Most GBS in the North Sea were built when decommissioning was not a design consideration or a regulatory requirement. They were not intended to be removed once they had been installed on the seabed. The Brent’s structures have either three or four concrete legs each, around 18m in diameter and up to 165m tall. The full height of the platforms – from the seabed to the top of the structure – is equivalent to the Eiffel Tower.
Considerable engineering challenges
There are considerable engineering challenges and safety risks associated with cutting through, lifting and transporting the reinforced concrete legs for removal. OSPAR recognises the difficulty in removing these concrete installations and has granted exemptions from removal to other operators of similar structures in the North Sea. In the last decade, Shell has thoroughly examined the options for decommissioning the GBS.
Comparative Assessments were conducted for the GBS to analyse options for the legs, such as partially removing them or leaving them in place. Studies also investigated the potential ways that the GBS structures could collapse and possible long-term effects. Many options presented specific technical challenges.
On balance, the recommendation is that the safest and most responsible solution is to leave the GBS legs and oil storage cells in place, marked with a navigation aid and on the FishSAFE database. This recommendation is consistent with what other operators have recommended and had approved. It is difficult to predict with accuracy how and when these structures will eventually collapse. Studies reveal that the visible part of the legs is expected to remain in place for 150 to 250 years. Once parts above sea level have degraded, the section of the legs under the sea is expected to last for another 300 to 500 years. The oil storage cells are also expected to remain largely intact for at least 1,000 years.