Mammoet has collaborated with Allseas on the decommissioning of major oil and gas topsides in the North Sea.
The joint efforts enabled the efficient removal of the 15,300t Heather Alpha and 11,640t Eider Alpha topsides, using integrated planning and specialised equipment.
In 2025, Mammoet managed the careful skidding of these massive structures for decommissioning. Both topsides were extracted and transported by Allseas’ heavy lift vessel, Pioneering Spirit, in single-lift operations.
After removal, they were transferred to Iron Lady, the purpose-built cargo barge, for further load-in operations.
Mammoet’s planning allowed the preparation of Iron Lady to handle both topsides simultaneously, resulting in significant time and cost savings. The decommissioning of Heather Alpha concluded in August, followed by the Eider Alpha in October.
The company’s execution included engineering management and the installation of skid tracks and interfacing equipment on the barge.
The operation relied on 40 push/pull units with hydraulic cylinders for stabilisation. Each topside was moved 15m per hour over 180m during load-in operations.
Mammoet project manager Leo de Vette said: “The clients wanted us to achieve the load-ins in 24 hours, but we managed it in half of that; a single shift.
“To achieve this required a lot of planning and preparation, with us working closely with Allseas to time the installation of our equipment with their skid shoes and ensure everything aligned and went smoothly.”
The Pioneering Spirit’s catamaran-style design facilitated the heavy lifts, supported by Mammoet’s equipment integration, which played a crucial role in ensuring smooth operations. The shared history of working on similar projects, such as the Brent topsides, contributed to the successful completion of these recent tasks.
Mammoet and Allseas’ combined efforts underscore the importance of collaboration and precision in decommissioning projects, ensuring safety through innovative planning and execution.
For more on lifting in the offshore sector see the ‘offshore sector’ feature in the current issue of Cranes Today magazine.
