The AUV market is getting caught in the middle of contradictory requirements Luc Simon of France-based RTsys told MJ. At its heart there’s a truly elemental conflict: tidal flow and positioning.
The idea of using multiple AUV’s in a swarm configuration is rapidly gaining ground, especially as it potentially raises the quality of the data as there’s no line pulling or other interference from a mothership. Further, it promises to dramatically shorten the length of time it takes to carry out scans, allowing rapid, 3D profiling and dropping costs considerably – some people putting day rate savings at over 80%. It has also considerable scope for use in shallow or otherwise difficult-to-access waters.
But there is an issue: as Mr Simon explained, a small AUV usually has very limited ability to handle turbulence, tidal flow or even accurate positioning “as there’s often just not that much room inside for power or complex systems”.
In short, more modest AUVs can run into difficulties when countering subsurface forces so that there may be issues in keeping them well-oriented enough to keep tracking a particular route across the seabed or to carry out accurate bathymetry readings.
However, he went on to say that scaling up lands the operation with another set of problems: “If you use a big AUV, it is much more costly, much heavier and more difficult to handle.” So it’s often just not practical for ‘swarm’ deployments.
This is where RTsys’ Comet project has started to pay off. On one side is its physical capacity: it’s around 2m long and weighs in at around 34kg making it single-man deployable said Mr Simon, although he admitted that “retrieval probably takes two people”. Onboard each of the AUV’s has its own inertial navigation system and an innovative communication and positioning protocol that can be combined with DVL for shallow water applications.
While the first model has around 1,200Wh of lithium ion batteries (linked to a BLDC permanent-magnet motor) to drive the propulsion, the second model has been fitted with an array that’s nearly 2,000Wh in size, so it can reach more than 10 kn speeds and it keeps going – if a lot more slowly – even in tidal environments.
This means the AUV can work on for eight hours at a stretch, day in, day out, by swapping operation and recharge between two battery packs (a matter of a five minute change-out) on alternative shifts, explained Mr Simon. There has also been a lot of work on the hydrodynamics: it has a bullet shape which nevertheless displays good stability even at top speed, a necessity given the need for accurate data collection.
But the long endurance would have limited use if it weren’t for that very robust set of communication protocols: this gives the AUV positioning accuracy down to a few centimetres.
Mr Simon explained that this lends a swarm huge flexibility: as well as data links with other AUVs, base stations or buoy-mounted communication ‘nodes’, it also “allows you to do things you wouldn’t think of doing with an AUV, such as keeping pace with a surface vessel, either staying at a set position out in front or following from behind”, he said.
The suitcase-sized ‘base station’ can easily be carried on and programmed aboard a mother ship, allowing it to command the AUVs to follow it like a shoal of dolphins – and since the batteries allow it to cover 150km distances on a single charge “it could actually survey its way across the Channel or the Mediterranean”, he explained.
More, if the deployment needs to be cut short or the working day is ending, Comet can be ‘called back in’ said Mr Simon, “coming home at the end of its pre-programmed pattern or on a signal”.
Further, a new innovation that is presently being worked on could make these AUVs almost completely autonomous: an automatic seabed data-connection and recharging point is being developed, meaning that these units could stay out at sea for weeks or even months at a time.
By Stevie Knight
