Not since Charlton Heston parted the Red Sea in “The Ten Commandments” has the dewatering of a large body of water captured the imagination of so many. Today, the star of this epic tale is Andrew Consulting Engineers (ACE), a structural engineering firm headquartered in Wilmington, N.C. The state of North Carolina hired ACE in 2014 to design a cofferdam and walkway to surround the USS North Carolina, a WWII battleship commissioned in 1941 and moored since 1961 in the Cape Fear River across from downtown Wilmington.
The battleship cofferdam will create a dry work environment to enable workers to repair and restore the ship’s corroded hull, and will also enable the state’s No. 1 tourist attraction to remain open while construction and repairs are underway. Since coming to her berth in 1961, more than 50 years of wind and water at the tide line have left the hull paper-thin in places. Her last major dry docking and repair work was done in November 1953.
The cofferdam, which will cost $8 million to construct, will be a permanent fixture around the battleship to allow for future repairs and maintenance as needed. The cost to construct the cofferdam and restore the hull is being funded by the state. The cost to construct the State Employees Credit Union (SECU) Memorial Walkway, estimated at $4 million, is being funded by a capital campaign managed by the USS North Carolina Battleship Commission. Entitled “Generations Campaign,” the fundraising effort is seeking the support of individuals, corporations, foundations, and organizations statewide and nationally. SECU Foundation donated $3 million for the naming rights to the walkway.
The SECU Memorial Walkway, which will be constructed simultaneously to the cofferdam, is a half-mile-long, 10-foot-wide timber walkway that is above the water line but below the main deck of the battleship. The walkway will completely surround the ship and enable visitors to see the entire hull for the first time (see Figure 1).
“This project is a once-in-a-lifetime opportunity for a structural engineer,” said Neal Andrew, president of ACE. “We’ve done a number of cofferdams, but the magnitude of this job is extraordinary. A cofferdam is typically a temporary structure used for underwater construction, but this cofferdam will be permanent and must not only be built to last, but be aesthetically pleasing since the battleship is a beloved Veterans Memorial and a historical icon for the City of Wilmington.”
The only other permanent cofferdam of its kind in place in the United States is the one surrounding the USS Alabama, a 680-foot-long battleship moored since 1964 in Mobile Bay.
The USS North Carolina’s cofferdam and SECU Memorial Walkway will be constructed over 18 months by Orion Marine Group of Norfolk, Va. The cofferdam will be comprised of a series of 50-foot-tall structural steel sheet pilings that form the wall and are held in place by a series of 55-foot to 60-foot-tall steel “H” pilings, half of which are placed vertically against the sheet to hold it in place and the other half placed at an angle to the sheet or “battered” (see Figure 2). Both the sheet pilings and the H pilings will be driven 30 feet to 40 feet below the mudline, with just 10 feet showing above the water line.
“The H pilings need to be 55 feet to 65 feet long due to the Jello-like consistency of the mud surrounding the ship,” Andrew said. “They have to extend through the mud and down to bedrock in order to provide the structural support needed. The same applies to the cofferdam sheet piles and steel piles. In essence, we’re creating a six-story building with about four to five stories of it underground and only one story visible above the waterline.”
The cofferdam’s purpose of dewatering the work area is made possible by four weir gates built into the structure. Typically, the weir gates will remain open unless repairs are underway and a dry work area is required.
“I’ve never seen a cofferdam around a ship and I think that it will be a marvelous tourist attraction both during and after construction,” said Wilbur Jones, member and former chairman of the USS North Carolina Battleship Commission. “This entire project is a terrific public-private partnership managed by a world-class staff under the leadership of Captain Terry Bragg and executed by local Wilmington firms who have all done an extraordinary job.”
In addition to ACE, the project team includes BMH Architects and McFadyen Engineers, both of Wilmington, and the Wilmington office of S&ME. BMH Architects is the lead design firm on the project and McFadyen is providing electrical engineering services for the walkway lighting. S&ME has provided geotechnical engineering and environmental testing services to assist in the permitting process and will continue to assist during installation of the cofferdam and walkway pilings.
“The cofferdam is an incredible feat of engineering, but the SECU Memorial Walkway will generate at least as much, if not more, excitement among the ship’s visitors,” Andrew said. “Until now, visitors have only been able to view the port side of the hull. When the walkway is complete, they will be able to walk around the entire ship. This will give an entirely new perspective of the ship, and will also let visitors enjoy the beauty of Eagles Island and the marshland habitat.”
The SECU Memorial Walkway will include a large, 50-foot by 35-foot event space called the quarterdeck, and five bump-out areas to honor each of the five branches of the military — Army, Navy, Marine Corps, Air Force, and Coast Guard. The Merchant Marine will be honored by flying its flag on the quarterdeck. Signage and displays will be placed at various points along the SECU Memorial Walkway to educate visitors about the battleship and the surrounding natural areas. The public will be able to access the SECU Memorial Walkway free of charge and will not be required to purchase a battleship tour pass.
“Andrew Consulting Engineers has done a fantastic job bringing this dream of ours to fruition,” said Captain Terry Bragg, executive director of the Battleship North Carolina. “We are very fortunate that a local firm has the experience and capabilities for this unique project, and that Neal Andrew has such a personal connection to the battleship and our mission to honor our military men and women.”
Andrew is a native of Wilmington and grew up visiting the battleship on numerous occasions with classmates and family members. “I come from a military family and I know firsthand how important the battleship is to our military, our community, and to all of North Carolina,” Andrew said. “My father is a retired Navy officer and my brother is a Marine Corps colonel and I have personally witnessed some of the sacrifices our veterans have made for our country. The battleship does an excellent job of honoring our veterans and I’m honored that our firm can contribute in some small way to its continued success. Many engineers work an entire career without having the privilege to work on a project of this significance. All of us at ACE are grateful and thrilled that construction will soon be underway.”
Information provided by Andrew Consulting Engineers (www.andrewengineers.com). Established in 2004, ACE provides structural, marine, and forensic engineering services.