Bit torque limiting device

Information

  • Patent Grant
  • 6182774
  • Patent Number
    6,182,774
  • Date Filed
    Wednesday, October 14, 1998
    26 years ago
  • Date Issued
    Tuesday, February 6, 2001
    23 years ago
Abstract
A torque limiting device that allows a drill string to rotate relative to the cutting structure of the bit when a predetermined torque is applied between the cutting structure of the drill bit and the drill string. The torque limiting device utilizes a retaining member which restricts rotational movement of a first component of the torque limiting device relative to a second component. When a sufficient torque load is placed on the cutting structure of the drill bit, the retaining member allows rotational movement of the first component relative to the second component and allows the drill string to continue to rotate relative to the cutting structure of the bit until the torque is sufficiently reduced. The torque limiting device may be an integral part of a drill bit, may be a separate device attached between the drill string and the drill bit or between the drill string and a downhole motor, or may be part of a near-bit sub or incorporated in a downhole motor.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




This invention relates generally to rotary drill bits used in drilling subterranean wells and, more specifically, to rotary drill bits employing a torque limiting device allowing the drill string to rotate relative to the crown of the bit when a predetermined reactive torque is experienced by the crown of the drill bit.




2. State of the Art




The equipment used in drilling operations is well known in the art and generally comprises a drill bit attached to a drill string, including drill pipe and drill collars. A rotary table or other device such as a top drive may be employed to rotate the drill string, resulting in a corresponding rotation of the drill bit. The drill collars, which are heavier and stiffer than drill pipe, are normally used on the bottom part of the drill string to add weight to the drill bit. The weight of these drill collars assists in stabilizing the drill bit against the formation at the bottom of the borehole, causing it to drill when rotated. Too much weight on bit (WOB), however, may cause the drill bit to stall.




Downhole motors may also be employed to rotate the drill bit and include two basic components: a rotor, which is a steel shaft shaped in the form of a spiral or helix, and a stator, which is a molded rubber sleeve in a rigid tubular housing, that forms a spiral passageway to accommodate the rotor. When the rotor is fitted inside the stator, the difference in geometry between the two components creates a series of cavities through which drilling fluid is pumped. In doing so, the fluid displaces the rotor, forcing it to rotate as the fluid continues to flow between the rotor and the stator. An output shaft connected to the rotor transmits its rotation to the bit.




A typical rotary drill bit includes a bit body secured to a steel shank having a threaded pin connection for attaching the bit body to the drill string or the output shaft of a downhole motor and a crown comprising that part of the bit fitted with cutting structures for cutting into an earth formation. Generally, if the bit is a fixed-cutter or so-called “drag” bit, the cutting structure includes a series of cutting elements made of a superabrasive substance, such as polycrystalline diamond, oriented on the bit face at an angle to the surface being cut. On the other hand, if the bit has rotating cutters such as on a tri-cone bit, each cone independently rotates relative to the body of the bit and includes a series of protruding teeth, which may be integral with the cone or comprise separately-formed inserts.




The bit body of a drag bit is generally formed of steel or a matrix of hard particulate material such as tungsten carbide infiltrated with a binder, generally of copper-based alloy. In the case of steel body bits, the bit body is usually machined from round stock to the shape desired, usually with internal watercourses for delivery of drilling fluid to the bit face. Topographical features are then defined at precise locations on the bit face by machining, typically using a computer-controlled, five-axis machine tool. For a steel body bit, hardfacing may be applied to the bit face and to other critical areas of the bit exterior, and cutting elements are secured to the bit face, generally by inserting the proximal ends of studs on which the cutting elements are mounted into apertures bored in the bit face. The end of the bit body opposite the face is then threaded, made up and welded to the bit shank.




In the case of a matrix-type drag bit body, it is conventional to employ a preformed so-called bit “blank” of steel or other suitable material for internal reinforcement of the bit body matrix. The blank may be merely cylindrical and tubular, or may be fairly complex in configuration and include protrusions corresponding to blades, wings or other features on the bit face. Other preform elements comprised of sand, or in some instances tungsten carbide particles, in a flexible polymeric binder may also be employed to define internal watercourses and passages for delivery of drilling fluid to the bit face, as well as cutting element sockets, ridges, lands, nozzle displacements, junk slots and other external topographic features of the bit. The blank and other preforms are placed at appropriate locations in the mold used to cast the bit body before the mold is filled with tungsten carbide. The blank is bonded to and within the matrix upon cooling of the bit body after infiltration of the tungsten carbide with the binder in a furnace, and the other preforms are removed once the matrix has cooled. The threaded shank is then welded to the bit blank. The cutting elements (typically diamond, and most often a synthetic polycrystalline diamond compact, or PDC) may be bonded to the bit face by the solidified binder subsequent to furnacing of the bit body. Thermally stable PDCs, commonly termed “TSPs”, may be bonded to the bit face by the furnacing process or may be subsequently bonded thereto, as by brazing, adhesive bonding, or mechanical affixation.




