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AUVs for Depth and Distance: Autosub6000 and Autosub Long Range
The National Oceanography Centre Designs and Builds Two AUVs To Meet a Need for Deep-Diving and Long-Duration Vehicles

AUTHORS:
Gwyn Griffiths
Chief Technologist
Stephen D. McPhail
Team Leader
National Oceanography Centre
University of Southampton Waterfront Campus
Southampton, England

The Autosub6000 and Autosub Long Range are the two latest AUVs designed, built and operated by the National Oceanography Centre, Southampton, England. It was in 1996 that the first member of the Autosub family of vehicles, Autosub1, was initially deployed and remained in service until 2000. Autosub2, its successor, made valuable contributions to ocean science, working on physical oceanography, plankton abundance and patchiness, sea ice and ice shelf studies. Meanwhile, Autosub3, with the same diving depth of 1,600 meters and the same alkaline primary battery technology as Autosub2, continues to undertake science missions in areas as diverse as the Bosporus Strait that bisects Istanbul, Turkey, and the Amundsen Sea in Antarctica, where in January 2009 it successfully carried out six missions beneath the Pine Island Glacier.

In response to the U.K. marine science community’s need for a deeper diving AUV capable of carrying a substantial payload of acoustic instruments for geophysical investigations down to 6,000 meters, the National Oceanography Centre designed and built the Autosub6000, which entered service in 2010 following engineering trials in 2008 and 2009.

A different part of the community also requires 6,000-meters-depth capability from an AUV, but also long endurance, up to 6,000 kilometers or six months. That requirement is being met with Autosub Long Range.

Technology Solutions and Limits
Ideally, all of these requirements could be met using one type of vehicle, reducing the design and operating costs. However, technology limitations mean that two different approaches are needed to provide the deep-diving and high-power capability within the Autosub6000, and the deep-diving, long-endurance and low-power capabilities of the Auto­sub Long Range. Interestingly, the battery technology that makes the Autosub6000’s depth capabilities feasible is completely impractical, today, to use in the Autosub Long Range. As a consequence, the pressure vessel choice for the two vehicles also has to be different.

Lithium polymer pouch cells provide the basis for the Autosub­6000’s batteries. As they are rechargeable, they keep mission costs low and have a high enough specific energy to make one- and two-day missions practical. The batteries’ specific power allows the submarine to carry bathymetric, sub-bottom profiling and side scan sonars with substantial power requirements. Fundamentally, because the pouch cells are pressure-tolerant to at least 600 bar, there is no need for a pressure vessel to house the batteries. Slots in a syntactic foam cylinder that forms the middle body of the vehicle can hold up to 12 batteries, providing up to 42 kilowatt-hours of energy at a peak power of up to 12 kilowatts. All of the instrumentation and vehicle systems are in their own pressure vessels in the free-flooding forward and rear-faired compartments of the vehicle.

The Autosub6000, ready for launch, equipped with side scan sonar, multibeam sonar, camera and physical and chemical sensors to map the environment of cold coral mounds. The AUV is depth-rated to 6,000 meters.

Unfortunately these lithium polymer secondary cells do not have the specific energy to provide for the long-endurance requirement. The only cell technology that can deliver on these requirements is primary lithium. Until lithium-carbon monofluoride cells are available at a cost-effective size and price, the choice is between lithium-thionyl chloride and lithium-sulfuryl chloride, neither of which is pressure tolerant. Consequently, a pressure vessel is needed for the battery pack, necessitating a very different approach for the Autosub Long Range.

Given the lack of substantive reliability data for glass sphere housings subject to hundreds of pressure cycles, the approach for the Autosub Long Range was to use forged aluminum spheres, which have proven themselves in long service for ocean-bottom seismometers. With a mass-to-displacement ratio of 0.73 and an internal volume of 142 liters, they provide sufficient buoyancy and space to result in the specification being met in a vehicle that is 3.6 meters long with a displacement of 600 kilograms. Of course, these are not the only differences between the two vehicles. The demands of low-power operation have resulted in a change from the Lon­Works-based distributed network architecture of the Autosub6000 and its predecessors to a system based on Windows CE. Highly accurate but power-consuming instruments, such as the fiber-optic inertial navigation system, have had to be replaced with lower specification modules with much lower power consumption. Experimental innovations in providing high-efficiency propulsion at low speed have included trials of a one-bladed propeller, as at low Reynolds numbers there are theoretical advantages to this approach.

Autosub6000 Science Missions
The Autosub6000’s contributions to marine science began during its second engineering trials in 2009. On five missions lasting a total of 104 hours and covering 521 kilometers, the vehicle’s sonars provided new high-resolution imagery of large-scale submarine density flows on the eastern margin of the North Atlantic. Surveys with the vehicle traveling at 100 meters off the seabed with tracks 300 meters apart delineated a giant scour (more than 3 kilometers wide and more than 45 meters deep) at a depth of about 4,600 meters. The science team hypothesized that the scour location, in a broad channel downstream of the Lagos and Portimao canyons on the Horseshoe Abyssal Plain, may be related to regional tectonics. Chevron-shaped bedforms less than 1 meter in height, which would have been impossible to image from a surface ship, suggested that the cohesive sediments were affected at times by substantial flows emanating from the canyons. At about 4,600 meters’ depth within the Whittard Canyon, the Autosub6000 bathymetric maps showed erosion of individual beds, sediment wave ridges and troughs, and spoon-shaped scours with clear layer-by-layer erosion. Knowledge of morphology at this fine scale is essential to understanding both the geological processes at work and the resulting habitats for deep-sea creatures. To continue this article please click here.



Gwyn Griffiths is chief technologist and head of the ocean technology and engineering group at the National Oceanography Centre, Southampton. He specializes in the technology and applications of AUVs, with recent emphasis on reliability and predicting risk of loss in harsh environments.

Stephen D. McPhail, who leads both the Autosub6000 and Autosub Long Range AUV projects, heads the autonomous systems group at the National Oceanography Centre, Southampton. His main specialization is the navigation and control of AUVs. McPhail has extensive experience of leading the technical team for AUV test and science cruises in locations from polar seas to the tropics.




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