Last year, Enel Green Power completed the Apiacas hydro complex in Brazil’s Amazon Rainforest, utilizing three small run-of-river facilities, which Enel says helped minimize environmental impact.
By Jonas Brito, Christian Mendes, Roberto Troiani and Ryan Berg
Rome, Italy-based Enel opened the 102-MW Apiacas hydroelectric complex in Brazil, on the Apiacas River, in November 2016 through its Brazilian renewables subsidiary, Enel Green Power Brazil.
Construction on the complex began in December 2014 and, including a US$115 million loan agreement from the Brazilian Development Bank, Enel invested about $287 million. The investment is supported by a 30-year power purchase agreement with Brazilian electrical energy agency (ANEEL).
Developing individual project sites for the Apiacas hydroelectric complex had many engineering challenges, and some of the challenges were made more difficult compared to other locations because of the complex’s remote location within the Brazilian Rainforest.
The main challenges for project development included:
• EGP’s desire to construct three different powerhouses and appurtenant structures simultaneously and redesign the previous layout and material of Cabeca de Boi Dam in order to reduce overall project costs; and
• Re-envisioning the original river diversion schemes for each powerhouse because construction at each site would begin during the wet season.
This article will discuss these challenges, as well as engineering solutions and techniques used to successfully construct and commission the Apiacas hydroelectric complex.
Apiacas hydroelectric complex
The Apiacas hydroelectric complex is located in Mato Grosso, a central western state. The complex has a drainage area of 7,230 km², and the river basin has a long north-south shape. It is limited by Serra dos Apiaca-Sucurundi in the north, in the south by Caiabis Ridge and Parecis Upland, and its western boundary is Dardanelos Plateau.
A tributary of Teles Pires River, Apiacas River is about 349 km long and its headwaters are located in the Caiabis Mountains at elevation 380 m above sea level (masl).
The Apiacas River flows through very rugged topography.
At a river distance of 122 km from the headwater, the river has an abrupt elevation change of 65 m in a river distance of only 6 km, resulting in a slope of 0.011 m/m. After this, the river flows in a flat section until the next elevation drop at river distance 190 km from its source. The mouth of the river is located at elevation 137 masl, resulting in a total vertical drop of 243 m.
The complex comprises a series of three run-of-river facilities situated in the following sequence from south to north: 30-MW Cabeca de Boi, 45-MW Salto Apiacas and 27-MW Da Fazenda (see Figure 1).
Overall, the project has seven Kaplan turbine-generator units, with three at Salto Apiacas and two each at Cabeca de Boi and Da Fazenda. Of note, the Da Fazenda powerhouse contains the world’s largest S-type Kaplan runner with a diameter of 3,665 mm under operation.
Enel estimates the Apiacas hydroelectric project will annually produce an estimated 490 GWh.
Figure 1: Apiacas Hydroelectric Complex
Cabeca de Boi
Cabeca de Boi powerhouse is located about 210 m upstream of the confluence of the Apiacas and Cabeca de Boi rivers. Project components include a rockfill dam with its structural axis spanning the Apiacas River and a roller-compacted concrete (RCC) dam that has a free-discharging spillway to the left margin. The rockfill dam is 27.6 m high and has a 465-m-long spillway. The powerhouse is located in the right margin of the river and includes an intake structure and two 4.2-m-diameter penstocks and a 255-m-long tailrace channel.
Cabeca de Boi Dam optimization
One of the most important phases of the overall development of the projects was optimizing Cabeca de Boi Dam, reducing the volume of clay with rockfill alternative.
Using the results of an additional geotechnical campaign conducted by Sondosolo, the layout of the dam was totally revised by Hatch and the structure was moved 200 m upstream to take advantage of a the newly discovered geologically sound rock formation of meta-arenite and meta-siltite. With this new solution, the main economical parameters of the overall project were improved.