In order for the cutting elements to properly cut the formation during a drilling operation, considerable torque is required to generate the necessary rotational force between the cutting elements and the formation under a WOB substantial enough to ensure an adequate depth of cut. The resultant or reactive torque on the bit from formation contact is translated through the drill string and must be overcome by the means used to rotate the drill string, such as a rotary table, top drive, or downhole motor. In some instances, such as drilling through harder formations, the resultant torque may result in the winding up and sudden release of the drill string under torque, manifested as so-called “slaps” of the drill string at the rotary table. In other instances, torque may be sufficient to actually stop the bit from rotating. The rotary table may continue to rotate the drill string for some time, in effect “twisting” the drill string and placing the bit under very high torque loads before an operator realizes that the bit is no longer rotating. This problem is of particular concern with drag bits, due to direct engagement of the formation by the fixed PDC cutters, but also manifests itself with rock bits. If such a condition occurs and the rotary table continues to rotate, the drill string, the bit and/or components thereof may be damaged, or the drill string may even part under the torque load. If failure of the drill string occurs, the portion of the drill string above the break must be removed from the wellbore. A “fishing” assembly inserted into the wellbore is then normally employed in an attempt to retrieve the remainder of the drill string. If retrieval is impractical or unsuccessful, a new drilling assembly must be deflected, “sidetracked,” or steered around the “fish.” Any such scenario adds to the cost of production and results in down-time of the drilling operation while the remainder of the broken drill string is “tripped” from the wellbore and replaced with other bottom hole assemblies.




When a downhole motor is being used to rotate the drill bit, a sudden rise in surface pressure of the drilling fluid may indicate that the motor has stalled. While other conditions may cause a rise in fluid pressure, such as a clogged motor or plugged nozzles, if the motor stalls because the bit is no longer rotating due to excessive torque on the bit and is maintained in a stalled condition, the elastomeric stator lining may be damaged, preventing a proper interface between the stator and the rotor, thus requiring the motor to be tripped out of the wellbore and replaced. At the least, the bottomhole assembly including the motor must be pulled off-bottom and drilling and circulation recommenced to start the motor before the formation is re-engaged by the bit.




In addition to damage to drill strings and bits, directional drilling presents its own set of problems when excessive torque is applied to the drill bit. A directional well must intersect a target that may be several miles below the surface location of the drilling rig, and laterally offset therefrom. In order to reach the target, the wellbore must be directed or steered along a predetermined trajectory. The trajectory of the bit is typically determined by the tool face orientation (TFO), which must be maintained during drilling in order to maintain the trajectory of the wellbore toward the desired target. If the TFO shifts due to a stalled drill bit, the drilling must stop and a new TFO set as a reference point for the direction of drilling. While a shift in TFO is quickly manifested to the operator due to the essentially real-time nature of the MWD (measurement while drilling) mud-pulse transmissions, nonetheless, loss of TFO and resetting thereof results in considerable reduction in the overall rate of penetration (ROP) of the drilling assembly.




It would thus be advantageous to provide a drill bit assembly that includes a torque limiting device that is either an integral part of the bit construction or is attached near the bit between the drill bit and the drill string, or is positioned between the downhole motor and the drill bit.




BRIEF SUMMARY OF THE INVENTION




According to the present invention, a torque limiting device is provided that allows the drill string to rotate relative to the cutting structure of the bit at a predetermined torque placed on the cutting structure of the bit. The torque limiting device may be incorporated into the structure of the bit itself, be a separate structure attached to a drill bit, or be near-bit positioned between the drill string and the bit. In any case, the torque limiting device prevents movement of the cutting structure relative to the drill string during normal operation. When a predetermined torque is applied to the cutting structure of the bit, the torque limiter allows the drill string to rotate relative to the stationary cutting structure until the torque is decreased below the predetermined level, typically by backing off the drill string to decrease the WOB.




In a preferred embodiment having the torque limiting device as an integral part of a drill bit, the fixed-cutter bit is comprised of a crown for providing a cutting face to which a plurality of cutting elements may be attached and a shank for supporting the crown and attaching the crown to a drill string. The crown has a substantially cylindrical internal chamber sized and shaped to mate with and effectively cap the proximal end of the shank, which also has a generally cylindrical configuration. The shank and the crown fit together in a snug arrangement without inhibiting rotational movement between the crown and the shank.




