Hull-wrap technique shows promise
The hull-wrapping trial of former navy frigate HMNZS Canterbury has proved the technique works, but was cut short when heavy tidal flows ripped the plastic PVC wrapping. This meant the wrapping had to be removed before it drifted into the harbour.
The Canterbury is being sunk in the Bay of Islands as an artificial reef and dive attraction by a charitable trust. A condition of the resource consent is that it be free of unwanted organisms before being towed north for sinking. One such unwanted organism, the sea squirt Styela clava, was found on the hull prior to the trial. Styela is well established throughout the Hauraki Gulf, and is likely to have attached itself to the hull during the two years the Canterbury was tied up at the Devonport Naval Base.
Biosecurity New Zealand Senior Marine Adviser Dr Peter Stratford says the $27,000 trial was pushing beyond the known limits of the technique. The largest vessel known to have been wrapped was a 60-metre barge in Australia. The Canterbury is 113 metres long.
"The idea is that the plastic wrapping deoxygenates the water between the plastic and the hull, suffocating any marine pests that might be attached. Water quality monitoring showed that happened, and was having the desired effect," Peter says.
"We've shown the technique works, which is what we set out to do. The vessel was always going to have to be dry-docked for final cleaning to meet the condition of the resource consent, but this was a good opportunity to investigate another potential technique for the biosecurity toolbox.
"This is certainly a tool we can consider for the future, and we have learned a lot from the trial. The trial was undertaken in 'worst case scenario' conditions so in future attempts we are likely to know what we are dealing with.
"We were hampered in this instance by the vessel being moored side-on to a strong tidal flow, in a high-traffic area, and it had several protrusions on the hull, including the propellers. We would bear those things in mind and make some adjustments to the technique next time," Peter says.
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Page last updated: 20 June 2008