THREADED JOINT WITH ENERGIZABLE SEAL FIELD OF THE INVENTION

The present invention relates to threaded joints, in particular for connecting tubes of predefined length 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 bottom. Prospecting for and exploitation of hydrocarbon fields makes it necessary to use hardware which is more resistant to environmental challenges such as higher loads and corrosion, which were less important in the past. 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 in specific situations, resilient seals are needed instead of or in combination with metallic seals, to prevent penetration of external fluids in 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 the fluids already penetrated with the surrounding fluids, in order to reduce corrosion velocity. To solve the mentioned problems various solutions have been provided, one of which consists of inserting a seal ring made of plastic material, e.g. an O-ring, at the extremity of the joint externally to the threaded portion. It is a widespread practice to use O-rings pre-mounted onto the box, but this practice has two disadvantages: the seal rings can be damaged during mounting of the ring and/or during make-up, and O-rings of simple cross-section cannot offer redundant sealing functions.

Seal rings with more sophisticated shape have also been proposed to increase efficiency and prevent external fluids from penetrating into the gaps of the joint. However such seal rings are subject to large stresses and adverse environmental

conditions, both during the initial make-up operation and during operational life, such that their efficiency is much reduced after some time. Another drawback of such elastic seal rings consists of the damages caused during the make-up operation because operators in the field can cause shocks between the tubes. The seal ring can thus be torn or suffer permanent deformations which later can jeopardize the sealing efficiency during operational life.

Document WO2004053376 discloses a threaded connection for connecting pipes for hydrocarbon wells with tapered male and female threads; an annular housing is provided in the vicinity of the free end of the female element, which housing receives a portion of a deformable sealing ring, which is radially compressed, while a retaining lip of the ring is immobilized in a further annular housing of the female element by a rib on the male element. However such lip is a weak and fragile element due to its small cross-section, its shape requires manufacture with a material having high slidability. It may easily break leaving the seal ring unretained and free to move axially. Moreover the seal ring is made of a material with low friction coefficient, reducing the available materials range that can be used. Such seal ring can be considered as a seal having multiple sealing mechanisms working as a whole. Another disadvantage of this solution is that there is a need to machine customised annular ribs and/or grooves on pin and box to position the seal ring which entails expensive and lengthy operations. Making the ribs and/or grooves requires high precision machining to determine the correct relative axial position with respect to the nose of the box after make up. Moreover the seal ring is prone to twisting during make up, which makes operation difficult. SUMMARY OF THE INVENTION

It is therefore a main object of the present invention to provide a threaded joint which overcomes the aforementioned drawbacks containing an innovative seal ring ensuring both a simple and secure initial make-up operation and a high sealing efficiency during operation.

The above mentioned objects are achieved in accordance with the present invention by means of a threaded joint defining an axis comprising a male threaded tube, defined as pin, and a female threaded tube, defined as the box,

and a sealing ring, the pin being adapted to be made up in the box, the sealing ring having a first base, a second face axially opposite to the first base, at least two protruding annular lips placed at an angle to the axis on the second face, an external cylindrical surface with at least two annular ribs, a reinforcement ring embedded at least partly in the sealing ring, the sealing ring being interposed between the pin and the box, placed in a portion of the surface of the pin after the end of the threaded section of the pin, in tight sealing contact with an internal surface of an annular groove of the box by means of the annular ribs and in tight sealing contact with an external surface of the pin by means of an internal surface. The innovative combination features of the invention that contribute to the improvements of the threaded joint are thus the V-sectioned mouth which ensures a better contact of the ring end with the surfaces of pin and box when pressure from the external fluid is exerted on that end. The external protruding ribs ensure a better specific pressure on the surfaces of pin and box to which they adhere. The sealing inner surface ensures a good positioning and contact in the ring housing made on the pin external surface. Advantageously the provision of a reinforcement ring ensures that the seal ring holds tight on the pin. The seal ring of the invention is advantageously pre-mounted onto the pin, thus avoiding damage of the seal ring during make-up. Besides, the seal ring boasts additional overlapped or redundant sealing functions, which secure tightness to the whole joint, even when one of the functions is singularly overcome by the external fluid. After completion of make up, the seal ring is retained in an annular housing by the inner surface of the box and the outer surface of the pin which can also be provided with a finishing and a coating layer. The housing has a geometry especially designed to co-operate together with the seal. The coating also protects the machined surface of both pin and box.

The joint of the invention does not require additional machining operations apart from the common threading operations. The provision of a reinforcement ring, preferably embedded in the sealing ring, prevents twisting of the ring, makes make up operation easier, and assures globally an easier handling and make up of the seal ring itself.

By virtue of the protruding ribs present on the external surface of the sealing ring in the presence of a wrapping force equal to that commonly used for state of the

art rings, there is produced a concentration of the stresses and higher localized contact pressures caused by the compression of the sealing ring between pin and box, which enhances sealability of the joint .

