retarded-torpedo.jpgHow does one study climate? The Earth’s atmosphere seems to make a sensible source for answers. But probing deeper and a whole century of proven misconceptions would tell otherwise. The oceans, whose top few meters alone store as much heat energy as the entire atmosphere, make a surprising yet logical starting place to predict climate changes. The vastness and depth of these waters, however, clearly make it a challenge for the scientific community to survey and observe changes at a cost-sensitive platform.

Enter Autonomous Underwater Vehicles. These are preprogrammed robots without tethers that swim through the shallow waters of the ocean or crawl along the bottom. The crawling varieties are used to survey and sample the ocean floor. There are two main types of these ocean-mapping devices. The propeller-driven AUVs and the buoyancy-driven AUVs, more popularly called gliders.

The propeller-drive AUVs, also the more traditional approach, use battery-powered engines and can swim programmed patterns such as constant depth or altitude surveys. Its body structure includes a single propeller at the stern with four steering fins alongside of the propeller. Lights and cameras that enable the vehicle to follow pipelines and search for underwater objects are located at the front. Some also have side-facing sonar that is programmed for determining position, receiving instructions, and transmitting data. An example of an AUV is the Alistar 3000 with a total of eight thrusters that enable the vehicle to follow pipelines and hover. It was once demonstrated for BP in the Gulf of Mexico.

Gliders, on the other hand, have inflatable bladders that give the vehicle positive or negative buoyancy. They have wings that allow them to glide forward while they rise or sink. They are configured to follow a saw tooth course in a vertical plane. To receive navigation information, gliders surface at the top of their navigational arc. Sensors are used to measure and record parameters of oceanographic interest, such as temperature, salinity, turbidity, and chemistry as a function of depth, latitude, and longitude. Although both seem to be perfect candidates, scientists have traditionally been more inclined to deploy buoyancy gliders.

Why Buoyancy Gliders?

To start with, AUVs have shorter flight durations due to high-energy requirements, usually derived from the batteries. They are unfit for missions which have a year-round run time. Also, they require underwater acoustic networks for precise navigation and significant human intervention for the vehicle to surface. Aside from subsurface docking stations or cranes, ships are sometimes utilized for deployment and maintenance, a technique that requires obvious expenditures.

Buoyancy gliders on the other hand, are robust systems made for traversing great distances for longer time periods. Most gliders still rely on battery-powered motors. A thermal glider however, a recent enhancement on the technology, draws its energy for propulsion from the differences in temperature—thermal stratification—between warm surface waters and colder, deeper layers of the ocean. Here’s how: The heat of the ocean warms wax-filled tubes inside the engine. The expansion of the warming wax converts heat to mechanical energy, which is stored and used to push oil from a bladder inside the vehicle’s hull to one outside, changing its buoyancy.

The latest of these thermal gliders was developed jointly Webb Research Corporation and Woods Hole Oceanographic Institution (WHOi) in December 2007. A research team led by oceanographers Dave Fratantoni of WHOI and Roy Watlington of the University of the Virgin Islands launched a prototype off the coast of St. Thomas. Since then the vehicle has been traveling uninterrupted, crisscrossing the 4,000-meter-deep Virgin Islands Basin between St. Thomas and St Croix more than 20 times. It has been said that the yellow torpedo can survey the oceans for as long as six months.

“We are tapping a virtually unlimited energy source for propulsion,” said Fratantoni. Although batteries might still be needed for the sensors, the data-logging system and the satellite communications system, this is definitely thus far a more viable alternative to the propeller-driven AUVs.

Wood’s Hole Oceanographic Institution is a dedicated research and education center for the advancement of marine understanding and its interaction with the Earth system, and communicating this understanding for the benefit of society. It is responsible for some of the main highlights in oceanographic history, including the discovery of RMS Titanic in 1985.

Robot Glider Harvests Ocean Heat

Image Credit: John Lund, WHOI