Jacket structures on their way to site. Photo: Alpha Ventus
No less than 104 different types of support structures competed when in 2009 the UK fund Carbon Trust set out to find the most cost effective. But even though the variety is large, support structure concepts are basically divided into two groups: Floating or Grounded.
Floating concepts imply that the support structure transfers loads and forces to the water, not the soil. Connection to the soil only ensures that the support structure stays in place.
Grounded concepts imply that the support structure transfers all loads and forces to the seabed.
The figure shows an overview of the types of structures and foundations. See the article 'What is the support structure' for more definitions, e.g. on the distinction between support structure and foundation.
The different types of foundations and support structures for Offshore Wind Turbines (OWT). The overall dominant one is the monopile, which is both piled in foundation and support structure. Illustration: LORC
The two concepts have strong differences, which make them applicable for various environments. Speaking generally, the pros and cons of floating concepts are:
- Pro: Large water depths – theoretically no limit
- Pro: Floating structures allow full fabrication at shipyard and transport to site in one piece.
- Con: Very expensive construction
- Con: Commercially uninteresting for many countries with large areas of shallow water
The pros and cons of grounded concepts are:
- Pro: Less expensive
- Pro: Large potential with water depths up to 50 meters or even deeper.
- Con: Expensive transportation and installation
- Con: Most types have only been installed in water < 25 meters.
Monopiles rule – so far
On the grounded concepts, the two major types used are the piled foundations and gravity-based foundations. The monopile is the most dominant, because of its simplicity in fabrication and installation. However, at large water depths the foundation is piled, but the support structure is more complex, i.e. a jacket or a tripod structure.
Gravity-based support structures are the second major type at the moment, but they lack the potential for large water depths. This is because when the water depth increases, the mass of the gravity-based foundation also has to increase, which leads to problems for transportation and installation.
However, the gravity-based foundations are simple to produce – although they require a large mass. Thereby they become very stable and experience no lifting between the seabed and the support structure.
For more details on each support structure concept, click the articles in the menu.