Qualifying rounds in Colombian project19 August 2004
Contractor pre-qualification bids for Colombia’s Porce III hydroelectric project have been invited until this September. A technical description of the scheme and the work carried out to date follows
RECOGNITION studies for the hydroelectric potential of the Porce river were made between 1974 and 1976 by the company Consultores Técnicos Ltda. Between 1982 and 1983, Ascón Ltda carried out feasibility studies on the hydroelectric potential of the river, and in December 1984 it submitted to Empresas Públicas de Medellín E.S.P. (EE.PP.M) the feasibility report of the Porce III project, one part of the development of the river basin, which included five likely hydroelectric projects – Porce I, Porce II, Porce III, Porce IV and Ermitaño.
In 1986, EE.PP.M undertook complementary studies to those made by Ascón and, in early 1996, contracted with the consortium Ingetec S.A. - Hidramsa - CCC the final complementary study for the technical, economic and environmental feasibility of the project.
In 2001, EE.PP.M carried out the conceptual design and updated the environmental impact assessment. In January 2002, the consortium Ingetec S.A. - klohn-crippen Consultants Ltd, was awarded work for the design and advice during the construction phase. On 16 April 2003, the national government declared the works influence zone a public interest area, and on 16 May that year, the Environmental Ministry approved the project environmental license by issuing Resolution No. 0561.
Porce III will be the largest power station in the EE.PP.M generation system, and is the culmination of a long-term planning process to develop Porce river hydraulic resources and to take advantage of existing works and facilities owned by EE.PP.M in the zone. With the completion of this project EE.PP.M will reach the maximum percentage allowed by law; equal to 25% of the Colombian installed capacity.
The Porce III hydroelectric project is to be located in the northeast of the Antioquia department, 147km from Medellín, with areas in Amalfi, Anorí, Guadalupe and Gomez Plata municipalities. EE.PP.M will use part of the existing infrastructure in the area for construction, and will take advantage of the regulation capacity of the Porce II and Riogrande II project reservoirs and the Troneras and Miraflores reservoirs, which are part of the Guadalupe system.
Porce III will have an installed capacity of 660MW, with an annual generation of 3106GWh. Its main source of water supply is the Porce river, (known as the Medellín river for the first 60km.) It streams on the south mountains of Medellín, about 2800m asl, and descends crossing the central part of the Antioquia department in a northeasterly direction, yielding its waters into the Nechí river, an affluent of the Cauca river, after a course of 227km.
Hydroelectric development of the river basin comprises the Troneras, Guadalupe III, Guadalupe IV, Niquía, La Tasajera and Porce II power plants, and the Riogrande, Troneras, Miraflores and Porce II dams. Deviation works of the Nechí river basin, which includes the deviations of the Nechí, Pajarito, Dolores and Tenche rivers are also part of this development.
The waters of the Negro, Buey, Piedras and Pantanillo rivers are diverted to the Medellín river, after being served by the Medellín metropolitan aqueduct. The reservoir storage capacity of this system is 647Mm3.
The reservoir will have a total capacity of 170Mm3 of which 127Mm3 corresponds to the useful reservoir; an area of 461ha and a hydrographic basin of 3756km2. The reservoir is to begin in the Puente Acacias site, about 14km upstream of the dam.
The concrete faced rockfill dam (CFRD) will have a height of 151m and a base width of about 400m. It will be constructed from rock from the spillway excavation. Its crest length will be 426m and it will have a rock fill of 4,155,000m3. The upstream concrete face of 57,000m2 will be used as a waterproof slab with a variable thickness between 0.3m–0.65m.
In order to start work on the dam, a river deviation tunnel will be located on its left side. It will be 653.3m long and 10.5m in diameter. The deviation tunnel will be closed by means of two 9m high x 4.5m wide wheel gates before the reservoir filling. It will then be closed permanently by means of a 23m long concrete plug. A 37.5m high low cement content RCC cofferdam will be constructed to create a dry zone for dam construction. A rockfill provisional cofferdam with its crest at 553m asl will also be constructed to drive the river waters towards the deviation tunnel.
The bottom discharge will be used to control reservoir filling; to partially drain the reservoir in case of emergency or revision conditions; and to supply 2m3/sec of ecological discharge. The bottom discharge tunnel will be constructed on the left side and controlled by a radial gate and a sliding guard gate 3.5m high x 2.5m wide, that will be located in an underground chamber, accessed by a 251.5m long access gallery.
