Field of the Invention

The invention relates to pipeline welding. More particularly, the invention relates to welding of pipe sections together to form a pipeline.

The invention has been devised particularly, although not solely, for welding of pipe sections together in the construction and laying of subsea pipelines in the oil and gas industry.

Background Art

It is common to construct and lay subsea pipelines using a lay barge or vessel on which pipe sections are welded one to another to form a pipeline which is progressively laid onto the seabed.

The pipe sections are typically welded together in stages at various welding stations on . the lay barge or vessel. The welding operation often involves an automatic welding system, such as a bare wire system. Automatic welding operations are particularly suitable for large pipeline installations as they allow the pipe sections to be welded together relatively quickly. However, automatic welding operations are not particularly effective, and can also be relatively expensive, for small pipeline installations.

It is against this background that the present invention has been developed.

Disclosure of the Invention

According to a first aspect of the invention there is provided a method of welding two pipe sections together in the construction of a pipeline, the method comprising applying a weld between the two pipe sections in stages involving root, hotpass, fill and cap stages, wherein the root stage is applied using a Surface Tension Transfer (STT) process.

The STT process is used for the weld root as this process allows a weld deposit to be made at high speed that avoids root weld lack of fusion and the associated cracking that is likely to occur with other processes because of high heat dissipation.

Preferably, the hotpass, fill and cap stages are performed using a semi-automatic welding process; typically, a gas-shielded flux-cored arc-welding process.

The fill may be applied in multiple phases, involving interlocked multiple layers.

The welding method according to the invention may be performed on a lay barge or vessel where the welding procedure is performed over four stations. The root and hotpass stages are performed at the first station. The fill stage is performed over two phases; a first phase performed at the second station and a second phase performed at the third station. The cap stage is performed at the fourth station.

The welding method according to the invention can produce welds compliant with Standard DNV-OS-F 101-2000.

Preferably, the method further includes inspection of the weld, the inspection being conducted in two stages, the first inspection stage being undertaken after completion of the root and the hot pass welding stages, and the second inspection being undertaken after completion of the weld (ie. after the cap welding stage).

Preferably, the inspection process is performed using standard radiography testing.

According to a second aspect of the invention there is provided a pipeline constructed by welding pipe sections together by a method according to the first aspect of the invention.

According to a third aspect of the invention there is provided a pipeline comprising a plurality of pipe section welded one to another in end-to-end relation, the weld

between adjacent pipe sections comprising a root, a hotpass, a fill and a cap, wherein the root is applied using a Surface Tension Transfer process.

According to. a fourth aspect of the invention there is provided a method of welding two pipe sections together in the construction of a pipeline, the method comprising applying a weld between the two pipe sections in stages involving root, hotpass, fill and cap stages, wherein the root stage is applied using a gas- metal-arc welding (GMAW) process.

A gas-metal-arc welding (GMAW) process is performed using an STT power source, such as that provided by Lincoln Electric Company.

According to a fifth aspect of the invention there is provided apparatus for welding a plurality of pipe sections together in end-to-end relation to form a pipeline, the weld between adjacent pipe sections comprising a root, a hotpass, a fill and a cap, the apparatus comprising a first welding station for applying the root and hotpass, a second welding station for applying a first phase of the fill, a third welding station for applying a second phase of the fill, and a fourth welding station for applying the cap.

According to a sixth aspect of the invention there is provided a lay barge or pipeline laying vessel equipped with apparatus for welding a plurality of pipe sections together in end-to-end relation to form a pipeline, the weld between adjacent pipe sections comprising a root, a hotpass, a fill and a cap, the apparatus comprising a first welding station for applying the root and hotpass, a second welding station for applying a first phase of the fill, a third welding station for applying a second phase of the fill, and a fourth welding station for applying the cap.

Brief Description of the Drawings

The invention will be better understood by reference to the following description of several specific embodiments thereof as shown in the accompanying drawings in which:

Figure 1 is a schematic fragmentary sectional view of adjacent ends of two pipe sections, with the ends beveled in preparation for welding of the two pipe sections together in accordance with the first embodiment;

Figure 2 is a schematic fragmentary sectional view of the adjacent ends of the two pipe sections welded together in accordance with the first embodiment; and

Figure 3 is a schematic view, on an enlarged scale, of the weld in accordance with the first embodiment between the two pipe sections.

Best Mode(s) for Carrying Out the Invention

The first embodiment shown in the drawings is directed to welding of pipe sections together to form a pipeline for the oil and gas industry. The pipeline is for subsea application, and is constructed and laid using a lay barge or vessel of known kind. Pipe sections are welded one to another to progressively form the pipeline as it is laid onto the seabed.

Referring to the drawings, there is shown a weld 10 according to the first embodiment between the adjacent ends of two pipe sections 11 , 12. The pipe sections 11 , 12 each comprise a clad pipe of known kind, having an outer pipe layer 15 and an inner pipe layer 17. The outer pipe layer 15 is sometimes referred to as an outer pipe, and the inner pipe layer 17 is sometimes referred to as a liner pipe. The inner and outer pipe layers are of different materials. It is common, for example, for the outer pipe section material to be a high strength carbon steel and for the inner pipe section material to be a corrosion resistant alloy. Such pipes are also referred to as bi-metal pipes.

