Decommissioning Insight 2017

4.3 Topside and Substructure Removal The removal of 206 platforms is forecast across the UK, Norwegian, Danish and Dutch Continental Shelves to 2025. These range from small, unmanned, steel installations weighing 350 tonnes to larger, manned, steel and concrete gravity-based structures that weigh more than 1,000 times that. The total weight of infrastructure to be removed is close to 860,000 tonnes of topsides and 510,000 tonnes of substructure.

Activity is greatest in the UKCS where 98 of the platforms are to be decommissioned, closely followed by The Netherlands with 77, 14 in Norway and 17 in Denmark.

Making Safe and Topside Preparation Before a platform can be decommissioned, it must be hydrocarbon free. This is referred to as ‘making safe’ and involves cleaning, freeing equipment of hydrocarbons, disconnection and physical isolation, and waste management. The topsides are then prepared for removal, which involves separating them from the process and utilities modules and appropriate engineering, such as installing lift points. Topsides are prepared in line with the removal method being used (see below for the varying removal methods). The overall activity levels for making safe and topside preparation mirror that for topside removal. Making safe can be carried out several years prior to removing the platform, while topside preparation typically occurs directly prior to removal and can either be contracted out separately or built into the removal contract.


Removal Methods The most common methods for topside removal are piece-small, reverse installation (piece-large) or single-lift.

• The piece-small method involves dismantling the topsides and using demolition techniques typically used onshore to produce small, manageable pieces that can be transported to shore.

• For reverse installation or piece-large, the topside modules are lifted separately onto a transportation barge or the deck of a crane vessel before being taken onshore.

• The single-lift method involves removing topsides in one piece and may involve extra engineering work to reinforce them in preparation for removal.

For the substructure, the removal method depends on the type, weight and configuration. In the southern North Sea, Irish Sea and The Netherlands, the substructures that are to be decommissioned are primarily shallow-water jackets that typically weigh less than 2,000 tonnes and are usually deployed in water depths of 55 metres or less; the single-lift method is suitable for these structures. For larger substructures (barge-launched, lift-installed and some self-floaters), the jackets may be cut into smaller sections in situ and removed in segments. These more complex projects are typically located in the central and northern North Sea and on the Norwegian Continental Shelf. The supply chain continues to innovate in cutting technology to undertake this task.


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