Abstract:
A method includes a rotary torque limiting assembly including a rotary input member having an internal cavity having sprag receptacles. A rotary output member is disposed within the internal cavity, the secondary rotary member having radial protrusions and recesses. Radial ratchet members are disposed radially between the input member and the output member, each ratchet member having a radially inner surface, and a radially outward surface that includes at least one radially protruding sprag. Bearings are disposed radially between the ratchet members and the output member. A retaining assembly is adapted to provide a compliant force to maintain contact among the ratchet members, bearings, and second rotary member. Each bearing is partly retained between one of the ratchet members and the second rotary member, and each sprag is partly retained within a corresponding sprag receptacle.

Description:
TECHNICAL FIELD 
     The present disclosure relates to systems, assemblies, and methods for protecting downhole tools (referred to as a tool string) attached to a “drill string” disposed in a wellbore, where adverse conditions may be present to challenge rotational movement of the tool string in the wellbore. 
     BACKGROUND 
     In oil and gas exploration it is important to protect the structural integrity of the drill string and downhole tools connected thereto. Referring to  FIG. 1 , in general, a drilling rig  10  located at or above the surface  12  rotates a drill string  20  disposed in the wellbore below the surface. The drill string typically includes drill pipe  22  and drill collars  24  that are rotated and transfer torque down the borehole to a drill bit  50  or other downhole equipment (referred to generally as the “tool string”)  40  attached to a distal end of the drill string. The surface equipment  14  on the drilling rig rotates the drill string  20  and the drill bit  50  as it bores into the Earth&#39;s crust to form a wellbore  60 . The drill bit, however, generally encounters variances across various geological formations that may provide differing amounts of resistance to the drill. In many instances, such resistance may be unanticipated and can result in an excessive amount of torque being delivered along the drill string from the surface, possibly causing the drill string or tool string connected to the drill string to be damaged and/or break. Such breakage results in additional work and expense needed to retrieve the section of the drill string and tool string below the break and repair the damage, in addition to the costs associated with the resulting downtime. 
     SUMMARY 
     In general, this document describes a rotary torque-limiting assembly used in conjunction with downhole drilling and or downhole completion tools. The rotary torque limiting assembly includes a rotary input member having an internal cavity having sprag receptacles. A rotary output member is disposed within the internal cavity, the secondary rotary member having radial protrusions and recesses. Radial ratchet members are disposed radially between the input member and the output member, each ratchet member having a radially inner surface, and a radially outward surface that includes at least one radially protruding sprag. Bearings are disposed radially between the ratchet members and the output member. A retaining assembly is adapted to provide a compliant force to maintain contact among the ratchet members, bearings, and second rotary member. Each bearing is at least partly retained between one of the ratchet members and the second rotary member, and each sprag is at least partly retained within a corresponding sprag receptacle. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of a drilling rig and downhole equipment disposed in a wellbore. 
         FIG. 2  is a perspective view of the lower portion of a drill string section that includes an example of a downhole torque limiting device. 
         FIG. 3  is a partial perspective view of an example of the downhole torque limiting device of  FIG. 2 . 
         FIG. 4  is a perspective view of examples of ratchet members. 
         FIG. 5  is a cross-sectional view of an example of a downhole torque limiting device. 
         FIG. 6  is a cross-sectional view of an example of a downhole torque limiting device in a torque limiting mode. 
         FIGS. 7A-7D  are cross-sectional views of an example of a downhole torque limiting device. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , in general, a drilling rig  10  located at or above the surface  12  rotates a drill string  20  disposed in a wellbore  60  below the surface. The drill string  20  typically includes drill pipe  22  and drill collars  24  that are rotated and transfer torque down the borehole to a drill bit  50  or other downhole equipment (referred to generally as the “tool string”)  40  attached to a distal end of the drill string  20 . The surface equipment  14  on the drilling rig rotates the drill string  20  and the drill bit  50  as it bores into the Earth&#39;s crust to form a wellbore  60 . The wellbore  60  is reinforced by a casing  34  and a cement sheath  32  in the annulus between the casing  34  and the borehole. The drill bit  50 , however, generally encounters variances across various geological formations  25  that may provide differing amounts of resistance to the drill. In many instances, such resistance may be unanticipated and can result in an excessive amount of torque being delivered along the drill string  20  from the surface  12 . In other situations in a deviated wellbore the drill string  20  or tool string  40  may become stuck in the wellbore  60  and cause excessive torque in the drill string  20  or tool string. In other situations the borehole walls may slough off formation material that packs around the drill bit  50 , tool string  40  or drill string  20  and cause one or more of these elements to become stuck and result in excessive torque in the drill string  20 . 
     In some implementations, the tool string  40  can be a drilling, completions or re-entry tool string. The drilling tool string includes tool elements such as rotary steerable tool systems, mud motors, under reamers, and/or measurement while drilling (MWD)/FEWD devices. In order to avoid over-torqueing the drill string  20  and/or elements of the tool string  40  a torque limiting device  110  may be inserted between the drill string  20  and the tool string  40 . 
