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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The present Invention relates to rotary Impact, torque intensifying apparatus for use with drill bits, particularly polycrystalline diamond compact “PDC” bits and methods of use applied to subterranean drilling. 
     BACKGROUND OF THE INVENTION 
     Conventional drill bits include roller bits which use compression to crush rock at the toolface when drilling a wellbore in a subterranean formation. It is known to apply axial impact assemblies for enhancing the compressive breaking action of percussive bits. 
     PDC bits, however, use a shearing action to break the material of the formation. Excessive axial force on a PDC bit is a known cause of failure of the cutters. 
     The PDC cutters and inserts of PDC bits are subject to failure through vibration and impact. Ideally, a PDC bit has continuous loading while shearing material at the toolface. However, when the rate of penetration suddenly slows, or when a hard interface is encountered, such as a stringer, the bit slows or hangs up, possibly even temporarily ceasing to rotate. Despite slowing or cessation of rotation of the drill bit, the drill string continues to rotate. Whether the bit is at the end of a rotating drill string, or at the end of a coiled tubing BHA, the rotary drive continues to wind up the drill string, building up torque and potential energy. Typically, the torque reaches a certain elevated level and the bit finally releases and spins violently, either due to the energy built up or due to a shortening of the drill string as it winds up. The sustained release of energy as the bit spins causes chatter or repeated impacts of the PDC cutters against the rock face—causing significant damage to the PDC bit cutters. 
     It Is an expensive process to trip out and replace a damaged PDC bit. 
     It is believed that PDC bit failure is caused by the chatter and impact associated with the sustained and violent release of the built up torque. Nevertheless, the lock up of a PDC bit is a known and persistent problem resulting in expensive down time and equipment cost 
     SUMMARY OF THE INVENTION 
     In a surprising discovery, PDC bit performance is improved and incidences of failure can be reduced by repeatedly applying increased torque at the PDC bit through the use of a rotary impact tool. So as to avoid large build up of torque and to suffer the associated sustained impact damage to a PDC bit on release, an assembly is provided for introducing a consistent series of smaller and localized rotary impacts to the bit, avoiding lockup and potentially damaging energy storage in the drill string. 
     The present invention implements a method and apparatus for increasing the drilling effectiveness of PDC bits while minimizing failures due to the release of energy following windup. 
     Simply, the method comprises increasing the effective torque of the drill bit by repeatedly and periodically intensifying the torque at the PDC drill bit. The periodic increases in torque avoid the potential for build-up of torque on bit lockup or sustained high torque incidences which are associated with PDC bit failure when the built-up of torque is released. Preferably, introduction of rotary impact is applied only during drilling. 
     In an apparatus aspect, a rotary torque impacting assembly is positioned between the drill bit and the rotary drive such as a rotary drill string or a downhole motor. The drill bit is adapted for rotation by the assembly which provides the nominal torque necessary to develop the shear forces used by the PDC bit to cut the formation. An energy source in the impacting assembly supplements the nominal torque provided by the rotary drive. Preferably, a drilling fluid driven turbine in the assembly drives a rotary hammer for periodic impacts with an anvil connected through to the drill bit. 
