ATB Captain Tried to Counteract Assist Tug, Resulting in Bridge Allision

The NTSB has released a report on  an allision between a tugboat and a bridge fender in Tacoma last year, which resulted in the destruction of the already-deteriorated fendering system. 

On the night of October 12, 2024, the tug Olympic Scout was assisting the ATB tug-barge combination Montlake/Sodo underneath the Hylebos Bridge, a roadway over a small waterway of the same name. Montlake/Sodo was departing and was outbound on the waterway. 

Olympic Scout was made up alongside the barge's port side near the bow, stern facing in the direction of motion. This arrangement is a common regional practice, but puts the tug close to the pivot point of the barge. 

Montlake/Sodo got under way from her berth at 2333 hours and paused when she reached the Hylebos Bridge, which had still not opened despite an earlier request. As the ATB waited for the bridge opening, the bow of the barge drifted to port, taking Olympic Scout with it. 

When the bridge opened at 2337, Montlake/Sodo resumed her outbound transit, building speed towards four knots. Olympic Scout's engines were idling and rudders were amidships. 

As they approached the bridge span, the ATB was set to port, towards the south side of the channel. Montlake's captain attempted to steer back to the middle of the channel, and as they got closer to the bridge, Olympic Scout's captain radioed to ask Montlake to maneuver to starboard to leave enough clearance for the assist tug to make it through the span. Montlake's captain instructed Olympic Scout's captain to "do what he needed to do" to get the ATB back to the center of the channel. 

Olympic Scout put rudders to starboard, port engine ahead and starboard engine astern to generate thrust to starboard. At the same time, Montlake's captain decided that he needed to counteract the turning force at his end, fearing that his stern would be pushed to port by the Olympic Scout's maneuvers. Montlake's captain put rudders to port, starboard engine aead and port engine astern. 

The bridge was rapidly approaching, and the situation was largely unchanged. Olympic Scout put both engines full ahead and Montlake put both engines full astern in an attempt to stop the ATB, but it was too late. At 2341, Olympic Scout's starboard quarter smashed the wooden fender on the south pier of the Hylebos Bridge. The ATB's speed over ground was three knots at the time of impact. 

Montlake/Sodo was undamaged, and Olympic Scout suffered paint scrapes. The bridge pier and bridge mechanism were undamaged, but the wooden fender - having done its job - was smashed. A previous engineering assessment had found that the old piles had "heavy fungal decay" and "heavy marine borer damage" in some areas. A below-water survey after the allision determined that there were "significant areas of wastage, rot, and marine worm and borer tracks." The level of damage caused by Olympic Scout was likely a result of "inherent loss of structural strength," and a new, non-rotten fender would likely have been less damaged. 

According to NTSB, Montlake's engines outclassed Olympic Scout's by a factor of two to one, and Montlake had the advantage of a longer distance from the pivot point. This gave Montlake a bigger "lever" to turn the barge. When Montlake's captain put rudders to port and "twisted" propulsion to port, he easily overpowered Olympic Scout's attempt to change the lineup; by the time he began backing down, it was too late. 

The formal probable cause of the casualty, according to NTSB, was the Montlake's captain's failure to stop or slow the ATB and correct the lineup before transiting out through the bridge span.