UK autonomous sub slips silently into the deep
The autonomous vehicle is the latest in the line of world-beating submarines developed by the National Oceanography Centre (NOC) in Southampton.
Autosub LR has a 6,000km range, endurance of six months and can dive to 6,000m.
“Although a third the weight of Autosub3 and Autosub6000, it will be able to travel for more than ten times the distance, and be deployed for over a hundred times greater duration. All this, and with a depth rating of 6000m,” said NOC. “The key to achieving this performance is efficient propulsion at slow speed [0.4m/s], and by keeping tight control of the power used by the AUV sensors and control systems. One area in which recent advances in technology has helped make this possible has been in the development of microprocessors for devices such as mobile phones which have ample processing power, but which use very little energy.”
Forged aluminium spheres (see transparent view), hold the batteries (front sphere) and the control computer (rear sphere). The propulsion motor is housed in a dry pressure vessel and coupled to the propeller through magnetic couplings – eliminating energy sapping and potentially unreliable rotating seals.
It runs with only 2kg buoyancy out a total displacement of 650kg – even the small amount of dissolved solids in the Southampton fresh water test tank affects buoyancy and must be accounted for (see picture below).
During its missions, it surfaces occasionally and transmits data back via an Iridium satellite data link.
Off Ireland, it is collecting scientific data down to 1,500m for the four-year FASTNEt (fluxes across sloping topography of the north east Atlantic) research programme into the little-known slopes of the ocean shelf edge – which aims to gain understanding of the water exchange between the UK’s shelf edge seas and the deep ocean.
“The ocean shelf edge is where the shallow coastal waters of the UK’s continental shelf meet the deep ocean water across the steep sloping sides of the shelf edge,” said NOC. “It is a nutrient-rich and productive area in terms of sea life and an important area for fisheries. It is also where there is a tremendous movement and exchange of water and nutrients.”
Autosub’s payload space allows the vehicle to be customised for bespoke science applications.
For FASTNEt, it has dual 600kHz acoustic doppler current profilers (ADCPs) which measure water velocities 50m above and below the submarine, a microstructure turbulence probe which measures small scale turbulence in the undisturbed water in front of the AUV, a fluorometer which provides information on the turbidity of the water; and a standard conductivity, temperature and pressure (CTD) sensor used to calculate water salinity and to identify different water masses.
“Autosub LR is an ideal vehicle to use with the FASTNEt programme. Its large payload capacity and very long range mean that it can be launched from a harbour in Donegal, undertake a 30 day mission and then return to the shore for recovery,” said Dr Maaten Furlong, head of marine autonomous robotic systems (MARS) at NOC. “This means the data can be gathered at a relatively low cost. The lower cost of this style of operation has significant potential to change how measurements are undertaken in the ocean, and could enable more routine monitoring of the oceans.”
It spent three days travelling nearly 100km to the working area, and is repeating a series of sections down and up the shelf edge at a constant altitude above the bottom. From its periodic Iridium exchanges, MARS will adjust the deployment and eventually navigate Autosub back to the Donegal.
Meanwhile remotely-operated sub-sea gliders will be mapping the upper water column.
There will also be more traditional ship-borne measurements and long term moorings. “This combined approach will allow the scientists to study the continental shelf edge in unprecedented detail,” said NOC.