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The application claims the benefit of U.S. Provisional Application No. 60/787,139, filed Mar. 29, 2006. This application is a continuation application of Ser. No. 11/706,143, filed Feb. 13, 2007, now U.S. Pat. No. 7,631,707 B2. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention pertains to drilling of wells in the earth. More particularly, apparatus and method are provided for controlling the direction of a drill bit using a Rotary Steerable System (RSS) having a shape memory alloy (SMA) for applying the controlling force. 
     2. Description of Related Art 
     Directional drilling in the earth has become very common in recent years. A variety of apparatus and methods are used. Hydraulic motors driven by a drilling fluid pumped down the drill pipe and connected to a drill bit have been widely used. Directional control is achieved by using a “bent sub” just above or below the motor and other apparatus in a bottom-hole assembly. In this mode of drilling the drill pipe is not rotated while direction is being changed; it slides along the hole. More recently, the use of “Rotary Steerable Systems” (RSSs) has grown. These systems are of two common types: “push-the-bit” and “point-the-bit” systems. The drill pipe rotates while drilling, which can be an advantage is many drilling situations such as, for example, when sticking of drill pipe is a risk. 
     An RSS using the “point-the-bit” method is disclosed in U.S. Pat. No. 6,837,315. The system includes a power generation section, an electronics and sensor section and a steering section. In the power generating system, a turbine driven by the drilling fluid drives an alternator. The electronics and sensor section includes a variety of directional sensors and other electronic devices used in the tool. In the steering section, the shaft driving the bit is supported within a collar and a variable bit shaft angulating mechanism, having a motor, an offset mandrel and a coupling, is used to change the direction of the bit attached to the shaft. Similar power generation and electronics sections are common to many rotary steerable systems. 
     An RSS using the “push-the-bit” method is disclosed in U.S. Pat. No. 6,116,354. Thrust pistons are attached to pads and when the thrust pistons are actuated the pad is kicked against the wall of the borehole. Hydraulic fluid driving the pistons is controlled by a battery-driven solenoid. 
     A simpler and more reliable actuation mechanism is needed for driving the mechanisms of both “point-the-bit” and “push-the-bit” systems. This mechanism should provide the force necessary for a wide range of drilling conditions. 
     BRIEF SUMMARY OF THE INVENTION 
     A Rotary Steerable System (RSS) is provided. Either a push-the-bit or point-the-bit mechanism is activated by a shape memory alloy that is changed in length. The change in length, caused by temperature change of the alloy, is converted to transverse movement of a mechanism. The temperature of the alloy is controlled by electrical current in the alloy or by heating of material in proximity to the alloy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of one embodiment of the rotary steerable drilling tool disclosed herein. 
         FIG. 2   a  is a section view of the rotary steerable drilling tool when not activated;  FIG. 2   b  is a section view of the tool when activated to push the bit. 
         FIG. 3  is an isometric view of the SMA actuator module. 
         FIG. 4   a  is a section view of the SMA actuator module when not activated;  FIG. 4   b  is a section view of the activator when activated to exert a force. 
         FIG. 5  is an illustration of the use of an SMA actuator to push a bit using pads on a sleeve. 
         FIG. 6  is an illustration of the use of an SMA actuator to point a bit using a flexible shaft. 
         FIG. 7  is a schematic of an actuator design with straight SMA wires or rods. 
         FIG. 8  is a block diagram of a directional drilling system using SMA actuators. The same part is identified by the same numeral in each drawing. 
         FIG. 9  is an illustration of the SMA wire wound about the guides. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an isometric view of rotary steerable tool  10  is shown. The tool consists of shaft  11 , up-connection pin or box  12 , non-rotating sleeve  13 , three pads  15  (one shown), three hatch covers  16  (one shown) and electronics section  18 . Shaft  11  may be connected to a drill bit and pin or box  12  may be connected to another segment of a bottom-hole assembly (BHA), which will be connected to the bottom of a string of drill pipe. Shaft  11  and connection pin or box  12  may rotate with the drill string while sleeve  13  is stationary. 
     Referring to  FIG. 2   a , sleeve  13  is constrained on shaft  11  through two bearing packs  19 . Sleeve  13  does not rotate with shaft  11  during drilling, although slow rotation may occur. The three SMA actuator modules  14 , which will be described in detail below, are bolted in the cavities evenly distributed along the circumference of sleeve  13 . Above each SMA actuator module, hatch cover  16  is screwed on sleeve  13  for protection. Pad  15  is hinged on sleeve  13  with pin  25 , and moves outwards as actuator  14  is being activated. In  FIG. 2   b , actuator  14  has been activated, forcing pads  15  outward. A bit attached to shaft  11  would thereby be forced in the opposite direction to movement of the pad, which would cause the creation of a curved trajectory of the borehole formed by the bit. 
