Doing Battle With Dam Deterioration

Raven Lining Systems

In 2001, the government of Uganda started to plan the construction of a new hydroelectric power plant at Bujagali Falls, which is about 8 km north of Lake Victoria, the source of the Nile. Within a short two years, the project was abandoned when the developer backed out due to objections from environmentalists. In 2007, a new group of developers formed a company called Bujagali Energy Limited (BEL), which was quick to act on the concerns of locals and environmentalists, developing a community action plan that was designed to put to rest the concerns of everyone. This action plan included BEL refurbishing schools, improving roads, installing water lines and relocating wildlife and locals to a new village. With those tasks underway, BEL could begin construction on the new hydroelectric dam.

The new dam would be the third-largest energy source in Uganda. After a long delay BEL finally broke ground in June 2007. The contractor for the project was an Italian Construction company named Salini Costruttori. The new earth-fill dam would house five 50-MW turbines with the ability to produce 250 MW of electricity. The new hydroelectric dam would ease Uganda’s need for electricity—approximately 97% of Ugandans had access to electricity. By June 2012, the dam was operating at full capacity with a total construction cost of $900 million.

Technical Situation

After only a few years of operation, the interior concrete walls of the spiral cases and draft tubes inside the Bujagali hydroelectric dam were showing severe deterioration. CLT Labs tested the concrete and found that the exposed aggregate surface was approximately CSP 7. Their findings were inconclusive, only stating that the top 50 mm (2 in.) were weaker than the rest of the structure. CLT could not give Bujagali a quantitative answer—only that it was weaker. The excessive deterioration was caused by this weakened state of the concrete combined with bacteria and the extremely high velocity of the river. A solution was needed quickly, as delayed repairs would cause excessive damage, resulting in astronomical costs.

Of the many challenges associated with this project, the biggest by far was the timeframe. The job would need to be completed during five separate outages. A single turbine could only be shut down for a seven-day outage once a year. This would be at an estimated cost to BEL of $6,000 to $7,000 per day.

Within these outages, an enormous amount of work would need to be completed. Each outage would consist of coating a combined 25,000 sq ft in both a spiral case (10,000 sq ft) and a draft tube (15,000 sq ft). The spiral case is a spiral-shaped concrete intake that guides the flow of water into the wicket gates located just prior to the turbine on the upstream side. It has a maximum height of 32 ft, which slopes and curves to the lowest height of 8 ft (ensuring the need for scaffolding). The draft tube on the downstream side is a curved shaft that drops 30 ft to release the water. It maintains a column of water from the turbine outlet and the downstream water level. Access to the draft tube was limited to a 2-ft square opening 30 ft above the floor. All scaffolding, materials and supplies would need to be brought in through a 3-ft opening four stories underground. The dam has a total of approximately 125,000 sq ft to be coated.

Other challenges included limited protective coatings in Uganda, limited applicators, no plural spray equipment, high humidity (constantly 87%) and limited supplies.

Solution

With the yearly planned outage approaching, BEL needed to find a product and applicator for its unique situation. Unable to find a coating or applicator within Uganda, it needed a manufacturer that could meet all its needs. Using the CLT Labs recommendations, BEL chose Raven’s AquataPoxy A-6.

Having dealt with many environmental issues early on, BEL wanted a product that was NSF/ANSI 61-certified to protect the river’s ecosystem. With the chambers maintaining a constant 87% humidity, it needed a product that was not moisture-sensitive. AquataPoxy A-6 offered the flexibility BEL needed to get the job underway. Raven would directly ship the AquataPoxy A-6 and any needed supplies that BEL was unable to obtain within Uganda. BEL decided to self-perform the coatings application during the first outage. Raven Technical Service Representative Ryan Bauman would travel to Uganda for the first outage and application of AquataPoxy A-6. He would be training local labor, consisting of brick masons, painters and mechanics, most of who worked on the construction of the dam. Special attention and considerable time would be spent with the two leads that would oversee the future applications.

Description of Application

During each outage both the spiral cases and draft tubes were drained and all remaining algae, fish, solids and debris were physically removed. A 4,000-psi hydro-blast with a zero-degree rotating tip was used over the entire structure to achieve a clean, sound surface while removing any remaining algae. After the surface was hydro-blasted, it was evident that a previous repair product was used to repair cracks during construction. It was documented that the previous product was Concresive 2200. After all areas that had Concresive on the surface had been ground with a hand grinder, a solvent wipe was conducted to ensure all dust was removed from the surface. After the substrate had dried, a single application of AquataPoxy A-6 Thick was applied. The 100% solid epoxy was mixed at room temperature and then applied by trowel. The finished millage was to be 60 mils. After the coating was tack-free, a visual check for holidays was performed. Any pinholes found after the visual holiday check were repaired within the recoat window. Pinholes discovered after the recoat window had expired were prepared and then repaired according to Raven recommended procedures.

“Raven is pleased to be part of providing our AquataPoxy A6 as a proven sustainable engineering solution to protect this delicate ecosystem on Lake Victoria and the Nile River,” said D. J. Wroble, senior vice president, Raven Lining Systems. “One of our goals is to continue expanding internationally to meet difficult challenges like we experienced in Uganda.”