Method for manufacturing a gravity based foundation for an offshore installation, and gravity based foundation

A method for manufacturing a gravity based foundation for an offshore installation,

• wherein the foundation comprises a caisson of concrete and a hollow shaft, the caisson having a bottom slab, a roof and a side wall extending between the bottom slab and the roof to define a hollow structure delimited by the bottom slab, the roof and the side wall, the roof having a passage for the shaft, the shaft extending with its lower part through the passage into the caisson;
• wherein the bottom slab is provided with a shaft support of concrete, the shaft support having embedded tensioning bars vertically projecting from the upper side of the shaft support;
• wherein the shaft has a flange at its lower part, the flange having bolt passages and being supported and mounted on the shaft support by means of the tensioning bars extending through the bolt passages;
• the method comprising the following steps:
• providing a concrete bottom slab,
• providing a full-length formwork configured for defining at least part of the shaft support and having a vertical length corresponding to the vertical length of the shaft support, and arranging the full-length formwork onto the bottom slab,
• providing a slip formwork configured for defining the side wall and having a vertical height smaller than the vertical length of the full-length formwork, and arranging the slip formwork onto the bottom slab,
• providing the tensioning bars and mounting the tensioning bars in a fixed position to the full-length formwork, and
• concrete pouring of the side wall and shaft support while raising the slip formwork.