Patent Description:
The pipe coupling of the present invention may be used to join pipes of any of a large range of diameters, made of any of a large range of materials, but has been designed in particular to join large diameter steel pipes of the type used in drilling equipment, where the forces on the pipes are high; the invention will be described with especial reference to this application.

At present, drill pipes commonly are joined together by a coupling of the general type shown in <FIG>, which shows an exploded side view of parts of two pipes and a coupling; and <FIG>, which shows a section on line L-L of <FIG> when the pipes are coupled together.

Referring to <FIG>, and <FIG> when required, a first pipe <NUM> is joined to a second pipe <NUM>, by a pipe coupling <NUM> which includes a male portion <NUM> welded to the free end of the first pipe <NUM>, and a female portion <NUM>, welded to the free end of the second pipe <NUM>. The male and female portions <NUM>, <NUM> are coaxial with the pipes <NUM>,<NUM> to which they are attached.

The male portion <NUM> is smaller in diameter than the female portion <NUM> (clearances are exaggerated in the drawing, for clarity) so that the male portion <NUM> is an easy sliding fit within the female portion <NUM>. The male portion <NUM> is formed with rigidly secured or cast in series of spaced keys <NUM>, spaced around the edge of the male portion <NUM> which is adjacent the first pipe <NUM> to which it is secured. The female portion <NUM> is formed with corresponding keyways <NUM>, which are cut-outs around the free edge of the female portion <NUM>. Each keyway <NUM> is sized so that each key <NUM> is an easy sliding fit into the corresponding keyway <NUM> when the male portion <NUM> is in position in the female portion <NUM>.

In this position, each of a series of spaced fasteners which include sets of apertures <NUM>, <NUM> on the male and female portions <NUM>,<NUM> respectively, are aligned. The male and female portions <NUM>,<NUM> are formed with a plurality of these fasteners, but for clarity, only one fastener shown in the drawing.

Within each male aperture <NUM> on the male portion <NUM> is a male socket 17a with an inner screw threaded boss 17b. Each male socket 17a is welded to the corresponding male aperture <NUM>. Each screw threaded boss 17b is countersunk into the male socket 17a, so that when each male socket 17a is in position in the corresponding male aperture <NUM>, the outer surface of both the male socket 17a and the boss 17b are flush with the outer surface of the male portion <NUM>.

Within each female aperture <NUM> on the female portion <NUM> is a complimentary female portion socket 18a.

The sockets 17a, 18a are circular in plan view, and the inner surface of each socket 17a, 18a is tapered to receive a plug 18b (<FIG>) the outer surface of which has a corresponding taper, the tapered portions of the sockets 17a,18a in combination form a fastener socket for the associated plug 18b. When assembled each plug 18b is a sliding fit within the fastener socket. Each plug 18b carries a central captive screw threaded connector 18d which is arranged and dimensioned to engage the screw threaded boss 17b when the apertures <NUM>, <NUM> of each fastener are aligned. The outer surface 17c of the male socket 17a is contoured to match the opposing surfaces of the female portion socket 18a and plug 18b when the apertures <NUM>, <NUM> are aligned.

Thus, to secure the pipe coupling <NUM> in position, the male portion <NUM> is slid into the female portion <NUM> with the keys <NUM> received within the keyways <NUM> and the apertures <NUM>, <NUM> of each fastener aligned. The screw threaded connector 18d in each plug 18b is then screwed into the corresponding boss 17b for each of the sets of aligned apertures <NUM>, <NUM>. It follows that the two pipes, <NUM>, <NUM> are connected together not only by the engagement of the keys <NUM> and the keyways <NUM>, but also by each of the fasteners.

Thus, each fastener is a tapered plug 18b inserted into a corresponding tapered fastener socket formed by the combination of male and female portion sockets 17a,18a. The plug 18b including a central void dimensioned to engage with a centrally located threaded boss 17b that extends from the base of the portion of the male socket 17a, and, in use, the screw threaded connector 18d acting to retain the plug 18b in the fastener socket.

