Misjudged crane height caused up to $2m-worth of damage to bridge

The US National Transportation Safety Board has said that an incorrect estimate of a crane’s boom height on a barge led to it striking the Houma Twin Span Bridge, Louisiana, in March 2022.

On March 6th 2022 towing vessel Robert Cenac (MMSI 367617690) was transiting the Gulf Intracoastal Waterway, pushing the crane barge Mr Dawg and another deck barge.

At 23:30 the master let his dispatcher know that he was comfortable with the tow, and he got under way towards Clovelly. The mate took over the watch at 23:40. He told investigators that he could not see the head of the boom in the dark.

At 00:38 the head of the crane boom struck a steel stringer on the underside of the Twin Span Bridge at about four knots. The stringer was bent inward by the impact, and the damage was significant enough to shut half the bridge until March 16th 2022.

No injuries or pollution were reported., but eastbound vehicle traffic, estimated at nearly 30,000 vehicles a day, was reduced from two lanes to one for 10 days. The contact caused damage to the bridge that would cost between $1.5m and $2m to repair.

The day before the contact, Sealevel Construction contracted with Al Cenac Towing to charter the Robert Cenac to tow the crane barge Mr Dawg and a deck barge from Houma to Clovelly, Louisiana.

Despite being asked at least twice about the height of the crane by the towboat operator before departure, Sealevel did not provide Al Cenac Towing with a verified crane height.

Without a verified crane boom height, the captain of the Robert Cenac, who had 23 years’ experience, estimated the total air draft of the tow, assuming the barge spuds to be 50 feet high (the local standard) and then estimating the portion of the crane boom above the spuds at 10 feet, for a total air draft of “roughly 60 feet.” However, the raised spuds on the Mr. Dawg had an air draft of about 56 feet, not 50 feet as the captain assumed. The captain was also judging the height in the dark, which most likely affected his ability to accurately determine the boom’s distance above the spuds, the NTSB said.

The previous construction crew on the barge had left the crane at an elevated angle, sticking up above the spuds.

The NTSB therefore determined the probable cause of the contact was the tow captain’s incorrect estimate of the crane boom height and his decision to depart before getting a confirmed height from the crane barge owner. Contributing to the incident was the crane barge owner not providing the accurate air draft information to the tow company.

“Tow operators are required to know the air draft of their vessel and tows and should not make assumptions,” the report said. “As the NTSB has recommended before, tow operators should have a detailed voyage plan with specific information concerning/about all known risks, including calculated overhead clearance limitations for tows. In this case, the captain should have waited to get underway until the exact air draft of the tow was established.”

The NTSB noted that it had recommended previously that the Occupational Safety and Health Administration revise its regulations to address “the placement and securing of crane booms for transit to and from marine construction and other sites.”

The OSHA declined so to do.