I.4 Anchor and mooring pre-installation

Function

The pre-installation and testing of anchors and mooring lines at site. Completing as much pre-installation as possible ahead of the installation critical path allows the hook-up of the floating offshore wind turbine to be completed faster.

What it costs

About £31 million for a 450 MW floating offshore wind farm.

Who supplies them

Bourbon Offshore, Bridon-Bekaert, First Subsea, Kvaerner/DOF Subsea JV, Lankhorst, PSG Marine & Logistics, Strainstall, TechnipFMC and Tronds Marine AS.

Key facts

The pre-installation begins by installing an anchor on the sea bed. The process depends on the anchor type (see B.3.1 for further information):

  • Drag embedment anchors are installed by placing the anchor on the sea bed and using the pulling force of the anchor-handling vessel (AHV) to obtain the necessary embedment which is normally between 1 and 2 m.
  • Suction embedment anchors are installed by lowering a suction pile to the sea bed which self-penetrates part-way into the sea bed under its own weight. The piles are then embedded to their full depth by evacuating seawater from inside the pile using a pump on the anchor operated by a specialised ROV.
  • A piled anchor is installed by lowering a pile to the sea bed which penetrates the sea bed under its own weight. A pile hammer is then used from the AHV to obtain the required penetration depth. Piled anchors can also be drilled and grouted into place, depending on ground conditions.

Anchors are proof loaded for 15 to 30 minutes, according to the standard used, using the pulling force of the AHV to ensure sufficient holding capacity. The use of an in-line tensioning device to pull against a temporary reaction anchor, or an opposing anchor of the mooring system, substantially reduces the bollard pull required.

The lower section of the mooring line, often anchor chain, is connected to the anchor during installation:

  • For drag embedment anchors it is attached before the anchor is embedded, as the attachment point is below the sea bed once it is embedded.
  • For suction and piled anchors, it is attached after the anchor is installed.

If there is a section of synthetic fibre rope in the mooring line, this section would not be connected at this time.

A submersible marker buoy is connected to the end of the mooring line to support retrieval for later hook-up operations.

What’s in it

Guide to a Floating Offshore Wind Farm