Off-shore drilling rig collects sea-bed samples

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An off-shore drilling rig is at work in the Rangitoto Channel off Mairangi Bay, collecting seabed samples, as part of the preparatory work for the new ocean outfall for North Shore City Council's Rosedale wastewater treatment plant.

The proposed new outfall, planned for completion by 2010, will take high quality treated effluent and discharge it 2.6 kilometres out into the Rangitoto Channel.

The council's water services planning and projects group manager, Adrian Vosloo, says the entire operation involves using the barge-mounted drilling rig and additional boats to move equipment and engineers. Drilling is a difficult exercise, he says, but the information gathered from analysis of the samples is vital for the project.

"These samples will tell us about the composition of the seabed so we can plan the most appropriate construction method for laying the outfall pipe," he says.

The rig is operating from a barge and will drill up to 10 boreholes, each about 10 metres deep along the proposed route of the outfall to collect sediment and rock samples.

It is estimated the off-shore geotechnical investigations will take about two weeks to complete, depending on weather and sea conditions.

"Obviously we need calm seas to be able to do this," Mr Vosloo says.

North Shore City Council is working in association with environmental consultants Maunsell, and Tonkin & Taylor.

The drilling operation is part of the council's wide-ranging assessment of the entire route, including the on-shore underground section of the outfall.

Information on water temperature, salinity and ocean currents recorded by data- sensing equipment on buoys moored in the Rangitoto Channel for six months last year, is currently being analysed both here and in the United States.

Two further surveys have also been carried out - one a seabed survey to measure seabed depths and contour levels, and the other a geophysical survey to investigate the composition of the seafloor.

Results from the buoys and the two surveys are expected later this year.

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