Mammoet completed the load-in operations for the Heather Alpha and Eider Alpha topsides in 2025, supporting Allseas on two offshore decommissioning projects in the UK North Sea.
The two topsides weighed 15,300 tonnes and 11,640 tonnes respectively. Both were removed by Pioneering Spirit, the motion-compensated heavy lift vessel operated by Allseas, in a single lift. After removal, the structures were transported to shallower waters and transferred to Iron Lady for load-in to disposal yards.
The removal of Heather Alpha was completed in August, while Eider Alpha followed in October. Although the projects were carried out separately, the extraction and load-in methodology was the same for both platforms.
A key part of the work was preparing Iron Lady to receive both topsides. Because the two removals were scheduled within a single summer campaign, Mammoet prepared the barge deck for both structures at the same time. This allowed the company to save time and cost during the overall operation.
Its scope included installing skid tracks, grillage, interfacing equipment and inner skid beams in Rotterdam. The inner beams were used to spread the load from the skid tracks onto the barge and to match the skid track height on the barge with the level on the quayside. Mammoet also had to ensure that its skid tracks were compatible with Allseas’ skid shoes.
Both load-ins were carried out as floating operations, with Iron Lady remaining afloat alongside the quay. This increased the complexity of ballasting, particularly when the topsides were partly on the quay and partly on the barge.
According to Mammoet, the clients wanted the load-ins completed within 24 hours, but each operation was finished in a single shift. Close planning with Allseas was required to align the installation of equipment with the skid shoes and keep the transfer sequence on schedule.
For the skidding works, Mammoet used 40 push/pull units in total, including 36 for Eider Alpha and 40 for Heather Alpha, with a combined push capacity of 3,652 tonnes. The topsides were moved at 15 m per hour over 180 m. The company also supplied hydraulic cylinders for stabilizer beams on the quaysides, creating temporary concrete moorings for Iron Lady during ballasting.
After load-in, Heather Alpha was shipped to Frederikshavn in Denmark, while Eider Alpha was transported to Vats in Norway for decommissioning.
