Fugro Seacore Ltd (FSCL) was sub-contracted by SBF to construct the marine element, consisting of drilled shaft, installation of 300t internal steel liner and 450t top r/c diffuser. FSCL’s campaign has spanned a period of 2 years- Year 1 incorporated additional site investigation and sub-seabed Televiewer analysis of the strata with additional core samples to assess drilled shaft stability, and a drilled 1m diameter pilot hole. Year 2 incorporated main shaft drilling and liner installation.
A new drill rig was required to drill the shaft, and a brand new 90 Tonne torque drill rig (T90) was designed and built with all new down-hole equipment. The build process from concept to commissioning took 8 months. The analysis from the Year 1 campaign concluded that casings were required to support the upper 18m of shaft, therefore the drill bit was adapted to under-ream from 5.85m out to 6.45m and two casings were installed using a telescoping technique. The rest of the shaft was drilled unsupported, however in order to cater for potential shaft wall instability, a shuttering device was designed and fabricated and ROV survey’s undertaken at key intervals to confirm wall integrity. The shaft was drilled to a depth of 63m. In total nearly 1750m3 (4900Te) of hard rock was excavated.
The total drill string weight was over 235Te including the 62Te bit which was dressed with tungsten carbide roller cutters. Following drilling for verticality and straightness the 300t steel liner was floated to Excalibur and the onboard Demag crane lifted it to vertical via a semi-buoyant assisted lift. The liner was then connected to 4 Fagioli strand jacks, mounted on the reconfigured T90 base and the liner was then flooded before lowering it to final elevation. Divers then confirmed position and cut the strands. Excalibur was remobilised with SBF concreting equipment and the FSCL team assisted with concreting the liner annulus.
Excalibur was remobilised in May 2009 to install the 450Te diffuser, which was lifted from a supply barge at Dielette Harbour using strand jacks and sea fastened for the tow to site. Once Excalibur had been positioned over the open shaft the diffuser was lowered into position on the strand jacks & the interface connection grouted to complete a watertight joint. The diffuser installation was successfully completed in June 2009.