Once the topsides have been recovered to shore, the removal of the substructure can take place. Substructures vary considerably in terms of their shape and size depending on the water depth and the size of the topsides they were designed to support. Substructures can be either concrete gravity-based (CGB) or steel piled jacket structures (SPJ). CGBs are large reinforced concrete structures which are constructed in near-shore locations and floated to site. SPJs are steel structures which consist of a lattice of steel circular hollow sections welded together. These jackets are fabricated onshore and can be installed from a barge, made buoyant and floated to site, or installed using a lift vessel.
Under the current OSPAR regulations (Decision 98/3), CGB structures may be left wholly in place at decommissioning. The footings of the largest and oldest SPJ structures may also be left in place, subject to regulatory approvals. Removal of SPJ structures, like topsides, can be conducted either by single-lift or by cutting the structure into sections and transported back to shore to for recycling. Prior to cutting and lifting, tasks such as reinstatement of lift points and strengthening for transportation will be carried out by specialist contractors.