Patent Publication Number: US-6209667-B1

Title: Drill string fitting

Description:
TECHNICAL FIELD 
     The present invention relates to a fitting for reducing friction between a drill string and the wall of a well. More particularly, but not exclusively, in a first aspect the present invention relates to a fitting having a plurality of longitudinal recesses enabling lubrication between the fitting and a drill string using drilling fluid. According to a second aspect there is provided a fitting having rotatable rollers to reduce both axial and rotational friction. According to a third aspect there is provided a fitting having an outer section rotatable relative to an inner section secured about a drill string. 
     BACKGROUND OF THE INVENTION 
     The depth to which and angle at which a well can be drilled are often limited by the degree of friction experienced by the drill string. The life of a drill string may also be reduced due to friction. With increasing environmental concerns it is also becoming less acceptable to reduce friction by injecting chemicals down a well. Using wellstream fluids as a lubricant results in drill string wear due to particulate matter carried in the fluids. Further, currently available similar fittings cannot be rebuilt or reconditioned. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide a fitting which reduces the friction on a drill string or at least to provide the public with a useful choice. 
     According to a first aspect of the invention there is provided a fitting for reducing friction between a fitting and a section of a drill string or further fitting on a drill string, said fitting comprising a body having a tubular bore provided with a plurality of longitudinally extending recesses spaced circumferentially about the bore, dimensioned to lubricate the interface between the fitting and the drill string or further fitting with fluid in a drilling well, wherein rollers are provided about the periphery of the body to reduce friction in the axial direction. 
     Preferably the bore of the fitting has a polygonal cross-section. In another embodiment the fittings may have a substantially circular cross-section and the sleeve may have a polygonal cross-section. The fitting may preferably be formed as a two part casing which can be secured to an assembled drill string. 
     According to a further aspect of the invention there is provided a fitting for engagement to a drill string or further fitting having a body with a bore therethrough provided with a plurality of roller means on the exterior of the body, each said roller means having one or more roller provided thereon and being rotatable relative to the body about an axis transverse to the axis of the bore, the arrangement being such that the roller means can rotate relative to said body to facilitate reduction of rotational and axial friction. 
     Preferably, the rollers have a substantially tapered cylindrical form and rotate about an axis transverse to the axis of rotation of the roller means. The body may be of two part construction and the rollers are preferably formed of a nylon or ceramic material. 
     According to a further aspect of the invention there is provided a fitting for engagement with a drill string or further fitting comprising an inner section for securement to a drill string or further fitting, an outer section for securement about said inner section, a bearing located between the inner section and outer section, seals provided at either end of the fitting between the inner section and outer section and pressure compensating means for maintaining the pressure within the bearing substantially the same as the external pressure. 
     Preferably a plurality of fins project radially from the outer section which are profiled to reduce drag in the axial direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects of the invention will become apparent from the following description which is given by way of example of possible embodiments with reference to the accompanying drawings in which: 
     FIG.  1 : Shows a two part fitting having a polygonal bore. 
     FIG.  2 : Shows the interface between a drill string and the interior polygonal bore of the fitting shown in FIG.  1 . 
     FIG.  3 : Shows a section of drill string having collars at either end for receiving the fitting of FIG. 1 therebetween. 
     FIG.  4 : Shows an end view of the fitting of FIG.  1 . 
     FIG.  5 : Shows an end view of the fitting of FIG. 1 engaged with the sleeve shown in FIG.  3 . 
     FIG.  6 : Shows a front view of the fitting of FIG. 1 showing a partial cross-sectional view. 
     FIG.  7 : Shows a fitting having rotatable rollers provided on the body thereof. 
     FIG.  8 : Shows a cross-sectional view of a rotatable roller shown in FIG.  7 . 
     FIG.  9 : Shows a cross-sectional view of a roller of a rotatable roller shown in FIG. 7 or FIG.  8 . 
     FIG.  10 : Shows a perspective view of a fitting according to a third embodiment. 
     FIG.  11 : Shows a cross-sectional view along the axis of the fitting shown in FIG.  10 . 
