A heavy weight of steel foundation with ballasted caissons

Due to the restrictions imposed by the BSH with regard to ram noise during the installation of offshore foundation structures, alternatives are being sought that do not require ram piles. One possibility is, for example, heavyweight concrete foundations, whose cavities can also be filled with sand if required, or foundation piles that are drilled or vibrated in. A heavyweight foundation made of steel has not been considered so far, although it is also possible to ballast with sand in a steel caisson. Together with a large lever arm, the necessary restoring moment against the wind and wave loads is activated.

Mr. P. Kelemen from Salzgitter has developed a foundation variant made of steel which acts as a heavyweight foundation. The aim of the design was to achieve a stable and fatigue-proof layout and dimensioning of the pipe sections and nodes of the K-tower offshore. The SKI Ingenieurges. mbH in Hanover was commissioned with this design.

The entire supporting structure is composed of two cylindrical tower sections and the K-Jacket, which is based on three caissons. Between the lowest tower section and the K-Jacket is the central tube node as an interface, with a pre-stressed ring flange connection serving to transmit power. Below the central tube node, the working platform is integrated into the foundation structure and thus used for load transfer. An adjustment is also integrated there in order to compensate for possible long-term misalignment of the K-jacket.

The jacket legs and struts connect to the upper edge of the caissons and also serve as guides for the compressed air and water pipes. The arrangement of the legs and struts allows each chamber of a caisson to be reached so that individual control of negative and positive pressure during installation is possible.

OWT - Suction Bucket

Further features of K-Tower Offshore

K-Turm-Offshore-Draufsicht

Foundation structure as K-tower

The tilting during installation is directly compensated by the lowering method, because the caissons can be pressed directly into the seabed by negative pressure and a temporary load. After the controlled lowering process, the caissons are permanently ballasted with a suitable and cloud-proof mineral mixture in order to obtain a sufficient gravity and restoring moment against the lifting forces due to wind and wave loads. Here, not only the ballast mass but also the lever arm is decisive.

Als Einsatzgebiete für den K-Turm-Offshore werden Standorte in der Nordsee und anderen Meeren bevorzugt, die eine geringe maritime Schicht aufweisen. Ein Eisgang sollte ausgeschlossen sein. Der tragfähige Baugrund muss mindestens mitteldichte Sande besitzen.

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
  • restlose Demontage der Gründungsstruktur inkl. der Caissons möglich

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).

Services

The SKI Ingenieurges. mbH accompanied these offshore developments with the following servic

Foundation variant: K-tower offshore
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