Mt. Coffee Hydropower Plant Rehabilitation Project
The Mount Coffee Hydropower Plant is located approximately 27 kilometers (km) northeast of Monrovia in Montserrado County, Liberia. The plant was originally designed as a run-of-river scheme. A dam and a 10-bay spillway structure with radial gates were located in the original river channel. Inflows to the reservoir were diverted through a channel to the power house located at a bend in the river. This arrangement, in combination with an excavated tailrace channel, allowed development of the maximum hydraulic head at the Mount Coffee site. The intake structure included provision for six units, although the powerhouse was only constructed for four units. Aside from partial excavation for the extension of the powerhouse substructure, none of the other civil works for units 5 and 6 was completed. The technical characteristics of the original four-unit hydropower plant were:
|• Gross Head||21.6 m|
|• Unit Rated Flow||85 m3/s|
|• Unit Rated Capacity Units 1 and 2||15 MW|
|• Unit Rated Capacity Units 3 and 4||17 MW|
|• Plant Rated Capacity||64 MW|
|• Commissioning Year Units 1 and 2||1966|
|• Commissioning Year Units 3 and 4||1973|
The original four units are being rehabilitated using the same type of turbine–a Francis turbine. The contract for the generating equipment (turbines, generators and associated electro-mechanical equipment for the powerhouse) has been awarded to Voith Hydro. The modernized turbine units will be reconnected to the grid with 22 megawatts each. In total these units will generate a much higher output than the previous machines–the power output per turbine will be raised by about one third.
During the period of civil unrest in Liberia, power generation at Mount Coffee had to be stopped and operation of the spillway gates was prevented by the hostilities. This resulted in surcharging of the reservoir until Forebay Dam No. 1 was breached in August 1990. Approximately 180 meters (m) of the 12.2 m high dam, which was founded on overburden, was eroded down to bedrock. Until construction, which included lifting of the gates, began in January 2014, uncontrolled discharge continued through the partially open spillway and, in the wet season, through the breach. With the shut-down of generation, the powerhouse was vandalized and virtually stripped of power generation equipment, and much of the gate equipment. The powerhouse cladding was also removed.
The major rehabilitation work was identified by Stanley Consultants in a 2008 feasibility study. The feasibility study determined that, while much of the electrical and mechanical equipment had either been removed or rendered inoperative, most of the civil works, with rehabilitation, can be reused. It was also concluded that the forebay dam breach was reparable and that the breach did not result in irreparable damage to the adjacent concrete structures. Reuse of the remaining structures was found to be much more cost effective than redevelopment of the site.
Engineering and preliminary site investigations required for preparation of tender documents for the Project were completed in February 2012 (partially revised in June 2012), with EU-Africa Infrastructure Trust Fund financing, implemented through the West African Power Pool (WAPP) Secretariat. The resulting design and other reports provide recommended solutions to restore the Mount Coffee HPP to an operating condition. The major rehabilitation issues are:
- Repair of the breached portion of the forebay dam, using a similar type of construction as used in the original dam (laterite construction with an internal drain system);
- Some movement of the Unit 4 turbine/generator block was identified and a system of rock anchors was recommended to be installed in the block to prohibit any additional movement;
- Repair of generator floors and columns for support of the powerhouse crane and turbine/generator equipment during erection and future maintenance.
Further investigations were conducted at site in early 2013 under PIU supervision, to provide information on the above-identified issues as well as design considerations. Detailed design, planning, and procurement began in early 2013 and were completed by early 2015 by the Owner’s Engineer, a joint venture of Norplan AS of Norway and Fichtner GmbH of Germany. Construction supervision is ongoing.
Currently, the WAPP Cote d’Ivoire, Liberia, Sierra Leone, and Guinea (CLSG) transmission interconnection project is also under implementation. It consists of a new 225 kV transmission line connecting the power networks of the CLSG countries.
Since the Mount Coffee HPP will be the largest generating facility in Liberia for years to come, it will be heavily depended upon to have high reliability and to provide frequency and load control for LEC’s system. It will also be required to operate as part of the CSLG Power Network which will be used for interchange of power with neighboring countries. The Project is being developed in a manner so that it will be compatible with future upstream developments, particularly storage developments (e.g. Via reservoir).