Abstract:
A screw-threaded pipe joint comprising a pin having at one end a male screw-threaded portion and a box having at one end a female portion having a complementary screw-thread. The pin and box inter-engage along the threaded portions with the male thread extending to a male stop shoulder adjacent a complementary stop shoulder on the other portion. The complementary stop shoulder comprises a recess in the form of a cone receiver having a radial surface adjacent a corresponding radial surface on the complementary stop shoulder. A curved sealing surface on the pin sealingly engages a corresponding curved sealing surface on the complementary stop shoulder of the box, each curved sealing surface lying on the arc of the circumference of a separate circle, and wherein the curvature of the box sealing surface is greater than on the sealing surface on the pin.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a National Stage Application of International Patent Application No. PCT/GB2010/002261, filed Dec. 13, 2010, which claims the benefit of and priority to United Kingdom patent application Ser. No. GB1005247.0, filed Mar. 29, 2010, the contents of each of which are incorporated fully by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to an improved seal between two pipe sections. The pipe sections particularly contemplated find application in the oil and gas drilling industries. 
     BACKGROUND TO THE INVENTION 
     The production of steel pipes for use in the oil and gas drilling industries has been the subject of a large amount of research and innovation. Apart from the materials and the means of forming a cylindrical tube, one aspect which has been a focus of attention is the coupling together of two separate pipes to provide a fluid-tight joint capable of withstanding both tensive and compressive forces. 
     The joints are normally formed by screwing together two pipes, each having complementary threads pre-cut into the ends of each of the pipes: addition of further pipes to the free end continuing, to build up a string. As an alternative but related method, a threaded coupling-sleeve is used to bridge across the ends of two pipes, but the principle remains the same. 
     To improve the sealing properties of the joint produced between pipes, and to give a seal capable of withstanding repeated handling under normal operating conditions, the profile and surface of the threaded part of the end of the pipes and the regions immediately around the threaded part towards the end of the pipe, are specifically designed to co-operate together to provide the seal. Typically the end of the pipe has a threaded portion cut either onto the outside of the pipe (to form a pin or male section) or into the inner surface of the pipe (to form a box or female section). The surface onto which the threaded portion is introduced can include a taper to assist the coupling process. 
     Alternatively the diameter of the pipe in the seal forming region can have been increased in comparison to that predominating along the length of the pipe, usually by cold forming, to allow a joint to be formed. 
     On the pipe, an unthreaded section is normally left between the end of the pipe and the threaded section, which unthreaded section is often referred to as a stop-shoulder. Particular emphasis has been placed in research on the stop-shoulder as this often constitutes the primary sealing region of the pipe joint. The stop-shoulder on the pin is usually profiled to engage a corresponding recess on the box section to form a strong seal. The particular profile is normally chosen to disperse efficiently the strain experienced in the pipe when torque is applied to form the joint and to retain the seal when the joint is in use. In addition to the profiling, coatings can also be applied to improve the fluid-tight nature of the seals. 
     However, as many reserves of oil and gas begin to run out and the price of oil climbs there is an increasing requirement and opportunity to extract oil and gas from reserves which would have hitherto been uneconomic. For example, extraction needs to take place from deeper or more inaccessible levels. Moreover, there is also a desire where possible to use existing bore-holes as a starting point to reach the more difficult deposits. 
     The outcome of this is that pipe joints need to be able to withstand high temperatures and pressures—both tensive and compressive—than has previously been the case. For example, many strings need to have bends of around 90° when going from a vertical orientation to a horizontal one. Such a bend obviously causes both compressive and tensive forces to act on the same joint. 
     One problem encountered on prior art joints is bending of the end of the pin on make-up of the joint. This can lead to galling: if not on the first time the joint is made up, then on subsequent make-ups. Although this problem can be addressed by increasing the tolerance on manufacture, this is an expensive solution and moreover does not completely remove the difficulties, particularly under normal working conditions. 
     It is an object of the present invention to seek to address the above problems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a pipe joint, the joint comprising: 
     a screw-threaded joint for pipes comprising a first pipe length or pin having at one end a male screw-threaded portion and a second pipe length or box having at one end a female portion having a complementary screw-thread, the portions being adapted to inter-engage along the greater part of the axial length of the threaded portions the screw threads thereof being inclined in the same direction and at an acute angle to the longitudinal axis of the pipe length, the male thread extending to a male stop shoulder adjacent a complementary stop shoulder on the other portion, the complementary stop shoulder comprising a recess in the form of a cone receiver having a rounded apex, the stop shoulder on the pin including a torque shoulder, frictionally engaging a corresponding shoulder in the complementary stop shoulder, 
     a radial surface adjacent a corresponding radial surface on the complementary stop shoulder, 
     a curved sealing surface on the pin sealingly engaging a corresponding curved sealing surface on the complementary stop shoulder of the box, the curved sealing surfaces being so shaped each to lie on the arc of the circumference of a separate circle, and wherein the curvature of the box sealing surface is greater than on the pin. 
