Bridge’s design fends off severe damage from tanker

The recent collision between a 685-foot oil tanker and a bridge in Portland, Maine, illustrates the importance of bridge design in minimizing damage and injuries.

The fender system of the Casco Bay Bridge cost about $7 million, or about 5 percent of the total cost of the bridge. The system sustained about $1 million in damages when struck by a tanker, but successfully protected the bridge itself.

At 1645 on May 6, Hawk, a tanker transporting 11.3 million gallons of No. 6 fuel oil from a refinery in the U.S. Virgin Islands, struck the Casco Bay Bridge. The 31,709-grt tanker hit the bridge on its port side between the No. 1 cargo tank and the forepeak. But a fender system protecting the bridge pier absorbed the impact. There were no injuries to the crew, no oil was spilled, and no damage was done to the Greek vessel.

Local officials were pleased at how well the fender system protected the 4,750-foot-long bridge and the harbor.

“It saved our bridge and all the headaches that go along with it if there was an oil spill,” said John Buxton, assistant bridge maintenance engineer for the Maine Department of Transportation.

The Casco Bay Bridge opened in 1997. Out of a total cost of $130 million for the bridge, about $7 million was spent on state-of-the-art fenders around all piers next to the navigation channel.

The Hawk incident was in stark contrast to a collision that occurred on Sept. 27, 1996, when a 560-foot oil tanker, Julie N, struck the old bridge in Portland, which was protected by timber fenders. The collision resulted in a 33-foot-long tear in the vessel’s hull and 168,000 gallons of oil spilled into Portland Harbor. The accident caused about $660,000 worth of damage to Julie N, and the resulting oil spill cost $46 million to clean up, according to the final report from the National Transportation Safety Board. The cause of that collision was the pilot’s mistaken order of port rudder instead of starboard rudder.

Capt. Jeffrey W. Monroe, director of Ports and Transportation for the City of Portland, praised the bridge’s fenders and the engineers who designed it. “Sometimes we have been able to think ahead and recognize that sometimes you do have human and mechanical failures, and those approaches (to the bridge) are able to make up for those failures,” he said.

The U.S. Coast Guard Marine Safety Office in Portland is still investigating the cause of Hawk’s collision.

As the tanker approached the bridge, the vessel was traveling at about 3 knots and wind speed was about 25 knots from the southwest, according to Lt. Mike McCarthy, Coast Guard spokesman.

Investigators are checking whether wind speed or the current was a factor in the accident. “Investigators are also examining human factors, specifically communications and maneuvering commands in the pilothouse of the tank vessel between the ship’s crew and the docking pilots,” the Coast Guard said in a release.

As the collision occurred, it appears that the tanker’s captain took control of Hawk from the pilot. As the tanker approached the bridge, the captain gave a command to drop the port anchor in an effort to stop the collision, according to Lt. McCarthy.

Although the tanker’s captain, the second mate and helmsman all speak Greek, they also understand English, so language was not a factor in the crash, McCarthy said.

At the time of the crash, the three officers were on the bridge, and the pilot and assistant pilot were on the bridge wing.

The tanker is operated by Pleiades Shipping Agents in Athens, Greece.

All three officers and the two pilots tested negative for drugs and alcohol, according to McCarthy.

When the state built the Casco Bay Bridge, improvements were made to help prevent accidents such as the Julie N incident. The channel under the new bridge is 197 feet wide, double the channel width under the old bridge.

To protect the bridge structure, four 60-foot-diameter steel pillars filled with gravel are located both upstream and downstream of each bascule pier.

Each bridge pier next to the navigation channel is also shielded by clusters of 100-foot-long, 36-inch-diameter steel pipes, filled with sand and sunk into the harbor bottom. Ten rows of horizontal steel I-beams are attached to the pipe clusters. These I-beams are coated with a thick, slippery plastic, so that vessels slide off better if the fender is struck.

Behind this system, attached directly to the concrete piers, are rubber bumpers. The system is designed to absorb the energy of a 50,000-dwt vessel traveling at 5 knots and striking the fenders at a 15° angle.

“This is pretty much a top-of-the-line system,” said Leon Huang, a vice president of Modjeski and Masters, the Harrisburg, Pa., engineering firm that designed the bridge.

   The Maine DOT has already contracted out the work to repair the fender, which will cost about $1 million, according to Buxton, the state bridge engineer. The work is expected to begin in August and be completed by mid-November.

By Professional Mariner Staff