A ‘dangerous’ and one-of-a-kind project: U.S.S. North Carolina cofferdam ahead of schedule despite obstacles

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WILMINGTON – Unique, challenging and more than a little dangerous – that’s how Neal Andrew, president of Andrew Consulting Engineers, described the cofferdam around the U.S.S. North Carolina. Yet, in spite of its difficulties, the project is reportedly ahead of schedule.

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Corrosion is a serious problem for the Battleship North Carolina. (Photo by Benjamin Schachtman)

The $8 million, state-funded cofferdam, designed by Andrew Consulting Engineers, will create a permanent structure around the battleship, allowing water levels to be raised and lowered. This will permit repair work on the battleship’s hull – described as “paper thin in some areas” by Andrew – which has not seen dry dock maintenance since 1953.

Related: Maintaining the Battleship North Carolina is a labor of love

According to Andrew, it is years of repeated exposure to salty water and air, caused by tidal waves, that have created a ring of corrosion in what he calls the battleship’s “splash zone.”

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The ring of corrosion around the hull, visible here. The 10-foot high band of rust has eaten deep into the ship’s structure. (Photo by Benjamin Schachtman)

Very few projects like this one exist. Aside from the battleship U.S.S. Alabama, in Mobile, Alabama, this is a unique situation, “probably the only one like this I’ll work on or even see in my lifetime,” Andrew said.

The Cape Fear Memorial Bridge, completed in 1969, eight years after the U.S.S. North Carolina was moored at Eagle Island, cannot be raised high enough to permit the battleship to leave, functionally locking it into place.

“This is a very interesting project,” Andrew said. “Ordinarily we’d tow the ship up to dry dock in Norfolk. But since it’s essentially stuck here, we have to bring the dry dock to the ship. And, since this is pretty much the ship’s location for good, the cofferdam we’re building is permanent – which you almost never see. So, it has to have the same functionality as a typical project, but it has to be built to last much, much longer. And it has to be aesthetically pleasing, it’s essentially becoming part of the attraction.”

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50-foot H pilings – named for the ‘H’ shape in cross section – are positioned and spaced carefully. The white hard hats are worn by safety inspectors. (Photo Benjamin Schachtman)

To create the cofferdam, the battleship will be encircled by massive steel beams, 50 to 60 feet in length, that reach through the mud and silt and into the limestone bedrock underneath the ship. Massive steel plates can then be fitted it place, creating what Andrew called “an enormous bathtub” around the battleship. But the geography of the battleship’s location has caused some issues.

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Depth markings on the H pilings, which are driven into the limestone. According to Andrew, to drive the pilings the last few feet can take as many as 48 blows from a massive diesel-powered pile driver to gain a single inch of depth. (Photo by Benjamin Schachtman)

“The limestone can be tricky,” Andrew said. “It’s incredible porous, which is what causes sinkholes in places like Florida. But it means that it’s more complicated to calculate how far we have to drive the steel into the rock.”

The limestone isn’t the only issue, the 30 feet of mud below the waterline is also an issue, both for construction and for workers.

“We have zero visibility, in terms of what’s going on at bottom,” Andrew said. “We have to be very accurate topside, because there’s no option to go down there and see – for example – if the sheets will be flush with the frame, or if the pilings have gone down straight.

“The other issue is that the mud itself is very thin. When we release the pilings, their own weight carries them all the way through 30 feet of silt. But, as a construction worker, if you were to step off a boat or one our temporary work spaces, you’d go right down into the mud.”

In general, the construction site was one of the most challenging – and potentially dangerous – Andrew had worked on.

“A major part of this project is driving the H pilings vertically through the mud into the limestone. These pilings have to be handled carefully, they can weight several tons. Then, once they’re driven into the rock, we use torches to trim the tops. So, you’re down near the water level, sometimes working on a boat, and everything is wet – you’re concerned with balance, and sharp edges of cut metal, and of course the torch. And, when the top of the piling is cut away, it’s hauled into the air by the crane – it’s a two-hundred-pound piece of metal moving through the air.”

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A worker uses a torch to cut through a piling. The top is secured to a crane – the three foot section being removed can weigh several hundred pounds and could be lethal isfit struck a worker. (Photo by Benjamin Schachtman)

Andrew’s company has its own safety inspectors on site, in addition to those who work with the contractors responsible for the construction. Andrew said, while safety inspectors are important, it is the crane operators who handle the most potentially treacherous aspects of the build. They are at the heart of safety concerns.

“A lot falls on their shoulders,” Andrew said. “That’s why they are the highest paid members of the construction crew. They’re not just responsible for a lot of delicate maneuvers, often with tons of steel moving around other workers, but they’re responsible for the actual cranes as well. All the mechanical safety and maintenance. It’s a massive responsibility.”

According to Andrew, the project – which began in the fall of last year – was given 18 months for completion, but now looks like it may be finished in 12.

“We’ve made incredible progress,” he said. “We have two teams, working 10 day shifts, it’s obviously a lot of work – especially when your entire work site floods twice a day.”

Andrew’s firm is also building the SECU Memorial Walkway, funded largely by the State Employees Credit Union Foundation.

“That will be a great way to see the battleship when it’s complete, being able to really size up the boat from pretty much water level. You get a sense of scale,” he said.

Andrew said he was not sure which project would reach completion first.

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Workers on a temporary platform measure the depth of wooden pilings to see if they have been driven deep enough. The wooden pilings mark the future path of the SECU Memorial Walkway. (Photo by Benjamin Schachtman)

Andrew, a native Wilmingtonian from a military family, said, “every project is important, of course. But I’d be lying if I said the stakes weren’t higher here. The Navy stills owns the battleship and they’ve essentially entrusted us with it. And, of course, it’s become an iconic part of Wilmington. The stakes are absolutely higher. This is a job that has to be done right.”

Watch one of the more dangerous jobs on the site and, below, see the plans for the completed cofferdam and SECU Memorial Walkway.

 

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The walls of the cofferdam, in yellow, will be supported by the pilings; the orange gates will allow water in and out. The SECU Memorial Walkway, in red, will be built on wooden pilings driven into to area outside the cofferdam. (Courtesy of Andrew Consulting Engineers)

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