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RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 61/422,396, filed Dec. 13 th , 2010, the full disclosure of which is hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The disclosure herein relates generally to the field of subterranean hydrocarbon production. More specifically the present invention relates to a system for facilitating desired orientation of a downhole string. 
         [0004]    2. Description of Related Art 
         [0005]    Many downhole tools, including perforating guns, comprise multiple elongated bodies joined end to end. If the elongated bodies are to be rotated or axially positioned, the elongated bodies must be able to rotate freely with respect to the adjacent body or bodies they are connected to. When a downhole tool is inserted within a deviated wellbore, gravity and other forces causes friction. Free rotation of the elongated bodies of a downhole tool is then hindered. If free rotation of the elongated bodies is hindered, they will not be able to be positioned into the desired orientation. Therefore, when the downhole tool consists of multiple perforating guns, perforations cannot be produced at the desired orientation along the wellbore. 
         [0006]    When perforating guns, are used in slanted or deviated wellbores it is often important that the tool be in a specific radial orientation. For example, orienting perforating guns in deviated wells enables the well operator to aim the shaped charges of the perforating gun at specific radial locations along the circumference of the wellbore. This is desired because the potential oil and gas producing zones of each specific well could exist at any radial position or region along the wellbore circumference. Based on the presence and location of these potential producing zones adjacent a deviated well, a well operator can discern a perforating gun orientation whose resulting perforations result in a maximum hydrocarbon production. Not only could a perforation aimed at the wrong angle not result in a preferred hydrocarbon production, but instead could produce unwanted sand production from the surrounding formation into the wellbore. 
       SUMMARY OF THE INVENTION 
       [0007]    Disclosed herein is an example of a device for attachment to a downhole string. In one example the device is a roller system for use in a wellbore downhole that is made of a body having a substantially cylindrical outer surface and that is selectively engaged by a couple to the downhole tool. A bore is formed axially through the body that is adapted to receive a portion of a downhole string. Also include is a swivel in the couple so that the body rotates with respect to the downhole string. Rollers are mounted on opposing lateral sides of the body that are rotatable about an axis that intersects the housing and that have diameters greater than a height of the body, so that when the downhole string is disposed in the wellbore, the rollers are rotatable with respect to an axis of the downhole string. The body can include lateral sections that bolt together. The portion of the downhole string that extends through the bore can be a mandrel having opposing ends adapted for coupling within the downhole string. In this example, the mandrel is retained in substantially the same azimuthal position as the downhole string. The swivel can include bearings between the housing and mandrel and that are adjacent shoulders on the mandrel defined where the outer surface of the mandrel projects radially outward at location that are spaced axially apart and wherein a spindle is defined on the mandrel between the shoulders. The portion of the downhole string that extends through the bore can be a downhole tool. In an example, the rollers have a hemispherically shaped convex outer surface and a concave inner surface that is partially hollow and receives a portion of the body therein. Indentations may be included on an outer surface of the rollers for promoting traction between the rollers and an inner surface of a tubular in the wellbore. A portion of the convex outer surface of the rollers can have a contour approximate to a contour of an inner surface of a tubular in the wellbore to thereby define a contact length between the rollers and the tubular. In one example, the portion of the downhole string that extends through the bore is a perforating gun. 
         [0008]    Also included herein is a downhole string that is selectively deployed in a tubular that is disposed in a wellbore. The downhole string is made up of a series of elongate members connected end to end with a swivel on an outer surface of a portion of one of the members. A housing is releasably coupled onto the swivel and is rotatable about an axis of the one of the members. Rollers are mounted onto lateral sides of the housing that have a diameter greater than a height of the housing, so that an outer circumference of the rollers is in contact with an inner surface of the tubular. One of the members can be a roller sub having opposing ends configured for coupling to other elongate members. In one example, the roller sub includes a mandrel having axially spaced apart shoulders defined where an outer surface of the mandrel extends radially outward and a spindle provided between the shoulders. Optionally, the swivel includes bearings between the housing and the spindle so the housing and rollers can rotate with respect to an axis of the roller sub. Optionally, the lateral sides of the housing are substantially planar and wherein the shoulders project past the lateral sides to define a recess in which the rollers are disposed. In an example embodiment, the one of the members is a downhole tool. Optionally, the rollers can have a hemispherically shaped convex outer surface and a concave inner surface that is partially hollow and receives a portion of the body therein. Indentations may be included on an outer surface of the rollers for promoting traction between the rollers and an inner surface of a tubular in the wellbore, and wherein a portion of the convex outer surface of the rollers has a contour approximate to a contour of an inner surface of a tubular in the wellbore to thereby define a contact length between the rollers and the tubular. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0009]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a side perspective view of an example embodiment of a roller assembly sub. 
