DOSSIER

 

Tug / tanker collision due to hydrodynamic forces

 

A collision between a tug and a tanker in the Corpus Christi Ship Channel, Texas was caused by hydrodynamic forces pulling the tug towards the tanker

The US National Transportation Safety Board has released its report into a collision between a tugboat “Mark E Kuebler” and a tanker “Nisalah” leading to $7m damages in January 2023, in the Corpus Christi Ship Channel, Texas.

There was $3m of damage to the tug and $3.9m of damages to the tanker. The tugboat’s hull was breached by the tanker’s propeller, which was also damaged in the collision.

It found that the tug was drawn towards the tanker by hydrodynamic forces.

NTSB recommends that operators of a small vessel like a tug should maintain a safe distance until the larger vessel slows and the hydrodynamic forces are reduced. Or it should ensure it has sufficient reserve power to counteract the hydrodynamic forces.

NTSB also recommends that speed limits should be set for “advanced manoeuvres such as stern-first approaches.”

Following the collision, the tugboat’s operating company instituted a policy limiting stern-first landings of tugboats on assisted vessels to speeds of seven knots or less.

 

The story
At the time of the collision, the tanker was moving at nearly ten knots.

Five tugboats were assigned to assist the tanker in coming into the harbour, with Mark E Kuebler assigned the “starboard quarter” position.

The mate of the Mark E Kuebler decided to turn the tug around 180 degrees to connect the towing line, for reasons of the geometry of the tanker’s hull and the location of the winch and cable on the tugboat, so the tug would be able to run alongside the tanker at a point of the tanker’s hull where the ship’s hull was vertical.

While doing this spin manoeuvre, the tug fell back near the stern of the tanker. As the tug  moved away from the tanker, the stern of the vessel was drawn in towards it by hydrodynamic forces and the two collided.

The maximum rated speed of the tug was thirteen knots. The tug was already moving into position at 11.6 knots. So, there was little reserve propulsion power available.

The mast and stacks of the tug struck the hull of the tanker first, then the stern of the tugboat hit the tanker.

The tanker’s propeller then struck the tugboat, slicing through its hull, and the tug crew were not immediately aware of this.

The tanker’s master and pilot went to the starboard bridgewing and observed the mast on the tug had bent over, so the pilot called the tugboat to confirm that the vessel was okay.

The tug captain responded that the tugboat’s mast had hit the tanker but was otherwise fine, and the tugboat would continue the job.

Shortly after, the tugboat captain radioed the Nisalah pilot again to state that the tugboat was getting a bilge high-water alarm.

The pilot ordered another tug boat, Connolly M, to switch out with the Mark E Kuebler at the starboard quarter position. No lines were passed between the tugboat and tanker at any time before or during the incident.

The tug crew inspected the tugboat and discovered water flooding into the machinery room. Because of the danger of sinking due to the flooding, the captain intentionally grounded the tugboat in the bank outside of the Corpus Christi Ship Channel.

The Nisalah pilots and crew saw no indications of damage to their vessel, and the tanker proceeded to the terminal and moored without further incident.

 

TankerOperator

 

 

 

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