Abstract:
Methods and apparatuses for drilling a borehole are disclosed. An electric motor electrically and mechanically coupled to a wired drill pipe is provided. The electric motor couples to a shaft that rotates when power is supplied to the electric motor. The shaft is couplable to a drill bit. The wired drill pipe transfers electricity to the electric motor from the surface. Operation of the electric motor rotates the shaft. The drill bit wears away earth to form the borehole in the earth.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to commonly owned U.S. provisional patent application Ser. No. 60/549,852, filed Mar.3, 2004, entitled “Rotating Systems Associated with Drill Pipe,” by Daniel D. Gleitman, Paul F. Rodney, and James H. Dudley, which is incorporated herein by reference for all purposes. This application is a continuation of U.S. patent application Ser. No. 11/071,823, filed Mar. 3, 2005, entitled “Rotating Systems Associated with Drill Pipe,” by Daniel D. Gleitman, Paul F. Rodney, and James H. Dudley, which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND  
       [0002]     In traditional systems for drilling boreholes, rock destruction is carried out via rotary power conveyed by rotating the drill string at the surface using a rotary table or by rotary power derived from mud flow downhole using, for example, a mud motor. Through these modes of power provision, traditional bits such as tri-cone, polycrystalline diamond compact (“PDC”), and diamond bits are operated at speeds and torques supplied at the surface rotary table or by the downhole motor.  
         [0003]     In some circumstances and under some drilling conditions when using these traditional techniques, the drilling rate (or rate of penetration, “ROP”) may be compromised. When that occurs, the operator has several options to improve the drilling rate. The operator can trip out the drill string for a new drilling assembly more likely to be successful in drilling under the existing circumstances. Alternatively, if a rotary table on the surface provides the drilling power, the operator can change the rotary speed within a relatively narrow range, such as approximately 60 to 250 revolutions per minute (“RPM”). If the drilling system includes a downhole positive-displacement motor (“PDM”), the operator can change the motor speed over a range, for example, of approximately 150 RPM to approximately 300 RPM (for a medium speed 6¾-inch motor). A change in motor speed, however, can produce proportionate flow rate changes that can have a profound effect on hole cleaning, pressure drop, and other factors. As yet another alternative, the operator can attempt to adjust the weight on bit by adjusting the hook load at surface.  
         [0004]     In all of these techniques the operator is remote, both in distance and time, from the changing bottom hole conditions that caused the compromised ROP. As a consequence, it may take some time for the compromised ROP to manifest itself at the surface and for the operator to recognize that the ROP has decreased. In addition, the operator&#39;s response actions, such as adjusting the rotary speed, hook load, or flow rate, are equally remote from the bit on bottom. Various load factors such as torque and drag may attenuate the operator&#39;s control action and compromise its effectiveness.  
         [0005]     Continuous movement, including rotation, of the drill string has important benefits in addition to transferring power to the bit. Torque and drag consumption along the drill string due to frictional losses may reduce the weight and rotary torque available to be transferred to the bit, which may cause the power available at the bit to be variable or unpredictable. This power variability may, in turn, compromise ROP. An important source of frictional loss is static friction, which typically occurs during non-rotary periods, momentary stoppages of the pipe during sliding due to stick/slip, and periodic stoppages during additions of drill pipe. In addition to the static friction, an immobile pipe string is more likely to become differentially stuck due to pressure differential between the hole and the formation. Further, pipe rotation is known to keep the cuttings mobile and off the bottom of the hole, especially in horizontal wells. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic illustration of an example drill string in a borehole.  
         [0007]      FIG. 2  is a schematic illustration of an example torque reaction sub.  
         [0008]      FIG. 3  is a schematic illustration of an example dynamic clutch sub.  
         [0009]      FIG. 4  is a schematic illustration of an electric motor, flywheel, and clutch housed within a drill string, with a shaft available for driving the bit, an alternator, and an optional rotating imbalance for creating a vibration sub.  
