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Feature Article

Underwater Glider for a Long-Term Virtual Mooring
JAMSTEC Glider Able to Monitor Ocean From Seafloor for Over a Year

Feature Author
Dr. Kenichi Asakawa
Feature Author
Dr. Masahiko Nakamura

Tsukuyomi at its first sea test in Sagami Bay.
The ocean has a strong influence on global climate. With a heat capacity thousands of times greater than that of the atmosphere, it absorbs about 30 percent of the artificially emitted carbon dioxide. The ocean environment is monitored by many means, including profiling floats, moored buoys, ships and satellites, but due to the ocean's vastness, gathering sufficient data on how it affects Earth's climate is still difficult.

To help address these gaps, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) began developing in 2009 a prototype underwater glider for virtual mooring, designed to stay on station and monitor designated waters from the seafloor to the sea surface for more than one year. The glider, named Tsukuyomi after a Japanese god from old myths who governs the moon and nights, can sleep on the seafloor to extend its monitoring duration. The goal of this project, conducted in cooperation with Research Institute for Applied Mechanics, Kyushu University, is to assess the possibility of underwater gliders for use in virtual mooring.

Although it is still in an early development stage, Tsukuyomi has proven its basic gliding performance in tank tests and the first sea test in March. The team is now expanding the glider software so it can carry out long-term monitoring in designated waters.

Existing Observing Platforms and Glider Capabilities
Even with 3,600 floats distributed worldwide, the Argo project has shortcomings in ocean-monitoring requirements. For example, the Argo floats float with currents and cannot stay in a designated area where data are needed, and it is difficult to increase their number to assess all oceans with adequate density.

In addition, a widespread change in seawater temperature has been observed even in waters deeper than the float's maximum profiling depth of 2,000 meters, which underscores the necessity of monitoring the ocean environment in deeper waters.

Other methods, such as moored buoys and research vessels, also have their benefits and shortcomings. Moored buoys can conduct long-term monitoring at a fixed point, but they cannot monitor the sea environment from the seabed to the sea surface, as they generally monitor waters shallower than several hundred meters. Increasing the number of deployed buoys is also impractical because of their construction and maintenance costs. Maintenance of buoys also requires large vessels, and research vessels provide a limited range of data.

Underwater gliders have recently attracted attention and have been used widely because they can travel autonomously over long distances gathering ocean data that cannot be obtained otherwise at a reasonable cost. However, their operating duration is shorter than one year. They cannot provide long-term data as Argo floats or moored buoys can, and their monitoring areas are generally restricted to coastal areas.

To study global environmental change, next-generation ocean observation systems should examine key areas long term to obtain the required data with limited resources. The data should be merged with computer simulations to elucidate the behavior of the whole ocean.

For example, the Arctic Ocean, Antarctic Ocean and their surrounding oceans are presumed to be key areas where seawater descends and starts to circulate in deep water throughout the world. Underwater gliders for virtual mooring would meet the requirements for their study.

Tsukuyomi Development
Common underwater gliders are designed to travel a long distance, but Tsukuyomi is designed to stay in designated waters and to monitor the ocean environment for a long time. Therefore, it can descend and ascend with a steep elevation angle, glide with a gentle elevation angle and travel a long distance. Capable of diving to 3,000 meters, Tsukuyomi will be able to touch down to the seafloor in various waters, where it can sleep to extend the monitoring duration.

Tsukuyomi, weighing 150 kilograms and measuring 2.5 meters long, glides underwater making use of a buoyancy engine (BE) and a gravity center controller (GCC).

The BE, developed by JAMSTEC and Tsurumi Seiki Co. Ltd. (Yokohama, Japan) for the profiling float Deep-NINJA, consists of a hydraulic piston pump and valves. A two-way valve was added between the oil bladder and the oil reservoir. It is rated to 4,000 meters depth. To continue this article please click here.

Dr. Kenichi Asakawa received a Ph.D. in engineering from Tokyo Institute of Technology in 1979 and joined KDD R&D Laboratories. In 1999, he joined JAMSTEC and since then has been engaged in developing underwater devices such as scientific cable systems, ceramic watertight housings and an optical salinometer.

Dr. Masahiko Nakamura joined the Research Institute for Applied Mechanics, Kyushu University in 1982 and received a Ph.D. in engineering from Kyushu University in 1990. He is studying control and dynamics of underwater vehicles and mooring systems.

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