The KPONE Independent Power Plant (KIPP) is located in the Kpone neighborhood of the port city of Tema which is approximately 32 km east of Accra, the capital and largest city in Ghana. The new power plant is designed with a seawater cooling system which requires therefore an underground connection into the open sea. BAUER Engineering Ghana Ltd. was awarded to excavate rock below sea level from a jack-up barge for the micro tunnel boring machine (MTBM). This contract includes for all offshore foundation works. The excavated rock shall form two
5 m x 19 m wide exit pits for the offshore break out of the MTBM. A BAUER BG 28 drill rig equipped for kelly drilling method was mobilized onto a jack-up barge and relocated approximately 350 m off the natural shore line for the foundation works. One auxiliary crane located on a second barge was assisting during the rock drilling works. The maximum required drilling depth measured from the jack-up barge level to the rock coring toe was 23 m. The rock excavation of the exit pits were achieved by multiple rows of five overlapping piles following the secant pile wall method. Each exit pit requires 105 rock cores, each drilled 6 m deep into strong to very strong gneiss. The works included only the removal of the rock, no other material had to be filled in afterwards below the sea level. A steel guide template which was attached to the side of the jack-up barge and a temporary casing string (1,180 mm diameter) which was installed down to the top of rock have ensured the correct location and overlap of each pile. The jack-up barge had to shift several times to reach each core location to complete one MTBM exit pit. The first MTBM exit pit was completed in December 2016, while the second MTBM exit pit was completed in March 2017.