           [0011]      FIG. 2  is another side perspective view of the roller assembly sub of  FIG. 1 . 
           [0012]      FIG. 3  is a side perspective partial sectional view of the roller assembly sub of  FIG. 1 . 
           [0013]      FIG. 4  is a side sectional view of the roller assembly sub of  FIG. 3 . 
           [0014]      FIG. 5  is a side perspective view of a roller assembly in a tubular. 
           [0015]      FIG. 6A  is an end view of the roller assembly  FIG. 5 . 
           [0016]      FIG. 6B  is an end view of an alternate embodiment of the roller assembly of  FIG. 6A . 
           [0017]      FIG. 6C  is an end view of an alternate embodiment of the roller assembly of  FIG. 6A . 
           [0018]      FIG. 7  is a side partial sectional view of a downhole string having a roller assembly. 
       
    
    
       [0019]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
         [0021]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
         [0022]    Referring now to  FIG. 1 , one example embodiment of a roller sub  20  is shown in a side perspective view. The roller sub  20  is made up of a body  22  and an elongate annular mandrel  23 . The body  22  is mounted in a mid portion of the mandrel  23 , which is a reduced diameter portion of the mandrel  23 . Opposing ends  24 ,  25  of the mandrel  23  are shown having profiles for coupling within a downhole string (not shown). Wherein in an example the profiles are threaded fittings, that can be male or female. The outer radius of the mandrel  23  projects radially outward adjacent opposing ends of the body  22  to define shoulders  26  on the mandrel  23 . A channel  27  is formed substantially along an entire circumference of one of the shoulders  26 . A channel  28  in the other shoulder  26  is shown formed along a portion of its circumference. An axial bore  29  in the roller sub  20  extends the length of the mandrel  23 . Recesses  30  are shown on lateral sides of the body  22  disposed at about a mid-portion of the body  22  and configured to receive rollers  32  therein. The recesses  30  have a substantially planar surface on the outer surface of the body  22  and terminate adjacent the shoulders  26 . The outer surface of the body  22  between the recesses  30  is generally curved. The rollers  32  rotate about an axis A X  shown intersecting the body  22 . 
         [0023]    Referring now to  FIG. 2 , another side perspective view of the roller sub  20  is provided that illustrate fasteners  34  set in counter bores formed through the outer wall of the housing  22  that depend downward from an upper surface of the housing  22 . In an example, the fasteners  34  are used for coupling together sections of the body  22  for mounting around the mandrel  23 . In the embodiments of  FIGS. 1 and 2 , the rollers  32  are rotatingly mounted onto the body  22  for facilitating movement of the roller sub  20  within a tubular. The rollers  32  are disklike members having a generally planar surface facing the body  22  and a hemispherically shaped surface facing away from the body  22 . The diameter of the rollers  32  of  FIGS. 1 and 2  exceeds the height of the body  22 , so that by positioning the axis of the rollers  32  at about the mid point of the height of the body  22 , the outer radius of the rollers  32  extends past both the upper end lower surfaces of the body  22 . In this example embodiment, the body  22  can be in more than a single orientation that allows the rollers  32  to engage an inner surface of a tubular in which the roller sub  20  is disposed. 
