Drop in the ocean

Early last month, to the delight of its UK developers, a deep-diving robot submarine designed to survey the hardest-to-reach regions of the ocean began to report its findings.

Autosub 6000, so-called because it can operate down to a depth of 6,000m, is the UK’s deepest-diving autonomous undersea vehicle (AUV). It is the brainchild of engineers from the Underwater Systems Laboratory (USL) at Southampton’s National Oceanography Centre(NOC) and is the latest stage in a programme that has been running for over 12 years.

The vehicle began life as Autosub, which was first trialled in Southampton’s Empress dock in 1996. Improvements in autonomy and range led to Autosub 3, which was able to operate at a depth of 1,600m. As well as being used to measure current and temperature in the Gulf Stream, this has carried out an under-ice mission measuring ice thickness and populations of krill in the Antarctic.

Autosub 6000, claimed to be suited to exploring 95 per cent of the world’s oceans, takes these capabilities a step further. On its current research mission it is fitted with a multi-beam bathymetric sonar to map the sea floor as part of an expedition under Dr Russell Wynn investigating potential threats from tsunamis, landslides and earthquakes on the western coasts of Europe.

‘We provide the AUV as a platform for scientists to run their experiments on,’ said project manager Dr Stephen McPhail, speaking from onboard the research vessel RSS James Cook. ‘It does the mission as designed, whether that is ploughing up and down through a water column to measure the properties of the water, or flying close to the seabed to map it.’

The fact that Autosub 6000 is in use so soon after its successful initial trials in September 2007 is a demonstration of the project’s success. It is supporting the scientists investigating historically large mud-flow events on the sea floor, mapping the areas in great detail and guiding coring operations from the ship.

‘The original plan was to do more engineering trials this year but when the opportunity for this cruise came along, it sounded ideal for us,’ said McPhail. ‘Right from the first mission, everything worked perfectly, we got very good image data back from its first dive when it surveyed the sea floor.’

Autosub 6000 is able to operate down to a depth of 6,000m and is fitted with a multi-beam bathymetric sonar to map the sea floor

The scientists can run the vehicle for just over 24 hours. The surveys take 20 hours, which allows some safety contingency. It takes about an hour and a half for the vehicle to dive through the water column to its target depth of 4,500m. ‘We check it really carefully as it’s going down using our acoustic communication system to check everything’s safe,’ said McPhail. ‘When it gets to the sea floor, we check the engineering systems, the batteries and that it’s controlling its depth and navigating correctly. Once we’re happy with all that, we send another command to the vehicle to tell it to start the survey.’

At the depths to which Autosub 6000 travels, navigation and communication pose new challenges. GPS cannot work underwater, so the AUV uses Doppler sonar to carry out dead reckoning, measuring its velocity relative to the sea floor to calculate its position. It also carries an expensive inertial navigation system, which acts as an extremely accurate gyro compass.

‘The navigation accuracy is pretty impressive — the drift is in the order of about 1m for every kilometre you run, so it’s accurate to within 0.1 per cent,’ said McPhail.

However, the Doppler sonar has a range of 200m, so only works close to the sea floor. As Autosub 6000 takes more than an hour to descend, the ocean current can push the position out at the bottom of its descent by several hundred metres.

To accurately fix its position on the sea floor, the scientists transmit a pulse of sound through the water. An onboard transponder receives it and replies with another. By timing the two-way travel, the range to the vehicle can be calculated and by taking a number of readings from different locations, Autosub can be accurately positioned.

Over a 20-hour mission the team surveys 5km x 4km sections by running the AUV up and down, building up about 15 overlapping swaths of sonar data in a process known as a lawnmower survey. The multi-beam sonar produces 111 beams of sound at right angles to the AUV’s path, each taking a depth measurement, which are put together to build up a picture of the ocean floor.

McPhail said Autosub’s acoustic communication systems are only effective up to seven or eight kilometres away, so the team relies on it to carry out its survey uninterrupted while the crew carries out other scientific tasks, such as taking cores from the ocean floor elsewhere. ‘We move the ship around 10 miles off-site to avoid spearing it with the core barrel,’ he said.

At the end of the survey, the AUV’s control systems direct it to circle at a depth of 1,300m from the sea floor at a pre-arranged rendezvous point. The crew measures its position to include in the survey data and improve future navigation, then sends a command for it to surface. ‘One of the most tense moments was getting the data off it after that first dive and seeing if there was anything useful there,’ admits McPhail, ‘So we were pretty happy when the images came back.’

The data is processed to make a 3D view, which the scientists can ‘fly around’ identifying features. These are far clearer than any that can be achieved from a ship, with a resolution of about 5m to accurately guide coring locations.

The missions are written via a user-friendly interface describing the path of the survey, way points, geographical locations, depths and special commands, such as waiting for further instruction from the operator. The script is generated from these commands and downloaded via a wireless link.

Autosub 6000 is close in design to its predecessor, Autosub 3, but the use of modern battery technology allows it to travel far deeper. The older vehicle has a centre section of heavy carbon fibre tubes to protect the equivalent of 5,000 torch batteries made up into packs. For Autosub 6000, USL developed its own battery packs based on lithium polymer battery technology, similar to that used in a laptop computer. As these cells are of solid construction, they can operate without a pressure case and are instead kept in oil-filled battery boxes that use pressure-balance technology, allowing the pressure from the outside ocean to distribute evenly between them. This means there is room for batteries to operate the vehicle for up to three days without compromising buoyancy.

The propulsion system is a slow-running, direct-drive brushless DC electric motor, which drives a conventional two-bladed propeller at the rear of the vehicle. It is an outrunner type motor, where the inner part of the motor stays stationary while the outer part rotates on a bearing. This makes it possible to operate the motor at pressure without having the axle come out of a pressure case through a seal, which would absorb energy through friction.

To go up and down, the vehicle is steered using two stern planes on the tail. ‘It’s made to dive from the surface like an aircraft taking off in reverse,’ said McPhail. ‘We run it on the surface to build up speed then we flip the stern planes, which forces the nose down, and it dives. It has an accurate pressure sensor on it to measure depth and an altimeter to measure its height from the sea floor. We can control the vehicle to fly at a constant altitude to do bathymetric surveys — in this case 100m off the sea floor.’

To steer it left and right there is a conventional arrangement of rudders and a control system, which adjusts the head of the vehicle so it follows the line between waypoints as accurately as possible.

Deep seawater compresses by about 2.5 per cent, getting denser and raising buoyancy challenges. The UAV’s stubby wings help it stay down more efficiently without introducing more drag. Most of the buoyancy is provided by the central section made from syntactic foam, which consists of tiny pressure-resistant hollow glass spheres embedded in a plastic matrix.

USL’s next challenge is a proposal from the Natural Environment Research Councilto use Autosub 6000 in the Cayman Trough to measure deep ocean turbulence. Next year it will further improve the navigation system and try out new sensors.

In future the vehicle could be developed to get much closer to the sea floor, slow down and even stop and hover to photograph small objects. Autosub 6000 could even start acting as an intervention AUV, finding interesting features and taking samples itself.