Unmanned underwater vehicles are becoming feasible for a wide variety of applications, from autonomous ship hull inspection to oil and gas exploration, while military leaders are developing UUVs for long-endurance underwater intelligence, surveillance, and reconnaissance missions.
Unmanned underwater vehicles (UUVs) in the past have been niche machines used for research and specific pre-programmed tasks. The difficulties of building and operating these submersibles made them less useful than their airborne and land-bound kin. With technology constantly moving forward, however, UUVs are expanding into the mainstream with the ability to complete a wider variety of missions than their research-specific predecessors.
UUVs face a unique challenge that other unmanned vehicles do not: the ocean. This is a corrosive environment in which high pressure often can be present. Marine animals as small as microorganisms and as large as whales can interfere with operation. In the ocean, communications are virtually non-existent, and ruggedizing these underwater craft, without exception, means they must be either waterproof, airtight, or both. As a result, UUVs need to be largely autonomous, able to withstand the rigors of the marine environment, and be among the most power-efficient vehicles designed.
|A Bluefin Robotics UUV sits just below the surface while gathering data.|
"The 1990s introduced the modern UUVs, which meant GPS capabilities on the surface, advanced navigation below the surface, good acoustic communications, Doppler velocity logs for navigation, and a sophisticated software infrastructure to support the autonomy features," explains David Kelly, president and CEO of UUV designer Bluefin Robotics in Quincy, Mass.
Size and endurance
UUVs are seeing a split in designs; some are growing larger than 21 inches in diameter, while some are as small as 7.5 inches in diameter. Most are long and skinny like torpedoes. Those larger than 21 inches in diameter officially are titled "large UUVs." The length of UUVs varies between several yards and a few feet.
"We're clearly seeing UUV size diverge in both directions," says Jason Stack, program officer at the U.S. Office of Naval Research in Arlington, Va. "Larger UUVs are appearing for when you need increased endurance, depth, or mission capability, while smaller UUVs are emerging and adding value in sectors where you need more affordability or larger numbers, and can live with less endurance and capability."
Large, long-endurance UUVs are more attractive to military customers than they are to commercial UUV operators. "From the Navy's perspective, there's interest in large-diameter vehicles, mostly for enhanced endurance and larger payloads," says Bluefin's Kelly. Several military research projects are in progress to improve UUV endurance and autonomy (see "Development of prototype propulsion and power system for future long-endurance UUVs is underway" on page 16).
|An Echo Ranger UUV awaits deployment. The Echo Ranger can dive up to 10,000 feet.|
UUVs must operate in some of the harshest environments on Earth. UUVs must be able to withstand the effects of corrosive saltwater, the pressure of the ocean's depths, extremes in temperature, unpredictable currents and weather, and wildlife in the maritime environment to perform their missions.
"The harshness of the ocean environment can never be understated," says the ONR's Stack, "Corrosion, biofouling, extreme pressure, and unpredictable marine environments place demands on UUVs that have no analogy on land or in air. Additionally, the physics of electromagnetic propagation underwater eliminates the use of GPS and radio, minimizes the value of optical imagery, and relegates UUVs to mainly acoustic sensing and communication."
Biofouling refers to corrosion, clogging, and other damage from microorganisms and other small sea life contacting connectors and other electronics aboard UUVs. Biofouling can involve the buildup of animals like barnacles and mussels. Although UUVs typically use radio waves for control and navigation, the submersible must raise an RF antenna above the surface to receive signals. Submerged, they must operate autonomously or take direction from sound waves.
Depending on the UUV's design, whether it is free-flood or fully pressurized, various components can be exposed directly to saltwater, pressure, and sea life.
The solution comes from rug- gedized parts designed specifically for UUVs. "Many sensor vendors are now making sensors that are designed for UUVs," says Bluefin's Kelly. "Sensors are smaller form factor, lower power, and better designed to fit into those vehicles." Unlike other vehicles where sensors are almost interchangeable, UUVs require their own set of sensors.
