Long-range autonomous underwater vehicle Tethys

Over the last three years MBARI engineers have developed a new class of autonomous underwater vehicles (AUVs) to support chemical and biological sensing missions covering ranges of 1,000 kilometers or more. The size and power consumption of desired chemical and biological sensors precluded the use of existing long-range gliders, and the endurance requirement precluded the use of more traditional propeller-driven AUVs. The concept for the new vehicle was a highly energy efficient propeller-driven AUV capable of operating at speeds between 0.5 and 1.0 meters per second. The new vehicle, named Tethys, conducted its first brief autonomous mission in December 2009, just offshore of Moss Landing.

Ben Yair Raanan, Brian Kieft, and Brett Hobson prior to the deployment of three long-range autonomous underwater vehicles from the R/V Paragon in Monterey Bay. Photo by Todd Walsh.

Ben Yair Raanan, Brian Kieft, and Brett Hobson prior to the deployment of three long-range autonomous underwater vehicles from the R/V Paragon in Monterey Bay. Photo by Todd Walsh.

The range and endurance of the new long-range AUV (LRAUV) greatly expands the types of observations and experiments possible with autonomous platforms. For instance, one of the institute’s AUVs carries a comprehensive suite of sensors out to MBARI’s M2 mooring and back. Tethys will carry a smaller, but still impressive suite of sensors 10 times farther, extending the reach of MBARI’s shore-launched AUVs into the California Current system. This will expand researchers’ non-ship observational capability beyond the upwelling shadow, well into the oligotrophic (nutrient poor) ocean. This capability also provides a foundation for studying phytoplankton blooms from boom to bust, by providing a mobile platform which can survey a bloom continuously through the two weeks to month-long lifetime of a bloom.

The LRAUV is designed to address the need for improved biological process experiments by providing a platform capable of sampling at the appropriate time and space scales with both in situ sensors and water samplers. The endurance of the vehicle and its variety of operating modes provide it with the endurance to wait in a low power configuration for biological events to occur, and the flexibility to respond to detected events to characterize processes from initiation to collapse of a bloom. Thus the LRAUV, combined with new sensors and samplers will enable a new generation of biological process experiments.

The LRAUV is, to the best of our understanding, exceeding the original proposal targets. Most notably, researchers estimated a range of 1,000 kilometers at 0.75 meters per second, and now project an ultimate range of more than 2,000 kilometers at one meter per second with primary batteries.

The Tethys

Researchers operate the Tethys near the bottom of MBARI's 10-meter-deep test tank. © MBARI 2009

Researchers operate the Tethys near the bottom of MBARI’s 10-meter-deep test tank. © MBARI 2009

In contrast to existing AUVs or gliders, Tethys is optimized around a “high power” payload power consumption of about 8 watts. Power management is integral to the vehicle, and the ability to operate sensors intermittently or not at all to reduce power consumption is key to achieving large ranges and endurances.

Thus high-power sensors which consume tens of watts can be operated without sacrifice of endurance provided that they are operated intermittently. Core electronics for the vehicle have been customized to minimize power consumption, and even the microprocessor selected can be “throttled down” to low clock speeds to minimize power consumption when feasible (at low speed or during drifting). Further, extensive efforts have gone in to minimizing the propulsion power. Strategies include reduced drag through the development of a low-drag afterbody, minimizing appendages, and the development of control strategies that minimize induced drag. A custom propeller design and a gearless propulsion motor optimized for efficient low-speed operations complete the low propulsion power efforts. Operationally, the Tethys is intended to be used much like a glider, using a satellite link to communicate with shore (perhaps a few times a day might be typical).

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