Abstract:
A shock absorber for attachment between a downhole motor and drill bit for drilling a well. Shafts within the tool&#39;s housing are supported by radial bearings outside of the shafts, and slideably and rotationally engaged with each other by mating splines. The lower shaft telescopically extends from around the outside of the upper shaft, the extension limited by an upper shaft mandrel ledge against a lower shaft internal shoulder. A biasing mechanism within the shock absorber&#39;s housing dampens shock between the motor and bit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional patent application 61/234,438 entitled “Downhole Motor Bearing Assembly with an Integrated Thrust Shock Absorber for Downhole Drilling and Method Thereof” filed on Aug. 17, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure generally relates to radial bearing systems and more particularly, to shock absorbers for radial bearing systems in downhole drilling assemblies. 
     In the drilling of wells for exploration and/or production of hydrocarbons, downhole drilling assemblies are often operably disposed near a drill bit in a sub-surface formation to rotate a drill bit rather than rotating an entire drill string. In such drilling operations, the drill string includes joined lengths of pipe that extend down into a wellbore. 
     These types of drilling assemblies usually contain a fluid-driven motor that is typically attached to the bottom end of the drill string. For example, a “Moineau” or progressive-cavity type motor may be operated by the flow of drilling fluids pumped down through the drill string from the surface. The motor drives an output shaft which is in turn coupled to a drill bit to rotate the drill bit. 
     Drilling fluid or mud is pumped down the drill string to the drilling assembly to drive the fluid motor. The mud is pumped into a casing at a predetermined pressure. The pressurized mud rotates the output shaft and correspondingly, the drill bit. The drilling mud leaving the motor is directed through the shaft to the bit and through well bore to cool the bit and remove rock fragments from the well. 
     Various components of the drill string are subjected to axial vibrations, thrust loads, and shocks during drilling operations. These typically high dynamic stresses and/or vibrations on the drill string may be substantial, particularly during drilling operations in hard and/or non-homogeneous formations. It is desirable to minimize the transmission of such vibrations to reduce the exposure of the components of the drill string to thrust loads. Specifically, it is desirable to dampen axial vibrations and shocks to components such as instrumentation that may be disposed along or within the drill string. Further, dampening axial shock is helpful in reducing bit bounce (i.e., the inability of a drill bit to maintain engagement/contact with the formation) thereby, increasing the rate of penetration of the drill bit and increasing the overall efficiency of the drilling effort. 
     Conventional approaches to dampen or otherwise absorb axial vibrations and shocks during drilling suffer from a number of disadvantages. Specifically, some conventional shock subs do not function optimally because the shock subs are typically disposed upstream of the bit at a distance that is too far from the drill bit to most effectively dampen axial vibrations. 
     In addition, conventional shock subs are typically incapable of fully reducing bit bounce. Excessive bit bounce typically results in reduced efficiency and shortens the lifespan of the drill bit. In addition, conventional shock subs may transmit excessive vibration along the drill string, damaging sensitive electronic components and other components of the drill string. Furthermore, conventional shock subs are sensitive to hydraulic flow through the downhole assembly. Specifically, hydraulic flow through the assembly significantly impedes the dampening characteristics of the shock sub. Still further, fluid flow through the shock sub significantly influences the telescopic extension of the shock sub. These effects limit the operating range of the shock sub and restrict the ability of the shock sub to function properly under certain conditions. In summary, these complexities adversely complicate the design and operation of the conventional shock sub. 
     Another disadvantage of some conventional shock sub designs is the excessive additional length that is introduced in the downhole assembly when a motor is attached to the drill string. This additional length may be particularly undesirable in instances where it is desirable to minimize the distance between the drill bit and the shock sub. As would be understood by those of ordinary skill in the art, it is desirable to locate the shock sub as close to the drill bit as possible to achieve maximum efficiency. 
     Accordingly, an improved shock sub design is needed to address the above-identified disadvantages of the prior art. 
     SUMMARY 
     In one embodiment, a downhole assembly includes a motor operatively coupled to a transmission, wherein the transmission is operatively coupled to an upper shaft disposed within a housing. The upper shaft is supported by a first radial bearing assembly disposed within the housing. A thrust bearing assembly including a ball bearing disposed between two races is provided. The upper shaft extends through and is supported by the first radial bearing. A distal end of the upper shaft has a first set of mating splines disposed thereon. A lower shaft supported by a second radial bearing assembly is disposed within the housing, wherein the lower shaft has a second set of mating splines disposed thereon. The second set of mating splines are adapted to mate with the first set of mating splines and the lower shaft is in coaxial relationship and telescopically extendable from the upper shaft. A catch is machined on an internal surface of the lower shaft to limit the extent of travel of the lower shaft. A biasing mechanism is disposed adjacent the ball bearing, wherein the biasing mechanism biases the lower shaft in an extended position with respect to the upper shaft, wherein the biasing mechanism comprises a series of disc springs. A drill bit that is operatively coupled to the lower shaft. 
