AN UNDERWATER service station that can recharge robotic submarines so they
never have to surface has been successfully tested in sea trials in Germany.
Robotic submarines鈥攗sed in commercial oil exploration and by marine
biologists鈥攏eed to recharge their batteries and download data they have
collected during a mission. Normally they surface to do this, but this takes
time and risks accidental damage to the subs as they are lifted aboard their
mother ship. Now the Eurodocker project, which is backed by the European Union,
has shown that all this can be done underwater.
The Eurodocker garage is a metal frame anchored to the sea floor. The
challenge has been to ensure the 鈥渁utonomous underwater鈥� vehicle (AUV) can find
its way back to the garage, despite the shifting currents. 鈥淭he trick of homing
is combining data from several sensors so the AUV knows by dead reckoning where
it is in relation to home all the time,鈥� says Anders Bjerrum, technology manager
at the company Maridan of H酶rsholm in Denmark, one of the partners in the
project.
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The 4.5-metre-long sub has sensors in its nose and tail that enable a Doppler
sonar to measure its speed over the sea floor. At the same time, a laser
gyroscope continuously records the direction of travel.
鈥淭he AUV can tell where it is to within 1 metre,鈥� says Bjerrum. 鈥淭his is
accurate enough because the entrance to the garage is 3 metres across.鈥� Once in
the funnel-shaped entrance, rails guide the robot into the docking station,
where it plugs into power and data sockets. Cables link these sockets to a ship
on the surface.
鈥淚t鈥檚 a significant demonstration of reliability and repeatability which is
required by the commercial offshore industry,鈥� says Gwyn Griffiths, an ocean
engineer at Southampton Oceanography Centre.