HDD a cost-effective choice

Davies said the decision to use HDD instead of tunnel boring also limited the impact on road and rail infrastructure. All HDD sections were completed by the Eastern Pipeline Alliance comprising AJ Lucas, Transfield Services, GHD and SunWater.Before commencing HDD, borehole testing had determined the best depth for the twin pipelines – each almost 1km long and 660-760mm in diameter – was 20m below the riverbed.The 311mm-diameter pilot hole, the only “steered” hole, was the first to be drilled. To ensure precision, down-hole steering tools registered electromagnetic signals calibrated to known positions on the surface. The pilot hole was steered to within 20cm of the target on the other side of the river. At all times, the driller and steerer knew the exact position, inclination, depth or length of the pilot hole. The pilot hole was then gradually enlarged by pulling increasingly larger reamers back through the hole. Meanwhile, the steel pipeline had been laid out across vacant land, then welded, internally lined, externally coated and hydro tested.When the final diameter of the tunnel was achieved, the pipeline string was attached to the final reamer and pulled back through the tunnel in one continuous operation – minimising the time the borehole was left open. Eastern Pipeline Alliance project manager Shane Gaudin said timing was critical. “The drill cab from where the drill rig is controlled is the centre of the operation and effectively drives all other tasks,” he said. “In this type of formation, where speed and accuracy are mitigation measures in themselves, there is little room for error in execution.“The pilot hole took about two weeks to complete but the pull-back operation had to be completed within six hours. This is because the material the team was drilling through is soft and there was a very real risk the hole could collapse, or the pipe could get stuck in the pull-back operation.”Clay, sand, gravel and weathered rock sections were encountered during the drilling. Gaudin said HDD superintendents Gary Hunter and Stephen Booth were two of the world’s leading authorities with more than 50 years combined global experience.“They have a wealth of down-hole drilling, formation and tooling knowledge and are the engine room of the whole operation,” he said.Gaudin said a critical phase of the pull-back was when the fabricated pipeline passed through a break-over structure. “The break-over is a series of cranes with roller cradles that train the pipe into a suitable radius as it is pulled into the borehole,” he said. “Its purpose is to ensure the pipeline maximum yield stress is not exceeded so that the pipeline does not sustain any permanent deformation.”

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