The provision of a coating on the seal ring contact surfaces ensures the necessary slidability for the housing, the seal ring being able to easily wrap, squeeze or move.

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: Figure 1 shows a section view on a longitudinal axial plane of the joint in accordance with the present invention;

Figure 1 a shows a section view on a longitudinal axial plane of another embodiment of the joint in accordance with the present invention;

Figure 2a shows an enlarged section view of an embodiment of a particular of the joint of fig.1 ;

Figure 2b shows an enlarged section view of another embodiment of a particular of the joint of fig.1 ;

Figure 3 shows an enlarged section view of another embodiment of a particular of the joint of fig.1 ; Figure 4a, b, c show other embodiments of a particular of the joint in accordance with the present invention.

DETAILED DESCRIPTION

With particular reference to the figures, there is shown a threaded joint indicated globally with reference numeral 1 , connecting two tubes, a male tube, or pin 3, with an external nominal diameter D, and a female tube or box 2 of external diameter D1.

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 thread.

The common axis of the tube, pin and box is indicated with A. The portion of the box 2 close to its nose 6 comprises a non-threaded surface 7.

Advantageously but not necessarily, a housing in the shape of an annular groove

10 is made on the internal surface of the box 2 comprised between the end of the thread 4 and the nose 6. This housing houses a sealing ring 1 1. In this embodi-

ment shown in fig. 1 , the housing 10 made in the internal surface of box 2 has preferably a cylindrical surface.

The sealing ring 1 1 has a main body on which there are externally one or more annular ribs 18 separated by annular grooves 17. At the extremity which is closer to the threaded portions 4, 5 the sealing ring 1 1 has a base 21 substantially, but not exclusively, flat.

Close to this base 21 there is provided a reinforcement ring 14, of another material which has greater stiffness, preferably, but not exclusively metal such as aluminium, copper, silver, nickel, steel, or stainless steel. Some reinforcement rings 14 are plated or coated with cadmium, copper, nickel, silver, gold, or indium. Others are heat-treated, age-hardened or annealed.

The reinforcement ring 14 can be completely embedded in the sealing ring 1 1 or can have one of its surfaces exposed to the exterior of the sealing ring 1 1 , either flush to the base 21 or protruding. The reinforcement ring can advantageously have various cross-section shapes, e.g. circular 14', square 14", rectangular or polygonal or T-shaped 14'", as shown by way of example in figures 4a, 4b, 4c. Preferably the radial thickness of the reinforcement ring 14 must always be smaller than the radial thickness of the elastomeric seal ring 1 1 , in order to allow deformation of the elastomer, when compressed in the space between pin and box.

At its longitudinally opposite end the sealing ring 1 1 has two annular lips 15, 16 which respectively protrude beyond the perimeter of the ring 1 1 in respective skewed directions with respect to the axis A, in unloaded position. In the space defined by the two annular lips 15 and 16 an annular groove is defined, which in axial section appears as a V-shaped mouth 12.

In a first embodiment the internal surface of the sealing ring 1 1 is shown in figure 1 , where this surface is ondulated in an axial direction and comprises a plurality of annular ribs 13 separated by annular grooves 20, when seen in section along an axial plane. In another embodiment the internal surface 19 of the seal ring is without abrupt variations. In this case it can have a plane cylindrical shape, as shown in figure 2 or alternatively a rounded or toroidal shape or a frusto-conical shape. In this embodiment the angle of conicity beta of the surface 19 with reference to the axis A

is about 3°. This facilitates mounting of the ring on the pin.

In the embodiments of figure 2a and 2b, the internal surface 19 is separated by a groove 19' of appropriate radius from the annular lip 15.

By virtue of the reinforcement ring 14, all embodiments of the seal ring 1 1 exert a wrapping force, which presses its internal surface against the external pin surface, thus enhancing tightness between seal ring 1 1 and the external surface of the pin. This wrapping stress exerted by the seal ring 1 1 is further enhanced by the small taper of the surface of the pin in the region designed for the ring after make up is complete. The surface 26 of the pin 3, on which rests the sealing ring 1 1 in its mounted position, is inclined to the axis A by an angle alpha smaller than 15°, preferably 3°. This has the function of preventing the seal ring 1 1 from sliding away from its position during operation by opposing an increased gripping force. In this case, the seal ring 1 1 is kept in place by both frictional forces and an increasing hoop stress due to the conical pin surface 26. With particular reference to the embodiment of the threaded joint of fig. 1 a, alternatively the portion of surface 24 on which the sealing ring 1 1 rests at the completion of make up can be made cylindrical. In this particular embodiment there is advantageously provided a frusto-conical surface 25 to facilitate sliding of the sealing ring 1 1 from the initial positioning surface 27 close to the threaded portion 5 to- ward the final position on surface 24 along the pin 3 during make up of the box 2 on the pin 3.

