Mammoet skids Brent Charlie topside onto quayside for decommissioning

Engineered transport and heavy lift specialist Mammoet, recently made sure the last and the heaviest topside of the Brent oil and gas field, Brent Charlie, was skidded onto the quayside for decommissioning. 

The Brent field, an oil and gas field in the North Sea, was operated by Shell and located approximately 186 kilometres northeast of the Shetland Islands. It comprised four platforms: the jacket-based Alpha platform and the gravity-based Bravo, Charlie, and Delta platforms. Production at Brent Delta ceased in 2011, followed by Alpha and Bravo in 2014. The Charlie platform remained operational until a later date.

Mammoet has performed the decommissioning load-ins of all three gravity-based topsides. Brent Charlie was the third, the last and heaviest, the company said.

Leveraging proven tech

The removal and transportation of the 31,000-ton topside from the Brent Charlie platform presented a complex engineering challenge. The structure, submerged in seawater for over four decades, required careful planning and execution to ensure a safe and controlled operation.

Allseas employed its Pioneering Spirit vessel to lift and transport the topside to shallow waters. The topside was then transferred to the Iron Lady, a specialized cargo barge equipped with skidding equipment provided by Mammoet.

The decommissioning process for the Brent Charlie topside followed a similar approach to the previous three Brent platforms, leveraging proven techniques and technology.

Skidding in stages

The barge delivered the topside to the Able Seaton Port near Hartlepool, UK, where it moored and settled into the seabed, allowing the team to 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 topside was skidded onto the quay in two stages. Initially, it was moved five meters aft on the barge, followed by a 130-meter skid onto the quayside. This process, which took place at a rate of 15 meters per hour, was made possible by a configuration of 76 skid shoes and 40 push-pull units.

To ensure safety, suspended netting was deployed to capture any falling debris. Additionally, remote control systems were used to minimize personnel exposure during the operation.

Balancing act

One of the biggest challenges during the operation was managing Brent Charlie’s four legs, as 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”.

To maintain structural integrity during the skidding process, it was crucial to ensure even pressure distribution across all four legs of the platform. This was achieved through a system of hydraulic cylinders that connected the legs, allowing for synchronized movement and preventing excessive stress on any individual leg.