         [0010]      FIG. 5  is a schematic illustration of an example vibration sub.  
         [0011]      FIG. 6  is a schematic illustration of a drill string turbine and flywheel. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  schematically illustrates a new drilling method and apparatus. A drill string  10  includes wired drill pipe  100 . Drill string  10  is located inside a borehole  20  in a formation  30 . Wired drill pipe  100  may include joints of pipe which contain conductors within the drill pipe walls. Wired drill pipe  100  may utilize tubing within the bore of the pipe (e.g., centralized down the center, or biased against the pipe bore inner diameter) to convey conductors. Wired drill pipe  100  may utilize, for example, center stab connectors at each pipe joint, male and female connectors making electrical contact as the drill pipe rotary shouldered connections are made up. In certain embodiments, wired drill pipe  100  may comprise continuous tubing to convey drilling fluid and hang the bottom hole assembly, with conductors either integral with the tubing wall, or contained within a smaller diameter tubing within the bore of the continuous tubing. Wired drill pipe  100  may, for example, convey on the order of 250 kw to 1 MW of electrical power downhole, so as to not depend upon surface rotation or the mud flow for steady power for use in drilling. Wired drill pipe  100  may additionally convey measurement and control signals between surface and various points downhole.  
         [0013]     A vibration sub  200  may be utilized at various points in the drill string, to ensure that the string is in a dynamic state even when not rotating or progressing down the hole. A typical logging-while-drilling (“LWD”) suite  300  may be utilized for directional and formation sensing. An electric motor sub  400  may be positioned below LWD suite  300  and above a bit  500 . Electric motor sub  400  houses an electric motor, not shown in  FIG. 1 , that drives the rotation of bit  500 . Example drill string  10  may alternatively include a fluid-driven motor sub in place of the electric motor sub  400 , discussed in greater detail later in this description. Drill string  10  may further include a torque reaction sub  600  and clutch  700 , both of which we discuss in greater detail later in this description. A real-time processor  800  may control the operation of drill string  10  and its components, as we also discuss in detail later in this description.  
         [0014]     Although not shown in  FIG. 1 , the electric motor inside electric motor sub  400  could be a brushless DC motor. This brushless DC motor could operate with commutation control as described in U.S. patent application Ser. No. 10/170960, filed Dec. 18, 2003, entitled “Digital Adaptive Sensorless Commutational Drive Controller for a Brushless DC Motor,” assigned to the assignee of this disclosure. That is, the brushless DC motor may be commutated by a digital adaptive controller circuit adapted to receive digital back electromotive force detector signals. The back electromotive force detector signals could be used to indicate whether voltages on windings in the brushless DC motor are above a threshold level. The voltages could be compared with previously detected levels to determine whether the winding voltages are as expected. Alternative known methods may instead be used to commutate the brushless DC motor.  
         [0015]     In one example drill string  10 , a housing  410  for electric motor sub  400  rotates with drill string  10  at, for example, approximately 60 to approximately 250 RPM. Bit  500  rotates relative to housing  410  at a much higher rate, such as approximately 1000 RPM to approximately 2000 RPM. Assuming the same approximate torque is available to bit  500  as would be available with a traditional drilling system (e.g. drilling with just surface-rotation, or with a mud-driven PDM), and the RPM is 10 times higher, the power available to break the rock would be 10 times higher than such a traditional system.  
         [0016]     In a conventional drill string, a 6¾-inch mud motor may provide a consistent 100 horsepower (HP) to the bit when drilling an 8½-hole, at 450 gallons per minute (gpm) mud flow rate and 500 psi pressure drop. If an electric motor were substituted for the mud motor to do the same job, this flow rate and pressure drop would correspond to around 74.6 kW of electrical power (not accounting for the efficiency factor of the electric motor, which is generally fairly high). Assuming a full 1 MW of electrical power can be made available to the electrical motor in drill string  10 , this increased power represents that full order of magnitude more power than the energy available to a typical mud motor. The operator may prefer, however, to limit the electric power being fed down drill string  10  to electric motor sub  400  to around 250 kW. Even this amount is several times the power available via a typical 6¾-inch mud motor, and the electric power in this case would be available without consuming 500 psi of mud pressure over a mud motor. This pressure is therefore available for other purposes, including increased hole cleaning at bit  500 .  
