In order to analyse and understand the invisible world of the
sea bed, oceanographic research uses laboratories which record data
at very great depths. Sirene, an autonomous craft, remotely controlled
from the surface, is now capable of positioning benthic stations down
to depths of as much as 4 500 metres with remarkable precision. A
technological achievement in the field of acoustic remote control.
Launching of Sirene from the afterdeck
of the carrier vessel during the ESSIR mission.
Sirene is a craft of four tonnes capable of
descending to a depth of 4 500 metres below the surface of the oceans
and positioning oceanographic research laboratories operating in a
deep-sea environment (known as benthic stations) with remarkable precision.
Sirene has no pilot on board and no cable to the surface; it is remote-controlled.
"Sirene has no trailing cables, communication is by means of sound
waves - hence its name, Sirene, since it responds to an acoustic song,"
points out Vincent Rigaud, Director of the Deep-Sea Robotics Laboratory
of the French Institute of Research into the Exploitation of the Sea
(IFREMER). But its name is also an acronym (in French) of its capabilities:
European, precision-positioning, remote-controlled, submarine robot.
Once it has reached the desired spot, much like a fork-lift truck,
Sirene sets down the observation station. Its movements can be monitored
continuously by teleacoustics and remote control. Having completed
its task, the craft resurfaces "under its own steam".
capability Before the creation of this pilot-craft, which can be
compared to a submarine space shuttle, stations designed to observe
the ocean depths were guided along the sea bed by means of cables,
thereby requiring high-tonnage support ships. "A laboratory released
in this way can end up anywhere - given the frequent unevenness
of this type of terrain," Vincent Rigaud goes on to explain. "Consequently,
it is then necessary to send out a submersible - crewed or remotely
operated and equipped with handling gear - in order to position
the platform at the required spot. The operation is laborious, costly
in time and the deployment of vessels, and lacking in precision.
Nowadays Sirene is carried by a single oceanographic vessel, some
60 metres in length. Once in the sea, with the laboratory secured
to it, it descends to the sea bed with the help of an integrated
advanced navigation system enabling it to pinpoint its position
with impressive accuracy - to within a few metres."
A concept not solely of interest to the
scientists, but potentially useful to the offshore sector
as well .
Sirene is a light prototype, small in size (4.2 metres long), capable
of shifting a load of 10m3 weighing some 3.5 tonnes, and is equipped
with two computers enabling its descent to be managed, in real time,
with extreme precision. This robot of the ocean deeps is the result
of a European research incentive in the framework of the Desibel
project (New methods for deep sea intervention on future benthic
laboratories, analysis, development, engineering and tests) under
the MAST II Programme. The acoustic transmissions, navigation and
control system were the work of the French scientific and technological
organisation IFREMER, the coordinator of the project. The mechanical
and electrical integration were achieved with the help of COMEX
Technologies (Marseilles, France), which has been entrusted with
the mechanical design and has played a coordinating role in integrating
all the component elements. The on-board computers to manage the
command and control system were supplied by the Lisbon Instituto
Superior Técnico (IST) and the aluminium structure by the Technical
University of Berlin, which also conducted the hydrodynamic tests,
on a mock-up, in its testing tank.
To the nearest metre
The prototype already has some 20 descents to its credit, one of
them to a depth of 2500 metres. Its existing propulsion systems
- the performance of which could easily be improved - enable it
to descend to 4500 metres, and its acoustic teletransmission system,
which is particularly powerful, allows it to be guided from a distance
of 9 km. Sirene has been designed to put down a wide variety of
stations, such as SAMO, which belongs to IFREMER. Under the MAST
II DESIBEL project it put down, by way of demonstration, a load
simulating a benthic station to a depth of 2500 metres.
"These stations designed to take physico-chemical measurements
are bristling with instruments. In certain cases, cameras can send
images to oceanographic vessels waiting at the surface, directly
above the station, by various acoustic teletransmission systems,"
explains the IFREMER official in charge. "The main attraction of
these benthic laboratories lies in the fact that they stay put at
a precise point for long periods, thereby enabling the scientists
to study the phenomena over time."
Sirene is not intended to be used solely by researchers. As Vincent
Rigaud points out, "The concept can be employed in other areas,
for instance in relation to offshore prospecting and exploration.
Oilmen, too, when they operate equipment on the sea bed, traditionally
work with cable equipment - Remotely Operated Vehicles (ROVs) -
which can descend to 1000 metres. However, new discoveries in West
Africa indicate that there are sites to be investigated down to
a depth of 2500 metres. Under these conditions, operational constraints
associated with cable mean that surface vessels supporting the ROVs
would need to be larger in size. Unattached craft operated by acoustic
remote control, such as Sirene, could be used as an economically