Solving the Puzzle of Long-Range Underwater Autonomy

Turns out making autonomous subsea drones is really hard. See how one robotics company overcame some of those challenges.

(Image courtesy of Cellula Robotics)

(Image courtesy of Cellula Robotics)

People have been touting the promise of autonomous underwater vehicles (AUVs) for the better part of three decades. Having a robot submarine quietly cruising the ocean, collecting and transmitting data and deploying payloads without surfacing is cause for excitement in a number of industries.

But in a world that has become expert at creating autonomous technology for use in the air, on the roads and in many industrial facilities, engineering the technology to operate successfully underwater for extended periods of time has been exceedingly difficult—that is, until July 15, 2022. 

On that day, in the deep, cold waters of the Indian Arm Inlet—a glacial fjord near Vancouver—Canadian AUV maker Cellula Robotics Ltd. successfully passed several milestones in the still burgeoning AUV industry. With an audience made up of representatives from the Canadian and Australian defense research sectors, Cellula deployed its 26-foot-long, hydrogen-fuel-cell-powered Solus-LR AUV from a nearby port.  

The Solus-LR made its way through the inlet where it autonomously—and without surfacing— deployed a foot-long micro AUV which then travelled to the surface and transmitted a status message to a command-and-control center by way of an Iridium satellite. This marked the first time such an operation was demonstrated successfully—both the autonomous launching of an AUV from another AUV, and the near-real time communication with a still-submerged AUV.  

This moment in the subsea autonomous vehicle industry has been a long time coming. Subsea “drones” are not new—since the late 1990’s, offshore mining and exploration companies have deployed small AUVs from ships for exploration missions of 24-72 hours before they are recovered. 

What makes the Solus-LR such a groundbreaking achievement is the method of deployment, its power source, the length of its missions and its ability to carry out multiple tasks on a mission using a variety of payloads. These attributes also caused many of the engineering challenges in building the Solus-LR. 

The first major difference is the method of deployment. The current generation of AUVs are smaller and must be deployed from a ship. This is an expensive undertaking that requires an ocean-going vessel with a crew numbering in the dozens. You must already know where you want the AUV to look, and the ship has to stay in place for the duration of the mission to monitor and recover the AUV.  

The Solus-LR is deployed from a port and needs only a skiff manned by a couple people to lead the drone through busy port waters into the open sea, where the Solus will head out on its mission. Recovery is easy too—once the mission is complete, it returns to port where it can be pulled from the water using a boat launch and a winch. This can save hundreds of thousands of dollars per mission, without even counting the cost of owning and maintaining an ocean-going ship.  

What makes this possible is the AUV’s hydrogen fuel cell power source. Current subsea AUVs are powered exclusively with lithium-ion batteries, which are limiting in terms of mission duration. Batteries are heavy, and the further you want the AUV to travel, the more batteries you need. This is fine for short missions, but there is a crossover point where the number of batteries required to go further would make the AUV too large to be practical. 

This is where the Solus-LR’s 1.2-kW hydrogen fuel cell comes in handy. The Solus had to be large enough to accommodate the fuel cell and the hydrogen and oxygen needed for power. It does not need to scale up in size to travel longer distances like a battery powered AUV. At 26-feet long and about four feet wide, there is plenty of room for the fuel cell and just about any typical AUV payload. These include sensors and other devices, as well as room for up to two micro AUVs.  

Standard AUV-based sensors and measurement devices are small because they are built for the current generation of smaller AUVs. And they don’t drain much power as they were engineered to run off batteries. This means that a relatively small 1.2kW fuel cell (hydrogen fuel cells in cars are generally in the 100-kW range) will power the Solus-LR and its payload for an astonishing 1,242 miles (2,000 km) over 15 days maintaining a speed of about three knots.  

Challenges, Workarounds and One-offs

Even though submarines and other autonomous subsea vehicles have been around for many years, the extended length mission set and power source for the Solus-LR created several engineering hurdles that needed to be overcome before it could be successfully tested at Indian Arm. While the fuel cell is the secret to the Solus-LR’s extended range, it also caused the Cellula team a few headaches and sleepless nights. 

“Developing equipment that’s able to go underwater is a big feat, and there’s no ‘Walmart’ where you can go buy all these pieces and make your own AUV—you are developing a lot of these things from scratch,” says Alex Johnson, Cellula’s Director of AUVs. “There’s an incredible amount of complexity in taking equipment designed to operate at ambient pressures and temperatures in a terrestrial application, put it in a housing and make it able to sustain the hydrostatic pressures that AUVs experience when they submerge—not to mention that saltwater is not good for the technology,”   

(Image courtesy of Cellula Robotics)

(Image courtesy of Cellula Robotics)

Johnson and his team overcame these challenges by using good design guided by many years of experience engineering products for underwater use. Cellula does a little of its own fabrication, but it mostly handles the design and engineering work, sending much of the part production to subcontractors. 

“One of the biggest challenges we face, and I have faced through my whole career in AUVs, is deciding if you want to design something yourself, or buy it from a third-party manufacturer,” says Johnson. “We aren’t in an industry with standard, reliable parts available from companies with huge production lines.” 

The long-range AUV industry is still developing, meaning commercial-off-the-shelf (COTS) parts aren’t readily available. “Everything you see in the subsea world that is COTS is possibly vaporware, possibly never been built before,” says Johnson. “Maybe they’ve built a prototype, written a data sheet and are hoping for their first real sale. It’s very hard to know that what you are buying has any kind of pedigree or history to it, so you run a lot of risk with every new supplier.” 

Johnson says there is a constant struggle with this decision, but if you can afford to design and build in-house at least you have control over the IP, the supply chain and production quality. “There’s a benefit to bringing it in house if you have the resources to do it. That’s one big lesson we have relearned over and over again.”  

The second major hurdle Johnson and his team had to tackle was the software and autonomy programming. Even in a market where autonomy has evolved from vacuum cleaners to passenger vehicles, programming autonomous underwater travel is tricky because it’s a communication-denied environment. 

“It’s not immediately obvious to people who haven’t done it for decades, but you have no real-time communication with the vehicle. You have no GPS and communication is limited to a few bytes per message, every 30 minutes or so,” says Johnson. Instead, his team had to design and program an inertial navigation system that uses an array of sensors to calculate position, orientation, and velocity using only direction and distance travelled as reference, a navigation method known as dead reckoning.  

Engineering the Market

Considering the test at Indian Arm, it’s clear that Cellula has thus far met the challenge of engineering and manufacturing its Solus-LR AUV. Which brings us to the last, and possibly most difficult, hurdle to overcome—ramping up sales.  

The long-duration subsea drone market is in its infancy. Only one company—Austin, TX-based Ocean Infinity—operates cruising AUVs at scale, and it only has 28. However, there are many commercial applications for this technology aside from the defense sector (surely a fleet of virtually undetectable, autonomous submarines cruising the ocean collecting data is on at least one Admiral’s wish list).  

From surveying the sea floor to extract minerals to monitoring offshore oil and gas rigs or wind farms, the business case for these AUVs looks strong: use AUVs instead of funding, operating and putting at risk fully crewed ships. In a world where the pressure on profit margins is constant, these fire-and-forget drones could make a big splash.