Understanding corrosion of flexible pipes at subsea oil and gas wells
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Energy Sector Analysis
High levels of carbon dioxide in production fluids corrode steel armor wires in flexible pipes connecting the subsea wells to floating production platforms. A better understanding of the corrosion mechanism, or a switch to composite pipes, could improve safety and reliability.
Understanding corrosion of flexible pipes at subsea oil and gas wells By Melissae Fellet Advisor: Rolf Nyborg
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lexible pipes installed at two offshore natural gas wells in Brazil’s pre-salt fields recently ruptured after only a few years of operation. These pipes were expected to last more than 20 years, so Petrobras, the company developing the fields, investigated the cause of the failure. They discovered damage to the pipes’ outer cover and found corroded steel armor wires in the pipes. Executives were surprised to learn that high concentrations of carbon dioxide contributed to the corrosion and eventual ruptured pipes. Failures, while infrequent relative to the thousands of flexible pipes in operation, have also occurred in at least four flexible risers connecting floating production vessels to subsea wells off the coasts of Norway and Africa. When flexible pipes were first introduced 30 years ago, it was thought that the interior of the pipes remained dry and noncorrosive. However, engineers now know that the unique environment that traps water vapor and carbon dioxide between layers of steel inside the pipe contributes to corrosion—even when its external cover is undamaged. Producers are beginning to use corrosion-resistant composite flexible pipes for subsea flow lines at deep wells. Companies are also testing and developing hybrid systems that combine steel and composite pipes to maximize the benefits of both systems. With oil and gas reserves in shallow waters running dry, producers are turning to fields in deep and ultra-deep waters off the coasts of Brazil, Norway, Angola, and the United States. Bringing fluids through 3000 meters of water to the surface poses new challenges for well operators: production fluids with increased temperatures and pressures that challenge welds; increased carbon dioxide—and sometimes hydrogen sulfide—that scour and weaken steel; deep ocean water pushing on pipes with increased pressures; and currents trying to drag pipes through the water. “Any one of those things would be manageable,” said Martin Jones, chief executive officer at Magma Global Ltd., a company that produces composite flexible pipe. “But the high temperatures, pressures, and corrosive fluids together make it very difficult for traditional materials.” The industry’s standard steel-reinforced flexible pipe starts with a corrugated liner made by interlocking S-shaped pieces of duplex stainless steel or high-strength steel alloyed with nickel or molybdenum. To make this structure gas- and hydrocarbon-tight, a thermoplastic polymer liner, typically polyamide, poly(vinylidine) fluoride (PVDF), or high-density polyethylene (HDPE), is extruded over the steel carcass. Then, layers of high-
strength carbon steel wire
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