A second tailrace at Manapouri is needed to optimise the efficiency of the original installation. The original 9.4m diameter horseshoe tailrace, designed by Bechtel of the US and excavated by Utah Construction of the US, is unable to sustain the full 5000GWh/year output, limiting efficiency of the seven 100MW units by 14% to 4300GWh/year. As there is no possibility of dewatering the first tailrace to correct its design faults, a second tunnel constituted the only possibility of increasing output.
Given the 56 month construction of the initial drill+blast tailrace, viability of the second tunnel was based on high-speed TBM excavation. A TBM feasibility study was commissioned in 1993 and, once confirmed by a review panel, the size of the tunnel was set at 10m diameter to allow the existing installation to operate on the second tailrace alone if necessary. The new tunnel is also designed to be dewatered, requiring the first tunnel to maintain station operation.
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Following intense economic viability studies, the project was approved in June 1996 based on a 42 month construction period and comprised: a largely unlined tunnel (33% having a permanent shotcrete lining,10% in-situ concrete); and a high-performance TBM.
The wholesale cost of electricity in New Zealand has since fallen significantly and it is unlikely that the tunnel would be approved under current economic conditions. However, “the philosophy for approval,” explained Brian Heer, assistant project director for Meridian Energy, “was the tunnel, yes, but not at any price. Of the three possibilities, ECNZ at the time chose ‘fast’ and ‘good’ and not necessarily ‘cheap’.
“We did not determine to select the lowest tender price and made every endeavour to mitigate and share reasonably the risk of the job. The delays, technological problems and contractual frustrations are very disappointing but we are hopeful yet of a much improved progress in 2000.”
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By GlobalData
