Executive summary
At the time of this incident the Taharoa Express and the loading operation at Port Taharoa were unique. Iron ore (including ironsand) was normally loaded dry, with particular care needed to ensure that the cargo moisture content was below what was called its transportable moisture limit. The transportable moisture limit was the moisture level at which the cargo was said to be safe from liquefaction.
The Taharoa Express was a bulk carrier that had been modified to load ironsand in the form of a slurry. The ship moored to a single mooring buoy off the port. The ironsand was mixed with fresh water and pumped out to the ship via pipelines on the seabed.
Once the slurry entered the cargo hold the ironsand sank to the bottom, while fresh water was removed using 2 different on-board processes, eventually leaving just the ironsand in the hold.
The ship arrived at Port Taharoa on 15 December 2009 to load a cargo of about 116 000 tonnes of ironsand for delivery to China. The ironsand was to be loaded into 5 of the ship's 9 cargo holds in 7 phases.
The management of the Taharoa Express had recently changed and the crew appointed by the new management company were conducting their first cargo-loading operation at Port Taharoa.
Cargo loading was in its fifth phase when suddenly and without warning the ironsand in more than one cargo hold shifted and the ship listed to an angle of 5 degrees. Cargo loading was stopped, but by the time the cargo lines had been cleared of slurry the Taharoa Express had listed to an angle of 9 degrees.
The Transport Accident Investigation Commission (Commission) determined that the most likely reason for the cargo shift in several holds was the ironsand being allowed to mound towards one side of the cargo hold’s centreline instead of being evenly distributed across the holds. Once the slope of the cargo reached a critical angle, the cargo slumped across more than one of the cargo holds.
The most likely reason for the ironsand mounding in the cargo holds was that the crew had not achieved an even sequencing for the direction of the cargo-loading nozzles.
The Commission identified 3 safety issues:
- New Zealand Steel Limited had not undertaken sufficient research on the properties of the Taharoa ironsand and the way it behaved during the slurry loading process
- the crew on the Taharoa Express did not maintain an understanding of the ironsand distribution across each cargo hold, and did not trim the cargo evenly in accordance with the operating procedures and industry best practice
- all of the resources that were available to the new crew to manage the first cargo loading operation were not used to best effect, which resulted in the first mate becoming fatigued.
New Zealand Steel Limited has since addressed the first safety issue, and the remaining 2 safety issues have been addressed with the Taharoa Express being withdrawn from service and replaced by a new, special-purpose bulk carrier that has different loading procedures. The Commission has therefore made no safety recommendations.
The key lessons learnt from this incident included:
- it is important to ensure that bulk cargo in any form is well trimmed in the cargo hold to prevent it shifting during the loading process and when at sea
- in any ship loading operation, care should be taken to ensure that sufficient operational experience is available and used to ensure a safe and efficient operation.