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Hydraulic Structures: Dam Safety Progress and Challenges at Electricite de France

To ensure the safety of its 450 hydroelectric projects, Electricite de France has used risk analysis to prioritize required maintenance. Work now underway will cost 500 million euros (US$716 million) and help ensure the continued reliable operation of these important hydraulic structures.

By Alain Petitjean and Bernard Denis

Dam safety is a priority for Electricite de France (EDF) that, if necessary, prevails over electricity generation. To meet this priority, in 2005 the utility employed a risk analysis method to identify the priorities for specialized maintenance for each category of equipment (such as dams, penstocks, gates, canals, and tunnels). And the following year, EDF created its SuperHydro program, which is intended to provide a framework for repair or refurbishment of the structures and equipment most critical to the continued safe operation of its hydro facilities.

As a result of this work and prioritization using the SuperHydro program, EDF plans more than 300 maintenance operations between 2007 and 2011, for a total cost of more than 500 million euros (US$716 million). Most of these operations involve gate or penstock work, such as replacing or refurbishing gates and replacing or painting penstocks.

Despite this progress, EDF's dam safety focus is not without challenges. In December 2007, a legal decree in France classified dams and dykes into four categories according to their height and volume. As part of this decree and classification, new documents must be submitted to the control authority within the next five years and updated every ten years, including danger studies and periodic safety reviews. At EDF, this creates the challenge of producing 239 danger studies and 149 periodic safety reviews within five years.

Launching the SuperHydro program

EDF operates about 450 hydro plants. Their construction dates stretch from the end of the 19th century to 2009, and the average age is about 60 years. In 2005 and 2006, several incidents at these plants showed the need to better detect the progression of aging and to schedule suitable maintenance operations.

In 2005, EDF introduced a risk analysis method intended to guarantee long-term generating efficiency at its hydro facilities. The method identifies the principal failure mechanisms, their probabilities, and potential consequences. For purposes of the risk analysis, EDF divided structures and equipment into 17 groups, also called families. For each family, the utility identified and described the main failure mechanisms. The consequences then were evaluated using a common method for all families.

From 2006 to 2008, every structure and piece of equipment was assessed. The resulting “risk maps” provide a roadmap for maintenance programs.

In the short term, measures have been taken to quickly and temporarily control the most important risks while a permanent repair solution is found. For example, one short-term measure might be to lower the elevation of the water in a reservoir behind a dam.

Over the longer term, EDF launched the SuperHydro program. This program is intended to guide the repair or refurbishment of the structures and equipment most critical to the continued safety of the hydro project, with a focus on penstocks and gates. The program has a total budget of 500 million euros (US$716 million) over five years.

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Replacement is underway on the lower section of the right bank penstock at the 180-MW Pragneres facility. This work is needed to ensure continued safety of the facility.

Penstock maintenance

There are about 600 penstocks associated with EDF's hydro projects, representing a total length of about 260 kilometers. This inventory includes different construction techniques, such as: riveted construction; self-hooped pipes to limit the thickness of the plates to be assembled; and welded structures.

At the end of 2006, EDF began a diagnosis campaign for penstocks. The first step was to develop a complete inventory of all penstocks, using information available from the operator. This inventory took into account both the risks peculiar to each structure (essentially the safety and economic aspects) and an overall assessment of the degradation modes (corrosion, leaks, deformations, etc.).

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Work to replace the eight gates at the 32-MW Tuilieres project was completed in January 2009. Electricite de France decided to replace the gates after experiencing a noncontrolled opening of one of the gates in January 2006.

Once the listing campaign was complete, the most critical penstocks were subjected to an in-depth diagnosis. This diagnosis included all or some of the following investigations: historical analysis, external and/or internal inspection, measurement of residual plate thickness, visual inspection of singularities, visual inspection of civil engineering structures, measurement of maximum pressures in the various operating configurations, calculation of the current safety factors, and analysis of the pathologies specific to certain technologies (local weakening of riveted assemblies by corrosion, stress corrosion of hoops, weld defects, etc.).

