This invention relates to a pipe coupling and more specifically to a pipe coupling for connecting metal pipe sections of pipe strings and more particularly to a pipe coupling for connecting tubular members such as tendons, conductor and casing connections and risers and flowline pipelines together in the oil and gas industries.

A riser is typically a pipe that connects an offshore floating production structure or a drilling rig to a sub-sea system either for production purposes such as drilling, production, injection and export, or for drilling, completion or workover purposes.

A flowline is typically a surface pipeline which is connected to the wellhead and carries total produced fluids such as oil, gas and production water from the well to the first piece of production equipment, such as a production separator for example.

There are various known methods for joining risers and flowline pipes together including welded forged connections and threaded and coupled connections.

In the former, typically pin and box connectors are machined out of forgings and then welded to either end of the riser and the flowline. The connectors are then assembled to attach the pipe and flowline together.

In the latter case, threaded male pin connectors are machined directly onto the ends of the riser and flowline and a coupling consisting of a back to back female box connector with cooperating threads. The threaded male pin connectors of the riser and flowline are connected to opposite ends of the female box connector to attach the riser and flowline together.

Threaded and coupled techniques offer a number of advantages over welded forged connections. No welding is required and therefore it can be a lower cost method for joining flowline and risers together. Also, the absence of welding makes it easier to meet the requirements for sour service which typically relates to well fluids which contain a significant amount of hydrogen sulphide when using higher strength steels. Hydrogen sulphide is hazardous to human health and could significantly impact on the performance of high strength steel drilling tubulars and pipes and therefore lower strength steels are typically used for these operations which thereby increase the overall weight of the system. Typical examples of high strength steel which may be suitable for sour service may be steel with a yield strength of between 85,000 and 110,000 psi.

However, threaded and coupled connections are not universally adapted for all connection profiles as some require a greater thickness of the pipe required for the threaded pin component in order to accommodate the external metal to metal seal and abutment face which is necessary for assembly, breakout, preload and sealing purposes.

So called MERLIN type pipe connectors which were invented by the applicant are known in the art for connecting pipes together and are disclosed for example in GB1573945, GB2033518, GB2099529. GB2113334 and GB2138089. An example of a known MERLIN connector is shown in Figure 1. The connection is formed by a tubular pin member 1 having a frustoconical outer peripheral surface 2 and a tubular box member 3 having a generally frustoconical inner peripheral surface 4 corresponding to the frustoconical outer peripheral surface of the pin member. In use, the two members each associated with a pipe section, are telescoped together and are axially locked together by mating projections and grooves forming teeth 5 provided on the said peripheral surfaces, the projections and grooves being spaced apart along the two surfaces.

The metal to metal seal between the pin member and the box member is provided by machining a nib or projection 6 into the end of the pin member and a corresponding groove 7 into the base of the box member. The connector is made up by stabbing the pin 1 into the box 2 until the teeth 5 begin to mesh, then injecting hydraulic pressure into the connection which expands the box 2 and/or contracts the pin 1 to allow the pin to be fully pushed into the box at which point the teeth 5 are fully engaged. Upon release of the pressure, the box shrinks around the pin. The wedging action of the teeth 5 converts radial preload into an extremely high axial preload that maintains connector stiffness and enhances axial and bending fatigue characteristics. Pressurised hydraulic fluid is also used to disengage the members by expanding the box and/or contracting the pin member to bring the annular teeth out of engagement with the annular grooves on the corresponding frustoconical peripheral surfaces which allows the pin member to be pulled from the box member.

Such a connection allows for the connector to be formed from relatively thin steel forgings which provides substantive savings in material costs.

However, such a connection cannot typically be adapted for use in a threaded and coupled pipe to pipe connection because as noted above as the pin component requires more than the thickness of the pipe to accommodate the external metal to metal seal and abutment face which is necessary for assembly, breakout, preload and sealing purposes.

The present invention aims to provide a pipe coupling which overcomes or at least mitigates the above problems with known couplings.