In theory, the above described pipe coupling <NUM> provides a strong and secure connection between the pipes <NUM>,<NUM>. In practice, pipe couplings <NUM> of this type frequently fail due to failure of the fasteners. In use, the longitudinal (compressive) forces are transmitted from the pipe <NUM> to the pipe <NUM> both by the engagement between the keys <NUM> and keyways <NUM> and, to a lesser extent between the engagement of the fasteners, but predominantly by the engagement of the shoulder 10a (male shoulder) formed at the junction between the male portion <NUM> and the pipe <NUM>, with the free end 14a of the female portion <NUM>, and by the engagement between the end 13a of the male portion <NUM> and the end 11a (female shoulder) of the pipe <NUM>. However, the torsional forces are substantially higher as the coupled pipes <NUM>,<NUM> are rotated. Typically, for a <NUM> millimetres diameter pipe, the longitudinal forces are of the order of <NUM> - <NUM> tonnes, whereas the torsional forces (torque) can be of the order of <NUM> tonne.

Torsional forces are transmitted from one pipe <NUM>,<NUM> to the other by engagement between the keys <NUM> and keyways <NUM>, and by the fasteners. Unfortunately, the keys <NUM> and keyways <NUM> tend to be a relatively loose fit, due to machining tolerances, and this passes a portion of the torsional forces onto the fasteners. As soon as wear occurs between the keys <NUM> and the keyways <NUM>, this increases the load on the tapered portions of the surrounds and plugs of the fasteners, and tends to cause excessive wear of these components. This wear can allow more rotary movement between the two portions <NUM>, <NUM> of the pipe coupling <NUM>, and this of course increases the wear and allows increasing movement:- the cycle continues until one or more of the plugs 18b fail, or one or more of the screw threaded connectors 18d does, leading to general failing of the fastener and potentially the failure of the coupling <NUM>.

Patent applications <CIT> and <CIT> are good examples of the prior art.

An object of the present invention is the provision of a pipe coupling which overcomes at least some of the above described drawbacks, or at least provides the consumer with a useful choice.

The present invention provides a pipe coupling which includes:.

In other words, the present invention provides a pipe coupling which includes:.

Preferably there are plurality of male portion fittings spaced around the male portion and a corresponding plurality of female portion sockets spaced around the female portion. Preferably the plurality of male portion fittings and female portion sockets are circumferentially equispaced around the male portion and female portion respectively. Preferably there is more than one fixing aperture in each plug. In a highly preferred form at least one securing bolt is captive in the corresponding fixing aperture. Preferably there are <NUM> to <NUM> fixing apertures in each plug.

Preferably the predetermined amount of movement between the plug and the corresponding central aperture is (WA-W) = <NUM> to <NUM>, where WA is the circumferential dimension of the central aperture and W is the circumferential length of the plug. In a more preferred form (WA-W) = <NUM> to <NUM>. In a highly preferred form (WA-W) = <NUM> to <NUM>.

Preferably there is a predetermined securing clearance between corresponding fixing apertures and the securing bolt which they receive. Preferably this predetermined securing clearance is between <NUM>. 05D and <NUM>. 2D, where D is the securing bolt diameter.

Preferably the plug is tapered on all faces that engage with the male portion fitting or female portion socket. In an alternative preferred form, the plug is only tapered for the portion that engages with the female portion socket. In a further alternative preferred form, the plug is tapered for the portion that engages with the male portion fitting but a lip engaged with a corresponding surface where it engages with the female portion socket. In an alternative form there is no lip present. Preferably, where possible, the taper is radially aligned to the female portion.

In one preferred option the circumferentially separated opposite faces of the central aperture, the central aperture end faces, when viewed in cross section, are parallel to a radial line extending from the centre of the female portion through the centre of the central aperture. Preferably the circumferentially separated opposite ends of the plug, the plug ends, in cross section are, in use, radially aligned to the female portion.

In an alternative preferred option the circumferentially separated opposite faces of the central aperture, the central aperture end faces, when viewed in cross section, are parallel to a radial line extending from the centre of the female portion and the circumferentially separated opposite ends of the plug, plug ends, in cross section are, in use, radially aligned to the female portion.

Preferably securing bolts are threaded fixing devices; namely elongate members with a drive section and a threaded section configured to engage with a corresponding threaded cavity/rod, said drive section including features configured to be engage with a tool so that it can be rotated. In a preferred form the securing bolts are a threaded fixing device selected from the list consisting of screws, set screws and bolts.

