AU701469B3 - Bore liner or casing - Google Patents

Description

p:\OPER\DH\53925-9 8 2 6 5 22/9/98 -2- BORE LINER OR CASING The present invention relates to a bore liner or casing.

Bore liner or casing, hereinafter referred to for simplicity as bore liner, conventionally comprises interconnected lengths of pipe which fit closely against the wall of the bore to maintain the bore against collapse. The bore may extend to a depth of several hundreds of metres and a typical liner will consist of threadedly interconnected lengths of pipe, for example each of about 6 metres in length. Conventionally, each length of pipe is formed at one end with a male thread and at the other end with a corresponding female thread.

Conventionally, these threads have been of square profile which require the ends of the pipe to be machined over a depth of in excess of half of the wall thickness. This causes substantial weakening of the pipe. Also the inherent nature of these square threads is such that the male o• thread of one pipe and the female thread of the adjacent pipe when interengaged do not 15 provide a substantial self-supporting function between the threads and under some circumstances the joint can fail under the tensile loading which, in the case of a liner of several hundred metres depth, can be very substantial. Also, due to the reduced wall thickness of the ends of the pipe due to the machining of the square section thread, the liner has poor collapse resistance to external forces in the zone of the joints.

According to the present invention, there is provided a coupling interconnecting lengths of pipe for use as a bore liner, said coupling comprising interengaging male and female parts of tubular form, the male coupling having an external taper and the female coupling having an internal taper, each taper having a thread of identical saw-tooth profile such that the threads, when engaged, are in mutually self-supporting relationship, wherein the threads are formed such that a ramped forward profile of the male thread will ride over an associated ramped forward profile of the female thread to snap engage the threads prior to relative rotation of the parts to complete secure coupling therebetween.

The tapers and the two threads are formed by machining on the same lathe using the same set-up on the lathe whereby to ensure exact co-operation between the interengaging threads.

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P:xoPER\RSH\P05230-9.SPE 16/2198 -3- The coupling can be provided by forming the male and female parts at opposite ends of each length of pipe, or by separate male and female coupling parts attached to the opposite ends of each length of pipe.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 is a cross-section through one end portion of a length of pipe for use as a bore liner and showing a male end fitting at that end portion; Figure 2 is a cross-section through the other end portion of the length of pipe and showing a female end fitting at that end portion; and Figure 3 is an enlarged detail showing the thread form on the male end fitting, an identical thread form being on the female fitting.

In accordance with the preferred embodiment of the invention, one end portion of a .15 length of pipe form forming a length of bore liner is machined to form an external taper with an external thread to form a male end fitting (Figure 1) and the other end portion is machined to form an internal taper with an internal thread to form a female end fitting (Figure For both end fittings the taper and thread are identical. The thread is of substantially saw-tooth profile (see Figure 3) having an inclined leading flank 2 which tapers in a direction towards 20 the adjacent end of the pipe. As shown, the trailing flank 4 of the profile is at right angles i to the axis of the pipe although other configurations are possible in which the trailing flank is inclined to the axis of the pipe by an angle other than 900. Importantly, the tapered male and female threads are machined with the same set-up on a lathe whereby substantially no tolerance will exist between the male and female threads when interengaged and as a result an exact match will exist between the interengaging threads whereby the threads can fit together tightly over their whole profile and over the whole thread length. This provides a substantial mutual self-supporting effect between the interengaging threads which provides substantial strength in the threaded connection.

The pipe can be fabricated from a suitable plastics material such as PVC or ABS.

p:\OpER\DH\53925-98.265 22/9/98 -4- To couple two lengths of pipe, the two lengths are pushed together so that the male thread of one pipe moves within the female thread of the other pipe. Due to the tapered form, the male thread can move within the female thread over a substantial part of the thread length before the threads start to meet. Towards the end of this movement as the threads start to meet, due to the saw-toothed thread form the ramped forward profile of the turns of the male thread will tend to ride over the ramped forward profile of the turns of the female teeth with a "click" type action to provide almost a snap-in fit. When the limit of this movement has been reached the connection can be completed by rotating the pipe through a few turns and this rotation will ensure complete engagement of the two threads over their entire thread length with the two threads being in firm, mutually supporting, engagement. It is to be noted that although each thread will consist of a large number of turns (by way of example 28 i turns), complete engagement can be achieved with only a few turns of the pipe (for example four turns) after the male thread has been pushed into the female thread. The lengths of pipe and at least the threads are preferably formed of a plastics material, which possesses a 15 sufficient resistance to allow the threads to ramp over each other for snap engagement.