Guidance and communications
Not only is the ocean dangerous to UUVs, but it also can limit the performance of UUV sensors and communications. Their operating environment requires UUVs to use a combination of inertial and acoustic navigation sensors while underwater. UUVs must surface now and then to verify their positions with GPS; their accuracy is not perfect.
"Unless the vehicles are operating within a few kilometers of each other (the range of acoustic communications), there is no in-mission communication," ONR's Stack explains. "Today's approaches often have vehicles surfacing periodically for communication and GPS localization, while significant research is going into making the vehicles autonomous enough to rely less on the operators."
Acoustic communications are notoriously low bandwidth, and have problems from multipath propagation and signal attenuation. "Acoustic communications are intermittent and have the bandwidth of a 1980s modem," Bluefin's Kelly explains. "There are people working on acoustic communications capability to be more reliable."
Although UUV experts have sought and tested other forms of submarine communication, none has been as versatile as acoustics. "They looked at laser, they looked at RF," Kelley explains. "Laser is viable for certain ranges, but typically not at distances for useful operation. For the foreseeable future, the choice communication mode for UUVs is going to be acoustics." Due to the difficulty of communication, the UUV industry aims at designing UUVs that can perform without human interaction.
|The Bluefin Hovering Autonomous Underwater Vehicle (HAUV) is designed to inspect the hulls of ships more quickly and efficiently than human divers.|
Applications and demand
UUVs typically function as mobile sensors, and to perform relatively simple tasks without putting human divers or manned vehicles at risk, like sampling, surveying, identifying objects, and performing undersea inspections. "Today, the vehicles collect data, which is processed after the mission is complete," says Bluefin's Kelly. "There's utility in those applications, but in the future UUVs are going to act on the data they collect.
"If you look at a military application like mine countermeasures, what they'd like to do is in stride detection, identification, and neutralization," Kelly continues. "Those are three different applications. The focus going forward is more complex missions and more integrated processing." Today's generation of UUVs can identify objects during mine-countermeasures missions, and report back potential threats. For the future, designers want UUVs to perform all three tasks involved in mine countermeasures.
One of today's most popular uses for UUVs is ship hull inspection. The U.S. Navy, as well as other foreign militaries, is showing interest in purchasing Bluefin's Hovering Autonomous Underwater Vehicle (HAUV), an autonomous UUV that is designed for ship hull inspection, Bluefin's Kelly says. "Today, they use divers to inspect their ships, which provides a low coverage rate," he says. "It's a tough job and takes a long time. What we've developed (the HAUV) is a fully autonomous vehicle for hull inspection without prior knowledge of the hull. Its search rates are vastly superior to human operators and provide a consistent repeatable result."
Hull-inspection UUVs offer faster and more accurate results than human divers produce. The Navy already has placed an order for the Bluefin HAUV for hull inspection.
|A 21-inch-diameter Bluefin Robotics UUV sits ready to deploy on the deck of a ship.|
"There is increased demand," says Bluefin's Kelly. "We've been working with the early adopters and concept exploration people for some time now. We are starting to see that UUVs are migrating into more general acceptance."
Free-flood vs. pressurized vehicles
Opinions among UUV designers today are split among two different architectures-free-flood and pressurized vehicles. Free-flood architectures enable the vehicle to fill with water while keeping some components in pressurized compartments. A pressurized vehicle, on the other hand, is entirely airtight and pressurized like a submarine.
"In an ideal world, you'd like a modular, plug-and-play UUV where you can easily swap batteries, data storage, and payloads with minimal impact on buoyancy and no impact on reliability," says ONR's Stack. "However, going back to the harshness of the marine environment, cycling cable connections and repeatedly opening pressure assemblies is a great way to introduce failures. We haven't seen the UUV developer community yet converge on the balance between pressurized and free-flooded."
Advocates of free-flood cite the ease of repair and replacement because they can swap out components on deck before everything is completely dry. This provides more up-time than a pressurized vehicle. "Our vehicles are free-flooded," says Bluefin's Kelly. "They are made up of several pressure-compensated and one-atmosphere modules so water can flood the system. The reason is so we can service our vehicles at sea quickly. We can swap out components that need to be serviced so you can keep your survey or mission time up without having to pull a pressure vessel."
