a prolate spheroid and a corresponding piston bore may be an elliptical cylinder. 
     Referring to  FIGS. 5A and 5B , a steering actuation mechanism  404  includes a pair of pistons  410  extending from respective piston bores  418 . The pistons  410  and the piston bores  418  are generally cylindrical in shape extending along piston axes A 5 , A 6  in a lateral direction. Hydraulic chambers  422  are defined in the piston bores adjacent each of the pistons  410 , which may be selectively pressurized to extend the pistons  410  radially from the piston bores  418  as illustrated in  FIG. 5A . The pistons  410  push on the steering pad  432  to pivot the steering pad  432  radially outwardly about the pivot axis A 7 . A cylindrical roller is  434  provided between the piston  410  and the steering pad  432  to facilitate pivotal motion of the steering pad  432  in response to lateral extension of the pistons  410 . The cylindrical roller  434  may be retained in a slot  434  of the steering pad  432  and may maintain rolling contact between the pistons  410  and the steering pad  432  as hydraulic pressure is applied and relieved from a hydraulic chamber  422 . Relieving the hydraulic pressure from the hydraulic chamber  422  permits the pistons  410  and steering pad  432  to return to radially retracted position as illustrated in  FIG. 5B . The pistons  410  each include a circumferential groove  444  therearound that may receive an elastomeric or other seal member therein (see, e.g.,  FIGS. 6A, 6B and 6C ). The elastomeric seal member establishes a sealing relation with a bearing  446  disposed with in the piston bores  418 . In other embodiments, a groove may be provided around any of the pistons  110 ,  210  or  310  described above to provide a back-up to the close fit of the respective piston  110 ,  210  or  310 . 
     Referring to  FIGS. 6A, 6B and 6C , the piston  410  is illustrated with at least one seal member disposed therein. The groove  444  may receive a single elastomeric o-ring  448  as illustrated in  FIG. 6A . Alternatively, as illustrated in  FIG. 6B , the groove  444  may be filled with wear resistant particles or balls  450  therein constructed of carbide, ceramics, diamond or other wear resistant materials. The interstitial spaces defined between the balls  450  may be filled with a filler material  452  such as grease or a rubber matrix in which the balls  450  are suspended. 
     As illustrated in  FIG. 6C , the balls  450  may be preloaded or energized so as to keep functioning as the balls  450  are worn. For example, as illustrated in  FIG. 6C  a spring  454  may be provided within the groove  444  to bias the balls  450  radially outward, e.g., in the direction of arrows  456 . The spring  454  biases the balls  450  into contact with the bearing  446  (see  FIGS. 5A and 5B ). The spring  454  may be a metallic spring or a compressed elastomer. In other embodiments, a fluid pressure may be applied to the groove  444  to press the balls  450  into contact with the bearing  446 . 
     Referring now to  FIGS. 7A and 7B , a steering actuation mechanism  504  includes at least one piston  510  extending from a piston bore  518 . The piston  510  includes a skirt  512  that extends below a circumferential groove  514  defined around the piston  510 . The skirt  512  is elongated on one lateral side thereof such that the piston  510  defines a first length L 1  on a first lateral side thereof and a greater second length L 2  defined on a second lateral side thereof. Since a circumferential gap “G” may be defined between the piston  510  and the piston bore  518  about a perimeter of the piston  510 , the greater length L 2  may operate to prevent tilting of the piston  510  within the piston bore  518 . Thus, the grater length L 2  maintains the piston  510  in general alignment with piston axis A 8 . 
     A key  524  is provided between housing  528  and the piston  510  to maintain a rotational orientation of the of the piston  510  about the piston axis A 8 . In the retracted configuration of  FIG. 7B , the skirt  512  extends into a hydraulic chamber  532  having a stepped floor  534 . The key  524  prevents the skirt  512  from impacting the stepped floor  534  in an orientation (not shown) that could prevent the piston  510  from reaching the retracted position of  FIG. 7B  where a steering pad  536  is fully closed. The stepped floor  534  accommodates the elongated skirt  512  within the limited space available in the housing  528 , e.g., without interfering with a longitudinal flow bore  538  extending through the housing  528 . 
     The piston  510  includes a ball  542  retained in a pocket  544  of the piston  510  by a pin  546 . The ball  542  rotates against a steering pad  536  as the piston  510  moves between the extended ( FIG. 7A ) and retracted ( FIG. 7B ) positions in the piston bore  518 . Retaining the ball  542  in the piston  510  may reduce impact loads of the ball  542  engaging the steering pad  536 . 
     Referring now to  FIGS. 8A and 8B , a steering actuation mechanism  604  includes at least one piston  610  extending from a piston bore  618 . The piston  610  has an angled or sloped skirt  612  such that the piston  610  defines a length L 3  on one first lateral side thereof and a greater length L 4  defined on a second lateral side thereof. The sloped skirt  612  may facilitate maintaining a rotational orientation of the piston  610  about piston axis A 9 . The skirt  612  may engage a flat or sloped floor  616  of a hydraulic chamber  618  to orient the piston  610 . 