The Salto Apiacas powerhouse is located about 1.7 km downstream of the Cabeca de Boi powerhouse. The project has an RCC dam 15 m high and a free-discharging 336-m-long circular segmented spillway. The powerhouse is located in the right margin of Apiacas river and includes an intake structure and three 4.2-m-diameter penstocks. The facility also features a 165-m-long tailrace channel.
The Da Fazenda powerhouse is located about 1.3 km downstream of Salto Apiacas. Da Fazenda dam is an RCC dam that is 15.5 m high and has a free-discharging 350-m-long spillway and two radial gates that are 4.8 m wide and 7.6 m high. The powerhouse is located on the right margin of Apiacas river and includes an intake structure, two short penstocks that are 5.5 m in diameter and a 138-m-long tailrace channel.
The Da Fazenda powerhouse has the largest S-type horizontal double regulated Kaplan turbine under operation.
The turbine runner was designed for a net head of 15.11 m and configured for three mobile blades, each blade having an external diameter of 3,674 mm.
The group generator was manufactured by WEG-HISA and was synchronized in July 2016. The generator couples directly to the turbine shaft, positioned upstream of the turbine rotor.
Due to the runner size and being one of the most critical components of the turbine assembly, Enel invested in developing detailed three-dimensional computational fluid dynamic (CFD) models to study possible optimizations and to verify all assembly interfaces. Finite element analysis (FEA) was performed on the most critical components for all operational conditions of the turbine.
The runner also underwent structural verification of its mechanical parts that included both static analysis (i.e., HUB, guides, bearings, etc.) and dynamic analysis (i.e., blades, activation, linkage, servomotor, etc.). In April 2015, Hydro Consulting GmbH performed validation of the turbine’s hydraulic design in Germany.
Based on proposed operational condition data for the turbine, CFD simulations verified the acting forces and the maximum deformations on its components.
The anticipated diversion works for the construction, originally designed for use during the dry months, were redesigned for use during the wet season due to delay on environmental license.
The Amazon River basin can increase in flow up to seven times its normal amount between the wet and dry seasons, which affected construction sequencing. Various flood return periods were calculated using daily data from seven hydrometric stations of Mato Grosso state. Data from similar hydrological behavior were used to determine what type of diversion structures would be required for construction. It is interesting to note, the monthly period between September and March is equal to the annual design flood. The evaluated periods are presented in Table 1.
The project was originally designed to have two temporary cofferdams that protected the construction site while the river remained in its natural channel. The cofferdams were designed to resist the calculated annual 50-year return period flood of 1,408 m³/s. Additionally, the river was to be diverted to the proposed spillway by constructing another cofferdam to allow the completion of the free-discharging spillway. The partially constructed free spillway of Da Fazenda could safely pass a dry period 10-year design flood.
Due to the change in the start of the construction season, it was necessary to revise the diversion sequence.
Quebec Engenharia was in charge of project construction and it determined the river would remain in its natural channel, with work areas being fully protected by cofferdams. Construction for this phase included: the water passage, bottom spillway, access bridge and part of the free spillway.
Also, as required by construction phasing, the remaining cofferdam construction was divided in two steps. First, the main cofferdam was breached to allow a flow equal to an annual return period flood of 10 years (1,160 m³/s) to flow through the access bridge and onto the bottom spillway and part of the partially completed free-discharging spillway. Second, after the wet season concluded in June and the bottom spillway could safely discharge all of the river’s incoming flow, the rest of the free discharging spillway was completed and the cofferdams were removed.
The original design used a series of cofferdams to protect the project site and was designed not to be overtopped during a 50-year storm event. During the dam construction phase, the cofferdam construction was re-designed to prevent overtopping by a 100-year storm event.
However, as construction began during the wet season, the diversion works had to be adjusted to accommodate much higher flow volumes. The cofferdam around the powerhouse was still constructed for a 50-year storm event, but for a wet season, a 50-year storm event increased the anticipated flood flow by more than 330% of the typical annual 50-year flood flow.
