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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase of the PCT International Application No. PCT/EP2008/058073, filed Jun. 25, 2008 and published as WO 2009/000851 on Dec. 31, 2008, which claims priority to European Application No. 07111204.9, filed Jun. 27, 2007. The disclosure of both prior applications is incorporated by reference in their entirety and should be considered a part of this specification. 
     FIELD OF THE INVENTION 
     The present invention relates to threaded joints, in particular for connecting tubes to create strings used in the hydrocarbon industry especially for use in the field of OCTG (Oil Country Tubular Goods) and pipelines in offshore applications. 
     BACKGROUND OF THE INVENTION 
     Searching for oil or, more generally, hydrocarbons has become more demanding in terms of hardware and devices in recent years because oil and gas fields or reservoirs are located deeper or in places difficult to reach and below the sea. Prospecting for and exploitation of hydrocarbon fields demands hardware which is more resistant to environmental challenges such as higher loads and corrosion, which were less important previously. 
     In some applications threaded joints can be subject to deformation of the joint seals. 
     Modern joints are generally designed with metal to metal seals made by the contact between two surfaces, usually at one end or at both ends of the threaded portion of the joint, interfering in the elastic range of the modulus of elasticity for an appropriate stress magnitude. However there are situations where resilient seals are needed instead of or in combination with metal seals, to prevent penetration of external fluids into the interstices of the threads. 
     It is therefore a design requirement that the joint seals resist penetration of external or internal fluids, or at least do not allow continuous exchange of fluids that have already penetrated the joint with surrounding fluids, in order to reduce the corrosion rate. 
     Currently a widespread technical solution to the problem of externally sealing a threaded connection is to use O-rings or resilient seal rings of various cross-sections, made of elastomeric or composite materials. 
     Complex resilient seal rings and simple O-rings perform their sealing function based on the diametrical geometric interference between pin and box, which is predefined with respect to at least one of the joint members. Said geometric interference appears after make up of the connection, to elastically deform the seal ring and thus induce contact pressures between the seal and each of the pin and box, defining a mechanical barrier which seals the joint. An additional energization of the seal ring is provided by the external fluid pressure which increases deformation and adherence to the seat where the seal ring is housed. 
     An example of an O-ring is disclosed in U.S. Pat. No. 6,173,968 for sealing a joint between a pin and box. An O-ring abuts an annular backup ring of substantially the same diameter. The annular backup ring is split to permit radial expansion and has a greater thickness on its outer periphery than on its inner periphery. When the joint being sealed is under high pressure, the seal ring urges the backup ring to expand radially to cover any gap between the members being sealed, maintaining sealing capacity, even under high temperature conditions, and preventing the sealing ring from extruding into the gap. The pressure varies with sea depth and seal efficiency is reduced when lower pressures act on the O-ring. 
     In this document the external pressure on the joint determines also the pressure acting on the O-ring. When higher contact pressures are needed for the O-ring, then higher geometric interference is required between the O-ring and joint members. This might cause seal breakage. 
     Another way to improve efficiency of the sealing capacity of the O-ring is by increasing geometric interference, which is achieved in most cases by making the seal ring radially bigger than its housing. However, the bigger the seal ring, the more exposed is it to damage during make up, especially when it is pre-mounted in the box member and it is forced to overcome the entire pin threaded area. 
     In this case other drawbacks may arise. Several geometric connection variables that originate during the manufacturing process, such as ovality, eccentricity, and rugosity, introduce uneven interference over the whole circumference of the sealing surfaces, thus producing uneven contact pressures and reducing the sealing capacity. 
     In practice, sealing capacity due to geometric interference is limited by the geometry, mainly radial sizes and length, of resilient elements in relation to their ability to be dragged during make up across the threads and any other interfering surface without being damaged. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a threaded joint which overcomes the aforementioned drawbacks. 
     The principal object of the present invention is to provide a joint having an innovative seal ring ensuring both simple and reliable initial make-up and a high sealing efficiency during operation. 