In one preferred embodiment, around the perimeter of the shank are a number of recesses positioned to match corresponding recesses formed in the wall of the internal chamber of the crown. A biasing member comprised of a resilient material or a spring is placed in each recess formed in the shank. A retaining member preferably made of a hard material such as steel is subsequently placed on top of (radially outboard of) each of the biasing members. When the shank and crown are assembled together longitudinally, the retaining member compresses the biasing member and is forced by the wall of the internal chamber of the crown into the recess formed in the shank. The lower portion of the retaining member may be tapered to facilitate assembly of the torque limiting device. When the shank and crown are completely engaged, the biasing member forces the retaining member into the recess in the internal chamber wall.




If sufficient torque is applied to the crown of the bit, the retaining member is forced against the biasing member out of the recess in the internal chamber wall of the crown. The shank can then rotate relative to the crown. If a single retaining member and recess are utilized as part of the torque limiting device, the shank will make a complete revolution before the retaining member can reengage the recess. If the torque is still sufficient, the shank will continue to rotate until the torque is sufficiently decreased and the retaining member is realigned with the recess. Preferably, there is more than one retaining member and more than one recess spaced around the perimeter of the shank. Thus, the retaining member or members may reengage with other recesses, depending on when the torque is sufficiently lowered. In addition, the retaining member may be longitudinally oriented or oriented at some angle relative to the bit axis. Engagement or disengagement of the retaining member or members with the recesses manifests itself as vibrations on the rig floor, alerting the driller to reduce WOB.




In another preferred embodiment where the torque limiting device is part of the drill bit itself, the crown is securely attached to a substantially cylindrical bit blank. The blank and the shank are then attached in a manner similar to the aforementioned embodiment, including the torque limiting feature. Such a configuration may be necessary if the crown is comprised of a relatively brittle material such as tungsten carbide, where forming recesses therein and engaging and reengaging a retaining member may cause the crown to crack. Thus, the blank is preferably formed of a more ductile material and the crown of a more abrasion resistant material, with the recesses necessary for engagement of the retaining member formed in the blank.




In either of the aforementioned embodiments, a standardized shank could be manufactured to accommodate a variety of crown and/or cutter sizes and configurations. In yet another embodiment, the crown is configured to be inserted into the proximal end of the shank with the proximal end of the shank having a substantially cylindrical chamber formed therein to mate with the distal end of the crown. The torque limiting device of the aforementioned embodiments is utilized in a substantially similar manner to limit the torque that may be applied to the bit crown.




In still another preferred embodiment where the torque limiting device is part of the bit itself, a pair of bands is positioned between the shank and the blank with one band attached to each. The bands maintain relative position due to a frictional interference fit but can slide relative to one another if a predetermined torque is applied to the crown of the bit. In addition, the bands may have various orientations including vertical, horizontal, or any angle therebetween. Moreover, one or both of the bands may be comprised of a resilient material such as synthetic elastomers, and the band material may be filled with particles or fibers of asbestos or other brake-material compounds. The location of the bands may be seated from wellbore fluids, or the band materials may be selected to operate in the wellbore environment. Such a torque limiting device would act in a clutch-like manner where the bands remain in stationary relationship so long as the force between them caused by torque on the crown does not exceed the static coefficient of friction between the bands. Moreover, the torque limiting device would have equal utility for tri-cone bits as well as coring or other bits used in rotational-type drilling.




In yet another preferred embodiment, the torque limiting device includes a plurality of load-driven rollers (clutch rollers) that allows rotational movement when a predetermined torque or load is placed on the cutting structure of the bit.




In another preferred embodiment, a ratchet-type torque limiter may be comprised of two substantially concentric rings of similar or dissimilar materials, each having teeth or projections in engaging contact with one another that disengage when a predetermined torque is applied to the cutting structure of the bit.




In an alternate embodiment where the torque limiting device of the present invention is separate from the bit, the device couples a typical drill bit to a drill string and/or downhole motor. The torque limiting device includes connecting structures, such as threads, at both ends, one for attaching the device to the bit and one for attaching it to the drill string. The device may be formed as part of a downhole motor, or as a near-bit sub. Similar to the construction of the drill bit embodiments, the torque limiter may be comprised of two connecting structures that are fitted together in a male-female interconnection and held together by retaining members engaged in recesses formed in the internal wall of one connector. If sufficient torque is applied to the bit by the formation, the torque limiting device will allow the drill string to rotate relative to the bit.




