According to Mammoet, which helps clients with smarter, safer and stronger solutions for heavy lifting and transport challenges, Brent Charlie has been successfully skidded onto the quayside for decommissioning.
Brent Charlie is the last and heaviest topside from the Brent oil and gas field, located 186 km northeast of the Shetland Islands. It is now to follow its siblings – jacket-based Alpha, and gravity-based Bravo, Charlie and Delta – into a decommissioned retirement having been the last one in operation since 2014.
Mammoet has now provided an update on the project, reporting that it has successfully performed the skidding of the 31,000t topside onto the quayside, overcoming complex engineering and safety challenges in the process.
Drawing on decommissioning experience
Brent Charlie was required to be removed and transported from the Brent field to the Able Seaton Port near Hartlepool, UK. In order to do so, the company followed a similar process from the previous three Brent topsides.
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 minimise the presence of people underneath the platform, and therefore maximise safety.
One of the primary challenges was managing the structures four legs, which cause it to be less stable during skidding. As Richard Verhoeff, Mammoet Sales Director, explained, “When you look at stability, three legs are always stable; four legs are not. 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.
Despite the various challenges, Mammoet drew from its extensive experience of performing similar operations to successfully complete the task. “We have a pragmatic approach, which is required on jobs like this,” remarked 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.”
