EPCM was contracted to conduct the design of a subsea temporary sheet-piled cofferdam for the retrieval of a Tunnel Boring Machine (TBM). Cenpower Generation Company Limited of Ghana (Cenpower) developed a new natural gas-fired power station in Tema, Ghana. Group Five International was appointed as the EPC contractor for the cooling water system of the power station.
The offshore components of the cooling water system broadly includes two large diameter marine pipelines (installed via horizontal tunnelling), an offshore submerged intake and an offshore submerged diffuser structure. Following pre-dredging tests with regards to the offshore extraction of the TBM, it was considered not feasible to dredge an unrestrained open pit for TBM receiving due to the high sediment transport rates at the offshore site (filling in a pit of approximately 500 m3 overnight). And so, EPCM was requested to assist with the design of a cofferdam using available material (sheet-piles) to retain the collapse of the pit and allow the safe extraction of the TBM with a custom made lifting barge.
After various option analyses, taking into consideration the difficulty with piling in the surf zone, a floating cofferdam concept was designed. Consisting of a skeletal frame with four spuds in the corner providing buoyancy, it floated into position, and driven into the sand through a series of intermittent steps of driving piles and lowering the stiffening rings, with the aid of diver intervention, as the excavation becomes deeper. No cross bracing was allowed inside the cofferdam as this would have hindered the extraction of the TBM. Posing a unique design challenge of deepening the cofferdam, while avoiding failure of the sheet piled retaining walls.