Abstract:
The present invention provides a roller sub for use downhole in oil or gas wells as part of a toolstring or drill string to reduce friction between the string and wellbore. The roller sub is a modular assembly of parts which assemble together to trap the roller wheels in place between them, avoiding need for grub screws to fasten the individual wheels and rendering the roller sub very compact.

Description:
FIELD OF THE INVENTION 
     The present invention concerns improvements in and relating to roller subs for use downhole in oil or gas wells as part of the tool string or drill string to reduce friction between the string and the wellbore. 
     BACKGROUND TO THE INVENTION 
     Roller subs are used widely throughout the oil industry but especially in wireline toolstrings, which rely on gravity alone to advance the toolstring, and are especially useful down wells that deviate substantially from the vertical. 
     Conventional roller subs are generally substantially solid circular cylindrical bodies that are milled to provide radial slots at intervals therearound and therealong. These slots each accommodate a respective roller wheel. Two example prior art configurations of multi-roller wheel sub are illustrated in FIGS. 1 and 2 below. 
     In the FIG. 1 example the cylindrical body  1 , formed with the plurality of slots  2 , holds each roller wheel  3  in place in its respective slot by means of a grub screw  4  which locks down onto a radius groove machined into the head of a caphead screw  5  that serves as the axle of the respective roller wheel  3 . 
     In the FIG. 2 example, the roller sub has substantially the same configuration but in this case the axle  6  of each roller wheel  3  has an undercut into which a fixing grub screw  7  locks. 
     A number of practical problems arise from the use of such conventional designs of multiroller wheel sub, perhaps the most important of which is that the axles and the grub screws and other locking fixtures for holding the roller wheels  3  in place are vulnerable to mechanical failure which may lead to jamming of roller wheels or their loss downhole. Loss of mechanical components such as these downhole is, of course, extremely undesirable since they may interfere with operation of the well and necessitate costly interruption of production to attempt to locate and fish them out. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a roller sub for use downhole in oil or gas wells as part of a toolstring or drill string to reduce friction between the string and wellbore, which roller sub carries at least one roller wheel, wherein the roller sub is a modular assembly of parts which assemble together to trap the at least one roller wheel in place between them. 
     Preferably the modular assembly comprises a body formed of segments. 
     Suitably the modular assembly body of the roller sub comprises six segments. 
     Advantageously each roller wheel has integral (i.e. integrally formed or assembled) pivot pin means. 
     Preferably the pivot pin means of each roller wheel comprise axle stubs that are domed or substantially hemispherical in shape to co-operatively engage with correspondingly shaped recesses in the body of the roller sub. 
     Suitably each roller wheel has a circumferential groove whereby the roller wheel has a dumbell-like shape in profile. 
     Advantageously a channel is provided extending longitudinally through the roller sub to serve as a conduit for fluids or electric line. 
     Preferably part of the channel is defined by the circumferential grooves of the roller wheels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of a first example of prior art roller sub and with insets showing transverse sections. 
     FIGS. 1A and 1B are, respectively, a transverse sectional view taken along the line I—I in FIG.  1  and along the line II—II in FIG.  1 . 
     FIG. 2 is a side elevation view of a second example of prior art roller sub and with insets showing transverse sections. 
     FIG. 2A is a transverse sectional view taken along the line III—III in FIG.  2 . 
     A preferred embodiment of the present invention will be now more particularly described, by way of example, with reference to FIGS. 3 to  6  of the accompanying drawings, wherein: 
     FIG. 3 is a perspective view of a preferred embodiment of roller sub fully assembled; 
     FIG. 4 is a perspective view similar to FIG. 3 but with two roller-mounting body segments of the roller sub disassembled therefrom. 
     FIGS. 5A and B are, respectively, a plan view of a body segment of the roller sub and a side elevation view of the same; 
     FIG. 6 is a transverse section along the line  6 — 6  of the body segment of FIGS.  5 A/B and shown schematically in-situ assembled with the other body segments and with a roller wheel shown in ghostline mounted thereto; 
     FIG. 7 is a transverse section of view of the body segment of FIGS.  5 A/B taken along the line  7 — 7  in FIGS.  5 A/B and again shown assembled together with the other segments; 
     FIG. 8 is an enlarged view of a portion of FIG. 6; 
     FIG. 9 is an enlarged view of a portion of FIG. 7; 
     FIG. 10 is a general assembly diagram of the roller sub, as fully assembled, and showing the top sub and bottom sub part cut away; and 
     FIGS. 11 and 12 are, respectively, a transverse sectional view taken along the line  11 — 11  in FIG.  10  and along the line  12 — 12  in FIG.  10 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 3 to  12  the roller sub of the present invention does not comprise a one-piece cylindrical body with machined slots for roller wheels as in the prior art. Instead, it comprises a modular assembly of body segments  12   a - 12   f  with the roller wheels  30  each having integrally formed pivot pin means  31  and being substantially encased and thereby locked within the roller sub body  1 ′ during assembly of the body  1 ′. 
     The sub body  1 ′ comprises a top sub  100 , a bottom sub  200  and an intermediate body part  300 . The intermediate body part  300  encases the roller wheels  30  in use and is composed of the body segments  12   a - 12   f  that fit together. In the illustrated form it is composed of six body segments  12   a - 12   f , two of which are seen disassembled from the sub body  1 ′ in FIG.  4 . 
