The Nam Theun 2 Hydroelectric Project is the most important project in a long-term collaborative effort between the Lao People’s Democratic Republic (Laos) and Thailand to develop up to 3000 MW of hydropower energy in Laos for export to Thailand. The Nam Theun River, a major tributary of the Mekong River, is situated on the Nakai plateau between the Mekong River (the Lao-Thai border) and the Annamite Mountains delineating the eastern border of Laos with Vietnam. The project entails a trans-basin diversion of waters from the Nam Theun River to the Project’s powerhouse located at the base of the Nakai escarpment and from there to the Xe Bang Fai River, another tributary of the Mekong.
Klohn Crippen Berger (KCB) was involved with the Nam Theun 2 Project since the mid 1990s when we first evaluated the project layout and recommended a roller compacted concrete (RCC) dam and integral spillway rather than a concrete face rockfill dam and separate spillway. Subsequently, KCB carried out value engineering reviews, site investigations and preliminary design for the RCC dam layout. In March 2002 the project development company, the Nam Theun 2 Power Company Ltd. (NTPC), led by Électricité de France (EDF), proceeded with project implementation by calling for competitive design-build tenders. The tender call was structured into three civil works packages (CW1, CW2 & CW3) and two electrical-mechanical work packages (EM1 & EM2).
KCB was retained by an International Design-Build Construction Consortium for bid design engineering of the two main civil works packages comprising the hydro facilities; namely, CW1 and CW2. KCB commenced final design services for the advance construction works in April 2004 in order to meet schedule constraints. KCB’s main final design effort commenced in late 2004 and continued until late 2008. Project in-service was December 2009.
Work Package CW1 facilities included the RCC Nakai Dam structure and integral reinforced concrete spillway, the diversion tunnel, upstream and downstream RCC cofferdams, ten saddle dams (four of which are major earth structures), site access and the headrace channel. The CW1 package earthworks are very extensive, comprising of more than 4.2 million m3/s excavated for the headrace channel and more than 0.5 million m3/s earth embankment design associated with the reservoir saddle dams.
Construction planning for the main dam and diversion system was problematic as the annual Nam Theun flood is large and could not be accommodated for the diversion design.
Consequently, KCB devised an innovative diversion scheme that had the construction of the Nakai Dam being done during dry-season windows over three years with the site area being flooded during the wet seasons. Using RCC construction for the main dam and both the upstream and downstream cofferdams provided the ability to overtop these structures, without failing, during the wet season. If RCC construction were not employed, it would have meant a requirement for a much larger capacity and higher cost diversion system. This would undoubtedly have impacted overall project economics.
Work Package CW2 facilities include the intake structure, intake gates, headrace tunnel, surge shaft, pressure shaft, pressure tunnel, penstock tunnel, units’ manifold, the main powerhouse and adjoining pelton units powerhouse, the tailrace and associated powerhouse area buildings and other civil structures. The CW2 underground works are large in scale and complex in design. This is especially true for the pressure shaft, pressure tunnel and penstock tunnel sections of the conveyance system where the normal pressure head is over 350 m (not including transients). The steel-lined portions require massive steel cans of considerable thickness. The steel manifold is a double bifurcation structure with critical support and pressure design requirements. The 7.2 m to 5.1 m-diameter main bifurcation designed for 500 m of pressure head is a world-class structure having the greatest pressure-diameter combination of any such structure in the world. The low-pressure headrace tunnel is a large diameter structure that is fully concrete lined.
The power from Nam Theun 2 will be supplied to Thailand under a Power Purchase Agreement that NTPC negotiated with the Government of Thailand through the state utility the Electricity Generating Authority of Thailand.