Abstract:
A technique facilitates control over the orientation of a bottom hole assembly. A planetary gearbox assembly incorporates a sun wheel which cooperates with planet wheels at a plurality of levels along the planetary gear box assembly. The sun wheel and the planet wheels cooperate to convert rotational input to rotational output through an output carrier. Torsional rigidity characteristics of the sun wheel and the output carrier are selected to distribute torque loading across the plurality of levels of the planetary gear box assembly. The distributed forces reduce the potential for component failure.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    1. Field of the Disclosure 
         [0002]    The subject disclosure relates generally to oilfield drilling, and more particularly to bottom hole assemblies and tools for orienting a bottom hole assembly (BHA). 
         [0003]    2. Background of the Related Art 
         [0004]    In conventional drilling, the BHA is lowered into the wellbore using jointed drill pipes or coiled tubing. Often the BHA includes a mud motor, directional drilling and measuring equipment, measurements-while-drilling tools, logging-while-drilling tools and other specialized devices. A simple BHA having a drill bit, various crossovers, and drill collars is relatively inexpensive, costing a few hundred thousand US dollars, while a complex BHA costs ten times or more than that amount. 
         [0005]    Many drilling operations require directional control so as to position the well along a particular trajectory into a formation. Directional control, also referred to as “directional drilling,” is accomplished using special BHA configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors, and special BHA components and drill bits. The directional driller can use drilling parameters such as weight-on-bit and rotary speed to deflect the bit away from the axis of the existing wellbore. In some cases, e.g. when drilling into steeply dipping formations or when experiencing an unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically. 
         [0006]    Direction control is most commonly accomplished through the use of a bend near the bit in a downhole steerable mud motor. The bend points the bit in a direction different from the axis of the wellbore when the entire drill string is not rotating. By pumping mud through the mud motor, the bit rotates though the drill string itself does not, allowing the bit alone to drill in the direction to which it points. When a particular wellbore direction is achieved, the new direction may be maintained by then rotating the entire drill string, including the bent section, so that the drill bit does not drill in a direction away from the intended wellbore axis, but instead sweeps around, bringing its direction in line with the existing wellbore. As it is well known by those skilled in the art, a drill bit has a tendency to stray from its intended drilling direction, a phenomenon known as “drill bit walk”. A device for addressing drill bit walk is shown in U.S. Pat. No. 7,610,970 to Sihler et al. issued Nov. 3, 2009, which is incorporated herein by reference. 
         [0007]    The use of coiled tubing with downhole mud motors to turn the drill bit to deepen a wellbore is another form of drilling, one which proceeds quickly compared to using a jointed pipe drilling rig. By using coiled tubing, the connection time required with rotary drilling is eliminated. Coiled tube drilling is economical in several applications, such as drilling narrow wells, working in areas where a small rig footprint is essential, or when reentering wells for work-over operations. 
         [0008]    In coiled tubing drilling, a BHA with a mud motor is attached to the end of a coiled tubing string. Typically, the mud motor has a fixed or adjustable bend housing in order to drill deviated holes. Because the coiled tubing is unable to rotate from surface, a so called orienter tool is used as part of the BHA to “orient” the bend of the mud motor into the desired direction. There exists a multitude of different designs for the drive systems of such tools. Some designs support continuous rotation such as electric motor and gearbox drives, while others only permit rotation by a certain limited angle. The orienter tool is typically a high-torque, low-speed device, wherein the design of the drive system provides a torque output which can at least match the reactive torque exerted by the drilling mud motor. 
         [0009]    For example, some orienter tools have utilized planetary gears in an effort to drive the output shaft. Basically, creating a torque on an output shaft means that a tangential force has to be exerted. By way of example, an output torque of 1,000 ft-lbs from a 2-inch diameter shaft means a tangential force of 12,000 lbs. This amount of force will quickly yield any material unless the tangential force is evenly distributed over a sufficient area to reduce the stress levels. In a conventional planetary stage with a size constraint on the order of 3 inches in diameter, the limits of how much bending force the gear teeth can take, and how much stress the planet carrier is capable of supporting will be much below 1000 ft-lbs of torque. 
       SUMMARY OF THE INVENTION 
       [0010]    A system and methodology are designed to facilitate control over the orientation of a bottom hole assembly. A planetary gear box assembly incorporates a sun wheel which cooperates with planet wheels at a plurality of levels along the planetary gear box assembly. The sun wheel and the planet wheels cooperate to convert rotational input to rotational output through an output carrier. Torsional rigidity characteristics of the sun wheel and the output carrier are selected to distribute torque load across the plurality of levels of the planetary gear box assembly. The distributed forces reduce the potential for component failure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that those having ordinary skill in the art to which the disclosed system appertains will more readily understand how to make and use the same, reference may be had to the following drawings. 
