The UK’s deepest-diving robot submarine, the University of Southampton’s Autosub6000, has completed its first science mission

“It formed part of a research expedition investigating potential threats from tsunamis, giant landslides and earthquakes to coastal communities along the west European margin,” said the National Oceanography Centre at Southampton.

This is the latest in a series of autonomous Autosubs, all developed at the Centre. 

“It is an entirely new-build vehicle that uses the proven control, navigation and sensing systems from Autosub3 and previous versions with a brand new design of energy source centre section that enables the vehicle to dive to 6000m," Professor Gwyn Griffiths told EW.

Visually similar, Autosubs 2 and 3 were powered by an astonishing 5,100 primary manganese alkaline D cells. To protect them from external water pressure, they are housed in four carbon fibre pressure vessels surrounded by syntactic foam.

“Autosub6000 is different. It uses secondary lithium polymer pouch cells - 405 of them forming one battery of circa 4.5kWh. This battery, to our own design, is housed in a thin walled titanium box that has a flexible lid, and the box is filled with oil,” said Griffiths. “The cells work well even when exposed to 60MPa external pressure, and the electronic components to support charging and battery management have been chosen to be pressure tolerant. Thus, we do not need a strong pressure-resisting case, the vehicle becomes lighter and can dive deeper with decent energy capacity.”

A maximum of 12 of the 4.5 kWh batteries can be fitted and the motor power can be varied. “A typical range being from 50 to 500W,” said Griffiths.

The motor is a DC brushless direct drive unit with the magnets external to the coils, enabling the motor to avoid needing a shaft seal at high pressure.

At maximum power, the vehicle travels at about 2m/s. “It can travel some 400km at 1.6m/s, 1,000km at 1m/s and 6000m deep,” said Griffiths.

The control system is based on a distributed network architecture using LONWorks, LonTalk and Neuron chips.
Sensors measure water properties, temperature and salinity, with speed information over the sea bed detected by a 300kHz acoustic Doppler velocity sensor. The Doppler sensor also measures the speed of water currents beneath the vehicle. “A multi-beam sonar at 200kHz provides swath bathymetry of the seabed over an angle of 120[deg] and a fibre optic gyrocompass provides precise heading,” said Griffiths.

In the initial scientific mission, Autosub6000 was released from the research vessel RRS James Cook and sent down almost 5,000m to investigate a submarine canyon north of the Canary Islands.

“Upon its return to the surface, some 24 hours later, the vehicle provided scientists with three-dimensional images showing holes in the seafloor the size of a football stadium,” said the University. “These holes were formed by giant submarine flows that ripped up huge volumes of seafloor sediment and carried the material up to 1,000km further offshore.”

The robot executed a so-called ‘lawnmower survey’, flying 100m above seabed while surveying the 16km[super2] area in 200m-wide strips with its high-resolution multi-beam echo sounder.

The research team is also sampling seafloor sediments to establish how often landslides occur in different areas of the deep ocean.

“We have found that giant landslides are actually quite rare around the Canary Islands, with no major activity in the last 15,000 years,” said Dr Russell Wynn, chief scientist on RRS James Cook. “However, in the coming days we will start working offshore southwest Iberia, where earthquakes and landslides have been occurring regularly throughout human history.”

During this next phase, off the Portuguese coast, the submarine will search for evidence of the devastating 1755 Lisbon earthquake, which, according to the team killed over 10,000 people and generated a tsunami that reached England. “The Lisbon earthquake was one of Europe’s worst natural disasters, and we hope that new data from the deep ocean will provide information about the potential future threat to coastal communities,” said Wynn.

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