Sea Technology

MAR 2018

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14 ST | March 2018 tromagnetic tab, with the aid of the French company BrightLoop. Glued to the drone before immersion, this tab transmits the ener- gy from the docking station to a coil connected to the drone's battery. The station itself can be powered via an electric cable connected to land, but other solutions are possible, for example, a lithium-ion battery station recharging the drone (with a smaller battery) for dozens of missions. Also envisaged is an energy production installation near the station, for example, a micro-watermill or a tide engine system. A Nest Protected from Biofouling When the drone is parked inside the station between missions, a major problem is how to prevent corrosion and biofouling on the drone. Fish can enter the dock- ing station, and biofilm, larvae and invertebrates tend to grow on the drone, risking disruption of its operation. To prevent this, the station developed by Naval Group is un- der a bell, open to the bottom. When the drone returns, this space fills with a liquid that inhibits corrosion and repels fish; it forces the seawater downward. This liquid is ecological and dissolves in seawater, in accordance with regulations. Before the drone's next mission, this liquid is pumped back into the station. Seawater reenters the bell, and the drone is released. It is possible to store the drone for a long period of time at the docking station without exposing it to an aggressive environment, an unprece- dented engineering feat. Successful Testing In the autumn of 2016, the engineers of Naval Group and RTsys tested the docking station in the ocean, near Brest, France, at the Celadon (Naval School of Lanvéoc) test site. These experiments proved that the drone can find the base and enter the docking station. After each mission, with journeys of several kilome- ters, the drone was able to find the station, which has a diameter of barely 1 m. Approximately 20 tests were per- formed, and they were satisfactory in terms of performing the mission, returning to the base, retrieving the data, re- charging and programming the following mission. Adapts to Most Drones The trials at sea focused on the implementation of the RTsys Comet AUV, a small drone approximately 1.8 m long with a weight of 30 kg. Cheap and easy to use, it can be launched by just one operator. Small drones are more sensitive to currents, but the tests with the Comet AUV yielded excellent results. Regardless of size, the docking system developed by Naval Group is designed to adapt to almost all drone models. Even with the largest drones, the results should be stable. The existing underwater positioning solutions, in par- ticular those using short baseline technology, are often quite large or have relatively high latency. By basing both communications and guidance on acoustics, we were able to create a multifunction system with a single transducer. The homing kit is a compact and economical solution that works in turbid water and is easy to adapt to numerous drone models for a wide range of applications. Varied Uses The applications initially envisaged are those requir- ing a permanent presence, notably, monitoring around ports and sensitive areas, detecting anomalies and mea- suring water quality for offshore facilities. At oil produc- tion sites, for example, the drones could be deployed regularly to check the condition of the equipment and foundations, or even to monitor leaks in the case of an alarm. Currently, when drones are used, they are de- ployed from boats. This operation can be difficult when the sea is rough and risks damage to the drone. Further, for scientific observation, a drone associated with a docking station has advantages in terms of mea- surement frequency or area studied. Contrary to a fixed seabed station that can take measurements or video of a surrounding environment of, say, tens of square meters, a station equipped with a drone can cover distances up to 100 km. The docking station developed by Naval Group and RTsys consists of a base placed on the seabed with a docking kit. Users can choose the drone they need and equip it as they like, for example, with sensors and cam- eras. Applications for such a system will be numerous, including long-term monitoring and environmental study. ST Marc Richard is the drone R&D projects manager at Naval Group. Raphaël Bourdon is chief technology officer and co-founder of RTsys. He has a master's degree in electronics. Alice Vettoretti collaborates with RTsys as a freelance science writer. She has master's degrees in oceanology and scientific communication. The Naval Group docking station and RTsys Comet AUV.

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