Going With the Flow
Losing the flow is a drag — literally. When a submersible or ship turns, or the current shifts, water flowing along one side of its hull pulls away. The resulting flow that looks like a large rotating and turbulent cavity produces a large force that takes a lot of energy for the vessel to overcome. Michael S. Triantafyllou has a solution: give vessels fish-like perception.
Fish have lateral lines of pressure sensors along the sides of their bodies. The sensors let fish sense flow, which allows them to maneuver efficiently, said Triantafyllou, the William I. Koch Professor of Marine Technology and director of MIT’s Center for Ocean Engineering.
As it turns out, he says, a well-developed technology is ideal for making artificial lateral lines: pressure sensor arrays made from microelectromechanical systems, or MEMS. Each sensor is about a millimeter in diameter.
Triantafyllou, whose work is funded by Chevron Energy Technology, is focusing his initial efforts on putting artificial lateral lines on autonomous underwater vehicles, or AUVs. Saving energy is critical for these battery-powered vessels. Despite ongoing advances in battery technology, AUVs are still limited to about six hours of operation.
As oil and gas companies increasingly operate in deep waters, AUVs are poised to play an important role in the future of the world’s energy supply. The vehicles promise to be more maneuverable than today’s tethered remotely operated vehicles when operating in dark, turbulent waters thousands of meters below the surface. Extending the vehicles’ operating times by making them more energy efficient is a key requirement for using them in deepwater oil and gas operations.
The energy savings are significant. “We’re estimating a 30 to 40 percent savings in energy by having these sensors,” Triantafyllou said. Add flow control, and you save more energy, he added.
Triantafyllou’s team is beginning to move beyond simply sensing flow to the ability to control it. Taking another lesson from fish, Triantafyllou is looking to give AUVs fins that can oscillate. “This oscillation causes the flow to change,” he said. Instead of being separated, the flow reattaches.
Saving energy isn’t the only advantage of artificial lateral lines. Fish also use their natural pressure sensors to sense nearby objects. Triantafyllou calls this ability touch at a distance. This sense is so finely developed that blind Mexican cave fish are able to dart among obstacles that are new to them. AUVs with this capability could maneuver in the equipment-cluttered environments typical of deepwater energy operations.
The technology is poised for rapid adoption. Triantafyllou noted that 15 years ago, a similar technology — a MEMS-based accelerometer — cost as much as $200. Today it costs 30 cents.This allows auto manufacturers to put 50 of them in a car, he said. Add MEMS-based pressure sensors to sea-going vessels and you get “a substantial immediate savings with very little investment,” Triantafyllou said.