Klohn Crippen Berger Nam Theun 2 Hydro

The 1,074-MW Nam Theun 2 Hydroelectric Project is part of a long-term collaboration between Laos and Thailand to develop up to 3,000 MW of energy in Laos for exporting to Thailand.

This project entails a trans-basin diversion of waters from the Nam Theun River to a powerhouse at the base of the Nakai escarpment, and from there to the Mekong River. lectricit de France, the majority owner of the Nam Theun 2 Power Company, envisions that the project will provide “inexpensive electricity to drive human development.” The hydro project will produce power for about three million Thai homes, and will contribute annual revenues equal to 7-9% of the Lao national budget.

On behalf of the International Design-Build Construction Consortium Joint Venture (ITD-NCC JV), the Vancouver office of Klohn Crippen Berger provided civil, hydrotechnical, structural, geotechnical, electrical and mechanical engineering services for the project’s two main civil works packages.

High Floods Called for Roller-Compacted Concrete Construction

The Nam Theun River has very large annual floods, so Klohn Crippen Berger designed an innovative method for flood handling that was economic and advantageous for the construction schedule. They convinced the ITD-NCC JV that rather than constructing two or more diversion tunnels to handle the flood flows during construction, it would be better to use one diversion “D” shaped tunnel and roller-compacted concrete construction for not only the main dam, but also the upstream and downstream cofferdams. Roller compacted construction has the same ingredients as conventional concrete (i. e. cement, aggregate, sand, water) but is a drier, stiffer mix suitable for compacting by vibratory rollers.

The RCC construction enabled the cofferdams and the main dam elements to be overtopped and flooded during the wet season. At the end of the wet season the structures were intact and all that was necessary before construction could resume was de-watering and cleanup of the work area between the cofferdams.

The main dam is 39 metres high, with a crest length of 320 metres, while the “D” shaped diversion tunnel is 8.6 metres high, 240 metres long and has a flow capacity of 550 m3/s.

The underground works are large in scale and complex, requiring Klohn Crippen Berger to exercise the most advanced design and analysis techniques. Especially challenging were the pressure shaft, pressure tunnel and penstock tunnel sections of the conveyance system.

The headrace tunnel is 9.2 m in diameter and the underground chamber for the main penstock bifurcation spans 25 m. Near the powerhouse, the normal pressure head is over 350 m (3.4 MPa) and up to 500 m (4.9 MPa) under transient load rejection conditions.

Huge Penstock Bifurcation

The main penstock bifurcation is one of the largest, if not the largest, pressure x diameter bifurcations (i. e. “Y Branch”) ever designed and constructed. Klohn Crippen Berger performed complex computer analyses using finite element methods to produce a design that establishes a precedent in the hydro industry for the use of the selected steel types and thicknesses. The main bifurcation uses HT80 high-strength, quenched and tempered steel (minimum ultimate strength 760 MPa) with plate thicknesses of up to 75 mm. The bifurcation’s internal stiffener is 170 mm thick, one of the thickest steel elements every produced in this type of steel.

Protecting the Landscape

Whether temporary or permanent, all the constructed facilities were designed according to best environmental practices. For example, ditches and settlement ponds were used to convey run-off waters to defined water courses without causing erosion of the landscape.

The project was completed in December 2009 and achieved commercial operation in April 2010.

Together with the World Bank, Laos is using revenues from the Nam Theun 2 project for reducing poverty. For the people in the project area on the Nakai Plateau, long-term benefits being provided include housing with electricity, schools, new roads and a clean water supply.

The project also provided 650 boats and fishing equipment for villagers to access the fishing grounds in the new 450 sq. km. reservoir.

Project name: Nam Theun 2 Hydro Project, Lao People’s Democratic Republic

Award-winning firm: (prime design consultant for main hydro civil works) Klohn Crippen Berger, Vancouver (Shane Johnson, P. Eng., Garry Stevenson, P. Eng., Neil Heidstra, P. Eng., Ryan Douglas, P. Eng., Bruno Bagnrs, P. Eng., Manuel Reyes, P. Eng., Simon Douglas, P. Eng., Dirk Duivestein, P. Eng., Lubos Petrik, P. Eng., Ron Braam, P. Eng.)

Project owner: Nam Theun 2 Power Company/Electricit de France

Client: JV Design-Build Contractor/ Italian-Thai Development/Nishimatsu Construction