As will be recognized, when the retaining members are disengaged from their respective recesses, the two connecting structures need not be axially mechanically attached to one another except for frictional forces applied by the retaining members on the internal wall of one connecting structure. Because the bit is being forced into the bottom of the wellbore, however, the two connecting structures are held together by the weight of the drill string. Thus, the two connecting structures will not become separated. The same is true for the embodiments where the torque limiting device is part of the bit construction. However, as required, additional structures as known in the art may be employed to help the two connecting structures secured together against longitudinal tensile forces encountered when tripping out of the wellbore.




It will be recognized by those skilled in the art that in any of the aforementioned embodiments, the configurations of the retaining and biasing members may vary. For example, the retaining member may simply be spherically shaped, cylindrically shaped, wedge shaped or otherwise suitably shaped including combinations thereof. Moreover, the retaining members may be biased by a segment of resilient material, a coil-type spring, a leaf spring, a belleville spring, or other means known in the art.




As noted above, a torque limiting device in accordance with the present invention will reduce the possibility of bit damage from excessive torque and will quickly signal the drilling operator through vibrations or shock waves that excessive torque is being applied to the drill bit.











BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS





FIG. 1

is a partial sectional view of a drill bit including a first embodiment of a torque limiting device in accordance with the present invention;





FIG. 2

is a cross-sectional view of the embodiment shown in

FIG. 1

;





FIG. 2A

is a cross-sectional view of a second embodiment of a torque limiting device in accordance with the present invention;





FIG. 3

is a partial sectional view of a drill bit including a third embodiment of a torque limiting device in accordance with the present invention;





FIG. 4

is a cross-sectional view of the embodiment shown in

FIG. 3

;





FIG. 5

is another cross-sectional view of the embodiment shown in

FIG. 3

;





FIG. 6

is a partial sectional view of a drill bit including a fourth embodiment of a torque limiting device in accordance with the present invention;





FIG. 6A

is a partial sectional view of a drill bit including a fifth embodiment of a torque limiting device in accordance with the present invention;





FIG. 7

is a sectional view of a sixth embodiment of a torque limiting device in accordance with the present invention;





FIG. 7A

is a cross-sectional view of a drill bit including a seventh embodiment of a torque limiting device in accordance with the present invention;





FIG. 8

is a partial cross-sectional view of an alternate embodiment of a retaining member and its associated biasing member positioned in a near-bit coupling device in accordance with the present invention;





FIG. 9

is a partial sectional view of a drill bit including an eighth embodiment of a torque limiting device in accordance with the present invention;





FIG. 9A

is a partial sectional view of a drill bit including a ninth embodiment of a torque limiting device in accordance with the present invention;





FIG. 10

is a cross-sectional view of a drill bit including a tenth embodiment of a torque limiting device in accordance with the present invention;





FIG. 11

is a cross-sectional view of a drill bit including an eleventh embodiment of a torque limiting device in accordance with the present invention;





FIG. 12

is a cross-sectional view of a drill bit including a twelfth embodiment of a torque limiting device in accordance with the present invention; and





FIG. 13

is a partial sectional view of a downhole motor including a torque limiting device in accordance with the present invention.











DETAILED DESCRIPTION OF THE INVENTION





FIG. 1

shows an exemplary drill bit


10


in accordance with the present invention attached by threads


12


to an end


14


of a drill string


16


. The drill bit


10


comprises a crown


18


attached to a shank


20


by the retaining members


22


. The crown


18


may have a typical rotary bit exterior configuration including a plurality of cutting elements


24


, nozzle exit ports


26


, and gage pads


28


. As with other similarly configured bits known in the art, the shank


20


includes a plenum


21


longitudinally extending through the shank


20


that is in fluid communication with the drilling fluid supply


15


of the drill string


16


and the nozzle ports


26


of the bit crown


18


.




The crown


18


has an internal chamber


30


defined by walls


32


and


34


and floor


36


. The internal chamber


30


is substantially cylindrically shaped and is sized to closely fit over the proximal end


38


of the shank


20


, which also has a substantially cylindrical shape. The shank


20


and the crown


18


form a male-female interconnection such that the shank


20


may rotate within the internal chamber


30


of the crown


18


.




As previously mentioned, the shank


20


is held in relative position to the crown


18


by retaining members


22


that protrude into recesses


40


formed in the wall


32


of the internal chamber


30


. The retaining members


22


may be formed of steel, bronze or any other suitable material known in the art. The retaining members


22


are radially biased by the biasing members


42


positioned in recesses


41


formed in the outer surface


44


of the shank


20


proximate its proximal end


38


. The biasing members


42


may be formed of a resilient elastomeric material such as natural or synthetic rubber compounds, polyurethane or other materials known in the art and may have varying durometer ratings, depending on the desired resiliency to accommodate the design torque limit. In order to keep drilling fluid from the plenum


21


or from outside the bit


10


from entering between the shank


20


and the crown


18


and into the recesses


40


and


41


, O-rings or other sealing structures


45


and


47


may be utilized to rotationally seal the crown


18


to the shank


20


.