     The assembly comprises an output bit shaft for connection to the drill bit, and a housing for connection to the rotary drive. The bit shaft has a lower connection to the bit and an upper shaft end which projects into the downhole end of the housing and is rotatably driven thereby. The upper shaft end is fitted with a rotary anvil. The housing further houses a motor which rotates a hammer about the bit shaft&#39;s anvil. The motor spins the hammer and builds up its potential energy. When the anvil and hammer connect, the potential energy is released into the upper shaft end and thus into the drill bit, increasing its instantaneous torque and hence to cut through the difficult formation. For increased effectiveness, the bit shaft is adapted for permitting limited rotational freedom relative to the driving housing so that the bit shaft receives substantially all of the rotary impact. Preferably, the hammer&#39;s motor is impeded from operation when the bit is off bottom and not drilling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of one embodiment of a rotary impact assembly of the present invention; 
     FIGS. 2 a  and  2   b  are cross-sectional views of the rotary impact assembly of FIG. 1; 
     FIG. 2 a  illustrates the assembly when the bit shaft is off bottom so that the rotary drive is rotationally restrained; 
     FIG. 2 b  illustrates the assembly when the bit shaft is on bottom so that the rotary drive is free to rotate and impart rotational impact into bit shaft; 
     FIG. 3 a  is a cross-sectional view of the housing and bit shaft interlocking castled interface during drilling operations prior to impact according to FIG. 2 b;    
     FIG. 3 b  is a partial cross-sectional view of the housing and bit shaft of FIG. 3 a  immediately after impact of the hammer and anvil; 
     FIG. 4 a  is a partial cross-sectional view of the hammer carrier, hammer and anvil of the assembly according to FIG. 2 b;    
     FIG. 4 b  is a cross-sectional view of the carrier according to the section S—S of FIG. 4 a , illustrating the hammer in full rotation prior to impacting the anvil; 
     FIG. 4 c  is a cross-sectional view of the carrier of FIG. 4 b  at impact of the hammer and anvil; and 
     FIGS. 5 a - 5   h  are sectional views according to section S—S of FIG. 4 a , illustrating the hammer, hammer carrier and anvil of the assembly and sequential views of the transfer of rotational impact energy from impact through to release of the hammer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Having reference to FIG. 1, a rotary impact tool of the present invention comprises an assembly  10  which is positioned between a rotary drive such as a rotary drill string or a downhole motor (not shown) and drill bit (not shown). The drill bit is typically employed to drill a wellbore through material in a subterranean formation. The assembly  10  comprises a driving housing  11  having a bore  12  and which is adapted for connection at a first end  13  to the rotary drive and at a second end  14  to a bit shaft  15  extending from the bore  12 . The bit shaft  15  has a downhole end  16  which is adapted for connection to a drill bit, such as a bit fitted with PDC cutters. The bit shaft  15  is fitted to the housing  11  so that rotation of the drive housing  11  also rotates the bit shaft  15 . Such co-rotation is achieved using a spline arrangement or interlocking castling  17  between the housing&#39;s end  14  and the bit shaft  15 . A rotary impact assembly  20  is fitted into the housing&#39;s bore  12 . 
     In one embodiment of an impact assembly  20 , depicted in FIG. 1, the assembly  20  comprises a turbine motor  21  which provides the impetus for rotating a mass and storing potential energy. The turbine motor  21  is located within the bore  12  and is supported on a stator shaft  22  guided at an upper bearing  23  and at a lower bearing  24 . The stator shaft  22  is enlarged at its lower end  25  for forming a hammer carrier  30  having a concentric cavity  31  formed therein. The carrier cavity  31  encircles an uphole end  32  of the bit shaft  15 . 
     Having reference also to FIGS. 4 a - 4   c , the bit shaft&#39;s uphole end  32  has a radially outwardly projecting dog or anvil  33 . 
     When the stator shaft  22  rotates, periodically, the rotating hammer  35  and the bit shaft&#39;s anvil  33  are coupled to impact and impart the potential energy of the moving hammer into the bit shaft. 
     The carrier  30  is fitted with an annular mass  34  having a radially inward projecting dog or hammer  35 . The annular mass  34  is pivotable about a first pin  36  fitted to the carrier  30  at a tangent of the annular mass  34 . The annular mass  34  has a first circular notch  37  at its tangent, the notch  37  being dimensionally sized so as to be pivotable about the first pin  36  and thereby permitting the annular mass  34  to move between concentric and eccentric positions about the bit shaft. 
     Diametrically opposite the first pin  36  is a second pin  38  secured in the carrier  30 . A second elongated notch  39  is formed in the annular mass  34 , diametrically opposite the first notch  37 . The second notch  39  is elongated circumferentially and, forming stops spaced at about the same angular dimension as the length of the radially inward projection of the hammer  35 . The second notch  39  is sized so that the annular mass&#39;s extreme eccentric position, the hammer  35  decouples or is released from the bit shaft&#39;s anvil. 
