Electric heating – Metal heating – By arc
Reexamination Certificate
2001-05-04
2002-08-06
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S061000, C219S125110, C219S125120
Reexamination Certificate
active
06429405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for welding pipes together. More specifically the invention relates to arc-welding together pipe sections when laying pipelines. The pipelines may be underwater pipelines or inland pipelines. The welding process used when laying such pipelines is commonly of the type where a continuous-wire arc welding torch is used. The present invention is particularly, but not exclusively, concerned with a welding process used when laying pipelines underwater.
2. Description of the Related Art
When laying a pipeline at sea it is customary to weld, on a lay-barge, individual pipe sections to a pipe string (the pipe string leading towards the seabed). The welding process takes place close to the surface of the water. The pipe sections may consist of a plurality of pipe lengths each welded together on the lay-barge to form the pipe sections when required.
The pipe-string, when being laid, is under great tension and weld joints must, of course, necessarily be sufficiently strong to withstand the high forces imposed on the weld joints. Each time a pipe is welded to another pipe extensive tests are made to ensure that the quality of the weld joint formed is sufficient. The strength of a weld joint depends upon various factors, one being the geometry of the path traced by the point of contact of the arc in relation to the surfaces of the pipes to be joined. If the point of contact of the arc is off target by as little as a tenth of a millimeter the quality of the joint may be reduced by enough that the pipe joint is rejected, when tested, as not being of sufficient quality. It is therefore important that the weld metal is laid down in the region of the joint with great accuracy.
Furthermore the radial distance of the electrode with respect to the pipes must change in relation to the depth of the weld joint. As the region of the joint between the pipes is filled with welded metal the surface of the welded metal gets closer to the welding torch.
There are therefore special considerations that must be taken into account when designing an apparatus for welding such pipes together.
A known method of welding two pipes together may be described as follows. The pipes to be joined are prepared prior to the welding process by beveling the ends of the pipes such that when the pipes are arranged immediately before the welding process commences (coaxially with respect to each other), an exterior circumferential groove is defined between the two pipes. The pipes are positioned ready for welding. A carriage is mounted on one of the pipes for movement around the circumference of the pipes to be joined. A welding torch is mounted on the carriage and the apparatus is so arranged that the end of the metal electrode of the torch is opposite and relatively close to the circumferential groove. The carriage is moved around the circumference of the pipe and the torch is operated so that an arc is directed into the groove. The arc is guided manually and/or by various mechanical sensors to guide the arc as accurately as possible along the length of the groove. The welding process generally takes several passes.
In the above-described method the resolution of the mechanical sensors is such that a human operator is required to assist in the welding process for guiding the arc with sufficient accuracy.
The time it takes to lay a given length of pipeline is, to a great extent, determined by the time it takes to perform all the necessary welding operations. There has therefore been a general desire to reduce the time it takes to weld two pipes together. Any attempt to speed up the welding process should not however lead to a significant reduction in the quality of the weld joint.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus and method for welding pipes together that mitigates at least some of the above-mentioned disadvantages associated with the known method and apparatus described above. A further object of the present invention is to provide an apparatus and method for welding pipes together that is faster at welding pipes together than the known method and apparatus described above but without significantly reducing the quality of weld joint.
Thus the present invention provides a welding apparatus for welding pipes together to form a pipeline comprising
a carriage carrying a plurality of arc welding torches,
a control unit for facilitating automatic guidance of the arcs produced by the torches, wherein the apparatus is so configured that it may be used to weld together two pipes laid end to end defining therebetween a groove, by arranging the apparatus so that
the carriage is mounted for movement around the circumference of the pipes,
the control unit receives signals representing electrical characteristics of the welding with regard to each pipe, respectively, whereby
the control unit facilitates automatic guidance of the arc of each torch along the groove by comparing the signals relating to one of the pipes with the corresponding signals relating to the other of the pipes.
Both the automation of the process and the provision of a plurality of torches on the carriage facilitate the speeding of the welding process, whilst not necessarily significantly increasing the complexity or cost of the welding apparatus. Costs may also be reduced because there is no need for there to be a skilled operator for manually guiding the welding apparatus.
The present invention also provides a method of welding two pipes together, the method comprising the steps of
arranging two pipes end to end, the pipes being so shaped that a circumferentially extending groove is defined between the ends of the pipes,
effecting relative movement of a plurality of arc welding torches at substantially the same speed around the pipes and operating the torches so that their arcs form a weld in the groove,
automatically guiding the arcs produced by the torches by ascertaining electrical characteristics of the welding with regard to each pipe, respectively, and comparing the electrical characteristics relating to one of the pipes with the corresponding electrical characteristics relating to the other of the pipes.
The present invention yet further provides a method of forming a deep sea pipeline or a cross country pipeline including a step of welding two pipes together, said step of welding two pipes together comprising the steps of
arranging two pipes end to end, the pipes being so shaped that a circumferentially extending groove is defined between the ends of the pipes, the angle of separation of the walls defining the groove being less than 10 degrees,
effecting relative movement of a plurality of arc welding torches arranged on a single carriage around the pipes and operating the torches so that their arcs form a weld in the groove,
each torch being moved around the pipe with substantially the same component of velocity along the length of the groove,
each torch being independently moved so that the respective positions of the arcs within the groove oscillate between the walls of the groove in a direction having a component parallel to the axis of the pipe, and
automatically guiding the arc produced by each of the torches by ascertaining, in respect of an arc, electrical characteristics of the welding with regard to each pipe, respectively, and comparing the electrical characteristics relating to one of the pipes with the corresponding electrical characteristics relating to the other of the pipes.
When the torches are first operated to form a weld, it is preferable for a first torch to start welding and for other torches to start welding only once they have reached the position at which the first torch was started. The torches may be shut down in order in a similar manner.
Many proposals have been made in the past to speed up the welding process with which the present invention is concerned. One such proposal is to provide more than one welding apparatus, each welding apparatus being operated by a respectiv
Belloni Antonio
Bonasorte Renato
Burns Doane Swecker & Mathis L.L.P.
Saipem S.p.A.
Shaw Clifford C.
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