Power giant

25 April 2008

Face drilling in the headrace tunnel.

Face drilling in the headrace tunnel.

The potential for Nam Theun, one of the tributaries of the Mekong River, to generate electricity was first identified by the Lao government in the mid-1970s. This followed the completion of the Nam Ngum 1 project north of the capital Vientiane in 1970, which was the country's first hydroelectric scheme, and at the time was the largest dam in south-east Asia.

Plans for bringing Nam Theun 2 (NT2) to fruition were developed in the 1990s by sponsors Electricité de France (EDF), a French energy company, and Italian-Thai Development (ITD), the largest construction contractor in neighbouring Thailand. However, the Asian financial crisis of 1997 brought things to a halt. When the scheme was resurrected in 2000, these two sponsors had been joined by Electricity Generating Public Company (EGCO), a Thai energy provider.

These three companies combined with Lao Holding State Enterprise (LHSE), owned by the Lao Government, to form the Nam Theun 2 Power Company (NTPC). The entity that is building NT2, and which will own and operate it over a 25 year concession period. EDF has the largest share at 35%, followed by LHSE and EGCO (25% each) and ITD (15%).

In June last year NTPC finalised financing for the US$ 1.58 billion project, including substantial loans from the World Bank and Asian Development Bank among others. The deal is structured with NTPC putting up 28% of the cost in equity, with the remaining 72% coming from loans. Construction is expected to cost US$ 1.25 billion, with the remaining US$ 330 million being set aside as a contingency and for additional financing costs.

When complete NT2 will have the capacity to generate 1070 MW of electricity. The bulk of this capacity - 1000 MW - will be exported to Thailand through long-term power purchase agreements with the Electricity Generating Authority of Thailand (EGAT). The remaining 70 MW (6.5% of production) will be taken by Electricité du Laos (EdL), the state-owned Laotian electricity company.

Overview

Located in Lao's central highlands, some 250 km east of Vientiane, the scheme comprises a 39 m high, 436 m long concrete gravity dam at the village of Nakai. This, along with 13 smaller earth saddle dams across various valleys, will create a 450 km2 reservoir, flooding the Nakai plain.

The Nakai Dam is at the north western end of the site. Some 70 km to the south east is the intake channel for the generating plant, where the abrupt change from mountains to Lao's lowland floodplains provides a net head of 348 m. Water will flow from the intake channel into the submerged entrance of the 1.48 km long, 80 m2 headrace channel, the first 844 m of which is inclined at 10%.

From here it will drop into the 200m high vertical pressure shaft before completing its journey to the turbine house in the steel lined 1.43 km long pressure tunnel. Just upstream from the pressure shaft, a surge shaft will be sunk from the hilltop above, down to the headrace tunnel - a kind of safety valve to prevent dangerously high pressures developing in the subsequent shaft and tunnel.

The high pressure tunnel itself starts with a 90 m2 cross section, narrowing to 60 m2 towards the end, before the flow is split into four tunnels by a manifold to supply the four main 250 MW turbines. The horizontal distance from the headrace inlet to the turbine house is 2.75 km, and the project comprises more than 4 km of tunnels when the various access and drainage tunnels are added.

After the turbine house a small dam will be used to regulate the flow into the 27 km downstream channel. This is being constructed to deposit water into the Xe Bang Fai River, which itself flows into the Mekong.

NT2 also includes the construction of a 138 km long double circuit 500 kV transmission line from the turbine house to the Thai border, which will be the means of exporting electricity to Thailand. An additional 115 kV transmission line will be built to distribute the Laotian portion of the electricity.

Progress

Preliminary construction work, such as the preparation of access roads, got underway in mid-2004, with work getting into full swing from June 2005, following the signing of the finance agreement. EDF is described as the main contractor for NT2, but its direct involvement only relates to the mechanical and electrical engineering.

The civil engineering work has been subcontracted out into three packages - CW1, the dam and surface work, CW2, underground construction, and CW3, construction of the downstream channel. ITD is involved in all three packages, and it's project manager for NT2 is Turdtam Pongpattana.

According to Mr Pongpattana, civil work got underway in May last year, and at the time of IC's visit to the site in April it was about 25% complete. “The project is roughly on schedule - plus or minus 1% to 2%,” he said.

One of the biggest difficulties with the project is coping with Lao's five-month rainy season. To call the June to October period the ‘rainy season' is a magnificent understatement. The summer is characterised by torrential rains which completely flood access roads and bring construction work to a virtual standstill.