     Shape Memory Alloy (SMA) is the family name of metals that have the ability to return to a predetermined shape when heated. Such materials are available from a variety of sources that may be identified with an internet search. When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape—which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure, which causes it to return to its original shape. During its phase transformation, the SMA either generates a large force against any encountered resistance or undergoes a significant dimension change when unrestricted. This characteristic of SMA is referred to as the “shape memory effect;” it enables SMAs to be used in solid-state actuators. There are SMAs having different transformation temperature, workout, and recovery strain. Fine adjustment of compositions of SMAs and manufacturing procedures will produce the desired properties of an SMA for specified applications. For the applications of the steering tool disclosed herein, the transformation temperature of SMA is chosen such that maximum ambient temperature is 20-30° C. below the transformation point of the material. Then the SMA can be activated only with the intentional addition of heat. The SMA can be heated by conducting electrical current through its length or by conduction effect of electrical heaters that are near or bonded to the SMA or by using environmental temperature, tool waste heat, drilling fluid temperature or a combination of sources. The SMA material used for the steering tool may be in the form of wires or rod. The dimensions and the number of the SMA wires or rods are chosen such that enough actuation force is ensured to push a drilling bit against the reaction resistance from side cutting. Due to the variety of the SMA forms and dimensions, there are various combinations of the SMA wires or rods suitable for the steering tool design. The example shown hereafter is just one of those possible design plans. 
     The SMA material to be used may be “trained” at a temperature above its transition temperature to have a length shorter than its length below the transition temperature. It is then installed in the RSS disclosed herein. When the material is heated above the transition temperature, length of the material decreases. In the embodiments discussed, this decrease in length is used to drive a pad or shaft in a direction transverse to the direction of the decrease in length. 
     A representative design of an actuator is shown in  FIGS. 3-4 , which is the same design as shown in  FIG. 2 . Referring to  FIG. 3 , the SMA actuator  14  comprises a linkage system ( 31 ,  33 ,  34 ,  35 , and  36 ), a motion transmission system ( 30 ,  32 ,  44  and  37 ), and an SMA winding system ( 32 ,  38  and  39 ). Stationary Guide Rail  38  of the winding system is held in place by pins  38 A. Guide  39  of the winding system is held in place by pins  39 A. Only a short segment of SMA strand  40 , which may be made of several thin SMA wires, is shown, to Provide greater clarity. Strand  40  winds around stationary guide rail  38  and movable guide rail  32  as shown in  FIG. 9 . The winding of SMA strand  40  and the its length are selected so that movable guide rail  32  slides a sufficient distance to ensure that pad  15  ( FIG. 2   a ) may push against the wall of the wellbore with a selected displacement amplitude and magnitude of lateral force when SMA strand  40  is heated above its transition temperature. Spring  43  may be used to pre-tension SMA strand  40  before activation and to reset the SMA after deactivation. The linear sliding motion of rail  32  is transmitted to the movement of slider  37 , spring  43  and rod  44 . Rod  44  is connected to rail  32  and slider  37 , and its movement is supported by bearing  46 . Rod  30  is attached to rah  32 , and slides on bearing  41 . To ensure a smooth sliding of slider  37 , sliding mail  42  is used to guide the slider. A long linkage  33  and short linkage  35  are hinged by a pin  34 . The other end of linkage  35  is hinged to stand  48 , which is bolted on sleeve  13  with bolt  49 . Hence, linkage  35  only rotates about the in  36 . Pin  31  connects slider  37  and long linkage  33  and allows linkage  33  to rotate relative to the slider. The lengths of the two linkages are chosen so that the pad moves a selected amount with a given displacement of rail  32 . Various modifications of the linkage system can meet the displacement amplification requirement. Stationary Guide Rail  38  and Guide  39  are mounted on a plate member  17 . 
     Upon electrical heating, which can be done by directly heating the SMA elements by passing electrical current through the elements or by using a heating element near or in contact with the SMA elements and/or using any other heat source available downhole, SMA strand  40  contracts as a result of crystal structure changes. The resultant contracting force overcomes the pre-tension force on spring  43  and pushes movable guide rail  32  toward stationary rail  38 . Through the transmission chain consisting of the rod  44 , slider  37  and linkages  33  and  35 , the displacement of the rail  32  results in the transverse movement of pad  15 . Comparison of the positions of the moving components in  FIGS. 4   a  and  4   b  clearly illustrates the actuation mechanism. 