           [0012]      FIG. 1  is a cross-sectional view of a multi-level planetary gear box assembly for an orienter tool of a bottom hole assembly in accordance with the subject technology. 
           [0013]      FIG. 2  is a schematic cross-sectional view of the multi-level planetary gear box assembly of  FIG. 1  taken along lines A-A, B-B and C-C. 
           [0014]      FIG. 3  is a qualitative plot of a twisting angle of the components of the planetary gear box assembly of  FIG. 1 . 
           [0015]      FIG. 4  is a schematic illustration of a drilling system having a bottom hole assembly utilizing the planetary gear box assembly. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    The present disclosure overcomes many of the prior art problems associated with providing torque in bottom hole assemblies. The advantages, and other features of the planetary gear box assembly disclosed herein, will become more readily apparent to those having ordinary skill in the art from the following detailed description of certain preferred embodiments taken in conjunction with the drawings which set forth representative embodiments of the present invention and wherein like reference numerals identify similar structural elements. 
         [0017]    All relative descriptions herein such as left, right, up, and down are with reference to the Figures, and not meant in a limiting sense. Unless otherwise specified, the illustrated embodiments can be understood as providing exemplary features of varying detail of certain embodiments, and therefore, unless otherwise specified, features, components, modules, elements, and/or aspects of the illustrations can be otherwise combined, interconnected, sequenced, separated, interchanged, positioned, and/or rearranged without materially departing from the disclosed systems or methods. Additionally, the shapes and sizes of components are also exemplary and unless otherwise specified, can be altered without materially affecting or limiting the disclosed technology. 
         [0018]    The subject technology generally is directed to a high torque planetary gear system for a bottom hole assembly. The planetary gear system includes a geometry where the torque load, resulting in a tangential force, is better distributed inside the structure and hence the stress levels are reduced throughout the planetary stage compared to conventional systems. In one embodiment, the planetary gear system stacks several planes of planetary gears in several levels. By matching the torsional rigidity of a sun gear with that of a carrier body, taking into account the transmission ratio, even engagement of all planetary wheels can be ensured by a principle of elastic averaging, which allows the design of a very high output torque planetary gear stage. It is envisioned that a gear box design for a downhole orienter tool in accordance with the subject technology has extremely high output torque. According to one embodiment, the gear output torque at least matches the stall torque of the mud motor which is driven/oriented by the orienter. 
         [0019]    The present technology also is directed to a high torque planetary gear box assembly for a bottom hole assembly (BHA) used in drilling. The gear box assembly comprises a housing having at least one stage with a plurality of levels, a sun wheel for connecting to an input shaft and having a gear portion within each level, at least one planet wheel coupled to the respective gear portion in each level, and a common carrier connected to the at least one planet wheel in each level. During operation, an external torque is transmitted by the sun wheel through the plurality of levels whereby tangential forces are transmitted from the gear portions to the respective at least one planet wheel, and, in turn, from the at least one planet wheel to the common carrier. The sun wheel is designed to match torsional rigidity characteristics of the common carrier to balance the tangential forces on each level. 
         [0020]    By way of example, the plurality of levels may be three levels and the at least one planet wheel may be two planet wheels in each level, although other numbers of levels and planet wheels may be employed. Torsionally flexible elements may be incorporated into the sun wheel, and the gear box assembly may include a housing gear for engaging the gear portions. According to one embodiment, the common carrier twists by an angle α as a result of torque applied thereto and the gear box assembly has a transmission ratio i such that a twisting angle β of the sun wheel is characterized by β=i*α, and a torsional rigidity of the sun wheel is about i 2  times less than a torsional rigidity of the common carrier to accomplish even engagement in all levels of the gear box assembly. 
         [0021]    The subject technology also may include a method for using a high torque planetary gear box in a bottom hole assembly (BHA). The method comprises providing a housing having at least one stage with a plurality of levels; and applying torque to a sun wheel, the sun wheel having a gear portion within each level that, in turn, applies torque to at least one planet wheel coupled to the respective gear portion in each level. The method also may comprise coupling a common carrier to the at least one planet wheel in each level whereby torque is transmitted from the planet wheels thereto; and matching torsional rigidity characteristics of the sun wheel to the common carrier such that tangential forces on each level are balanced. 
