An Overview of Underwater Robotics Technology

By Ankit SinghReviewed by Susha Cheriyedath, M.Sc.Oct 14 2024

Underwater robotics has become an essential field, enabling a wide variety of marine operations, from exploration to industrial and research activities. These robots, typically referred to as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), are outfitted with advanced sensors, cameras, and manipulation tools that allow them to operate in deep-sea environments that are otherwise inaccessible to humans.

Image Credit: Humberto Ramirez/Shutterstock.com

Core Technologies of Underwater Robotics

Underwater robotics is a multidisciplinary field integrating mechanical engineering, computer science, electronics, and oceanography. These robots are engineered to operate in extreme underwater environments, relying on several key technical components and systems to function effectively.

Hull Design and Materials

The hull of an underwater robot must be able to withstand the immense pressure of deep-sea environments, where pressures can exceed 100 MPa (megapascals). Most underwater robots use materials like titanium or aluminum alloys for the main frame, as these materials offer excellent strength-to-weight ratios and resist corrosion in saline water.

Buoyancy management is equally critical. Syntactic foam, composed of hollow glass microspheres embedded in an epoxy resin matrix, is commonly used to maintain neutral buoyancy, minimizing energy expenditure during movement. The hull's hydrodynamic design also reduces drag, allowing the robot to move efficiently through dense water environments while conserving energy.¹

Propulsion Systems

Underwater robots use thrusters and propellers optimized for thrust in water, which is more viscous than air. Multiple thrusters are typically employed to enable six degrees of freedom (DOF) movement: forward/backward, left/right, up/down, as well as rotational movements (pitch, yaw, and roll).

Modern AUVs feature energy-efficient thrusters that allow for extended operational periods. For deep-sea missions, low-noise electric thrusters are often preferred to minimize acoustic interference with sonar systems. Proper placement of these thrusters is vital for maintaining balance and stability during complex maneuvers in unpredictable underwater currents.¹

Navigation and Control

Unlike surface or aerial robots, underwater robots cannot rely on global positioning system (GPS) signals due to signal attenuation in water. Instead, they employ a combination of Inertial Navigation Systems (INS), Doppler Velocity Logs (DVL), and acoustic positioning systems to determine their location. INS systems use accelerometers and gyroscopes to estimate changes in the robot’s position.^1,2

DVL systems, on the other hand, measure velocity relative to the seabed, providing more accurate short-term location tracking. For long-distance missions, underwater robots often rely on acoustic positioning systems like ultra-short baseline (USBL) or long baseline (LBL) systems, where fixed transponders on the seabed provide location references. Control systems utilize advanced algorithms for real-time path planning, obstacle avoidance, and autonomous decision-making, critical in dynamic and cluttered underwater environments.^1,2

Power Supply and Energy Management

Energy constraints significantly limit the operational range of underwater robots, especially AUVs, which rely on onboard power. Lithium-ion batteries are commonly used due to their high energy density and relatively lightweight design. However, the demand for longer missions has spurred research into alternative energy sources, such as fuel cells or ocean thermal gradient energy harvesting.

Robots operating at great depths optimize energy consumption by using low-power sensors and efficient thrusters. A growing trend is the deployment of underwater docking stations, allowing AUVs to recharge mid-mission. This innovation enables continuous operations without human intervention, extending both mission duration and depth.¹

Sensors and Data Acquisition

Underwater robots use a variety of sensors for navigation, data collection, and task execution. Sonar systems, such as multi-beam and side-scan sonar, are essential for underwater mapping and obstacle detection, emitting acoustic pulses and measuring their reflections to generate 3D maps.

Optical sensors, including cameras and laser scanners, are used for close-range inspection tasks, although their performance decreases with turbidity and depth. Environmental sensors measure parameters like temperature, salinity, pressure, and turbidity, providing essential data for marine research. These robots also require robust real-time data acquisition systems to handle the large volumes of data they generate, transmitted via acoustic communication or stored for later analysis.^1,3

Advanced Navigation: Solutions for GPS-Denied Environments

Manipulation and Actuation

Some ROVs are fitted with robotic arms for manipulation tasks like object retrieval, sample collection, and underwater repairs. These manipulators require advanced actuators capable of withstanding underwater pressure, and they are typically operated using precise control mechanisms to perform delicate operations in challenging conditions.¹

Game-Changing Applications

Underwater robotics technology is integral to several industries, enabling tasks that were previously impossible or highly challenging. The following are key applications where underwater robots play a significant role.

• Marine Research and Exploration: AUVs and ROVs are widely used in oceanographic research to explore the deep-sea ecosystem, study marine biodiversity, and monitor underwater geological processes.^4,5

• Oil and Gas Industry: ROVs are indispensable for subsea oil and gas operations, including pipeline inspections, maintenance, and repairs in extreme underwater environments.⁴ 
  Ensuring Safer Subsea Oil Operations With Robot Inspections

• Defense and Security: Navies around the world use underwater robots for tasks such as mine detection, surveillance, and anti-submarine warfare.⁶ 

• Environmental Monitoring: ROVs and AUVs play a crucial role in tracking pollution levels, monitoring coral reefs, and gathering data for climate change studies.^4,5

