October 03, 2009
Tunnels And Penstocks of The Nam Theun 2 Hydroelectric Project
Abstract
The 1,070 MW Nam Theun 2 Hydroelectric Project is located in central Lao PDR. The tunnels and penstocks convey the power flow from the reservoir impounded by the Nakai Dam, down to the surface powerhouse located at the base of the escarpment on the Nam Kathang River. The gross head of the project is 360 m. The maximum power flow is conveyed through a concrete-lined headrace tunnel, down a concrete-lined pressure shaft, through a steel-lined high pressure tunnel to steel-lined bifurcations where the water is conveyed to four Francis turbines, each 250 MW, and two Pelton turbines, each 35 MW. The headrace tunnel, 1,500m long, was excavated through near horizontally bedded sandstone and siltstone rock at the upstream end which then gradually transformed to steeply dipping sandstone and siltstone rocks. The 270 m deep vertical pressure shaft and the 1,000m long near horizontal, high-pressure steel-lined tunnel were excavated through these steeply dipping sandstone and siltstone rocks. The bifurcations and penstocks are designed for a peak transient pressure of 4.9 MPa. A concrete-lined surge shaft at the downstream end of the headrace tunnel limits the maximum transient pressures in the high-pressure tunnel. The project presented some unique design challenges for the design of the steel-lined high-pressure tunnels, bifurcations, tunnel plugs, hydraulic throttle in the surge shaft and an access door for maintenance in the downstream end of the headrace tunnel.