The spillway will be an open channel structure, excavated on the left side of the dam, with a discharge capacity of 10,850m3/sec; the flow at the site after the maximum calculated possible flood of 11,700m3/sec. It comprises an intake curved channel; a flow control structure formed by a spillway with the top edge at 664.2m asl; four radial gates 12.5m x by 16.5m high; a discharge channel 62m constant wide, 206.5m long, and a radial baffle to dissipate the spilled water energy and discharge back into the river.
Water intake system
The water intake system will consist of the submerged type intake structure, the upper headrace tunnel section and the gate shaft. The water intake will be placed on the left side of the river and will be built as a steel reinforced concrete structure with steel racks. The entrance level will be placed at 604m asl and was designed for a 235m3/sec discharge.The intake system works will comprise: submerged water intake structure; gates shaft; two wheel gates 5m wide x 7.8m high; upper headrace tunnel; and the gates shaft drainage gallery.
Power station works
Water will be driven from the reservoir to the four generation units through the headrace tunnel, which is divided into: an upper section 12.301m long, 10.2m in diameter and with a 1% slope; and a lower section 304m long, 10.2m in diameter and with a slope of 7.7%.
Both sections are connected by a 149m long vertical shaft of 10.3m diameter. The lower headrace section will connect the four generating unit branches through a steel-lined tunnel 6.1m in diameter. To ease headrace tunnel construction, three work fronts will be opened; the first at adit no. 1 which is 520.6m long, the second at adit no. 2 which is 648.9 m long, and the third at the 465.2m long adit no. 3.
The underground power house will be placed upstream of El Boqueron stream, on the left side of the Porce river. It will be 121.15m long, 18.2m wide and 40.7m high.
The power house will have four generation groups, each with a 172MW Francis turbine for 322m net head and 234.8m3/sec total discharge, and a three phase synchronous generator 218MVA for 13.8 or 16kV, that will turn at 360 or 400rpm. Each turbine will have one spherical guard valve 2.5m in diameter.
The turbine will have a vertical shaft with its runner centre line placed at 306m asl. In the power house, there will be two travelling bridge cranes of 190t capacity and 18m span for equipment erection, assembly and maintenance.
Two transformer banks and one spare transformer will be erected on the transformer cavern. Each bank will be formed by three 145MVA mono-phase transformer 13.8 or 16kV/500kV.
Along the cables and ventilation gallery two three phase circuits will be installed with 500kV cross linked polyethylene (XLPE) cables, which will conduct power from the transformers up to the cables connecting frame, standing between the power house access tunnel and the cable and ventilation gallery entrances. To transmit the power from the connecting frame to the main switchyard, there will be two 500kV and 6.1km long aerial lines.
The power house will have an access tunnel 493.25m long, with a vertical wall horseshoe section, 7m excavation diameter and 7.6% slope.
After leaving the turbine, water will be returned to the Porce river by means of a tunnel system with the following features:
• Draft tube tunnels – four horizontal draft tube tunnels for each generating unit will be constructed, with a variable length between 50.25–57.05m, and vertical wall horseshoe section of variable height, from 7.94m upstream to 7.75m downstream.
• Discharge header tunnel – the discharge header tunnel will be 76.41m long, with a vertical wall horseshoe section of variable width between 5.7m and 10.4m and will be full section
• Tailrace tunnel – the tailrace tunnel will be 827.45m long, have a slope of 1.627% slope and vertical wall horseshoe section with 4.9m high at the walls and semicircular vault of 5.1m radius. The tunnel exit level will be 310.3m asl, at the top edge of the discharge structure spillway. The discharge structure will be built with reinforced concrete at the tunnel end with lateral walls to drive water toward the Porce river.
• Downstream surge tunnel – Construction gallery no. 1 will be used as a downstream surge tunnel or tank which will be 348.03m long, have a slope of 10.3% toward the tailrace tunnel, with a vertical wall horseshoe section 8.5m wide and 8.7m high, 4.45m high at the walls and semicircular vault of 4.25m radius. The downstream surge tunnel will start at the power house access tunnel and will be used as a construction tunnel.
Power transmission works
The 500kV switchyard will be placed 6.1km away from the power house access tunnel entrance, on San Benigno hill. It will be of the conventional type with main and transfer bars and will have four connecting fields; two fields for connecting the power lines coming from the generation units and two for connecting the lines coming from the San Carlos and Cerro Matoso switchyards.