Those who prefer pressurized vehicles point out they can use different components that don't have to be ruggedized to withstand maritime conditions. Instead of using large and extremely rugged connectors, they can use smaller connectors designed for relatively benign environments to space and open more options for the vehicle's internal architecture.
|A Bluefin Robotics UUV is lowered into the water for its next mission. The silver bands connect different pieces of the modular UUV design used by Bluefin, allowing for UUVs of different lengths by adding or subtracting sections of the vehicle.|
The future of UUVs
UUVs today are still an emerging technology that is far from mature. Virtually any technology improvements related to UUVs can improve the entire system, whether improvements come in software, computer hardware, acoustic communications, or inertial navigation, the UUVs of the future will be capable of performing vastly different missions.
"For larger UUVs, breakthroughs in power and endurance will fundamentally change how we think about them and how we use them," explains ONR's Stack. "These breakthroughs could come in the form of new onboard energy generation technologies or even unmanned, practical approaches to in-situ recharging or energy transfer. For smaller UUVs, a major aspect will be the continual commoditization of the vehicles and their base capabilities. Just as we didn't envision the original telephones becoming mobile cameras, GPS, and computing platforms, we'll likely see the aggregation of processing, power, autonomy, and increasingly capable payloads lead to unexpected and innovative uses for UUVs."
Currently UUVs are not used with great frequency, but developing technology is making UUVs part of more and more organizations normal routine. "The future is bright, there are increasing uses and applications in the field," Bluefin's Kelly says. "In the future, we'd like to see routine operational use for UUVs."
Development of prototype propulsion and power system for future long-endurance UUVs is underway
The U.S. Office of Naval Research (ONR) in Arlington, Va., has asked engineers at two underwater vehicle designers to develop a prototype propulsion and power system for the next generation of unmanned underwater vehicles (UUVs) for long-endurance operations.
The companies, NexTech Materials Ltd. in Lewis Center, Ohio, and Lynntech Inc. in College Station, Texas, will build UUV prototype power and propulsion systems that measure 30 inches long by 18.5 inches in diameter, and that deliver 42 to 68 kilowatt hours of power for no less than 30 hours. The vehicle that the propulsion and power system is being designed for will be a pressurized vessel rather than free-flood.
NexTech Materials specializes in high-performance complex oxides for solid-oxide fuel cell (SOFC) and catalysis applications. The company has a dedicated pilot facility to build SOFCs and electrochemical sensors.
Lynntech focuses on energy and power, embedded systems, materials, industrial sciences, and life sciences with resources that include analytical laboratories, computational modeling, instrumentation, machining, and fabrication.
"One of the real challenges is that conventional battery technology that has been used for UUVs limits the mission duration," explains Scott Swartz, chief technology officer at NexTech Materials. "As you increase the demand on how long they need to go out there, you need to develop a higher energy density to do that."
The system that NexTech and Lynntech are developing is based on fuel cell technology rather than traditional batteries. The fuel cells being used have a membrane of zirconia with electrodes on either side. Designers place oxygen on one side and JP10 fuel on the other. The reaction of these two components creates electricity, with a biproduct of hydrogen, carbon monoxide, water, and carbon dioxide. The current that the reaction creates provides power as long as the membrane has fuel and oxygen.
"The primary challenge is fitting everything in the weight constraints they have provided us," Swartz says. "The energy system has to utilize fuel-JP10 hydrocarbon fuel and liquid oxygen. We have to pack it efficiently. Mission duration isn't as dependent on the discharging of the battery but on how much fuel you can store."
The technology involved in this system will be scalable, allowing it to be used for UUVs that are larger and smaller than the 21-inch-diameter test vehicle that is being used.
Company designers are looking to increase system efficiency. "Our approach is making [fuel cells] more efficient; efficiency drives this application because it drives how much fuel and liquid oxygen we have to store," Swartz explains. "Fifty percent of the system is taken up by liquid oxygen and fuel." Due to weight constraints, there is a limit on how much fuel can be stored on the vehicle, making efficiency the best way forward.
The system is not expected to be complete for at least four and a half years.