LAST June, a tugboat crashed into the general cargo vessel Thorco Crystal ripping an 80mm breach in the ship’s hull, and the Australian Transport and Safety Bureau recently released a report, finding that hydrodynamic interaction forces were the main culprit in the crash.
The incident occurred as the Thorco Crystal was heading into the Queensland port of Weipa, and just after the tug Arafura Sea Delta was made fast to the port side of the vessel just aft of the accommodation.
Seconds later, the tug took a sheer to starboard towards the larger ship and the master used the tug’s two main engines, rudders and bow thrusters in an attempt to avert a collision, but was unable to do so, and the tug contacted Thorco Crystal’s hull.
The ATSB found that the tug was affected by hydrodynamic interaction forces before it collided with the ship.
The Bureau’s analysis pointed to the fact that the tug’s position near the ship’s port quarter placed it in the low-pressure zone surrounding the larger vessel’s stern, thus subjecting the tug to hydrodynamic interaction forces.
“The ship’s speed of 4.5 knots and the estimated ebb tidal stream of about 1.8 knots (obtained after the collision) were both factors that had an influence on the strength of the interaction forces generated,” the report reads.
“Furthermore, tidal streams on the ebb tide are known to reach rates of up to 4 knots in the vicinity of the collision, potentially intensifying the interaction forces generated.”
As a ship makes headway, the flow of water around the hull causes areas of different water pressure surrounding it. Generally, areas of positive pressure develop around the bow, while the flow of water down the ship’s side creates a low-pressure or suction area, and according to the ATSB report, this aft suction area is enhanced by the flow of water into the propeller when the engine is turning ahead.
Speed is critical in managing hydrodynamic interaction, as the strength and reach of the forces can increase dramatically not only with a small increase in speed, but also if the ship passes into narrow or shallow waters.
In response to the incident, the tug owner and operator has reviewed its operational work instruction manual and updated the Arafura Sea Delta’s on-board job hazard analysis.
The company has also instituted pre-planning meetings between the pilot and the master of the lead tug, and tugs now adjust the fendering arrangement based on the pre-planning meeting.