A team of researchers at MIT have come up with a solution that might prevent a Deepwater Horizon-like oil rig disaster (caused by a mixture of frozen water and methane, called methane clathrate). It may also prevent blockages inside oil and gas pipelines that can lead to expensive shutdowns to clear a pipe, or worse, to pipeline rupture from a buildup of pressure.
The new method of preventing the icy buildup is described in a paper in the journal ACS Applied Materials and Interfaces by associate professor of mechanical engineering Kripa Varanasi, postdoc Arindam Das, and recent graduates Taylor Farnham SB '14 SM '16 and Srinivas Bengaluru Subramanyam PhD '16.
The key to the new system is coating the inside of the pipe with a layer of a material that promotes spreading of a water-barrier layer along the pipe's inner surface. This barrier layer, the team found, can effectively prevent the adhesion of any ice particles or water droplets to the wall and thus thwart the buildup of clathrates that could slow or block the flow.
Unlike previous methods, such as heating of the pipe walls, depressurization or using chemical additives, which can be expensive and potentially polluting, the new method is completely passive — that is, once in place it requires no further addition of energy or material. The coated surface attracts liquid hydrocarbons that are already present in the flowing petroleum, creating a thin surface layer that naturally repels water. This prevents the ices from ever attaching to the wall in the first place.
Existing prevention measures, known as flow assurance measures, "are expensive or environmentally unfriendly," said Varanasi, and currently the use of those measures "runs into the hundreds of millions of dollars" every year. Without those measures, hydrates can build up so that they reduce the flow rate, which can reduce revenues, and if they create blockages then that "can lead to catastrophic failure," Varanasi said. "It's a major problem for the industry, for both safety and reliability."
The problem could become even greater, says Das, the paper's lead author, because methane hydrates themselves, which are abundant in many locations such as continental shelves, are seen as a huge new potential fuel source, if methods can be devised to extract them. "The reserves themselves substantially overshadow all known reserves [of oil and natural gas] on land and in deep water," he says.
But such deposits would be even more vulnerable to freezing and plug formation than existing oil and gas wells. Preventing these icy buildups depends critically on stopping the very first particles of clathrate from adhering to the pipe: "Once they attach, they attract other particles" of clathrate, and the buildup takes off rapidly, says Farnham. "We wanted to see how we could minimize the initial adhesion on the pipe walls."
The approach is similar to that being used in a company Varanasi established to commercialize earlier work from his lab, which creates coatings for containers that prevent the contents—anything from ketchup or honey to paint and agrochemicals — from sticking to the container walls. That system involves two steps: first creating a textured coating on the container walls, and then adding a lubricant that gets trapped by the texture and prevents contents from adhering.
The new pipeline system is similar to that, Varanasi explains, but in this case "we are using the liquid that's in the environment itself," rather than applying a lubricant to the surface. The key characteristic in clathrate formation is the presence of water, he says, so as long as the water can be kept away from the pipe wall, clathrate buildup can be stopped. And the liquid hydrocarbons present in the petroleum, as long as they cling to the wall thanks to a chemical affinity of the surface coating, can effectively keep that water away.
"If the oil [in the pipeline] is made to spread more readily on the surface, then it forms a barrier film between the water and the wall," Varanasi said.
In lab tests, which used a proxy chemical for the methane because the actual methane clathrates form under high-pressure conditions that are hard to reproduce in the lab, the system performed very effectively, the team says.
"We didn't see any hydrates adhering to the substrates," Varanasi said.