As better shown in

FIG. 2

, the cross-section of the bit


10


illustrates the position of the junk slots


43


and the gage pads


28


relative to a plurality of retaining members


22


and biasing members


42


which are shown equidistantly placed about the perimeter


46


of the shank


20


. The embodiment shown in

FIG. 2

includes four torque limiting assemblies


48


. As will be recognized by those skilled in the art, the number of assemblies


48


is not critical and may include one or more. It is advantageous, however, to place a plurality of the torque limiting assemblies


48


equidistantly around the perimeter


46


of the shank


20


so that any one retaining member


22


may engage with any other recess


40


.




For example, as further illustrated in

FIG. 2A

, each torque limiting assembly


70


may engage with a plurality of different recesses


71


. Moreover, while each retaining member


72


, in the form of a substantially spherical ball, is illustrated as being forced into a recess


71


formed in the crown


73


, those skilled in the art will recognize that the recesses


71


may with equal utility be formed in the shank


74


with each torque limiting assembly


70


fitted within the crown


73


.




When a sufficient amount of torque is placed on the crown


18


of the bit


10


to load the retaining members


22


and force them radially into the biasing members


42


a distance that allows the retaining members


22


to clear the perimeter of interior wall


32


of the crown


18


, the shank


20


will rotate relative to the crown


18


. In every quarter turn of the shank


20


relative to the crown


18


, the retaining members


22


will reengage with the recesses


40


. If the torque applied to the crown


18


is still sufficient to overcome the forces applied by the biasing members


42


on the retaining members


22


, the shank


20


will continue to rotate. If not, the retaining members


22


will reengage with the next closest recess


40


, and the crown


18


will then rotate along with the shank


20


.




The retaining members


22


of the embodiment shown in

FIGS. 1 and 2

have a substantially cylindrical cross-section with a flat side


50


used to provide uniform contact by the biasing member


42


along the length and width of the retaining member


22


. It should also be noted that the rounded side


52


of the retaining member


22


must not extend a distance into the crown


18


such that the retaining member forms a mechanical lock between the crown


18


and the shank


20


. That is, the rounded side


52


must be able to slide out of the recess


40


when a predetermined torque is applied to the bit crown


18


. In addition, for assembly purposes, the retaining members


22


have a tapered portion


56


to slidedly engage with the beveled edge


60


of the crown


18


. Thus, when the shank


20


and the crown


18


are slid together during assembly of the drill bit


10


, the tapered portion


56


is assisted into the recess


41


by the beveled edge


60


.




Similar to the embodiment shown in

FIG. 1

, the drill bit


100


depicted in

FIG. 3

is attached to a drill string


102


by a threaded portion


104


. The bit


100


, however, includes a substantially cylindrical tubular blank or crown insert


106


longitudinally extending along a length of the bit


100


positioned between the crown


108


and the shank


110


proximate its proximal end


114


. The crown


108


is securely attached to the insert


106


, which attachment may be assisted by protrusions


112


to mechanically hold the insert


106


relative to the crown


108


.




The torque limiting assemblies


116


are located between the shank


110


and the insert


106


and proximate the proximal end


114


. In this embodiment, however, it is not critical that the torque limiting assemblies


116


be located at or near the proximal end


114


, and could therefore be positioned at any point along the interface


118


between the insert


106


and the shank


110


. As in the previous embodiment, each torque limiting assembly


116


includes a retaining member


120


and a biasing member


122


(in this case a coil spring). Moreover, the retaining member


120


, which is held into the recess


124


by the biasing member


122


, has a tapered edge


126


at its proximal end


128


. During the assembly process, when the shank


110


is slid into the insert


106


, this tapered edge


126


contacts the beveled recess


130


located on the inner distal edge


132


of the insert


106


and helps to force the retaining member


120


into the insert


106


. As better shown in

FIG. 5

, a cross-sectional view of the drill bit


100


taken through the interface between the insert


106


and the drill string


102


, there are four such recesses


130


positioned to correspond to each torque limiting assembly


116


.




Referring now to

FIG. 4

, depicting a cross-section of the drill bit


100


through the torque limiting assemblies


116


, the insert


106


has a number of radially extending blades


150


corresponding to the external blades


152


of the crown


108


. The insert


106


provides structural support for the crown


108


so that the crown


108


does not fracture during drilling. The retaining members


120


have a wedge-shaped cross-section with a tapered edge


154


which, when positioned in the recess


124


, extends into the recess


156


to provide a sliding surface between the retaining member


120


and the edge


157


of the recess


124


at the inner surface


158


of the insert


106


. Again, there are four, equidistantly spaced torque limiting assemblies


116


. As one skilled in the art will recognize, however, there may be as few as one torque limiting assembly


116


or as many as will fit within the given space, depending on their size and configuration.




