The floating substructure provides buoyancy to the turbine and, in conjunction with the mooring system, maintains the turbine’s verticality and movements within acceptable limits. It also provides secondary functions of allowing access from vessels and accommodating ancillary equipment.

What it costs

About £430 million for a 450 MW floating offshore wind farm.

Who supplies them

Floating innovators: BW Ideol, Principle Power, Saitec and Stiesdal.
Engineering consultants: Kent and Ramboll.
Project developers: Equinor.
Steel fabricators: Bladt, EEW, Harland & Wolff, Lamprell, Navantia, Sif, Smulders and Welcon.
The contract for supply may be directly with the steel fabricator, or it can be through an EPCI contractor such as Aker Solutions, DEME, or Jan de Nul.

Semi-submersible floating substructures used at the WindFloat Atlantic project.
Semi-submersible floating substructures used at the WindFloat Atlantic project. Photo of the WindFloat Atlantic project courtesy of Principle Power/Ocean Winds.

Key facts

There are four main types of floating substructures:

  • Semi-submersible
  • Barge
  • Spar buoy, and
  • Tension leg platform (TLP).

Semi-spar is the term sometimes used to describe a semi-submersible with a suspended mass to provide additional stability. In this Guide, it is considered to be a subset of semi-submersibles.

This section of the Guide describes a steel semi-submersible with three columns because this type is the most developed type to date.

A typical steel semi-submersible for a 15 MW turbine has an unballasted mass of about 3,500 t and dimensions of about 80 × 90 × 35 m.

The substructure can move along three axes (heave: up/down, sway: right/left, and surge: forwards/backwards) or rotate about three axes (pitch: tilt from front to back, roll: tilt from side to side, and yaw: rotate when seen from above). Accelerations from all six degrees of freedom contribute to the loads on the wind turbine, so it is vital that the substructure (in combination with the mooring system) controls these to within acceptable limits for a range of metocean and wind turbine load cases.

Once the wind turbine and the substructure type have been selected, a process of jointly optimising the substructure, mooring system, wind turbine and its control algorithms is carried out. Joint optimisation is a complex process and takes many months, but it is expected to shorten with experience.

Designs are based on those used successfully in the oil and gas market, but significant developments have been necessary to address the different loads and requirements of floating offshore wind turbines, and to optimise for serial manufacturing, installation, and support operations.

The diverse fabrication requirements and the logistical challenges of producing such large structures in volume may result in supply from several different locations or suppliers, with the final floating substructure assembly at the wind farm construction port.

The high labour requirements of floating substructures, particularly concrete designs, may make them an attractive opportunity for providing local content.

Guide to a Floating Offshore Wind Farm