The Nereus AUV

Deep-ocean submarines: voyage to the bottom of the sea

E&T discovers that it takes a special kind of submarine to dive seven miles beneath the surface of the Pacific Ocean.

On 31 May 2009 a unique and versatile hybrid vehicle dived 10,902m below the surface of the Pacific Ocean to the bottom of the 'Challenger Deep' in the Mariana Trench. The Nereus is only the third vehicle ever to have descended this far - to the deepest part of the world's oceans.

The revolutionary unmanned craft was designed by engineers at Woods Hole Oceanographic Institute (WHOI) in Massachusetts on a budget of only $6m. Nereus is set to open up the exploration of the extreme depths to science, and its design concept will have extensive repercussions on 'conventional' ROV design in the oil and gas and military sectors.

Project manager and principal developer Andy Bowen talked to E&T about what makes it special. "Its hybrid nature means that Nereus can be operated as a pre-programmed, free-flying autonomous underwater vehicle (AUV) to survey broad areas. Conversion into its tethered, piloted ROV (remotely-operated vehicle) mode only takes six hours and allows it to inspect specific areas of interest identified in the flyby. This makes it especially flexible and versatile. Freed of the constraints of heavy steel cables, with a robot like Nereus we can now explore virtually anywhere in the ocean."

Appropriately named after the half man, half fish, shape-shifting Greek god of the sea, the vehicle was seven years in the design and testing phase, and went through three major iterations. A series of test dives from the University of Hawaii research vessel Kilo Moana during May culminated in the dive to the Challenger Deep.

Every element of Nereus had to be designed to be capable of withstanding the 1,000-bar pressure at the bottom of the ocean, or housed in a non-compressible container.

"Here at Woods Hole our biggest resources are our team of highly motivated, clever and creative engineers and technologists," says Bowen, "combined with-our experience in building deep sea vehicles like Jason Jr, the Titanic-exploring ROV, where I started my career. At the beginning of the Nereus development, we decided to focus on those technologies that would have the greatest impact on the system. In particular, we focused on the tether and our use of ceramics as having the most significant impact in realising the objective of a relatively small and lightweight system."

Fibre-optic cable

Nereus weighs just under 3t in air and is neutrally buoyant in the water, meaning that the vessel will neither rise nor fall once in position. The revolutionary patent-pending tether system that links Nereus in ROV mode to its mother ship is a fibre optic cable sheathed in plastic with a total diameter similar to that of a human hair. Incredibly it has a breaking strength of only four kilos. The ultra thin tether is only slightly negatively buoyant and is stored in two containers, one onboard Nereus, the other near the mother ship, with 20km of cable in each.

The tether is Nereus' umbilical, passing control data from its pilot downwards and transmitting live video or stills feedback up to the surface. "One of our partners is the US Navy Space and Naval Warfare group in San Diego. We leveraged up on vessel-to-torpedo communications systems, and developed the tether from that," says Bowen.

The cable is needed because transmitting large amounts of digital information through the ocean is difficult as it is essentially opaque to the high bandwidth signals required to transmit video.

Because WHOI have been able to use such lightweight cabling, the craft can be smaller, lighter, and less expensive than conventional submersibles. The Nereus team started with the control system software from Jason, the ROV that was use to explore the Titanic. But the challenge was to bring two control systems together, one autonomous, the other pilot-driven.

Conceptually this is similar to the complex boundary between the automated systems and the role of the pilot on a passenger jet. Given the fragile nature of the cable, the vehicle could conceivably lose communications with its pilot and have to drop instantly into AUV mode. It has to be able to interpret the situation and carry out a series of manoeuvres to self rescue. As Andy Bowen says, "This is an area of much ongoing R&D; computers are good at doing what you tell them, less good at doing what you want," and continues with something of an understatement, "Codifying desire into actionable code is quite a challenge," he laughs.