As illustrated in

FIG. 3

, O-rings


134


and


136


, or other seals as known in the art, placed in races


138


and


140


, respectively, seal the torque limiting assemblies


116


from drilling fluid contained in the plenum


142


and drilling fluid located outside the drill bit


100


. A top view of the O-ring race


140


is shown in FIG.


5


.





FIG. 6

is a partial sectional view of an alternate preferred embodiment of a drill bit


160


in accordance with the present invention. In this embodiment, a portion


162


of the crown


164


actually fits in an internal chamber


166


defined by the proximal end


168


of the shank


170


in a male-female interconnection. Additionally, the torque limiting assembly


172


is comprised of a substantially spherically shaped retaining member


174


and a substantially cylindrical biasing member


176


. Thus, the shank


170


can rotate relative to the crown


164


when a sufficient torque on the crown


164


forces the retaining member


174


toward the biasing member


176


enough that the retaining member


174


clears the wall


178


defining the internal chamber


166


. O-rings


180


and


182


positioned in O-ring races


184


and


186


, respectively, substantially seal the torque limiting assembly


172


from drilling fluid.




Likewise, in

FIG. 6A

, the torque limiting feature of the bit


271


operates in a similar manner to that illustrated in FIG.


6


. The retaining member


270


and biasing member


272


, however, are vertically oriented between the crown


274


and the shank


276


.





FIG. 7

illustrates that many modifications and/or combinations of the aforementioned embodiments of the torque limiting assembly


200


can be made without departing from the spirit of this invention. For example, the retaining member


202


may include a semi-spherical or semi-cylindrical portion


204


at its proximal end


206


for engagement with an insert or crown


208


, as the case may be, and a guide rod or fin


210


to keep the portion


204


from rotating during disengagement and reengagement from the recess


212


. The biasing member or coiled spring


214


sits in a first recess


216


formed in the shank


218


. The first recess


216


is followed by a second recess


220


which is smaller and sized and shaped to accommodate the rod or fin


210


through its full range of motion. Additionally, as illustrated in

FIG. 7A

, the retaining member and biasing member may be a single integral retaining component such as spring


230


. Such a spring


230


could hold the crown


231


relative to the shank


232


while engaged with engagement portions


233


in the outer surface


234


of the shank


232


. As shown, the engagement portions


233


are comprised of recesses in the outer surface


234


, but could just as well be flattened portions that would require deflection of the spring


230


to allow rotation of the crown


231


relative to the shank


232


.




While other preferred embodiments of the torque limiting assembly according to the present invention have been illustrated as including a biasing member and a retaining member, other devices which provide releasability between two drilling related structures are also contemplated. For example, as illustrated in

FIGS. 9 and 9A

, the torque limiting assembly


280


includes a pair of circumferential bands


282


and


284


, at least one of which is comprised of an abrasion-resistant yet resilient material, the bands


282


and


284


being frictionally held in relative relation and adhesively or mechanically attached to the crown


286


and shank


288


, respectively. The bands


282


and


284


remain in one relative position to one another so long as the force between the two bands


282


and


284


does not exceed the force holding the bands


282


and


284


together based on the coefficient of static friction between the two bands. Once the force holding the bands


282


and


284


together is exceeded, however, the bands will move relative to one another, allowing the crown


286


to rotate relative to the shank


288


. In addition, the bands may be substantially vertically oriented as illustrated in

FIG. 9

, substantially horizontally oriented, or oriented at any angle thereinbetween as further illustrated in FIG.


9


A.




As further illustrated in

FIG. 10

, the torque limiting assembly may be comprised of a single friction band


290


interposed between the crown


292


and the shank


294


. The band


290


may be attached to either the crown


292


or the shank


294


or not be attached at all. Accordingly, the crown


292


can rotate relative to the shank


294


when a torque placed on the crown


292


results in a force in excess of the static frictional force between the crown


292


and band


290


or the shank


294


and the band


290


. Materials employed in brake linings and pads for motor vehicles may be especially suitable for band


290


.




