High-pressure job 45m under water

Adelaide Advertiser
Tuesday 6/7/2010 Page: 14

WORKERS on the Adelaide desalination plant's underwater tunnels face conditions comparable to those of deep-sea divers as the sophisticated project takes shape. The tunnels - which will suck sea water into the plant from Ikm out to sea and discharge brine 1.5km from shore - are well advanced as the December deadline looms to produce first water. The tunnels being built by consortium AdelaideAqua will be nearly 45m below sea level at their end points.

Because of the depth, workers engaged in the routine task of replacing the cutter heads on tunnel boring machines in each of the intake and outfall tunnels work under pressurised conditions. "This is to allow maintenance to take place without risk of the rock face collapsing," AdelaideAqua spokeswoman Bernie Auricht said. "There is no water in the tunnel but the pressure created is similar to that experienced by diving at the same depth. "And, just like diving, you must surface in stages and at a certain pace to ensure that your body releases dissolved nitrogen from the blood."

To date, the tunnels have been bored between 830m and 880m out to sea and 20m deep, mostly through sandstone but also through thick black clay. While the tunnels are being bored, work continues at a rapid pace onshore, with the major buildings completed and racks of reverse-osmosis cylinders installed. Chemical plants for dosing the water also are nearing completion. "The water's so pure you have to reintroduce minerals to it," AdelaideAqua project director Duncan Whitfield said.

Desalination is energy intensive and the plant will use 500GW hours a year when running at full capacity to produce 100 gigalitres a year of water. The Government has signed a 20-year contract with AGL Energy to provide renewable energy only to power the plant. In 2009-10, SA generated about 2500GWh of electricity from wind farms, so the desal plant would soak up one-fifth of that capacity. The plant also will add considerably to water costs. Water Minister Paul Caica said legislation was being drafted to allow prices to be set by an independent body.