     The above mentioned objects are achieved in accordance with the present invention by means of a threaded tube joint defining an axis comprising a male threaded tube, defined as a pin, and a female threaded tube, defined as a box, the pin being adapted to be made up in the box. A seal ring has an external cylindrical surface, an internal surface comprising one or more annular ribs protruding from the internal surface, a first base with at least a portion of a frustoconical shape, a second base with at least a portion of a frustoconical shape axially opposite to the first base. The first and second bases are slanted towards the axis whereby the external surface is wider than the internal surface. The seal ring is interposed between the pin and the box in tight sealing contact with an internal surface of an annular groove of the box by the external cylindrical surface and in tight sealing contact with an external surface of the pin by the annular ribs. Hydraulic actuating means can pressurize the seal ring against said pin to provide improved scalability of the joint. 
     In the threaded joint of the invention the seal ring is housed within the box member, and is actuated and energized by means of the pressurization of an external injected fluid, injection and pressurization of this fluid being effected via a non-return valve, fixed to the box member. 
     The housing for the seal ring is an annular groove formed in the box member of the threaded joint. It houses and protects the seal ring against shocks before it is activated by the pressurizing fluid. This housing is configured to provide a fluid tight cavity embracing the seal ring and allowing its energization. 
     The box of the threaded joint incorporating this seal is made in such a way that the seal ring actuates on a cylindrical surface, advantageously that of the tube body, and thus the pin does not need to be modified. In this manner the joint&#39;s performance under tension loads remains intact. 
     Both the annular groove, non-return valve and seal ring are completely contained within the box wall thickness, this having the advantage of preserving coupling design and performance as well as protecting the sealing system. Full design via FEA and full scale testing yielded optimized interaction of the seal ring with the annular groove, and the threaded joint of the invention can withstand high axial loads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects will become more readily apparent by referring to the following detailed description and the appended drawings in which: 
         FIG. 1  shows a perspective view of a partial section of the joint in accordance with the present invention, 
         FIG. 2  shows a section view along an axial plane of an embodiment of the joint in accordance with the invention, 
         FIG. 2   a  shows a section view along an axial plane of a detail of the box of the joint in accordance with the invention, 
         FIG. 3  shows a section view along an axial plane of an embodiment of an element of the joint in accordance with the invention, 
         FIG. 4  shows a section view along an axial plane of another embodiment of an element of the joint in accordance with the invention, 
         FIG. 5  shows a section view along an axial plane of another embodiment of an element of the joint in accordance with the invention, 
         FIG. 6  shows a section view of an enlarged particular of the joint according to the invention in a first working stage, 
         FIG. 7  shows a section view of an enlarged particular of the joint according to the invention in a second working stage, 
         FIG. 8  shows a section view along an axial plane of another embodiment of the box of the joint in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With particular reference to the figures, there is shown a threaded joint indicated overall by reference numeral  1 , connecting two tubes, a male tube  3 , also called a pin, with a nominal external diameter D, and a female pipe  2 , also called a box, of external diameter D 1 . 
     The pin  3  has a threaded portion  5 , with male threads of appropriate profile, e.g. trapezoidal, and the box  2  has an internal threaded portion  4  with female threads. The common axis of the pipe and the pin and box is indicated by A. 
     The box  2  ends with a nose  6 . The portion of the box  2  close to the nose  6  comprises a non-threaded surface  7 . 
     Preferably, but not necessarily, the joint  1  has an internal metal-to-metal seal between the pin nose  6 ′ of the pin  3  and box shoulder  8 . 
     With particular reference to the embodiment of  FIG. 1 , there is provided a housing configured as an annular groove  10  in the box  2  between the end of the thread  4  and the nose  6 . The groove  10  houses a ring  11  forming an external seal when the joint  1  is made up. 
     In some embodiments, the seal ring  11  performs the function of an external seal placed between the box  2  and the pin  3  or the tube body, such that it prevents leakage of external fluids into the threaded zone of the joint  1 . However, the seal ring  11  could be used in other parts of a threaded joint. 
