Function
Removal and shipment to shore of anchors and mooring systems.
What it costs*
About £18 million for a 450 MW floating offshore wind farm.
Who supplies them
Expected to be the same as the anchor and mooring installers.
Key facts
Decommissioning plans may define specific requirements for removal of components below the mud line which may drive the choice or design of anchors and installation methods.
The removal process is likely to involve the use of a work-class ROV fitted with a vision system and a range of cutting and drilling tools. These include guillotine saws, hydraulic hole cutting tools (for making lifting holes) and abrasive waterjet cutting.
Mooring lines are disconnected from the floating substructure, then disconnected from anchors, then brought onto the deck of an AHV (see I.4 for further information). Where the connection to the anchor is not accessible, the mooring line may be cut. As they are brought on board any buoyancy modules, clump weights and load-reduction devices are removed.
The removal of anchors depends on their type and the commitments made in the decommissioning plan:
- Drag embedment anchors are relatively easy to remove, with several different techniques being used. These vary according to the design of the anchor but include pulling the anchor upwards and in the opposite direction to its operational loading.
- Suction-embedment anchors are also relatively easy to remove. A high-pressure line is connected to the anchor using an ROV and water is pumped into the anchor to reverse the embedment process.
- Piled anchors are most likely to cut off at an agreed height, or the pile driven under the sea bed and left in position. Initially, the process is likely to draw on the fixed offshore industry’s experience of decommissioning monopile structures.