The Brent oil and gas field, a cornerstone of the North Sea’s energy industry for decades, has reached another milestone. The last and heaviest of its topsides, Brent Charlie, was successfully skidded onto the quayside at Able Seaton Port, near Hartlepool, UK, for decommissioning. This significant move underscores both the engineering prowess and methodical planning that characterize major offshore decommissioning projects.
Operated by Shell and situated 186 kilometers northeast of the Shetland Islands, the Brent field once boasted four platforms: Alpha, Bravo, Charlie, and Delta. Production ceased for Delta in 2011, followed by Alpha and Bravo in 2014, leaving Charlie as the final platform to close operations. The decommissioning of Charlie marks the conclusion of a chapter in the Brent field’s extensive history.
Mammoet, renowned for its expertise in heavy lifting and transport, has been integral to the removal process. The company managed the load-ins of all three gravity-based topsides, with Brent Charlie being the last and most challenging due to its 31,000-ton weight. This final phase presented complex engineering challenges, not least because its concrete legs had endured over 40 years of submersion.
Executing a load-in operation of this scale required careful precision and control. To bring the topside from the Brent field to Able Seaton Port, Allseas deployed its heavy lift vessel, Pioneering Spirit. The vessel performed a single-lift removal at sea, transporting the structure to shallower waters where it was shifted onto Iron Lady, a specially designed cargo barge fitted by Mammoet.
Preparations at the port were meticulous. Iron Lady was equipped with skidding gear comprising 45 truckloads of materials and mooring winches set to precise specifications. Once moored and stabilized on the seabed, Mammoet’s team began laying down the skid tracks on the quayside. Alignment of the 12 skid tracks between the barge and quayside was crucial for a seamless transition.
The operation unfolded in two main stages. Initially, the topside was skidded five meters aft of the barge. After a 12-hour pause to ensure stability, it was moved the remaining 130 meters to its final position on land. To achieve this, 76 skid shoes and 40 push-pull units were employed, collectively pushing with a force of 3,320 tons and boasting a total lift capacity of 51,000 tons.
Safety remained paramount throughout the process. Suspended netting captured potential debris and marine growth dislodged from the structure. Remotely controlled operations further reduced risks by minimizing the need for personnel near the platform.
One of the major engineering hurdles involved ensuring stability across Brent Charlie’s four legs. Unlike three-legged structures, which are inherently stable, four-legged platforms pose challenges due to potential deflection. Hydraulic coupling provided a solution, connecting the hydraulic cylinders under each leg to equalize pressure and maintain balance, similar to how beer mats stabilize an uneven table.
Richard Verhoeff, Mammoet’s Sales Director, elaborated: “When you look at stability, three legs are always stable; four legs are not. You try to keep a three-point suspension… hydraulic grouping comes in very handy.” This intricate control was essential to prevent uneven force distribution during the skid.
With its extensive experience, Mammoet ensured the project’s success, showcasing the importance of a coordinated team approach. Leo de Vette, Mammoet’s Project Manager, emphasized, “It’s really a team operation, you must do it together… Once the topside is on the barge, there is only one priority – get it off as safely and efficiently as possible.”
