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Construction of Nga Awa Purua, the second power station on the Rotokawa geothermal resource north of Taupo, has just been completed. ALAN TITCHALL reviews its unique design and construction.
At a cost of $430 million, and a joint venture between Mighty River Power and the Tauhara North No.2 trust, Nga Awa Purua was constructed by Sumitomo Corporation in partnership with Fuji Electric (the main suppliers) and Hawkins Construction. The alliance arrangement has proved to be very efficient. The construction contact was signed in April 2008 and key elements of the power station built in 10 months. From start to finish, including two months of test running the turbine, it took less than two years to finish the project. During peak construction the workforce numbered 400 – many had already worked on Mighty River’s Kawerau Geothermal Power Station project and were relocated to Taupo. Taking on the full ‘liquidated damages’ risks of up to $190,000 a day, the design-build contractor Hawkins Construction set a target programme of 98 weeks – the contractual programme was 104 weeks – and construction kept four to six weeks in front of that. The project was finished six weeks early.
Harsh environment
The design of this turbine-generator pedestal involves a broad range of engineering disciplines to tackle both the massive size of the turbine machinery and the harsh geothermal environment. A very seismic area, structures had to be designed to withstand large earthquakes – not only for stability under a very large quake, but the smaller earthquakes the plant will experience during its 35-year expected lifetime. In a geothermal environment the groundwater, soil and atmosphere are chemically aggressive – with humidity, heat, hydrogen sulphide, carbon dioxide, sulphates and chlorides in abundance. At Rotokawa, the main challenge for the builders was the heat – up to 80 degrees C at bottom of the deepest foundation piles.
A lot of concreteThe largest and most technically challenging structure at Nga Awa Purua Nga was the turbine hall building and its huge turbine pedestal that contains 1500 cubic metres of concrete.
A grillage foundation supports the top of the turbine structure that is 10 metres above ground level. This grillage is supported on six reinforced concrete columns that sit on a 1.6 metre deep (and 25 metre square) reinforced concrete foundation slab at seven metres below ground. The slab is supported by bored piles 0.9 metres in diameter that are founded at around 25 metres below the ground surface. The pedestal has been designed with the 50-year design life and is tough enough to withstand a ‘1000-year’ earthquake. The generator arrives
The journey was one of many for the freight/transport industry, as over the construction of the power house, more than 65 shipments of special equipment was imported from 12 countries during construction. The generation parts are stacked vertically, with the turbine and generator sharing the top level (about 21 metres high) and the condenser sitting beneath the low-pressure end of the turbine. The condenser rapidly condenses the exhaust steam leaving the turbine, creating a vacuum that effectively ‘sucks ‘ the low-pressure steam through the turbine that is being ‘pushed’ from the input side. Condensing the exhaust steam converts it into water (referred to as steam condensate), which releases a lot of heat energy as it changes from gas to liquid. Two large 1.4MW hotwell sumps/pumps on the bottom of the condenser collect the liquid condensate and pump it (at between 10 and 15 bar) to the top of a forced-air evaporative cooling tower in a circuit where the condensate is reused in the steam generator, or the excess pumped to the re-injection wells. The triple system produces 26,000 cubic metres per hour of condensate.
After hand-over in April, some six weeks ahead of target, the country’s newest geothermal power station started pumping 132MW into the grid, or sufficient electricity to power 130,000 homes. It is likely that the design and construction, having completed their second project for the Rotokawa Joint Venture between Mighty River Power and the Tauhara North No.2 will have their work cut out for them again. Mighty River Power, again in partnership with the trust, lodged consents earlier this year for another $400 million geothermal power station at Ngatamariki, 17 kilometres northeast of Taupo.
Contractor Vol.34 No.4 May 2010 |
No two geothermal power stations are the same design and Nga Awa Purua near Taupo generates its 140 gross megawatts on just one, single-shaft, condensing steam turbine driven by three pressures of steam from a triple ‘flash’ system – the largest such operation in the world.
Beca was engaged by Hawkins to provide the engineering design for the civil and structural components of the plant, which is made up of a vast array of pipe and plant support structures, a series of cooling towers, and the main turbine hall. The turbine hall features a whopping, reinforced, 17 metre concrete pedestal supporting the turbine, generator and condenser with their combined mass in excess of 400 tonnes.
Concrete designs involved dense, heavily enforced, high-quality concrete mixed with fly-ash; a very fine pozzolanic material that is a by-product from the burning of pulverised coal. It’s a nice irony that the durability one of the country’s renewable energy sources is dependant on material produced by thermal generation. When added to concrete mixes, the fly-ash particles react with the cement and fill void spaces in the concrete, reducing the permeability of the cured concrete to water and aggressive chemicals. The concrete also had to contend with high temperatures generated by hydration reactions in the cement from within the thick concrete structures that cause cracking in the young concrete and impair its water-tightness. A special mix design along with the fly-ash slowed down this reaction heat, and insulation was used during the curing to reduce differential temperatures.
To eliminate vibration in the turbine hall building, the pedestal and the surrounding building is structurally separated and, in order to reduce the overall height of the turbine hall, about half of the pedestal is set below ground.
The 177 tonne generator for the power station made dramatic news as it was cautiously transported by Tranzcarr Heavy Haulage via state highways and through the Kaingaroa Forestry roads to the Rotokawa site on specialised transporters. The painstaking journey took three days and careful planning to avoid a number of the bridges on the state highways that were not capable of supporting a load of 177 tonnes.
The four rejection wells that were drilled a couple of kilometres southeast of the plant and are between two and three kilometres deep. It is always a big challenge for engineers to determine the depth of re-injection point – too far and it won’t work its way back into production and if too close, it cools down the production field.