In yet another preferred embodiment illustrated in

FIG. 11

, the torque limiting assembly


300


includes a band


302


of resilient material, such as an elastomer, that is mechanically attached to or molded onto and fitted around a plurality of protrusions


304


radially extending from an outer surface


306


of the shank


308


. Accordingly, the band


302


is restricted from moving relative to the shank


308


. The band


302


includes a layer


310


of wear-resistant material provided on its outer surface


312


that follows the contour of the outer surface


312


of the band


302


. The outer surface


312


of the band


302


, and more specifically the contour of the layer


310


, is configured to substantially matingly match with the contour of the inner surface


314


of the crown


316


. In this example, the inner surface


314


of the crown


316


is comprised of a zig-zag or corrugated, ribbed pattern that uniformly repeats around the inner surface


314


. Thus, when a sufficient torque is applied to the crown


316


, the crown


316


can rotate relative to the shank


308


with the layer


310


protecting the band


302


from being damaged or destroyed by the inner surface


314


of the crown


316


. It will also be understood that while illustrated in a zig-zag configuration, the interface between the band


302


and the crown


316


may be similar to a sinusoidal wave, saw teeth, or any other desired pattern. Such an arrangement may be formed using an elastomer of one durometer for band


302


having molded thereon a second, higher-durometer layer


310


. Polyurethanes are especially suitable for such an arrangement.




Moreover, in

FIG. 12

, the torque limiting assembly


320


may include one or more rotatable clutch elements


322


held in fixed relation to the shank


324


but rotatable along an inner surface


326


of the crown


328


when sufficient torque is applied to the crown


328


.




It is also contemplated that the torque limiting device of the present invention may be incorporated into a near-bit coupling device


250


as illustrated in

FIG. 8

which incorporates a torque limiting assembly


252


as previously described. The coupling device


250


is comprised of two interface structures or connectors


254


and


256


. The first connector


254


would typically be attached to a drill string as known in the art and the second connector


256


would be attached to a typical drill bit. As with other embodiments described herein, the torque limiting assemblies


252


are releasable and allow rotational movement of the first interface structure or connector


254


relative to the second interface structure or connector


256


. The coupling device


250


also includes a plenum


255


to allow passage of drilling fluid from a drill string to a drill bit. O-ring


258


placed in race


260


and another O-ring placed in race


262


could help seal the torque limiting assemblies


252


and the coupling device


250


relative to a connected drill string and bit. Such a coupling device


250


incorporating a torque limiting assembly


252


would allow a typical bit to have torque limiting abilities without modifying the bit itself or the manufacturing of such a bit.




It will be appreciated by those of ordinary skill in the art that use of the present invention facilitates the use of drag bits having aggressive PDC cutters, such as those with minimal or no back rake or even a forward (positive) rake of the cutting faces. Prior art bits in part employ negatively-back raked cutters to limit torque, but this also limits ROP, so runs take longer for a given borehole interval in the interests of preserving the bit and string against damage.




During a drilling operation utilizing a drill bit incorporating a torque limiting device in accordance with the present invention, if the crown of the bit ceases rotation, the vibrations generated by the disengagement and reengagement of the torque limiting device will quickly signal the operator that the crown is not rotating. Drilling parameters can then be promptly adjusted to decrease the WOB applied on the bit crown, or in the case of a downhole motor, the drilling fluid flow as well as WOB.




It will be appreciated by those skilled in the art that many modifications and combinations of the preferred embodiments can be made without departing from the scope of the invention and particularly the appended claims. More specifically, features of the torque limiting device that have been illustrated as an integral part of the drill bit could be incorporated into a near-bit torque limiting device or anywhere between the drill string and the drill bit. For example, as illustrated in

FIG. 13

, a torque limiting device could be incorporated at a variety of locations along a downhole motor


330


. A torque limiting device according to the present invention may have utility at point A between a downhole motor


330


and bit


332


, at point B between motor


330


and drill string


334


, or even at point C within motor


330


as, for example, within bearing housing


336


below the rotor/stator section


338


and connecting rod assembly


340


. In addition, the torque limiting device, while being illustrated with respect to a fixed-cutter bit, will have equal utility when used with or as an integral part of a roller cone bit (also called “tri-cone” or “rock” bit) as well as coring or other bits used in rotational-type drilling. Moreover, those skilled in the art will appreciate that configurations of the components could be interchanged between embodiments such as changing the type and/or shape of the retaining member and/or the type and/or shape of the biasing member. Further, the arrangement of torque limiting assemblies may be reversed so that the retaining members are radially inwardly biased by biasing members carried by the crown (or blank) into cooperating recesses formed in the shank. Thus, it is believed that the essence of the invention is to provide a torque limiting device in a drill bit or between a drill string or downhole motor as is known in the art and a bit so that the drill string or motor drive shaft can continue to rotate while the crown of the bit remains stationary once a predetermined torque is exceeded by the drill bit.