By way of example only, a preferred embodiment of the present invention is described in detail with reference to the accompanying drawings, in which:-.

Referring to <FIG> of the drawings, a pipe coupling <NUM> in accordance with the present invention differs from the pipe coupling <NUM> shown and described with reference to <FIG> and <FIG> only in respect of the design of the fasteners, as described below. Thus, in <FIG>, the same reference numerals are used for the first and second pipes <NUM>/<NUM>, the male and female portions <NUM>/<NUM> of the coupling <NUM>, the keys <NUM> and keyways <NUM>. In addition, the male shoulder 10a engages the free end 14a of the female portion <NUM>, and the end 13a of the male portion <NUM> of the coupling <NUM> engages the female shoulder 11a. In <FIG> the male shoulder 10a and the keys <NUM> are shown formed as part of the male portion <NUM> which is rigidly attached to the first pipe <NUM>, and the female shoulder 11a is an internal shoulder formed inside the female portion <NUM>. In other variants the male shoulder 10a may be the exposed end of the first pipe <NUM> and the female shoulder 11a the end of second pipe <NUM> as shown in <FIG>.

The female portion <NUM> is formed with a female aperture <NUM> which has a female portion socket <NUM> rigidly secured thereto (generally by welding). The female portion socket <NUM> is generally rectangular in plan view, and has a central aperture <NUM> the sides of which angle inwards towards the inner surface of the female portion <NUM>. A plug <NUM> has sidewalls <NUM> which are angled to match the angle of the central aperture <NUM>, and is dimensioned so that the plug <NUM> is a sliding fit within the central aperture <NUM>. However, the plug <NUM> has a width W which is less than the width WA of the central aperture <NUM>. This leaves a gap (WA-W) between the sides of the plug <NUM> and the adjacent walls of the central aperture <NUM> of approximately <NUM> millimetres (preferably <NUM> to <NUM>, though this can be anything in the range of <NUM> to <NUM>), so that the plug <NUM> can move sideways (circumferentially) relative to the sides of the female portion socket <NUM>. This movement allows the portions <NUM>,<NUM> to move relative to each other until the keys <NUM>/keyways <NUM> engage and the pipes <NUM>/<NUM> rotate together.

It is preferred that (WA-W) = <NUM> to <NUM>, where WA is the circumferential dimension of the central aperture <NUM> and W is the circumferential length of the plug <NUM>. This wide range takes into account the wide range of pipe diameters and wear that may occur in keys <NUM>/keyways <NUM>. For most pipes (WA-W) will be from <NUM> to <NUM>, though it is felt (WA-W) = <NUM> to <NUM> will be the optimal range.

The centre of the plug <NUM> is formed with a counterbored fixing aperture <NUM> which receives a securing bolt <NUM>. The lower end of the securing bolt <NUM> is screw threaded, and in use engages the screw thread formed on a corresponding boss 17b, as described below. In some configurations one or more securing bolt <NUM> is a captive bolt similar to that used in the prior art. Though referred to as securing bolts <NUM> these may be any form of threaded fixing device or fastener including bolts, screws, set screws, etc; namely an elongate member with a drive section and a threaded section configured to engage with a corresponding threaded cavity, said drive section including features configured to be engage with a tool so that it can be rotated.

The female portion socket <NUM> is radiused such that when rigidly attached to the female portion <NUM> it matches the radius of the female portion <NUM>, as shown in <FIG>. The plug <NUM> is radiused to match the female portion socket <NUM>, as shown in <FIG>.

Because the female portion socket <NUM> and plug <NUM> are rectangular in plan, and thus can easily be radiused to fit the curve of the pipe coupling <NUM>, these components can be made larger than the normal non-radiused circular sockets and plugs used in the prior art coupling. The prior art sockets are ground to allow the male and female portions <NUM>,<NUM> to engage. The larger the dimensions of each socket and plug, the greater the area on which tensile forces can bear.

The inner surfaces 22c, 24c of the female portion socket <NUM> and plug <NUM> respectively (i.e. the surfaces which in use lie adjacent the male portion <NUM> of the coupling <NUM>) are contoured as shown in <FIG>.

