The anchors of a mooring system provide fixed points in the sea bed which can resist the loads from the mooring lines for the lifetime of the project. The reference configuration includes three anchors, one for each mooring line.
What it costs
About £17 million for a 450 MW floating offshore wind farm using drag embedment anchors.
Difficult ground conditions require the use of piled or suction anchors which could result in anchor costs that are several times higher.
Vertical and multi-directional loading from other foundation types, or shared anchors also increase anchor costs.
Who supplies them
Suction pile anchor, drag embedment anchor and driven pile anchor. Images courtesy of Acteon, Principle Power and Acteon. All rights reserved.
Anchors are used across many different industries and so the existing design types are well established, although new devices are coming on to the market at low technology readiness levels, providing the opportunity to anchor in a wide range of ground conditions. The anchor types expected to be used most for floating offshore wind turbines are:
- Drag embedment
- Driven pile, and
- Suction pile.
|Drag embedment||Driven pile||Suction pile|
|Where used||Best suited to cohesive sediments that are not too stiff to impede embedment. Used where possible as lowest cost.||Can be used in a wide range of conditions, including where there are boulders or hard ground.||Requires sea bed conditions that are firm enough to hold suction but not so hard that penetration is impeded|
|Loading||Uni-directional, horizontal only||Multi-directional, horizontal, and vertical||Multi-directional, horizontal, and vertical|
|Installation||Simple, requires pre-tensioning||Driving by vibro- or impact-hammer causes noise||Relatively simple process: self-weight starts embedment, followed by suction|
|Removal||Are designed to be recoverable||Difficult to remove||Removal is the reverse of installation|
The choice of which type of anchor to use is driven primarily by the set of loads it will encounter and ground conditions.
Floating offshore wind introduces larger numbers of anchors per site than are used by other markets and so the total installed cost is a major consideration.
An array of floating turbines has the potential to share some anchors, which would reduce the overall cost, as has been demonstrated at Hywind Tampen where 11 turbines share 19 anchors. Shared anchors must be designed to resist loading from multiple directions, and the consequence of cascade effects resulting from single/multiple line failures needs to be addressed.
A drag embedment anchor for a 15 MW turbine has a typical mass of 35 to 50 t.