     FIG.  12 : Shows an enlarged view of the seal arrangement shown in FIG.  11 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring firstly to FIGS. 1 to  6 , there is shown a fitting for reducing friction on a pipe string. The fitting comprises a body formed of two parts  1  and  2  which may be secured together by bolts which pass through apertures  3 . A plurality of rollers  4  are provided about the outside of the fitting to reduce longitudinal friction on the pipe string. The bore  5  of the body sections is polygonal so as to provide a number of longitudinal recesses  6  between the body  1  and a sleeve  7 . 
     Drilling pipe is usually forged from high tensile steel. The outside surface is typically rough. The preferred method of securing the fitting of the invention to a drilling pipe is as follows. Firstly, a section of the drilling pipe is machined so as to have a relatively smooth outside surface. The two halves  7   a  and  7   b  of the sleeve shown in FIG. 3 are then secured to the drilling pipe by bolts etc passing through the apertures of collars  8   a ,  8   b ,  9   a  and  9   b . Once the sleeve has been secured to a section of pipe, the two halves  1  and  2  of the fitting are secured about sleeve portions  7   a  and  7   b  and secured by bolts passing through apertures  3 . 
     Collars  8  and  9  restrict the longitudinal movement of the fitting. The fitting is however free to rotate about sleeve  7 . Accordingly, friction due to rotation of the drilling rig is minimised due to the fluid lubricant provided in recesses  6  between body sections  1  and  2  and sleeve  7 . Axial friction is reduced by rollers  4  which minimise friction between the wall of the well and the fitting in the longitudinal direction. 
     It would be possible to secure the fitting directly about a section of pipe. This would however not reduce friction to the same extent as by providing smooth sleeve  7 . Where a new section of pipe is being manufactured, it may of course be provided with a smooth section having separate collars  8  and  9  integrally formed at either end thereof for receiving the fitting. 
     Although the interior bore  5  of the fitting has been described as polygonal, it will be appreciated that other shapes of internal bore (eg: sinusoidal) may be provided as long as suitable recesses are provided between the sleeves and the body of the fitting to minimise friction. In some applications the bore  5  of the fitting may be spiralled to minimise the effect of transitions from one recess to another and to promote fluid flow through the fitting. Filtering means, such as wire mesh may preferably be provided at either end of the fitting to prevent large debris entering the recesses. 
     In viewing FIG. 5 it will be seen that each roller  4  is secured to body  1  by a pin  10  passing through roller  4 . Pin  10  may pass through aperture  12  in body  1  into recess  11 . The aperture  12  may then be welded closed to prevent the pin  10  be removed. 
     Roller  4  may preferably be formed of a ceramic or nylon material. Ceramic materials have the advantage that they exhibit excellent wear properties and have a low friction coefficient. Newly developed ceramics have acceptable “ductility” properties and are easily formed. Ceramics are also very stable at high temperatures and are self lubricating, so do not require oil-based lubrication. Ceramics materials are not susceptible to Theological failure or welding either. One of the key advantages, however, is that the density of ceramic materials is such that if a roller breaks the pieces can be circulated out of the well bore, unlike steel fragments which sink to the bottom of the well and interfere with drilling. 
     As shown in FIG. 6 a protective section  13  may be provided between the collars  8   a ,  8   b  and  9   a ,  9   b  and between the rollers  4  to create a smooth exterior profile so that parts of the fitting do not catch as the fitting is moved up and down in a well. 
     It is estimated that using fittings as herein before described about drill pipe joints will reduce the drag by at least 30%. This enables wells to be drilled to greater displacements and at higher angles. Further, expensive drill pipe is protected and the fitting is exposed to most of the wear. The fitting is designed for easy retrofitting to existing pipe and so avoids the need for large expenditure on new pipe strings. 
     FIGS. 7 to  9  show a second embodiment of the invention. The aim again is to reduce longitudinal and rotational friction on a pipe string or fittings employed therewith. A simple one part construction is described although it will be appreciated that a two part body as previously described, may be employed. 
     Body  20  is provided with a plurality of rotatable roller means  21 , shown in more detail in FIG.  8 . Rotatable roller means  21  are substantially disc-shaped and have a cylindrical recess  22  located at the centre thereof. Pin  23  of body  20  engages in recess  22  so that the rotatable roller means  21  is rotatable about pin  23 . Circumferential flange  24  is secured after roller means  21  has been inserted and retains the roller means  21  in place in use. Circumferential flange  24  may be secured firmly in place by welding etc. The rotatable rollers  25  are secured off-centre from pin  23  so that the rotatable roller means  21  may be rotated as it is exposed to different types of frictional force (i.e. longitudinal or rotational). 