The male portion <NUM> is formed with a fitting aperture <NUM> which has a male portion fitting <NUM> rigidly secured thereto (generally by welding), though the male portion fitting <NUM> can be formed into the male portion <NUM>. Like the female portion socket <NUM>, the male portion fitting <NUM> is generally rectangular in plan view and its outer surface 31c is contoured to match the contouring on the inner surfaces 22c and 24c, so that the female portion socket <NUM>, male portion fitting <NUM> and plug <NUM> engage when the fasteners are secured.

In the centre of the male portion fitting <NUM> is a boss <NUM>, with a central internally screw threaded aperture which is sized to engage the lower end of the securing bolt <NUM>. Said securing bolt <NUM> is shown as free but it can be a captive bolt as shown in the prior art configuration (see <FIG>).

<FIG> show a variant design in which the female portion sockets 22a are enlarged to accommodate two bosses <NUM> on the male portion <NUM> of the pipe <NUM>. In the example illustrated, the female portion socket 22a is enlarged so that it covers two bosses <NUM>; the plug 24a is similarly enlarged to provide two fixing apertures <NUM> for receiving a bolt <NUM> in each aperture <NUM>. Because the female portion socket 22a and the plug 24a are radiused to match the curve of the portions <NUM>,<NUM>, the size of the fastener can be circumferentially enlarged without any drop in efficiency. The ability to provide a circumferentially larger fastener means that it may be possible to reduce the length of the coupling <NUM>, because it is possible to provide a large number of securing bolts <NUM> in a relatively short coupling <NUM>. Though the securing bolts <NUM> are shown as free one or both in each pair may be captive, though this will depend on the circumferential separation of said securing bolts <NUM>.

<FIG> shows a section through a coupling <NUM> which provides three pairs of securing bolts <NUM> of this type, with the pairs of securing bolts <NUM> equidistantly spaced around the circumference of the coupling <NUM>.

<FIG> shows a detail of the fastener on the male portion <NUM> of the coupling <NUM>:- a pair of bosses <NUM> are mounted on a male portion fitting <NUM> which is generally rectangular in plan view with a contoured outer surface 31c providing a pair of deep V-cross section grooves <NUM> which receive, and match, the contouring on the plug inner surfaces 24c on each of the plugs <NUM> (see <FIG>).

It will be appreciated that further variant designs are possible in which the fasteners on the male and female portions <NUM>,<NUM> of the coupling <NUM> are enlarged to accommodate more than two bosses <NUM> on the male portion <NUM> of the pipe <NUM>.

The above described coupling <NUM> is used as follows (Referring to any of <FIG> where appropriate):- the male portion <NUM> of the coupling <NUM> is formed as described above and when fully inserted into the female portion <NUM> of the coupling <NUM> engages with the female portion <NUM> of the coupling <NUM> in at least the following ways:.

The majority of the compressive loading on the coupling <NUM>, under normal conditions, is absorbed by engagement of the male shoulder 10a with the free end 14a of the female portion <NUM>, and/or the engagement of the end 13a of the male portion <NUM> of the coupling <NUM> with the female shoulder 11a. With the torsional loading on the coupling <NUM> transmitted, under normal conditions, primarily by the engagement between the keys <NUM> and keyways <NUM>. This part of the connection system is extremely strong, since loading is taken over the full length of each key <NUM>, and the keys <NUM> are normally cast-in structures. If necessary, for unusually high loading, both keys <NUM> and keyways <NUM> can be increased in size.

<FIG> show only two sets of female portion sockets <NUM> and plugs <NUM>, but it will be appreciated that female apertures <NUM>, each with an associated female portion socket <NUM> and plug <NUM>, and each corresponding to fitting apertures <NUM> formed in the male portion <NUM> of the coupling <NUM>, are formed at spaced intervals around the coupling <NUM>.

Referring to <FIG> a further variant of the female portion <NUM> in pictorial form is shown. In this variant the female portion socket <NUM> is formed into the female portion <NUM> rather than being a separate component welded or otherwise rigidly fixed into a female aperture <NUM> (see <FIG>). This variant avoids the need to weld or otherwise fix a separate female portion socket <NUM>/22a to the female portion <NUM>.

