Clair was the third field west of Shetlands to be developed in that area to use a fixed platform. Weighing 8800t the four legged steel jacket was built by Aker Kvaerner’s Verdal yard in Norway, and was installed in 139m of water. The 165m high jacket was made up of 14 piles, each 51m long, 2.4m diameter and weighing 290t. The piles were passed through 12m long pile sleeves on the jacket, gathered in two clusters of three and two clusters of four at the base of the jacket legs.
The seabed at Clair contained boulder clays resulting from glacial deposits, unusually hard soils with shear strengths up to 2200 kPa. When a pile encounters a really hard section it requires assistance by driving it down to the embedded depth of 29m, which is the reasoning behind the utilisation of Fugro Seacore’s reverse circulation drilling system. The client, Saipem UK contracted Fugro Seacore to design, build and operate a relief drilling facility on board. The T10-40 drilling system is designed to drill down inside the pile, cleaning it out to relieve soil friction and also remove any obstructions which potentially block the toe of the pile. The drill bit in itself weighs 14t with the complete 16m long bottom hole assembly weighing in at 70t. The drill assembly is capable of delivering 250t of pull-back or 150t of thrust.
In operation, pipe sections are added or removed to the drill string by way of an articulated access ramp from the static deck to the compensation platform on the rig. As the compensator platform requires crew to make up or break out the flanges, they must be protected from unexpected acceleration, in the unlikely event the 120t drill string should be ‘lost’. The compensator is equipped with a safety system which Fugro Seacore describes as its ‘slingshot valve’.
The Teredo system was successfully used in a planned operation in the BP operated Shah Deniz field in the Caspian Sea, to install fifteen 1.6m diameter pre-conductor well casings.