When pipes are joined and used in for example a downhole, sub-sea or remote environment it is advantageous to log the position of individual pipes or to detect the position of individual pipes when the pipe and/or surrounding hole is re-entered. This is useful, to e. g. allow accurate placement of side-tracks, packers and perforations.

At the moment logging of this nature is done by measuring the distance a tool has travelled within or adjacent to the pipe, counting the number of connections or by positioning easily identifiable markers, such as magnetic or radioactive markers, in the hole alongside, and attached to, the pipe, which may'be a production tubing and/or casing which marker may be found using specialized equipment. In addition special pup joints are often used for this purpose. The existing methods for marking pipe joints are expensive, time consuming and may lead to a quick wear of the markings and replacement of worn markings may require retrieval of the entire pipe string from a well, which is very expensive.

An object of the present invention is to provide a cheap and reliable method of placing markers along strings of tubulars, such that the markers are well protected and not vulnerable to. wear or dislodging after installation.

Summary of the Invention The method according to the invention comprises creating a cavity into an end surface of one of the tubular ends that are to be joined, inserting a marker into said cavity and subsequently joining the tubular ends.

The tubular ends may be joined by welding, such as forge welding, or may be pressed together by a screw thread connector.

The thus inserted marker may comprise an electronic tag such as a passive radio frequency identification'tag, or a magnetic or radioactive material and the cavity may be machined at or near the center of said end surface.

The invention also relates to a string of joined tubulars that are marked in accordance with the invention, so that a marker is permanently arranged in a cavity adjacent to at least one joined tubular joint.

The tubular string may comprise a plurality of joints that are provided with markers, wherein each marker transmits a radio, magnetic, radioactive or other detectable signal, which is detected using appropriate equipment., and may be different to the signal. transmitted by any other marker.

The tubular string may be a string of oil field and/or well tubulars.

Thus, the present invention provides an improved method of marking joint locations when used with screw thread connections or welding techniques, such as forge welding, fusion welding, diffusion welding, amorphous bonding, friction welding or other techniques in which a metallurgical bond is formed between abutted pipe ends.

It preferably involves positioning a marker mid-wall in a

tubular joint such that it is an integral part of the joint and cannot be dislodged.

Description of a preferred embodiment These and further. features and advantages of the method according to the present invention will be described in more detail with reference to the accompanying drawings in which: Fig. 1 is a longitudinal sectional view of prepared and mated ends of pipe joints suitable for welding in which a marker has been inserted in accordance with the method according to the invention; Fig. 2 is an enlarged longitudinal section view of the prepared and mated ends and marker shown in Fig. l ; and Fig. 3 is a longitudinal sectional view of a pair of tubulars that are interconnected by a screw thread connector and an end surface of one tubular comprises a cavity in which a marker is inserted.

As shown in Figures 1 and 2 the method according to the invention involves preparing the tubular ends 1 and 2 of a tubular joint for welding, and machining a slot or hole 3 into an end face of one of the tubular ends 1.

A small electronic tag, or amount of radioactive or magnetic material may then be placed securely into the slot 3 to act as a permanent marker 4. When welding takes place the area containing the marker is forged and the marker 4 becomes trapped inside the made-up string of pipes, which are lowered into the hole. When necessary it may then be detected using an appropriate logging device.

Fig. 3 depicts an embodiment of the joint marking method according to the. invention wherein a pair of tubulars 10,11 is interconnected by a screw thread connector 12 such that inner and outer disk-shaped end

surfaces 13 and 14 are pressed together after the screw thread connection has been made.

A cavity 15 has been drilled into the outer disk- shaped end face of the upper tubular 10, in which cavity 15 a marker 16 has been inserted. The marker 16 is embedded in a sealant 17 which seals off the entrance of the cavity.

The marker 16 is adequately sealed off from fluids in the interior and exterior of the string of tubulars 10,11 and is adequately protected from impacts and forces exerted on the inner and outer walls of the string of tubulars 10, 11.