Referring to <FIG> a pictorial view of a matching plug <NUM> and male portion fitting <NUM> is shown, in this variant one of the securing bolts <NUM> is a captive bolt. Both securing bolts <NUM> can be captive, but this may make assembly/disassembly more difficult. In this variant the two bosses <NUM> are formed (or pressed) into a longitudinally aligned central rib <NUM> that when the male portion fitting <NUM> is installed is circumferentially aligned. Immediately adjacent each side of this central rib <NUM> are engagement grooves <NUM> which are similar to the V-shaped grooves <NUM>. The engagement grooves <NUM> have a flat base <NUM> and only one angled face <NUM>, the face that does not form part of the central rib <NUM>, when viewed in cross-section. Each engagement groove <NUM> is dimensioned and contoured to engage with a longitudinally aligned engagement ribs <NUM> which are part of the plug <NUM>. In use the engagement ribs <NUM> engage with the engagement grooves <NUM> and the securing bolts <NUM> lock them together.

Referring to <FIG> a coupling <NUM> incorporating the variant female portion <NUM> shown in <FIG> is shown in side view and cross-sectional view along the line E-E.

In <FIG> a plug seal <NUM> is shown between each end of the plugs <NUM> and female portion socket <NUM>. These plug seals <NUM> are in place to act as seals to prevent unwanted material entering the spaces between the plug ends <NUM>, the longitudinal terminal ends of the plugs <NUM>, and the circumferentially separated opposite faces of the central aperture <NUM>, the central aperture end faces <NUM>. The plug seals <NUM> are an elastomeric material shown as oval in cross section. The plug seals <NUM> are intended to compress or expand to maintain this seal as the plug <NUM> moves within the female portion socket <NUM>.

In <FIG> the central aperture end faces <NUM> are shown parallel to a radial extending through the centre of the central aperture <NUM> and the plug ends <NUM> are shown radially aligned to the female portion <NUM>, thus the plug seals <NUM> are trapped in a substantially trapezoidal void.

Referring to <FIG> a variant of the plug <NUM> is shown with three securing bolts <NUM> within corresponding fixing apertures, the central securing bolt <NUM> is a captive. The centre securing bolt <NUM> being captive is believed to aid the installation/removal of the plug <NUM>.

With the coupling <NUM> of the present invention, significantly less of the torsional load is exerted on the plugs <NUM>/24a and securing bolts <NUM>, because of the built-in play between the plugs <NUM>/24a and the corresponding female portion sockets <NUM>/22a. As torsional force is exerted on the coupling <NUM>, the portions <NUM>, <NUM> initially move relative to each other until the keys <NUM> and keyways <NUM> engage, the keys <NUM>/keyways <NUM> then transfer this torsional force between pipes <NUM>,<NUM>.

In some configurations, as shown in <FIG> with a single securing bolt <NUM>, each securing bolt <NUM> is a loose fit. By loose fit in this case, it means there is a securing clearance <NUM> of between <NUM>. 05D and <NUM>. 2D (where D is the securing bolt <NUM> diameter) between the securing bolt <NUM> and the corresponding fixing aperture <NUM>. The presence of this securing clearance <NUM> permits a small amount of movement between the portions <NUM>,<NUM> of the coupling <NUM> before the sides of each plug <NUM>/24a engage with/contact the adjacent sides of the female portion socket <NUM>/22a, as such there is additional movement between portions <NUM>,<NUM> available to allow the keys <NUM> and keyways <NUM> to engage prior to any torsional load being transferred to the securing bolts <NUM>.

The above described design allows for the absorption by the coupling <NUM> of high torsional forces without causing undue wear (and therefore damage) to the sockets <NUM>/22a, plugs <NUM>/24a or securing bolts <NUM>.

Referring to <FIG> a cross-sectional view of the plug <NUM> shown in <FIG> along the line H-H is shown. In this variant the centre securing bolt <NUM> is a captive bolt and it includes the securing clearance <NUM>.

In some variants there is minimal free play between the securing bolts <NUM> and the corresponding fixing aperture <NUM>, there is little or no securing clearance <NUM> (see <FIG> or <FIG>).

Referring to <FIG> a number of cross-sectional variations of the plug <NUM> engaged with the complementary female socket <NUM> and male fitting <NUM>.