     The SMA material may be heated by a variety of methods. For example, an oil bath surrounding the SMA material may be heated electrically. Alternatively, a separate resistance wire in thermal contact with the SMA material may be heated to heat the SMA material. 
     Referring to  FIG. 5   a  and  FIG. 5   b , fully deployed pads  15  may be designed to extend outward to a diameter greater than the nominal diameter of the wellbore. As pads  15  touch wall-of-the-wellbore  50 , they may be not fully activated, and continuously heating of SMA strands  40  ( FIG. 3 ) will produce large holding force on the pads. At this moment, pads  15  function like stabilizers, and sleeve  13  is stationary (not rotating). The combination of reaction forces from the three pads determines the steering force and direction. If the three forces are equal, a drill bit attached to shaft  11  remains at the center of the well, as illustrated in  FIG. 5   a . To make a deviation of the drilling trajectory, under command from the electronics package, a feedback control loop coded in the electronics may regulate the electrical current applied to the three actuators to adjust their actuation forces so that the combined reaction pushes the attached drill bit sideways (transverse to the axis of the wellbore) and in the desired direction, as shown in  FIG. 5   b . One or two pads may be activated to apply greater sideways force and one or two pads may be deactivated to an extent to apply less force. This steering approach is called the “push-the-bit” mode. 
     The SMA actuator may also be used for “point the bit” RSSs, as illustrated in  FIGS. 6   a  and  6   b . For this system, three steering pads  51  are directed inwards to apply sideways force between sleeve  53  and bearing  55 , which supports shaft  52 , instead of outwards to wall-of-the-wellbore  50 . As illustrated in  FIG. 6 , as the three pads are deployed, they control the axial alignment of the shaft by means of bearing  55 . Similar to the former, the resultant steering force may be applied to shaft  52  to cause  FIG. 6   b  to point the bit for deviation of the wellbore, as shown. 
     The SMA used to generate the actuation force can be used in different combinations and arrangements, including SMA rods, wires, cables, pre-formed elements, and/or a combination thereof to achieve different forces, different expansion and contraction lengths, different stroke lengths and different actuation cycle times for generation of force and for the subsequent relaxation period of the SMA. The direction of the generated force can also be varied by using different assemblies of pulleys, linkages, levers, springs, rods, in different forms and combinations. For example, the schematic in  FIG. 7  shows an actuator using straight SMA wires or rods  70  instead of strands of SMA materials that pass around pulleys. The wires or rods are attached at one end to slider  72  and at the other end to support  76 . The linkage system remains, but the actuator force comes from two groups of SMA wires or rods symmetrically placed at the two sides of the linkage system. The linkage system is moved by rod  74 , which is attached to slider  72 . Without pulleys, this design eliminates the potential friction of the SMA wires and the rail used in the alternate embodiment, and requires more strain recovery capability of SMA materials. 
     The same principle of generating a substantial force using SMA material in different forms and shapes and alloys and combinations thereof, can also be used in different temperature ranges and environments; for example, the actuator unit disclosed herein may be used as a valve actuator or for other applications. 
     The disclosed system when used for rotary steerable drilling may be controlled with an algorithm, as illustrated in  FIG. 8 . The electrical current to heat the SMA may come from 3-phase alternator  80 , which may be either driven by a turbine from drilling fluid flow or from relative rotation of shaft  11  in stationary sleeve  13  ( FIG. 1 ) of a drilling assembly. Closed loop control system  82  controls the steering of the device, which may receive downlink commands using well known methods such as industry standard mud pulse telemetry or drill string rpm coding. Once the tool receives commands from the surface, electronics package  84  and software work to immediately implement automatic steering continuously, using heating elements and temperature and force sensors  86 , until another command is sent. Alternatively, commands may not be downlinked from the surface but may be generated when downhole instruments that measure direction of the bit, such as an accelerometer and gyroscope or magnetometer, compare that direction to a pre-selected direction and send a signal to the rotary steerable system disclosed herein. 
     Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except as and to the extent that they are included in the accompanying claims.

Summary:
A rotary steerable apparatus is provided having an actuator for pushing the bit or pointing the bit that includes a shape memory alloy. An elongated form of the alloy, such as a wire or rod, is employed in a mechanism that applies force in a direction transverse to the wellbore in response to a change in length of the alloy. Temperature of the alloy is controlled to change shape and produce the desired force on pads for operating the apparatus. The apparatus may be used with downhole power generation and control electronics to steer a bit, either in response to signals from the surface or from downhole instruments.