Flexible Couplings for Tubular Members

Technical field

[0001] This invention relates to connectors for coupling two pipes together. In particular for the use of connecting lengths of casing together for use downhole in the oil and gas industry.

Background art

[0002] It is standard practice in the gas and oil industry to complete construction of a well by lowering a large diameter pipe known as the casing into an open hole and then cement it in place in order to protect the wellbore and reinforce the structure against collapse.

[0003] Casing is typically formed by connecting a number of tubular sections

(joints) together to form a long string. Most casing joints are fabricated with male threads at each end, and short-length tubular casing couplings with female threads are used to join the individual joints of casing together to form the string. Alternatively, joints of casing may be fabricated with male threads on one end and female threads on the other in a similar manner to drill pipe. Connector designs are explored in the Journal of Petroleum Technology Article on pages 74-75 of May 2006. Further examples of casing couplings can be found in US 5,263,748 and WO 2006/076194.

[0004] Guiding the casing string to the desired setting depth within the wellbore is not always achieved. This is particularly true with directional and deviated wells. The main reason for this is that the casing string is usually stiffer than the drill string used to drill the well and therefore cannot follow the well path created by the drill bit.

[0005] Observations on casing strings that have been recovered from wells has shown that it can be possible to distinguish between the two ends of the same coupling due to damage, when in fact it should have been symmetric. This indicates that the casing string is behaving not as a single catenary, but as a succession of mini-catenaries between the casing couplings. Thus the load on the casing coupling is not symmetrical which leads to the differing types or degrees of damage. [0006] Resonance vibration is not a common method of freeing stuck tubular strings, however, it has been noticed that after a while, the faulty couplings glow red which suggests that there is an abnormal stress concentration at these points.

[0007] With API couplings, it is well known in the art that making up the field end pin influences the behaviour of the mill end pipe. With semi-premium coupling connections this influence can still be detected, whereas with premium coupling connections where the pipes are well separated, there is no influence of the make up of one pipe on the other.

[0008] The premium coupling owes its extra stiffness, in relation to the adjoining pipes, to both the larger outside diameter (OD) and also to its extra thickness, which is approximately double that of the pipe's body. Premium coupling connections allow for matched strength connections where there are no weak members.

[0009] Previous evidence suggests that the manner in which the connection

between the casing joints and the coupling can have a big impact on the behaviour of the connection in bending due to the difference in stiffness between the casing joint and the coupling.

[0010] This invention aims to provide a coupling which avoids the problems

associated with this difference in stiffness.

Disclosure of the invention

[0011 ] This invention provides a connector for connecting threaded ends of

adjacent pipes, comprising a cylindrical body having threads formed at each end, the threads corresponding to the threads on the pipes to be connected;

wherein a circumferential region of an outer wall of the body has a reduced diameter so as to provide a region of reduced wall thickness between the threads such that the bending stiffness of the connector is substantially similar to that of the pipes to be connected.

[0012] In one embodiment of the invention, the threads on the connector are formed on the internal surfaces of the body for engagement with external threads on the ends of the pipes to be connected.

[0013] The circumferential region of the outer wall of the body which has a

reduced diameter can be an axially extended region. [0014] Alternatively, the circumferential region of the outer wall of the body comprises at least one groove.

[0015] In this case, the circumferential region of the outer wall of the body

comprises a number of grooves.

[0016] In one embodiment of the invention, the grooves are adjacent to one

another.

[0017] This invention also provides a method of manufacturing a connector for connecting threaded ends of adjacent pipes, comprising:

- forming threads at each end of a connector comprising a cylindrical body, the threads corresponding to those on the pipes to be connected; and

- reducing the diameter of a circumferential region of an outer wall of the body between the threads so as to provide a region of reduced thickness such that the bending stiffness of the connector is substantially similar to that of the pipes to be connected.

[0018] This invention also provides a method of modifying a connector for

connecting ends of adjacent pipes comprising a cylindrical body having threads formed at each end, the threads corresponding to the threads on the pipes to be connected, the method comprising reducing the diameter of a circumferential region of an outer wall of the body between the threads so as to provide a region of reduced thickness such that the bending stiffness of the connector is substantially similar to that of the pipes to be connected.

[0019] One embodiment comprises removing material from the circumferential region of the outer wall of the body so as to reduce its diameter.

[0020] An alternative embodiment comprises reducing the diameter by cold, warm or hot rolling.

Brief description of the drawings

[0021] Figure 1 shows a first embodiment of the invention;

Figure 2 shows a second embodiment of the invention;

Figure 3 shows further detail of the embodiment of Figure 1 when connected to the end of a casing joint;

Figures 4 and 5 show part-sectioned views of the embodiments of Figures

1 and 2 when connecting two casing joints; Figure 6 shows a third embodiment of the invention; and

Figure 7 shows the embodiment of Figure 6 in a bending mode.

Mode(s) for carrying out the invention

[0022] Figures 1 and 2 show side views of embodiments of a casing connector according to the invention showing the region of reduced wall thickness that is provided around the cylindrical body.

[0023] In the embodiment of Figure 1 , the connector 10 comprises a cylindrical body 12 having a region of reduced wall thickness 14. In this embodiment, the region 14 is extended axially for a length A centred on the mid-line 16 of the body 12. This results in the region 14 having the form of a shallow annular depression. In the case of Figure 2, the region of reduced wall thickness is in the form of a steep sided groove 18 having a limited axial extent B.

[0024] While Figures 1 and 2 show specific profiles for the region, the profile can be selected according to requirements. Likewise, the position of the region of reduced wall thickness is also not critical. However, maintaining symmetry of the region around the mid-line allows the symmetry of the connector to be maintained such that its orientation is not important in use.

[0025] Figure 3 shows further detail of the connector of Figure 1 with a casing joint 20 coupled to one end 22.

[0026] Threads 26 are formed inside each end of the connector body 12 for

engagement with threads 28 on the ends of the casing joints 20a, 20b. The threads 26 are shown in Figure 3 as being on the internal surface of the connector 10 (female, or box configuration) in order to complement the external threads 28 on the casing joint 20 (male or pin configuration).

These configurations can be reversed such that when the joints are internally threaded, the connector is externally threaded. Furthermore, the configuration of one end can be different to the other where different types of casing are to be connected. The thread profile can be one of any number of known profiles commonly used for connecting tubular in oil and gas operations.

[0027] The space 30 between the threads 26 means that the ends of the casing joint 20 are spaced apart when engaged in the connector. The region of reduced wall thickness corresponds with this space. [0028] Figure 4 shows the embodiment of Figures 1 and 3 with both casing joints 20a, 20b engaged. Figure 5 shows the corresponding view for the embodiment of Figure 2.

[0029] Figure 6 shows a third embodiment of a connector according to the

invention. In this embodiment, the region of reduced wall thickness comprises a series grooves 32 arranged about the mid-line of the connector.

[0030] Figure 7 shows deformation of the connector around the region of the reduced wall thickness. The flexibility provided by the groves 32 means that there is less deformation around the threaded end of the connector, reducing the likelihood of damage or failure. The addition of the region of reduced wall thickness reduces the stress concentrations in the threaded regions and minimises abnormal wear at the point of the coupling when a bending force is applied.

[0031] The region of reduced wall thickness reduces the bending stiffness of the connector compared to one with a constant wall thickness. Since known connectors typically have a higher bending stiffness than a casing joint, this makes it possible to reduce the bending stiffness of the connector to be closer to that of the casing joints. In this way, it may be possible to match the bending stiffness of the connector to the casing joints.

[0032] The region of reduced wall thickness can be formed in a number of

different ways. If the removal of material is to be avoided, warm rolling to the desired thickness can be used. Alternatively the metal can be removed at the desired position of the region of reduced wall thickness, for example by machining.

[0033] The connector according to the invention can be manufactured with the reduced wall thickness ab initio, or an existing connector can be modified by reducing the wall thickness. The exact technique used to reduce the wall thickness and the profile of the reduced area can be selected from a number of calibrated options to give a predetermined bending stiffness according to the stiffness of the casing joints to be connected.

[0034] It is understood that other changes can be made whilst staying within the scope of the invention. For example, the description given above considers connecting casing joints. Other tubular members can also be connected in this fashion, for example production risers, production tubing, drill strings (e.g. casing drilling), completion strings, etc.

[0035] Furthermore, each side of the coupling can have the same direction

threads, for example both sides being left or right handed, or alternatively each side of the coupling can have opposite threads, with one side being left handed and the other being right handed.

[0036] Having opposite threads on each side of the coupling can be used to ease the make up of the joint by requiring only the coupling to be rotated to connect the tubular members.

[0037] Having the same direction threads on each side of the coupling will be required where the coupled tubular members need to be rotated, so that the coupling join does not disconnect itself during rotation.