<FIG> shows a cross-sectional view of a plug <NUM> that has a lip <NUM> and parallel section <NUM> where it passes through the female portion <NUM> and a taper similar to that shown in <FIG>, <FIG>, <FIG>, <FIG> or <FIG> in the male portion <NUM>. The lip <NUM> sits on top of a complementary shelf <NUM> in the female portion socket <NUM>/22a. In this variant the female portion socket <NUM>/22a is formed into the female portion <NUM>. This configuration allows the tensile clamping forces holding the male portion <NUM> to the female portion <NUM> to be distributed along the shelves <NUM> whilst the taper locks the plug <NUM> into the male fitting <NUM>. The lip <NUM> can move relative to the complementary shelf <NUM> as there is a coupling gap (WA-W).

<FIG> shows a cross sectional view similar to <FIG>, however there is a primary gap <NUM> between the plug <NUM> and the female portion socket <NUM>/22a that allows the plug <NUM> to move co-axially with respect to the portions <NUM>,<NUM> when there is a securing clearance <NUM> present. The securing clearance <NUM> in combination with the primary gap <NUM> allow a predetermined amount (<NUM> to <NUM>) of coaxial movement between the pipes <NUM>,<NUM> so that the engagement of the male shoulder 10a with the free end 14a of the female portion <NUM>, and/or the engagement of the end 13a of the male portion <NUM> of the coupling with the female shoulder 11a can occur.

<FIG> shows a cross -sectional view similar to <FIG>, however the taper in the male portion <NUM> is an asymmetric truncated 'V' shape.

<FIG> shows a cross-sectional view similar to <FIG>, however in this case the parallel section <NUM> extends the entire thickness of the female section (there is no lip <NUM> as shown in <FIG>). This variant provides the same extended contact area between the plug <NUM> and the female portion socket <NUM>/22a during extraction of the pipes <NUM>,<NUM> joined by the pipe coupling <NUM>. As can be seen the plug <NUM> engaged with the female portion socket <NUM>/22a allows the torsional, compressive and extraction forces to be transmitted even though there is no direct clamping force applied to connect the portions <NUM>,<NUM> together.

<FIG> shows a cross-sectional view similar to <FIG>, however there is no taper on the plug <NUM> or the male fitting <NUM>. This variant is shown in two different configurations, the first with the parallel section of the plug <NUM> extending the full length of the plug <NUM> and, in dashed lines <NUM>, the plug <NUM> (in cross-section) being 'T' shaped (the plug <NUM> steps down in size within the male fitting <NUM>). These two options do not have the expected advantage of the taper.

It should be noted that individual compatible features of the variants shown in <FIG> can be combined without deviating from the inventive concept.

Referring to <FIG> a cross-sectional view of a further variant is shown, in this variant the male portion <NUM> is tapered where it engages with the female portion <NUM>, and the female portion <NUM> incorporates a complementary taper. These complementary tapers are not intended to transmit the compressive forces between the male and female portions <NUM>,<NUM> they are incorporated to make the assembly and disassembly of the pipe coupling <NUM> easier. The complementary tapers may pass compressive forces due to wear or specific purposes.

Referring to <FIG> a variant plug <NUM> with an extraction socket <NUM> is shown in plan and cross-sectional view. The extraction socket <NUM> includes an extraction aperture <NUM> and an extraction cavity <NUM>. The extraction aperture <NUM> is dimensioned to accept one end of a slide hammer <NUM>, and the extraction cavity <NUM> is a void within the plug <NUM> that is the swept cross-section of the portion of the slide hammer <NUM> that engages with the extraction socket <NUM>. To extract a plug <NUM> that is stuck in a coupling <NUM> (see <FIG> for example) the extraction hammer <NUM> is inserted through the extraction aperture <NUM> and then turned within the extraction cavity <NUM>, the slide <NUM> portion of the slide hammer <NUM> is then used to generate force impulses to extract the plug <NUM>. If one of the securing bolts <NUM> is captive then this can also be used to assist removal of the plug <NUM>, alone or in combination with the extraction socket <NUM>.

In a further variant, not shown, the side walls of the pipes <NUM>,<NUM> and coupling <NUM> include a fluid cavity that extends through all components and includes the required sealing to allow a fluid to pass along the length of a joined pipeline using these couplings <NUM>.

