Fugro Seacore successfully drilled two large sewage outfall shafts in Hong Kong for the $72m sewage treatment works built by Philipp Holzmann, Franki Contractors and Dredging International joint venture on reclaimed land at StonecuttersIsland.
FSCL used their own Teredo 40 pile top drill to drill the two shafts. This was mounted onto a 4.5m casing which constituted a 26 meter long first section of double skinned casing followed by 10m sections as required. The casing was supported laterally on large cantilever casing top guide frames hung off one side of the JUB and pinned into position with vertical spud legs driven into the seabed under their own weight all in 30m water depth. An additional hydraulic casing manipulator was lowered in round the two spud pins through which the casing was fed.
The Teredo 40, a 330kW reverse circulation pile top hydraulic drill was designed, built and operated by Fugro Seacore. The modular drill bit used utilised a central hexagonal-shaped core capable of accepting interchangeable cutter wings allowing the use of three different types of cutter throughout the project. The casing advanced as the socket was drilled through the overlying soft overburdens and added as required until it seated into the top of the bedrock layer. The resulting 70 meter long casing was then grouted into the underlying granite and boring the blind hole continued another 50 meters to full depth.
At full penetration the 4.1 meter diameter shafts were bored to a 70 meters depth in a single pass and nearly 120 meters of drill pipe, complete with bit and weighing about 170 tonnes was suspended from the rig. Holding the drill string in tension to act like a massive pendulum, combined with hole stabilisers on the drill pipe, enabled Fugro Seacore to achieve hole verticality of 0.2˚, well within the specified tolerance.
Both shafts were then lined with 400 tonne steel risers, which connected at their tops to a 1.2km long manifold with 24 domed head, eight port diffusers. The manifold and diffuser system was installed in a 20 meter deep sloping sided trench, about 150 meter wide at the top, covered and protected with rock armouring. The toe of each riser was linked by short cross passages to a 5 meter diameter, 1.7km long precast concrete lined tunnel bored from a 100 meter deep shaft at the treatment plant in 30 meters of water.