Patent Publication Number: US-6655477-B2

Title: Friction-reducing drill pipe component

Description:
This invention relates to a drill pipe component, and in particular to a component to be placed in a string of drill pipe to reduce the friction between the string and the hole wall. 
     In industries where long holes or bores are drilled, such as the oil and gas exploration and extraction industries, the friction which occurs due to contact between the drill string and the bore wall may result in a substantial increase in the torque required to rotate the string and the drill bit. Such contact also causes wear and damage to the steel casing used to line sections of the bore. 
     In an effort to avoid these difficulties there have been various proposals for friction reducing components to be mounted in or on the string. U.S. Pat. No. 5,261,498 (The Red Baron (Oil Tools Rental) Limited) describes a typical friction reducing component or sub, in which a bore wall contacting sleeve is mounted on the lower part of a mandrel via bearings and is axially retained on the lower mandrel part between a shoulder and an upper mandrel part. While this and other subs have been used successfully in numerous operations, the cost and complexity of such subs has limited their widespread adoption and use. 
     It is among the objects of embodiments of the present invention to provide a friction-reducing drill pipe component which is relatively simple in construction and is thus less expensive to manufacture and maintain. 
     According to the present invention there is provided a friction-reducing drill pipe component for forming part of a drill string, the component comprising a tubular mandrel having first and second ends for connection to adjacent components of the drill string, a sleeve mounted on the mandrel, and first and second stops on the mandrel for restraining the sleeve against axial movement relative to the mandrel, at least the first stop being removable from the mandrel to permit the sleeve to be removed over the first end of the mandrel. 
     In use, the major parts of the component may be disassembled simply by removing the first stop and then lifting the sleeve over the first end of the mandrel. This contrasts with conventional arrangements in which removal of the sleeve, if possible, requires, for example, the dismantling of the mandrel or heat treatment and expansion of the sleeve. Thus, maintenance and repair of components made in accordance with embodiments of the present invention is relatively simple and in many instances may be carried out on-site at a drilling location. 
     The sleeve may be rotatable relative to the mandrel or may be non-rotatable on the mandrel. In this area, components or subs in which the sleeve is fixed relative to a mandrel are described as “rotating” subs, as the sleeve rotates in the bore with the drill string. If the sleeve is rotatable on the mandrel such subs are described as “non-rotating” subs, as the sleeve remains stationary relative to the bore. 
     In non-rotating subs, bearings may be provided between the sleeve and mandrel, or the sleeve and mandrel may define bearing surfaces. Where bearings are provided these may be introduced into the gap between the sleeve and the mandrel through a port in the sleeve. Bearing lubricant may be trapped between the mandrel and sleeve, however it is preferred that the fluid in the bore provides the necessary lubrication, and to this end the spacing of the stops may be selected to provide a flow path between the stops and the sleeve ends. One of the upper stop and the upper end of the sleeve may be configured to permit flow of fluid therebetween in the event that the contact between the sleeve and bore wall causes the sleeve to be pushed upwardly into contact with the upper stop, for example the upper end of the sleeve or the stop may be scalloped. Alternatively, ports may be provided in the upper end of the sleeve. 
     Preferably, the first stop is in the form of a collar. The collar may engage with a screw thread formed on the mandrel or may be retained on the mandrel by releasable connectors. The releasable connectors may be in the form of bolts or pins or, most preferably, are in the form of sprung pins or dogs which normally extend radially from the mandrel to engage and retain the collar. The collar may define ports therethrough to allow the dogs to be pushed inwardly to allow removal of the collar. The collar may also define slots in communication with the ports so that the collar may be rotated to cover the pins. Where sprung dogs are utilised to retain the sleeve, the dogs may be pushed inwardly to permit removal of the sleeve. Thus, with this embodiment of the invention it is possible for unskilled personnel to remove and replace the sleeve using only very simple tools, such that components may be repaired on-site without requiring specialised assistance or equipment. 
     Preferably also, the second stop is in the form of a stop ring. The ring may be removable but is preferably integral with the mandrel. In the preferred embodiment the mandrel, the mandrel end connections and the second stop are machined from a single piece of metal. The first stop and the sleeve may each also be formed of single pieces of metal. Accordingly, the resulting connector has only a small number of parts and is therefore easily assembled and disassembled and may be of robust construction. 
