Action stations at Bakun AC8 October 1998
At the Bakun AC project in the Philippines lack of access is not delaying construction. Bill Austin and Peter Buchanan* report
The National Power Corporation (NPC), the government-owned body responsible for power production in the Philippines, is following the path of privatisation. This has allowed private developers to enter the power generation market, and one such scheme is Bakun AC in Luzon, which has been awarded to Luzon Hydro Corporation. The project uses the Bakun river for power generation and will have an installed capacity of 70MW and an average annual production of 216GWh.
Luzon Hydro Corporation (LHC) is a consortium of Pacific Hydro of Australia, Aboitiz Equity Ventures of the Philippines and Pacific Corp Generation of the US. LHC has an agreement with NPC to design, build and operate Bakun AC for 25 years. In turn, LHC has entered into a turnkey contract with Transfield Philippines (TPI) to design and build the plant and Halcrow Water Power (now Halcrow Pacific) was engaged by TPI as designer. Construction began in the first quarter of 1997 and the project is due for completion at the end of 1999.
The total cost of the project is US$150M, which includes all pre-engineering, development and financing costs. Debt/equity ratio is 70/30 and equity is provided equally by the LHC partners. Debt provision has been provided by ING Baring, AB Capital, Philippines National Bank and Solid Bank, who have since syndicated on to ten international banks.
The project is located on the island of Luzon about 250km north of Manila. The project layout that was first identified, during feasibility studies undertaken for the World Bank, included a series of three run-of-river schemes on the right-hand side of the river. These were identified as Bakun A, B and C and had a total capacity of 65MW. The project went to bid as the Bakun A/B and the Bakun C project, with the two upstream schemes combined.
Halcrow Water Power (HWP) was initially appointed by Pacific Hydro to act for the developers to review the feasibility study and advise on the engineering issues during the bidding phase. Halcrow identified a single 70MW scheme on the left-hand side of the river, which was put forward as an alternative with the developers’ bid. This was later accepted by NPC and is the layout being developed as Bakun AC.
Project site and layout
The project is located in the provinces of Benguet and La Union in the Central Cordillera mountain range.
The topography of the project site is typically steep narrow gorges, particularly at the upstream end. The geology of the weir site is mainly agglomerates with some interbedded sandstones, siltstones conglomerates and mudstones. The agglomerates are typically massive, with high-strength igneous particles, of gravel to boulder size, and a medium strength matrix. At the power station site there are exposures of interbedded sandstones and there are conglomerates both upstream and downstream, but a greater depth of weathering was encountered at the power station site.
The site is within the vicinity of the Philippines fault system and is potentially subject to very large seismic events. This has been reflected in the design: a 2000-year event has been adopted as the maximum design event, with a peak ground acceleration of 0.6g at the weir site and 0.53g at the power station site.
The climate is dominated by the southwest monsoon. There are distinct wet and dry seasons, and over 80% of annual rainfall is from June to November.
The layout as adopted consists of a 20m high concrete intake structure, two 80mx6m underground desanding chambers, a 9.3km headrace tunnel and a surface power station.
The intake structure is in a narrow gully on the Bakun river — the hillside minimises the size of the structure. Incorporated into the weir is a crest intake (elevation of 686.8m asl) which extends across most of the crest. Its form was chosen because of the high sediment load. The reservoir formed by the weir will have very limited storage that is expected to silt up early in the project’s life. No means of flushing the reservoir have been included in the structure.
There is a gated spillway on the left-hand side of the weir to allow smaller flows to bypass the intake should access to the crest and intake be required. The intake will have a capacity of about 20m3/sec, which provides the 16.35m3/sec divertible flow and additional flow for the operation of the desander flushing system.
To ensure that the intake would have sufficient capacity and would operate at all flows a model study was carried out. As a result the intake length was refined, and reduced in length by about 5m from the original design which was based on published data. The results of the study provided data to determine water levels in the desander for various river flows.
