The National Transportation Safety Board (NTSB) has released an investigation report on an incident where a tugboat made contact with Hylebos Bridge in October 2023.
The incident
On October 12, 2023, at 2341 local time, the tugboat Olympic Scout was assisting the articulated tug and barge Montlake (tugboat) and Sodo (barge) as it headed outbound on the Hylebos Waterway in Tacoma, Washington (see figure 1 and figure 2). As the vessels attempted to transit through the Hylebos Bridge, the starboard quarter of the Olympic Scout struck the bridge’s protective fender system. There were no injuries, and no pollution was reported. The Olympic Scout sustained superficial damage; the south side of the bridge’s fender system was damaged beyond repair and was replaced at a cost of $2.43 million.
Analysis
While made up to and assisting the ATB Montlake/Sodo as it transited outbound on the Hylebos Waterway, the tugboat Olympic Scout contacted the south fender of the Hylebos Bridge, significantly damaging the fender. Before getting underway from the SeaPort Sound Terminal, the captain of the Montlake/Sodo radioed the Hylebos Bridge operator requesting that the bridge be opened. Shortly after, the Montlake/Sodo got underway, but the bridge had not yet opened, prompting the captain to again request it be opened.
While awaiting the bridge opening, the ATB had to pause, and the bow of the ATB drifted to port, toward the south side of the channel. Once the bridge was opened a few minutes later and the Montlake/Sodo began to move forward, the ATB was set further to port. The Olympic Scout was made up on the port bow of the Sodo with its stern facing in the direction of the ATB’s travel. Initially, the Olympic Scout’s engines were at idle and its rudders midship. In its position on the bow of the Sodo, the Olympic Scout caused drag on the forward port side of the ATB. The captain steered to starboard attempting to line up for the bridge, but likely due to the drag from the Olympic Scout on the port bow, he was unable to move the ATB appreciably to starboard. Consequently, the ATB was on the port side of the channel as it approached the bridge, with the Olympic Scout in danger of hitting the fender.
In an attempt to avoid hitting the Hylebos Bridge fender, the Olympic Scout captain used a starboard twist on his vessel’s rudders and engines. Soon after, the Montlake captain initiated a port counter twist out of concern that the stern of the Montlake would approach the southern bank of the waterway. According to the Olympic Scout captain, the starboard twist on his tugboat “wasn’t giving us anything.”
When a vessel is underway and steering and propulsion are applied, a turning moment is created, and the vessel will pivot around a point, generally located centerline aft of the bow. The actual location of the point varies depending on the shape of the hull, speed, and other factors, but a common tenet is that, for a vessel moving forward under its own power, the location is about one-third of the length of the vessel aft of the bow. When a vessel moves astern, the pivot point will move aft.
Figure 7 (below) shows the Montlake/Sodo with the Olympic Scout made up as it was during the Hylebos Bridge casualty. In this simplified illustration, the pivot point is assumed to be one-third of the total length of the Montlake/Sodo aft of the bow of the barge, and both the Montlake and Olympic Scout are applying a turning moment through the centerline of the ATB via their respective rudder and engine orders. Note that the moment arm for the Montlake is longer than the moment arm for the Olympic Scout, and thus the Montlake would require less effort to impart a turning moment on the ATB.
In addition to requiring less effort to turn the ATB, the Montlake had almost twice the engine power of the Olympic Scout (4,200 hp compared to 2,250 hp). Given the advantages in moment arm and power (turning moment), the Montlake’s port counter twist effectively negated the efforts of the Olympic Scout to move the ATB’s bow back toward the center of the channel. The Olympic Scout captain applied progressively more rudder angle and engine speed, and the Montlake captain began backing down on his vessel’s engines, but by the time these actions were taken, the fender was too close for the Olympic Scout to avoid contact.
The width of the channel between the Hylebos Bridge fenders was 150 feet. The combined breadth of the 78-foot-wide Montlake/Sodo and the 26-foot-wide Olympic Scout was 104 feet, leaving a maximum clearance of 23 feet on either side of the combined unit if it was perfectly centered in the channel. The Montlake captain stated that his normal practice was to center the ATB in the channel (not the ATB and assist tugboat combined), which would have left only 10 feet of clearance between the Olympic Scout and the bridge’s south fender if the bridge transit was executed as intended. With such a tight clearance, the margin for error was slim.
The distance between the bow of the Montlake/Sodo at its berth and the bridge was 1,276 feet, providing a short distance for the ATB and its assist tugboat to line up to pass through the narrow opening between the protective fendering for the bridge. After getting underway, while the ATB waited for the bridge to open, the bow of the ATB moved to port. As the ATB began moving forward and accelerating to 4.1 knots, it was set further to port and, consequently, was not lined up properly with the bridge. Because of the short distance to the bridge and the speed of the ATB, there was insufficient time to correct the lineup before the Olympic Scout struck the fender.
Given the slim margin of error for making the bridge transit and the short distance to make the approach, slowing or fully stopping the ATB’s forward motion earlier would have provided the operators more time to correct the lineup and successfully transit through the opening between the bridge’s protective fendering. Protective fender systems are installed to prevent damage to abutments, piers, and other critical bridge structures by redirecting errant vessels back toward the navigable channel.
After striking the south bridge fender, the Olympic Scout and Montlake/Sodo moved back toward the center of the channel, and the combined unit continued through the bridge opening. The Hylebos Bridge was undamaged, but the fender was catastrophically damaged. Precasualty inspections of the bridge’s fenders in 2017 and 2022 noted significant deterioration of the fender piles from marine borer damage and fungal decay. A postcasualty inspection found similar damage, and a marine surveying and consulting firm stated, “the damage caused by the [Olympic Scout contact] resulted in significantly more damage being sustained than would have been had the fendering structure been of sufficient structural strength.”
It is notable that the entire fender sustained catastrophic damage when the Olympic Scout struck it at one end (at an angle—not directly), yet the tugboat sustained almost no damage. The Hylebos Bridge fender system prevented damage to the bridge structure by the Olympic Scout; however, the system’s degraded condition contributed to its extensive damage.
Probable Cause
The National Transportation Safety Board determines that the probable cause of the contact of the assist tugboat Olympic Scout with the Hylebos Bridge fender was the captain of the articulated tug and barge (ATB) Montlake/Sodo not stopping or slowing the ATB’s forward motion to correct the ATB’s lineup before attempting the bridge transit. Contributing to the severity of damage to the bridge’s fender system was the system’s deteriorated condition.
