A routine harbor-assist operation near Ingleside, Texas, turned hazardous when a tugboat collided with a large crude oil tanker on January 22, 2023. While no one was injured, both vessels sustained millions in damage, highlighting the risks posed by hydrodynamic forces in narrow channels and prompting procedural changes for safer operations.
What Happened
At around 3:30 p.m. local time, a tugboat was assisting a tanker transiting inbound through the Corpus Christi Ship Channel. The tanker, in ballast condition with significant draft, was en route to load crude oil. As part of the maneuver, the tugboat was assigned to the tanker’s starboard quarter.
To facilitate hawser connection, the tugboat’s mate performed a 180-degree turn using the vessel’s Z-drives, preparing for a stern-first approach. During this repositioning, the tugboat fell back near the tanker’s stern. The mate applied maximum engine power to regain position—but unknowingly entered a zone of low pressure created by the tanker’s hull and propeller intake.
This hydrodynamic suction, intensified by the vessel’s ballast draft and forward speed, overwhelmed the tugboat’s available thrust. Despite efforts to steer away, the tugboat was drawn in and struck by the tanker’s propeller.
The collision tore open the tugboat’s stern, flooding engine compartments and warping the hull. The tanker’s propeller also sustained damage to all four blades. To prevent the tug from sinking, its captain grounded the vessel nearby. A minor hydraulic oil spill was quickly contained. Damage estimates reached $3 million for the tugboat and $3.9 million for the tanker.
Probable Cause
Investigators found that the tugboat was maneuvered near the tanker’s starboard quarter—an area prone to strong hydrodynamic suction. The tug had insufficient reserve power to counter these forces due to the vessels’ transit speed. As a result, the tugboat was drawn in and collided with the tanker’s propeller.
Lessons Learned
Hydrodynamic Forces Between Vessels in a Channel
When a large vessel moves through a confined waterway, especially in ballast condition, it generates low-pressure zones near its hull—most significantly near the intake side of the propeller. These suction forces increase exponentially with speed and can pull in smaller vessels operating nearby.
Operators of smaller vessels, particularly tugboats, must:
- Maintain a safe distance during close-proximity operations.
- Ensure they have adequate reserve power to counteract hydrodynamic suction.
- If necessary, delay approach until the larger vessel reduces speed and the suction effect diminishes.
Speed During Harbor-Assist Maneuvers
Following the incident, the tugboat’s operating company instituted a policy limiting stern-first maneuvers to speeds of 7 knots or less.
Harbor-assist operators using Z-drive tugs are advised to:
- Set speed thresholds for complex maneuvers like stern-first landings.
- Tailor these limits based on the design and capabilities of the tugboat.
- Communicate these limits to ship pilots and masters in advance of the operation.
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Source: NTSB