         [0017]     In drilling some boreholes, sufficient power may be available downhole, but the power is not in useable form. For example, power available downhole may not be available as speed. An electric motor is especially appropriate for circumstances in which the extra bit speed can be used to more effectively break and remove the rock. Existing diamond bit technology is particularly effective at high speeds, and electric motors would be ideal for driving them.  
         [0018]     Whether the higher bit rotation speed is accomplished with the same level of power as is currently used, such as around 100 HP, or at the higher power levels that can be produced as a result of increased electrical power provided to the motor, an optional flywheel may be used to provide even further increased power, or torque at that high speed, for a few moments to minutes when needed to break through a hard spot in a formation. We discuss this flywheel in greater detail later in this description.  
         [0019]     The operator may steer bit  500  by maintaining electric motor sub housing  410  in a non-rotating mode, while at the same time biasing the bit. This action may be completed by “pointing” bit  500  with a pair of eccentrics (not shown in the figures), as described in U.S. Pat. No. 6,640,909, entitled “Steerable Rotary Drilling Device,” assigned to the assignee of this disclosure. When steering, the operator may then prefer to maintain the motor housing in a sliding mode, with its orientation referenced to the borehole.  
         [0020]     In certain circumstances, extreme torque may be desired or required, even just for a moment, to break through a hard region in a formation. To accommodate such an increased torque requirement without excessively winding up drill string  10 , a torque reaction sub  600  may be provided to transfer torque into the formation immediately above bit  500  and electric motor sub  400 . This transfer would be practical only when the lower portion of the borehole assembly (“BHA”), such as electric motor sub housing  410 , is sliding.  
         [0021]      FIG. 2  schematically illustrates an example torque reaction sub  600  in cross-section with center line  601 . Example torque reaction sub  600  may include wheels  610 , which may be actuated via solenoids  611 . For illustrational purposes only,  FIG. 2  illustrates one wheel  610  in its retracted position, while another wheel  610  is in its extended position. Wheels  610  may have a hard cutting edge of a material such as carbide or diamond for digging into formation  30 . In this case, wheels  610  may align with the axis of borehole  20  and have preferred rolling directions parallel to the borehole axis so as to restrict rotation of the housing of torque reaction sub  600 . Alternatively, wheels  610  may include a hard broad area for contact with the wall of borehole  20  and utilize a significant radial force from, for example, solenoids  611 . In either case, torque reaction sub  600  may transfer significant torque through wheels  610  while allowing drill string  10  to travel in the axial direction.  
         [0022]     In some circumstances, the operator may wish to maintain electric motor sub housing  410  in a sliding mode, when steering or during other operations, such as transferring torque into the formation as referenced above. At the same time, the operator may wish to continue to rotate drill string  10  to remove cuttings and to prevent the drill string from experiencing static drag and sticking in borehole  20 . To accommodate both concerns, drill string  10  may optionally include a clutch  700 . In particular, drill string  10  may include a dynamic clutch sub, as described in a United States Patent Application filed on Mar. 4, 2004, entitled “Providing a Local Response to a Local Condition in an Oil Well”,attorney docket number 063718.0523, by the same inventors (referred to hereafter as the “Local Response Patent Application”).  
