Function
The mooring system provides the station keeping capability for the floating offshore wind turbine and contributes to the stability of the substructure and turbine.
What it costs*
About £80 million for a 450 MW floating offshore wind farm.
Who supplies them
Bridon-Bekaert, Bruce Anchor, Delmar Vryhof, InterMoor, MacGregor, NOV and Vicinay.
High-level mooring system options: plain catenary, multi-catenary, buoyant semi-taut and taut, clockwise from top left. Image courtesy of BVG Associates. All rights reserved.
Typical mooring system components for floating offshore wind turbines. An actual system would not use all of these at the same time. Image courtesy of BVG Associates. All rights reserved.
Key facts
There are four major mooring system options for a semi-submersible structure, shown in Figure 25 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 sea bed 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. It has a smaller footprint than other mooring systems with a radius from turbine to anchor of approximately two times the water depth.
Mooring solutions for floating offshore wind turbines have been developed from technology proven for floating oil and gas platforms. They differ as, generally, floating offshore wind turbines are located in shallower water, have a different set of loads, and have lower consequences of failure as there is no oil spillage risk.
Mooring lines connect to a substructure at an angle to the vertical. The horizontal component of tension keeps the substructure on station and the vertical component of tension provides a restoring force that contributes to the stability of the substructure and turbine.
Most early mooring systems have been designed to be compliant and so reduce the extreme loads. Compliance can be achieved in several different ways:
- A catenary shape straightens out and lengthens under increasing load, according to its mass.
- The length of ground chain progressively lifts with increasing load, according to its mass.
- A taut or semi-taut mooring line provides some compliance, depending on its length and material properties.
- In-line dampers and other load reducing devices are being developed for use with taut and semi-taut moorings, which are typically located in the upper section of the line.
Mooring system designs can also be “restrained”, which means that they minimise motions.
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 have to cope with this movement.
A three-line mooring system has been the preferred design on early demonstration projects.
Redundancy is a commonly used term, but one which can be misunderstood in the context of mooring system design – it does not mean that no failures will occur. The relevant standards for floating offshore wind turbine moorings provide guidance. It is up to engineers to either design by following the guidance or to persuade insurers that their novel mooring system designs are fit for purpose, which in turn will give confidence to investors and lenders.
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.