Laser-welded pipes for deep sea deployment

VENAREY LES LAUMES, FRANCE - In its production facility, Vallourec Umbilicals demonstrates how to use laser welding to produce pipes with high mechanical characteristics to reduce the wall thickness. With an 8kW Rofin CO2 laser, this factory produces approx. 10m of laser-welded pipes per minute.

The company is the world leader in manufacturing high-quality steel pipes that are used primarily in the oil and gas industry. In only a few years, they have developed a specialized area, that of the so-called "subsea umbilicals"—underwater pipelines that serve as conduits for various media such as supply lines, cables, or fiber optics. Umbilicals are an essential component of underwater installations and also serve to transport fluids, supply power, or transfer information between offshore platforms and their control station. Being deployed at depths of several thousand meters, it is obvious that these supply pipes have to be extremely robust—but little is known about how they are actually made. In 2011, the group founded its subsidiary Vallourec Umbilicals, which is dedicated exclusively to the production of special pipes that are conducted through the interior of the umbilicals.

As they have to meet extreme demands, pipes that run through umbilicals are usually produced seamlessly. Their manufacturing costs consist of 50 percent material, with the other 50 percent going towards production. Using the laser welding process, both material as well as production costs can be reduced and the technological properties can be improved.

Pipes that are conducted through media such as sea water require materials with an extremely high corrosion resistance. Rustproof Duplex steels—for example, stainless steels with equal parts of austenite and ferrite—offer high corrosion resistance combined with great strength characteristics. With its special alloy composition (25 percent chromium, 7 percent nickel, 4 to 5 percent molybdenum, and 0.3 percent nitrogen), so-called Super-Duplex steel has the best characteristics with regards to the corrosive media of the offshore environment. The welding process takes care of the splatters and porousness to avoid affecting the tube quality.

The extremely high demands on these special stainless steel pipes are also reflected in the complexity of the production line. In the first production step, the material for the continuous process comes directly from the coil-individual coils are joined together by tungsten inert gas (TIG) welding. Following the edge preparation by a scraping process, the stainless steel band is run through the molding of the roll former before reaching the laser welding area. To achieve the required weld quality, the subsequent welding process requires positioning of the laser beam exactly in the middle of the welding seam. A Rofin laser welding system, consisting of an 8kW CO2 laser and the company's Profile Welding System (PWS), is equipped with a proprietary Weld Sensor to detect the welding gap position. Thanks to the Weld Sensor, even the tiniest deviations in the gap position and the welding position are detected and corrected, keeping the laser beam precisely positioned at all times and thereby achieving the highest level of process reliability.

After welding, an in-line seam heat treatment ensures a uniform microstructure. With completion of the final brushing process, the welding seam is no longer visible to the naked eye.

The entire manufacturing process—from the coil to the finished pipe—is controlled by Vallourec, with the help of various nondestructive methods of measuring and an intelligent, quality in-line process sensor system. In case of any quality impairment, the respective pipe sections are marked, cut out, and rejoined by orbital welding before being rewound on a coil as continuous pipe. A pressure test of up to 2000 bar (2,000,000 hPa) concludes the quality control process. The finished pipes have diameters in the range of 16 to 40mm and wall thicknesses ranging from 1 to 3mm.

With the production of supply pipes for umbilicals, Vallourec has not only expanded its product line, but has developed a new and innovative manufacturing process for this special type of pipe. Laser welding has proven to be the ideal method by which to provide continuously welded pipes of extremely high corrosion resistance and high mechanical strengths. Last but not least, the intense cooperation and collaboration between all project partners has resulted in considering the integration of further laser processes into the production line.

Dipl.-Ing. Corinna Brettschneider, Rofin-Sinar Laser GmbH, Hamburg

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