     The above arrangement enables movement of the seal along the curved surface to take place when stresses are applied to the joint. 
     Preferably, the centre of the circle on which the sealing surface of the pin section lies, has its centre on a line which is 0.30-0.40″ (0.762-1.016 cm) from the intersection point of lines extending from the torque shoulder and the radial surface on the pin. 
     Advantageously, the radius of the circle on which the sealing surface of the pin section is described is from 0.25-0.35″ (0.635-0.890 cm). 
     Preferably the angle between the line on which the centre of the circle describing the sealing surface of the pin, and the line from the centre of the circle and the initial contact point of the two sealing surfaces is from 7.0-9.0°. 
     Preferably the angle between the line on which the centre of the circle describing the sealing surface of the box, and the line from the centre of the circle and the initial contact point of the two sealing surfaces is from 10.0-12.0°. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be now described with reference to the accompanying drawings which show one embodiment of a joint. In the drawings: 
         FIG. 1  is a section of the longitudinal axis of a pipe section, showing the sealing region of a box seal; 
         FIG. 2  is a section of the longitudinal axis of a pipe section, showing the sealing region of a pin seal; 
         FIG. 3  illustrates the location within a pipe section of the box seal in  FIG. 1 ; 
         FIG. 4  illustrates the location within a pipe section of the pin seal in  FIG. 2 ; 
         FIG. 5  illustrates a torque graph obtained during make-up of a joint; and 
         FIGS. 6 and 7  illustrate datum points and line used to determine the position of a radius sealing surface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In designing the sealing region of a joint between the pin and box sections of steel pipes, a large number of features have been utilised and varied to try and achieve the objectives set for a particular seal. The present invention is concerned with the use of what is often referred to as a radius, on both the pin on the box section, said radii engaging each other on make-up of the joint and allowing some movement of the pin and the box relative to each other, said movement still however maintaining a seal. The seal can also be maintained when the longitudinal axes of the pin and the box sections are not in complete alignment, provided said misalignment is not too great. In a cross-section along the longitudinal axis of a pipe, the radii appear as curved lines and are so shown in the attached figures. 
     Although such radii are known in the prior art it has been surprisingly found that the use of two radii—one on the pin section and one on the box section—co-operate together to provide a seal which is stable under both tensive and compressive forces and also at elevated temperatures. The seal produced is therefore suitable for use in difficult environments such as those at greater depths, where high curvature of a string is required, where there is a large pressure differential between the internal volume of the pipe and the outside and where temperatures can reach more than 200 C. 
     It has been found firstly that the two radii preferably have different curvatures to each other. Secondly, each curved surface can be defined as being part of the arc of a circle whose centre is in the main body of the pipe. It has again been found that results are improved for those situations where on contact between the surfaces, the centre of those circles are offset longitudinally from each other and from the point of contact. In this case the centre of the circle for the curved surface of the box section lies in the longitudinal direction of the bulk of the pipe of which the box section is a part. Similarly the centre of the circle for the curved surface on the pin section lies longitudinally to the opposite side of the point of contact. 
     The invention is now illustrated initially with respect to  FIGS. 1 and 2  which show the stop shoulder recess of a box section and the stop shoulder of a pin section of a pipe respectively. 
     Dealing firstly with the box section  10  of a pipe, this comprises a recess  11  defined by the metal from which the bulk  12  of the pipe is formed. The pipe body itself is shown in hatched shading to visually assist the interpretation of the features. The recess  11  has a torque shoulder  13  which in use engages a corresponding surface on the pin on make-up of the joint. Although this engagement is defined in many prior art documents to be a sealing engagement, this is not necessary or desirable in the present invention. As can be seen from  FIG. 1 , the torque shoulder  13  defines an angle of 15° with respect to the plane perpendicular to the longitudinal axis of the pipe. It will be recognised by the skilled person that this angle can be varied within limits, without detracting from the performance of the main seal. A curved surface  14  of radius 0.02″ (0.0508 cm) joins the torque shoulder  13  to a further tapered surface  15 . The surface  15 , in three dimensions is conical, but in the cross-sectional representation of  FIG. 1  appears as a straight line. The surface  15  is at an angle of 7.12° (tan −1  (⅛)) to the longitudinal axis of the pipe. 
     At point A, the cross-sectional profile of the box  10  becomes curved between point A and point B. Along this section the curved surface  16 , between A and B, describes the arc of the circumference of a circle whose centre lies in the bulk material of the pipe  10 . The radius of the circle in the exemplified figure is 0.6 cm. 
     Beyond the point B, a further conical surface  17  at an angle of 65° to the plane perpendicular to the longitudinal axis of the pipe  10 , joins the curved surface  16  to the final crest of the threaded portion of the pipe  10 . 
