The Manapouri hydro power station is New Zealand’s largest hydro facility with a current maximum output of around 730MW. The project is located in an area that has received World Heritage status, and as a result the project must be operated under strict environmental standards
The Manapouri power station, located in Fiordland National Park on New Zealand’s South Island, was designed by Bechtel-Pacific Corporation. The majority of the construction was carried out between 1963 and 1969 by the joint venture of Utah-Williamson-Burnett for the owner New Zealand Electricity Department, which in 1988 became the Electricity Corporation of New Zealand (ECNZ).
The power station utilises water and snow catchments from Fiordland. Water flows from Lake Te Anau down the upper Waiau river to Lake Manapouri. Water from Lake Manapouri is drawn into the West Arm power station, where the powerhouse is located more than 200m underground in a cavern measuring 111m long, 18m wide and 39m high. Other project features included the construction of a 2km long power station access tunnel, a 170km long transmission line and control structures on Lakes Te Anau and Manapouri.
Soon after the Manapouri power station was commissioned, observations confirmed that the hydraulic head loss was higher than had been estimated during the design stage of the project. The potential hydraulic head had been estimated at 178m, but the actual hydraulic head was only 148m. The result was less net head available to the station for generation and the losses were eventually traced to the tailrace system comprising the 10km long tunnel 1 and the 1.6 km exit channel. High tailwater levels in the manifold caused the Manapouri power station to restrict generation from a design using 700MW to a much lower 590MW. Safety concerns were also raised due to possible flooding of the facilities in the event of a sudden load shedding event.
Further concerns also focused on the fact that the head losses in tunnel 1 were the result of a partial blockage due to a lining failure or movement along one or more of the geological faults that cross the tunnel. It was feared that conditions could deteriorate at any time. Possible solutions to remedy the problem included enlarging the existing 9.4m horseshoe tunnel out to 10m or more in diameter using a mechanical reamer. After consideration this was abandoned as it was too costly, too time consuming, potentially dangerous and required a long station shutdown.
An additional tunnel, parallel to tunnel 1 which could be used as a back up in the event of a tunnel 1 lining failure, was considered to be beneficial and more realistic due to tunnel boring machine (TBM) technology advances. In the early 1990s the construction of a parallel tunnel using a TBM became the preferred option. ECNZ established the Manapouri Tailrace Investigations Project after a feasibility study confirmed a second tunnel was practical and would require only a short station shutdown.
In addition to safety and the higher production associated with TBM mining, consultants and expert reviewers confirmed that a hard rock tunnel excavated by machine could remain unlined except through faulted/fractured ground, thus resulting in considerable savings.
In 1997 ECNZ awarded a construction contract worth US$85M to a joint venture of Fletcher Construction (New Zealand), Dillingham Construction (US) and Ilbau (Austria). The JV then awarded the contract to Robbins for one 10.05m diameter, 500m long Main Beam TBM to excavate the tunnel.
The TBM was designed by Robbins for the mixed face hard rock conditions in the tunnel, and was then built by Kvaerner-Markham in the UK and shipped to the job site. The cutterhead featured 68x432mm cutters with loading from either the front or back – 4084 cutters had to be replaced throughout the project. Eleven two-speed electric motors powered the cutterhead with 3463kW. The 470m long back-up system, built by Rowa Engineering, included a secondary rock-bolting station and a robotic shotcrete station. The Robbins TBM began boring in June 1998 and finished in 33 months. The Average advance rate achieved was 10m per day with the best advance rate being achieved in April 2000 at 20m per day.
TBM mining also meant minimal risk to the existing facilities and the power station could remain operational except when connecting the two tunnels. Removal of the final ‘rock plug’ separating tunnels 1 and 2 by conventional excavation (drill and blast) was accomplished during a shutdown of the Manapouri power station limited to 11 days.
