The mooring system provides the station keeping capability for the floating offshore wind turbine and contributes to the stability of the substructure and turbine.

Who is involved

Mooring systems are manufactured by specialist suppliers contracted by the developer.

Key facts

There are four major mooring system options for a semi-submersible structure, which provide compliance in different ways. The optimum design for each site is a technical and economic trade-off.

  • Plain catenary: A system that uses free hanging chain mooring lines, whose self-weight leads to the catenary shape. These connect the substructure to the anchors. A length of ground chain means that the anchors are loaded almost horizontally and, where ground conditions allow, use drag embedment anchors. It is the simplest mooring system design, with the least expensive anchor type, and is used at shallower sites. The radius from turbine to anchor is approximately six to eight times the water depth.
  • Multi-catenary: A system that uses chain mooring lines and may include rope sections. Compliance is provided first by the catenary chain sections and by the elasticity of the rope section, where used. The compliance properties can be tuned by the addition of clump weights and floats. Where ground conditions allow, it is expected to be used with drag embedment anchors.
  • Buoyant semi-taut: A system that uses a combination of chain at the top and bottom with a rope mid-section on each line. The ground chain ensures that the loads seen by the anchors are predominantly horizontal and buoyancy modules lift the rope sectors above the seabed to prevent damage. Compliance is provided predominantly by the elasticity of the rope section.
  • Taut: A system that uses rope lines connected under tension between substructure and anchors. Short sections of chain may be used at the top and bottom to make connections and adjust tension. Compliance is achieved through the properties of the rope section and from a load reduction device, if used. This option sees higher loads including high vertical loads on the anchors and so piled or suction anchors of greater capacity are needed.

Mooring solutions for floating offshore wind turbines have been developed from technology proven for floating oil and gas platforms.

A typical design value for excursion is 30 to 35% of the water depth. This means that the substructure could move up to 30 to 35 m away from its station, for a water depth of 100 m, and systems such as the dynamic array cables must cope with this movement.

The minimum cost of a mooring system is seen at depths of about 100 to 150 m. At shallower depths, the cost increases as complexities to do with relative wave height to water depth increase and substructure compliance is harder to manage. At greater depths, the cost increases because the lengths of mooring line are greater.

The mooring system restricts certain types of activity within the wind farm, such as fishing, depending on its detailed design.

The mooring system design must allow for ease of installation and hook-up, and ease of disconnection to allow for any major repair events.