Statement of Invention

According to one aspect of the present invention there is provided a pipe coupling comprising a tubular pin member having an outer peripheral surface and a tubular box member having an inner peripheral surface corresponding to the outer peripheral surface of the pin member and which overlies the outer peripheral surface of the pin member when the members are fully engaged together, the members being provided with inter engaging projections and grooves on said peripheral surfaces for axially locking the members together when they are fully engaged together, a first metal to metal seal being provided at the end of the peripheral surfaces proximate the free end of the pin member and a second metal to metal seal being provided at the end of the peripheral surfaces proximate the free end of the box member when the members are fully engaged together, the box member comprising a chamber extending from the end of the peripheral surfaces adjacent the second metal to metal seal into the free end of the box member behind the second metal to metal seal.

Advantageously the chamber comprises a tapered channel. The provision of the chamber within the free end of the box member provides a hook-like termination at the free end of the box member.

Preferably the outer peripheral surface of the pin member is generally frustoconical. Preferably also the inner peripheral surface of the box member is generally frustoconical.

Alternatively, the outer peripheral surface of the pin member and the inner peripheral surface of the box member may be generally straight.

Preferably, the projections and grooves on the peripheral surfaces of the pin and box members are substantially annular.

Advantageously the substantially annular projections and grooves are spaced apart along the surfaces.

Conveniently the substantially annular projections and grooves are equally spaced apart along the surfaces.

Alternatively, the projections and grooves on the peripheral surfaces of the pin and box members are helical.

Conveniently a projection or nib may extend from the free end of the pin member adjacent the first metal to metal seal.

Preferably the nib extends around the free end of the pin member.

Advantageously a groove is provided in the box member to receive the nib of the pin member when the pin and box members are fully engaged together.

Conveniently the groove is annular and extends around the inner surface of the box member. In some embodiments, additional sealing may be provided by one or more resilient members between the pin and box components.

Conveniently the one or more resilient members may comprise one or more O-rings.

Advantageously the frustoconical surfaces taper inwardly towards the free ends of the pin and box members.

Conveniently a seating point for an installation clamp may be provided on the box member of the coupling.

Advantageously the seating point comprises an annular groove extending around the outer surface of the box member.

Conveniently the box member comprises an inlet port for the injection of pressured fluid between the peripheral surface of the pin and box members.

According to a further aspect of the present invention there is provided a pipeline comprising a plurality of pipe sections connected together through couplings as described in accordance with the first aspect of the present invention.

Embodiments of the present invention will now be described with reference to and as shown in the accompanying drawings in which:

Figure 1 is a schematic cross sectional view of a prior art pipe coupling;

Figure 2 is a schematic cross sectional view of a pipe connection according to one aspect of the present invention,

Figure 3 is a schematic view of a clamp attached around the pipe coupling of figure 2 during assembly and disassembly, and

Figure 4 is an enlarged view of the pipe to clamp interface.

Turning now to Figure 2 there is depicted a pipe coupling according to a first embodiment of the present invention. In this embodiment a MERLIN type connection is adapted for a threaded and coupled pipe coupling. A first tubular or pipe 10 is connected to a second tubular or pipe 11 through a coupling 12. The first pipe has a tubular pin member 13 provided at the free end of the pipe 10. The pin member is preferably machined into the end of the pipe in accordance with a known procedure.

The outer peripheral surface 14 of the pin member is preferably frustoconical and tapers towards the free end of the pin member. In the illustrated embodiment annular projections 15 are provided along the frustoconical peripheral surface 14 of the pin member. The annular projections are spaced apart along the frustoconical surface and form annular teeth around the outer peripheral surface of the pin member separated by annular grooves. In other, non-illustrated embodiments, the projections may be helically formed on the outer peripheral surface of the pin member.

The pin member 13 has an axially aligned nib or projection 16 machined into the end of the pin member and which projects beyond the free end of the pin member. In the embodiment shown, the nib or projection extends from the outer surface of the pin member and extends around the outer end of the pin member.

