VESSEL REVIEW | Reefcat – Electric autonomous catamaran for Great Barrier Reef monitoring missions

The Australian Institute of Marine Science (AIMS) has begun operating a new catamaran autonomous surface vehicle (ASV) to support its marine research activities.

The fully electric Reefcat was developed entirely within Queensland, having been designed by Pipar Automation of Brisbane in collaboration with AIMS’ engineers and built by Norman R. Wright and Sons of Murarrie.

AIMS said the purpose-built, mid-sized ASV can support a wide range of scientific payloads with multiple safety systems including obstacle avoidance, high bandwidth communication capacity and energy efficiency within a compact length, allowing it to fit on an AIMS research vessel deck.

The institute has described Reefcat as a “just right” ASV: larger and more sophisticated than most off-the-shelf ASVs, but not as large as full-sized vessels that had been converted into autonomous vehicles. According to AIMS Technology Transformation Research Team Leader Melanie Olsen, the ASV can navigate a variety of trajectories, including covering an entire reef in a “mow-the-lawn” pattern carrying cameras or a sonar system for mapping the ocean floor, or it can also be used like a mooring, staying in one place with instruments taking measurements.

“Reefcat was designed from the keel up, in response to a specification issued by AIMS, for a stable autonomous platform that they could use to expand the scope and extend their current monitoring operations on the Great Barrier Reef (GBR), as a central part of their mission to identify trends in the condition of the reef ecosystem,” Russ Morrison, Senior Principal Engineer at Pipar Automation, told Baird Maritime.

“Monitoring missions on the GBR are challenging, particularly in the presence of tidal currents, waves and wind that are regularly encountered on the reef, especially in the more exposed, outer sections of the reef. It is generally difficult under these conditions for a vessel to maintain a sufficiently accurate track, to satisfy both operational and safety requirements.“

Morrison said that, operationally, the ASV needs to navigate in close proximity to reef structures. In particular, it needs to be close enough to capture the imagery required while maintaining a safe separation to avoid potential reef structure collisions.

Long endurance coupled with robust construction

The AIMS specification required the ASV to be capable of accommodating a broad range of instrumentation payloads, many of which had high drag characteristics. The need to structurally connect and support these payloads, and to integrate them electrically with relevant ASV systems, has also had a significant impact on the craft’s configuration, on installed drive power and on onboard energy storage.

"Another important design constraint was the requirement that the ASV had to be suitable for transport out to the measurement site, on the deck of one of AIMS’ research vessels, and to be deployable and recoverable from these RVs using their service cranes or A-frames," said Morrison. "Again, because of the very large cost and time associated with the transport logistics to offshore reefs, there was a major focus on improving the reliability of critical ASV systems, to minimise the potential for missions being aborted due to ASV equipment failures."

Morrison explained that in the context of these requirements, the design of Reefcat embodied a number of special features that, in combination, make it uniquely capable for the AIMS application of monitoring coral reefs.

The ASV is a catamaran of three metres (10 feet) in length and 2.1 metres (6.9 feet) in beam with a draught of only 0.5 metre (1.6 feet). This envelope was dictated by the requirement to transport the vessel on the deck of an AIMS research vessel. It has a maximum speed of about eight knots and a maximum survey speed of about five knots and, although relatively small, it has an operational weight of about 460 kg, of which 100 kg is batteries.

“The hulls, electrical box and towing frame are all carbon fibre laminates, and the hulls are foam filled, making them particularly robust, being able to survive sharp coral impacts without water ingress even if a hull is penetrated locally,” Morrison told Baird Maritime. “The hull form itself is unique, optimised to minimise pitch response at a survey speed of three knots in the short-period waves typical of the GBR (significant wave height about 0.8 metre, mean period about 4.5 seconds). Minimising pitch is important because it limits motion-induced blurring of images captured by cameras mounted on or from the ASV.”

The ASV is fully electric, with six lithium-ion batteries connected in parallel for complete redundancy. Total installed power is 18 kWh, which Morrison said is sufficient to support mission durations of more than 12 hours.

Propulsion is by two Torqeedo 6kW electric drives, with steering by electric linear actuators. In the event of a drive failure, steering control automatically switches to a “return to home” mode in which the remaining healthy drive is used for direction control, allowing the ASV to return safely to the support RV without assistance. A large-capacity thermal management system meanwhile dissipates the high thermal loading produced by the onboard electrical equipment by using passive convective heat transfer externally together with a bank of internal circulating fans.

“This has proven extremely effective in holding internal equipment temperatures well below acceptable limits — an essential requirement for an ASV operating in tropical waters, where high solar loads regularly occur in combination with high ambient air and water temperatures,” said Morrison.

Reefcat also incorporates a fast response, robust and accurate navigation system, using two independent GNSS receivers, each having dual antennas and integrated IMUs, with RTK corrections received via the Starlink internet connection. Positional accuracy should be better than +/- 100 mm anywhere on the GBR, and the ASV can follow any pre-defined path generally to an accuracy of +/- 200 mm during typical GBR operations, including the effects of cross tidal currents, waves and wind. To achieve this accuracy, steering control algorithms employ progressive learning to identify external wind, wave and tidal disturbances, and the pre-emptive steering corrections needed to minimise course deviations.

