Looking back at decades of underwater inspection history—from divers making dive after dive, laden with heavy gear weighing dozens of kilograms, to technicians seated at consoles, operating everything at the click of a mouse; from groping in total darkness, relying solely on their hands, to panoramic 3D sonar imaging, bringing everything into clear view and under full control; from a 45-day maintenance duration to completing all work in just one week—every step in the evolution of underwater inspection technology is a testament to how technological innovation evolves in tandem with these strategic national assets.

The 67.5 kg Diving Gear: A Diver's "Heavy" Memory

In 2008, ZHAO Chunlu joined the Overhaul and Maintenance Factory (OMF) of CYPC. At that time, underwater inspection relied entirely on manual diving.

"Wearing nearly 67.5 kg of gear, climbing down and then back up the steel ladder was very tiring," recalled ZHAO Chunlu, now the Deputy Director of the Ship and Diving Division of OMF.

What exactly do divers do underwater? Mainly three types of tasks: inspection, desilting, and retrieval. For instance, if a gate fails to close properly because branches or debris are lodged in the underwater gate slot, clearing and retrieval work is required. If scour holes or other defects appear in the dam's underwater concrete, their dimensions need to be measured. All these seemingly mundane tasks serve one purpose: ensuring the safe and stable operation of this strategic national asset.

However, manual diving has obvious limitations: Take a 45-day underwater inspection project, for example. Each dive allows only a very short window of effective work, forcing divers to dive multiple times a day, spending long hours in heavy diving suits under immense water pressure. What makes it even harder is that it is pitch black underwater with near-zero visibility, and compounded by the dam's labyrinthine underwater structure, every single dive pushes a diver's experience, courage, and physical endurance to the limit.

As underwater inspection work extended from the Gezhouba and Three Gorges dams to the high dams and large reservoirs at Xiangjiaba, Xiluodu, Baihetan, and Wudongde, operating depths soon exceeded the safe limit of air diving. In air diving, divers breathe ordinary air supplied from the surface, with a safe depth generally not exceeding 60 meters; beyond that, the body is at risk of nitrogen narcosis and other serious conditions. Even with more advanced "mixed-gas diving" (breathing a helium-oxygen mixture), each breath a diver takes can cost as much as 10 yuan, and the safe depth can only be extended to around 120 meters. Manual diving has reached its limit, yet the Xiluodu Dam—the deepest among these upstream reservoirs—has an operating depth of over 250 meters.

At the time, we kept thinking, if only we had a machine that could dive for us," said ZHAO Chunlu. It was this pressing need that drove CYPC's OMF to resolve to make a breakthrough by developing underwater inspection and repair robots. An R&D team was formed, tasked with harnessing the power of technology to address real-world challenges.

 The First Robot: A Single Underwater Trial Run That Convinced Everyone

In 2017, at the early stage of the project, there were no robots in China specifically designed for underwater inspection and repair of hydropower station dams, and there was no successful precedent for independent development to draw upon. The R&D team started from scratch, conducting surveys, feasibility studies, and design reviews, feeling their way forward step by step, like crossing a river by feeling the stones. To achieve a breakthrough from "0 to 1" for the underwater robot as quickly as possible, the team closely tracked every detail of R&D, design, assembly, and testing in the workshop. The toughest challenge was the robot's deployment and retrieval method—the team had to ensure the hundred-kilogram robot could be safely launched into the water, while also solving the puzzle of getting it to detach itself automatically from the lifting hook underwater. After countless simulations, the team ultimately settled on a "buoyancy-triggered release latch" solution, which allowed the robot to automatically unhook itself the moment it entered the water, without the need for a diver.

In 2018, the first dedicated underwater inspection and repair robot was successfully developed. With an operating depth of up to 300 meters, its main body is equipped with four replaceable mission modules. When fitted with a manipulator arm, it can cut steel cables and retrieve heavy objects. Swapping to a cleaning and grinding module enables rotary surface treatment. In confined spaces, it can deploy a small-diameter sub-robot to probe inside. When necessary, its crawler track module allows it to climb inclined dam surfaces. Much like a Transformer, the robot changes its weapons and armor to suit the demands of each mission, rapidly reconfiguring itself into the optimal working form.

