The most difficult seabed overburden conditions encountered by UK specialist marine contractor Seacore, did not prevent the company from successfully drilling large diameter holes deep into the Baltic Sea bed, midway between the south east coast of Sweden and the southern tip of the island of Oland.
Seacore had to remove huge boulders from the seabed and densely compacted glacial till overburden, to allow the drilling of five 3.5m diameter sockets in the exceptionally hard underlying quartzitic sandstone bedrock.
These accepted the steel monopile foundations for Sweden's 10MW Yttre Stengrund offshore wind farm.
The five 2 MW turbines, made by NEG Micon A/S in Denmark, have a hub height 60m above sea level and 72m diameter, three bladed rotors. They are spaced about 250m apart and supported on tapered tubular steel towers standing in about 9m of water.
Each tower is flange bolted to an ice cone protected, 3.5m diameter steel monopile grouted into a mating rock socket drilled up to 20.5m through 12m of overburden and 8.5m of rock into the seabed.
Seacore worked closely with main civil works contractor AMEC Special Projects on the approximately £750,000 specialist sub-contract to drill the rock sockets and install the turbine tower monopile foundations.
The contractor operated round the clock from AMEC's Wijslift 6 jack-up platform, which was fitted with Seacore's in-house designed and built 40t/m torque reverse circulation hydraulic rotary drill rig mounted on adjustable pile gates straddling two finger pontoons on the jack-up's stern.
INFOPLUS www. seacore. co. uk