A heavy weight of steel foundation with ballasted caissons

Due to the restrictions of the BSH in terms of pile driving noise during the installation of offshore foundation structures alternatives are sought that do not require piles. One possibility, for example, are heavyweight concrete foundations, whose cavities are filled, if necessary, with sand or piles that are drilled or vibrated. A gravity foundation of steel has not yet been considered, but can be ballasted in a steel caisson with sand. Together with a large lever arm the necessary restoring torque against the wind and wave loads is activated.

Mr. P. Kelemen from Salzgitter has a type of foundation steel developed which acts as a gravity foundation. The stand-and fatigue-safe design and dimensioning of the pipe sections and pipe nodes of K-Tower Offshore was the goal of the draft designs with the SKI Ingenieurges. mbH was commissioned in Hanover.

The entire supporting structure is made up of two cylindrical tower sections and the K-Jacket, which is founded on three caissons. The central pipe node is located between the bottom tower section and the K-Jacket as an interface, with a prestressed annular flange is used for power transmission. Below the central tube node, the working platform is integrated into the foundation structure and thus be used for load transfer. There also an adjustment is integrated to compensate for any long-term conditional misalignment of the K-Jackets.

The jacket legs and braces connect to the top of the caissons and serve as guides for the compressed air and water lines. The arrangement of the legs and struts each chamber of a caisson is achieved, so that individual control over negative pressure and positive pressure during installation is possible.

Further features of K-Tower Offshore

The misalignment during installation will be directly compensated by the lowering method, because the caissons can be pressed by vacuum and a temporary ballast specifically in the seabed. After the controlled lowering a permanent ballast of the caisson is carried out to obtain against the contrasting forces due to wind and wave loads a sufficient heavyweight and-resetting moment with a suitable and crater safe mineral mixture. Here not only the ballast mass, but also the lever arm is crucial.

As applications for the K-Tower offshore sites in the North Sea and other seas are preferred which have a low marine layer. A floating ice should be excluded. The sustainable building must have at least medium dense sands.

Further features of K-Tower Offshore:

  • Deep water 20 to 50m 
  • No ice
  • Building with medium dense and dense sands
  • Reduced volume and reduced depth of the ground investigation required
  • Silent installation work, as no piles available (also mass savings)
  • No grout connections friction bonding of steel parts and to compensate for the misalignment required
  • Foundation structure is immediately after lowering stable for wave climate during the installation phase 
  • Residue-free disassembly of the foundation structure including the caissons possible 

    

 

      

Level of development

The stability for a site in the North Sea has been confirmed by the TÜV Nord in Hamburg. The draft design of SKI was rigorously tested in addition to the stress and stability and for the proof of fatigue. The geotechnical evidence such as Settlement calculation, tilt and slip were reviewed by a geotechnical expert. Further information on the level of development can be obtained from Mr. P. Kelemen (kelemen-salzgitter(at)t-online.de).