The various variants (please refer to the accompanying drawings <FIG>) can be combined without deviating from the inventive concept, that is to form a pipe coupling <NUM> that allows a certain amount of circumferential movement between the male and female portions <NUM> of the coupling <NUM> so that the keys <NUM> and keyways <NUM> transmit the majority if not all of the torsional forces. This circumferential movement may be due to a predetermined spacing between the securing bolts <NUM> and the corresponding fixing aperture <NUM> (the securing clearance <NUM>), the different circumferential lengths of the plug <NUM> and central aperture <NUM>, or a combination of the two. In addition there may be a predetermined amount of co-axial (longitudinal) movement allowed for between the pipes <NUM>,<NUM> so that the majority of the compressive forces can be transmitted by the engagement of the male shoulder 10a with the free end 14a of the female portion <NUM>, and/or the engagement of the end 13a of the male portion <NUM> of the coupling <NUM> with the female shoulder 11a.

Referring to <FIG>. the pipe coupling <NUM> is shown in use. Firstly in <FIG> with the second pipe <NUM>, attached to the female portion <NUM>, being rotated in the direction of arrow R as it is being moved in the direction of arrow P (towards the first pipe <NUM>); and secondly, in <FIG>, with the second pipe <NUM> being moved in the direction of arrow T (away from the first pipe <NUM>).

Referring to <FIG> the female portion <NUM> moves in relation to the male portion <NUM> until the engagement of the male shoulder 10a with the free end 14a of the female portion <NUM>, and/or the engagement of the end 13a of the male portion <NUM> with the female shoulder 11a. At the same time the female socket <NUM>/22a moves in the direction of arrow R until the keys <NUM> engage with the keyways <NUM>, this occurs before the plug <NUM> contacts the female socket <NUM>/22a. The plug <NUM> prevents the pipes <NUM>,<NUM> from separating, but it is not intended to transfer the compressive or torsional forces applied by the driven pipe <NUM>,<NUM> to the undriven pipe <NUM>,<NUM>.

Claim 1:
A pipe coupling (<NUM>) which includes:
• a male portion (<NUM>) which in use is rigidly secured to one end of a first pipe (<NUM>), coaxial with said first pipe (<NUM>);
• a female portion (<NUM>) which in use is rigidly secured to one end of a second pipe (<NUM>), coaxial with said second pipe (<NUM>);
• the male and female portions (<NUM>,<NUM>) being sized so that the male portion (<NUM>) is an easy sliding fit within the female portion (<NUM>) when the first and second pipes (<NUM>,<NUM>) are connected together;
• the male portion (<NUM>) being provided with a series of spaced keys (<NUM>) protruding from the outer surface thereof;
• the female portion (<NUM>) being provided with a series of spaced keyways (<NUM>) which are dimensioned and arranged so that the keys (<NUM>) are a sliding fit within the keyways (<NUM>) when the male and female portions (<NUM>,<NUM>) are connected together;
• at least one fastener, each of the at least one fastener including:
• a male portion fitting (<NUM>) rigidly secured to, or formed in, the male portion (<NUM>) and providing at least one internally screw-threaded bore;
• a female portion socket (<NUM>,22a) rigidly attached to, or formed into, the female portion (<NUM>), said female portion socket (<NUM>,22a) including a central aperture (<NUM>) which is shaped to receive a plug (<NUM>,24a) therein, said female portion socket (<NUM>,22a) and said plug (<NUM>,24a) being radiused to match the radius of the female portion (<NUM>), and being substantially rectangular in plan;
• a securing bolt (<NUM>) dimensioned to fit in a corresponding fixing aperture (<NUM>) formed in said plug (<NUM>,24a), said securing bolt (<NUM>) being releasably engageable with a corresponding bore; and
• the male portion fitting (<NUM>) is shaped, at least in part, to receive a portion of the plug (<NUM>,24a);
wherein said plug (<NUM>,24a) is sized relative to the female portion socket (<NUM>,22a) such that the plug (<NUM>,24a) has a predetermined amount of movement relative to the female portion socket (<NUM>,22a) in either direction around the circumference of the female portion (<NUM>), even when each securing bolt (<NUM>) present is engaged with the corresponding bore.