Designing Underwater Robots for Deeper Dives

By Matthew Braga

The quest to find robot-building materials that balance buoyancy, weight, and strength.

In May, the remotely operated underwater vehicle Nereus descended 10,000m to the bottom of the Kermadec Trench, one of the ocean’s deepest, and never came back. It’s believed that Nereus—a hybrid remotely operated vehicle, or ROV, that could also operate autonomously—likely imploded. The pressure at such depths can be as great as 16,000 pounds per square inch.

What’s weird is that Nereus was *designed* to withstand such pressure. That’s what made it unique. Unlike most other ROVs, which get their buoyancy from a material called syntactic foam, the Woods Hole Oceanographic Institute (WHOI), which designed and built Nereus, opted for a radical new design involving hundreds of ceramic spheres instead.

Photo credit: WHOI

While we still don’t really know how or why Nereus failed–it completed numerous previous dives, some to deeper depths, without issue–there’s no denying that its novel design allowed Nereus to dive deeper, be built lighter, and stay underwater longer than probably any other ROV in existence. So, implosion aside, why aren’t we yet building more ROVs like Nereus—even the ones that aren’t destined for places as deep or pressures as intense as those of the Kermadec Trench?

Putting anything underwater requires a delicate balance between buoyancy and weight, explains Andy Bowen, director of the WHOI’s National Deep Submergence Facility, and maintaining that balance becomes more difficult the deeper you go down.

“You want the vehicle to be slightly positively buoyant, or at least neutrally buoyant. So all the stuff that weighs something has to be offset by something that doesn’t weigh as much–or, in fact provides, a buoyancy offset,” Bowen says. “You can broadly divide these things into parts that float or parts that don’t.”

Syntactic foam block machined for ROV use.

Obviously, batteries, cameras, lights and motors are the things that don’t, and it’s the job of people like Bowen to make them float. Traditionally, manufacturers have used a material known as syntactic foam, which is composite material filled hollow microscopic glass bubbles. These bubbles lower the material’s density, making it buoyant. It’s flexible, well-understood, and has been in use for decades. When you look at a photo of a typical ROV, it's the brightly colored material mounted to the top of the robot's frame. "You can make syntactic foam to go just about anywhere you want it to go,” says Bowen, “but with a price.”

The deeper you go, the stronger syntactic foam has to be to withstand the increasing pressure, and adding strength also adds size and weight.

Going underwater is sort of like going into space. Every pound counts, and to keep your ROV maneuverable and energy efficient (if running on battery), it has to be relatively small and lightweight. However, the deeper you go, the stronger syntactic foam has to be to withstand the increasing pressure, and adding strength also adds size and weight. For vehicles such as Nereus, syntactic foam becomes a less practical choice past 6,000m, and into the ocean’s Hadal zone.

“When one starts to go very deep, and they go into areas that are basically on a battery powered type system, then you have to get very very efficient in how you build that vehicle otherwise you burn through your battery power too fast,” says Dave Lovalvo, project manager for the NOAA Office of Ocean Exploration and Research’s Deep Submergence Group.

2003 Patent for using hollow ceramic spheres for low cost deep water buoyancy.

One way to increase efficiency is to reduce the weight of your buoyant material, so while designing Nereus WHOI began to investigate materials other than syntactic foam. Ceramics, it turns out, are around five times stronger than steel when compressed, but weigh about a third as much. When molded into the shape of a sphere, the ceramic can distribute pressure equally across its surface, and only gets stronger as the pressure increases.

"You're getting a lot of buoyancy for a lot less physical weight,” Lovalvo explains. “So it's a very attractive way to go, especially when you have vehicles that run on battery." Lovalvo, who works with the ROV on NOAA’s Okeanos Explorer ship and weighs about 9200 pounds in air, estimates you could easily shed a few thousand pounds by transitioning to ceramic spheres.

But building a vehicle that’s smaller, lighter and more energy efficient, would have lots of benefits at shallower depths too. By using small ceramic spheres, you could theoretically enable an ROV to carry more weight by using a lighter buoyant material with the same buoyancy as syntactic foam – say, more complex or stronger gripping tools. And by reducing the weight of an ROV in air, you could build and launch a larger, more powerful ROV from a smaller ship (the reason ROVs are typically launched from larger ships is because a strong crane is required to support the ROV’s weight in air). "It's certainly going to be the way of the future,” says Lovalvo.

Photo credit: WHOI

But the future may still be a long ways off. Though ceramic material is cheap and well-understood, its use as a buoyant replacement for syntactic foam has not been tested widely as yet. Implosions–much like what is suspected to have happened to Nereus–remain a big concern.

"What you're always worried about is an implosion, because when it implodes, you pretty much lose the vehicle,” Lovalvo explains. “There has to be a compelling reason for one to go to these exotic materials.”

Photo credit: WHOI

And for most people, there’s not often a compelling reason to go below 6,000m depth. In fact, so few vehicles venture into the Hadal zone that the industry’s definition of “full-ocean” depth only accounts for vehicles capable of traveling to depths of 6,000m–a depth that represents 80% of the ocean bottom. For most people, the risks associated with using a relatively new material as ceramic spheres don’t yet outweigh the benefits of ditching syntactic foam in shallower depths.

In fact, WHOI may even abandon ceramic spheres too, Bowen says, for a newly advanced syntactic foam, when it comes time to construct the successor to Nereus. This new syntactic foam would theoretically be capable of withstanding the pressures of the Hadal zone’s depths. But whether we build future ROVs with ceramic spheres or better syntactic foam, you can bet we’ll be building more ROVs like Nereus. The material may not be the same, but the intent certainly will be: vehicles that are lighter, more efficient, and capable of diving deeper than what we have today.