Simple but clever

Friday, 4 December, 2015 - 16:15
Six sand-filled ballast tanks will anchor the ­Texbase to the seabed. (Graphic: MOG)
Six sand-filled ballast tanks will anchor the ­Texbase to the seabed. (Graphic: MOG)

The principle of gravity foundations may be simple but the ­concept is ­having a hard time gaining a ­foothold in the market. The Texbase aims to change this. Its big advantage is that the sand silos that provide a secure anchorage can be filled on site.

Gravity foundations are currently outsiders in the foundation business. They are too heavy and too bulky, and therefore too expensive to install. These are precisely the weaknesses that Texbase tackles. To put it simply, Texbase replaces the heavy concrete bases of current gravity foundations with flexible textile silos which only need to be filled with sand at the offshore location.

The Maritime Offshore Group (MOG), the steel construction company Conferdo, the Mittelhessen University of Applied Sciences and the Franzius-Institute of Leibniz University in Hanover began collaborating on the development of this new type of offshore foundation system in July. The project participants plan to conduct research and development for two years and achieve a detailed concept by mid-2017.

The basic characteristics are already clear: the Hexabase foundation developed by MOG and ­ThyssenKrupp Mannex, a jacket with a six-sided cross-section designed for serial production, will be equipped with a new base. At the bottom will be six flat elements with a strong textile weave stretched across them. Six ballast tanks made of geotextiles and filled with sand will hold the foundation securely on the seabed.

Relying on gravity

Texbase – the path to the prototype
> Base construction and structural calculations: Mittelhessen University of Applied Sciences
> Detailed development of the ballast tanks and ground supports: Maritime Offshore Group
> Analysis of geotechnical ­effects: Franzius-Institute of Leibniz University, Hanover
> Planning of production and after-sales maintenance: Conferdo
> Manufacturing and sales: ThyssenKrupp Mannex
> Coating: BASF Coatings

The inventors aim to make use of a property of sand to fill the sandbags: while sand is extremely stiff and has a specific weight close to that of concrete when it is compacted, a sand/water mixture is so fluid that it can be pumped. This means that the Texbase can be transported to the installation site empty and lowered into the water before the ballast tanks are filled with a sand/water mix.

Because the geotextiles to be used for the ballast tanks are permeable to water, it will be squeezed out of the silos by the foundation’s own weight and the remaining sand will be compressed. Its own weight will also cause the Texbase to slowly sink into the ground. It will go quite quickly through the first 50 cm or so of sludge that forms the upper layer, but because the seabed under it is also compressed by the weight of the foundation, Christof Schramm, Research and Development at MOG, calculates that the Texbase will sink by a further 50 cm. “Foundations usually stand in the water for a while before the turbine is installed anyway. By then the base should have settled”, he says.

Texbase’s developers see its greatest advantage in the quick and cheap installation. “A normal crane vessel suffices for the installation, while concrete gravity foundations generally require four tugs”, says Schramm. But the Texbase will not just have to measure up to other gravity foundations. In order to succeed on the market, the developers will have to offer a true alternative to the monopile. Schramm sees potential in that area too. “The larger monopiles become, the more difficult it is to install them”, he says. Furthermore, using the Texbase not only saves time. Because it does not require any preparation of the seabed, it is also more environmentally friendly. It can be used in deep waters and “is very well adapted to most offshore sites in Europe”, as Schramm emphasizes. The only terrain on which it makes little sense is solid rock.

To ensure that the wind turbines have a stable base, several thousand tonnes of sand will be needed. Schramm estimates that 4,000 to 5,000 t will be required for a 6 MW system. Exactly how much is actually needed will be calculated over the two years of the research project. It is clear that a foundation for a 6 MW turbine will be the basis for the calculations. However, in the future, Texbase will also support turbines as large as 8 MW in depths of up to 50 m.

Katharina Garus

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