     The seal ring  11  is actuated when an external fluid is injected and pressurized through a valve  20  into the housing  10 . This pressure is generated in a chamber  13  and acts onto a surface  25  of the ring, thus deforming and forcing the seal ring  11  both against the sides  14 ,  15 ,  16 ,  17  of the housing  10  and against the pin  3 , to produce a mechanical barrier which seals the joint  1 . 
     The groove or housing  10  holds, hides and protects the seal ring  11  before injection of the actuation fluid, corresponding to the position as shown in  FIG. 6 . Together with the seal ring  11  it constitutes a fluid tight annular pressure chamber when actuation fluid is injected and pressurized through the valve  20 . The valve  20  is a non-return valve and is fixed to the box  2 , the valve tip being advantageously flush with the box surface, to avoid damage to the valve or danger to operators. Injection of the actuation fluid and subsequent energization and activation of the sealing system is performed after the joint  1  is in its final made up position. 
     The seal ring  11  actuates against a cylindrical surface of the pin member  3  or on the tube body, depending on the position of the housing along the box  2 . This surface where the seal ring  11  contacts the pin  3  is either rolled or machined, and corresponds advantageously, but not exclusively, to the zone comprising the thread run out part. In this manner the pin  3  geometry is not modified, thus the tensional efficiency of the joint remains unaffected. 
     The actuating fluid may be either a high or low viscosity fluid, such as oil, grease, dope, gel, etc, or a polymer which solidifies after injection, or indeed the fluid present outside the connection, i.e. the same fluid the seal intends to prevent from leaking into the joint. 
     An advantage of the invention is that the seal ring  11  is protected, since it remains hidden during make up, and is prevented from undergoing any kind of damage, this objective being accomplished without affecting the joint performance. 
     Another advantage of the invention is the fact that contact pressures deployed by the sealing system are proportional to the pressure of the actuating fluid, these being well known and defined at the moment of injecting and pressurizing the actuating fluid. On the other hand, the magnitude of this actuation pressure is completely independent of the pressure generated by the fluid external to the joint. Furthermore, contact pressures produced by the seal ring  11  are also independent of geometrical parameters of the joint itself, such as ovality, eccentricity, rugosity, and type of connection, and are uniform over the whole circumference of the joint  1 , as geometry, shape, length of both seal and contact areas are variables independent of the joint type. 
     The internal surface of the seal ring  11  is configured with a plurality of protruding ribs  9 , thus adding sealability, as the adjacent ribs  9  offer multiple subsequent barriers against external fluid when the seal ring  11  is loaded. If one or more barriers leak, subsequent barriers can still withstand the external pressure and ensure sufficient sealing capability. 
     In a first embodiment of the threaded joint  1 , shown in  FIG. 2 , the annular groove geometry has a dovetail or wedge shape in axial section, i.e. its shape is such that the section width increases as the radial distance from the axis A increases, with two lateral surfaces  16 ,  17  inclined towards each other. 
     In this embodiment the groove also has advantageously two annular plane surfaces  14 ,  15  opposite and parallel to each other. This particular shape of the groove  10 , when the joint  1  is made up, forces the seal ring  11  to shrink and be energized when pressed by the actuating fluid in the annular chamber  13 . 
     The section of the seal ring  11  on an axial plane is configured to be housed in the groove  10  and has various alternative shapes, one of which is shown in  FIG. 5  complementary to that of the groove  10 . It has a first base  26  with a portion of a frustoconical shape, a second base  27  with a portion of a frustoconical shape axially opposite to the first base, first  26  and second  27  bases being slanted towards the axis A so that its external surface  25  is wider than the internal surface of the seal ring with the ribs  9 . 
     In an alternative embodiment of the groove  10 , shown in  FIG. 2   a , the groove  10  has a simple dovetail section and the associated seal ring  11  has preferably a shape like the ones shown in  FIGS. 3 and 4 . 
     In the embodiments of  FIGS. 3 and 5 , the seal ring  11  has also a portion  28 ,  29  of each first and second base which has the shape of an annulus. 
     These shapes of the seal ring  11  yield optimized contact surfaces and optimized seal ring deformation. 