     A roller  620  on the piston  610  is provided to roll against a steering pad  626  as the piston  610  moves between the extended ( FIG. 8A ) and retracted ( FIG. 8B ) positions in the piston bore  618 . As illustrated in  FIG. 8C , the roller  620  is retained in a pocket  630  defined in the piston  610  by an axel  632  extending through the roller  620 . The axle  632  and a convex outer diameter of the roller  620  facilitates rolling of the roller  620  in a single plane with respect to the piston  610 . The arrangement of the roller  620  may also facilitate maintaining the piston  610  in a particular rotational orientation about the piston axis A 9 . 
     The aspects of the disclosure described below are provided to describe a selection of concepts in a simplified form that are described in greater detail above. This section is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     According to a first aspect, the disclosure is directed to a rotary steerable apparatus. The rotary steerable apparatus includes a housing defining a longitudinal axis and having at least one piston bore extending from a hydraulic chamber within the housing along a piston axis oriented in a lateral direction with respect to the longitudinal axis of the housing. A drill bit is supported at a distal end of the housing and at least one steering pad is laterally extendable from the housing to thereby urge the housing in an opposite lateral direction in a wellbore. At least one piston is movable within the at least one piston bore in response to an increase in hydraulic pressure within the hydraulic chamber to thereby laterally extend the at least one steering pad. A gap defined between the at least one piston and the piston bore, the gap extending along the piston bore from the hydraulic chamber to an exterior of the housing. 
     In one or more embodiments, the at least one piston defines a convex cross-section in a plane extending through the piston axis. The convex cross-section of the at least one piston may be generally circular such that the at least one piston includes a generally spherical portion. 
     In some embodiments, the steering pad is pivotably coupled to the housing about a pivot axis generally parallel to the longitudinal axis. The at least one piston may be retained to the at least one steering pad and selectively movable between radially retracted and extended positions along the piston axis. The at least one piston may be retained in a T-slot defined on the steering pad, the at least one piston movable with respect to the steering along the T-slot in an oblique direction with respect to the piston axis. The at least one piston may be retained in the T-slot by a circular flange of the piston, and the circular flange may be rotatable in the T-slot such that the at least one piston is rotatable about the piston axis. 
     In one or more embodiments, the at least one piston includes a pair of pistons spaced from one another along the longitudinal axis. In some embodiments, the at least one piston includes a circumferential groove receiving at least one seal member therein. The at least one seal member may include at least one of the group consisting of an elastomeric o-ring, a plurality of wear resistant particles embedded in a filler material and a plurality of wear resistant particles suspended in grease. The at least one seal member may include a plurality of wear resistant particles energized by a spring to be biased radially outward with respect to the circumferential groove. In some embodiments, the at least one piston bore is at least one of the group consisting of cylindrical, elongated cylindrical and elliptically cylindrical, and wherein the at least one piston is at least one of the group consisting of spherical, spheroidal and spherocylindrical. 
     In another aspect, the disclosure is directed to a steerable drilling system. The steerable drilling system includes drill string extending from a surface location into a borehole, the drill string operable to rotate about a longitudinal axis of the drill string. A housing is supported within the drill string, the housing defining a hydraulic chamber therein and at least one piston bore extending from the hydraulic chamber. A drill bit is supported at a distal end of the housing, and at least one steering pad is pivotably coupled to the housing and extendable laterally from the housing to engage a side of the borehole and thereby urge the housing in an opposite lateral direction. At least one piston is selectively extendable through the at least one piston bore in the lateral direction and in engagement with the at least one steering pad to urge the steering pad to pivot radially outward from the housing. A gap is defined along the piston bore between the at least one piston and the housing about a perimeter of the at least one piston. 
     In some embodiments, the at least one piston is retained to the at least one steering pad and is slidable along the steering pad in an oblique direction as the steering pad pivots. In some embodiments, at least one piston is disconnected from the at least one steering pad. The steerable drilling system may further included a roller retained on the at least one pad and rollable between the at least one pad and the at least one piston as the at least one piston is extended. 
     In one or more embodiments, the at least one piston defines an arcuate convex cross-section in a plane through a piston axis extending in the lateral direction. The at least one piston may include a skirt elongated on one lateral side thereof such that the at least one piston defines a greater length along a first lateral side than an opposite lateral side thereof. The skirt is sloped between the first lateral side and the opposite lateral side of the at least one piston. In some embodiments, the skirt is stepped between the first lateral side and the opposite lateral side of the at least one piston, and the piston may be keyed to the housing such that the piston maintains a rotational orientation with respect to the housing. In some embodiments, the at least one piston includes at least one of the group consisting of a ball retained in a pocket defined in the at least one piston, the ball rotatable against the at least one steering pad and a roller retained in a pocket defined in the at least one piston, the roller retained in the pocket to rotate in a single plane with respect to the piston. 
     The Abstract of the disclosure is solely for providing the United States Patent and Trademark Office and the public at large with a way by which to determine quickly from a cursory reading the nature and gist of technical disclosure, and it represents solely one or more examples. 
     While various examples have been illustrated in detail, the disclosure is not limited to the examples shown. Modifications and adaptations of the above examples may occur to those skilled in the art. Such modifications and adaptations are in the scope of the disclosure.