         [0023]      FIG. 3  is a cross-sectional, side, schematic drawing of an embodiment of an example dynamic clutch sub  1000  having a center line  1001 . The sub has a box connector  1002  at the top for making up to pipe string. A housing  1003  is threaded onto the exterior of the box connector  1002  wherein o-ring seals  1004  complete the connection. An electronics insert  1005  may be connected to the interior of the box connector  1002 . A printed circuit board (“PCB”)  1006  may be housed within the electronics insert  1005 . The printed circuit board may be controllable by surface real-time processor  800 , not shown in  FIG. 3 . Processor  800  may be located outside sub  1000 , such as at the surface. PCB  1006  may include one or more sensors, preferably for sensing rotational orientation, rotary speed, tangential accelerations, or torsional strains, as may be useful in control of a dynamic clutch sub. A balance chamber  1010  may be defined between the box connector  1002  and the housing  1003 . The balance chamber  1010  may be split into a mud fluid section in the top and a hydraulic fluid section in the bottom by a balance piston  1011 . The upper section of the balance chamber  1010  fluidly communicates with the exterior (annulus between the sub and casing, not shown) of the sub  1000  via balance port  1012 . Hydraulic fluid may be injected into the balance chamber  1010  through a fill plug  1013 . The balance chamber  1010  may also have a spring in the upper mud portion to bias the balance piston  1011 .  
         [0024]     A rotating mandrel  1015  may be made up to the inside of the box connector  1002  and the housing  1003 . The rotating mandrel  1015  may have two parts, a friction section  1016  and a pin connector  1017 . The friction section  1016  and the pin connector  1017  may be threaded into each other and o-rings  1018  may complete the connection. A friction plate  1019  may have a ring-like structure and may be attached to an upward facing surface of the friction section  1016 . A radial bearing  1020  may be positioned between the friction section  1016  and the box connector  1002 . A thrust bearing  1022  may be positioned between the bottom end of the friction section  1016  and a housing flange  1021  that extends radially inward from a lower end of the housing  1003 . A radial bearing  1023  may be positioned between pin connector  1017  and the housing flange  1021 . A thrust bearing  1024  may be positioned between an upward face of the pin connector  1017  and the housing flange  1021 .  
         [0025]     A bearing chamber  1025  may be defined between the housing  1003 , the box connector  1002 , and the rotating mandrel  1015 . An upper end of the bearing chamber  1025  may be sealed by rotary seals  1026  between the friction section  1016  and the box connector  1002 . A lower end of the bearing chamber  1025  may be sealed by rotary seals  1027  between the pin connector  1017  and the housing  1003 . The bearing chamber  1025  may be fluidly connected to the balance chamber  1010  via gap  1028 . The balance chamber  1010  enables hydraulic fluid to be maintained in and around the bearing regardless of the pressure being generated on the exterior of the sub  1000 .  
         [0026]     An array of solenoids  1007  may be connected to the bottom of the box connector  1002 . A communication/power bus  1008  communicates control signals between PCB  1006  and the array of solenoids  1007 , and in one embodiment also communicates rotary electrical interface  1030  between the opposing faces of the box connector  1002  structure and the rotating mandrel  1015 . This rotary electrical interface may comprise simply a relative rotation sensor.  
         [0027]     In other embodiments, the communication power bus  1008  also extends through this rotary electrical interface  1030  into the rotating mandrel  1015  for connection to a sensor set (not shown) which may preferably sense similar parameters to those named earlier which may be included with printed circuit board  1006 , but here such parameters associated with the rotating mandrel. This extension of communication/power bus  1008  may further extend along the mandrel  1015  and connect to other drill string elements connected to the bottom of the sub. In such embodiments the rotary electrical interface  1030  may comprise an inductive type or brush type interface.  
         [0028]     An array of pistons  1009  may extend from the array of solenoids  1007  and have clutch plates  1014  attached thereto. The clutch plates  1014  may be positioned opposite the friction plate  1019  so that when the array of solenoids  1007  is engaged, the clutch plates  1014  extend to contact and press against the friction plate  1019 . This action restricts relative rotational movement between the rotating mandrel  1015  and the box connector  1002 . A return spring  1029  may be positioned between a flange on the housing  1003  and the clutch plates  1014  to release the clutch plates  1014  from the friction plate  1019  when the array of solenoids  1007  is deactivated. The clutch plates  1014  may also engage in a spline  1028  between the clutch plates  1014  and the housing  1003  to prevent rotational movement while allowing axial movement.  
