Faulty assumption about crane height cited in Louisiana bridge strike

Damage to the span after the crane strike.
The Houma Twin Span Bridges cross the Gulf Intracoastal Waterway in Houma, La
The Houma Twin Span Bridges cross the Gulf Intracoastal Waterway in Houma, La

With the clock approaching midnight, the captain of the towboat Robert Cenac contacted the home office with a question: How tall was the crane boom on the barge he was hired to push?

An Al Cenac Towing official sent text messages to a representative from the barge’s owner seeking that information. Minutes later, without getting a firm response of the crane boom’s height, Robert Cenac got underway from Houma, La., with the crane barge Mr. Dawg and another deck barge in tow.
They didn’t make it very far. 

At 0038 on March 6, 2022, about an hour after leaving the dock, the crane boom on Mr. Dawg hit the steel support beam for the eastbound side of the Houma Twin Span Bridge. The impact caused at least $1.5 million in damage and limited traffic on the bridge for more than a week. 

National Transportation Safety Board (NTSB) investigators cited the captain’s decision to get underway without knowing the crane boom’s height as the primary cause of the bridge strike. The failure of barge owner Sealevel Construction to provide accurate air draft information about the crane height was a contributing factor. 

“Despite being asked at least twice about the height of the crane by the towboat operator before departure, Sealevel staff did not provide Al Cenac Towing with a verified crane height,” the NTSB said in its report. 

Al Cenac Towing of Houma did not respond to an inquiry about the NTSB findings. Sealevel Construction could not be reached for comment. 

The 150-foot crane barge Mr. Dawg was built in 2020 and owned by Sealevel Construction of Thibodeaux, La. The vessel had two retractable spuds and a crawler crane secured to its deck with a 180-foot-long boom that was positioned about 75 feet above the water. 

Sealevel Construction hired Al Cenac Towing at about 2000 on March 5 to bring Mr. Dawg and the deck barge HMT 26 to an oil storage and pipeline facility near Galliano, La. The towing agreement, according to the NTSB, was made verbally over the phone.

Sealevel initially said that the crane boom had been lowered past the height of the barge spuds, which extended about 56 feet off the barge deck while they were in the “up” position. However, that was not accurate: The boom still extended above the spuds, which the Robert Cenac captain recognized while building the tow. 

At about 2330 on March 5, the Al Cenac Towing official contacted his counterpart at Sealevel Construction to verify the crane boom height. According to the NTSB report, the two texted back and forth several times but the actual height was never shared. Soon afterward, the towboat captain decided to get underway despite not knowing how tall the crane boom was.

The mate aboard Robert Cenac began his watch around midnight and the captain went to sleep. Some 40 minutes later, at 0038, the crane boom struck the bridge at about 4 mph. The impact damaged a steel girder supporting the span. It also caused HMT 26 to break free. 

“The mate stated that from his vantage point in the wheelhouse, he saw the red and green navigation lights on the bridge, but he couldn’t see how high the boom was,” the NTSB said in its report. 

The captain returned to the wheelhouse soon after the bridge strike. He reported the incident the U.S. Coast Guard, the Houma police department and the towing company’s home office. The four people on the vessel were not hurt and there was no pollution. 

The Houma Twin Span Bridge was built in 1996. It carries about 30,000 vehicles a day along Main Street between Houma and East Houma. Its vertical clearance was 72.8 feet above the Gulf Intracoastal Waterway. Robert Cenac’s captain knew the clearance for this and other bridges along the route toward Galliano. 

Mr. Dawg had 60-foot-tall spuds that allowed it to work in the Lower Mississippi River. At their fully retracted height, the spuds stood 56 feet out of the water. The captain aboard Robert Cenac, however, believed the spuds were only 50 feet tall, which was standard for spud barges. 

Based on that assumption, he believed the crane still had ample room to clear the Houma bridge even though it stood higher than the spuds.

“The captain’s ability to accurately judge the height of the crane boom above the spuds was also likely affected by the dark nighttime conditions impacting his ability to make an accurate estimate of height from a distance given the 180-foot length of the crane boom,” the NTSB said in its report. “These factors resulted in an inaccurate and subjective assessment of the crane barge’s air draft, which was the highest in the tow.”

“As a result of the incorrect estimate,” the NTSB report continued, “the captain felt comfortable getting underway, and the crane contacted the bridge, damaging the steel girder and impacting vehicular traffic for 10 days.”

The bridge strike also highlighted shortcomings in the towboat captain’s voyage planning. For instance, captains are required to know the air draft of their vessels before getting underway. According to the NTSB, that information should be part of any voyage plan that also would include bridge clearances along the route and other potential risks along the way. 

“In this case,” the NTSB said, “the captain should have waited to get underway until the exact air draft of the tow was established.”