Another technological breakthrough in the design of Nereus was the extensive use of ceramics. The critical, hollow 9cm diameter buoyancy spheres are five times stronger than steel, have very high compressive strength, but at only 2mm thick give considerable weight advantage over steel. Some 1,700 of them are mounted in the two hulls and provide 400kg of lift for neutral buoyancy, important to be able to maintain a stable hover, and to minimise power demand for the eight-hour ascent.

Ceramics are also used in the camera pressure housings and to house the control systems and battery. Because Nereus is a hybrid, operating both autonomously and tethered, the craft had to be equipped with its own power source to drive propulsion, lights and the robotic arm.

Lithium ion batteries

Daniel Gomez-Ibanez, a specialist in battery system development for deep submergence vehicles at WHOI says, "We went for mass-produced rechargeable Lithium Ion batteries, 4,000 of them, because the high volume manufacturing process has consistency and good quality control. Even so each individual battery was pressure tested to 1,000 bar."

Lithium Ion batteries also represented the best energy density to weight ratio. All of the batteries plug into a single power cord to form one big battery. But 4,000 lithium ion batteries hold a lot of energy, and could be a hazard if they heat up and catch fire. Protection features include heat sensors that will turn off the battery if it gets too hot. The final battery can power Nereus for up to a 20-hour mission and takes eight hours to recharge fully on the mother ship's deck.

Below 1,000m there is absolute darkness, so Nereus had to be equipped with a light source that would allow its pilot to see what was going on, and for the cameras to capture images. LEDs were selected because they are bright and they tolerate great pressures. The small point sources of light are grouped together in an array so they can be aimed precisely where they are needed, and can be turned on and off quickly, so power can be conserved.

"Our LEDs are grouped together on a shiny parabolic reflector in sets of 16, called pucks. The whole lighting system no longer needs to be housed as the LEDs are capable of withstanding ambient pressures, giving us another weight advantage. They are a huge advance over flashes that used to take 12 seconds to recharge between shots and the area lit can be identical to the camera's angle of view and shutter speed," explains Jonathan Howland, WHOI research engineer responsible for the cameras and lighting systems.

In keeping for a hybrid vehicle, the onboard camera is something of a hybrid too. "Rather than have both video and stills cameras, to save payload weight, Nereus has a stills camera that can be run at 30 frames per second to give us video-type images," says Howland. In AUV mode the camera used is sensitive to blue-green light, which has better water penetration than other wavelengths. The LEDs used with this camera emit only blue green light (consuming less power than full spectrum light), and record a black and white image for analysis of the broad span of the survey.

Robotic arm

But when Nereus switches to ROV mode the LEDs switch to normal broad spectrum white light, and are operated continuously as the pilot needs to see what he is doing when operating the manipulator arm at close range. Also the human brain discerns detail and recognises things better in a colour environment. In ROV mode a larger area is lit to enable the pilot to see both the tool sled mounted under the vehicle and ahead of the craft so that he can fly it. The aluminium tool sled can carry samples of up to 25kg and is also used to mount scientific instruments, probes and sensors for temperature, salinity and conductivity of the water and corers that can be used to sample into the sediment below.

The ROV's hydraulic, sample-gathering robotic arm was specially modified to operate under extreme pressure and is very power efficient. It was jointly designed by engineers at WHOI and Kraft Telerobotics. Research engineer Matt Heintz at WHOI says: "We have a 'master' control arm on the surface ship that we move, and then the subsea 'slave' arm mimics what the master does. We pick up rocks, collect biological samples, open and close boxes and push corers into the substrate to sample the sediment."

Asked how he sees the next generation of submersibles, Andy Bowen says: "Within the next ten years we will be seeing a much greater degree of autonomy, so much so that we may be able to eliminate support vessels altogether. Instructions will be transmitted wirelessly to the vehicle using a mixture of communication strategies such as high speed acoustics and other emerging technologies.

"Onboard software will be much more sophisticated. I can imagine a database on the vessel that contains all known marine biological life forms, and that the robot is able to detect new species, sample their DNA and bring it back. And with the huge investment going on in the development of battery technology, I also expect the power sources in use will be lighter, and much more efficient."

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