The primary structure consists of the large structural elements which provide buoyancy and resist the loads from the mooring system and the base of the wind turbine tower.

What it costs

About £360 million for a 450 MW floating offshore wind farm.

Who supplies them

Large steel or concrete floating substructures and their major components must be produced by specialists capable of fabricating large and heavy items, which requires the manufacturing facility to be located port-side. If the final assembly of the floating substructures is in a different location this must also be located port-side to move complete substructures into the water. Experience in batch volume production is highly desirable for orders of many units, which limits the choice of supplier options.

Fabricators: Aker Solutions, Bladt, EEW, Harland & Wolff, Lamprell, Navantia, Sif, Smulders and Welcon.

The final assembly of the primary structure of a steel semi-submersible floating substructure.
The final assembly of the primary structure of a steel semi-submersible floating substructure. Photo of the WindFloat Atlantic project courtesy of Principle Power/Ocean Winds.

Key facts

The primary structure is made of the following main components:

  • Columns
  • Pontoons
  • Trusses, and
  • Transition piece.

Semi-submersibles typically have a triangular arrangement, with three columns connected by horizontal pontoons at the base, with further trusses (also known as braces) between them. The major structural elements are:

  • Columns and pontoons: provide most of the buoyancy, and because they are located away from the centre of the substructure, they provide stability. The pontoons may have square edges and heave plates to reduce heave motion (up and down) in the water by creating drag.
  • Trusses: the triangular shapes made by trusses, in conjunction with the columns and pontoons, provide rigidity.
  • Transition piece: some designs include a transition piece to distribute the concentrated loads from the base of the tower into the floating substructure.

On a semi-submersible substructure the turbine may be positioned on one corner, in the centre, or halfway along one of the sides. A centrally-mounted turbine results in the most symmetrical loading and reduces the need for active ballasting to maintain verticality. A disadvantage is the longer crane reach needed to assemble the turbine onto the substructure.

Important considerations for efficient manufacturing include the space required, the cycle time (a shorter cycle time allows a higher delivery rate), and the total manufactured cost.

Some designs seek to use manufacturing processes and facilities developed for other purposes to reduce manufacturing costs, like tubular fabrication used for turbine towers (for example Stiesdal Offshore’s TetraSpar) or panel production lines used for shipbuilding (for example Gusto MSC’s Tri-Floater). The latter may result in columns with square or hexagonal cross-sections.

Some designs are fully welded whereas others use joints for the final assembly of the major items of the floating substructure. A design suited to final assembly allows manufacturing firms to focus on major component manufacture and suitably located ports to focus on the final assembly of the floating substructure.

The mass of a typical primary structure, at 3,500 t, is greater than the maximum lift capacity of the largest mobile cranes. Rail systems or self-propelled modular transporters are options for moving them on land. Ring cranes, vessel-mounted cranes, or semi-submersible barges can be used to move a primary structure from land into the water. A dry dock addresses both issues at the same time, but large dry docks are scarce (see I.5 for further information).

Some fabricators are exploring the use of electron beam welding to reduce the time, cost, and energy consumption associated with the more commonly used submerged arc welding.

What’s in it

Guide to a Floating Offshore Wind Farm