Tripod (foundation)

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Tripod in Bremerhaven

A tripod (from the Greek  τριπους for "tripod") is a form of foundation for offshore structures in construction .

Tripods are assembled from round structural steel tubes and welded so that a stable tripod is created. The structure is lowered to the seabed and anchored with piles that are driven through the sleeve-shaped feet. The supporting structure is usually completely submerged. The loads of an offshore structure, e.g. B. a wind turbine , are derived via the extension of the central foundation pipe to the tripod. The design differs in this point from the three-legged Tripile foundation . Tripods are suitable for water depths from 25 m on soils in which piles can be inserted with a load bearing capacity.

For example, six of the twelve wind turbines in the first German offshore wind farm, alpha ventus , were founded on tripods. Jackets were used for the other six systems .

Development history

Onshore tripod

Offshore foundation structures have their origins in the oil & gas industry. Jacket constructions on oil platforms, for example, have been tried and tested for decades, and tripods have also been used in this environment. However, different forces act on an offshore wind turbine than on a heavy oil platform, and the water depths of more than 25 m also presented a challenge. The foundation design was then adapted to the OWT requirements and a tripod design specially for OWEA developed (by OWT - Offshore Wind Technologie ).

In 2006, after a long development period, a tripod "onshore demonstrator" was designed, manufactured and installed in Bremerhaven on behalf of Weserwind GmbH for Multibrid GmbH. The first operation was accompanied by the research project IMO-Wind. The main focus of the measurements on the prototype was, among other things, the determination of voltage curves, the so-called “hot spot” measurement, in order to enable comparison with calculation models.

Horizontal manufacturing at Aker Yards (2008)
Transport with Taklift 4 to the Alpha Ventus test field (2009)
Series production in an upright position at WeserWind
Three tripods on board the Stanislaw Yudin, ready for transport

In 2008 the tripod was built as a foundation structure for six Multibrid M5000 OWTs in the alpha ventus project. alpha ventus was planned as a test field for the offshore use of wind energy. Parallel to the construction, the BMU supports a number of research projects in the RAVE initiative ( R eSearch at A lpha VE are summarized NTUs). This is intended to provide a broad base of experience and knowledge for the construction and operation of further offshore wind farms. Project participants: EWE , eon , Vattenfall , Areva , Senvion , OWT .

The tripods were then planned as the first commercial use on a larger scale in the offshore wind farm Borkum West II. 40 OWTs have been in operation in what is now known as Trianel Windpark Borkum , since 2015 - based on tripods at a depth of 26–33 m. According to the company, another 40 plants should be built from 2017. 33 municipal utilities under the management of Trianel GmbH are participating in the offshore project with a total investment of 1 billion euros.

In 2012–2014, 80 tripods were manufactured and installed in 40 m water depth for the Global Tech I offshore wind farm. It is currently (as of September 2015) the most distant offshore wind farm from the coast.

Special technical characteristics

Suitability and conditions of use

The specialty of the tripod lies in the combination of the surface structure of a single-pile solution (low exposed surface, robust behavior in risk scenarios, easy transition to the tower side, production comparable to monopiles and towers) with the load-bearing effect and performance of a broken structure.

In wind energy, the coordination of the dynamics of the structure, i.e. H. In which frequencies it mainly vibrates, of particular importance. The tripod lies between the monopile, which tends to be softer, and the fully resolved framework structure jacket, which in turn is stiffer.

The area of ​​application is geometrically determined to be on the order of at least 25 m water depth up to currently 50 m.

Comparable to the half-timbered structures “jackets”, the tripod requires floors in which piles can be set. Whereas monopiles, especially those with a high load-bearing capacity, are preferably used in densely packed sandy soils or the like.

The connection to the post is usually achieved via a so-called grout. It is a technique in which special concrete is poured into the joint gap between the pile and the pile sleeve. The resulting composite effect transfers the loads from the sleeve to the pile and thus into the ground.

Structural background

Carrying effect

The load-bearing effect is based on the deflection of the tower's bending moment onto the piles, which are then essentially only pulled or pushed. This requires a combination of upper and lower legs that build the leverage for it. Alternatively, a suction bucket can be used instead of the stake . In comparison, the monopile carries its loads by supporting itself laterally in the ground.

For pipe joints, it is preferred that incoming pipes remain in certain ratios of the diameter (around 0.8) to the continuous pipe and then achieve higher load-bearing effects. This effect leads to the existing dimensional relationships.

The thickness of offshore foundations in general belies the actual load-bearing effect. Tripods and monopiles are shell structures. Their wall thickness, although 100 mm sheet metal thickness is used in parts, is relatively small compared to the diameter. You must therefore be proven against shell dents .

The service life is a key issue in design. Wave loads have already been taken into account in the classic offshore oil and gas industry. In wind energy, ongoing operation causes high operating loads (see Growian project). When used at sea, both effects combine.

The cylindrical sections from which the tower, central tube and legs are assembled are 3 to 4 m long. The wall thicknesses are graded according to the respective requirements. In the central tube, the wall thickness is in the range from 40 to 60 mm, on a few meters in particularly stressed areas up to 100 mm. The legs connect 25 to 30 mm to the central tube.

Calculation methods

FEM methods are mainly used for the calculation . Only they can reproduce the stress curves in detail and with the accuracy required for the design. Due to the increasing computing speeds, the computation times have been reduced considerably, which can be used to increase the iterations and thus the optimization.

Web links

Commons : Tripods for Offshore Wind Turbines  - Collection of pictures, videos and audio files

Individual evidence

  1. a b foundation structures. offshore-stiftung.com, accessed on November 16, 2013 .
  2. Foundations and foundation structures. offshore-windenergie.net, archived from the original on January 10, 2015 ; Retrieved November 16, 2013 .
  3. With three legs on the high seas. In: Deutschlandfunk.de. Retrieved on February 10, 2016 (German).
  4. C. Heftrich: Integral Monitoring and Evaluation System for Offshore Wind Energy Plants (IMO-WIND). In: www.mb.uni-siegen.de. Retrieved February 10, 2016 .
  5. https://www.trianel.com/trianel/beteiligungen/
  6. Tripods. (No longer available online.) In: owt.de. Archived from the original on February 10, 2016 ; accessed on February 10, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / owt.de