Mr Pongpattana said, “We stop work - 90% of the activities have to stop. We are in a very wet area and all the roads - in fact everything - floods. It should be better this year because we should be able to do some of the rock excavation in the wet season.”

Tunnelling

The CW2 contract is the most technically demanding, and ITD is carrying out this tunnelling work in partnership with Japan's Nishimatsu Construction Co. (NCC). The joint venture is using the drill & blast method to tunnel through the site's sedimentary mudstone, sandstone and limestone.

Atlas Copco has supplied a fleet of equipment and consumables for the work on a full maintenance contract with a fixed cost per drill metre guarantee. Makoto Oi, a deputy project manager for NCC and operations manager for the ITD-NCC joint venture said, “I prefer this kind of arrangement. It has meant we have had no problems with the underground rigs.”

Given NT2's remote location, about three hours' drive on unpaved roads from the nearest town, it makes sense that NCC has opted for the security this type of contract provides. It also means a steady and predictable flow of work for Atlas Copco over the course of the project, but the company has also had to invest in the site.

Its Thai sales company has set up a workshop close to the headrace access tunnel, and has several staff on site full-time, including one for each of the four tunnelling jumbos that will eventually be working there. According to Manrat Payapanon, business manager for Atlas Copco Thailand, the company will invest THB 20 million (US$ 520000) in the on-site workshop over the course of the scheme.

The workhorses for the underground work are three two-boom L2C tunnelling drill rigs. When IC visited the site two of these were at work on the headrace tunnel, where two faces are being advanced simultaneously.

The first boomer has been on site since last May, where it first excavated the access tunnel to the downstream end of the headrace tunnel, before progressing upstream to advance this structure. In just under a year it had clocked-up 1130 percussion hours for the two rock drills combined, and achieved 164400 linear metres of drilling using a 48 mm diameter bit and 4.88 m long, 35 mm diameter R35 rods. The second L2C is advancing the headrace tunnel at the upstream face. With each drilling 4 m deep rounds, the total rate of advance being achieved is about 150 m per month.

While the sedimentary rocks do not contain any silica, and so are not particularly abrasive, there is a need to reinforce the tunnels. Mr Oi said, “We are mainly using bolting with shotcrete and mesh, although we have used some arches in the weak zones.”

The tunnels are being driven using the benching method, which is to say in this case the boomers are leaving the bottom 2.5 m of the cross section un-excavated. Surface drill rigs will follow on behind to drill into this bench, which will then be blasted to take the tunnels to their final diameter. This is a popular technique for driving large diameter drill & blast tunnels because it means the desired cross-section can be achieved without having to use an impractically large boomer.

Besides the technical challenges inherent in the construction of the scheme, NT2's remote location, and the fact that it is in an under-developed communist country presents a range of additional headaches.

Mr Oi said, “It's difficult to find skilled workers and difficult to get materials and spare parts because everything has to be imported. It takes months. We try to keep lots of spare parts here, but the distance is still a problem. For example, we have had to wait four months for a ventilation fan from Sweden.”

Finding good machine operators for the tunnelling work has also been very difficult, and the joint-venture has had to bring in people from all over the south-east Asia region. “We have employed miners from the Philippines, Malaysia and Nepal. It's very difficult to find skilled workers locally,” said Mr Oi.

Downstream

After the turbine house, water will flow into a pond controlled by a regulating dam before being discharged into a 27 km long channel that will feed into the Xe Bang Fai. Rock excavation here is being carried out with a fleet of ECM 580 surface drill rigs from Atlas Copco, and ITD has also invested in a huge fleet of new Volvo machines to work here and on the Nakai Dam site.

Last year Volvo supplied 66 machines to the scheme, including 49 excavators. Most of these (33) were 20 tonne class EC210B LC models, but there were also 29, 36 and 46 tonne machines in the package. The fleet also includes eight graders, four 25 ton (22,7 tonne) haulers and five wheeled loaders.

Tunnel construction is expected to take another three years, with completion due in late 2009. At the upstream end of the project, the diversion tunnel was completed at the end of last year, allowing work on the Nakai Dam to get underway. This is scheduled to carry on over the next two dry seasons, with completion expected in mid-2008.

The overall construction time is put at 54 months, starting from June last year, so completion of NT2 is due in late 2009. At the peak of activity, the scheme will have 400 people working on it.

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