At the end of 2008, EDF concluded there was a need to replace 30 sections of penstock. In addition, about 60 penstocks will undergo maintenance operations, including renewal of the anti-corrosion protection or restoration of the civil engineering structures (such as anchor blocks and support pillars).

Certain replacements are under way. For example, EDF is replacing the lower section of the right bank penstock of its 180-MW Pragneres facility. The plant supplies electricity during periods of peak demand and thus is strategic for the region. After an incident in 2005, EDF undertook an expert analysis of the entire penstock. To ensure safety of the facility, EDF decided to cease operation and rebuild 700 meters of the 1.6-meter-diameter penstock. This work is taking place in a difficult environment. The plant is located at more than 2,100 meters in altitude. About 1,000 tons of steel must be installed on slopes of more than 45 degrees, and it is only possible to transport loads via helicopter or a renovated cableway. EDF expects the facility to return to full capacity by the end of 2009.

Gate maintenance

The 32-MW Tuilieres project is an example of gate work being performed by EDF. In January 2006, EDF experienced a non-controlled opening of one of the eight gates at Tuilieres. Tuilieres is a gate-structure dam fitted with eight lift gates. The powerhouse contains eight Kaplan units that operate with an average head of 12 meters. Fortunately, there were no negative consequences of this gate opening on the people downstream from the dam.

Immediately after this incident, EDF decided to renovate the dam completely. This work included replacing the gates and their operating devices and replacing the upper lifting equipment support gantry. To increase spillway capacity at this dam, EDF raised the upper gantry crane and all the companionways. This enabled the utility to increase the maximum discharge through the gates by 775 cubic meters per second (cms), to 6,725 cms from 5,950 cms.

In 2006, EDF demolished the eight gates and the upper metal gantry crane. The subsequent renovation work was carried out during two distinct periods, to allow control of the inflow to the plant. In 2007, EDF renovated the four gate bays on the right bank, installed a new upper metal gantry, and installed the first flood gates. In 2008, EDF renovated the four gate bays on the left bank, installed the remaining new flood gates, built a fish passage facility on the right bank, and modernized the plant. The total cost of the work at this facility was about 37 million euros (US$53 million), including 26 million euros (US$37.3 million) for renovation of the dam and 6 million euros (US$8.6 million) for the generating plant. EDF completed work on this facility in January 2009.

Challenges posed by new dam safety regulations

In 2006, a new law on the aquatic environment modified the legislative framework governing the safety of hydraulic structures in France. This law was completed with a decree dated December 11, 2007, which introduces new requirements for dam and/or levee owners with respect to public safety around dams and use of lands along rivers. Under this decree, dams and dykes are classified in four categories, according to their height and volume:

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Owners or operators of dams classified as A must provide a risk assessment study and periodic safety study audit, with an update every ten years.

The goal of a risk assessment study is to identify and quantify risks for the public (fatalities, injuries, losses) upstream, around, and downstream from dams and levees. All existing and potential risks arising from the presence of the dam or levee must be assessed, including the magnitude and effects of recognized dangerous situations: disaster due to extreme (dam failure) or less serious (injuries) hazards, as well as less rare situations of risk (such as gate failure). The risk assessment study also states how owners and operators identify, monitor, and overcome dangerous situations and how they implement appropriate solutions to mitigate issues and reduce consequences.

The safety review is intended to “draw up an assessment report on safety of the dam and its components.” It reexamines and supplements previous regulatory practices (monitoring reports, ten-year visits, behaviour analyses) by extending the field of safety review to the safety components of the dam, and particularly to the gates.

As a result of this new regulation, EDF faces the challenge of producing 239 risk assessment studies and 149 periodic safety audits. These documents are due before the end of 2012 for dams classified as A and before the end of 2014 for dams classified as B. To reach that aim, EDF has created a project involving the different engineering and hydraulic operation contributions to dam safety. The first studies have not revealed safety risks that were not identified previously. However, they are providing a more global assessment of safety, including all risks and taking into account the gravity of their consequences.

Alain Petitjean is head of the risk management and sustainable development department and Bernard Denis is deputy head of the civil engineering department with Electricite de France's Hydro-engineering Centre.

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