Submarines & Biomimetics
Ocean Technologies Laboratory

Fjord Explorer

An autonomous underwater vehicle for coastal oceanography research

Background

The aim of this project is to design & build an autonomous underwater vehicle to conduct continuous oceanographic surveys of coastal fjords, such as Rivers Inlet, British Columbia, Canada, where I run a field research group. The inlet is a typical fjord, about 80km long, 4km wide and 200-300m deep, with steep walls on both sides. The inlet once hosted highly abundant salmon runs, but years of overfishing & upland logging have all but eliminated most of them. Despite the closure of the commercial fishery and increased enhancement efforts, the salmon stocks have yet to recover.

What is required is a year-round survey programme. Hydroacoustic surveys of the inlet need to be done on a regular basis throughout the year in order to determine the distribution of fish in the inlet. Oceanographic information such as salinity, temperature, productivity, and chemical composition need to be monitored throughout the inlet throughout the year. Movements of young fish out of the inlet, and of adult fish into the inlet need to be mapped out in space and time. Abundance of fish and prey need to be monitored throughout the season, so that management decisions can be made based on sound data rather than on model predictions.

To conduct such a survey programme using surface ships is costly and subject to unpredictable weather conditions. The result is that research, if it is done at all, is confined to at most a few weeks per year. Hence the need for an AUV which will be able to conduct surveys of the inlet continuously, year-round. The AUV would travel the inlet on a continuous basis, covering the distance from mouth to head and back in approximately 60 hours.

The Sub

Externally the sub will resemble a manta ray, with a streamlined body, broad wings and long tail. Vertical rudders and a propeller will be placed at the base of the tail, which will house antennae. The overall length will be approximately 2m, and dry weight less than 100kg. The fibreglass hull will be designed to withstand depths of 400m.

Propulsion will be provided by a combination of a variable buoyancy system and a novel biomimetic drive. In the former mode, the sub will glide downwards or upwards on the buoyancy difference between itself and the surrounding water, while in the latter, it will propel itself in a manner reminiscent of a manta ray.

The sub will be powered primarily by onboard batteries, though energy harvesting is also an option. It will communicate with the outside world via satellite, and be equipped with an emergency recovery system. Navigation will be by GPS and dead-reckoning, and it will use a sonar system to avoid obstacles. Once functional, the unit will eventually be fitted with oceanographic research sensors.

The Hull

The hull will need to withstand the pressure at depths up to 300m. Part of the hull will need to be open to the outside environment, in order for the variable buoyancy drive to be effective. I envisage the external hull being made of fibreglass, but it may be necessary to use a stronger material for the internal hull. It will be necessary to have access to the internal parts of the sub, so properly designed hatches will also be required.

Variable Buoyancy Drive

The variable buoyancy system will require a change in density of the device. This has been accomplished in other devices by pumping oil between bladders inside and outside the main air-filled body of the AUV. The external bladder is contained in an open compartment in the hull so that water can flow in and out as the bladder contracts or expands. A design similar to that would work - however other possibilities exist, including the use of variable density oil and the temperature difference between surface & deep waters.

Rajiform Drive

The biomimetic propulsion is based on the flight of manta rays, which swim by undulating their wing-like pectoral fins. The AUV's wings will consist of a set of bars joined by an elastomeric sheet. As in fish, the fin rays will rotate about their bases. Each ray will be individually actuated so that the wing can be shaped to provide a lifting surface for gliding or undulated to provide propulsion.

Navigation & Control

Control will be effected through a combination of fins, weight shifting, and differential thrust from the two wings. Basic navigation will be done with a GPS at the surface and by dead-reackoning underwater using a Hall effect compass, depth gauge and speedometer. The unit will be equipped with forward and downward looking multi-beam sonars to detect obstacles and find fish. It will additionally be equipped with a directional hydrophone system so that it can home in on a signal from a boat or the dock. All three systems will be coordinated by an onboard computer using artificial intelligence algorithms.

Power & Communication

Main power for the AUV will come from a bank of batteries. A solar panel built into the top of the hull will provide power while the unit is at the surface, and a propeller-driven generator will recharge batteries underwater.

The device will communicate using cellular or satellite telephony. Bandwidth will limit the information which can be transmitted. The rest will be downloaded via infrared when the machine returns to base at periodic intervals.

In case of major onboard failure, the AUV will deploy an air-filled buoy which will bring it to the surface. It will be fitted with a satellite-enabled locator device and VHF radio beacon so that it can be tracked and recovered.

Sponsors

Initial work on this project was funded by the Royal Society. Current work is being funded by the Engineering & Physical Sciences Research Council and British Maritime Technologies.