On 24 June 2017, the tug Arafura Sea Delta collided with the general cargo ship Thorco Crystal about 2 nautical miles west of Humbug Point Wharf, Weipa, Queensland. Arafura Sea Delta was one of two tugs assigned to assist Thorco Crystal’s berthing.
The incident
On June 24, the pilot of Thorco Crystal informed the tug masters that once clear of the channel, the ship’s speed would be reduced to about 5 knots.
The tugs were then to make fast on the ship’s port side while ensuring no weight was placed on the tow lines. The pilot’s instructions were acknowledged and he was advised that the tugs would be making fast to the ship using a single fixed tow line of 30 m length from the tugs’ bows. Sea Hotel’s master, the second tug, also advised the pilot that because of the tugs’ design, with no forward winch and a fixed tow line, there would be a time delay when changing between pushing and pulling modes.
He also said that the tugs might have difficulty holding station in the outgoing ebb tide. The pilot acknowledged these concerns and requested that the tugs inform him should they encounter any difficulty and to ensure they did not put any weight on the tow lines which could make difficult the pilot’s efforts to control the ship’s movement. The pilot then informed the tugs that the forward tug would make fast to the port bow while the aft tug would have to make fast ‘underneath the accommodation’.
Later, the pilot cleared the tugs to come alongside and make fast to the ship. The order was acknowledged and both tugs started to close on Thorco Crystal. Arafura Sea Delta’s master manoeuvred to about 50 m off the ship’s port side and adjusted speed to about 4.5 knots to match Thorco Crystal.
Arafura Sea Delta’s master manoeuvred to where the ship’s crew were standing, at a set of bitts just aft of the accommodation. As the tug got closer, a heaving line was passed down from the ship. The tug’s crew attached the heaving line to the tow line, which was heaved up and secured to Thorco Crystal’s bitts by the ship’s crew.
A few moments later, Arafura Sea Delta took a sudden sheer to starboard towards Thorco Crystal. The tug master used the two main engines, rudders and bow thruster in an attempt to manoeuvre away but was unable to do so. The impact of the tug’s contact was felt by those on Thorco Crystal’s bridge and an was noticed indent in the ship’s hull.
After that, the pilot began swinging the ship using the tugs and the ship’s bow thruster. The ship was then manoeuvred stern first towards Humbug Point Wharf using the main engine, bow thruster and assisted by the tugs. During the manoeuvre, the tugs experienced difficulty holding station and staying perpendicular to the ship’s side in the outgoing ebb tidal stream.
Arafura Sea Delta’s master after inspecting the damage to the tug discovered that the top of the steel frame for the fender on the tug’s bow was offset slightly and the starboard bitts had some paint damage.
Thorco Crystal had a breach about 80 mm long in the hull and several indentations in the vicinity of the breach. Temporary repairs were performed on board and the ship sailed for its next port without any delays.
Probable cause
While making fast to Thorco Crystal, the tug Arafura Sea Delta was affected by hydrodynamic interaction forces and subsequently collided with the ship.
Recommendations
After the accident, ATSB reminds masters of ships and tugs, pilots and others involved in handling vessels in confined waters or in proximity to other vessels of the hazards posed by hydrodynamic interaction.
An awareness of the pressure zones around a moving vessel and an appreciation of the effect of speed on interaction forces is important to help vessel handlers predict a potential interaction situation developing and allow them to deal with it. The effects of interaction can be worsen by an increase in vessel speed, the presence of another vessel or if the flow of water around the vessel is influenced by tidal streams or by entering a narrow or shallow channel.
Following the accident occurred, Bhagwan Marine took the following measures:
- Operational work instructions manual
- Arafura Sea Delta’s job hazard analysis
- Pre-planning meeting
- Fendering arrangement
You can find more details into the accident in the PDF below
