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New Welding Method Shows Appeal In Reducing Costs.

This story has its origin in the fact that the Netherlands Oil Company (NAM) specification for welding duplex stainless steel pipelines states that the first two layers should be welded using the (Gas Tungsten Arc Welding) GTAW (TIG) (Tungsten Inert Gas) method, with a backing gas system being used to protect the inside of the weld against oxidation. In the past, duplex stainless steel pipelines were also welded using coated electrodes (Shielded Metal Arc Welding) (SMAW method), but because of corrosion problems in SMAW-welded pipelines, the welding method prescribed for the first two layers was changed from SMAW to GTAW.

Max Klein, welding, materials and corrosion expert with NAM, said that duplex steel is used by NAM as a corrosion-resistant material for constructing pressure vessels, pipelines and pipeline systems. The aforementioned welding specifications were laid down to guarantee the quality of the weld on the inside. The use of the GTAW method for the first two layers, combined with the use of the correct backing gas procedure, prevents welding splatter or unacceptable oxidation on the inside of the weld. The minute cracks that develop at the position of the welding splatters or oxidized layers make it possible for split corrosion to develop under certain conditions. The possibility of split corrosion on the inside of a duplex or other type of stainless steel pipeline is unacceptable to NAM.

A major disadvantage of GTAW welding using a backing gas is that it requires time-consuming preparations before the actual welding can even start. It is necessary to install a special backing gas chamber inside the pipe, complete with the cables and hoses for the controls, the supply of backing gas and the associated measurements. It requires a high degree of accuracy and means lengthy delays and waiting periods prior to and during the welding process.

In practical terms, this procedure means a significant increase in the construction costs of stainless steel pipelines. Compared to the old method of welding using coated electrodes (SMAW method) the cost of a pipeline increases considerably. This is largely because of the completely different and more complex logistics and the longer construction time required.

Incentive

For Wim Dijkstra, mechanical engineer, Gas Land Business Unit, NAM, the higher costs were an important incentive to start looking for a cheaper alternative; if possible, one based on welding with coated electrodes (SMAW method). According to Dijkstra, experience has shown that pipelines welded using coated electrodes never gave problems under standard process conditions. Problems, if any, only arose after their decommissioning, particularly if the steel had received inadequate preservation treatment.

From Backing Gas Via Ceramics And Glass Fiber To Copper

To investigate whether there were other, cheaper alternatives for the welding method, Dijkstra got in touch with Jan Schepers, welding expert at Hulsink de Roo. After consulting with Max Klein, Schepers was assigned to conduct a variety of tests. In mutual consultation, a number of experiments were carried out with various methods and materials, such as ceramic, glass-fiber and copper backing strips on the inside of the welding seam. Tests were also carried out with coated electrodes. In the end, it was found that a copper backing strip gave the best results for the proposed application. In the meantime, the idea had also been postulated of using a standard internal (hydraulic) pipe clamp to press the strip of copper against the weld on the inside of the pipe. Such welds have been used in the pipeline industry for aligning pipe sections for quite some time. The manufacturer of these clamps is the Danish firm Weld-Tech ApS, a supplier to the pipeline industry. Rene Gronmark, managing director, modified the internal pipe clamps in accordance with Schepers' suggestions.

Grooved Shoe

The standard aligning clamp was modified and a six-piece copper shoe, which could be pressed hydraulically against the inside of the pipe to form a continuous ring, was added. It is essential that a central U-shaped groove should be milled in the copper shoe to act as a duct with a width of 15 mm.

The clamp construction, mounted on wheels, can be driven along the inside of the pipe to the position where the weld is to be made. The copper shoe must be lined up in such a position that the duct can be pressed exactly across the welding seam. During the welding process, the duct then serves to direct the flow of the backing gas, with the gas being supplied from the welding apparatus itself. This direct supply of inert gas provides the required protection in a trouble-free manner.

Corrosion Tests: Positive Results

The developers of the new welding method obviously did not do things by halves. A number of welds produced with this innovative welding method were subjected to corrosion tests at the Shell Research Laboratory. The results of these tests were positive and inspire confidence for using this new welding method in practice.

Advantages

According to Schepers, the advantages of the new GTAW welding method are obvious. It is very easy to install the clamping shoe in the pipe and to position it correctly. Welding can start as soon as the clamp has been positioned. There are no delays and waiting periods, because the time-consuming backing gas procedure is no longer required.

A not altogether unimportant additional advantage is that the adapted clamp still retains its original purpose as an aligning clamp. The clamp can also be used for a range of pipe diameters.

The application of the new procedure results in significant time savings and because waiting periods are no longer necessary, it is possible to make more welds per day. This means, of course, lower costs: over a stretch of pipeline it is possible to achieve cost savings of 10 to 15 percent.

Broader Market For Weld Tech ApS

The innovative adaptation of the existing aligning clamp has broadened the product range of Danish partner Weld Tech ApS. The new use, which results in faster and cheaper GTAW welding, consequently opens up interesting prospects in the international pipeline market.

Interim Stage

The favorable results of the new GTAW welding method are no reason for Dijkstra to rest on his laurels. He says he sees this as an interim stage. The new method does produce significant savings in overall pipeline costs, but it still requires an `in-pipe mechanism'. Only when we are able to do without this will we have achieved complete success, he says. Which means that the industry is still faced with a challenge: the `in-pipe mechanism' must go. Work must still be continued in order to lower pipeline costs even further.

Application

The positive results provided confidence for applying the newly developed method in practice. It was decided to do so on the previously planned 10-inch Saaksum-Munnekezijl pipeline. The approximately 11-km long pipeline will be constructed of 13 chrome steel and will be used to transport wet gas to the NAM's Grijpskerk gas treatment plant. Construction work has already begun and it will soon be determined how the new method performs in practice.
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Title Annotation:Netherlands Oil Co. specifications for pipelines
Comment:New Welding Method Shows Appeal In Reducing Costs.(Netherlands Oil Co. specifications for pipelines)
Publication:Pipeline & Gas Journal
Geographic Code:4EUNE
Date:Dec 1, 2000
Words:1168
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