         [0029]     The amount of torque translated from one side of the dynamic clutch sub to the other depends on the control signals applied to the array of solenoids  1007 . The control signals may be provided by an independent controller on PCB  1006  or may be provided through the PCB  1006  by real-time processor  800 , discussed later in this description. A set or series of clutch and friction plates operating together (not shown) may alternatively be employed, to increase the contact area and thereby reduce the contact pressure requirement in achieving the mechanical torque capacity required. In another embodiment (not shown), the return springs  1029  may be positioned so as to create a default contact condition between clutch plates  1014  and friction plates  1019 , thus allowing for slippage and relative rotation only when the solenoids are activated.  
         [0030]     Returning to  FIG. 1 , drill string  10  could be rotated from surface at a relatively low RPM, with clutch  700  engaged in a dynamic manner to continuously and precisely offset reactive torque from the electric motor inside electric motor sub  400  and bit  500  and to carry that reaction up drill string  10  to the surface and into the wall of borehole  20  through frictional losses. This precise offsetting of motor torque allows the operator to maintain electric motor sub housing  410  at an approximately constant orientation within borehole  20 —or at least prevent the orientation of electric motor sub housing  410  from varying too quickly for the eccentrics pointing bit  500  to readjust bit  500 .  
         [0031]     Should bit  500  encounter a particularly hard formation top that requires more torque than drill string  10  can safely accommodate, torque reaction sub  600  can activate rudder wheels  610  to engage the wall of borehole  20  and provide a torque short circuit into formation  30 . The BHA can still advance even when rudder wheels  610  engage formation  30 . Clutch  700  would disengage fully or maintain a torque transmittal level up drill string  10  that is below the safety threshold of drill string  10  but that still allows the string to be rotated from surface.  
         [0032]     A real-time processor  800  may be coupled to drill string  10  and provide real-time control to electric motor sub  400 , clutch  700 , and torque reaction sub  600 . As shown in  FIG. 1 , processor  800  may be located at surface, if desired. Processor  800 , or portions of processor  800 , may be located downhole. Processor  800  may comprise two or more processing units that may be distributed within the elements of drill string  10 . Processor  800  could control the current available to electric motor sub  400 , or torque capacity. Also, processor  800  could control the motor speed for the electric motor in electric motor sub  400  and actuate rudder wheels  610  of torque reaction sub  600  to engage with or disengage from the wall of borehole  20 . Processor  800  could also control to partially or fully engage clutch  700 . Drill string  10  would require appropriate sensors downhole to help realize these control functions. Any of the control functions of the electric motor sub  400 , clutch  700 , and torque reactor sub  600  may be performed by distributed controllers that themselves are under the control of processor  800 . For example, drill string  10  may include torque and RPM sensors (not shown) at the two sides of clutch  700  and displacement sensors on rudder wheels  610  (also not shown). Further, drill string  10  could feed motor current and back-electromotive forces into the controls.  
