A new method of making plastic-coated steel pipeline sections promises a 10% reduction in installed costs of risers, pipelines, and flowlines for sour wells.
This is the claim of Design Unique System (DUS) developer Avesta Sheffield AS, Oslo, which acquired North Sea application rights from Swedish engineer Dag Thulin, who invented and patented DUS.
Sten-Goran Johansson, Avesta's DUS project leader, said, "The typical temperature in North Sea wells is getting higher, pressures are getting higher, and oil is getting more aggressive.
"Titanium components are expensive, and carbon steel can't withstand such tough operating conditions, yet the market requires cost-efficient solutions. We have bought the exclusive right to a cheap and efficient solution."
With Thulin's technique, a length of steel pipe is wrapped in layers of glass fiber sheeting, between which small diameter tubes are laid lengthwise for injection of plastic resin.
Resin is pumped down the injection tubes as they are being withdrawn from the fiber glass wrapping. The resin suffuses evenly through the glass fiber to create a reinforced outer coating for the pipe, which is then cured.
A DUS pipe length consists of a 1-30 m, 6-40 in. pipeline section made of any of a number of types of steel coated in fiber-reinforced plastic.
Avesta says coating the steel pipe in composite materials enables considerable savings in the amount of steel required and improves corrosion resistance.
For a 10 in. diameter pipe rated at 500 bar, Avesta calculates a DUS pipe consisting of a 6 mm thick Duplex stainless steel wall and a 30 mm thick glass reinforced plastic are required.
A 10 in. diameter pipeline rated to 7,250 psi made only of Duplex stainless steel would require a 22 mm wall thickness, said Avesta.
Since DUS pipe weighs two thirds that of solid steel pipe, it can be assembled onshore and towed out to the installation site by tug, avoiding the high cost of using a specialized pipelay barge. Johansson said DUS is currently being qualified for operating temperatures as hot as 150° C. and pressures of 1,450-43,500 psi by independent certification bureau Det Norsk Veritas, Oslo.
Long term tests on DUS samples are to begin shortly. The product was checked out theoretically using finite element analysis and is expected to overcome the brittleness problem associated with other plastic-coated steel pipeline techniques.
"The DUS pipe is likely to be available on the market in 1 year," said Johansson. "We have visited the Den norske stats oljeselskap AS (Statoil) laboratories at Trondheim with this product, and they are very interested.
"We are also working with Norsk Hydro AS to develop the product further, and we have been in contact with lay barge contractors that have said they are interested because of DUS' short welding time and lightness."
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