         [0022]    It should be appreciated that the present technology can be implemented and utilized in numerous ways, including without limitation as a process, an apparatus, a system, a device, a method for applications now known and later developed. These and other unique features of the system disclosed herein will become more readily apparent from the following description and the accompanying drawings. 
         [0023]    In brief overview, the subject technology includes a gear box design for a downhole orienter tool having an extremely high output torque. In some embodiments, the gear output torque at least matches the stall torque of the mud motor which is driven/oriented by the orienter. For example, the output torque for a 3-inch size orienter tool can be on the order of 1,000 ft-lbs and above. Conventional planetary gear boxes are normally not capable of such a high torque in the desirable small sizes. Limiting factors include the strength of the gear teeth as well as the planet carrier. For multi-stage designs, the last planetary stage at the high-torque side typically endures the highest loads and will normally break first. 
         [0024]    To evenly distribute the stress over a sufficient area, one embodiment of the subject technology uses a principle of elastic averaging to spread the torque imposed onto the output shaft over several levels of planet wheels. Preferably, fewer planets per level are used rather than the maximum number that could normally be fitted before the planets start overlapping. By using relatively fewer wheels per level, the number of windows cut into the carrier is reduced, and the carrier will be much stronger against twisting deformation. 
         [0025]    Referring generally to  FIG. 1 , a multi-level stage  102  of a planetary gear box assembly  100  in accordance with the present technology is shown. The planetary gear box assembly  100  may include a plurality of stages or simply be a single stage as shown within a housing  104 . The multi-level stage  102  has an input shaft  106 . The input shaft  106  may be part of a motor or even the output of a previous stage. A sun wheel  108  is connected to or an extension of the input shaft  106  and has a gear portion  110   a - c  within each level  112   a - c , respectively. One or more torsionally flexible elements  114  may be incorporated in the sun wheel  108  intermediate each sun gear portion  110   a - c.    
         [0026]    In the example illustrated, the multi-level stage  102  includes two planet wheels  116   a - c  in each of the three levels  112   a - c , respectively. There could be more or less planet wheels per level, perhaps even up to as many planet wheels as can be fitted in each gear box level without overlapping, depending on the design. Further, there could be more or less levels. Each planet wheel  116   a - c  connects to and is supported by a respective planet axle  118   a - c . The planet wheels  116   a - c  of all levels  112   a - c  are all connected to a common carrier  120 . The common carrier  120  may be an output carrier, such as an output shaft or an input shaft connected to another stage (not shown). The planet wheels  116   a - c  also engage a housing gear  122  mounted within the housing  104 . As would be known to those of ordinary skill in the art, each of the sun gear portions  110   a - c , planet wheels  116   a - c , housing gear  122 , and common carrier  120  include force transfer members, e.g. teeth (not explicitly shown) that engage and interact to transmit forces therebetween. 
         [0027]    During operation, an external torque being transmitted by the input shaft  106  through the multi-level stage  102  results in a series of tangential forces occurring between the surfaces of the gear teeth that are interacting with each other. Tangential forces are transmitted from the sun gear portions  110   a - c  to the planet wheels  116   a - c , and, in turn, from the planet wheels  116   a - c  to the common carrier  120 . Tangential forces are also being transmitted to the housing gear  122 . 
         [0028]    Because the pairs of planet wheels  116   a - c  are divided into several levels  112   a - c  rather than all being in one level, the total torque exerted onto the common carrier  120  (e.g., output shaft) is the result of all the tangential forces acting in the different levels. The resulting tangential forces will cause the common carrier  120  to twist by a certain amount. 
         [0029]    Referring generally to  FIG. 2 , a somewhat schematic cross-sectional view of the multi-level planetary gear box assembly  100  of  FIG. 1  taken along lines A-A, B-B and C-C is shown to representatively indicate operational effects in each level  112   a - c . In each level, the common carrier  120  will twist by a certain angle α as a result of the torque applied. Due to the inherent gear ratio of the planetary gear box assembly  100 , the angle α of the common carrier  120  will require a twisting angle β=i*α of the sun wheel  108  in order to satisfy geometric compatibility, where i is the transmission ratio of the planetary stage  102 . On the other hand, the torque seen by the sun wheel  108  is reduced by a factor of the transmission ratio i as compared to the torque seen by the common carrier  120 . The torque applied to the sun wheel  108  is represented by the arrow “T sun ” and is equal to T Carrier /i. These calculations assume that the housing gear  122  is substantially infinitely stiff with no appreciable twisting. In practice, the housing gear  122  may twist and such twisting should be taken into account, but to simplify for illustrative purposes, this assumption may be utilized. 
