The National Transportation Safety Board (NTSB) has issued an investigation report regarding an incorrect estimate of a crane’s boom height led to contact with the Houma Twin Span Bridge last year in Houma, Louisiana.
On March 6, 2022, about 0038 local time, the towing vessel Robert Cenac was transiting the Gulf Intracoastal Waterway, in Houma, Louisiana, pushing ahead the crane barge Mr. Dawg and another deck barge.1While passing the Houma Twin Span Bridges, the crane aboard Mr. Dawg contacted the eastbound span of the bridge. Eastbound automobile bridge traffic was reduced from two lanes to one for 10 days. No pollution or injuries were reported. Damage to the bridge was estimated at $1.5 to $2.0 million.
Analysis
Unbeknownst to Sealevel staff, who chartered the tow, their last crew to use the crane aboard the Mr. Dawg did not lower the boom to an angle typically used for transport—with the boom lowered below the barge’s raised spud tops. Both the towboat operator (Al Cenac Towing) shoreside staff and their towboat crew aboard the Robert Cenac were concerned about the height of the crane, and the captain observed that the head of the crane boom was higher than the crane barge spuds, contrary to what Sealevel had initially communicated to Al Cenac Towing. 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.
Without a verified crane boom height, the captain of the Robert Cenac estimated the total air draft of the tow, assuming the spuds to be a standard 50 feet high and then estimating the portion of the crane boom above the spuds at 10 feet, arriving at 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.
Conclusions
The National Transportation Safety Board determines that the probable cause of contact of the Robert Cenac tow with the Houma Twin Span Bridge was the tow captain’s incorrect estimate of the crane boom height and his decision to depart before getting a confirmed height from the chartering company. Contributing to the incident was the crane barge owner not providing the accurate air draft information to the tow company.
Lessons Learned
Navigation Assessments
When operating in higher risk conditions, operators should ensure that they have the most accurate and objective data before getting underway. Bridges pose a risk to vessels and tows with high air drafts. Owners and operators should develop voyage plans that assess operational risks and hazards, to include air draft relative to bridge vertical clearances along the intended route.
