A towing vessel’s captain mistaken estimate of a crane’s height caused $1.5 to $2 million in damage and offers a safety lesson for workers in any industry.
The National Transportation Safety Board (NTSB) found that the probable cause of the incident was the captain’s incorrect estimate of the crane boom height and his decision to leave port without getting confirmation of the height from the crane barge’s owner.
Captain sets out without knowing exact height of crane
On March 6, 2022, the towing vessel Robert Cenac was transiting the Gulf Intracoastal Waterway in Houma, Louisiana, preparing to go under the Houma Twin Span Bridges. This twin-span bridge saw more than 30,000 vehicles cross it daily, according to the U.S. Department of Transportation.
Robert Cenac, which is owned by Al Cenac Towing, was pushing a crane barge known as the Mr. Dawg, owned by Sealevel Construction. A crawler crane with a 300-ton lift rating was secured on Mr. Dawg’s aft deck with its 180-foot-long boom facing forward.
The captain of the Robert Cenac estimated the crane boom’s lowered, travel height was about 10 feet higher than the 50-feet-long through-deck pilings on the starboard side of the barge. Fifty feet was the standard length of these pilings, in the captain’s 23 years of experience.
This estimate led him to believe the crane’s boom head was about 60 feet above the water. He also knew that all of the bridges on the passage had 72 feet of vertical clearance.
However, the captain wanted to be sure, so he contacted Sealevel via text message to get the exact height of the crane boom. When he, and his employer, didn’t get an answer, the captain decided to set out, using his estimate on the crane boom’s height.
First mate can’t see boom’s height in the darkness
The Robert Cenac was near the Houma Twin Span Bridges at about 11:40 p.m. with the first mate in the wheelhouse for his watch. The captain was in bed at this time. In the darkness, the first mate could see the navigation lights on the bridge, but he couldn’t see how high the crane boom was.
At 12:38 p.m., the head of the crane boom struck the outmost stringer of the eastbound bridge at about 4 mph. The captain returned to the wheelhouse, reported the incident to police and called Sealevel to have a crane operator come out and lower the crane boom.
The crane operator arrived around 9 a.m. and found the boom was elevated between 26 and 30 degrees. He lowered the boom to about 15 degrees to allow it to safely pass under the bridge.
There was $1.5 to $2 million in damages done to the bridge, which sustained a bent girder, damaged deck and fractures to the pier caps. The bridge was closed to vehicular traffic until 11 a.m. on the day of the incident, although the second lane couldn’t be re-opened for almost two weeks.
Captain’s estimate made on incorrect assumption
NTSB investigators found that the captain had:
- 23 years of experience
- worked for Al Cenac Towing for 13 years
- operated the Robert Cenac for two years
- been under these same bridges “countless” times with crane barges in the past, and
- never towed the Mr. Dawg before.
He also stated that throughout his career he’d never received written documentation on the height of any barges.
Investigators learned that the last Sealevel crew who used the Mr. Dawg didn’t lower the boom to an angle that was safe for transport. Further, the length of Mr. Dawg‘s pilings, which the captain used to estimate the height of the crane, were actually 56 feet long rather than the typical 50 feet that the captain assumed.
The captain’s incorrect estimate of the crane boom height and his decision to depart before getting a confirmed height from Sealevel were the probable causes of the incident, according to the NTSB.
“When faced with uncertainty, such as the height of the crane boom or (pilings), humans typically draw from experience, relying on mental heuristics, or shortcuts to get to an acceptable solution,” the NTSB report states.
However, the U.S. Coast Guard “has recommended that ‘assumptions are not made regarding a vessel or its cargo’s (height) or of bridge heights. Specific data must be known when planning transits.’ In this case, the captain should have waited to get underway until the exact (height) of the (crane) was established.”
This can happen in any industry
While this is a maritime incident, consider how sometimes an incident or a near-miss occurs when a forklift operator underestimates the load they’re trying to lift or the height of their mast compared to an overhead obstacle near a pallet they’re attempting to reach.
The same can be said of truck drivers and trailer heights when passing under bridges. In construction, poorly calculated load weights and heights can also lead to trouble.
And to get away from equipment and vehicles, how about a roofer who fails to estimate the proper distance between the metal ladder they’re setting up and some energized overhead power lines that may be nearby?
Exact measurements are best in certain situations
This doesn’t mean that a worker should have to stop and take exact measurements every single time they perform a typical daily task. But, if an experienced worker, such as the captain in this incident, is questioning the situation then more precise measurements are required before moving forward.
For example, a warehouse that’s going to handle product for a new client should be certain that its forklifts are capable of handling the loads. The heights of the storage decking, if used, would also need to be measured precisely and adjusted as needed.