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SANDVIK'S DUPLEX AND SUPERDUPLEX STAINLESS STEELS BECOME INDUSTRY STANDARD FOR DRILLING RISERS
Replacing standard 316L stainless steels, Sandvik SAF 2205 and SAF 2507 duplex grades offer increased corrosion resistance but more especially high tensile and yield strength. The result is weight and cost savings in the hydraulic tubes used by drilling riser producers. Drilling risers contain a package of tubes, comprising a heavy gauge, large diameter carbon steel central tube, through which the drill is lowered to the sea bed, and hydraulic tubes. Normally measuring between 38 mm and 127 mm outside diameter, the hydraulic tubes are secured around the central tube. Their function is to control valves, such as to exclude water during drilling, and operate at pressure, often at 300 bar or more. The risers, each of 20 or 30 metres in length, are joined together to achieve depths in excess of 2000 metres. The high strength benefits of duplex steels, compared to standard stainless steel grades, means that a much thinner hydraulic tube wall thickness can be used for the same pressure with a resultant weight saving. As an example, a 316L stainless hydraulic tube with an outside diameter of 2 7/8 inches, operating at 250 bar, would require a wall thickness of 0.376 inches. In duplex stainless steel SAF 2205 this could be reduced to 0.220 inches and in super duplex SAF 2507 to 0.173 inches - a potential weight saving approaching 50%. Although the high strength steels cost more per kilo, the reduced wall thickness means less cost per meter of tubing. Every 20 to 30 metre section of the riser is fitted with a flotation collar. The lighter tubes reduce the buoyancy aid requirement. Since this cost can be calculated in terms of cost per kilogram of buoyancy aid, customers again save money. Magnus Nystrom, Sandvik Steel's Technical Marketing Manager for oil and gas products comments, 'The trend is for oil to be extracted at ever greater depths, which points to the weight of drilling risers becoming more and more critical. Duplex steel has now become the industry standard for most of the major drilling riser producers'.
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