In this embodiment, the pin provides the frusto-conical surface 25 with a conicity lower than 15°, and is intercalated between two cylindrical surfaces 24 and 27 . In this case, the cylindrical surface 24 is the sealing ring housing after make up and the seal ring is kept in position on the surface 24 by the axial friction forces produced by a constant hoop stress or wrapping force.

Advantageously, this internal surface of the sealing ring can also be a combination of a portion having a cylindrical part 19 and of a portion with one or more protruding ribs 13, intercalated by annular grooves 20, to improve sealing effectiveness, as is shown in the embodiment of figure 3.

The sealing ring 1 1 can be made of various elastomeric materials, e.g.: Acryloni- trile-Butadiene (NBR), Carboxylated Nitrile (XNBR), Ethylene Acrylate (AEM), Ethylene Propylene Rubber (EPM, EPDM), Butyl Rubber (NR), Butadiene Rubber

(BR), Chlorobutyl Rubber (CIIR), Chloroprene Rubber (CR), Chlorosulfonated Polyethylene (CSM), Epichlorohydrin (CO, ECO), Fluorocarbon (FKM), Fluorosili- cone (FVMQ), Hydrogenated Nitrile (HNBR), Perfluoroelastomer (FFKM), Polyac- rylate (ACM), Polyurethane (AU, EU), Silicone Rubber (Q, MQ, VMQ, PVMQ), Sty- rene-Butadiene (SBR), Tetrafluoroethylene-Propylene (AFLAS®).

The joint 1 defines an inner space or duct 9, containing the axis A of pin 3 and box 2, in which a fluid, for example a hydrocarbon, such as natural gas or petroleum or other similar fluid, flows and an outer space 8 which can be in contact with fluids of various kinds such as sea water in specific applications. The sealing ring 1 1 offers an additional sealing capacity to the joint 1 which prevents external fluids present outside the joint from leaking into the interstices 23 of the threads of the joint.

The external nominal diameter D of the tubes, in the area distant from the joint between pin 3 and box 2 can be smaller than the external diameter D1 of the end portion of the box 2 in the case of a sleeve joint or can be equal in the event of an integral "flush" type joint.

As mentioned above, this seal ring 1 1 boasts more than one sealing function. One of the functions is performed by the annular groove 12, which opens out in radial direction under the action of the external pressure, constituting a self-energizable seal, by reacting proportionally to such pressure, increasing the adherence of the groove's internal and external lips 15, 16 to the surfaces of respectively pin 3 and box 2.

Yet another sealing function is performed by the external annular ribs 18 protruding radially, which behave as multiple O-rings, ensuring tightness between seal ring and the interior surface of the box 2.

The method of mounting the joint with the sealing ring 1 1 in accordance with the invention is as follows. Prior to the make-up of the connection, this seal ring 1 1 is pre-mounted onto the pin 3 over the threaded portion 5 and placed on the surface 27. Having the sealing ring in place avoids damage of the sealing ring 1 1 during make up, which is an advantage over most of the state of the art sealing rings, which are pre-mounted onto the box and are forced to drag across the threads of the pin. After the sealing ring 1 1 is mounted on the pin 3 the standard make-up operation is performed. When make-up comes close to the end, the seal ring is

slightly pushed by the abutment 28 of the box 2 on the base 21 to its final position. In this last part of the make up, the seal ring 1 1 is pushed against the final rest surface, either 24 or 26 depending on the embodiment of the joint, thereby gradually increasing adherence of the seal ring 1 1 on the pin 3. On the opposite side, the seal ring 1 1 enters in the housing 10 on the box, if this groove is present, which ensures that the ring does not slide in an axial direction under either internal or external pressures.

When the joint 1 has reached its final made up position, the sealing ring 1 1 assumes its final geometry and reaches its final design stresses and deformations. The ring 1 1 fulfils then the three sealing functions mentioned above and is energized to working levels after the make up operation is completed and the ring 1 1 is ready to take up further deformations under the fluid pressure. The seal ring 1 1 is an improved alternative to using the classic O-rings which can offer only one level of protection. It gives the threaded joint several advantages: - a capability to offer successive barriers to leaks, thanks to the multiple sealing functions provided by the seal ring 1 1 ;

- additional overlapped and redundant functions which maintain tightness even if one of them should fail, thanks to the combination of the self-energizable V- shaped annular groove 12, external protruding ribs 18, and the reinforcement ring 14 embedded completely inside or partially outside the seal ring 1 1.

In yet another advantageous embodiment of the invention the seal ring 1 1 can also have the internal surface 19 completely cylindrical, as shown in figure 2 or alternatively a part of the internal surface 19 can also have internal protruding ribs 13. The seal ring is preferably used on the outside of the joint between the internal surface of the box and the external surface of the pin. Such joints may also have an internal metal-to-metal seal near the nose of the pin.

Thanks to these features the joint of the invention can nearly completely block leaks, providing full tightness. The invention is to be used preferably in the field of OCTG and pipeline connections for the oil & gas industry, especially in offshore applications.