     From the above it will be apparent that when body  20  experiences pure rotation relative to the wall of a well, rollers  25  will not be able to rotate (in the position shown in FIG. 7) and will cause the rotatable roller means  21  to rotate 90° so that the axes of the rollers are aligned with the axis of the drilling rig. When in this position, the rollers can freely rotate to minimise friction. When the drill string is moved purely in the longitudinal direction, the rollers will stay in the position as shown in FIG. 7 so that they may freely rotate to reduce longitudinal friction. It will be appreciated that when there is a combination of rotational and axial movement the axis of the rollers will be somewhere between the two positions described above. 
     Referring to FIGS. 10 to  12  a third embodiment will be described. The fitting of the third embodiment comprises an inner section  30  and an outer section  31  which is rotatable about inner section  30 . Inner section  30  is adapted to be secured about a drill pipe which passes through bore  32 . Inner section  30  may be of two part construction (similar to that shown in FIG. 3) where the two parts are secured together by bolts or similar fastening means. Outer section  31  may similarly be of two part construction and be secured about inner section  30 . Outer section  31  is provided with a plurality of fins  33  extending radially from body  34 . 
     FIG. 11 shows a cross-sectional view along the axis of the fitting shown in FIG.  10 . In this case the fitting is secured to a drill pipe  35 . A layer of friction reducing material  36  is provided between faces  42  to  47  to reduce friction as outer section  31  rotates about inner section  30 . Layer  36  will preferably be formed of a plastics material such as nylon (zytel 70633L for example). 
     Seals  37  and  38  are provided at either end of the bearing formed by the inter-engaging faces  42  to  47  of inner section  30  and outer section  31  and friction reducing layer  36 . These seals serve to prevent the ingress of fluid from a well into the bearing. This greatly reduces friction on bearing surfaces, thus reducing wear and decreasing the torque required to drive a drill string. 
     Due to the sealed nature of the bearing a pressure compensating system  39  is provided to compensate the pressure within the bearing as the external pressure varies. The pressure compensating system comprises a diaphragm  40  containing grease within region  41  which moves in and out of the bearing as external pressure varies. This prevents external fluid being drawn into the bearing as the external pressure increases. 
     The bearing journals  42  and  43  are preferably precision ground. Bearing sleeves may be provided if required. Sections  44  and  45 , and  46  and  47  of the bearing minimise friction when the outer section  31  is forced in the axial direction relative to the inner section  30 . 
     Referring now to FIG. 12, seal  37  is shown in detail. The seal is seen to include a resilient seal  48  located within a recess  49  in outer section  31 . Seal  48  is preferably formed of a fibre reinforced PTFE. 
     The profile of the fins  33  is shown to be semi-circular in FIGS. 10 and 11. It is to be appreciated that other profiles may be employed which reduce drag in the axial direction. The curved profile shown is preferred due to its drag reduction in both directions. It is to be appreciated that rollers could be provided upon fins  31  to assist in the reduction of axial drag. The fins are preferably coated with a ceramic coating such as CERAM-KOTE™. 
     Bearing surfaces  42  to  47  are preferably coated with a hard material such as Technogenia “technopoudre” or similar. Channels are preferably provided in bearing surfaces  42  and  43  to facilitate the flow of lubricant. These channels will preferably be semicircular in profile and will preferably spiral along the length of the journals (similar to the recesses  6  shown in FIG.  2 ). 
     This fitting may be mounted directly onto a drill pipe during production or may be retrofitted to an existing drill pipe. Alternatively, the fitting may be provided on its own separate “sub” or mandrill, in which case the “sub” or mandrill may be screwed into the drill string between two lengths of drill pipe. 
     It will thus be seen that the invention provides a number of simple inexpensive fittings for reducing the friction experienced between a drill string and the wall of a well. The fittings may be used to protect the joints of pipe strings or fitting tools as required. The invention reduces friction and thus the required torque to drill a well. Reduction of friction also reduces drill string vibration and thus fatigue in the drill string. The invention also minimises environmental damage by using a water-based mud lubricant. 
     Where in the foregoing description reference has been made to integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth. 
     Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope of the invention as defined in the claims. 
     Industrial applicability 
     The present invention may find particular application in the reduction of friction experienced by drilling strings.