     The groove  10  radial depth and the seal ring  11  thickness are defined in accordance with the box  2  wall thickness and taking into account the requirement of completely hiding the seal in the groove  10  when the chamber  13  is not pressurized. 
     The groove  10  has a width of about 15 mm and the seal ring  11  has a similar width, smaller, greater or equal to the groove&#39;s width, depending on the geometry of the groove, the shape and material of the seal ring  11 , the type and pressure of the actuating fluid. 
     The pressure in the chamber  13  can be set to a value between about 100 psi (6.89 bar) and 5000 psi (344.73 bar). This range gives a good seal ring deformation pattern, suitable for the expected range of operation conditions, both in the transient and in the steady state and optimal deformation/stress ratio at concentrating points, necessary to assess seal integrity. Another advantage of a pressure of such magnitude is a good contact pattern between seal and groove, necessary to assess fluid-tight cavity, and a good magnitude and contact pattern between seal and pin, necessary to assess joint sealing capacity. 
     Actuating fluids for pressurizing seal ring  11  may have either a high or low viscosity fluid, such as oil, grease, dope, gel, etc, or a polymer which solidifies after injection, or indeed the fluid present outside the connection, i.e. the same fluid the seal prevents from leaking into the joint. 
     An additional advantage of the threaded joint  1  is that its tensional efficiency can be improved by means of this sealing ring  11 . The threaded joint  1  allows, if required for structural reasons, the lengthening of the threaded zone or use of truncated or vanishing threads to be avoided, because the seal ring is mounted and protected in the groove  10  of the box  2  which hides and protects the seal ring  11  within the box  2 . The threaded joint  1  renders superfluous the making of a thread run-out zone (i.e. threads with truncated crests) at the extremities of the thread, when this is made to prevent damage of a seal ring during pre-mounting onto the pin, because the seal ring has to be dragged over the threads. One disadvantage of a thread run-out zone is that the worst stress condition is produced on the seal ring  11  in the last thread of the zone, which has a larger diameter than the nose of the pin  3  and is a reason why threads are truncated. Therefore, in such case valuable thread height is lost for protecting the seal ring  11 . 
     With particular reference to  FIG. 8 , there is shown another embodiment of a joint according to the invention having two seal rings  11 ,  11 ′ inserted in two annular grooves  10 ,  10 ′ formed in the box  2 . In this embodiment one seal ring  11  is positioned in the portion of the box  2  near its nose  6  which, after make up, faces the portion of the pin  3  after the threaded portion  5 . The second seal ring  11 ′ is positioned in the threadless portion of the box  2  which, after make up, faces the foremost portion of the pin  3  near its nose. 
     In this embodiment the seal ring  11 ′, like the seal ring  11 , can be formed according to one of the variants as described above. Alternatively, if appropriate for obtaining the best sealing results, the seal ring  11  and  11 ′ can be formed differently from each other: the first seal ring according to one variant among those described above and the second seal ring according to a different variant. 
     In some specific embodiments of the joint the two rings can be actuated with different pressures of the actuating fluid so that the pressure exerted by one seal ring is either greater or lower than the other. 
     Alternatively in another embodiment not shown in the figures the joint  1  can be used with the seal ring  11  alone in the position shown in  FIG. 8 , but without the seal ring  11 ′. This embodiment can be used for example, but not necessarily, in combination with a metal to metal seal located in the vicinity of the pin  3  nose. The joint of the invention in its various embodiments can almost completely prevent leaking, providing full tightness. 
     The invention is to be used preferably in the field of OCTG and line pipe connections for the oil &amp; gas industry, especially in offshore applications.

Summary:
A threaded joint comprises a pin coupled to a box with an annular groove, one or a pair of seal rings, and hydraulic actuating means to pressurize the seal ring or rings against the pin to provide sealability of the joint. The seal ring or rings comprise a frustoconical surface, an external surface, and an internal surface with one or more protruding annular ribs. In the unpressurized state, the seal ring or rings do not protrude from the groove. In the pressurized state, the frustoconical surface of the sealing ring is in sealing contact with an internal surface of the annular groove of the box, and the annular ribs of the sealing ring are in sealing contact with the external surface of the pin.