The second phase of the diversion works was divided into three steps. The first step was to excavate under a temporary access bridge, allowing the river to flow in its natural channel. In this step, another part of the spillway was built. The second step began the closure of the Apiacas River, and all of its flow had to pass through the access bridge galleries and the partially constructed spillway. In this step, the rest of the free-discharging spillway was constructed. The final step involved removing all of the cofferdams and the temporary access bridge and filling the reservoir according to the environmental and safety guidelines of Brazil.
Cabeca de Boi
Cabeca de Boi is just on the confluence
of the Cabeca de Boi and Apiacas rivers, which further complicated the temporary diversion works.
Originally, the two rivers were to flow in their natural riverbeds while the powerhouse, part of the spillway and the right margin rockfill dam, transition wall, diversion structure and forebay channel were built. The natural site topography was supposed to provide protection. Once completed, the water would then be diverted through the diversion structure and cofferdams would be constructed that would allow the remaining parts of the dam to be constructed.
Both rivers would be closed using cofferdams designed to resist a 50-year flood event during the dry season. This would allow for building the left margin concrete dam while the hydraulic circuit and powerhouse equipment were installed.
The first phase of the river diversion did not change from the original version because the site was protected by the natural topography, even during the wet season. During the second phase, a cofferdam was used to close the diversion galleries, connection channels and the Cabeca de Boi River. During this phase, part of the free-discharging spillway, the right margin concrete dam, upstream cofferdam to protect the right margin rockfill dam and access from downstream of the dam were constructed.
The second step of this phase included cleaning and treating the spillway foundation over the Cabeca de Boi riverbed, constructing a cofferdam downstream of the tailrace channel and final excavation of the tailrace channel. After that, the cofferdam over Cabeca de Boi River was removed in order to increase the discharge capacity of the site to accommodate the design flood return periods.
The last step of the diversion works was to construct the cofferdam in the Cabeca de Boi River in order to finish the concrete spillway during the dry season, to minimize the height needed for the cofferdams. In parallel with construction of the free-discharging spillway, the cofferdams that were constructed downstream of the tailrace channel were removed and the rockfill dam was completed to its design crest elevation. Once the dam was completed, the excavation of the intake channel entrance and removal of the access bridge and upstream cofferdam were finalized before filling the reservoir.
By the end of 2016, all seven turbine-generator units were commissioned and all three power plants were in commercial operation.
During the river diversion stage, even with the unpredicatable weather conditions at each of the facilities, there were no instances of cofferdam overtopping. Additionally, hydraulic structures that were responsible for diverting the Apiacas River performed as designed without the need for additional modification or added expense.
For the Cabeca de Boi Dam optimization, there were no situations related to water percolation in the dam, something common in rockfill dams when not built under strong criteria, indicating the adopted solution
The Da Fazenda project, with the large S-Kaplan turbine, reached its expected results related to its efficiency which was above 91%. No cavitation indicators were presented and the stresses applied in its components were within the results of the developed CFD simulations and calculations.
Overcoming the main challenges were only possible by the strong synergy between the design and construction teams, especially the hydrology and geotechnical departments of Enel Green Power Brazil. But also, the machine designers from WEG-HISA foresaw the design challenges and the required protections to manufacture and successfully operate such a large S-Kaplan turbine at the Da Fazenda project.
These three hydro plants were built in the middle of the Amazon Rainforest, with almost no environmental impact provided by their small reservoir size. This project is a demonstration and an example of how it is possible to harvest energy from nature without harming its sensitive ecosystem.
Jonas Brito is a member of the Hydro Centre of Excellence and Cristian Mendes is a hydro main machinery specialist at Enel Green Power in Brazil. Roberto Troiani is director of Hydro Design at Enel Green Power in Italy. Ryan Berg, P.E., is a principal engineer at Enel Green Power in the U.S.