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After years of planning, the 24,200 tonne topside of Shell’s iconic Brent Delta platform was removed in a single lift, making it the world’s heaviest offshore lift. The topside was supported by the three-legs of a concrete gravity base structure (GBS) in140m of water.
Arup supported the decommissioning of the platform by designing the shear restraint system between the topside and the legs, and also the concrete caps which sealed the top of each leg after the topsides had been removed.
Revolutionising the decommissioning industry
Shell’s Brent Field is approximately 186km off the North-East coast of Shetland, and has 4 platforms – Alpha, Bravo, Delta and Charlie.
Allseas was appointed by Shell to complete the topside lift, after constructing the world’s largest vessel, the Pioneering Spirit, which enabled the single lift process. The vessel has completely changed what is possible in the decommissioning industry.
The Brent Delta platform. ©Shell
A complex challenge
To prepare for the single lift operation, the legs had to be cut just below the topside. Our engineers worked with Allseas and Shell to assess the performance of the GBS in the post-cut condition. The assessment determined that the cut-interface between the legs and the topside required strengthening to ensure the topside would be secure between cutting and lifting.
Minimal offshore intervention for a safer lift
Following the assessment, we designed the shear restraint system for the cutline interface. Five options were eventually shortlisted from many to undergo scoping calculations by our team. We used key selection criteria to choose the final option to take forward into detailed design. Using skills such as nonlinear structural analysis to simulate the in-place condition, our designers focussed on installation with minimal manual intervention offshore, automatic disengagement during lift and withstanding the harsh North Sea wave conditions.
A strengthening design that could cope with all season wave loading and no manual intervention required to disengage, enabled Shell to make the cut and de-man the platform well in advance of the lift, reducing the offshore risk exposure for the workforce.
Concrete leg caps were placed on top of each of the three legs of the GBS. ©Shell
Ensuring future safety
Following the topside removal by Allseas’ Pioneering Spirit, concrete leg caps were placed on top of each of the three legs of the GBS. We undertook the detailed design of the leg caps ensuring they were simple to install. Our design ensured the leg shafts were closed off, whilst providing the foundation for a navigation aid required to warn shipping.
First of a kind – decommissioning success
The Pioneering Spirit took around 12 hours to position itself and prepare for the lift. Following completion of the 10 second lift, the world’s largest vessel then transported the topside close to shore where it was transferred to the vessel’s bespoke barge, the Iron Lady, before being delivered to the Able UK decommissioning yard in Hartlepool. Arup was also involved in the harbour works, on behalf of Shell, to ensure the infrastructure was capable of receiving the topsides.
Arup was involved in supporting Shell, to ensure the harbour infrastructure was capable of receiving the topside performing an independent technical review of the detailed design of the receiving quay as well as the critical structural interfaces with the Iron Lady barge. Our specialist maritime team attended the quay upgrades works, including witnessing piling, and reviewing site investigations.
