The tower is typically a tubular steel structure that supports the nacelle. It also provides access to the nacelle, houses electrical and control equipment, and provides shelter and storage for safety equipment.

What it costs

About £3 million for a 15 MW floating offshore wind turbine.

Who supplies them

CS Wind, Gestamp Renewable Industries, GSG Towers, Haizea Wind Group, Titan Wind Energy, Windar and Welcon.

Offshore wind turbine towers being stored at the quayside of a port.
Offshore wind turbine towers being stored at the quayside of a port. Image courtesy of TMS. All rights reserved.

Key facts

Fabricators manufacture towers to designs provided by wind turbine suppliers, sometimes using free-issue materials (both steel and internal components).

Towers are normally made at coastal locations.

Once fabricated, the tower sections are shot-blasted, metal sprayed and painted before fit-out with other internal components then prepared for transport and storage.

The hub height is about 135 m above mean sea level minimum depending on the rotor diameter, so each tower is about 120 m high and has a mass over 800 t.

Towers on early stage floating projects have had almost double the mass of the towers of equivalent fixed offshore wind turbines. This is to cope with the increased loads and different resonant frequencies experienced by the floating structure (wind turbine and floating substructure) including excitation from wave loading. Improved floating foundation designs and wind turbine control algorithms could reduce this additional mass.

About 90% of the mass is steel plate with forged steel flanges making up most of the rest.

Towers are generally tapered, with a top diameter of about 6 m and a base diameter of about 10 m for a 15 MW turbine.

Design is driven by fatigue and extreme loading, plus natural frequency requirements and avoidance of bucking.

The optimum tower height is normally as low as is needed to comply with maritime safety regulations for blade clearance above the water. This is because the wind shear is low offshore (the wind speed does not increase significantly with increasing the hub height), meaning there is not enough cost benefit to use a taller tower. The tower height to achieve blade clearance does not need to take account of the tidal range for floating offshore wind turbines using semi-submersible floating substructures because they rise and fall with the tides. Permitting at some sites has required taller towers to reduce the risk of bird strikes.

Integrated design of substructures and towers is increasingly seen as desirable with the transition from substructure to tower predicted to be less distinct. The tower continues to be a discrete component supplied with the wind turbine.

Towers for floating substructures that yaw around a single mooring point, or have more than one rotor, have the potential to be significantly different from the established norm.

The tower internals provide means of access, lighting and safety for maintenance and service personnel, plus means of transferring hand tools and components to the nacelle. They provide support for control and electrical cables and housing of switchgear, transformers, and other elements of power take-off.

Tower internals also provide storage for survival equipment. A tuned damper may be located at the top of the tower to aide damping of tower and structure resonances.

What’s in it

Guide to a Floating Offshore Wind Farm