A technique facilitates steering of rotary steerable systems when conducting directional drilling operations. A rotary steerable system is combined with a pressurized oil system which delivers oil to a piston actuated mechanism. The pressurized oil provides precise, long-lasting control over the orientation of the bottom hole assembly and the drill bit to facilitate directional drilling of boreholes through subterranean formations.

CROSS-REFERENCE TO RELATED APPLICATION 
     The present document is based on and claims priority to U.S. Provisional Application Ser. No. 61/356,442, filed Jun. 18, 2010, incorporated herein by reference. 
    
    
     BACKGROUND 
     Rotary steerable drilling systems for drilling deviated boreholes into the earth are generally classified either as point-the-bit systems or push-the-bit systems. In point-the-bit systems, the axis of rotation of the drill bit is deviated from the local axis of the bottom hole assembly in the general direction of the new portion of the hole being drilled. The borehole is propagated according to customary three-point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and a lower stabilizer results in a non-collinear condition required for a curve to be generated. In this type of system, the drill bit tends to have less sideways cutting because the bit axis is continually rotated in the direction of the curved borehole. 
     In push-the-bit rotary steerable systems, there is usually no specially identified mechanism to deviate the bit axis from the local bottom hole assembly axis. Instead, the requisite non-collinear condition is achieved when either upper or lower stabilizers are used to apply an eccentric force or displacement in a direction oriented with respect to the direction of borehole propagation. Steering is again achieved by creating non co-linearity between the drill bit and at least two other touch points. In this type of system, the drill bit is required to cut sideways to generate the desired, curved borehole. 
     In many of these rotary steerable systems, pistons may be used to create force against a borehole wall or to cause angular displacement of one steerable system component with respect to another to cause the drill bit to move in the desired direction of deviation. The pistons are deployed in a piston actuated mechanism and forced to their desired displacement to achieve the directional control. The pistons are manipulated via drilling mud pumped down through the bottom hole assembly. However, such systems may be subjected to internal wear from the flowing mud and also may be limited with respect to the forces which may be applied to steer the drill bit. 
     SUMMARY 
     In general, the present invention provides a technique which facilitates steering of steerable systems when conducting directional drilling operations. A directional drilling system (e.g. a rotary steerable system) is preferably combined with a pressurized oil system which delivers oil to a piston actuated mechanism. The pressurized oil provides precise, long-lasting control over the orientation of the bottom hole assembly and the drill bit to facilitate directional drilling of boreholes through subterranean formations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a schematic illustration of a rotary steerable system coupled with a pressurized oil system, according to an embodiment of the present invention; and 
         FIG. 2  is a schematic example of one type of bottom hole assembly and rotary steerable system incorporating pistons which are controlled by the pressurized oil system, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     The disclosure herein generally relates to a steering system for directional drilling. A bottom hole assembly incorporates a rotary steerable system having a piston actuated mechanism. However, the piston actuated mechanism is controlled by pressurized oil supplied from a pressurized oil system rather than being controlled by flowing drilling mud. The flow of oil to pistons of the piston actuated mechanism is controlled by a valve system. The valve system allows the pressurized oil to be ported to the pistons of the rotary steerable system to, for example, point the bit in the desired steering direction. The force required to manage the reactive forces from weight on bit (WOB), bottom hole assembly (BHA) mass and other drilling loads is provided by the pressure differential between the annulus and the pressurized oil acting across the piston area of the pistons. The separate pressurized oil system may be used with, for example, point-the-bit type systems, push-the-bit systems, or other types of steerable drilling systems. 
     Referring generally to  FIG. 1 , an embodiment of a drilling system  20  is illustrated. In this embodiment, drilling system  20  comprises a bottom hole assembly  22  coupled with a pressurized oil system  24 . The bottom hole assembly  22  comprises a drill bit  26  connected to a rotary steerable system  28  having a steering section  30  which is selectively manipulated via a piston actuated mechanism  32  having a plurality of pistons  34 . Pressurized oil system  24  is employed to route pressurized oil to piston actuated mechanism  32  and to selected pistons  34  via oil supply lines  36 . The pressurized oil is used to move specific pistons  34  which changes the orientation of the drill bit  26 , e.g. changes the drilling axis orientation, with respect to the longitudinal axis  38  of the bottom hole assembly  22 . For example, the pistons  34  may be employed to control at least one of the directional bias and the axial orientation of the drill bit  26 . The pistons  34  may be arranged, for example, to point the drill bit  26  or to push the drill bit  26 . By way of specific example, the drilling system  20  utilizes rotary steerable system  28  which rotates with the plurality of pistons/actuators  34 . Additionally, the rotary steerable system  28  may be used in conjunction with stabilizers, such as non-rotating stabilizers. 
     Pressurized oil system  24  may comprise an oil pump  40  which pressurizes the oil supplied through oil supply lines  36  for controlling the drilling orientation of the rotary steerable system  28 . The pressurized oil from pump  40  may be routed through a valve system  42  used to control the flow and pressure of the oil supplied to pistons  34  of rotary steerable system  28  and piston actuated mechanism  32 . 
     In the embodiment illustrated, oil pump  40  is driven by a shaft  44  which, in turn, may be driven directly by flowing drilling mud flowing through a turbine  46  or other device designed to power oil pump  40 . Alternatively, the oil pump  40  may be powered by an electric motor  48 . In the case of an electric motor, electrical power may be provided to motor  48  by an alternator  50 . By way of example, the alternator  50  may be driven by drilling mud, e.g. driven by drilling mud via a mud turbine or mud pump (PDM)  52 . In the embodiment employing electric motor  44 , a speed control system  54  may be implemented to maintain a constant pump pressure. In the embodiment in which pump  40  is a direct mud driven pump, the pressure may be maintained by an internal pressure relief valve  56 . It should be noted that electrical power may be supplied to motor  48  from other sources, e.g. from a surface supply coupled to electric motor  48  via cable or other conductors routed downhole. 
