What an ROV Cannot Do
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama The intake gate at the Conowingo Dam came off its track at 8:40 AM. We had a VideoRay Pro 4 on site, and the owner wanted a look. The camera went down and the screen went brown—just silt and turbulence. The ROV couldn't hold station in the 3.2-knot current, and after twenty minutes we pulled it out with a bent tether clamp. I suited up and went down with a gloved hand and a steel brush. The gate's bottom roller was jammed on a chunk of concrete that had dropped out of the sill. I found it by touch, not sight. That's the first thing you need to know about ROVs. They're good for a lot of things, but an underwater inspection is more than looking. It's feeling, listening, smelling, and judging. This is a how-to for anyone planning an underwater inspection, so you don't waste a day and ten thousand bucks learning what I learned in a week. I've got 28 years as a dive supervisor, most of it on dams, bridges, water storage tanks, and power plant intakes. I've done more than 2,000 inspections. I've also flown ROVs—small ones like the VideoRay and Deep Trekker, and a couple of work-class ones. I'm not anti-ROV. They keep divers out of danger. But they have real limitations. Here's what they can't do.
Turbid Water Is the Killer
The camera is the ROV's best and worst tool. In clear water, you can see a fine crack from two feet away. In turbid or muddy water, the camera is just a blind eye in a muddy river. Light from the ROV's lamps reflects off suspended silt, creating backscatter, and the image goes white or brown. Sonar helps you see objects, but it's a low-resolution blob. You can't tell a 2-millimeter crack from a hairline. I've been in a potable water tank where visibility was three inches. No ROV could do a thing. A diver can wipe the surface with a gloved hand, use a flashlight as a surgical light, and inspect inch by inch. In planning an inspection, check the water clarity first. If it's above a certain turbidity, save the ROV for open-water work and put a diver in. Give or take, a Secchi disk reading of 3 feet or less is a diver job. Some ROVs have a downward-looking sonar that can map a shape, but for a structural inspection, you need to see cracks, corrosion pitting, and weld details. I've used a Teledyne BlueView sonar to map a bridge footing, and it's great for finding debris. But it won't tell you if the rebar is exposed or if the concrete has spalled. That requires a camera or a diver.
Manipulators Aren't Hands
ROV manipulators have come a long way. The Schilling Orion 7F on a work-class ROV can lift 250 pounds and has a sophisticated joint system. But it's still a claw on a stick. You don't have the tactile feedback to know how much force you're applying. You can't torque a bolt to specification. You can't file a burr. You can't do a magnetic particle inspection because you can't hold the probe steady. The typical inspection task is to clean a surface and look at the steel. An ROV manipulator can't scrape a patch of marine growth without gouging the substrate, unless you use a high-pressure water lance, which is a blunt tool. I've seen ROVs scratch a penstock surface so badly that we had to do a repair pass. An ROV can deploy a thickness gauge, but the operator has to position it exactly, and it's tricky. A diver can press an ultrasonic transducer against a steel member, sweep the area, and note variations in a way that an ROV can't yet. For visual inspection, you can sometimes get away with an ROV, but for close inspection, you need a diver's hand.
Current, Tether, and Tempo
ROVs have power limits. In a current over 2 knots, even a work-class ROV with thrusters will burn fuel or battery just to hold position. The tether catches on everything. I once lost two hours at Pickwick Landing Dam because the ROV tether wrapped around a trash rack. A diver would have just swum under it. And ROV deployment is not fast. You need a launch system, a control van, a generator, a pilot, and a navigator. We typically spend four hours setting up a fly-away ROV for a bridge inspection. A diver crew can be in the water in one hour for a simple job. When there's an emergency, speed matters. That's not to say ROVs aren't worth it. They are, for deep water, for long durations, for work in hazardous environments like nuclear plants or raw sewage. But if your job is a 30-foot inspection of a dam gate, you can probably have a diver do it quicker, cheaper, and with more information. The current rating depends on the plant, but 2 knots is a good rule of thumb.
The Human Hand and the Backup Plan
The biggest limitation of an ROV is that it can't think on the fly. A diver sees a crack, touches it, and knows if it's a surface crack or a through-thickness. He taps it with a hammer and hears a hollow sound, indicating delamination. He can feel for sharp changes in profile. He can smell hydraulic oil in a sump. He can hear a hiss in an air pocket. An ROV gives you a video feed and a bunch of telemetry, but not these observations. I've done inspections where the ROV footage looked perfectly fine, but the diver found a 1/8-inch offset that the camera missed because it was in the shadow of a weld. I had a mentor who said, "A diver who doesn't feel is a photographer." He meant that you don't inspect with your eyes alone. The hand is an instrument. In a lot of industrial tanks, the water is zero-visibility, and the diver works entirely by touch. You get so good at it that you can tell a sharp edge from a rounded one, a fine crack from a surface scratch, a loose bolt from a seized one. An ROV can't do any of that. So the best plan for a tricky inspection is to use both: an ROV for reconnaissance and a diver for the detailed look. The ROV goes in first, maps the layout, and then you put the diver in with a checklist. That's worked for me for decades.
Frequently Asked Questions
What are the biggest limitations of an ROV for underwater inspection? Visibility in turbid water, lack of tactile feedback, limited manipulator dexterity, and poor performance in currents or confined spaces. ROVs are best for clear-water reconnaissance, not for close-up structural assessment. Can an ROV work in cloudy or muddy water? Not effectively. Silt and suspended material scatter light, so the camera sees backscatter. Sonar can detect shapes but not fine details like cracks or pitting. In water with Secchi depth less than 3 feet, you're better off with a diver. When should I choose a diver over an ROV? When you need to touch, clean, or physically test the surface, when the water is turbid, when the current is high, when the structure is complex with confined spaces, or when you need the job done fast. Divers can go in quicker and give more information. What can a diver detect that an ROV cannot? A diver can feel for rough edges, loose coating, or a crack's depth. He can tap concrete to hear delamination, smell hydrocarbons, and hear gas leaks. He can also make on-the-spot judgments about whether a defect is active.