Understanding corrosion of flexible pipes at subsea oil and gas wells

11 Oct 2018

“Flexible 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.

However, there is a way to eliminate stress-induced corrosion cracking: Eliminate the steel from flexible pipes. Companies such as Magma produce flexible pipes for the oil and gas industry by winding layers of composite reinforced with glass or carbon fibers around a polymer liner. Each layer of the thermoplastic composite is fused to the one below it. Composite pipes are much lighter than steel flexible pipes, so operators can use simpler, less expensive equipment to install them.”

Extract from MRS Bulletin,43(9), 654-655. Understanding corrosion of flexible pipes at subsea oil and gas wells. Author Fellet, M., & Nyborg, R. (2018). doi:10.1557/mrs.2018.214

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