     The motive fluid for steering rotary steerable system  28 , e.g. oil supplied through oil supply lines  36 , works between a high-pressure source and a low-pressure reservoir. The low-pressure reservoir may be pressure balanced with the pressure internal to the drill string. The power required to provide the cyclic steering forces as the drill string rotates often requires a mechanical source of energy of several kWatts. For example, 5000 pounds acting over 0.25 square inches requires 141 joules at four times a second (240 rpm)−mechanical power of 1700 Watts assuming there is no energy recycling/storage. 
     The valve system  42  employs valves to control the flow of pressurized oil into/out of the pistons  34 , and those valves may comprise bistable actuators (low energy fluid flow switches) or piezo restrictive actuator valves. As an alternative, the valve system  42  may employ a rotary valve format, such as the rotary valve format used in the PowerDrive rotary steerable system available from Schlumberger Corporation. In either case, the system is designed to track the local gravity vector to enable the system to determine which valves are to be activated to achieve the required steering response taking into account the various tool face offset effects that exist due to friction, bit response, bottom hole assembly load, formation tendencies, and other potential factors. The gravity and valve data may be provided by suitable sensors. However, other types of valves and sensor systems may be employed in pressurized oil system  24  to control the flow of pressurized oil. 
     By using the separate pressurized oil system  24  to control the orientation of rotary steerable system  28 , less internal wear results which enables extended runtimes and a reduction in tools for each drilling job. The pressurized oil system also is amenable to higher pressure which, in turn, enables actuation by smaller pistons  34 , thereby providing more flexibility with respect to both packaging and actuation. The pressurized oil system  24  further enables use of higher forces while eliminating the coupling of actuation force and flow rate of the drilling mud. Additionally, the system  20  no longer requires relatively high bit drop pressures. The pressurized oil also can be combined with oil needed for other drilling systems. Depending on the specific application, the pressurized oil system  24  may be located in whole or in part downhole along the drill string. For example, the oil pump  40  and the valve system  42  may be located anywhere in the bottom hole assembly. The rotary steerable system  28  and the pressurized oil system are designed so that the pistons  34  can be actuated independently to achieve a straight ahead steering. Additionally, the design of the system enables modulation of the piston displacement and forces in synchronism with the phase of drill string rotation to achieve intermediate steering curvatures. 
     The pressurized oil system  24  may be used in combination with a variety of bottom hole assemblies  22  and rotary steerable systems  28 . However, one example of a suitable bottom hole assembly is illustrated in  FIG. 2 . A similar bottom hole assembly is described in U.S. Pat. No. 7,188,685. In this example, bottom hole assembly  22  combines both point-the-bit and push-the-bit technologies. It should be noted, however, the pressurized oil system  24  may be combined with a variety of other types of steerable bottom hole assemblies for use in directional drilling. For example, the rotary steerable system  28  may be a purely point-the-bit system or a purely push-the-bit system. 
     In the example illustrated, bottom hole assembly  22  comprises an upper stabilizer  58  mounted on a collar  60  which may be positioned adjacent rotary steerable system  28 . A lower stabilizer  62  is attached to an upper section  64  of rotary steerable system  28 . A steering section  65  is connected to drill bit  26 . A surface control system  66  may be utilized to communicate steering commands to electronics in upper section  64 . In some embodiments, the rotary steerable system  28  rotates with the pistons/actuators  34  and the stabilizers  58  and/or  62  may comprise non-rotating stabilizers. 
     The drill bit  26  is tilted about a swivel  67  which may be in the form of a universal joint  68 . In this embodiment, the steering section  65  is selectively actuated (e.g. pivoted/rotated) about swivel  67  with respect to upper section  64  to actively maintain a bit axis  69  pointing in a particular direction while the bottom hole assembly is rotated at a desired rotational speed of the drill string. Pistons  34  act on a periphery of the steering section  65  to apply a force for tilting the drill bit  26  with respect to the bottom hole assembly or tool axis  38 . The direction or orientation of the drill bit  26  broadly defines the direction of borehole formation. In a push-the-bit type system, the pistons  34  can be configured to act against the surrounding wellbore wall. 
     In one example, pistons  34  are sequentially actuated by virtue of the pressurized oil from oil system  24  as steering section  65  pivots/rotates. This enables the desired tilt of the drill bit  26  to be actively maintained to ensure drilling in a desired direction through the formation. In other embodiments and situations, the pistons  34  may be intermittently actuated in a random manner by the pressurized oil supplied through oil supply lines  36  to, for example, drill straight ahead as discussed above. In still other embodiments and situations, the pressurized oil from oil system  24  is used to actuate pistons  34  in a directionally-weighted semi-random manner to provide for less aggressive steering as the steering section  65  pivots/rotates. In some situations, the pressurized oil system  24  may be used to activate either all or none of the pistons  34  simultaneously to lock the steering sleeve, e.g. steering section  65 , in a drill ahead configuration and/or to reduce wear on the steering actuators. A variety of methods may be employed to measure the sleeve angle so as to improve control over the toolface and to improve control over the direction in which the sleeve is oriented. As described above, the steering may be achieved by synchronously modulating the pistons  34  in both force and displacement in phase relationship with the desired toolface pointing direction. Accordingly, the pressurized oil system  24  provides great flexibility for controlling directional drilling in a variety of applications and with many types of bottom hole assemblies  22  and rotary steerable systems  28 . 
     Although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims.