Abstract:
A pipe joint comprises a pin having a male screw-threaded portion and a box having a complementary screw-threaded portion. The threaded portions of the pin and box inter-engage along the greater part of the axial length of the threaded portions, the male thread extending to a male stop shoulder adjacent a complementary stop shoulder on the other portion. The pin further comprises a radial surface adjacent a corresponding radial surface on the complementary stop shoulder of the box, a curved sealing surface of the radial surface on the pin sealingly engaging a corresponding curved sealing surface on the corresponding radial surface on the complementary stop shoulder. Each of the curved sealing surfaces is so shaped to lie on an arc of a separate ellipse. The parameters of the ellipses are carefully chosen to maximize the strength and robustness of the seal.

Description:
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. 
     Additionally, 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 joint region of the pipe when torque is applied to form the joint between pipes 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 these resources 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. In addition, modern methods of drilling often utilise a single, generally vertical, downhole which is then diverted in the horizontal direction to pass through the oil or gas deposit. 
     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 opposite sides of the same region of the 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. 
     A further problem encountered in the production of oil pipes lies in the production methods used to produce pipe sections which are intended for different uses within the industry. These are typically required to be made having different characteristics depending on the use to which the sections are to be put: for example tubing, casing etc. The present invention contemplates a pipe joint in which the sealing region is common across a wide range of pipe diameters resulting in a lowering of costs of manufacture. 
     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,
 
the stop shoulder on the pin further comprising a radial surface adjacent a corresponding radial surface on the complementary stop shoulder of the box, a curved sealing surface of the radial surface on the pin sealingly engaging a corresponding curved sealing surface on the corresponding radial surface on the complementary stop shoulder, the curved sealing surfaces each being so shaped to lie on an arc of a separate ellipse.
 
     Preferably the long axis of the ellipse described by the curved sealing surface on the pin is at an angle of from 10.0-16.0° to the main axis of the pin and further preferably at an angle of from 11.0-14.0°. 
     The centre of the ellipse described by the curved sealing surface of the pin is preferably at a distance of from 0.2550-0.2800″ in a direction toward the main body of the pipe and parallel to the longitudinal axis of the pipe, the distance being measured from the intersection point of the line extending from the torque shoulder and the radial surface. 
     The centre of the ellipse described by the curved sealing surface on the box is preferably at a distance of from 0.2550-0.2800″ in a direction towards the torque shoulder of the box section and parallel to the longitudinal axis of the pipe, the distance measured from the intersection point of a line extending from the torque shoulder of the box section and from the radial surface. 
     Conveniently the centre of the ellipse described by the curved sealing surface of the pin is at a radial distance of from 0.0040-0.0065″ from a line through the intersection point and parallel to the longitudinal axis of the pin section. 
     Conveniently the centre of the ellipse described by the curved sealing surface of the box is at a radial distance of from 0.0165-0.0200″ from a line through the intersection point and parallel to the longitudinal axis of the box section. 
     Preferably, the long axis of the ellipse described by the curved sealing surface on the box is at an angle of from 5.0-8.0° to the main longitudinal axis of the box section and is further preferably at an angle of from 5.5-7.0°. 
     The long axis of the ellipse described by the curved sealing surface on the box section is preferably from 0.1420-0.1650″ in length. The length of the short axis of the ellipse is preferably from 0.0180-0.0205″ in length. 
     The long axis of the ellipse described by the curved sealing surface on the pin section is preferably from 0.1250-0.1550″ in length. The length of the short axis of the ellipse is preferably from 0.0230-0.0300″ in length. 
    
    
     
