Intake Crib Inspection on the Great Lakes

By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Dana Whitcombe, technical reviewer.

Underwater Inspection | Commercial Diving Contractors Alabama Last October I sat on the gunwale of a 28-foot aluminum workboat, watching a diver named Mike go over the side in a dry suit. The water was 41°F, the wave height was two feet, and the intake screen differential was sitting at 14 inches of mercury. We had a four-hour window before the plant needed the intake fully back in service. That crib feeds 160,000 gallons a minute to the condenser loop. It's a reinforced concrete box, roughly 40 by 60 feet, sitting in 38 feet of water a half-mile offshore. Out of sight, out of mind — until the pressure gauge tells you the screens are plugging or the vacuum is sagging. Then you remember you haven't seen the underwater side for years. The whole job comes down to one question: what's the condition of that structure below the waterline? And the only honest answer is to get eyes on it, either a commercial diver or an ROV. This is how we plan it, run it, and read the results.

Why the Crib Gets Neglected

Most plants don't think about intake cribs until something starts going wrong. The crib is just a concrete box in the lake, drawing water. There's no moving machinery, no alarm beacon, no status light on a control board. But a plugged or broken crib can take a plant offline just as fast as a boiler trip. The main culprit is biofouling. Zebra mussels love the Great Lakes, and they'll build a crust an inch or two thick on any hard surface inside the crib. That cuts the effective open area of the screens, which raises the differential across the structure. Once that exceeds the design limit, you're losing flow, maybe cavitating your circulating water pumps. Ice is the other big one for us. Winter ice can grind against the concrete, spall the surface, and crack the intake roof. You can't see any of that from the shoreline. So we inspect the crib every three to five years. Depends on the plant — some run on a fixed schedule, some go by differential pressure trending. We do both. Call it every four years on average, but we're not married to that.

Before Anyone Gets Wet

Prep work is where you actually do your thinking. First, review the original drawings and any prior inspection reports. You need to know where the trash racks are, what the openings are, and what you expect to see. Then pull the last few months of differential pressure charts. That tells you whether the fouling rate is accelerating. Next, pick your tool. For a first pass, we use a small ROV — a VideoRay Defiant, older model, but it gets the job done. It gives you a quick look at the outside of the crib, the surrounding lakebed, and any obvious damage. But it can't get inside the intake slots well, and it can't feel for organic buildup. For that, you need a human. The dive contractor handles the crew and the gear. We make sure they're commercially certified and that their dive plan matches our requirements. We also take a sediment sample around the crib — you'd be surprised what's in there. One year we found a chunk of old tire. Another year, a fishing net wrapped around a support column. So we bring a steel hook attached to a pole, which sounds crude, but it clears enough to give the diver a clean path.

What You're Looking For Underwater

The diver's job is to methodically cover the entire structure and log everything. We ask him to start at the top, work down one face, then the next. Thickness of zebra mussel layer on the screens is a key measurement. We have him use a stainless scale to measure a couple of spots, and we record the video. If the layer is over an inch, we know we're losing 20 to 30 percent of the open area, give or take. Also look for concrete spalling, cracks wider than 1/8 inch, and exposed rebar where the cover has come off. That's a serious red flag. And you have to look for the stupid stuff. We once had a dock bump that knocked off a section of the concrete ledge. Another time a contractor's anchor left a chain wrapped around one support. The diver carries a knife and a cutting tool, and we log any debris we remove. We also check for settlement around the base — uneven gaps between the crib and the lakebed. That could mean scour from the discharge flow. So we take depth measurements at a set grid pattern along two sides. It's easy to get caught up in the neat engineering parts, but the real skill is knowing what's worth fixing now versus what you can schedule later. Small cracks don't sink a plant. A crack that runs all the way through the roof will.

The Day of the Dive

We time the window for low current and calm water, usually in late spring or early fall. The day starts with a phone call to the plant operator to confirm the intake is still online. You don't shut it down for a normal inspection — you just reduce flow if you need a lower velocity near the diver or ROV. The workboat gets into position, and we toss a transponder and a surface buoy. Then the ROV goes in first, runs a quick lane around the perimeter, maybe twenty minutes. That gives the diver a heads‑up so he doesn't bump into anything unexpected. When the diver goes down, he stays on a surface-supplied air hose, not scuba. That gives him unlimited bottom time and constant communication. He carries a GoPro-style camera on the helmet, plus a handheld camera for close-ups. We talk to him throughout, asking for a second look at anything that doesn't match the drawings. We time each section — typically ten minutes per face — and we log start/stop times for each part of the structure. After he surfaces, we download all the video and stills, mark up the drawing with the findings, and write a two-page memo with our recommendations. That becomes the inspection record. Sometimes we'll go back with a pressure washer or a chemical coat of chlorine for zebra mussels, but only if the inspection says it's worth the money. The inspection itself is cheap. The repair is not.

What the Findings Actually Mean

In the end, you're looking for a trend, not a snapshot. I keep a binder of past reports, and the new inspection gives me a way to see if the cracks are growing or the mussel layer is thickening. That's what decides when we need to do a full outage with a cofferdam or just a cleaning cycle. The funny thing is, the intake crib gets almost no respect until it fails. Then it gets all the respect in the world. If you're asking how to inspect one, the answer is simple: stay out of the water and use a good dive contractor. But the real work is plan, watch, and document. Do that right, and the crib will give you another five years of quiet service.

Frequently Asked Questions

How often should an intake crib inspection be done? For most plants on the Great Lakes, every three to five years. We base it on differential pressure trends and the last inspection's findings. If you're seeing a rapid rise in screen pressure, do it sooner. Can an ROV fully replace a diver for intake crib inspection? No. An ROV is great for a quick perimeter sweep and getting a video from a variety of angles. But a diver can measure buildup, feel for loose concrete, and clear debris. We use both, with the ROV going in first. What are the biggest threats to an intake crib? Zebra mussels and winter ice. Mussels clog screens and reduce flow; ice can scrape and crack the concrete. Also, boat anchors and debris can ding the structure more than you'd expect. Do you have to shut down the plant to inspect the intake crib? Normally no. The crib is designed to operate while we inspect. In some cases we reduce flow a bit to lower the current around the diver. Only if we need major repairs would we take an outage.