     In another embodiment a shock absorber assembly includes a housing and a biasing mechanism disposed within the housing. A rotatable shaft assembly includes an upper shaft that is supported by a first radial bearing. A lower shaft is supported by a second radial bearing and is concentrically disposed around at least a portion of the upper shaft. The lower shaft is telescopically extendable from the upper shaft. A drill bit is coupled to a the lower shaft. 
     In yet another embodiment, a method of dampening axial shock on a drill bit includes the steps of providing a housing, wherein a biasing mechanism is disposed within the housing. The method further includes a step of providing a rotatable shaft assembly that includes an upper shaft supported by a first radial bearing and a lower shaft supported by a second radial bearing. The lower shaft is concentrically disposed around at least a portion of the upper shaft and the lower shaft is telescopically extendable from the upper shaft. Still further, the method comprises the steps of providing a thrust bearing assembly disposed adjacent the biasing mechanism and providing a drill bit that is coupled to a the lower shaft. 
     The features and advantages of the present disclosure will be apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying figures, wherein: 
         FIG. 1  depicts a front elevational view of a drilling assembly; 
         FIG. 2  illustrates a partial cross-sectional view of a drilling assembly; 
         FIG. 3  illustrates a magnified view of an upper portion of the partial cross sectional view of  FIG. 2 ; and 
         FIG. 4  illustrates a magnified view of a lower portion of the partial cross-sectional view of  FIG. 2 ; 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Radial bearing assemblies disclosed herein stabilize and support rotating shafts in downhole drilling assemblies. In certain embodiments, the radial bearing assemblies of the present disclosure produce less friction compared to conventional bearing assemblies. Less friction is desirable because less heat is generated by rotating components that experience less friction, and thereby results in higher efficiencies of power output. Further, reduced friction and the resulting lower heat generation is desirable to reduce wear and tear on the bearing assembly components. Accordingly, certain embodiments of the radial bearing assemblies disclosed herein experience longer life spans due to reduced wear and tear. Consequently, advantages of certain embodiments of the present disclosure enable significant cost reduction over the life of rotating equipment compared to conventional bearing assemblies. 
     To facilitate a better understanding of the present disclosure, the following examples of certain embodiments are given. In no way should the following examples be read to limit, or define, the scope of the invention. 
     For convenience of reference, when referring to components in longitudinal relation to one another on the drill string, the term “lower” refers to components closer or proximate to the drill bit whereas “upper” refers to components away from or distal from the drill bit. 
       FIG. 1  illustrates drilling assembly  10  including motor  12  and transmission  14  that are operatively coupled to threaded upper end  16  of shock sub  18 . Drill bit  60  is operatively coupled to lower end  17  of shock sub  18 . In one embodiment, drill bit  60  is a PDC drill bit. 
     With continuing reference to  FIGS. 2 ,  3 , and  4 , shock sub  18  includes housing  20  that is adapted to receive rotatable shaft assembly  22  and thrust stack  24 . Thrust stack  24  includes ball bearings  26  that are disposed between stationery races  28   a  and rotating races  28   b . Each pair stationery and rotating races  28   a ,  28   b  are capable of operating under up to approximately 15,000 lb force. Rotatable shaft assembly  22  includes upper shaft  30  having upper end  30   a  that is supported by first radial bearing  32  as would be understood by those of skill in the art. Lower shaft  40  is disposed around and extends downwardly from lower end  42  of upper shaft  30 . Lower shaft  40  is supported by second radial bearing  43 . Upper mating splines  44  are disposed around an outer surface of upper shaft  30 . Upper mating splines  44  operatively mate with corresponding lower mating splines  48  that are disposed on inner surface  49  of lower shaft  40 . Lower shaft  40  is adapted to telescopically extend from upper shaft  30 . 
     Lower mandrel stop  51  extending between first end  53  and bottom end  54  extends from lower end  42  of upper shaft  30 . In one embodiment, lower mandrel stop is integral with upper shaft  30 . In yet another embodiment, first end  53  of lower mandrel stop  51  may be threadably attached to lower end  42  upper out put shaft. Those of ordinary skill in the art will appreciate other methods that may be utilized to couple the first end  53  to lower end  42 . A ledge  56  extends outwardly around the bottom end  54 . 
