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46 able to cover larger areas within a given operating time, the quality of images taken in the nadir beneath the UUV’s route is high enough to conduct automated target detection and recognition. It will therefore be easier for the US Navy and other users to detect mines, crashed vehicles and other critical points of interest on the seafloor. Port maintenance C-Job Naval Architects has unveiled a concept for an autonomous underwater maintenance dredger (AUMD), which the Netherlands-based company views as a more sustainable solution for reducing build-up of silt in port waters and ensuring they remain navigable. The anticipated design of the vehicle would measure 80 m long and be powered by a 16 MWh battery pack, enabling 12 hours of harbour dredging at roughly 2 knots (with a top speed of 11 knots for transit between jobs). “The current concept was developed for depths down to 35 m,” says Rolph Hijdra, the company’s research leader for autonomous vessels. “Theoretically, the AUMD will be able to surface [up to a freeboard – the distance from the waterline to the upper deck level – of about 1 m] even during dredging, and therefore minimum operational depths would be about 10 m.” Using a large UUV instead of a conventional dredging ship has two advantages. Being underwater, the AUMD would not have to fight against waves to move or hold its position, so only two 500 kW thrusters would be installed, as opposed to two 1.1 MW thrusters in a conventional dredger. Also, since it would be much closer to the seabed, less vacuum power is needed by the dredging pumps to excavate the seafloor. Two 175 kW pumping systems are used in the design; by comparison, a typical vessel might use two rated at 675 kW. “As the AUMD’s 3.43 m 3 hopper will be filled with seawater during normal operations, the only buoyancy concern is the difference in density once silt starts entering it. The change in density and buoyancy will be handled by removing water from ballast tanks located in several separate compartments around the hopper,” Hijdra explains. “The ballast tanks will also eject water when we need to surface. And in terms of the hopper’s capacity, we consider the silt’s density to be around 1.3 t/m 3 , so 3.43 m 3 of silt would equal 4459 t.” The research team had considered using hydrogen fuel cells for the energy density they could provide the AUMD, but chose to use battery packs and wait for fuel cell technology to mature further. As Hijdra notes, “Each of our two stacks of batteries are expected to measure about 10 x 3 x 3 m, and together have a mass of 125 t. “Currently, we are looking to collaborate with several parties to continue the AUMD’s development. Several technical and regulatory challenges need to be investigated further by research institutes, suppliers, operators and regulatory bodies.” Search & rescue The 6000 m-rated SeaRaptor AUV from Teledyne Marine (featured in UST 26, June/July 2019) has completed sea acceptance trials for its unnamed client, a government agency which is expected to acquire the UUV for deep-sea search & rescue missions such as those following aircraft crashes or oil rig accidents. “The vehicle started off testing in shallow waters, and gradually worked down to near full rated depth for a number of missions. The pass/fail criteria for all the equipment, sensors and data quality was determined with the client during the original bidding process,” explains Stefan Reynisson, general manager of Teledyne Gavia and project manager for the SeaRaptor. Much of the SeaRaptor’s architecture is based on that of its predecessor, the Gavia, and the sea testing criteria for the former was based partly on in-house criteria for the latter. Differences in design and testing parameters were made to fit the contract and specifics of the platform to long- range, deep-ocean survey, such as the SeaRaptor having three to six times the Gavia’s depth rating, more than three times its volume and three times its endurance. “Notably, during the trials we found one February/March 2020 | Unmanned Systems Technology The Teledyne SeaRaptor has completed its final sea acceptance trials for deep-sea search and survey (Courtesy of Teledyne Gavia)