It will cost an estimated $2.5 million to repair the Sarah Mildred Long Bridge, which was damaged Monday by a 470-foot tanker adrift in the Piscataqua River.

Cianbro, a construction company based in Pittsfield, Maine, has been awarded the contract to repair the bridge, according to the N.H. Department of Transportation.

New Hampshire officials say the 73-year-old bridge will be closed for two to four weeks while repairs are made.

“We are confident the Long bridge can be repaired and reopened,” said Bill Boynton, spokesman for the N.H. Department of Transportation.

The attorney generals’ offices in Maine and New Hampshire are working together to seek reimbursement after they determine who is at fault for the accident.

Steffen Thate, a spokesman for TB Marine Shipmanagement in Hamburg, Germany, the company responsible for the ship’s management, said the company is working with U.S. authorities to determine what caused the tanker to break away from the N.H. State Pier in Portsmouth.

Boynton said the engineers have obtained original detailed fabrication drawings, dating back to the late 1930s, from Harrington & Cortelyou/BMcD Inc. of Kansas City, Mo., which was involved in the bridge’s construction. He said the drawings will expedite steel fabrication and repairs.

Ted Talbot, spokesman for the Maine Department of Transportation, said the $2.5 million estimate is the cost of repairs if engineers restore the bridge to its pre-impact state. Repairs to make the bridge safe for vehicle traffic may require less work.

The bridge carries about 14,000 vehicles a day on the Route 1 Bypass between Kittery and Portsmouth, N.H., and is owned by both Maine and New Hampshire.

It was damaged when the tanker Harbour Feature broke away from its moorings in Portsmouth, where it was tied up to refuel before sailing to a destination in Europe. The swift incoming tide of the Piscataqua River carried the ship to the bridge and pinned it there.

Two tugboats moved the ship back to the pier about four hours later, during slack tide.

The collision damaged four steel beams on the bridge. In addition, chunks of granite broke off the base of the supports and landed on the ship. A crane on the ship cracked the bridge’s sidewalk, which was already closed.

The accident left a crack in the ship about 18 inches long, about 15 feet below the water surface, said the Coast Guard, which is investigating the accident. The crack is allowing water to leak into a ballast tank used to stabilize the vessel when it is empty. The ship has not spilled any fuel or cargo.

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