Mammoet, Allseas, complete dual platform topside lifts

One of the platform topsides during load-in procedures (Photo: Mammoet)

Mammoet has worked with Allseas, a Dutch-Swiss heavy-lift and pipeline construction company, for many years.

The two companies recently completed the load-in to shore of two oil drilling platform topsides, the Heather Alpha and Eider Alpha. These megastructures weigh in at 15,300 and 11,640 tonnes respectively.

As these topsides were being removed one after the other it was possible to prepare the deck of the Allseas’ purpose-built cargo barge, named Iron Lady, to accommodate both topsides, saving both time and cost.

There were additional challenges; for example, Mammoet needed to ensure its skidding tracks could accommodate the skid shoes used by Allseas.

The Pioneering Spirit, a motion-compensated heavy-lift vessel from the Allseas fleet, was deployed to the North Sea to remove each structure.

The Pioneering Spirit has a catamaran design which allows lifting beams to directly interface with the underside of the topside, raising it clear of the jacket foundations to suspend it between the twin hulls of the vessel.

Following these lifts, the topsides were transported to shallower waters for transfer to the Iron Lady in preparation for load-in at the disposal yard. The Heather Alpha was delivered to Frederikshaven in Denmark for decommissioning, while the Eider Alpha was unloaded at Vats in Norway.

On the skids

The legs of the topsides rested in skid shoes slotted within Mammoet’s skid tracks. On top of these sat the grillage and interfacing equipment.

Inner beams were used to spread the load weight, while als omatching the height of the skid tracks on the barge with those on the quayside.

At no point was the Iron Lady grounded for the transfer, meaning that the vessel was free-floating during the off-lift.

This presented some difficulties when the topsides were both on the quay and the barge. But quick completion of the move helped, as explained by Leo de Vette, project manager at Mammoet. “The clients wanted us to achieve the load-in in 24 hours, but we managed it in half of that; a single shift.”

“To avhieve 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.”

Using 36 push/pull units for the Eider Alpha and 40 units for the Heather Alpha, a combined push capacity of 3,652 tonnes moved the topsides 15 metres per hour over 180 metres.

Mammoet provided hydraulic cylinders for the stabiliser beams at the quayside, creating temporary moorings for the Iron Lady while ballasting.

“[Moving topsides] has now become a standard operation for Mammoet,” added de Vette. “We know the optimum time and steps to set up our equipment on the barge and on the quay.”