Environmental Engineering Reference
In-Depth Information
few offshore rigs available that can handle the lifting, lowering and offshore transport
of such heavy loads. Furthermore, the deployment of these rigs is subjected to a global
demand that results in high charter prices and timetable constraints. And the fact that
the use of such offshore plant is highly dependent on the weather means that calculating
the charter costs is a very risky business.
The reduction in costs that can be achieved with concrete designs therefore essentially
depends on the logistics concepts and the offshore plant available for those concepts.
5.4.2.1 Special design criteria
Besides the typical design criteria associated with wind turbine support structures, for
example the actions due to turbine operation, wind and waves plus the dynamic
behaviour of the total structure, the design of concrete substructures calls for other
criteria to be considered as well:
- Wave loads on compact concrete designs (Section 2.6)
- Bearing capacity of the subsoil, especially near the surface of the seabed
- Scouring and measures required to prevent scouring
- Offshore logistics concept
- Flotation stability while transporting and lowering float-out substructures
- Behaviour in the event of vessel impact
- Area of seabed sealed
- Number of substructures (exploitation of series effects and use of prefabricated
elements)
5.4.2.2 Construction
Conc rete substructures can be const ructed on l and, on a floa ting pontoon or in a dry
dock. When building on land it is generally necessa ry t o ensure that t he su bsoil i s
able to bear the high dead loads; the su bsoil below t he fa brication ya rd ma y need to
be upgraded. A concrete substructure can be transported to the edge of the quay or a
launch barge on skidways or modular heavy-load trailers, for example. Figure 5.16
Fig. 5.16 Gravity bases for the Thorton Bank offshore wind farm. (source:
www.c-power.be
)