The actual dimensions of the thread are to an extent a compromise between a number of different considerations. Whereas increased thread depth will result in increased strength and reduce the risk of thread stripping under the tensile load, on the other hand the greater the thread depth the greater is the tendency to cross thread during engagement and also the greater the thread depth the more turns will be required to complete the connection when the male thread has been pushed down into the female thread. For a typical liner having an outside diameter of 140mm and a wall thickness of 10mm we have determined that satisfactory results can be obtained when the thread depth is between about 0.5mm and 1.5mm. At thread depths of less than 0.5mm the strength would be reduced to an extent that the thread might strip under some tensile loading and we have determined that at thread depths of about 2mm and above there is a tendency for cross-threading to occur during connection. Bore liner for use in bores for water are conventionally produced in a range of outer diameters from 50 630 mm and the above detailed considerations regarding thread depth would be applicable throughout that range.

In one practical example the thread extends over a pipe length of 90mm with about p:\OpER\RSH\P05230-9.SPE 162I98 28 turns of thread machined at 8 turns per inch. The taper angle is about It will be noted from Figures 1 and 2 that the external and internal tapers commence at a small projecting shoulder 6,8 at the exterior and interior surface of the pipe respectively.

However it is important to note that when the two threads are fully tightened, the outer end edges of the male and female tapers do not abut against the adjacent internal or external shoulder but, instead are spaced therefrom by a small distance. This allows the depth of insertion of the male end fitting into the female end fitting to be restricted solely by the interengagement between the male and female threads and due to the exact compatibility between these threads substantially without tolerance a very strong mutually supporting connection is obtained between the interengaging threads.

It is to be noted that due to the manner in which the male and female threads interengage to provide a mutual self-supporting effect over the entire thread length the S. 15 primary zone of weakness will occur in the zone of the shoulders 6,8 which are not contacted by the other thread for the reasons just discussed. However in the zone of the shoulder the reduction in wall thickness of the pipe is only about 15% in comparison within excess of *.*with the square threads as used hitherto with the result that a substantial improvement in collapse strength is obtained in relation to equivalent pipes using a conventional square thread.

20 However in addition to the increase in collapse strength a substantial increase in tensile strength is also obtained. The configuration also provides substantially higher torsional strength for the threaded connection whereby the connection can withstand higher torque when coupling the two lengths together.

In some existing pipe liners with square threads, a metal band is applied around the end of the pipe for strengthening purposes. With a threaded end configuration as now described such a strengthening band becomes unnecessary.

It is also to be noted that due to the form of interengagement which is achieved between the male and female threads the joint becomes waterproof and self-sealing so that if p:\OpER\RSH\P05230-9.SPE 16/2/98 -6required the liner can also act as a pipe for passage of water through the bore without the need to install a separate water pipe through the liner. However for most applications it is envisaged that a separate water pipe would be installed.

Although as particularly described the connection between two lengths of pipe is provided by tapered male and female threads machined at the opposite ends of the pipe, in an alternative configuration a connector can be formed by co-operating male and female sockets machined with tapered male and female threads as described with the sockets then being attached to the ends of the pipe using a suitable form of welding, for example solvent 10 welding, a mechanical fastening, or a combination thereof.

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The embodiment has been described by way of example only and modifications are possible within the scope of the invention.

15 Throughout this specification and claims which follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

*4 9 a P:\OPER\DH\53925-9 8 .265 22/9/98 -7- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:- 1. A coupling interconnecting lengths of pipe for use as a bore liner, said coupling comprising interengaging male and female parts of tubular form, the male coupling having an external taper and the female coupling having an internal taper, each taper having a thread of identical saw-tooth profile such that the threads, when engaged, are in mutually selfsupporting relationship, wherein the threads are formed such that a ramped forward profile of the male thread will ride over an associated ramped forward profile of the female thread to snap engage the threads prior to relative rotation of the parts to complete secure coupling therebetween.

2. A coupling according to claim 1 or claim 2, wherein the tapers and the two threads are formed by machining on the same lathe using the same set-up on the lathe whereby to ensure exact co-operation between the intergengaging threads.

3. A coupling as claimed in claim 1 or 2, substantially as hereinbefore described with reference to the drawings.

DATED this 22nd day of September, 1998.

EURATECH

LIMITED

By its Patent Attorneys: DAVIES COLLISON CAVE

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