     The body segments  12   a - 12   f  are formed as metal bars that are of cross sectional shape that is generally a segment of a circle, whereby the assembled sub intermediate part  300  is substantially circular cylindrical. The bars  12   a - 12   f  are suitably cast, but may be machined to have a pair of longitudinally spaced apart recesses  40   a ,  40   b , angled at different radial orientations. Each recess  40   a ,  40   b  defines half of a cavity to receive a roller wheel  30  and which mates with a recess  40   a ,  40   b  of an adjacent one of the bars  12   a - 12   f  to define a full cavity. 
     The six body segments  12   a - 12   f  between them define six cavities, each to accommodate a respective one of six roller wheels  30 , each wheel  30  oriented radially outwardly at a different orientation from each other and the wheels  30  between them substantially covering the full  3600  circumference around the sub. 
     Each of the roller wheels  30  is seated within a respective cavity and is held within the cavity by co-operative engagement of the axle stubs  31  of the roller wheel  30  with corresponding sockets  32  in the wall of each opposing recess  40   a ,  40   a ′ defining the mounting cavity for the roller wheel  30 . Each axle stub socket  32  is suitably a substantially hemispherical recess to receive a corresponding hemispherical shape of axle stub  31 . 
     The simple act of assembling two adjacent body segments  12   a ,  12   b  together around a roller wheel  30  traps it in place between the two. When all six body segments  12   a - 12   f  are assembled together as the intermediate body part  300  trapping all six wheels  30  in place, they are secured together in assembled state by screw thread mounting of the top sub  100  to the upper end of the intermediate body part  300 , and with the lower end of the intermediate body part  300  being threadedly coupled with the bottom sub  200 . 
     By screw threaded engagement of the intermediate body part  300  with the top and bottom subs  100 ,  200  it is possible to completely avoid use of any grub screws or other means of locking the parts together. However, individual grub screws may additionally or alternatively be used for this purpose while still achieving a very marked improvement over the prior art arrangement of roller sub, using only a pair of grub screws  34 , one for coupling with the intermediate part  300  with the top sub  100 , and the other for coupling with the bottom sub  200 . 
     The provision of the roller wheels  30  with their own integrally formed pivot pin means, i.e axle stubs  31  confers a number of technical benefits. The axle stubs  31  of the roller wheels  30  occupy little volume in comparison to the axles and locking grub screws of the prior art. This provides the opportunity of forming the roller wheel  30  with a broader profile than the conventional wheel, and which suitably has a pulley-like shape, as illustrated, with a prominent rim portion  35  at each end separated by a median groove portion  33 . 
     This profile of the wheel  30  provides for a wide stable wheel while minimising surface contact area. The wheel  30  configuration as a whole is more robust and more stable and spans a greater proportion of the circumference of the roller sub  1 ′, enabling provision of roller wheel  30  contact with the well bore around the full circumference of the roller sub with as few as six wheels  30  and, therefore, within a relatively short length of roller sub, making the whole device far more compact in all respects than the prior art roller sub and utilising less roller wheels  30  as well as avoiding the need for the various other fixing components. 
     With six wheel-mounting segments and between them carrying six wheels  30 , each of wide span, substantially any orientation around the full circumference presents at least a part of a roller wheel  30  to the well bore. This enables, at the simplest level, a tool string to be supported by the one short roller sub at each end of the tool string, avoiding the need for many subs or subs of extended length. 
     Through avoiding use of separate axles and locking screws and the like, a number of yet further technical benefits ensue. In particular, not only does the assembly of the present invention avoid risk of loss of components down hole but maintenance is also made much simpler. In the prior art configuration great trouble has to be taken in the assembly of the roller wheels to the roller sub to minimise the risk of their falling out, in use, and the locking screws are commonly bonded into place with adhesive, whereby stripping of the tool for maintenance is made awkward and often leads to damage to the screw threads and the need to clean and replace not only the roller wheels, but also the axles and screws. In the case of the present invention the roller sub is disassembled very easily, simply by unscrewing the top and bottom end subs  100 ,  200  and the service engineer need only clean and replace the roller wheels  30 , where necessary. 
     By forming the axle stubs  31  of the roller wheel  30  to be domed and suitably substantially hemispherical in shape, they are able to support the roller wheel  30  effectively under heavy lateral loads, further enhancing the substantial improvement in strength of the axles. 
     The roller wheels  30  by virtue of their ‘pulley-like’ or ‘dumbell-shaped’ profile, are able to more easily traverse debris downhole. Furthermore, the manner of mounting of the roller wheels within the connector body provides a wheel cavity with better clearance for egress of any debris that might otherwise enter and interfere with operation of the wheel. 
     The pulley-like or dumbell-shaped profile of the roller wheels  30  has a yet further benefit in that where the roller wheels  30  come together back-to-back in the roller sub, the median groove portion  33  of each wheel  30  combines with the groove  33  of each of the adjacent two wheels  30  to define a substantially sized generally circular or polygonal space  41  which may form part of a conduit that extends through the length of the roller sub. This can best be seen in FIGS. 11,  12  and may be exploited for a number of purposes including accommodating an electric line extending the length of the sub. Indeed, in one embodiment the roller sub may be adapted specifically for this purpose and have a male electrical connector at one end and a female electrical connector at the other end, linked by electrical wire extending between the ends through the conduit  41 . Such a configuration is original and is facilitated through the existence of the median grooves  33  of the roller wheels to define the basis for the passage/conduit  41  without wastage of space and enabling maintenance of the compact design of the whole assembly. 
     The roller sub of the present invention has been tested and established to work efficiently to angles of well bore deviation as much as 88°—e.g. where the toolstring extends initially substantially vertically but turns to extend substantially horizontally downhole. 
     The compact configuration of the roller sub of the present invention enables it to be scaled down to a diameter as small as 2 inches, if necessary, and still be effective.