The power generated will be supplied to the ISA interconnected grid by splitting the existing 500kV line San Carlos - Cerro Matoso, which crosses over the Guadalupe IV power plant, and two new 23km lines.
The development of Porce III will require the construction of roads, bridges, camps and power lines for the works. Flooding of existing roads will make the construction of a 17.1km-long substitute road necessary. Additionally, in order to provide access to the main features of the project, as well as to the camps, substation, quarries and disposal areas, construction of 51.5km of new roads, plus the rehabilitation of another 60km of existing roads will be needed.
Camps will house EE.PP.M staff, while supervision and consulting personnel will be accommodated within the Primavera area. These facilities will later lodge the required offices, housing for 280 people, repair shops, restaurants, storage facilities and recreational areas. Additionally, use will be made of the existing Los Cedros and El Tablon camps to house staff and administrative personnel.
The contractor will be located, according to its own logistics, in areas near the project's main structures – that is dam and power house, respectively. In order to supply electricity to the various facilities, a 27km long, 44kV transmission line will be erected between the Guadalupe IV power plant substation and the Porce III power house.
To dispose of 14,383,900m3 from excess construction materials, 15 disposal areas have been selected and designed, with a combined storage capacity of 15,472,000m3.
Social and environmental management
In compliance with the environmental license and the project environmental management plan, EE.PP.M designed a Social and Environmental Management Plan aiming to prevent, mitigate, repair and compensate damage and enhance the positive effects produced by the project during its construction phase. This involves seven programmes:
• Restoration of living conditions – the purpose is to fully restore the living conditions directly affected by the project through a participative process and decisions agreed by the communities and EE.PP.M within the scope of in-force regulations.
• Harmonious coexistence – the aim is for the project and various social groups to co-exist, based on the timely and anticipated management of environmental impacts inflicted by construction activities.
• Coordination of municipal management – the objective is to coordinate project management with each municipality located within the affected area through mechanisms such as land ordaining schemes, municipal development plans and the promotion of training in resource management.
• Communications – the purpose is to promote communication with individuals, groups and institutions related to Porce III, in order to ensure project feasibility in the region by means of disclosure strategies, direct communication and communicative strengthening of communitarian leaders.
• Environmental education – the objective is to ensure environmental education of construction personnel and to contribute with formal and informal environmental education actions developed by the Autonomous Regional Corporation (Corantioquia), municipal administrations, learning institutions, communities, contractors and personnel.
• New jobs – based on its experience in projects such as the La Sierra thermoelectric project and Porce II and La Herradura hydroelectric projects, EE.PP.M has developed a methodology for employment implementation that enables the participation of the local population in a fair manner and benefits the communities influenced by the project. Unskilled manual labour required for the works will be provided by the municipalities of Amalfi, Anori, Guadalupe and Gomez Plata, specifically by the hamlets located in the area under direct influence of the project. It is estimated that the project will directly employ some 2500 during its construction phase, plus indirect jobs generated during the development of related services.
• rchaeological programme – the objective is to implement measures that allow the retrieval and preservation of the archaeological heritage that could be affected by construction of the infrastructure works.
A quantitative and qualitative monitoring system for social management, similar to that used in the Porce II project, will contribute to the following goals:
• To perform, in a coherent and planned manner, the social management plan to deal with social, economic and cultural impacts, established in the environmental impact plan.
• To evaluate, during the different project phases, the results attained in compliance of such plan under diverse scenarios of the biotic and social environment.
Surveillance and epidemiological control system
The epidemiological surveillance system is an environmental management and public health programme that turned out to be quite successful during the construction and the beginning of operation of Porce II. It will continue to focus on project personnel and the population located within the area of influence of Porce III. It will develop preventive and timely intervention actions in order to monitor the health conditions of the inhabitants of the region during the construction stage and start of operations of the project. The application of this system started off in Porce III with the setting of the base line for the municipality of Anori.
Physical / biotic component
The objective is to establish and develop a follow-up and monitoring plan for the physical-biotic and social component during project construction and future operation. This follow-up is performed on aspects such as climate, hydrology, sedimentation, erosion, land vegetation layer, aquatic vegetation, land fauna, ichtic fauna, quality of air, instability and erosion phenomena, quality of water, landscape and water table draw down in streams located over the pressure tunnel.
This information was provided by the Empresas Publicas de Medelling E.S.P. Communication and Generation Projects Departments. Contact Juan Carlos Gutierrez at: email@example.com