     Shoulder  58  is machined on lower shaft  40 . Shoulder  58  is adapted to engage ledge  56  of lower mandrel stop  51  when lower shaft  40  is in the fully extended position as shown in ( FIG. 4 ). Consequently the lower shaft  40  can be retracted along upper shaft  30  when an opposing axial force from the formation is transmitted to the lower shaft  40  by drill bit  60 , thereby allowing lower shaft to travel axially over a distance R. 
     Biasing mechanism  50  is disposed within housing  20  as shown in  FIGS. 2 and 3 . Stationery spacer  37  is disposed between biasing mechanism  50  and housing  20 . Upper shaft  30  extends through biasing mechanism  50  and rotating spacer  39  is disposed between upper shaft  30  and rotating spacer  39 . Stationery spacer  37  and rotating spacer  39  are provided to enable biasing mechanism  50  to be preloaded and torqued and also to serve as a protective surface to prevent biasing mechanism  50  from rubbing against housing  20  and/or upper shaft  30 . Arm  47  extends outwardly from rotating spacer  39  and supports a bottom end of biasing mechanism  50 . In one embodiment, biasing mechanism  50  comprises a plurality of disc springs  52  manufactured by Bellevile Springs of Redditch, United Kingdom. Springs  52  are arranged in series configuration to bias lower shaft  40  in an extended position with respect to upper shaft  30 . In certain embodiments, biasing mechanism  50  comprises springs  52  having varying spring constants. For example, spring  52 A has a first spring constant that is different form a second spring constant of spring  52 B. In another embodiment of the present disclosure, the biasing mechanism may be a coil spring or a wave spring as will be understood by those of ordinary skill in the art. In some embodiments, a dynamic fluid may be utilized as the biasing mechanism. 
     In operation, when motor  12  is operated to rotate upper shaft  30 , upper mating splines  44  and lower mating splines  48  engage and cause lower shaft  40  to rotate thereby transmitting rotational energy to drill bit  60 . As will be understood by those of skill in the art, drill bit  60  experiences opposing axial forces from the formation during drilling operations. These axial forces are transmitted directly to lower shaft  40 . Hitherto, the axial forces will be transmitted from lower shaft  40  to other components of the drill string including sensitive instrumentation components that may be damaged by such forces. Also severe axial forces may reduce the rate of penetration of drill bit  60  due to “bit bounce” as discussed above. 
     However, in drilling assembly  10  disclosed herein, the axial forces are transmitted from lower shaft  40  to biasing mechanism  50  which aids in preventing or minimizing bit bounce, thereby increasing the rate of penetration of drill bit  60 . The biasing mechanism also dampens vibrations and absorbs axial shocks preventing such vibrations and/or axial shocks from impacting other components of the drill string and motor  12  because the biasing mechanism is disposed between the drill bit and motor  12 . Therefore, biasing mechanism  50  is able to absorb/dissipate vibrations and/or axial shocks before motor  12  and/or other components experience the vibrations and/or shocks. Further, biasing mechanism  50  engages thrust stack  24  when lower shaft  40  transmits axial forces to biasing mechanism  50  to further dissipate axial forces without compromising the integrity of other components installed in the drill string. It is contemplated that the ability of drilling assembly  10  to dampen vibrations and absorb axial shocks can be varied by varying the spring coefficients of springs  52 . 
     Consequently, it is contemplated that the configuration of biasing mechanism  50  and/or thrust stack  24  disposed downstream of motor  12  helps to increase the serviceable life of drill string components. This configuration allows for a more compact drilling assembly  10 . In addition, this configuration of components downstream of motor  12  enables vibration dampening and shock absorption closer to drill bit  60  thereby allowing a greater percentage of vibrations and shocks to be dissipated away from components of the downhole assembly. 
     It is believed that incorporation of the above-described assembly  10  in a drill string reduces bit bounce and enables absorption and/or dissipation of axial shocks and/or vibrations experienced by a drill bit and prevent such axial shocks and/or vibrations from damaging components of the drill sting and the motor that drives the drill string. In addition, incorporation of the assembly  10  results in a compact and more efficient drill sting. 
     It is explicitly recognized that any of the elements and features of each of the devices described herein are capable of use with any of the other devices described herein with no limitation. Furthermore, it is explicitly recognized that the steps of the methods herein may be performed in any order except unless explicitly stated otherwise or inherently required otherwise by the particular method. 
     Therefore, the present disclosure is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined herein.