     The sleeve may have a cylindrical outer surface, or may define axial or helical blades with slots therebetween, to facilitate passage of drilling fluid through the annulus between the drill string and the bore wall. The blades may be of resilient material, such as PTFE, PEEK polymeric material, or vulcanised neoprene, most preferably reinforced with metal or some other rigid structure. Alternatively, the blades may be of metal, such as steel or alloy. The metal blades may be integral with the sleeve or welded or otherwise bonded to the sleeve. The slots may be undercut. 
     According to another aspect of the present invention there is provided a friction-reducing drill pipe component for forming part of a drill string, the component comprising a tubular mandrel having first and second ends for connection to adjacent components of the drill string, and a sleeve mounted on the mandrel, the sleeve defining external blades with undercut channels therebetween. 
     According to a further aspect of the present invention there is provided a friction-reducing drill pipe component for forming part of a drill string, the component comprising a tubular mandrel having first and second ends for connection to adjacent components of the drill string, a sleeve mounted on the mandrel, and spring-mounted lock dogs mounted on the mandrel and operatively associated with the sleeve for releasably retaining the sleeve on the mandrel. 
    
    
     These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a part-sectional view of a friction-reducing drill pipe component in accordance with a first embodiment of the present invention; 
     FIG. 2 is a part-sectional view of a friction-reducing drill pipe component in accordance with a second embodiment of the present invention; 
     FIG. 3 is a sectional view of the mandrel of the component of FIG. 2; 
     FIG. 4 is a sectional view on line  4 — 4  of FIG.  2 ; 
     FIG. 5 is a part-sectional view of a friction-reducing drill pipe component in accordance with another embodiment of the present invention; 
     FIG. 6 is a sectional view of the mandrel of the component of FIG. 5; 
     FIG. 7 is a perspective view of the sleeve of the component of FIG. 5; and 
     FIG. 8 is a perspective view of an alternative sleeve for the component of FIG.  5 . 
    
    
     Reference is first made to FIG. 1 of the drawings, which illustrates a friction-reducing drill pipe component in the form of a sub  10  forming part of a drill string (not shown) for location in a drilled bore. The sub  10  comprises a tubular body or mandrel  12  provided with conventional conical threaded pin and box connections  14 ,  15  to permit the sub  10  to form part of a drill string. A stop ring  16  is formed on the mandrel  12  and locates the lower end of a sleeve  18 . The upper end of the sleeve  18  is located by a stop comprising a set of sprung lock dogs  20  biassed to extend radially from the mandrel  12  into corresponding slots  22  defined on the inner surface of the sleeve  18 . In this embodiment the sleeve is a “rotating” sleeve, in that it rotates with the mandrel  12 . 
     To remove the sleeve  18  from the mandrel  12  an operator depresses the lock dogs  20  by pushing on the lock dogs  20  through the sleeve ports  24  which communicate with the slots  22 . When the lock dogs  20  are pushed inwardly the sleeve  18  may be lifted over the depressed dogs  20 , and removed from the end of the mandrel. 
     Reference is now made to FIGS. 2,  3  and  4  of the drawings, which illustrate a friction-reducing pipe component in the form of a sub  30  in accordance with a further embodiment of the present invention. The sub  30  is somewhat similar to the sub  10  described above, in that it comprises a tubular mandrel  32  defining corresponding pin and box connections  34 ,  35  and carrying a stop ring  36  to retain a sleeve  38  on the mandrel  32 . Further, the upper end of the sleeve  38  is retained by lock dogs  40 . However, the lock dogs  40  engage with a lock collar  41  rather than with the sleeve  38 , which is thus free to rotate on the mandrel  32 . Those working in the area would describe the sleeve  38  as of the “non-rotating” type, as in use the sleeve  38  remains stationary relative to the bore wall, while the mandrel  32  and the remainder of the drill string rotates. 
     The lock dogs  40  engage slots  42  in the collar  41 , and ports  44  provide operator access to the lock dogs  40 . Each port  44  is located at one end of the respective slot  42 , such that the collar  41  may be rotated on the mandrel  32  until the dogs  40  engage the other “closed” end of the slot  42 . Of course the ports  44  and slots  42  are arranged such that rotation of the sub  30  in a bore tends to result in rotation of the collar  41  to bring the dogs  40  to the closed ends of the slots  42 . 