     The pin  20 , has a configuration which is in some aspects complementary to that of the recess  11  in the box  10 . The pin  20  has a torque shoulder  21  at an angle of 15° to the plane perpendicular to the longitudinal axis of the pin  20 . The torque shoulder  21  is joined by a curved surface  22  of radius 0.0415″ (0.1054 cm) to a conical surface  23  which has 0° taper with respect to the longitudinal axis of the pin  20 . 
     At point C, the cross-sectional profile of the pin  20  becomes curved between points C and D, with the curved surface  24  lying on the arc of the circumference of a circle of radius 0.6 cm. Beyond point D, a further conical surface  25  links the curved surface  24  to the final crest of the threaded portion  26  of the pin  20 . The notch  27  shown in  FIG. 2  is simply the base of the thread portion  26 . 
     The pin  20  has a chamfered surface  28 , said chamfered surface  28  having a straight cross-section as shown and at an angle of 10° to the longitudinal axis of the pin  20 . It will be appreciated by the skilled person that said surface  28  can also be concave (when viewed from the internal volume of the pin), without departing from the scope of the invention. The relationship of the portions of the pin and box shown in  FIGS. 1 and 2  to the rest of the respective pipe are shown in ellipses in  FIGS. 3 and 4 . 
     In use therefore, on make-up of the joint, the pin and box section are brought together through engagement of the threaded sections of the pin and the box, by the application of a torque. Torque is applied until a seal is formed between the two sections. This situation is usually recognised by monitoring the magnitude of torque applied and a typical graph is shown in  FIG. 5 . At the point of engagement of the torque shoulders  13 ,  21  and formation of the seal, the torque rises discontinuously from a formerly steadily rising value. The primary seal is formed between the two curved surfaces  16 ,  24 . 
     The advantage of using two curved surfaces to provide a seal which is capable of maintaining integrity under tensive or compressive forces, including bending of the joint can now be seen. If such forces act, the curved surfaces ‘roll’ against each other, and although the sealing point may move, its integrity remains intact. In addition, it is recognised that the end of the pin has a tendency, due to the forces exerted thereon to bend inwardly. If this process goes too far then the deformation can become so acute that on subsequent make-ups, galling can occur as the end of the pin cuts into the box section. 
     A further feature of the invention relates to the positioning and aspect of the curved sealing surfaces on the pin and the box. This is illustrated in  FIGS. 6 and 7 . Dealing firstly with the curved surface  16  on the box in  FIG. 6 , the location of the centre of the circle on whose circumference the circle lies is defined with respect to an imaginary point P within the bulk metal of the box  10 . The point P is obtained as the intersection of a line drawn extending the torque shoulder  13  and a line drawn extending the flat section  15  (the extensions being shown as broken lines in  FIG. 6 ). Once this point is obtained, the centre of the circle then lies on the line R, perpendicular to the axis of the box which has a minimum distance of 0.2″ (0.508 cm) from the intersection point P, in the direction of the threaded portion. This, line R is shown dotted in  FIG. 6 . The radial location of the centre of the circle on this line is governed by the circumference of the circle drawn therefrom to ensure that the curve meets the surface  17  smoothly at B; i.e. the surface is tangential to the circle at that meeting point. It has been found that the value of the minimum distance of R from P can be from 0.15″-0.25″ (0.380-0.635 cm). For example for a 4½″ (11.43 cm) box section a distance of 0.193″ (0.4902 cm) has been found to be suitable and for a 7″ (17.78 cm) box section, 0.2002″ (0.5085 cm). 
     The location of the centre of the circle which is used in respect of the curved surface  24  on the pin  20 , is similarly defined. Firstly an imaginary point Q, the intersection of a line drawn extending the torque shoulder  21  and a line drawn extending the flat section  23  is found. The centre of the circle then lies on the line S perpendicular to the axis of the box which has a minimum distance of 0.35″ (0.889 cm) from the intersection point Q in the direction from Q towards the threaded portion. It has been found that the value of the minimum distance of S from Q can be from 0.30″-0.40″ (0.7620-1.016 cm). For example for a 4½″ (11.43 cm) pin section a distance of 0.33″ (0.8382 cm) has been found to be suitable and for a 7″ (17.78 cm) pin section, 0.3298″ (0.8377 cm). 
     The radii of the circles formed need to be carefully chosen to ensure good seal integrity and also that the seal remains intact under the forces acting thereon. A further feature which is important is the angle made between the lines joining the initial seal contact point, and the centre of a circle, and the line perpendicular to the longitudinal axis of the pipe on which the centre of the circle lies. 
     The curvature of the sealing surface on the pin is preferably less than that of the corresponding surface on the box. For the pin a suitable radius is selected to be in the range of 0.25″-0.35″ (0.63-0.89 cm) and the above-defined angle to be from 7-9°. In respect of the box, the radius selected is from 0.05-0.07″ (0.12-0.18 cm) and the angle from 10-12°. It will be appreciated therefore that the radii of the circles are set on the pin and box to be different to each other. 
     It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention.