BACKGROUND OF THE INVENTION Downhole tubulars, such as bore-lining casing and liners, are typically coupled by means of threaded connectors, or by providing adjacent ends of tubulars with male and female, or pin and box, threaded ends. For conventional applications, such coupling arrangements are generally satisfactory, providing secure, pressure tight connections. However, where strings of tubulars are to be diametrically expanded, it has been found that the integrity of the coupling may be compromised.

It is among the objectives of embodiments of the present invention to provide coupling arrangements for tubulars which will retain mechanical and pressure integrity following diametric expansion of the-tubulars.

SUMMARY OF THE INVENTION According to a first embodiment of the present

invention there is provided an expandable coupling arrangement for first and second expandable tubulars, the coupling comprising: a male thread portion on an end of a first tubular; and a female thread portion on an end of a second tubular, the thread portions comprising dovetail threads and having flanks, roots and crests, wherein the flanks are inclined at an angle of greater than 10'.

On expansion of the engaged thread portions, the high angle dovetail threads maintain the integrity of the coupling. The invention has particular application in couplings which are to be subject to expansion by a rotary expansion tool. Such a tool expands the male or pin thread portion by reducing the wall thickness of the portion, resulting in a corresponding increase in circumference and diameter of the portion, which tends to be accompanied by axial elongation of the thread portion. However, the female or box thread portion is expanded by contact with the expanding male or pin thread portion. This tends to induce axial contraction of the female thread portion. In a conventional thread, this differential expansion tends to produce an adverse effect on the thread integrity, however in the present invention the opposite is true; the differential expansion tends to lock the thread portions together.

Preferably, the thread portions define a thread which is cut in an opposite direction to the intended direction of rotary expansion of the coupling, such that any torque applied to the coupling by the rotating expander will tend to tighten the coupling.

Each thread portion has stab flanks and load flanks, and it is preferred that both the stab flanks and the load flanks of each respective thread portion are inclined at substantially the same angle to the respective thread root.

Preferably, the flanks are inclined at an angle of greater than 15°.

Preferably, the flanks of the male thread portion are inclined at an angle of less than 80°, and most preferably less than 75°, to the male thread portion roots.

Preferably, the flanks of the female thread portion are inclined at an angle of less than 80°, and most preferably less than 75°, to the female thread portion roots.

The thread portions may be parallel, tapered or stepped.

Preferably, the first tubular has a leading end portion or nose adapted to be radially constrained by the second tubular. For example, the second tubular may define an undercut slot, recess or groove in which the nose is received. This prevents the nose from separating from the second tubular, and in particular from encroaching into the

internal diameter of the coupling following expansion, as might otherwise occur due to"end effects", where the free end or nose tends to diametrically contract more than adjacent portions of the tubular. Alternatively, or in addition, the groove may extend axially and be dimensioned to accommodate axial extension of the first tubular relative to the second tubular. The groove may accommodate a deformable sealing material, such as an elastomer, in particular an elastomer o-ring or the like which will be energised by relative axial extension of the male thread.

Preferably, the free end of the first tubular is not threaded, to permit axial movement of the nose relative to the second tubular, and thus to energise, or further energise, the deformable seal, where provided. Preferably, the groove features a rounded recess angle, to prevent stress concentration and to alleviate stress-induced cracking upon expansion.

Preferably, the first tubular comprises at least one sealing member for sealing engagement with an opposing surface of the second tubular, most preferably for sealing engagement with an opposing surface adjacent a free end of the second tubular. Conveniently, the sealing engagement is provided with a surface spaced sufficiently from the free end of the second tubular to accommodate axial shrinkage of the tubular following expansion. The end effect of the free end also serves to energise the sealing member. Most

preferably, the sealing member is in the form of an elastomer. At least two axially spaced elastomers may be provided. The sealing members may be located in appropriate grooves in the first tubular.

Where sealing members are provided, it is preferred that these are formed of swelling elastomers, or other materials which swell or expand when exposed to a selected fluid, most preferably the fluid or fluids contained by the sealing member. Thus, a seal will be re-energised in the event of a leak. The use of adjacent sealing members which swell in response to contact with different fluids ensures re-energisation of a leaking seal in an environment where the seal may be exposed to a number of different fluids, for example a water-swell elastomer and an oil-swell elastomer in adjacent grooves provides for re-energising a seal in a water and oil environment.

The material properties of the male and female threads may be selected to facilitate the creation of a secure engage between the threads following expansion.

Preferably, at least some of the crests of the threads are adapted to extend axially on expansion of the coupling.

On expansion of the coupling, the root portions of the threads, particularly the root portions of the male thread portion, will tend to elongate to a greater extent than the other, thicker portions of the threads. There is thus a possibility of a loss of contact between the flanks of the

engaged thread portions. To counter this possibility, the crests may be configured to splay outwardly on experiencing the radial compression associated with expansion of the coupling. In one embodiment this is achieved by providing a relief or slot in the crest. This effect may be enhanced by providing a rib. or spreader on at least some of the roots.

The various features described above may be incorporated in other couplings, in addition to couplings made in accordance with the first embodiment of the present invention.

BRIEF DESCRIPTION OF DRAWINGS This and other aspects of the. present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional drawing of a tubular coupling in accordance with an embodiment of the present invention; and Figure 2 is an enlarged view of area 2 of Figure 1 ; Figure 3 is a sectional drawing of a pin connector forming part of a coupling in accordance with a further embodiment of the present invention ; Figure 4 is an enlarged view of area 4 of Figure 3 ; Figure 5 is an enlarged view of area 5 of Figure 3 ; Figure 6 is a sectional drawing of a box connector

forming part of a coupling in accordance with said further embodiment of the present invention, and configured to be coupled with the pin connector of Figure 3; Figure 7 is an enlarged view of area 7 of Figure 6 ; and Figure 8 is an enlarged view of area 8 of Figure 6 ; and Figure 9 is an enlarged view of an area of part of a coupling in accordance with a still further embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWING Figures 1 and 2 of the drawings illustrate an upset threaded coupling 10 connecting the ends of first and second downhole tubulars 12, 14. The end of the first tubular 12 features a male threaded portion 16, or pin connection, while the adjacent end of the second tubular 14 features a corresponding female threaded portion 18, or box connection.

The male and female threads 20,22 are parallel, that is the roots and crests of the threads extend parallel to the longitudinal axis 24 of the coupled tubulars 12,14, and as may be seen more clearly from Figure 2, the threads 20,22 are dovetailed. In particular, each thread has flanks 26, roots 28 and crests 30, and the flanks 26 are inclined at an angle 27 of greater than 15"to the roots 28 and the crests 30.

The leading end of the first tubular 12 has a nose 32

which, when the tubulars 12,14 are coupled, is located in an axially-extending circumferential groove 34 at the base of the female threaded'portion 18. The groove 34 accommodates an elastomeric O-ring 35. The free end of the second tubular 14 is similarly located in a corresponding groove 36 at the base of the male portion 16, which also accommodates an o-ring 38, and the free end of the second tubular 14 is also pinned to the male portion 16 to prevent relative rotation.

In use, the coupling 10 is made up on surface in the usual manner, that is the ends of the tubular will be brought together, the first tubular 12 then rotated relative to the second tubular 14 to make up the threads, and the made-up ends then pinned. In this manner a tubing string will be created, and will be run into a bore. Once in position in the bore, a rotary expansion tool is run through the string, in direction"A", to diametrically expand the tubing. When the tool encounters a coupling 10, the male threaded portion 16 is subject to diametric expansion, by virtue of the wall thickness of the portion being reduced, with a corresponding increase in circumference. The increasing outside diameter of the male portion 16 causes the surrounding female portion 18 to experience a corresponding increase in diameter. Furthermore, the increase in diameter of the male portion 16 is accompanied by axial extension, whereas the increase in diameter of the

female portion 18 is accompanied by axial contraction.

Whereas in a conventional thread profile such deformation tends to have an adverse affect on the integrity of the coupling, in the coupling 10 the deformation of the high angle dovetail thread locks the male and female portions 16,18 together, such that mechanical and hydraulic integrity is retained, and indeed enhanced. Furthermore, any tendency of the male portion 16 to axially extend relative to the female portion 18 is accommodated by the groove 34, and is not restrained at the free end of the first tubular due to the absence of inter-engaging threads.

Thus, the differential axial expansion and contraction of the tubulars 12,14 is utilised to further energise the o- ring 35 in the groove 34. The groove 34 also prevents the nose of the first tubular 12 from encroaching on the internal diameter once the expansion tool has passed.

The male and female thread portions 16,18 may be formed of the same or different materials, and the material properties, such as the yield strength, may be selected to facilitate creation of a secure lock.

Reference is now made to Figures 3 to 8 of the drawings, which illustrate a pin connector 50 (Figures 3,4 and 5), and a box connector 52 (Figures 6,7 and 8), together forming a coupling in accordance with a further embodiment of the present invention. Reference is made first to Figure 3, which is a sectional drawing of the pin

connector 50, which may be welded to the end. of an appropriate expandable tubular, or which may be machined in the end of a tubular. From a maximum outer diameter portion 54, the pin 50 steps down in diameter first to a seal portion 56 and then to the male threaded portion 58. The "quick-start"thread is left-handed and dovetailed, the details of the thread being more readily visible in Figure 4, which is an enlarged view of area 4 of Figure 3.

The thread features parallel roots 60 and crests 62, and both the stab and load flanks 64,66 are angled at 75 degrees to roots 60. The free end or nose 68 of the pin 50 is rounded and defines a negative angle of 45 degrees.

The seal portion 56 is substantially cylindrical, but defines two circumferential grooves 70,72, one of which is illustrated in greater detail in Figure 5. The grooves 70, 72 receive respective 0-rings (not shown) of swelling elastomer: one o-ring swells in response to contact with water, while the other swells from contact with oil.

Reference is now made in particular to Figure 6, which illustrates the box connector 52. Like the pin 50, the box 52 may be machined into the end of a tubular, or may be formed separately and then welded to the end of a tubular.

The box 52 has an upset portion 80 which contains the female thread portion 82. The left-handed dovetail thread (Figure 7) is spaced from the free end of the box 52 by a smooth- walled portion 84 for cooperating with the seal portion 56

of the pin 50 (Figure 3).

At the inner end of the thread 82 is a rounded undercut groove 86 (Figure 8) adapted to receive and retain the pin nose 68 when the threads are made up.

The coupling 50,52 is used in a similar manner to the coupling 10, that is tubulars provided with the pin and box ends are made up on surface to form a string and then run into a bore. A rotary expansion tool is then run through the string, the tool being rotated clockwise as the tool is advanced axially through the string. When the tool encounters a coupling 50,52, travelling in direction"A", the tool will induce compressive yield in the pin 50, reducing the pin wall thickness and thus increasing the pin diameter. This increase in diameter forces the surrounding portions of the box 52 to diametrically expand. The different natures of the expansion mechanisms of the pin 50 and box 52 are such that the expanded pin 50 tends to extend axially, while the expanded box 52 tends to contract. This locks the threads 58,82 together. However, where relative axial movement is permitted, that is between the pin seal portion 56 and the smooth bore box portion 84, the relative dimension of the portions and the locations of the seal grooves 70,72 are selected to ensure maintenance of the seal between the opposing surfaces. In addition, the end effects experienced by the free end of the box 52, which will cause the free end to tend to diametrically contract to

a greater degree following passage of the expansion tool, also serves to maintain a seal.

The pin nose 68 experiences a similar end effect, however the nose is prevented from contracting by the engagement between the nose 68 and the groove 86.

Once the rotary expansion tool has passed through the pin 50 and engages directly the wall of the box 52, the box 52 will experience a clockwise torque. As the pin 50 and box 52 define a left-handed thread, this applied torque will therefore tend to tighten the threads; it has been found that diametric expansion of threaded couplings made up to a specified torque may lead to loosening of the coupling, and if this should occur the use of a thread direction which is opposed to the direction of rotation of the expander will serve to mitigate this effect.

Reference is now made to Figure 9 of the drawings, which illustrates details of a coupling in accordance with a still further embodiment of the invention. On expansion of the coupling by means of a rotary expander, the male thread 120 axially extends in addition to the desired diametric expansion. Furthermore, the relatively thin- walled portions of the thread 120, that is the root portions 128, tend to experience a greater degree of extension.

This introduces a risk that there will be a loss of contact between the male and female thread flanks 126. To avoid this possibility, the thread crests 130 feature a

relief 131, and the roots 128 feature a raised rib 129.

Thus, when the coupling is expanded, and the male thread 120 is radially thinned and urged outwardly to expand the surrounding female thread 122, each crest 130 is urged into the opposing root 128, the relief 131 and the rib 129 co- operating to splay the crest 130. The axial extent of the crest 130 therefore increases, as does the flank angle, such that the thread flanks 126 are maintained in engagement.

Those of skill in the art will appreciate that the above described embodiments are merely exemplary of the present invention and that various modifications and improvements may be made thereto, without departing from the scope of the present invention.