Claims
  • 1. A torque limiting device for use in conjunction with transmitting torque to a subterranean drill bit, comprising:a first connector having a first connecting portion at a distal end and a first interface portion at a proximal end; a second connector having a second connecting portion at a proximal end and a second interface portion at a distal end proximate said first interface portion; the first connector and the second connector being positioned generally axially opposite each other; and a releasable structure proximate said first and said second interface portions, said releasable structure rotationally retaining said first and said second connectors together until a torque exceeding a predetermined torque is applied between said first connector and said second connector.
  • 2. The torque limiting device of claim 1, wherein said first and said second interface portions fit together in a male-female relationship.
  • 3. The torque limiting structure of claim 1, wherein at least one of said first connecting portion of said first connector and the second connecting portion of said second connector comprises a threaded region.
  • 4. The torque limiting structure of claim 1, wherein said at least one retaining member has a shape selected from the group consisting of a substantially cylindrical shape, a substantially wedge shape, and a substantially spherical shape.
  • 5. The torque limiting device of claim 1, wherein said first connector comprises a first plenum and said second connector comprises a second plenum and wherein said first and second plenums are in fluid communication with each other.
  • 6. The torque limiting device of claim 5, wherein said first and second plenums are generally longitudinally aligned with each other and wherein at least one fluid seal is provided intermediate said first connector and said second connector to seal said first and second plenums.
  • 7. The torque limiting device of claim 1, wherein said releasable structure includes at least one retaining member and at least one biasing member associated therewith.
  • 8. The torque limiting structure of claim 7, wherein said at least one retaining member is biased in a generally radially oriented direction by said at least one biasing member.
  • 9. The torque limiting device of claim 7, wherein said at least one retaining member comprises a plurality of retaining members and said at least one biasing member comprises a plurality of biasing members respectively associated with said plurality of retaining members.
  • 10. The torque limiting device of claim 9, wherein said plurality of retaining members and respectively associated biasing members are substantially circumferentially equidistantly spaced.
  • 11. The torque limiting device of claim 7, wherein said at least one retaining member is engageable with a first recess formed in one of said first and second connectors.
  • 12. The torque limiting device of claim 11, wherein said first recess comprises a plurality of first recesses, each of which being engageable by said at least one retaining member.
  • 13. The torque limiting device of claim 11, wherein said at least one biasing member is positioned within a second recess formed in the other of said first and second connectors and said retaining member is biased by said at least one biasing member toward said first recess.
  • 14. The torque limiting structure of claim 13, wherein the at least one retaining member extends partially into said first recess until said predetermined torque is applied between said first connector and said second connector.
  • 15. The torque limiting structure of claim 13, wherein said first connector is adapted to rotate relative to said second connector upon application of a torque exceeding said predetermined torque therebetween by compression of said at least one resilient member sufficient to permit said retaining member to exit said at least one resilient member sufficient to permit said retaining said retaining member to exit said at least one cooperating recess.
  • 16. The torque limiting device of claim 15, wherein said first connector is adapted to rotate relative to said second connector until said torque does not exceed said predetermined torque.
  • 17. A torque limiting device for use in conjunction with transmitting torque for subterranean drilling, comprising:a first interface structure; a second interface structure proximate said first interface structure; the first interface structure and the second interface structure being positioned generally axially opposite each other; and a releasable structure between said first and said second interface structures, said releasable structure rotationally retaining said first and said second interface structures together until a predetermined torque is applied between said first and second interface structures.
  • 18. The torque limiting device of claim 17, wherein said first interface structure and said second interface structure are incorporated in a drill bit.
  • 19. The torque limiting device of claim 18, wherein said first interface structure is associated with a bit shank and said second interface structure is associated with a cutting structure.
  • 20. The torque limiting device of claim 19, wherein said cutting structure includes at least one roller cone.
  • 21. The torque limiting device of claim 17, wherein said first interface structure and said second interface structure are incorporated in a downhole motor.
  • 22. The torque limiting device of claim 17, wherein said first interface structure and said second interface structure are incorporated in a near-bit sub.
  • 23. The torque limiting device of claim 22, wherein said first interface structure is attached to a drill string.
  • 24. The torque limiting device of claim 23, wherein said second interface structure is attached to a drill bit.
  • 25. The torque limiting structure of claim 23, wherein said second interface structure is attached to a downhole motor.
  • 26. The torque limiting device of claim 17, wherein said first interface structure comprises a first plenum and said second interface structure comprises a second plenum and wherein said first and second plenums are in fluid communication with each other.
  • 27. The torque limiting device of claim 26, wherein said first and second plenums are generally longitudinally aligned with each other and wherein at least one fluid seal is provided intermediate said first connector and said second connector to seal said first and second plenums.
CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 08/821,465, filed Mar. 21, 1997, now U.S. Pat. No. 5,947,214.

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