     Returning to FIGS. 1,  2   a  and  2   b , the turbine motor  20  comprises a plurality of turbines  40  affixed to and spaced axially along the stator shaft  22 . Each turbine  40  occupies an annular space  41  in the bore  12 , formed between the stator shaft  22  and the housing  11 . A plurality of complementary diffusers  42  are arranged, one per turbine  40  and are affixed in the annular space  41 . Five turbines and four diffusers are shown. 
     A flow path is formed through the housing  11  and bit shaft  15  for conducting drilling fluids through the assembly  10  and to the bit. Drilling fluid flows into the assembly  10  from the rotary drive and into the bore  12  of the housing  11 . Fluid then flows through the annular space  41  housing the diffusers  42  and turbines  40 . Ports  43  are formed in the stator shaft  22  above the carrier  30  and conduct the drilling fluids from the turbines&#39; annular space  41  and centrally into a bore  44  formed in the stator shaft  22 . The bore  44  in the stator shaft  22  is contiguous with a bore  45  formed in the bit shaft  15  for conducting drilling fluid to the bit. 
     In an optional embodiment, it is advantageous to minimize assembly component wear by limiting the rotary impact operation to the actual drilling operations. There is little advantage in having the rotary impact operation occurring during running in and tripping out of the drill string. Accordingly, an arrangement is provided for arresting rotation of the turbine motor  20  until such time as the drill bit is on bottom of the drilled wellbore. 
     Having reference to FIGS. 2 a  and  2   b , the bit shaft  15  has limited axial movement responsive to weight on bit such as when contacted on the bottom of the wellbore being drilled. As shown in FIG. 2 a , when off bottom, the bit shaft  15  is biased downwardly, binding the turbine motor  20  against rotation. In FIG. 2 b , when on bottom, the bit shaft  15  is forced uphole which releases the turbine motor  20  for rotation. 
     Referring to FIG. 2 a , while the bit shaft is not drilling and off bottom, an annular spring  50  biases the bit shaft  15  downhole. The spring  50  acts between an annular stop  51  and a shoulder  52  on the bit shaft  15 . A cap  53  threaded onto the uphole end  32  of the bit shaft  15  has a base  54  which engages a shoulder  55  on the carrier  30 , also biasing the stator shaft  22  downhole. When biased downhole, each turbine  40  shifts freely and axially within the annular space  41  and within an axial tolerance provided between diffusers  42 . At the top of the stator shaft  22 , a capping nut  57  moves axially downhole with the stator shaft  22  and engages a braking surface or frictional interface  58 . Even through the shaft  22  is frictionally restrained, drilling fluid can continue to flow substantially unimpeded through the turbines  40  and through to the bit shaft  15  and bit. 
     Referring to FIG. 2 b , when the bit shaft  15  is on bottom and drilling, the reactive force F overcomes the spring  50  and shifts the bit shaft  15  axially uphole. A thrust bearing  60  is fitted to the top of the cap  53 . A complementary thrust bearing  61  is fitted into the carrier cavity  31 . One suitable set of bearings  60 ,  61  include facing PDC surfaces. The uphole axial shift of the bit shaft  15  also drives the carrier  30  and stator shaft  22  uphole, lifting and disengaging the capping nut  57  from the frictional braking surface  58 , freeing the stator shaft  22  for rotation when drilling fluids flow through the turbines  40  and diffusers  42 , and initiating rotary impact operation. 
     Having reference to FIGS. 4 a - 4   c  and FIGS. 5 a - 5   h , in operation, the rotating stator shaft  22  rotates the carrier  30  and annular mass  34  (FIG. 4 b ). Each revolution of the stator shaft  22  brings the hammer  35  into impact contact with the bit shaft&#39;s anvil  33  (FIG. 4 c ) for periodically and rotatably impacting the bit shaft  15  for intensifying the torque applied to the drill bit. Each impact converts the potential energy of the rotating annular mass  34  into increased torque. The momentum of the annular mass  34  is transferred into the bit shaft  15  and the bit, briefly yet energetically aiding in bit rotation despite resistance encountered by the bit. 
