Topside Decommissioning Projects Streamlined With Combined Planning

Oil and gas topsides are colossal offshore structures that can weigh tens of thousands of tons, so their safe removal requires specialist equipment and careful planning.

Mammoet has been working with Allseas for many years, supporting it to offload some of the world's biggest topsides from its specialist vessels onto quaysides for disposal.

Previously, Mammoet assisted Allseas with the load-in to shore of the Brent, Bravo, Charlie and Delta topsides. In 2025, it managed the skidding of the 16,865-ton Heather Alpha and 12,831-ton Eider Alpha topsides for decommissioning.

These latest projects presented additional challenges — Mammoet had to ensure its skidding tracks could accommodate Allseas' skid shoes. It also was crucial that the topsides be offloaded quickly to minimize risk during the load-in operations.

As the topsides were being removed one after the other, it presented a unique opportunity during the planning stage — Mammoet could prepare the deck of Allseas' purpose-built cargo barge, Iron Lady, for both topsides simultaneously, saving considerable time and cost.

Helping Iron Lady Deliver Twins

While the decommissioning of Heather Alpha was completed in August 2025, and Eider Alpha in October 2025, the extraction and load-in methodology was the same for both platforms.

Allseas deployed its motion-compensated heavy lift vessel Pioneering Spirit to remove each structure from the UK North Sea in a single lift.

Pioneering Spirit features a catamaran-style design with powerful lifting beams that interface with the underside of a topside, raising it clear of its jacket foundations and suspending it securely between the vessel's twin bows.

Following removal, the topsides were transported to shallower waters, where they were transferred to Iron lady for load-in to the disposal yard.

The topside's legs rested on skid shoes slotted inside Mammoet's skid tracks. On top of these shoes sat the grillage and interfacing equipment.

Mammoet's scope was to outfit Iron Lady with all this equipment — as well as the inner skid beams that would later be used for transporting Eider Alpha — in Rotterdam.

The inner beams were used to spread the load from the skid tracks onto the barge, as well as match the height level of the skid tracks on the barge with those on the quayside.

Executing both topside's removals within a single summer campaign enabled Mammoet to optimize the configuration of Iron Lady to accommodate the two structures simultaneously.

Once safely transferred onto Iron Lady, Heather Alpha was shipped to Frederikshavn in Denmark and Eider Alpha to Vats in Norway for decommissioning.

In addition to preparing the barge, Mammoet managed all engineering for the load-in operations.

Both load-ins were floating operations, meaning Iron Lady wasn't grounded in front of the quay but floating in the water when the cargos were offloaded.

This presented additional challenges during ballasting, especially when the topsides were partially on the quay and partially on the barge — which is why timing was of the essence.

"The clients wanted us to achieve the load-ins in 24 hours, but we managed it in half of that; a single shift," said Leo de Vette, project manager of Mammoet. "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".

Pushing, Pulling

Using a total of 40 push/pull units (36 for Eider Alpha and 40 for Heather Alpha), a combined push capacity of 4,025 tons moved the topsides 49 ft. per hour over 590 ft.

Mammoet provided hydraulic cylinders for the stabilizer beams for the quaysides, creating makeshift concrete moorings for the Iron Lady to dock inside and stay secure during ballasting.

"This has now become a standard operation for Mammoet," said de Vette. "We know the optimum time and steps to set up our equipment on the barge and on the quay.

Naturally, supporting Allseas with the Brent topsides helped us, because it was a very similar process. This meant Allseas had full confidence in our method and equipment".

Mammoet's ability to integrate its tools, teams and equipment with those of its clients is a huge benefit in projects like these, where close cooperation and communication at every stage is essential for safety and success.

For more information, visit mammoet.com.