The second pipe 11 has a box member 17 formed at the free end of the second pipe 18. The wall thickness of the box member 17 is greater than that of the pin member 13. As with the pin member, the box member profile may be machined into the end of the pipe in accordance with a known procedure.

The inner peripheral surface 19 of the box member is preferably frustoconical and tapers towards the free end of the box member. Annular projections 15 are also provided along the peripheral surface of the box member and form annular teeth around the inner peripheral surface of the box member spaced apart by annular grooves between the teeth. The teeth of the pin and box members are designed such that the projections on the peripheral surface of one of the members can be received within the annular grooves of the peripheral surface of the other of the members such that the peripheral surfaces are inter-engaged along the entire length of the surfaces. The number of teeth provided along the peripheral surface may vary but it is proposed that a number of teeth between 10 and 60 may be provided. In preferred embodiments, between 50 and 60 may be provided. A shoulder 20 is formed in the inner end of the box member where the box member joins the pipe 11 , remote from the free end 18 of the second pipe. At the inner edge of the shoulder an axial groove 21 extends into the body of the pipe 11 away from the box member 17. The width and depth of the groove is selected such that when the pin and box members are coupled together, the extending nib 16 of the pin member can be received within the axial groove 21 of the box member as will be described further below.

The inner profile of the box member tapers from the end of the peripheral surface 19 closest to the end of the pipe 18 towards the outer surface of the box member to form a region of reducing thickness of the box member. A tapered channel 22 is provided in the end of the box member within the free end of the pipe 18 and extends from the end of the region of reducing thickness. The tapering region of the inner profile of the box member and the tapered groove within the free end of the pipe 18 defines one side of a chamber 23 within the box member of the coupling as will be described further below.

As clearly seen in figure 2, the end of the box member adjacent the free end of the pipe 18 resembles an inwardly directed hook with the tapered channel 22 provided within the box member.

A groove 24 is provided in the outer surface of the box member 17, or alternatively on the second pipe 11 close to the proximity of the box member to accommodate a clamp for use in assisting the make-up of the coupling. The profile of the groove 24 will be determined by the profile of the clamp as will be well understood by a person of skill in the relevant field.

Due to the relationship between the hardness and the strength of steel used, the higher the strength of the steel, the less suitable the steel becomes for use in sour service. As the box member of the coupling must be thicker than the pin member, in order to allow engagement of the pin member within the box member, the box member is formed of a lower strength steel than the pin member for use in such a sour service environment. In the present invention, the pin and box members are formed of high strength steel. The minimum yield strength of the first pipe with the pin member machined therein is preferably around 110,000 psi and the minimum yield strength of the box member of the second pipe is preferably around 85,000 psi.

When it is required to couple together the first pipe 10 and the second pipe 11 , the pin member 13 on the free end of the first pipe 10 is stabbed into the box member 17 on the free end of the second pipe 11.

The outer peripheral surface 14 of the pin member 13 engages with the inner peripheral surface 19 of the box member 17 until the teeth 15 along the peripheral surfaces of the two components begin to mesh.

Pressurised hydraulic fluid (not shown) is then injected into the coupling through one or more inlet ports in the box member 17 (not shown). The fluid is injected between the peripheral surfaces 14,19 of the pin and box members which expands the box member 17 and or contracts the pin member 13 as with a standard MERLIN connector to allow the pin member 13 to be fully pushed into the box member 19.

A clamp 27 will typically be engaged around the outer surface of the box member 17 to assist in drawing the pin member 13 and box member 17 into full engagement. The clamp engages in groove 24. Since the pin member 13 is machined directly onto the end of the first pipe 10, a conventional clamp groove is not provided in the outer surface of the first pipe 10 as this would result in an unacceptable loss of wall thickness around the coupling 12. Figure 3 illustrates the application of a clamp 27 to assist in assembly and disassembly of the coupling. A plurality of annular grooves 28 are provided in the outer surface of the first pipe 10 behind the pin member 13. These grooves provide a suitable anchoring point on the first pipe 10 for the clamp 27 which has corresponding annular projections 29 which extend into the annular grooves on the outer surface of the first pipe 10 when the clamp is engaged.