     In repeated and periodic cycles, and having reference to FIGS. 5 a - 5   h , after each impact, the annular hammer  35  is able to recover and rotate once again to raise its potential energy for the next impact. Despite the periodic impact which, for each cycle, arrests the annular hammer&#39;s rotation, the hammer  35  is caused to disengage from the anvil  33  and begin the annular mass&#39;s cycle of rotation once again. 
     In FIG. 5 a , in a first step of the cycle, the impact of hammer and anvils  35 , 33  is depicted. In FIG. 5 b , the energy of the impact causes the annular hammer  35  to begins to pivot about the first pin  36 . As shown in FIGS. 5 c - 5   f , the annular hammer  35  continues to pivot about the first pin  36 , enabled by a shifting of the elongated second notch  39  along the second pin  38 , permitting pivoting to continue unchecked. The center of the annular hammer  35  progressively shift so that eventually the hammer and anvils  35 , 33  separate radially. As shown at FIG. 5 h , at the end of the impact cycle, the hammer and anvils  35 ,  33  have fully disengaged and the turbine motor  30  is free once again to rotate the annular hammer  35  through the next rotation to initiate the next impact cycle. 
     Having reference to FIGS. 2 a ,  3   a  and  3   b , the energy released into the bit shaft  15  is most effective if it is directed substantially entirely into the materials being drilled. The least effective energy transfer is that which is imparted and absorbed by the mass of the entire drill string. Accordingly, the bit shaft  15  is partially decoupled rotationally from the housing  11  for permitting limited rotational freedom. As shown on FIG. 2 a , the bit shaft  15  forms a shoulder  63  at the interface of the bit shaft  15  to an end face  65  of the housing  11 . This housing end face  65  and bit shaft shoulder  63  interface is fitted with complementary castled faces of alternating axially projecting dogs. 
     Turning to FIGS. 3 a  and  3   b , in one embodiment, four axial bit shaft dogs  66 , each having a 45° arc, are circumferentially spaced on the bit shaft shoulder forming four annular gaps  67  of about 45° each. Four corresponding axial housing dogs  68 , each having a 40° arc, are also circumferentially spaced on the housing&#39;s end face  65  forming four annular gaps  69  of about 50° each. When drilling, the 40° housing dogs  68  advance to engage the bit shaft&#39;s 45° annular gaps. Correspondingly, the 45° bit shaft dogs  66  advance to engage the housing&#39;s 50° annular gaps  69 . The housing&#39;s bit shaft dogs  68  rotationally drive the bit shaft  15  which drives the bit to drill. Accordingly, the bit shaft  15  has a limited independent rotational capability. 
     Each impact of the hammer and anvils  35 ,  33  causes the bit shaft  15  to be driven momentarily and rotationally ahead of the housing&#39;s rotation, the bit shaft shoulder dogs  66  advancing ahead of the housing&#39;s dogs  68  so as to absorb substantially all of the energy in the annular hammer  34  and imparting it into the drill bit without involving the assembly or the drill string.

Summary:
Apparatus is provided for introducing a consistent series of small and localized rotary impacts to a PDC bit during drilling, to improve PDC drill bit performance. Rotary impact supplements the nominal torque supplied by the rotary drive thereby avoiding lockup and potentially damaging energy storage in the drill string following windup, should the bit slow or hang up when drilling in difficult formations. The apparatus comprises a rotary hammer which is rotated about a bit shaft&#39;s anvil, preferably by a drilling fluid driven turbine. As the hammer rotates, potential energy is built up. When the hammer and anvil connect, the energy is released into the bit shaft and thus into the bit, increases its instantaneous torque and allows it to more effectively cut through difficult formations.