In this embodiment, the outer pipe layer 15 is of high strength carbon steel such as X65, and the inner pipe section 17 is of corrosion resistant alloy (CRA) such as 316L stainless steel.

With pipe sections 11 , 12 of the type shown in the drawings, the end of each inner pipe layer 17 terminates inwardly of the end of its corresponding outer pipe layer 15, with a weld layer 19 being provided to seal the ends of the inner and outer pipe layers. The weld layer 19 comprises an "over-alloyed" stainless steel, such as 309 MoL.

The adjacent ends of the two pipe sections 11 , 12 are beveled so as to provide so-called "knife edges" 20, as shown in Figure 1 , to receive the weld 10. The ends of the pipe sections 11, 12 are positioned in a slightly spaced apart relationship, which in this embodiment is about 4mm.

The weld 10 is applied in stages, comprising a comprising a root 21 , a hotpass 23, a fill 25 and a cap 27.

In this embodiment, the root 21, hotpass 23 and fill 25 are formed using ER 2209 welding consumables.

The weld cap 27 requires an all positional welding consumable. ER2209 consumables, while suitable for the roof, hotpass and fill stages of the weld, were found to be unsuitable for the cap pass as a lack of surface tension in the molten weld pool (particularly in the "5 and 8 o'clock" positions) meant the weld molten pool became unstable. To alleviate this problem and maintain the integrity of the weld, an all positional 309 MoL Stainless Steel consumable is used for the capping pass to provide the weld cap 27.

The root 21 comprises a single "slug" of metal applied using a GMAW process, performed using a STT power source, such as that provided by Lincoln Electric Company, which is a high-frequency, current controlled machine wherein power to the arc is based on instantaneous arc requirements.

The hotpass 23, fill 25 and cap 27 are applied using a semi-automatic welding process, specifically a gas-shielded flux-cored arc-welding process.

The hotpass 23 is a single pass.

The fill 25 is applied in multiple phases, involving interlocked multiple layers.

The welding procedure is performed over welding four stations on the lay barge or vessel. The root 21 and hotpass 23 are applied at the first station. This provides a bond between the two pipe sections 11 , 12 of sufficient strength to allow the two pipe sections to then be advanced so that their ends register at the second station at which a first phase of the fill 25 is applied. Two pipe sections 11 , 12 are then advanced so that their ends register at the third station where the second phase of the fill 25 is applied. The two pipe sections 11 , 12 are then advanced again so that their ends register at the fourth station where the cap 27 is applied. At this stage, the weld 10 is complete. From the fourth welding station, the pipeline progresses to a laying station from which it is progressively laid onto the seabed.

The welding procedure further includes inspection of the weld. The inspection is conducted in two stages. The first inspection stage is undertaken after completion of the root and the hot pass at the first station. The second inspection stage is undertaken after completion of the weld (ie. after application of the cap at the fourth station).

The inspection process is performed using standard radiography testing.

This approach ensures that any root repair required could be identified and performed immediately without the need to back-up the installation barge. Additionally, any defect found in the completed weld could be confirmed as being contained within the fill or cap and evaluated for repair accordingly.

The operation producing the weld 10 is a semi-automatic, manually based procedure. It is particularly appropriate for installation of pipelines, such as for example small pipelines, where the cost of automatic welding systems are not warranted.

The weld 10 is compliant with Standard DNV-OS-F101-2000 and is suitable for "hot pipe" applications as well as ambient temperature applications.

The second embodiment is directed to a weld between adjacent ends of carbon steel pipe. Such a weld is similar to the weld illustrated schematically in Figure 3 and so corresponding reference numerals are used to identify similar parts.

In the weld according to the second embodiment, carbon steel consumables are used to match the carbon steel pipe.

The root 21 comprises a single "slug" of metal applied using a GMAW process which involves a solid wire consumable with CO ₂ gas shield. The GMAW process _^ is performed using a STT power source, such as a Lincoln Ultramag S6.

The hotpass 23, fill 25 and cap 27 are applied using a semi-automatic welding process, specifically a gas-shielded flux-cored arc-welding process. In certain applications, this process may be performed using a Lincoln Pipeliner G70M, using an arc welding wire of 1.2mm diameter and a gas shield comprising a mixture of Argon and CO ₂ (typically about 80% Argon/20% CO ₂ ).

Such a weld would only require inspection at completion. In other words, it does not require inspection in two stages as proposed for the weld according to the first embodiment.

It should be appreciated that the scope of the invention is not limited to the scope of the embodiments described. The invention is not, for example, limited in application to clad pipe of the type described in the first embodiment, having a weld layer sealing the ends of the inner and outer pipe layers. Indeed, the invention is applicable to welding of other types of clad pipes, including mechanically lined and metallurgically lined clad pipe. The invention is in fact applicable to welding of pipe sections generally, with the particular consumables selected for use being dependent upon the pipe materials.

Modifications and improvements can be made without departing from the scope of the invention.

Throughout the specification, 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.