Upon release of the pressure the box member 17 contracts around the pin 13 to the position shown in Figure 2 such that the annular teeth 15 of the pin member 13 are fully engaged in the annular grooves of the box member 13 and the annular teeth 15 of the box member 13 are fully engaged in the annular grooves of the pin member 13.

The wedging action of the teeth 15 converts radial preload of the coupling into an extremely high axial preload that maintains coupling stiffness and enhances axial and bending fatigue characteristics.

When the pin member 13 is fully engaged in the box member 17, the nib 16 extending from the free end of the pin member 13 is received within the inner axial groove 16 at the inner end of the shoulder 20 of the box member 17. This forms a first metal to metal seal 25 between the pin and box members at the inner end of the peripheral surfaces of the pin and box members of the coupling 12.

A second metal to metal seal 26 is provided between the pin and box members at the outer end of the peripheral surfaces 14, 19 of the pin and box members.

With the pin member 13 fully engaged in the box member 17 and the teeth 15 of the peripheral surfaces 14, 19 fully engaged in the cooperating grooves, the chamber 23 is sealed by the outer surface of the first pipe 10 out with the inner peripheral surface 14 of the pin member 13. between the pin and box members at the outer end of the peripheral surfaces.

Pressurised fluid injected into the coupling during make up fills the chamber 23 and flows into the tapered channel 22. As this tapered channel surrounds the second metal to metal seal at the outer end of the coupling 12, pressure is applied internally from the channel to discourage any deflection of the free end of the box member 17 away from the outer surface of the pipe 10 and force the end of the box against the outer surface of the first pipe 10 which enhances the outer metal to metal seal of the coupling.

During disassembly of the coupling 12, a seal must be maintained in order to allow sufficient build up on injection pressure to radially disengage the teeth 15 of the peripheral surfaces 14,19 of the pin and box members. As pressurised fluid fills the chamber 23 and forces the end of the box member further into contact with the outer surface of the first pipe 10, this means that the metal to metal seal between the components of the couple is maintained even during disassembly as the box member is prevented from deflecting excessively and losing contact with the outer surface of the pipe 10 in the region of the second or outer metal to metal seal.

Similarly, in the event of any loss or failure of the first or inner metal to metal seal 25 at the inner end of the coupling, for example due to damage to the extending nib 16 of the pin member 13, the pressurised fluid within the channel will act to force the hook shaped portion at the end of the box member 17 against the outer surface of the first pipe to maintain the outer metal to metal seal 26 of the coupling. Thus during service of the pipe, the coupling 12 provides an environmental seal between the pin and box components of the coupling.

An example of the present invention is further described below:

Connector specification

Machined pin on pipe and box

Box outer diameter (OD) 9.247 inches

Box stab length 11.25 inches

Pin/pipe minimum yield strength 110.000 psi

Box minimum yield strength 85,000 psi

Make/break injection pressure 20,000psi

Tension capacity 1 ,000,000lbs

Pressure rating 20,000 psi

Hydrostatic test pressure 30,000 psi

Number of fully formed teeth 56

Tooth engagement 0.0135 inches

This example was found to perform well under modelling conditions.

The coupling described in the application provides both an environmental seal against hydraulic pressure during service of the components coupled together but also provides enhanced operation during assembly and disassembly of connections formed using the coupling which provide significant advantages over known pipe to pipe couplings. The embodiments described above with reference to the drawings of the application refer to annular projections on the outer peripheral surface of the pin member and similar annular projections on the inner peripheral surface of the box member. However in some, non-illustrated embodiments, it is envisaged that the projections on each of the peripheral surfaces of the pin and box member may be helical.

In a further embodiment, the grooves at the extreme ends of the set of grooves 28 in the pin body for mounting the clamp 27 may have a different profile to the remainder of the grooves 28. For example, the outer grooves may have be wider that the central grooves and may have an elliptical rather than a semi-circular profile.