The expansion of power supply is an urgent issue in the economically emerging Ghana on the South West African Atlantic coast. The capacity of the hydropower plants on Lake Volta, for example, is decreasing during persistent drought periods. As a consequence, both the government and businesses are increasing investment in additional energy sources.
With the new „Kpone Independent Power Plant“ (KIPP) Ghana is strengthening the national electricity grid. The 350 megawatts or 2,700 GWh per year capacity is enough to reliably supply approximately 1 million households with electricity. In the operation of the oil-gas power plant located about 25 kilometers east of the Ghanaian capital Accra directly on the coast, it will be cooled with sea water from the Atlantic Ocean via two cooling water lines.
For the construction of these underground sea water intake and outfall tunnels with an inner diameter 2 meters a Herrenknecht AVND2000AB utility tunnelling machine is delivered to the African continent. Herrenknecht specially designed the tunnel boring machine with a salvage module for this special application: In the sea-outfall method pipelines are constructed from the coastline into the open sea. There, the machine has to be recovered under water. The salvage module prevents the machine and the tunnel from flooding. In addition, for cutter tool changes under high groundwater pressure Herrenknecht equipped the microtunnelling machine with a hyperbaric chamber.
Two shafts on the coast served as starting point for the jobsite crew and the Herrenknecht AVND for a total of four pipe jacking drives between May and July 2017. First, the tunnelling specialists produced two tunnels from the two shafts toward the power plant. These 545 and 520 meter long tunnels run under the mainland and connect the sea-outfalls and the power plant. Afterwards, construction of the challenging tunnelling sections under the sea floor started. Initially, the AVN machine excavated a 1,085 meter long tunnel from the coast to the target spot on the seabed for the extraction of the cooling water (“sea-intake”). There, the TBM was recovered at a depth of 17 meters. Finally, the machine created a parallel 380 meter long tunnel toward the sea which will later take the cooling water back into the Atlantic ("sea-outlet"). In total, it covered a distance of 2,530 meters.