         [0033]      FIG. 4  schematically illustrates a detailed view of a portion of the above-described drill string, with electric motor sub  400 . An electric motor  420  inside electric motor sub  400  couples to a shaft  425 . Shaft  425 , in turn, may couple to bit  500 , not shown in  FIG. 3 . Shaft  425  may alternatively or additionally couple to a vibration sub, discussed later in this description. An example electric motor  420  may include windings to form a stator  430  that is fixed within a collar  440 . Given the form-factor requirements of the drilling environment, stator  430  may comprise multiple stators  431  in series driving a single rotor  432 . Rotor  432  may include sets of magnets  436  arranged around the rotor, with a magnet set  436  corresponding to each of the multiple stators  431 . The multiple stators  431  may be configured with the multiple rotor magnet sets  436  to provide for establishing a closed magnetic circuit at each stator “stage.” Such an arrangement may enable electric motor  420  to provide a greater power output than a single-stage electric motor could provide. Rotor  432  may be on radial and thrust bearings  433  (shown schematically) and may have a channel  434  for mud flow. An inner sleeve (not shown) may optionally be used on bearings within rotor  432  and fixed from rotation from a key above or below, to prevent mud flow from interacting with rotor  432  as it rotates at high speeds. The motor windings may be wired to via hanger interface  435  to a sonde  450  centralized within collar  440  above electric motor  420 . Sonde  450  may optionally contain elements of motor control circuitry, and communications interface to real-time processor  800 , not shown in  FIG. 4 . Processor  800  may be located outside sonde  450 ; for example, processor  800  may be located on the surface. Hanger interface  435  may provide an electrical interface while permitting the mud flow to transition from annular flow around sonde  450  to center flow through rotor  432 .  
         [0034]     Rotor  432  may be fixed to an optional flywheel  900  below or above rotor  432 . Flywheel  900  may provide rotor  432  with an inertia that allows the electric-motor-flywheel combination to provide a power output on an impulse or a short-term basis that is greater than the output by electric motor  432  alone. Such increased power may be useful for a number of purposes, including breaking a particularly hard rock section embedded in an otherwise drillable formation. For example, electric motor  420  can drive bit  500  and flywheel  900  at speeds of approximately 1000 RPM to approximately 3000 RPM. The electric motor, bit and flywheel combination can thereby develop much greater power (as calculated by multiplying speed by torque) for breaking and clearing formations than the power generated through traditional rotary- or mud-motor-based drilling.  
         [0035]     An example flywheel  900  for use in a 6¾-inch collar might be 5 feet long and have a 4.6-inch outside diameter and 3-inch inside diameter. If, for example, flywheel  900  is made of steel, and spinning at 3000 RPM, it could provide kinetic energy on an “as needed” basis of 10,300 ft-lbs, or 18.7 HP-seconds. As bit  500  engages a hard spot in the formation, and the torque requirement subsequently increases impulsively corresponding to approximately one bit revolution at 3000 RPM (i.e., 0.02 seconds), the energy supplied by flywheel  900  would represent an extra 935 HP for that brief interval.  
         [0036]     Various design parameters of flywheel  900  can be adjusted to provide greater stored energy. A 25-foot flywheel may be implemented within a standard length, or 30-foot, collar; if made of steel, such a flywheel would provide 95 HP-seconds of energy. If flywheel  900  is made of a heavier substance such as tungsten, it could provide more than double the energy that a comparably-designed steel flywheel  900  could provide. We have thus far discussed flywheels of relatively small diameters. To drill larger holes, drill string  10  may employ a flywheel  900  with a significantly larger outside diameter. A 9⅝ inch outside diameter sub could be used in drilling 12 l/4-inch or larger holes and could employ a flywheel with a 7-inch outer diameter and a 5-inch inner diameter. That change would increase the energy capability of flywheel  900  by a factor of four times, other design parameters being equal.  
         [0037]     Flywheel  900  could alternatively be clutched in and out of the rotation path.  FIG. 4  illustrates a clutch assembly  750  that could be used for engaging the flywheel to the shaft or engaging the motor to the flywheel (not shown), as described earlier in this description.  
         [0038]     Flywheel  900  also can be used for other purposes. During connections, such as when operators add new drill pipe at the surface, the electrical power supplied through wired drill pipe  100  may be disconnected. By using flywheel  900  to drive an alternator (not shown in  FIG. 4 ), or simply allowing flywheel  900  to back-drive electrical motor  420 , ample electrical power can be made available for most functions. The drilling would probably not be taking place during the addition of pipe, as the mud flow and the weight on bit  500  from the surface will also be interrupted. However, circumstances may require that drill string  10  keep moving, and flywheel  900  may be used to maintain the dynamic state of drill string  10 .  