     When the torsional forces between the drill string  20  and tool string  40  across the torque limiting device  110  exceed a predetermined amount, the device  110  will rotationally decouple the two sections until the forces drop back below the predetermined amount. 
       FIG. 2  is a perspective view of the downhole torque limiting device  110 . Referring to  FIGS. 1 and 2 , in some implementations, the torque limiting device  110  limits the amount of torque that is transmitted from a rotary input member (e.g., external housing  114 ) connected to the drill string  20  (see  FIG. 1 ) and receiving rotational torque from the drill string  20 , to a rotary output member (e.g., drive shaft  112 ). In some implementations, the torque limiting device can be used to limit the amount of torque that is developed along the tool string section  40  (see  FIG. 1 ) as the external housing  114  is rotated to drive a drill bit  50  (see  FIG. 1 ) directly or indirectly coupled to the drive shaft  112 . 
     The torque limiting device  110  includes a radial ratchet assembly  120 , a spring support assembly  122   a  disposed adjacent to an uphole axial end of the radial ratchet assembly  120 , and a spring support assembly  122   b  disposed adjacent to a downhole axial end of the radial ratchet assembly  120 . A spring section  124   a  axially provides axial compression between the spring support assembly  122   a  and the radial ratchet assembly. A spring section  124   b  axially provides axial compression between the spring support assembly  122   b  and the radial ratchet assembly. 
     A bearing assembly  130  is provided to constrain the relative motion of the drive shaft  112  and rotationally decouple the drive shaft  112  from an outer housing (not shown) of the torque limiting device  110 . A sealing housing  140  is provided to or at least minimizes the intrusion of contaminants (e.g., drilling debris, particulate suspensions, dirt, mud, sand) from entering the interior components of the torque limiting device  110 . 
       FIG. 3  is a partial perspective view of the example of the downhole torque limiting device  110 . The radial ratchet assembly  120  includes a collection of roller bearings  202  and a collection of radial ratchet members  204 . In the view of  FIG. 3 , the radial ratchet assembly  120  of  FIG. 2  is shown with one of the radial ratchet members  204  removed to provide a view of the roller bearings  202 . 
       FIG. 4  is a perspective view of examples of the radial ratchet members  204  and roller bearings  202 . As further detailed below, the collection of roller bearings  202  are at least partially disposed within a collection of corresponding recesses  302  formed within a radially interior surface  304  of each of the radial ratchet members  204 . 
       FIG. 5  is a cross-sectional view of an example of the downhole torque limiting device  110 . As illustrated in  FIG. 5 , the collection of radial ratchet members  204  is constrained radially by the spring support assemblies  122   a  and  122   b . The constraint is compliant, in which an angular face  210  of each radial ratchet member  204  is in sliding contact with a corresponding angular face  212  of the spring support assemblies  122   a  and  122   b . These components will be discussed in additional detail in the descriptions of  FIGS. 7A-7C . 
       FIG. 6  is a cross-sectional view of an example of the downhole torque limiting device  110  in a torque limiting mode. In operation, when the torque forces developed across the downhole torque limiting device  110  are substantially zero, the radial ratchet members  204  will be in a generally compressed configuration such as that shown by the example of  FIG. 5 . In operation, as the amount of torque developed across the downhole torque limiting device  110  increases, the radial ratchet members  204  are urged radially outward, as depicted in the example of  FIG. 6 . This process of radially outward expansion is discussed further in the descriptions of  FIGS. 7A-7C . 
     The spring sections  124   a - 124   b  compress the spring support members  122   a - 122   b  axially toward each other. Such compression compliantly urges the radial ratchet members  204  radially inward. In use, torque forces developed along the downhole torque limiting device  110  act to urge the radial ratchet members  204  radially outward. This outward expansion causes the angular faces  210  to impart an axial force against the angular faces  212 , urging the spring support members  122   a - 122   b  axially away from the radial ratchet assembly  120 , which in turn compresses the spring sections  124   a - 124   b.    
     In some embodiments, the spring sections  124   a - 124   b  can each include a collection of one or more frusto-conical springs (e.g., coned-disc springs, conical spring washers, disc springs, cupped spring washers, Belleville springs, Belleville washers). In some implementations, the springs can be helical compression springs, such as die springs. In some implementations, multiple springs may be stacked to modify the spring constant provided by the spring sections  124   a - 124   b . In some implementations, multiple springs may be stacked to modify the amount of deflection provided by the spring sections  124   a - 124   b . For example, stacking springs in the same direction can add the spring constant in parallel, creating a stiffer joint with substantially the same deflection. In another example, stacking springs in an alternating direction can perform substantially the same functions as adding springs in series, resulting in a lower spring constant and greater deflection. In some implementations, mixing and/or matching spring directions can provide a predetermined spring constant and deflection capacity. In some implementations, by altering the deflection and/or spring constant of the spring sections  124   a - 124   b , the amount of torque required to cause the downhole torque limiting device  110  to enter a torque limiting mode can be likewise altered. 
