THE 70m-deep shaft which leads to two 3m x 2.3km horizontal pipes heading seaward.
THE 70m-deep shaft which leads to two 3m x 2.3km horizontal pipes heading seaward.

Awesome desal plant on schedule

IT looks like a hole to the centre of the earth. At the construction site for the first super-size desalination plant on Australia's east coast, next to Gold Coast Airport at Tugun, there is an assortment of heavy engineering machines and equipment you might expect to see on a massive $1.2 billion infrastructure building project employing almost 700 people. Large trucks, huge piles of concrete and steel, towering cranes and scaffolding, numerous demountable buildings and men in hard hats. And then there is the mother of all holes. A 70-metre-deep shaft, 10 metres in diameter, connects to two vast underground tunnels. The three-metre-diameter tunnels, about 2.3km long, will bring sea water to the desalination plant for conversion to drinking water by a complex filtering process of reverse osmosis, and then take salty water remaining after the filtering back out to sea. There are very legitimate environmental concerns about the Gold Coast desalination plant, on schedule to be completed at the end of next year, which the plant construction consortium is at pains to rebut. But the sheer scale of the plant's engineering is awesome. And it's the giant shaft which leads down to the mega-tunnels that is the most impressive sight on-site currently visible from ground level. About 30 crewmen work deep inside the desalination plant's twin tunnels, entering at the bottom of the shaft. Each shift is transported by a small locomotive along the completed tunnel sections, lined with thousands of thick concrete discs, until they arrive at the German-imported Herrenknecht tunnelling machines, which use computer controlled laser drilling and giant cutting discs. It is dangerous work, and although the tunnellers are very experienced (some worked on the undersea Channel Tunnel connecting England and France and on Sydney's ill-fated Lane Cove Tunnel), they are also superstitious. Tunnelling machines on big construction projects are usually given nicknames. Without a nickname, a tunnel-boring machine is considered unlucky. Local schools ran competitions to name the desalination plant tunnelling machines. The winning names were Terra Nautilus, after Jules Verne's undersea vessel in 20,000 Leagues Under the Sea, and Pipi, after small local shellfish. These two tunnelling machines will never return to the surface, although some of their components and oils will be recovered. Once their work at Tugun is complete, the tunnel borers will be sealed off under the sea bed, because they are just too big and cumbersome to bring back out. Drilling work on the twin tunnels has passed the halfway mark, with the tunnels now excavated from near the airport to about 300 metres past the shoreline. About one kilometre offshore, a "self-elevating platform barge" sits in the water, from where a giant pile-driver has been punching two three-metre diameter shafts down under the seabed to meet up with the tunnels. Intake and outlet "risers" will sit on the sea floor at the top of the undersea shafts, taking water in and out of the tunnels. The outlet riser has been installed, and the barge was recently moved to install the intake riser. The intake riser will be about four metres high, with a water flow of about 0.05 metres per second, slow enough not to trap any fish swimming nearby, according to the desalination plant builders. The outlet riser is a metre-diameter reinforced pipe, 175 metres long, sitting on the sea floor, with eight diffusers which will squirt out salty water, about twice as salty as sea water, left after the desalination process. This saltier water will be squirted about six metres out from each diffuser, dissipating in a "mixing zone" covering about 225 metres x 120 metres. Levels of dissolved oxygen and salt concentrations in the mixing zone, which the desalination plant builders say will not harm sea life, will be monitored in line with Queensland Environment Protection Agency approvals, requiring monthly analysis reports. Gold Coast Desalination Alliance spokesman Alan Davie said about 25 megawatts of power is required to operate the plant, equivalent to every Gold Coast household running a 60-watt light bulb eight hours a day.



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