         [0024]    Shown in  FIG. 3  is a side perspective and partially exploded view of the roller sub  20  of  FIGS. 1 and 2 . In this example, the body  22  of  FIGS. 1 and 2  is removed from the assembly  20  so the mid portion of the mandrel  23  is visible. As noted above, the mandrel  23  has a reduced diameter portion to define a spindle  35  over which the body  22  mounts. In the embodiment of  FIG. 3  a multiplicity of spherical bearings  36  are shown set within a groove  37  that circumscribes the outer circumference of the spindle  35 . The groove  37  and bearings  36  are shown at an end of the spindle  35 . Although not shown in  FIG. 3 , another set of groove  37  and bearings  36  may be included at the opposite end of the spindle  35 . Other embodiments exist wherein the groove  37  and bearings are formed at any axial distance along the length of the spindle  35 . 
         [0025]    Referring now to  FIG. 4 , a side sectional view of the assembly  20  from  FIG. 3  is provided. In the embodiment of  FIG. 4 , a pair of bearing assemblies made up of the bearings  36  set in grooves  37  are illustrated at distal locations on the spindle  35 . The body segments  22  are shown set over the spindle  35  and in contact with the bearings  36 . As will be described in more detail below, the roller sub  20  can be used in conjunction with any thing or device that is insertable within a subterranean well. The things used with the roller sub  20  can be passive or active; examples include a downhole string, downhole tools, completion strings, and any device used in wellbore operations. Also, a component of a tool or string can be used with the roller sub  20 , such as a valve, a packer, a whipstock, a sleeve, and the like. An axle (not shown) couples the rollers  32  to the housing  22  and is rotatable with respect to the housing  22  so that the rollers  32  are freely rotatable as well with respect to the housing  22 . Thus, when set within a tubular within a well, or in an open hole configuration, the rolling action of the rollers  32  introduces less drag than does a downhole string sliding through the well. In an example embodiment, the body segments  22  are positioned on the mandrel  23 , then the bearings  36  are fed into grooves via a slot at each end. A cover  39  is provided for retaining the bearings  36  within the body segments  22  after the bearings  36  are inserted therein. The cover  39  is a substantially solid L shaped member with an elongate portion that inserts into the slot. A lower end of the cover  39  is curved to accommodate the shape of the bearings  36 . 
         [0026]    Moreover, addition of the groove  37  and bearings in the sub  20  enables the housing  22  to axially rotate with respect to the mandrel  23 . As such, orientation of the mandrel  23  along with any associated or attached downhole string or string members experiences a substantially reduced resistance to turning. Thus when a downhole string is to be oriented, such as from an eccentric weight, the likelihood that the desired or selected orientation occurs is substantially increased. 
         [0027]      FIG. 5  illustrates an example of a roller assembly  40  that can be coupled with a downhole string or element of a downhole string. In the embodiment of  FIG. 5 , the roller assembly  40  includes a housing  42  made up of a pair of lateral segments  44 ,  46  that can be coupled to one another in a clam shell fashion for defining the housing  42 . The lateral segments  44 ,  46  of  FIG. 5  are bowl shaped members having a convex outer surface on one side and is concave and hollowed out on an opposite side. The concave sides of the lateral segments  44 ,  46  are facing one another with the convex sides facing radially outward. Each of the lateral segments  44 ,  46  is equipped with a hemispherical roller  48  on the convex outer surface of the lateral segments  44 ,  46 , wherein the roller  48  is adapted to freely rotate with respect to either of the lateral segments  44 ,  46 . The roller assembly  40  of  FIG. 5  is shown set on a cut away of a tubular  50 , wherein the tubular  50  can be a wellbore casing or a section of tubing. As discussed above, the addition of the rollers  48  enables movement of the roller assembly  40  along the axial length of the tubular  50  and substantially parallel with the direction of an axis A L  of the tubular  50 . The hollowed out concave sides of the lateral segments  44 ,  46  defines a bore  52  when the lateral segments  44 ,  46  are coupled as shown in  FIG. 5 . An example downhole device  54  (shown in dashed outline) projects through the bore  52 , the downhole device  54  can rotate about axis A L  of the tubular  50  as illustrated by arrow A R . An optional opening  56  is shown extending through the housing  42 . 