       FIGS. 7A-7C  are cross-sectional views of an example of the downhole torque limiting device  110 , taken across an axially central point of the radial ratchet assembly  120 . Referring to  FIG. 7A , the downhole torque limiting device  110  includes an outer housing  602  (corresponding to the housing  114  of  FIG. 2 ). The outer housing  602  includes an internal cavity  604 . The internal cavity  604  includes an internal surface  606 , which includes a collection of receptacles  608 . 
     The radial ratchet members  204  include one or more projections (“sprags”)  610  that extend radially outward from a radially outward surface  612 . In use, the sprags  610  are at least partly retained within the receptacles  608  (hereinafter referred to as “sprag receptacles”). It will be understood that the sprag  610  is illustrated as triangular shaped. However it will be understood that other geometric configurations of the projection and a matting receptacle may be used and that “sprag” and sprag shape is not limited to a triangular configuration 
     As discussed previously, the radial ratchet members  204  also include a radially inner surface  614 . The radially inner surface  614  includes at least one semicircular recess  616 . Each semicircular recess  616  is formed to partly retain a corresponding one of the collection of roller bearings  202 . The collection of roller bearings  202  is substantially held in rolling contact with the drive shaft  112 . 
     The drive shaft  112  includes a collection of radial protrusions  620  and radial recesses  622 . Under the compression provided by the spring sections  124   a - 124   b  (e.g.,  FIGS. 3 ,  4 ,  5 , and  6 ), the radial ratchet members  204  are urged radially inward. As such, under conditions in which the downhole torque limiting device  110  is experiencing substantially zero torque, the roller bearings  202  will be rolled to substantially the bases of the radial recesses  622  (e.g., allowing the spring sections  124   a - 124   b  to rest at a point of relatively low potential energy). 
       FIG. 7B  illustrates an example of the radial ratchet assembly  120  with some torque (e.g., an amount of torque less than a predetermined torque threshold) being developed between the drive shaft  112  and the outer housing  602 . In use, the outer housing  602  (also referred to as  114  in  FIG. 2 ) is rotated. This rotational force is transferred to the roller bearings  202 , to the radial ratchet members  204 , and to the drive shaft  112 . 
     As torque forces between the outer housing  602  and the drive shaft  112  increase, the roller bearings  202  are partly urged out of the radial recesses  622  toward neighboring radial protrusions  620 . As the roller bearings  202  are urged toward the radial protrusions  620 , the radial ratchet members  204  comply by extending radially outward in opposition to the compressive forces provided by the spring sections  124   a - 124   b  (not shown). As the radial ratchet members  204  extend outward, contact between the sprags  610  and the sprag receptacles  608  is substantially maintained as the sprags  610  penetrate further into the sprag receptacles  608 . 
     In implementations in which the torque developed between the drive shaft  112  and the outer housing  602  is less than a predetermined torque threshold, rotational forces can continue to be imparted to the drive shaft  112  from the outer housing  602 . In some implementations, the predetermined torque threshold can be set through selective configuration of the spring sections  124   a - 124   b.    
       FIG. 7C  illustrates an example of the radial ratchet assembly  120  with an excess torque (e.g., an amount of torque greater than a predetermined torque threshold) being developed between the drive shaft  112  and the outer housing  602 . In use, the outer housing  602  is rotated. The operation of the radial ratchet assembly  120  substantially decouples the transfer of rotational energy to the drive shaft  112  from the outer housing  602  when torque levels are in excess of the predetermined torque threshold. 
     In operation, an excess torque level causes the roller bearings  202  to roll further toward the radial protrusions  620 . Eventually, as depicted in  FIG. 7C , the present example, the radial ratchet members  204  comply sufficiently to allow the roller bearings  202  to reach the peaks of the radial protrusions  620 . In such a configuration, the rotational force of the outer housing  602  imparted to the radial ratchet members  204  is substantially unable to be transferred as rotational energy to the roller bearings  202 , and as such, the drive shaft  112  becomes substantially rotationally decoupled from the outer housing  602 . 
     In the examples discussed in the descriptions of  FIGS. 1-7C , the radial ratchet assembly  120  may be bidirectionally operable, e.g., the torque limiting function of the downhole torque limiting device  110  can operate substantially the same under clockwise or counterclockwise torques. In some implementations, the radial ratchet assembly  120 , the outer housing  602 , and/or the drive shaft  112  may be formed to provide a torque limiting device that is unidirectional. 
     In some implementations, as illustrated in  FIG. 7D , the roller bearings  202  may be replaced by sliding bearings  702 . For example, one or more of the radial ratchet members  704  may include semicircular protrusions extending radially inward from the radially inner surface of the ratchet member  704 . These semicircular protrusions may rest within the radial recesses  622  during low-torque conditions, and be slidably urged toward the radial protrusions  620  as torque levels increase. 
     In some implementations, multiple sets of radial ratchet assemblies may be used together. For example, the torque limiting assembly  110  can include two or more of the radial ratchet assemblies  120  in parallel to increase the torque capability available between the drilling rig  10  and the drill bit  50 . 
     Although a few implementations have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.