Non rotating centraliser

Abstract: A fitting for engagement with a drill string or further fitting comprises an inner section (30) for securement to a drill string or further fitting, an outer section (31) for securement about said inner section and rotatable relative thereto, a bearing (36) located between the inner section and outer section, and seals (37,38) provided at either end of the fitting between the inner section and outer section. Pressure compensating means (39) are provided for maintaining the pressure within the bearing substantially the same as the external pressure.

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, the present invention relates to 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 an angle at which a well can be drilled is 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. US4102552 shows a tandem roller stabiliser having eccentric journals. A sealed antifriction bearing is employed and the rollers have outer hard wearing surfaces. US4606417 shows a pressure equalised stabiliser for a drill string having an outer roller mounted on an inner mandrel via a sealed bearing. 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. In accordance with the present invention there is provided a fitting for engagement with a drill string or further fitting comprising an inner section for securement about a drill string or further fitting and an outer section secured about said inner section and rotatable relative thereto. A sealed bearing is provided between the inner and outer sections. Pressure compensating means are provided for maintaining the pressure within the bearing substantially the same as atmospheric pressure, and the bearing includes a layer of friction reducing material. Preferably a plurality of fins project radially from the outer section which are profiled to reduce draft 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: Figure 1 shows a two part fitting having a polygonal bore. Figure 2 shows the interface between a drill string and the interior polygonal bore of the fitting shown in Figure 1. Figure 3: Shows a section of drill string having collars at either end for receiving the fitting of Figure 1 therebetween. Figure 4 : Shows an end view of the fitting of Figure 1. Figure 5 : Shows an end view of the fitting of Figure 1 engaged with the sleeve shown in Figure 3. Figure 6 : Shows a front view of the fitting of Figure 1 showing a partial cross-sectional view. Figure 7 : Shows a fitting having rotatable rollers provided on the body thereof. Figure 8 : Shows a cross-sectional view of a rotatable roller shown in Figure 7. Figure 9 : Shows a cross-sectional view of a roller of a rotatable roller shown in Figure 7 or Figure 8. Figure 10 : Shows a perspective view of a fitting according to a third embodiment. Figure 11 : Shows a cross-sectional view along the axis of the fitting shown in figure 10. Figure 12 : Shows an enlarged view of the seal arrangement shown in figure 11. Best mode for carrying out the invention Referring firstly to Figures 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 7a and 7b of the sleeve shown in Figure 3 are then secured to the drilling pipe by bolts etc passing through the apertures of collars 8a, 8b, 9a and 9b. 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 7a and 7b 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 Figure 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 rheological 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 Figure 6 a protective section 13 may be provided between the collars 8a, 8b and 9a, 9b 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 draft 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. Figures 7 to 9 show a second system for reducing 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 Figure 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 Figure 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 Figure 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 figures 10 to 12 an embodiment of the invention 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 figure 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. Figure 11 shows a cross-sectional view along the axis of the fitting shown in figure 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 figure 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 figures 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 semi-circular in profile and will preferably spiral along the length of the journals (similar to the recesses 6 shown in figure 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.

Claims: A fitting for engagement with a drill string or further fitting comprising: an inner section (30) for securement to a drill string or further fitting; an outer section (31) for securement about said inner section and rotatable relative thereto; a bearing (36) located between the inner section and outer section; seals (37,38) provided at either end of the fitting between the inner section and outer section; and pressure compensating means (39) for maintaining the pressure within the bearing substantially the same as the external pressure; characterised in that the bearing comprises a layer (36) of friction reducing material. A fitting as claimed in claim 1, wherein the pressure compensating means is in the form of a diaphragm (39). A fitting as claimed in claim 1 or 2, wherein the friction reducing material (36) is a plastics material. A fitting as claimed in claim 1, 2 or 3, wherein the friction reducing material (36) is nylon. A fitting as claimed in any preceding claim, wherein the bearing comprises first journal surfaces (42,43) on said inner and outer sections (30,31) extending in the axial direction of said fitting and second and third journal surfaces (44,45,46,47) on said inner and outer sections extending radially outwards to restrict movement of said outer section (31) relative to said inner section (30) in the axial direction. A fitting as claimed in any preceding claim, wherein a plurality of fins (33) project radially from said outer section (31) which are profiled to reduce drag in the axial direction. A fitting as claimed in claim 6, wherein the fins have a curved profile. A fitting as claimed in any preceding claim, wherein rollers are provided on the periphery of said outer section to reduce friction in the axial direction.

Inventors: FERNANDES DENIS ROBERT, MURRAY GEOFFREY NEIL, FERNANDES, DENIS, ROBERT, MURRAY, GEOFFREY, NEIL
Applicants: WEATHERFORD LAMB, WEATHERFORD/LAMB, INC.
Application year: 1996
Main classification: E21B17
IPC codes: E21B17