       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 pin seal; 
         FIG. 2  is a section of the longitudinal axis of a pipe section, showing the sealing region of a box seal; 
         FIG. 3  is the sealing region of the pin seal of  FIG. 1 , showing dimensional references; and, 
         FIG. 4  is the sealing region of the box seal of  FIG. 2 , showing dimensional references. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The sealing region of a joint formed between the pin and the box section of the joining pipe sections is a critical one and many features have been introduced to provide a good seal. Said seals need to not just withstand and remain fluid tight where there is a pressure differential across the pipe wall, but also often need to maintain their integrity when the pipe string is describing a curve of up to 90° or more. The present invention addresses the problem by the provision of curved sealing surfaces on both the pin and the box sections, with each curved surface being separately able to be described in terms of an ellipse. In the cross-sectional illustrations utilised in this description, the curved surfaces are represented as two dimensional curved lines. 
     Although curved surfaces are known within the art, it has been found that the use of two elliptical surfaces, and in particular elliptical surfaces as defined and delimited herein provide a means of obtaining a seal which can be applied to pipes having a wide range of diameters such that the pipes can be used as tubing or casing. 
     The elliptical surfaces as herein defined can be easily engineered by conventional methods known in the art. It is preferred that the sealing surfaces utilised will be subject to a pre-treatment to enhance the structural strength of the surface, particularly against galling on make-up of the joint. For example, the pin section is typically pre-treated with phosphoric acid or by peening (such as with Aluminium or glass). Preferably, treatment with a molybdenum strengthening agent is also carried out. 
     The box section can also be pre-treated with phosphoric acid and/or with the molybdenum agent. Peening, although in principle suitable for the surface would not typically be used due to the location of the sealing surface within the body of the pipe section. 
     In addition to the above, a lubricant of a type known in the art would usually be used on make-up of the joint to reduce galling. 
     With reference to  FIGS. 1 and 3 , which illustrate the sealing region of a pin section, the following features should be noted. The pin, generally referenced  10 , has an end torque surface  11  which in use engages a corresponding torque surface  21  on the box section. Although these two surfaces do produce on engagement, a seal, this seal is not a critical one between the two pipe sections. The torque surface  11  is joined to the planar radial surface  12  by a curved nose surface  13 . The sealing surface  14  provides, in conjunction with the corresponding surface  24  on the box section  20 , the main fluid-tight seal between adjoining pipe sections. The sealing surface  14  extends at one end from the surface  12  and is operatively connected to the first thread crest  15 . 
     As indicated above the sealing surface  14  lies on a portion of the curve of an ellipse. It is important that the position and orientation of the ellipse be well-defined as detailed below. 
     In defining the ellipse on the pin section, the first point of reference does not lie in the pin  10  itself but is a point defined as the intersection between a line extending from the torque surface  11  and the radial surface  12 . This is shown as an X in  FIG. 3 . The distance, d 1 , from the point X to the centre of the ellipse as measured in the direction parallel to the main longitudinal axis of the pipe and towards the main body of the pipe is 0.2696″ (0.6848 cm). A range of values for this parameter, which is suitable has been found to be from 0.2550 to 0.2800″ (0.6477-0.7112 cm). Radially, the centre of the ellipse is in the line parallel to the main axis of the pipe but displaced , d 2 , by 0.0053″ (0.0135 cm) radially outwards from that axis. Typically the displacement, d 2 , can be from 0.0040-0.0065″ (0.0102-0.0165 cm). The length, d 3 , of the long axis of the ellipse is 0.1356″ (0.3444 cm) although a length, d 3 , of from 0.1250-0.1550″ (0.3175-0.3937 cm) has been found to be suitable. The length, d 4 , of the short axis is 0.0257″ (0.06528 cm), although a length, d 4 , of from 0.0230-0.0300″ (0.0584-0.0762 cm) has been found to be suitable. 
     As is evident from the  FIGS. 1 and 3 , the long axis of the ellipse is, in order to produce the required surface, set at an angle , θ 1 , of 12° to the main axis of the pin section. However, a range for θ 1  of from 10.0-16.0° and further preferably from 11.0-14.0° has been found to be suitable. 
     As set out above, the sealing surface  14  follows a portion of the thus-defined ellipse. Towards the distal end of the pipe, the sealing surface  14  joins the surface  12  whilst at the proximal end it smoothly joins the surface  16  linking the crest  15  with the sealing surface  14 . 
     The corresponding sealing surface  24  on the box section  20  can be defined in similar fashion to that described for the pin  10  and is shown in  FIGS. 2 and 4 . The box section  20  has a recess, complementary in shape to the end of the pin section  10 , which recess is defined by the surfaces as follows: a torque surface  21  is of the same or similar orientation relative to the main axis of the box section  20  as the torque surface  11  and engages frictionally or sealingly with the torque surface  11  on make-up of the joint between the pin and box sections. The torque surface  21  is joined by curved surface  23  to the planar surface  22 . The sealing surface  24  then extends smoothly from the planar surface  22  and is operably connected to the first crest  25  of the thread on the box section  20 . 
     The sealing surface  24  again follows a portion of an ellipse which is defined as follows. Firstly, the surfaces  21  and  22  are extended to define a point which actually lies within the body of the box section  20 . This point is shown with an X in  FIG. 4 . The centre of the ellipse is then set to be at a distance, d 5 , as measured in the direction of the line parallel to the main longitudinal axis away from the main body of the box section  20  of 0.2652″ (0.6736 cm). It has been found that a suitable range of values for the distance d 5  is from 0.2550-0.2800″ (0.6477-0.7112 cm). Radially, the centre of the ellipse is on the line parallel to the main axis of the pipe but displaced, d 6 , by, 0.0178″ (0.0452 cm) radially outwards from that axis. Typically the displacement d 6  can be from 0.0165-0.0200″ (0.0149-0.0508 cm). 
     The length, d 7 , of the long axis of the ellipse is 0.1503″ (0.3818 cm), although a length d 7  of from 0.1420-0.1650″ (0.3607-0.4191 cm) has been found to be suitable. The length, d 8 , of the short axis is 0.0192″ (0.0488 cm), although a length d 8  of from 0.0180-0.0205″ (0.4572-0.5207 cm) has been found to be suitable. 
     As with the elliptical surface on the pin section, the long axis of the ellipse is set at an angle, θ 2 , to that of the main longitudinal axis of the pipe. In the case of the box section, the long axis of the ellipse is at an angle θ 2  of 6.195°, although a range of from 5.0-8.0° has been found to be suitable and a range of 5.5-7.0° particularly suitable. 
     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 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. 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. The present invention utilises two elliptically curved surfaces in which the curvature, the dimensions and the orientation of the ellipses is selected to provide an effective seal which is capable of retaining its integrity as the two pipes move or are subjected to unequal internal and external pressures. 
     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. 
     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.