         [0039]     For example, flywheel  900  could directly engage a mechanical vibration sub  200  through clutch  750 , as shown in  FIG. 3 . Vibration sub  200  may be a limber sub with external outside-diameter reliefs to reduce stiffness. This sub could contain another smaller offset flywheel  220  on bearings about shaft  425  but with its center of mass offset from the center of collar  440 . As flywheel  900  engages through clutch  750 , offset flywheel  220  represents a rotating imbalance and would shake collar  440  and a significant part of drill string  10 . Through gearing, the shake frequency of vibration sub  200  could be designed to be low, or even intermittent yet periodic, so as to conserve the energy of flywheel  900  and provide a longer period of utility until electrical power is reestablished. Drill string  10  can also employ vibration subs  200  or other rotating imbalances up and down drill string  10  during drilling to help maintain consistent weight transfer from surface and reduce the likelihood of drill string  10  sticking to the side of borehole  20 . Multiple vibration subs  200  could be employed at several locations along drill string  10  to keep it dynamic.  
         [0040]     As discussed earlier in this description, flywheel  900  can be used to generate electricity. The electric power can be used to drive vibration sub  200 . An example of an electrically powered vibration sub  200  might be a piezo-vibration sub, as described below.  FIG. 5  illustrates schematically an example vibration sub  1100  in cross-section with center line  1101 . A portion of a pin sub  1102  is also shown to which the vibration sub  1100  is made up. The vibration sub  1100  has a housing  1103  made of two sections which are threaded together. The upper housing  1104  has a female thread into which male threads on the lower housing  1105  are threaded. O-ring seals  1106  complete the connection. An electronics insert  1107  may be positioned between the upper housing  1104  and the lower housing  1105 , and may be clamped in and keyed to the upper housing  1104  via locking ring  1109 . A printed circuit board  1108  may be contained within the electronics insert  1107 . A connector  1112  extends from the pin sub  1102  for electrical communication with the electronics insert  1107 . The printed circuit board may be controllable by the surface real-time processor  800 . The printed circuit board may include one or more of the sensors discussed earlier in this description for use with dynamic clutch sub  1000 ; the PCB may preferably include an axial vibration sensor or accelerometer useful for control of the vibration sub. A balance chamber  1110  may be defined between upper housing  1104 , lower housing  1105 , and electronics insert  1107 . The balance chamber  1110  may be divided into a mud portion above and a hydraulic portion below by a balance piston  1111 . The mud portion of the balance chamber  1110  above the balance piston  1111  communicates with the borehole annulus mud via balance port  1112 . The oil side of the balance chamber  1110  below the balance piston  1111  communicates with the inner diameter of the vibration sub  1100  via balance port  1108 . Hydraulic fluid is inserted into the balance chamber  1110  through fill plug  1113 .  
         [0041]     A mandrel  1114  may be made up within a lower housing  1105 . The upper portion of the mandrel  1114  is inserted between lower housing  1105  and electronics insert  1107 , wherein o-ring seals  1115  seal the connection between the mandrel  1114  and the electronics insert  1107 . A stack chamber  1116  may be defined between the lower housing  1105  and the mandrel  1114 . The stack chamber  1116  may be in fluid communication with the balance chamber  1110  via a gap  1117  between the mandrel  1114  and the lower housing  1105 . The two chambers may be in further fluid communication to the balance chamber  1110  (oil side) through port  1118  in an upper portion of the lower housing  1105 .  