         [0028]      FIG. 6A  provides a partial sectional end view of the roller assembly  40  of  FIG. 5 . In this embodiment, a multiplicity of bearings  58  are shown packed in a circumferential assembly within the housing  42  and across an inner periphery of each of the lateral segments  44 ,  46 . The bearings  58  provide a frictional reduction for relative motion between the housing  42  and downhole string  54  coaxially set within the housing  42 . Referring back to  FIG. 5 , the bearings  58  enhance movement along curved arrow A R . The bearings  58  can be spherical as well as cylindrical roller bearings and can either be individually set within a recess provided on an inner circumference of the housing  42  or within respective inner and outer races (not shown). 
         [0029]    Still referring to  FIG. 6A , a side of the rollers  48  facing the lateral segments  44 ,  46  can be recessed in order to receive therein outer radial portions of the lateral segments  44 ,  46 . The hemispherical outer surface of the rollers  48  is shown having a contour similar to the contour of the tubular  50  so that a larger contact length L and area can be realized between the rollers  48  and inner surface of the tubular  50 . Moreover, spacing the rollers  48  apart a designated distance provides stability of the roller assembly  40  and reduces chances of tipping over in the tubular  50 . Indentations  59  are optionally provided on the hemispherical surface of the rollers  48  that in one example can increase traction between the rollers  48  and tubular  50  and promote rotation of the rollers  48  when the roller assembly  40  moves through the tubular  50 . 
         [0030]      FIG. 6B  illustrates an alternate embodiment of a roller assembly  40 B in an end partial sectional view. The tubular  508  of  FIG. 6B  has a diameter that is less than the diameter of the tubular  50  of  FIG. 6A . As such, the width W and diameter D of the rollers  48 B of  FIG. 6B  are respectively reduced over that of the width W and diameter D of the rollers  48  of  FIG. 6A . Whereas the housing  42  of both roller assemblies  40 ,  40 B may have substantially the same dimensions. Referring now to  FIG. 6C , an example of a roller assembly  40 C is shown in an end partial sectional view, where the roller assembly  40 C is coupled with a downhole tool T and disposed within a tubular  50 C. Here the tool T has an outer diameter of about 85% the inner diameter of the tubular SOC. In this example, the rollers  48 C are dimensioned so that clearance is provided between the lower surface of the tool T and inner surface of the tubular  50 C. 
         [0031]      FIG. 7  provides a partial side sectional view an example of a downhole string  60  set within a subterranean wellbore  62 . In the example of  FIG. 7 , the downhole string  60  is made up of a number of individual string members  64  attached in series. Example members include perforating guns, sensors, acoustic devices, submersible pumps, and the like. A wireline  66  is shown suspending the string  60  within the well  62 . A surface truck  68  is provided for manipulating and controlling the string  60  via the wireline  66 . Alternatively, coiled tubing, drill pipe, or other elongate tubulars could be used for deploying the string  60  in the well  62 . In the example of  FIG. 7 , roller subs  20  as well as roller assemblies  40  are shown either combined within the string  60  or coupled on an outer surface of the string  60  for facilitating movement throughout the well  62 . In the embodiment of  FIG. 7 , the string  60  is optionally equipped with eccentric weights for strategically orienting one or more of the string members  64  within the string. Optionally, springs or motors (not shown) could be used for the step of orienting the string elements. An advantage of the device described herein is that because the reduced friction of axial movement of the string in a wellbore, longer perforating strings can be deployed and properly oriented that in the past. Moreover, as embodiments exist wherein the rollers  32 ,  48  respectively project past the outer surface of the body  22  and housing  42 , bodies  22  and/or housings  42  in the string  60  can be azimuthally rotated with respect to other bodies  22  and/or housings  42  in the string  60  so that rolling engagement between the string  60  and tubular (not shown) in the well  62  can occur at any angular position about an axis of the string  60 . 
         [0032]    The improvements described herein, therefore, are well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments have been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present disclosure and the scope of the appended claims.

Summary:
A downhole system includes a downhole string insertable within a subterranean wellbore and a roller assembly coupled with the string. The roller assembly includes rollers mounted on lateral sides of the downhole string for reducing the resistance of deploying the string within the wellbore. The string is rotatable about its axis with respect to the roller assembly; bearing surfaces, or low torque surfaces, are included in the roller assembly to further reduce rotational friction so the string precisely positions itself to a designated orientation.