         [0030]    Referring again to  FIG. 1 , the twisting angle of the common carrier  120  with respect to itself in cross section along line A-A will be larger than in cross section along line C-C because if the sun wheel  108  was infinitely stiff in torsion, most of the output torque would be taken by the components of level  112   c  only. As a result, the components of level  112   c  would wear out quickly. However, in the present approach the sun wheel  108  is designed to match or otherwise address the torsional rigidity characteristics of the common carrier  120  so the principle of elastic averaging will ensure that the tangential forces on all planet levels  112   a - c  are distributed, e.g. balanced. When the load distribution is balanced, the loading is taken by all planet levels  112   a - c  approximately evenly. If the sun wheel  108  is inherently too stiff to support the desired flexibility, torsionally flexible elements  114  can be used to increase the flexibility. 
         [0031]    Referring again to  FIG. 2 , it is possible to quantify the required balance of torsional rigidities to ensure elastic averaging. Preferably, the sun wheel  108  twists by an angle i times larger with a torque which is i times less than that of the common carrier  120 . Hence, the torsional rigidity of the sun wheel  108  should be about i 2  times less than that of the common carrier  120  to ensure even engagement in all levels  112   a - c  of the gear box assembly  100 . 
         [0032]    Referring generally to  FIG. 3 , a qualitative plot  124  of a twisting angle of the components of the planetary gear box assembly  100  is shown. The plot  124  shows the torsional displacement situation inside the gear box assembly  100 . For illustrative purposes, it is assumed that the planet carrier or common carrier  120  is fixed to ground and a torque is applied to the input shaft  106  to create the internal twisting deformations. The twist angle is then measured with respect to ground. In each section, the twisting angle of the sun wheel  108  is approximately i times that of the common carrier  120  for geometric compatibility. 
         [0033]    It should be noted that the lines in  FIG. 3  are purely qualitative. In reality, the twisting angle as a function of position of the common carrier  120  and sun wheel  108  may be more complex, and in addition, such factors as the twisting of the housing  104  can be taken into account. However, the plot  124  well illustrates that by matching the torsional rigidities of the components involved, taking into account the gear ratio, elastic averaging is accomplished which enables the design of a planetary gear stage capable of much greater torque than conventional 1-level-per-stage designs. 
         [0034]    Referring generally to  FIG. 4 , an example of a well system  126  is illustrated as deployed in a well  128  defined by at least one wellbore  130  having at least one deviated wellbore section  132  being formed. Although the planetary gear box assembly  100  may be utilized in a variety of downhole systems to provide improved control over the orienting of a variety of components, the drilling example is illustrated in  FIG. 4 . In this example, the well system  126  comprises a drilling system having a bottom hole assembly  134  delivered downhole by a suitable conveyance  136 , such as coiled tubing. 
         [0035]    In the embodiment illustrated, bottom hole assembly  134  comprises an orienting tool  138  containing the planetary gear box assembly  100 . The orienting tool  138  and its planetary gear box assembly  100  may be used to ultimately control the drilling orientation of a drill bit  140 . In some drilling operations, the drill bit  140  is powered by a motor  142 , such as a mud motor. Depending on the application, the motor  142  may work in cooperation with a bent housing  144  and the orienting tool  138  to control the desired direction of drilling. As known to those of ordinary skill in the art, bottom hole assembly  134  may comprise a variety of other components, including steering components, valve components, sensor components, measurement components, drill collars, crossovers, and/or other components. The actual selection of components depends on, for example, the specifics of the drilling application and/or the characteristics of the environment. 
         [0036]    As would be appreciated by those of ordinary skill in the pertinent art, the subject technology is applicable to use in a variety of applications with significant advantages for bottom hole assembly applications. The functions of several elements may, in alternative embodiments, be carried out by fewer elements, or a single element. Similarly, in some embodiments, any functional element may perform fewer, or different, operations than those described with respect to the illustrated embodiment. Also, functional elements shown as distinct for purposes of illustration may be incorporated within other functional elements, separated in different hardware or distributed in various ways in a particular implementation. Further, relative size and location are merely somewhat schematic and it is understood that not only the same but many other embodiments could have varying depictions. 
         [0037]    Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Such modifications are intended to be included within the scope of this invention as defined in the claims.