VESSEL REVIEW | SeaDragon – Unmanned carrier vehicle and technology demonstrator for German research institute

The Fraunhofer Center for Maritime Logistics and Services (Fraunhofer CML), a research organisation based in Hamburg, has introduced a new type of unmanned surface vehicle (USV) into service.

The 5.5- by 2.1-metre (18- by 6.9-foot) SeaDragon is characterised by a modular catamaran design and sensor technologies to help advance research in the field of autonomous systems. The technologies are intended to support research in the areas of obstacle avoidance, target tracking, autonomous docking, and controlling the craft with the aid of augmented reality.

“The USV was also designed to function as a carrier vehicle for smaller unmanned aerial vehicles or underwater remotely operated vehicles (ROVs),” Fraunhofer CML told Baird Maritime. “This fleet of vehicles will be used in research to provide services that are still carried out by a human operator today. Examples of duties in which such craft will be employed are ship and bridge inspections.”

Improving upon an earlier design

In designing the USV, a specialised software stack was created. It is a containerised ROS 2 software stack that can be used on different USVs and to which different projects can contribute.

“It’s a plug and play solution combining perception, guidance and control with the vehicle and its sensors,” the institute said.

SeaDragon is a further development of SeaLion, an earlier USV also built and operated by Fraunhofer CML.

“This ensures that the craft can easily be equipped with additional hardware for testing purposes. It is versatile, being able to transport about 350 kg (770 pounds) of payload, performing Lidar and multibeam echosounder (MBES) scans, as well as being capable of adding a launch and recovery system (LARS) for underwater ROVs.”

The hulls were taken from an existing sailing catamaran. Fraunhofer CML said this reduces development cost, increases the speed of development, and ensures that the USVs dimensions are within road regulations for ease of transport.

Data collection craft that also serves as a technology testbed

SeaDragon was designed to be capable of providing real-time data collection and analysis, ensuring that information is available when needed as an alternative to having to rely on inaccurate data acquisition methods.

“Additionally, the USV serves as a versatile platform for testing and evaluating new marine technologies and products, facilitating innovation and development in the maritime industry,” added Fraunhofer CML. “Notably, it can employ a LARS for ROVs, filling a significant gap, as LARS are not broadly available for USVs. This then enhances operational efficiency and safety during underwater missions.

The USV can also support scientific research by enabling extensive data collection in various marine environments, contributing to a better understanding and management of marine ecosystems.

The craft is initially being used to support the development of autonomous path planning and docking. Path planning is mostly used for surveying tasks using lidar and MBES as well as developing algorithms to implement collision avoidance guidelines including COLREGs.

Electric propulsion coupled with full sensor suite

SeaDragon is powered by electric pod drives that can rotate up to 320 degrees, ensuring enhanced manoeuvrability and precise navigation in restricted waters. Power for the drives is supplied by two 5.4kWh lithium polymer batteries.

“The use of electrically-powered Rim pod drives in catamarans offers numerous advantages that significantly enhance performance and efficiency,” Fraunhofer CML told Baird Maritime.

The institute added that electric drive technology not only ensures a smooth and comfortable ride but is also environmentally friendly, while efficient energy transfer and low-vibration functionality help optimise energy consumption. The compact design of the pods also allows for optimal space utilisation on board.

Communication with the craft is via wifi or LTE. There are two wifi configurations available: one for high bandwidth at distances of less than one kilometre and one for long-range communication over distances greater than one kilometre. An additional emergency link for operators is made through a 2.4GHz radio.

A lidar provides information that is used for situational awareness, and this sensor can be complemented by cameras and radar to ensure complete short- to long-range detection of surrounding marine traffic.

There are two kill switches: one consisting of a radio-controlled relay, and the other an MOB switch that commutes the same relay.

The USV is equipped with an SOA MBES capable of performing bathymetric scans even in shallow and turbid waters, noisy environments, and silty seafloors. It is also capable of scanning underwater structures, such as quay walls or ship hulls.

“There is no final version of the boat,” Fraunhofer CML replied when asked about challenges encountered in the design work. “Instead, small improvements were gradually incorporated with recurrent testing.

"A particular challenge was [related to] making the craft's hardware components – which are not designed for maritime use – safe and watertight. The positioning of the batteries and thrusters was severely restricted by the narrow hull shape. This had to be done while maintaining a good balance of the hulls along the longitudinal axis."

The institute found that, with the low weight of the craft, it was also essential to distribute the other components evenly between the two hulls to prevent heeling.

Also, creating a clean layout within the hulls and connections to the mast as well as ensuring possible integration of additional components without redesigning everything posed additional challenges that needed to be overcome.

“[We learned to create] a fully electrical USV, capable of navigating sea states three and four and for a nominal endurance of 10 hours (at two knots operating speed),” Fraunhofer CML told Baird Maritime.

“Secondly, a navigation stack was developed with the vessel’s capabilities in mind, but scalable to any USV within its size class. For this, understanding the dynamics and the impact of environmental forces (such as wind and waves) was quite challenging, especially given its low draught, making it very prone to sideways drifting (as the area above water is a considerably high percentage of its total area).

“Lastly, [there is] the modularity of the platform, so that it can be fitted with different sensory systems, covering a plethora of use cases. Particularly important are the underwater sensors (such as MBES, Doppler velocity log, side-scan sonars) that impact the hydrodynamics and therefore the control of the vehicle.”