The present invention relates to a coupling for use in the forming of drill strings and downhole pipe drilling assemblies . Drill strings are used in drilling operations, where a single drilling tool is not sufficient to reach the depth of drilling required. A drill string is formed by joining together a number of drill tool sections to the length required, by a coupling joint which is normally a single start, screw thread type joint. Where a new drilling operation is undertaken, it is common practice to continually lengthen the drill string as the depth of the drilling increases, by continuingly adding drill tool sections. This can be a time consuming, arduous process, as, each time a drill tool section is to be added, the drilling operation must be stopped, a new tool added and the operation restarted.

Where the drilling operation is finished and it is necessary to dismantle the drill string, each coupling joint between adjacent drill tool sections must in turn, be broken and the particular drill string section removed. The sections are normally removed at the drilling platform, whether it be on land or at sea.

Great difficulty is often encountered in "breaking" the coupling between drill tool sections, as the coupling is normally much tighter at the conclusion of drilling, than when the drill string was initially formed, due to the torque transmitted through these joints by continual drilling. It is often found that the torque needed to break these joints is more than that which the joint is subjected to, to initially form the joint. The torque needed to break these joints can be of such magnitude that it can be dangerous.

In some instances, the torque needed to break a joint is so great, that it exceeds the tensile strength of the metal forming the joint and the joint fails. It is normally found that the failure point is at the base of the threaded section of the male member, which severs, thereby ruining the particular coupling and rendering the

associated drill tool sections useless.

A further problem, is that of thread "popping".

This occurs when a thread is stripped on either the male or female part and the coupling can no longer be threaded together. Again, this is caused when the tensile strength of the threads is exceeded.

It is an object of the present invention to obviate or at least alleviate some or all of the above problems.

According to the present invention there is provided a drill string coupling for use in coupling together sections of a drill string, said coupling comprising a male part connectable to a drill tool section and a female part connectable to another drill tool section, said male and female parts having interengageable multiple start V-screw threads, for releasably interconnecting the male and female parts and to thereby couple the drill string together.

Further, the present invention provides a method of forming a composite drill string from separate drill tool sections, said method comprising connecting a first member to one end of one of said drill tool sections and connecting a second member to one end of another of said drill tool sections, said first and second members including interengageable, multiple start V-screw threads the method further comprising forming said drill string by threadably connecting said male and female parts.

The V-screw thread as described above is a common thread arrangement in which both sides of opposide threads diverging from a trough in the thread arrangement diverge at substantially the same angle from the centre-line of the trough.

One advantage of a multiple start V-screw thread in a drill string coupling, is that the "thread-up" time to connect separate drill tool sections is substantially reduced over that of a single start thread. The "thread-up" time is the time taken to thread together the male and female parts of the coupling. Conversely, the "thread-down" time when dismantling the drill stirng is also reduced in the same proportion. The time reduction

is directly proportional to the number of thread starts, with the time reduction increasing with the number of starts, ie for a two-start V-screw thread, the threading time is reduced by half, over a single-start thread with similar pitch properties; for a three-start V-screw thread, the threading time is reduced to a thrid of that for a single-start thread, etc.

A further advantage of the multi-start V-screw threaded coupling, is that the "breaking" torque, (ie that torque which is required to break the coupling between drill tool sections) can, to some extent, be governed by the number of thread starts and the thread dimensions, ie pitch size and pitch diameter. It is possible, for example, to reduce the breaking torque to a value lower than the make-up torque (the "make-up" torque being the torque to which the coupling is subjected to effect the initial connection) . This contrasts with single start threads used in existing couplings where the breaking torque is typically higher than the initial make-up torque. By reducing the braking torque to a level lower than the make-up torque, the drilling operator can dismantle the drill string, knowing that the torque needed to do this will not exceed the torque needed to assemble the string, thereby avoiding the problems referred to before.

The multi-start V-screw thread has also been found to resist greater stress levels than single start threads, thereby allowing greater torque levels to be used to form the joint and providing greater safety when "breaking" the joint. It is also found that stress is more evenly distributed across the entire threaded area. These properties reduce the likelihood of "popping", as greater stress levels must be subjected to a multiple start thread over a single start thread, in order to "pop" a thread. Where, for whatever reason, an excessive torque is applied to the drill string, the multiple start V-screw thread has also been found to fail in an advantageous manner compared to that of a single start thread. A single start threaded coupling tends to fail at the base

of the male threaded section. If the failure is complete, the threaded section of the male member will have been severed from the remaining portion of the member and will remain in threaded connection with the female member down the hole. If the failure is partial, then applying further torque to the coupling will cause the failure to creep until the failure becomes complete. Having severed the coupling, the continuity of the drill string is broken. However, a multi-start V-screw threaded coupling tends to fail along the spiral of the thread and, while this requires replacement of the drill tool sections, the continuity of the drill string is not always broken, thereby reducing down time and possible loss of drilling tools down the hole. In order to assist in arriving at an understanding of the prevent invention, a preferred embodiment is illustrated in the attached drawing. However, it should be understoon that the following description is illustrative only and should not be taken in any way as a restriction on the generality of the invention as described above.

Figure 1 shows a partially cut away section of a drill string coupling according to one preferred embodiment. Referring to Figure 1, there is generally shown a male part 1 and a female part 2. As shown, the external shapes of both the male and female parts 1 and 2, are circular, however, they may be made to any shape which suits their use. Male part 1 includes abutment surfaces 4 and 5, while female part 2 include abutment surfaces 7 and 8.

Abutment surfaces 4 and 7 are adapted to meet when male part 1 and female part 2 are threadably connected together to provide a locking step. In a similar manner, abutment surfaces 5 and 8 are also adapted to meet on connection of male and female parts 1 and 2. Only one set of abutment surfaces 4 and 7 or 5 and 8 need be provided, although it is desirable to have both.

The coupling shown is hollow, having an internal

diameter 10 and an external diameter 11. However, the connector need not be hollow.

It can be seen that male part 1 has a tapered threaded area 12. This threaded area as shown, is a two-start V-screw thread. The threads may be of any form suitable for the particular loading which the coupling will be subjected. A person skilled in the art will be familiar with such thread forms, however, a range of suitable thread forms is shown in the TRI-STATE OIL TOOL INDUSTRIES, INC. catalogue although these are not shown as multiple start threads. The first thread start 13 is shown, while the second thread start 14 is located diametrically opposite first start 13 and cannot be seen. So thread start 13 and thread start 14 alternate as individual threads as shown. It is to be appreciated that any number of thread starts is possible and dependent only on the size of th connector and on the properties to be achieved. For example, while a two-start V-screw thread will reduce thread-up and thread-down times and reduce the breaking torque over the make-up torque, in relation to a one-start thread, a three-start V-screw thread will reduce these properties further and so-on for four, five and six, etc start threads.

The threaded area 16 located on the female part 2 is constructed to be threadably connectable to male part 1. As such, the number and location of thread starts in threaded area 16 must equal those of threaded area 12, as should other thread characteristics such as thread form, diameter and pitch. Those skilled in the art will appreciate that there may be many variations and modifications of the configuration described herein which are within the scope of the present invention.