A small tributary of the Bakun river which discharges naturally about 200m downstream is to be diverted upstream of the intake to increase the effective intake catchment area by about 5%. In the steep narrow gorge in which the intake is located there is not enough space on the surface for the desander. It has been sited underground on the left-hand side of the weir, arranged as an intake tunnel, which bifurcates to two parallel chambers each 80m long, 6m wide and 10m deep. Under normal operation flow is split evenly through both chambers, but one chamber can be taken offline without interrupting flow to the power station.
The water level is controlled from the power station and is maintained such that under normal operation it does not fall below el 685.0m asl. Sediment is flushed from the settling basins using the Bieri Vertical Flushing System. This was adopted for two reasons: it requires fewer auxiliary tunnels to achieve flushing, and it uses less flushing water than other methods, and therefore minimises loss of power production. The flushing water and intercepted sediment is returned to the river about 150m downstream of the weir via a tunnel.
The desander discharges the clean water to the headrace tunnel, which is 9.3km long with a horseshoe cross-section of 3.4m diameter and 2.6m invert width. The tunnel will be nominally unlined for most of its length, but will have 2.1m diameter steel lining for its final 1.3km. In situ stress measurements using hydro-splitting are being used to determine the final length of lining required. The tunnel is being excavated by drill and blast and is currently about 50% excavated.
The geology is massive and moderately strong conglomerates and agglomerates, with some thin beds of sandstones, siltstones and mudstones. To meet the construction programme the tunnel is excavated on six faces, one at each end and two each from intermediate construction adits. The upstream drive starts just above the weir and runs parallel to the desander chambers until it has passed them. After startup this will be used as access to the desander. The upstream adit, known as the ‘fly-in’ adit, is 650m long and will be permanently plugged. The other adit is 730m long and will become permanent access to inspect and clean the rock trap, with a manhole in the plug.
The power station will have a steel portal framed superstructure, with a reinforced concrete substructure and a total area of 42.5mx17.5m. It has been designed to remain watertight up to the 200-year flood. The tailrace will discharge back into the Bakun river.
The station will operate with flows of 0.5-16.35m3/sec and a gross head of 535m, using two twin 17.5MW horizontal shaft twin jet Pelton turbines coupled to a 13.8kV, 44MVA synchronous generator. The switchyard has two 44 MVA, 230/13.8kV power transformers, and a 30kV double circuit steel tower transmission line will supply the NPC line about 18km away.
Because of the difficult terrain, pop-ulation development in the region is limited to the coastal plain and a number of small communities throughout the ranges. Both the power station site and the intake site are remote, with difficult access. Neither site is directly accessible by vehicle from the other: a trip of over 100km is required.
The power station site does not have permanent vehicle access. The road is partly in the bed of the Bakun river and is impassable during the wet season. The intake site has vehicle access via low quality roads to within about 5km, to the village of Bakun Central. This point is about 400m above the weir site. The topography makes the construction of access roads from Bakun Central extremely difficult and this has not been attempted. All plant and materials are brought from Bakun Central to site using a skyline.
These restrictions have required the construction programme to be tailored to suit. For the power station, for example, design and fabrication has to be far enough advanced so all materials required during the wet season can be brought to site during the dry season. The situation at the intake site is also restricted by site access. All plant and materials are brought down on the skyline, which has a limited capacity. A further restriction is the laydown area which is limited by topography, preventing any significant stockpiling of materials.
Options are being considered for the weir design to minimise the quantity of concrete in the structure.
A further restriction is the ‘fly-in’ adit, the upstream construction adit for the tunnel. It was named because the only means of transporting plant and materials to the portal is by helicopter. It was decided at the beginning of the construction that the additional two faces for the tunnel excavation were required to achieve a suitable rate of progress and this location provided the most suitable site for the adit. A track allows construction workers to walk from the power station.
At various times during construction heavy lift helicopters have been used to transport materials to overcome these access problems.