FACT BOX
Advantages:
- No transition piece
- Durable
- Can be sailed afloat to site
Disadvantages:
- Heavy structure
- Expensive to install at depths above 10 meters
Installing gravity based support structures at Roedsand 2, Denmark. The barge ‘Eide Barge 5’ was fitted with an 1800 ton lifting capacity crane for the purpose. Photo: Aarsleff Bilfinger Berger Joint Venture
The first offshore wind farm in the world was placed on gravity bases. 11 large concrete structures weighing in average 908 tonnes were placed in the waters near the shore of Lolland, Denmark back in year 1991. They hold the wind turbines that constitute the Vindeby offshore wind farm.
Today, the gravity based support structure is basically constructed the same way. The principle of the gravity base is that the weight of the structure and ballast holds the tower and wind turbine in place, thus no drilling or hammering into the soil is needed. However, the seabed has to be prepared with dredging, gravel and concrete.
The gravity based structure is typically constructed out of steel reinforced concrete. One case of steel gravity base is known. At low depths it is very affordable, but above 10 meters depth it generally is not competitive with other types of structures. The cost of the completed structure is in general proportional with the depth squared.
Largest in the world on gravity bases is the Thornton Bank field in Belgium, who despite the high costs has put gravity based support structures as deep as 27.5 meters (23.5 if one measures after installation and application of scour protection). This implies that the largest of the structures is 44 meters high and weighs 3000 tonnes. 2000 m3 of sand is filled inside the structure as ballast.
Design
There is a lot to be gained if the design of the gravity-based structure is right.
The design of early gravity-based structures was influenced by the round shapes of wind turbines and towers. But round shapes are difficult to construct when casting in concrete, and thus the design moves toward rectangular sections.
An example: If the base of the support structure is circular, the casting of the concrete has to be done using specially built formwork. A conical design makes the process even more difficult, and the construction of the formwork becomes a design challenge in itself.
However, if the design uses rectangular sections, the casting can be done with standard formwork from the construction industry. There is – roughly speaking – no great difference between casting a rectangular-shaped gravity base and casting a concrete building.
Angular sections were used partly in the design at the Roedsand II wind farm in Denmark, and a new design from Strabag Offshore Wind GmbH in Germany takes the idea a step further. Strabag is one of the world’s leading construction companies with its 73,600 employees and €12.8 billion output volume, according to 2010 key figures, and they have been more inspired by the construction industry than by the previous gravity-based designs.
The Strabag solution. Rectangular shapes are considerably easier to produce than round designs. Illustration: Strabag Offshore Wind GmbH
With a footprint that looks more like a Christmas tree stand than a support structure for an offshore wind turbine, the Strabag design is definitely something new. The four rectangular cross-sections form a footprint that can transfer the same forces and loads as a round base. Strabag has also experimented with only three rectangular feet – this was used on a wind measuring platform at Arkona Basin South-East wind farm – but eventually four feet proved to deliver the needed stability.
The rectangular box sections and the shaft are made from pre-stressed reinforced concrete. Base plates are made from reinforced concrete.
Along with the support structure, Strabag has developed a design for serial production, which includes a port facility, where the whole structure including wind turbine is assembled, and a custom-made vessel that transports the structure to site.
Another reason for constructing a custom-made vessel is the sheer size and weight of the structure. The various measurements are:
- Weight of concrete structure: 6500 t
- Weight of ballast: 3000 t
- Water depth range: 20-60 m
- Complete height of foundation: 45-80 m
A full-scale prototype of this design has been built at the factory in Cuxhaven, Germany.
Another design that tries to innovate on the gravity-based structure is the GBF, a joint venture between Gifford, BMT and Freyssinet. They also have designed a custom-made vessel along with the support structure so that the wind turbine and support structure can be transported to site in one piece.
The GBF was chosen for demonstration in the Carbon Trust offshore wind accelerator foundations competition in the UK. The GBF has not built its demonstrator yet (June 2011). There is more about the design in this article about GBF.
Installation
Gravity based supports structures need accurate preparation of the seabed. A layer of gravel and concrete has to be leveled before placing the structure. Comprehensive dredging is usually needed before filling with gravel and concrete. After installation, protection around the structure is needed in order to avoid soil erosion – so-called scour protection is applied.
In some cases, the support structure is self-floating and can be tugged to site. This allows for production in dry docks – when the construction is finished, the dock is filled with water, and the structure can be tugged. This off course can reduce costs for heavy lifting barges.
But in most cases, the gravity based structures are transported on barges. At site the structures are lifted in place using heavy-lifting cranes i. e. the Rambiz or Eide Barge 5.
After installation, ballast is pumped into the gravity base structures or layed on the base of the structure. The ballast can constitute up to two thirds of the final weight.
When installed, a final advantage of the gravity based support structure appears. The concrete construction can last up to 100 years with little maintenance.