Environmental Engineering Reference
In-Depth Information
Fig. 5.20 Hybrid gravity base in high-strength lightweight concrete
grbv Ingenieure im Bauwesen
GmbH & Co. KG Hannover Germany
Once a spread foundation has been positioned, the surrounding seabed must be
protected against scouring, for example with sand-filled geotextiles. Figure 5.20 shows
an example of a hybrid gravity base with scouring protection. The logistics concept in
such a situation presumes that the concrete substructure is fabricated in a dry dock.
After controlled floating of the concrete substructure in the dock, it is attached to
heavy-lift pontoons and towed from the dock to a nearby quayside. This is where the
steel shaft can be mounted on the concrete foundation, aligned and the space between
steel and concrete filled with a high-performance concrete (grouted joint). Alterna-
tively, the tower shaft can be in prestressed concrete.
The hybrid substructure is then towed to its offshore location. The necessary stability
during the floating procedure is guaranteed by the heavy-lift pontoons. At the site,
the foundation can then be lowered to the seabed in a controlled operation with the
system of winches installed on one of the pontoons. Once in position on the seabed
and ballasted for the final condition, the anti-scouring measures can be carried out.
The extent of the measures necessary to prevent scouring can be estimated by way
of numerical analyses and investigated and specified with the help of testing. The
geometry of the substructure can have a considerable influence on scouring, or rather
the scouring protection measures required. A review of the various anti-scouring
measures used for different offshore designs and the experience gained with such
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