Underwater Robots in Deep Sea Mining

Challenges in Underwater Robotics

Despite the impressive capabilities of underwater robots, the field still faces several challenges, which limit its performance in tough and unpredictable underwater conditions.^1,5

• Communication Limitations: Water severely restricts radio wave transmission, making real-time communication with underwater robots difficult. Acoustic communication is often used, but it has limited bandwidth and is susceptible to interference.^1,5 

• Energy Efficiency: The battery life of AUVs is a critical constraint, especially for long-duration missions. Advancements in energy storage and harvesting systems are needed to extend mission times.^1,5 

• Navigation Accuracy: Underwater robots face difficulty in achieving precise navigation, particularly in complex and cluttered environments, due to the absence of reliable positioning systems like GPS.^1,5

Recent Research Breakthroughs

Recent advancements in underwater robotics have dramatically improved the scope of what these machines can achieve. For instance, a new Hybrid Remotely Operated Vehicle (HROV) was developed, combining the best features of both AUVs and ROVs. This HROV operates in multiple modes—allowing it to switch between autonomous and remote-controlled functions—and features advanced communication technologies that enhance data transmission efficiency.⁷

Another exciting innovation is DeepStalk, a bioinspired soft robot designed for deep-sea exploration. Modeled after the eyestalks of deep-sea snails, DeepStalk uses shape memory alloy (SMA) springs to mimic muscle movements, enabling it to navigate high-pressure environments with exceptional precision. This technology could revolutionize how we explore the ocean's most extreme depths.⁸

Underwater Robots in Marine Research

Leading Companies in Underwater Robotics

Several companies are driving innovation in underwater robotics, pushing the limits of technology for commercial and research purposes.

• Oceaneering International Inc.: Known for its ROVs and hybrid AUV systems, Oceaneering is a leader in underwater technology, particularly for oil and gas operations. Its ROV systems, such as the Magnum and Millennium series, are equipped with advanced manipulation and sensor technologies for deep-sea operations. The company has also developed the Freedom AUV, which operates in hybrid mode, combining autonomy and manual control to reduce operational costs in subsea maintenance. 
• Blue Robotics: Blue Robotics focuses on low-cost underwater robot platforms, enabling broader access to marine robotics technology. Their BlueROV2 is a popular choice for researchers, educational institutions, and hobbyists due to its modular design and affordability. Despite its lower price point, the robot offers professional-grade performance with its high-end sensors.

Future Prospects and Conclusion

The future of underwater robotics promises exciting developments, particularly with advancements in autonomous navigation, machine learning for adaptive control, and energy-efficient systems. As ocean exploration becomes more critical for both environmental and commercial reasons, underwater robots will play an increasing role in data collection, resource extraction, and ecosystem monitoring. Innovations in communication technologies, energy harvesting, and AI-driven decision-making will enable more autonomous and sophisticated robots that can carry out missions previously deemed impossible.

Despite current challenges like energy constraints and communication limitations, ongoing research is driving us closer to a future where underwater robots can operate autonomously for extended periods. The potential for innovation in this field is vast, and the oceans are waiting to be explored.

References and Further Reading

1. Neira, J. et al. (2020). Review on Unmanned Underwater Robotics, Structure Designs, Materials, Sensors, Actuators, and Navigation Control. Journal of Robotics, 2021(1), 5542920. DOI:10.1155/2021/5542920. https://onlinelibrary.wiley.com/doi/full/10.1155/2021/5542920
2. Bao, H. et al. (2023). A review of underwater vehicle motion stability. Ocean Engineering, 287, 115735. DOI:10.1016/j.oceaneng.2023.115735. https://www.sciencedirect.com/science/article/abs/pii/S0029801823021194
3. Cong, Y. et al. (2021). Underwater robot sensing technology: A survey. Fundamental Research, 1(3), 337-345. DOI:10.1016/j.fmre.2021.03.002. https://www.sciencedirect.com/science/article/pii/S2667325821000522
4. Bae, I. et al. (2022). Survey on the Developments of Unmanned Marine Vehicles: Intelligence and Cooperation. Sensors, 23(10), 4643. DOI:10.3390/s23104643. https://www.mdpi.com/1424-8220/23/10/4643
5. Agarwala, N. (2020). Monitoring the Ocean Environment Using Robotic Systems: Advancements, Trends, and Challenges. Marine Technology Society Journal, 54(5), 42–60. DOI:10.4031/mtsj.54.5.7. https://www.ingentaconnect.com/content/mts/mtsj/2020/00000054/00000005/art00008
6. Terracciano, D.et al. (2020). Marine Robots for Underwater Surveillance. Curr Robot Rep 1, 159–167. DOI:10.1007/s43154-020-00028-z. https://link.springer.com/article/10.1007/s43154-020-00028-z
7. Café, R. et al. (2024). Hybrid ROV/AUV underwater robotic system for safe and robust remote operations. IEEE Xplore. DOI:10.1109/OCEANS51537.2024.10682144. https://ieeexplore.ieee.org/abstract/document/10682144
8. Xu, Y. et al. (2024). A Bioinspired Shape Memory Alloy Based Soft Robotic System for Deep-Sea Exploration. Advanced Intelligent Systems, 6(5), 2300699. DOI:10.1002/aisy.202300699. https://onlinelibrary.wiley.com/doi/full/10.1002/aisy.202300699

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