Decommissioning a Gravity Base Structure (GBS)

Following the earlier removal of the topside, the next phase in the decommissioning of the Halfweg platform, located in the Q field near Den Helder, has begun: the removal of the concrete Gravity Base Structure (GBS). This is a significant challenge, as there are no standard solutions or comparable cases to draw from.

Petrogas has appointed Iv as client engineer to oversee the tender process, assess the proposed removal method from a technical perspective, and carry out additional analyses. In doing so, Iv provides independent confirmation of the technical feasibility and safety of the operation.

The Gravity Base Structure differs from the usual foundations used in steel jackets. It is a hollow concrete structure weighing nearly 3,500 tonnes, partially filled with water and firmly anchored in the seabed at a depth of over 24 metres.

Due to the anchoring and the volume of ballast water, the effective mass of the structure changes, which affects its dynamic behaviour during lifting. This increases the risk of structural damage or implosion. Iv analysed various scenarios to determine the maximum load the structure can withstand.

Iv also reviewed the motion analyses conducted by the offshore contractor, which examined the interaction between the Gravity Base Structure, the lifting barge and the vessel. These analyses considered the effects of wave action, currents and vessel movement on the structure, in order to ensure lifting could be carried out safely and that the resulting forces would remain within the structural limits of the GBS. These limits are critical in determining the feasibility and scheduling of the operation.

The studies serve as a safe and technically sound foundation for a responsible approach to this complex operation. By applying its offshore expertise, performing analyses and conducting technical reviews, Iv helps reduce risk and supports the feasibility of the operation.