Mammoet, Allseas cut load-in time after integrating North Sea topside removals

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Two major North Sea decommissioning projects have demonstrated how integrated planning can significantly compress offshore removal schedules. Mammoet and Allseas aligned operations for the Heather Alpha and Eider Alpha topsides, reducing load-in durations and improving overall execution efficiency.

The topsides, weighing 15,300 tonnes and 11,640 tonnes respectively, were removed in separate campaigns during the 2025 summer season. However, both projects were engineered as part of a single operational framework, allowing shared preparation and synchronized execution.

Instead of treating each removal as a standalone operation, both companies coordinated vessel configuration, equipment setup, and load-in methodologies in advance. This approach enabled parallel readiness for both structures and reduced downtime between campaigns.

The removal process began offshore, where Allseas’ Pioneering Spirit executed single lift removals of each topside from the UK North Sea. The vessel’s twin hull design allowed it to lift entire platform topsides in one motion, eliminating the need for offshore dismantling.

Once lifted, each structure was transported to shallower waters and transferred onto the cargo barge Iron Lady. From there, the focus shifted to onshore load-in operations, where timing and stability became critical.

Unlike grounded operations, Iron Lady remained afloat during load-ins. This introduced complexity during ballasting, particularly when the topside weight was distributed between the barge and the quay.

As one engineer involved in similar projects once remarked, “It is like moving a grand piano while standing on a floating dock. Every shift matters.”

A key efficiency gain came from preparing Iron Lady to accommodate both topsides simultaneously. Mammoet installed skid tracks, grillage systems, and inner beams in advance, ensuring compatibility with Allseas’ skid shoes.

These inner beams distributed loads across the barge while aligning height differences between vessel and quay. By configuring the barge once for both projects, the teams avoided redundant mobilization and equipment adjustments.

This decision allowed Heather Alpha and Eider Alpha to follow identical load-in methodologies despite being delivered months apart.

Operational execution reflected the benefits of this planning. Both topsides were moved using hydraulic skidding systems powered by up to 40 push pull units, generating a combined capacity of 3,652 tonnes.

The structures were transported at a rate of 15 meters per hour across a total distance of 180 meters.

Clients had set a 24 hour target for each load-in. According to Leo de Vette, Project Manager at Mammoet, operations were completed in a single shift.

“To achieve this required a lot of planning and preparation,” he said, noting the close coordination between both companies to align equipment interfaces and timing.

The project builds on previous collaboration between the two firms, including the Brent field topside removals. That experience contributed to a standardized approach to barge setup, skidding operations, and load transfer.

Mammoet also deployed hydraulic stabilizer systems to secure Iron Lady during operations, effectively creating temporary mooring structures to maintain alignment during ballasting.

With decommissioning activity increasing across the North Sea, such integrated approaches are becoming more common. Operators are under pressure to reduce offshore exposure time while maintaining safety and cost control.

The Heather Alpha and Eider Alpha projects suggest that combining campaigns and aligning engineering strategies can deliver measurable gains, particularly when handling assets of this scale.