     The lock dogs  40  are illustrated in greater detail in FIG. 4 of the drawings. Each lock dog  40  is located in a respective threaded hole  46  which accommodates a stepped and threaded lock dog retainer  48 . A larger diameter stop  50  is provided on each lock dog  40  to engage a retainer shoulder  52  and limit the outward radial extension of the dog  40 . A compression spring is provided between each lock dog and the base of the hole  46 , to urge the dog radially outwardly. The dogs  40  are ported to prevent the dogs being pushed inwardly by the elevated pressures experienced downhole. 
     In use, a number of subs  30  will be provided in a drill string, and as the string is rotated in a bore the sleeve  38 , which is of larger diameter than the other parts of the drill string, will contact the casing which lines the bore and the mandrel  32  will rotate relative to the non-rotating sleeve  38 . In this particular embodiment the mandrel  32  and the sleeve  38  each define plane bearing surfaces, however lubrication is provided by the drilling mud which, during a drilling operation, will flow upwardly through the annulus between the sub  30  and the bore casing. This drilling mud will find its way between the stop ring  36  and the lower end of the sleeve  38 , pass between the mandrel and the sleeve, and then flow out between the upper end of the sleeve  38  and the collar  41 . To ensure that the fluid may flow out between the upper end of the sleeve  38  and the collar  41 , the collar  41  is scalloped. 
     From FIG. 3 of the drawings it will be noted that the mandrel  32  and stop ring  36  are formed from a single piece of metal. Further, the sleeve  38  and collar  41  are also each formed of a single piece of metal. The sub  30  is therefore very robust, and tests have revealed that the various parts of the sub  30  experience very little wear under normal circumstances. However, if it is desired to remove the sleeve  38  from the mandrel  32 , this is achieved by depressing the lock dogs  40  to allow removal of the collar  41 , and then depressing the lock dogs to allow removal of the sleeve  38  from the end of the mandrel. Similarly, the sleeve  38  may be refitted on the mandrel  32  with equal ease. 
     Reference is now made to FIGS. 5,  6  and  7  of the drawings which illustrate a friction-reducing drill pipe component in the form of a sub  60  in accordance with a further embodiment of the present invention. The sub comprises a tubular mandrel  62  provided with conventional pin and box connections  64 ,  65  to permit the sub  60  to form part of a drill string. The mandrel also defines a stop ring  66  which locates the lower end of a sleeve  68 , the upper end of the sleeve  68  being located by a collar  70  which engages a thread  72  cut on the outer surface of the mandrel  62 . Like the sub  30  described above, the sleeve  68  is rotatable on the mandrel  62 , and in this embodiment various bearings  74  are provided between the sleeve  68  and the mandrel  62 , the opposing faces of which are shaped to define appropriate bearing races or tracks  76 ,  77  (it should be noted that the dimensions of the bearings  74  and the tracks,  76 ,  77  are shown somewhat exaggerated in the Figures). 
     Reference is now made in particular to FIG. 7 of the drawings, which illustrates the sleeve  68 . It will be noted that the sleeve  68  defines four axially extending blades  78  with channels or slots  80  therebetween. The blades  78  are formed of steel, a metal alloy or a resilient material, such as PTFE, moulded or otherwise formed or secured around a steel reinforcing body. To facilitate assembly and disassembly of the sub  60 , various ports  82  are provided in the sleeve  68  to allow bearings to be placed in or removed from the appropriate bearing tracks  76 ,  77  between the mandrel  62  and the sleeve  68 . 
     To disassemble the sub  60 , the ports  82  are opened and the bearings  74  removed therethrough. The collar  70  is then disengaged from the thread  72  and removed from the mandrel  62 . The sleeve  68  may then be lifted over the upper end of the mandrel  62 . To reassemble the sub  60  these steps are simply repeated in the reverse order. 
     Reference is now also made to FIG. 8 of the drawings, which illustrates an alternative sleeve  88  defining three helically extending blades  90  with undercut channels  92  extending therebetween; the undercut channels  92  provide a larger flow area between the blades  90  while not reducing the contact area provided by the blades  90 . 
     It will be clear to those of skill in the art that the above-described embodiments are merely exemplary of the present invention, and that various modifications and improvements may be made thereto, without departing from the scope of the present invention. In a further embodiment, the sub  30  described above may be modified by the provision of a sleeve defining a series of blades, and in a still further embodiment the sleeve may include means to permit for filling of the gap between the sleeve  38  and the mandrel  32  with bearings, which may be in the form of a large number of glass balls.