Arafura Sea Delta (2017)

Classification

Vessel Details

Incident Details

Key facts

• Arafura Sea Delta was the aft tug and Sea Hotel was the forward tug for Thorco Crystal's berthing.
• At about 07:31, the pilot cleared both tugs to come alongside at about 4.5 knots.
• Arafura Sea Delta suddenly sheered to starboard while near Thorco Crystal's port quarter.
• At about 07:35, the tug's starboard bow contacted the ship's port quarter.
• The ATSB identified hydrodynamic interaction, ship speed and the outgoing ebb stream as significant factors.

Narrative

On 24 June 2017, the general cargo ship Thorco Crystal was approaching Humbug Point Wharf in Weipa Harbour, Queensland, with a pilot on the bridge. Sea Hotel was assigned as the forward tug and Arafura Sea Delta as the aft tug. At about 07:23, the pilot told the tug masters that the ship would reduce speed to about 5 knots and that the tugs should make fast on the port side without putting weight on their tow lines. At about 07:31, the pilot cleared the tugs to come alongside. Arafura Sea Delta approached the ship's port side, adjusted to about 4.5 knots and passed its tow line to the ship's crew. A few seconds later, the tug master felt a sudden sheer to starboard toward Thorco Crystal. He used the tug's main engines, rudders and bow thruster but could not prevent contact. At about 07:35, Arafura Sea Delta's starboard bow struck Thorco Crystal's port quarter. The ship's hull sustained a breach about 80 millimetres long and adjacent indentations. The tug's bow fender frame was offset slightly and its starboard bitts sustained paint damage. At about 07:55, the pilot obtained a tidal-stream estimate of 1.8 knots, and the tugs later reported difficulty holding station in the outgoing ebb stream. The ATSB concluded that Arafura Sea Delta's position near the ship's stern placed it in the low-pressure zone around the quarter, where hydrodynamic interaction likely caused the sudden sheer. Ship speed and the ebb stream increased the strength of the interaction forces. The report does not state normalized casualty counts or an economic-loss figure.

Sequence of events

Contributing factors

Investigation

The Australian Transport Safety Bureau investigated the collision in report 332-MO-2017-005. Its analysis states that hydrodynamic interaction forces likely caused Arafura Sea Delta's sudden sheer toward Thorco Crystal. The report identifies the tug's position near the ship's port quarter, the ship's speed and the outgoing ebb stream as factors affecting the interaction forces. It records the physical damage to both vessels but does not provide normalized casualty counts or a monetary loss estimate.

References

• ATSB Report 332-MO-2017-005: Collision between Arafura Sea Delta and Thorco Crystal

Related

• Arafura Sea Delta
• Thorco Crystal
• Sea Hotel
• Australia
• Australian Transport Safety Bureau (ATSB)
• Hydrodynamic Interaction

Similar incidents

• 2015-06-24: Virtus (2015)
• 2010-11-11: Fairplay 22 (2010)
• 2008-04-04: WILHELMINE (2008)