The ASV has robust and redundant collision avoidance systems for both above and below water obstacles — an essential requirement for vessels navigating in close proximity to coral reef structures. The above water system employs dual-redundant, multi-beam lidar scanners, providing obstacle detection in all lighting conditions, including at night, with a detection range of at least 40 metres even in low visibility such as rain or fog. Underwater obstacle detection meanwhile uses a combination of 3D acoustic cameras and scanning sonars mounted on deployment mechanisms that lower the sensors below keel level to allow unobstructed scanning to the front and sides of the ASV.

Both systems are integrated with the ASV’s navigation systems, allowing course correction to steer around detected obstacles.

Capable of carrying a range of sensors and related equipment

The ASV also carries an AIS transceiver and radar reflectors, together with a system allowing safe access for operators approaching by tender: a group of lights indicating the current status of the ASV, and a remotely operated radio safety stop which de-energises the ASV power circuits, making it safe for operators to approach and interact.

“Operating the ASV from a supporting RV mothership motivated the adoption of a single-point-lift system, which allows rapid launch and recovery without the need to have personnel in or on the water,” said Morrison. “The system has a single connection to the ASV, located close to its centre of gravity, enabling the ASV to be lifted using the RV’s deck crane or A-frame. The lift connection includes a bespoke locking mechanism, remotely operated to release the ASV from the lift cable after launch and to re-connect prior to recovery, and incorporates an interlocking technique that prevents disengagement while there is tension in the lift cable, preventing accidental release during the lift operation.”

Reefcat has been designed to accommodate a number of different payloads, including towed camera arrays, pole mounted cameras, towed under-water platforms, side-scan and multi-beam echosounders, and water samplers. Many of these payloads have sensors that need to be lowered below the ASV keel to improve the field of view for scanners, or image quality for cameras. These sensors are mounted in articulation mechanisms with stepper motor or electric linear actuators and position encoders, allowing deployment and accurate positioning at the required depth, with automatic retraction to avoid sea-bed impacts when under-keel clearance is insufficient.

“All payloads are supported from a dedicated carbon-fibre towing frame towards the stern, which has the structural competence to accommodate the loads and moments required to tow payloads at survey speeds of up to five knots,” Morrison added. “Payloads are also supported electrically by the ASV, which provides power and communication links between the payload and the ASV systems, including data processing and data storage.”

Redundant onboard electronics for reduced downtimes

The ASV has multiple redundant communications systems — wifi, 4G/5G, 900MHz and Starlink — which switch automatically depending on operational circumstances, delivering very reliable communications even on the most remote parts of the GBR. The Starlink system in particular provides a high-bandwidth, very robust connection, enabling the ASV and onboard payload systems to be monitored from anywhere with internet access.

On board, the ASV carries a large data processing and storage capacity, including dual-redundant, high-speed processors in a master-slave configuration that transitions seamlessly in the event of a failure, and 12 terabytes of redundant mass storage.

“These processors manage all ASV systems — positioning, navigation, steering control and communications, as well as payload deployment and payload data management,” Morrison remarked. “Redundancy is a key feature throughout: almost all sensors, power supplies, processing and interfacing equipment items are duplicated to ensure continued operation in the event of equipment failure or malfunction.”

A comprehensive mission planning and monitoring system is used to develop and load ASV mission plans and to monitor their subsequent execution and performance, with software residing partly on the ASV processors and partly on the ASV control centre. Normally located on-board the support RV, this control centre is portable, enabling missions to be controlled from any convenient location with internet access, either on another support vessel or on land.

“Probably the greatest challenge in designing the ASV was development of simulation modelling tools for rapidly evaluating the key performance metrics of stability and tracking accuracy of the ASV when operating in waves and tidal currents,” Morrison told Baird Maritime. “For these simulation modelling tools to be useful in the design process, they needed to be sufficiently accurate, but also fast enough, to enable the key performance metrics to be evaluated rapidly, as the hull form and other aspects of the ASV design were adjusted iteratively to progressively improve performance.”

Evaluation of these performance metrics became a focus of the design process, when the potential impact of wave and tidal conditions in limiting the ability of the ASV to meet the specified performance targets became apparent. It was decided, as a result, that before committing to build the ASV, an assessment would be undertaken to evaluate the fraction of time during which the proposed ASV design would be able to meet the specified criteria for payload stability and tracking accuracy, when operating on the GBR.

“The technique adopted to assess both the stability and tracking accuracy issues was to develop dynamic simulation models that predicted the dynamic response of the ASV and payload, when travelling through a wave field, with and without tidal currents. This required the development of a new, fast pseudo-CFD hydrodynamic model coupled with dynamic models of the ASV and payload, as well as a model of the steering control system and the electric drives. The integrated model was then calibrated against a full CFD-dynamic model for several representative transient dynamic events.”

Morrison said that, following successful calibration, the fast pseudo CFD model was then employed to run a large number of simulated operational conditions corresponding to a broad range of different wave conditions and tidal currents, with the ASV traversing the wave field in different directions relative to wave and tide directions.

“For each simulation case, both payload motion and tracking accuracy were generated. Finally, this information was combined with wave height and direction statistical data for the whole of the GBR to produce a distribution of the time for which performance criteria would be met, for all points on the GBR. This analysis predicted that both payload stability and course tracking accuracy criteria should be met by the final ASV design for more than 95 per cent of the time, for operations anywhere on the GBR. Later, this simulation capability was developed into a standalone product.”