60 In operation | SeaCAT and the SeaClear project thrusters capable of 17 kg of force each and two vertical ones each rated to 10 kg of force. At the bow is the Mini Tortuga, a 19 kg ROV that travels at up to 4 knots. As with the Tortuga at the stern, it is tethered to the SeaClear, its umbilical providing power and control signals from the USV as well as key telemetry from the ROV. The UAV chosen for the project is a DJI Matrice 210, its low cost and weight (6.14 kg MTOW) being felt to be enough for the purposes of the SeaClear trials. The SeaCAT weighs 1075 kg, and 500 kg of payload can be carried on top of this. A diesel generator supplies electrical power to the onboard thrusters, ROVs and subsystems such as its embedded control PC, DGNSS, radar, AIS, inertial systems and wi-fi antennas. “The diesel-hybrid powertrain gives us an endurance of almost 7 days, which also happens to be largely in line with what we want for this kind of operation,” Chardard says. Containerised transport Despite being large enough to host two ROVs and a UAV, the SeaCAT fits into a standard 20 ft shipping container (once deflated), which as Chardard explains was not something that could be done with other USVs large enough to fit three other vehicles onboard. “Transporting things in a 20 ft container isn’t exactly cheap but it’s simple, because you can fit everything needed for the mission on any standard container ship,” he says. “Making the SeaCAT containertransportable was a key target during the design phases, and we emphasised that to our naval architects early on, as well as making clear that the USV had to withstand sea states 4 to 5. That all culminated in an architecture that’s quite long, and uses the inflatable hulls to enhance the navigability and stability of the vessel in rough seas.” The two ROVs and the UAV are transported with the USV in a 20 ft container to the launch point. There, the doors are opened and a system of rails with a motorised winch inside the container enables the USV to be pulled out safely (after the container has been connected to a power source of course). While the Tortuga is always stowed at the USV’s stern, the Mini Tortuga sits between the deflated hulls in the container and is therefore installed at the stern, along with its lifting frame and the UAV platform after the SeaCAT is out of the container. Also, the USV has two masts for mounting its navigation sensors and antennas, which are also unfolded to their operating height after removal and before a launch, at about the same time as the pontoons are inflated. “Once we’re on site, preparations for launch take just under an hour,” Chardard notes. “That includes pre-planning the waypoints – generally standard lawnmower patterns like a survey UAV – for the SeaCAT to follow on a simple tablet, to scour a given area for litter. We also fuel the USV and carry out a navalstandard checklist of visual and digital subsystems to make sure everything is working as intended.” Clearing the sea As the USV follows its mission path, its Norbit multi-beam echosounder generates a 3D bathymetric map of the seafloor, looking for large refuse October/November 2023 | Uncrewed Systems Technology A DJI Matrice 210 flies overhead to provide obstacle avoidance for the USV and spot underwater litter when the water is clear
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