Tunnel 2 became fully operational at approximately 0500 hours on 5 May 2002 and by 0700 hours Manapouri power station had ramped up to an output of 714MW. This became the first time the station’s generation output was more than 600MW since being commissioned in late 1969. Measured tailwater level in the manifold at 700MW was 10.7m which compared favorably with levels of 12.5m predicted by the designer, and 30m previously experienced at 585MW output with only tunnel 1 operating.
Furthermore, the use of a highly sophisticated remote operating vehicle to inspect tunnel 1 between Deep Cove to the station manifold in May 2000 revealed no obvious damage. The competent nature of the concrete lining meant the head loss was attributed to friction and not a partially blocked tunnel.
The scale of a project to construct a 10m diameter, 10km long tunnel in one of New Zealand’s most remote and pristine environments presented a major engineering and environmental challenge. The detailed environmental standards adopted for the project were incorporated into the contract documents for the second tunnel to ensure construction activities complied with the Department of Conservation’s requirements for operating within Fiordland National Park.
Before construction work could commence onsite the contractor was required to prepare a detailed Environmental Protection Plan (EPP). This had to cover all matters necessary to carry out and monitor the construction works in accordance with the environmental standards set. Compliance also meant obtaining pro-construction approvals for the provision and operation of all on site facilities.
Meridian undertook extensive consultation to agree on environmental outcomes for the project. These included defining the physical limits of disturbance, sewerage and construction water discharge limits, waste management practices, noise and dust control and site rehabilitation. Measures to lessen the environmental impact of the project accounted for 10% of the budget and were given an equal first in priority alongside engineering requirements.
The project presented a wide range of challenges, from technical and engineering issues to the demand for a cost-effective and environmentally sensitive solution. Its geographical isolation and pristine environmental conditions, including rock material and terrain, needed to be preserved. The paramount requirement was to protect the World Heritage Area, a site of international interest that attracts thousands of tourists each year.
A strict environmental specification was required to ensure that no unacceptable pollutants were brought in and designated waste was removed from the park on completion. Potential pollutants, such as dirty tunnel water discharges, diesel fuel and oil, were managed carefully and met all wastewater clarity standards. The carefully engineered waste rock dumps were contoured and replanted with locally sourced native plants. All construction waste and debris that could cause environmental problems, except for the tunnel spoil, was transported out of the national park. A diving programme was implemented to monitor rare black coral to ensure that sediment released during channel excavation did not harm underwater plants or marine life.
With the Deep Cove portal located below sea level, a significant sheet pile cofferdam and deep well pumping system were required to provide construction access to the tunnel, as groundwater inflows had been a major problem during construction of the original tunnel. Design studies indicated that the second tunnel could be positioned within a groundwater ‘pressure shadow’ around the existing tunnel to reduce total inflows. Its positioning was also calculated to prevent the risk of tapping leakage flows from the existing tunnel.
Preparing for the unexpected
The potential for differing site condition claims is inherent in underground construction, and nothing can eliminate the risk of encountering unexpected conditions. In the past, tender documents for tunnel construction projects provided factual data gathered from nearby construction and subsurface exploration programs but left it to the tenderers to evaluate and cope with the risk that variable conditions might be encountered. For the tailrace tunnel, an evaluation was provided in document called a Geotechnical Baseline Report (GBR). It was the first time such a document was used for a major project in New Zealand. The purpose of the GBR was:
• To set forth the designer’s interpretation of the available subsurface data and previous construction records, which were detailed in the accompanying Geotechnical Data Report.
• To describe how the anticipated subsurface conditions had influenced the construction plans and specifications.
• To provide clear baseline statements of the conditions likely to be encountered during construction which would be used as a basis for contractors in the development of their tenders and as a key tool in assessing the merits of differing site condition claims if they should arise during the work.
The baseline statements in the GBR did not represent a guarantee that those conditions would actually be met; they represented a set of contractual assumptions to be applied to all tenderers equally and to assist in the administration of the differing site conditions clause in the contract. Some aspects of the project particularly those that resulted from the contractor’s work methods were not assigned baseline parameters.
