North Sea topside decom ops benefit from integrated load-ins

April 20, 2026, by Melisa Cavcic

Netherlands-based heavy lifting and transport provider Mammoet has streamlined two topside decommissioning projects in the North Sea by employing combined planning to curb costs and delivery time.

North Sea topside decom ops; Source: Mammoet

While explaining that oil and gas topsides’ safe removal requires specialist equipment and careful planning, as these are colossal offshore structures that can weigh tens of thousands of tonnes, Mammoet, which has been supporting Allseas in offloading some of the world’s biggest topsides from its specialist vessels onto quaysides for disposal over the years, elaborated that the load-ins for EnQuest’s Heather Alpha and TAQA’s Eider Alpha topsides were integrated to save time.

Leo de Vette, Project Manager at Mammoet, commented: “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.”

Previously, the Dutch firm assisted Allseas with the load-in to shore of the Brent, Bravo, Charlie, and Delta topsides. The company managed the skidding of the 15,300-t Heather Alpha and 11,640-t Eider Alpha topsides for decommissioning in 2025. However, Mammoet claims that the latest projects presented additional challenges, as it had to ensure its skidding tracks could accommodate Allseas’ skid shoes, with the topsides being offloaded quickly to minimize risk during the load-in operations.

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Since the topsides were being removed one after the other, the company underlined that the situation presented a unique opportunity during the planning stage to prepare the deck of the other player’s purpose-built cargo barge, Iron Lady, for both topsides simultaneously, said to enable considerable time and cost savings. While the decommissioning of Heather Alpha was completed in August 2025 and Eider Alpha in October last year, 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. The vessel 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, while the grillage and interfacing equipment sat on top of these shoes. The Dutch player’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.

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“Executing both topside’s removals within a single summer campaign enabled Mammoet to optimise 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,” highlighted Mammoet.

Aside from preparing the barge, the company managed all engineering for the load-in operations, with both load-ins being floating operations; thus, Iron Lady was not grounded in front of the quay but floating in the water when the cargos were offloaded, which presented additional challenges during ballasting, especially when the topsides were partially on the quay and partially on the barge, making timing essential.

A combined push capacity of 3,652 tonnes moved the topsides 15 meters per hour over 180 meters, using a total of 40 push/pull units: 36 for Eider Alpha and 40 for Heather Alpha. The Dutch firm 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.

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“This has now become a standard operation for Mammoet. 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,” added de Vette.