New Zealand bulk carrier mishap traced to poor welding of rudder

The Transport Accident Investigation Commission of New Zealand (TAIC) has published its report on its investigation into the loss of rudder on the bulk carrier Achilles Bulker off the Port of Tauranga on July 24, 2023.

On the said date, Achilles Bulker, fully loaded and under pilotage, was outbound from the Port of Tauranga.

The ship deviated from the centre of the channel shortly after clearing the harbour entrance. As the crew and onboard pilots tried to steer it back on course, the ship’s rudder broke off. With steering impossible, the vessel drifted towards shallow water.

TAIC said grounding was averted thanks to the swift coordinated action by the pilots and the crew successfully deploying both anchors.

Why it happened

Fixed into the base of the rudder assembly was a heavy hinge pin (pintle), which could rotate freely in a matching socket (gudgeon), enabling the rudder to turn.

During 2021 drydock maintenance, the pintle was removed and refitted, but the securing parts installed had weak, porous welds. After the ship returned to service, those welds broke under vibration.

With its securing parts broken, the nut that retained the pintle as part of the rudder assembly unwound itself. Sometime before the incident off Tauranga, the pintle dropped unnoticed from its casting and sank.

Without a pintle, the bottom of the rudder increasingly moved side to side and fore and aft. This exerted sideways forces beyond the designed strength of the coupling plate (palm) at the top that joined the rudder to the solid shaft (stock) that connected to the ship’s internal steering mechanism.

Lastly, the rudder palm developed fatigue cracks that grew steadily until the palm fractured completely on both sides of the rudder stock, and the whole rudder blade dropped off.

Safety issue and recommendations

TAIC said the total rudder loss occurred because the rudder system experienced damaging forces, the result of insufficient quality assurance during reinstallation of the pintle. Pintles are critical components of steering systems and must be installed with precise fit and robust quality control.

In principle, owners and operators of ships must maintain oversight of critical system repairs, including during shipyard work.

TAIC recommends the following:

• The shipyard that did the drydock work on this ship must implement robust quality assurance procedures for rudder pintle installation (TAIC has notified the Maritime Safety Administration of China of this recommendation).

• Maritime New Zealand should work with the International Maritime Organisation to promote enhanced global standards for quality assurance of rudder systems during installation, maintenance, and repairs.