Andrew Consulting Engineers was hired in 2014 by the state of North Carolina to design a cofferdam and SECU Memorial 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.
- A cofferdam is a structure built within a body of water to allow the enclosed area to be de-watered to create a dry work environment.
- Cofferdams are typically temporary construction structures, but the Battleship cofferdam will be permanent to allow for future repairs and maintenance.
- The Battleship cofferdam will enable workers to repair and restore the ship’s corroded hull, which is paper-thin in places.
- The Battleship’s last major dry docking and repair work was done in November 1953.
- The cofferdam will be constructed from a series of 50’ tall structural steel sheet pilings that form the wall and are held in place by a series of 55’ to 60’ 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.”
- Both the sheet pilings and the H pilings will be driven 30’ to 40’ below the mudline, with just 10’ showing above the water line.
- Four weir gates will be installed in the cofferdam. The water tight gates can be opened and closed to pump water out and allow water in.
- The dam will be painted to match the color of the hull.
- The cofferdam will cost $8 million to construct and will be paid by the state of North Carolina.
- The cofferdam will be constructed over 18 months by Orion Marine Group of Norfolk, VA.
- Project team:
- Andrew Consulting Engineers – design and structural engineering services
- BMH Architects – design services
- McFadyen Engineers – electrical engineering for SECU Memorial Walkway lighting
- S&ME -- geotechnical engineering and environmental testing services for permitting process and installation of the cofferdam and SECU Memorial Walkway pilings.
- The only other permanent cofferdam of its kind in place in the United States is the one surrounding the USS Alabama, a 680’ long battleship moored since 1964 in Mobile Bay.
By the Numbers:
- 50’ tall, with 10’ visible above the waterlinE
- 1,809 feet long around the perimeter, or just over 1/3 of a mile
- 1,206 steel sheet piles, each measuring 25” wide and 15” deep in a corrugated “z” shape
- 229 vertical steel H-piles, each 55 feet long
- 229 battered steel H-piles, each 60 feet long
- 4 weir gates measuring 10’6” tall x 6’ wide to allow water in and out
- 4.6 million pounds (or 2,300 tons) of steel
- $11 million to construct
- 18 months to build