The acceptance test for this robot was conducted in the plunge pool of the Xiangjiaba Hydropower Plant. The robot went down for a spin and immediately discovered a large scour pit, precisely mapping its detailed contours in just one afternoon. If the same task were given to a human diver, even with a week's effort it would be nearly impossible to piece together such an accurate picture.

"The contrast was striking," said ZHAO Chunlu. "Before, whenever divers were in the water, the generating units had to be shut down. Now, with the robot work under the water, the units can keep generating electricity as normal. The work period is drastically shorter, and both costs and safety risks have come down." Having seen firsthand the difference technology could make, the team grew even more determined to follow the path of robotic maintenance.

Advancing into Deeper Waters: From "Getting It Done" to "Working Smart"

The first robot's umbilical cable—the cable linking it to the surface control console, carrying both power and data—was only 450 meters long. Faced with the drainage tunnels and culverts stretching several kilometers deep within high dams and huge reservoirs, this length was far from sufficient.

Needs drive iteration. In 2019, a second robot specially designed for long-distance underwater travel was unveiled. With its umbilical cable reaching 1,800 meters, it could maneuver with ease through deep, lengthy tunnels, filling a gap in China's underwater inspection capabilities for long-distance tunnels.

Long-range capability means little without clear vision. The first two generations of robots used 2D sonar, which produced images like a flashlight beam—illuminating only a single flat slice—and the pictures were far from intuitive. In 2022, the third robot was equipped with 3D sonar technology. Like turning on a light bulb in the dark underwater world, it could instantly reveal the entire space and generate real-time, three-dimensional images. Sitting at the control console, the operator feels as though they have "X-ray vision", able to clearly observe the robot's every move underwater and the surrounding environment in vivid detail.

The fourth unit was an even more "specialized" robot dedicated to clearing debris from gate slots. ZHAO Chunlu still vividly recalls the days of manual clearing: "Inspection, obstacle removal, and clearing silt—it was all done by hand: feeling, digging, and scooping. The water would get muddy as soon as it was stirred, and the silt just cleared would drift back in no time." Today, the robot comes equipped with a dozer blade, a high-pressure water jet, a siphon, and an electromagnet, and can clean a gate slot thoroughly in just 30 minutes.

Dual-Robot Collaboration: A Leap from "Inspection" to "Operation"

By this point, underwater inspection, debris removal, and retrieval had all been taken over by robots. However, the most critical and difficult task—underwater concrete defect repair—remained a tough nut to crack. What is underwater concrete repair? You can imagine it as performing "surgery" on the concrete surface of a dam underwater. Divers first need to clean the damaged area, then drill holes, install rebar, set up formwork, and finally pour special concrete or grout, which must set to restore strength. These tasks are simple on land, but in the dark, high-pressure underwater environment, every cut and every drill hole is extremely difficult. Could robots handle it?

By the end of 2025, the fifth unit provided the answer. This was no longer a single robot, but a pair: a scout robot plus a main robot. The agile scout deployed first, submerging to precisely locate the defect requiring repair with high-precision detection equipment, and sharing all data in real time with the main robot waiting behind. Then, the main robot—weighing 7 tons and standing about 4 meters tall—made its official debut. This robot integrated all repair functions: it carried three types of repair materials that could be automatically mixed underwater, and it was capable of autonomous drilling, installing rebar, erecting formwork, sealing and grouting cracks, and even injecting flexible waterproof materials into structural joints. From detecting the problem to completing the "surgery," nearly all steps were performed autonomously.

"Previously, underwater repair costs were high; now this robotic system improves efficiency and saves costs," said XIAO Rong, Director of OMF. DING Rongjun, an academician of the Chinese Academy of Engineering, commented: "This has pioneered a new model for underwater robotic inspection and repair of dams."

Leveraging the development, application, and iterative upgrades of underwater inspection and repair robots, CYPC can currently perform underwater inspection and repair operations at depths of 300 meters, as well as operations in underwater tunnels reaching nearly 2,000 meters in length. In 2026, under the National Key Research and Development Program, a new generation of intelligent inspection robots achieved millimeter-level measurement of dam surface defects.

Looking ahead, CYPC will continue to adhere to an approach that combines a problem-oriented focus with strategic guidance, advancing the digital and intelligent transformation of maintenance for cascade hydropower plants along the river, and providing robust support for the safe and stable operation of the world's largest clean energy corridor.