Specific geological problems were anticipated to be met along the way, including poor ground conditions. To counter those conditions, the design incorporated specific ground support systems for various ground classes and provided for probe drilling to investigate and pre-drain the rock and formation grouting if required. Also, a specific type of support system was designed to protect against the possibility of rock spalling.
The form of contract used for the construction of the second Manapouri tailrace tunnel was a lump sum contract with a schedule of unit prices. The General Conditions of Contract were based on NZS 3910:1987 Conditions of Contract for Building and Civil Engineering Construction, modified to suit project specific provisions. The contract recognised the need for great sensitivity due to the high risk nature of underground works, the isolation and environmental sensitivity of the site and included risk sharing provisions such as for the different categories of tunnel stabilisation and lining.
Manapouri is an important project. It is New Zealand’s largest hydropower power station both in terms of installed capacity and annual generation. In order to improve the efficiency of generating units and to extend the life of generating plant for an additional 30 years, the Manapouri Half Life Refurbishment Project was undertaken in 2001. Its objectives were to:
• Address generating plant reliability and availability issues.
• Realise the power and energy output uprating potential of the generating plant made available with the second tailrace tunnel, by increasing unit output from 122MVA to 135MVA.
• Improve overall station water to wire efficiency.
• Improve ease of maintenance and reduce maintenance costs.
The project was completed in 2007 and comprised:
• Replacing the generator excitation systems from static excitation to rotating brushless excitation systems.
• Upgrading the generator Isophase buswork.
• Upgrading the rewound generator stators from 122MVA to 135MVA.
• Replacing the 220kV cables from oil filled to XLPE from the underground powerhouse to the above ground switchyard.
• Replacing the turbine runners achieving a 3-4% turbine runner peak load efficiency increase.
• Completing the general mechanical refurbishment/overhaul of each turbine generator unit.
Other significant upgrade and refurbishment works which were completed include rewinding and uprating the main unit 13.8/220kV transformer; refurbishing the station AC supply system; installing remote control facilities to allow full remote control of the facility from the operational control centre located 400km away.
Typically, annual energy output from Manapouri power station prior to construction of the second tailrace and the Manapouri Half Life Refurbishment Project was approximately 4100GWh per annum. Following completion of these two projects in 2007, typical annual energy increased to 4700GWh; a 15% increase. Indeed, reliability and availability improvements are shown in table 1.
As per the description and figures shown in Table 1; Manapouri power station has realised increased power and energy output, improved generating unit reliability and availability, increased unit efficiency, and improved maintenance and reduced maintenance costs. By undertaking the second Manapouri tailrace tunnel and the Manapouri Half Life Refurbishment Project, the hydraulic constraints posed by the original single tailrace tunnel, which limited power station output to 585MW, have been eliminated. The additional power output was achieved without constructing any new power generation facility and is of immediate benefit to the New Zealand power consumer.
Amended discharge proposal
The installed capacity of Manapouri power station is now 945MW, but is limited to 731MW due to water flow discharge restrictions into Deep Cove at Doubtful Sound. Proposals are underway to increased discharge from the project with the aim to increase the maximum permitted flow discharge through the power station (from the current maximum of 510m3/sec to 550m3/sec) to increase maximum continuous power output from the station to 840MW.
If this is possible it will result in a 255MW increase from the 585MW original maximum station output. However, in order to increase the maximum tailrace discharge to 550m3/sec, new resource consents will be required. The quest for these has become known as the Manapouri tailrace amended discharge proposal and consent hearings are due to take place in the first quarter of 2009. The proposal involves:
• Increasing the maximum rate at which water is discharged through the power station to Deep Cove.
• Changing the pattern of lake level fluctuations in Lake Manapouri and Lake Te Anau.
• Changing the discharge of water released through the Manapouri Lake control structure to the Lower Waiau river. Overall the power station will discharge an additional 2.6% (or 10m3/sec). In wet years it will discharge 5.1% and 0.6% in dry years.
Meridian’s proposal will increase electricity generation by 89GWh annually. The new consents will not replace the existing ones but will seek new conditions allowing Meridian to change the way it operates the system.