         [0042]     Within the stack chamber  1116 , an annular stack of piezo electric crystals  1119  may be secured to the mandrel  1114 . An annular tail mass  1120  may be positioned immediately on top of the piezo electric crystals  1119 . Tension bolts  1121  may extend through the tail mass  1120  and the piezo electric crystals  1119  and thread directly into the bottom of the stack chamber  1116  defined by the mandrel  1114 . The tension bolts  1121  keep the piezo electric crystals  1119  and tail mass  1120  in compression. An electrical communication/power bus  1122  extends from the electronics insert  1107  to the piezo electric crystals  1119 . As before, the characteristics of the dynamic vibration sub may be controlled via the circuit board  1108  by surface real-time processor  800 .  
         [0043]     A spring chamber  1123  may also defined between the lower housing  1105  and the mandrel  1114 . A spring  1124  may be positioned within the spring chamber  1123  to engage the mandrel  1114  at the bottom and the lower housing  1105  at the top. The spring chamber  1123  may be sealed by o-ring seals  1125  at the bottom. The spring chamber  1123  may be in fluid communication with the stack chamber  1116  through a gap  1126  between the mandrel  1114  and the lower housing  1105 . A spline  1127  may be configured in the gap  1126  to prevent relative rotational movement between the mandrel  1114  and the lower housing  1105  while allowing relative movement in the axial direction.  
         [0044]     An upper portion of the mandrel  1114  may have a notch  1128  for receiving multiple keys  1129  which extend from the lower housing  1105 . The keys may be secured in the lower housing  1105  by sealed plugs  1130 . The keys  1129  prevent rotation and retain the mandrel  1114  within the housing  1103  when the vibration sub  1100  is in tension. The vibration sub  1110  is placed in tension, for example, when pipe string is made up to the pin connector  1131  and suspended below the vibration sub  1100  and especially when the pipe string is being tripped in or out of the borehole.  
         [0045]     The vibration sub  1100  may also include a mini-sensor set  1132 . The sensors of the sensor set  1132  are positioned in the exterior of the mandrel  1114  where the mandrel extends below the housing  1103 . The sensor set  1132  may be electrically connected to the communication/power bus  1122  by copper with a seal plug, and preferably includes the sensors as noted above that might be useful in monitoring and/or controlling the vibration sub.  
         [0046]     In certain implementations of the drilling apparatus, a fluid-driven motor may be substituted for the electric motor sub  400 . A fluid-driven motor may be of a positive displacement type or may be a drill string turbine.  FIG. 6  illustrates schematically a cross-section of a portion of drill string  10  with a turbine  1200 . Drill string turbine  1200  may include multiple stages of rotors  1201  and stators  1202 , the rotors  1201  coupled to drive the shaft  425 , and the stators  1202  coupled to the housing  1203  of drill string turbine  1200 . Drill string turbine  1200  may be implemented without conveying significant electrical power from surface, as the power for drilling is derived from the mud flow: each of the multiple rotors  1201  extracts some of the power from the mud flow, and together they drive shaft  425 . Although not shown in  FIG. 6 , drill string turbine  1200  may include 50 to 100 or more rotor/stator stages, and shaft  425  may be driven at, for example, around 1000 RPM. Such drill string turbines are used today in certain drilling situations, often with diamond bits. Drill string turbine  1200  may be coupled with a flywheel  900  as per earlier descriptions, and the turbine-plus-flywheel combination may be used in overcoming hard-to-drill circumstances as described earlier for electric motor sub  400 . Moreover, flywheel  900  could drive an alternator (not shown in  FIG. 6 ) to provide electrical power to LWD suite  300 , vibration sub  200 , or for other electrical needs drilling-stoppage periods when mud flow has also stopped.  
         [0047]     The term “couple” or “couples” used herein is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect electrical connection via other devices and connections.  
         [0048]     The present invention is therefore well-adapted to carry out the objects and attain the ends mentioned, as well as those that are inherent therein. While the invention has been depicted, described and is defined by references to examples of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration and equivalents in form and function, as will occur to those ordinarily skilled in the art having the benefit of this disclosure. The depicted and described examples are not exhaustive of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.