Function
Removal and shipment to shore or cut-off at sea bed level and making safe.
What it costs*
Around £131 million for a 1 GW offshore wind farm.
Who supplies them
Foundation installation contractors such as Ashtead, Boskalis, Decom Engineering, Geosea (DEME Group) and Jan de Nul can provide foundation decommissioning.
Likely also other offshore operators will enter the space, including players with offshore oil and gas decommissioning experience.
Key facts
Decommissioning plans may define specific requirements for removal of components below the mud line which in turn may drive the choice or design of foundations and installation methods. It is possible that in some circumstances, structures could be left in position where they support ecosystems that in the opinion of the regulator must be safeguarded.
For monopiles or jackets, all elements above the sea bed will probably need to be removed with piles cut off at an agreed height (typically 1 m below the top of the sea bed). Initially, the process is likely to draw heavily on the oil and gas industry’s experience of removing subsea structures and then be optimised for the offshore wind industry.
Removal of foundations is likely to involve the use of a work-class ROV fitted with a range of cutting and drilling tools including guillotine saws, hydraulic hole cutting tools (for making lifting holes) and abrasive waterjet cutting. Gravity base manufacturers stress the ease of decommissioning as the structures can be de-ballasted and lifted/floated off to be broken down or used as breakwaters, the basis for artificial reefs or similar.
The use of suction caissons has been put forward as a means of reducing fabrication and installation costs. Decommissioning could also be straightforward, using the suction system in reverse to raise the foundation from the sea bed. Early trials have shown positive results, but removal after many years of fatigue loading has not been tried yet in wind.