The Brent field, one of the largest oil and gas fields in the North Sea, was a Shell-operated field located 186km northeast of the Shetland Islands.
It was once home to four oil and gas platforms: one jacket-based (Alpha) and three gravity-based (Bravo, Charlie and Delta). Brent Delta ended production in 2011, Alpha and Bravo in 2014, leaving Charlie as the final one in operation.
Performing skidding of the 31,000t topside onto the quayside, whose concrete legs were submerged in seawater for over 40 years, brings complex engineering and safety challenges.
Careful engineering ensured the stability needed to carry out this operation safely and with precise control.
Performing offshore removal, load-in and decommissioning on repeat
The steps to remove and transport the Charlie topside from the Brent field to the Able Seaton Port near Hartlepool, UK, for decommissioning were largely unchanged from the previous three Brent topsides, providing the peace of mind that comes from a tried-and-tested solution.
Allseas used its motion compensated heavy lift vessel Pioneering Spirit to remove the structure at sea in a single lift and transport it to shallow waters, where the topside was transferred onto Iron Lady, Allseas’ purpose-built cargo barge.
Mammoet had already fitted Iron Lady with skidding equipment (some 45 truckloads of materials), which would be needed to offload Brent Charlie at the port. Mammoet also provided mooring winches for Iron Lady within specific guidelines provided by Allseas on lengths and drum load capacities.
Once the barge had moored at Able Seaton Port and settled into the seabed, the team could determine the starting height of the skid tracks on the quayside and begin laying them down.
The topside was skidded over twelve skid tracks, which needed to be perfectly aligned with the skid tracks installed on the barge.
The skidding operation was performed in two stages. First, the topside was skidded five meters to the aft of the barge. Then, after 12 hours to allow for further settling, it was skidded the remaining 130 meters onto the quay, to its final position.
A configuration of 76 skid shoes - divided between the four legs of the platform - and 40 push-pull units were used to skid the topside 15 meters per hour. The combined pushing capacity was 3,320t; the total lift capacity was 51,000t.
Suspended netting was used to collect any falling debris and marine growth that might come away from the structure. All movements were remotely controlled from a control room to minimize the presence of people underneath the platform, and therefore maximize safety.
The importance of hydraulic coupling, topsides and beer mats
How many times have you been to a restaurant and needed to place beer mats under the legs of a four-legged table to stop it from wobbling?
Similarly, one of the biggest challenges in the operation was managing Brent Charlie’s four legs, which meant the structure was less stable and more likely to deflect during skidding.
“When you look at stability, three legs are always stable; four legs are not,” explains Richard Verhoeff, Mammoet Sales Director.
“You try to keep a three-point suspension when performing a load-in, and still need to achieve that even with four legs. That’s where hydraulic grouping comes in very handy”.
But there can also be some level of deflection between the legs, so the force needs to be able to communicate between the different hydraulic groups.
This is why there were hydraulic cylinders under each leg, and why the cylinders between both pairs of two legs had to be connected – to ensure the pressure on each remained the same.
The experience to perform a successful decommissioning
Mammoet’s experience of performing the successful load-ins of all three of the gravity-based topsides ensured a successful project. As was the method used to safely skid the topside onto the quay, which allowed it to be moved as a single unit.
“We have a pragmatic approach, which is required on jobs like this,” explains Leo de Vette, Project Manager at Mammoet. “It’s really a team operation, you must do it together.
Time is of the essence, so equipment can be moved to the next job. Once the topside is on the barge, there is only one priority – get it off as safely and efficiently as possible”.
