The mooring lines connect and transfer loads from the substructure to the anchor system for station-keeping and stability. The reference configuration uses three mooring lines.
What it costs
About £50 million for a 450 MW floating offshore wind farm.
Who supplies them
Bexco, Bridon-Bekaert, Dynamica Ropes, Lankhorst and Vicinay.
A mooring line is unlikely to be made of the same material and specification along its full length. It can be thought of as having three sections:
- The upper section, which attaches to the substructure, is subject to the splash zone and sees the greatest loads.
- The middle section, in the free hanging section, is not subject to the splash zone or thrash zone.
- The ground line, which normally rests on the sea bed and attaches to the anchor. It needs to be heavy and stand up to the abrasion from movement across the sea bed under heavy loads, known as the thrash zone.
Typical pre-tension loads in a catenary system are 200 to 300 t.
Steel chain considerations:
- Stud-link chain is stronger and heavier than stud-less chain and better at preventing knot formation. Stud-less chain is cheaper for a given load and is less sensitive to fatigue loading.
- Steel chain specifications, for example, R2, R3, R4 and R5, determine the strength and material properties, although higher ultimate strength material will not necessarily have higher fatigue strength.
- Chain needs to be larger in the upper section, which sees the greatest loads. In addition, the link in a chain stopper receives higher loads than the rest of the chain.
- Steel chain is manufactured to several standard sizes, defined by the diameter of the steel rod it is made from, according to its loading and to enable compatibility with other mooring system components.
- Steel chain has considerable size and mass for the 185 to 220 mm diameter chains expected to be used for floating offshore wind turbines. For example, a single link of a 220 mm diameter chain has a mass of 700 kg and is over a metre long.
- Steel chain’s high mass limits its use to shallower sites, generally less than 200 m deep.
Synthetic fibre rope considerations:
- Nylon has a long history of use in mooring systems. It is the most compliant synthetic mooring material, which could help to limit loads at shallow sites but would lead to larger excursions. There are concerns about its ability to accommodate fatigue loads and recent advances are focused on extending its lifetime.
- Polyester is expected to be used most often in the near term as it is more proven for permanent moorings than high modulus polyethylene (HMPE) and nylon. Polyester has a moderate level of compliance.
- HMPE is stiffer than polyester and offers high load capacity. It is ideally suited to taut mooring designs.
- Spliced eyelets are made at the end of each section of synthetic fibre rope, and steel eyelets are introduced. This is a high-skill process which allows connection using shackles or H-links to adjacent mooring components.
- A high-density polyethylene or polyurethane jacket is used to provide resistance from abrasion, in conjunction with a sand barrier to prevent abrasive sand particles from entering the body of the rope.
- A typical synthetic rope mooring line made from nylon, polyester or HMPE is significantly lighter, per metre, than a steel chain line for the same load.
Wire rope considerations:
- Wire ropes are lighter than chains with the same breaking load, have higher elasticity than chain and are easier to handle. The higher strength to weight ratio makes wire rope a potential alternative for deep water mooring systems. However, the drawbacks are its lower stiffness in the water column caused by its low weight and structural degradation when laid onto the sea bed without additional protection.
A high-density polyethylene or polyurethane jacket is normally used to provide corrosion protection and some resistance from damage.