4.3.2.3 Spar Buoy
A spar buoy is a slender vertical buoy with a large submerged
volume that provides the buoyancy for the topside
that is located on top of the buoy (S. Chakrabarti,
2005). The stability is provided by a low center of gravity,
which is achieved by ballasting the lower end of the buoy.
A spar buoy is characterized by its high stability (R. Gianville
et al., 1991). This makes it very suitable for the use of
sensitive equipment onboard. For this reason a spar buoy
is used for the Hywind floating wind turbine developed by
Statoil, shown in Figure 4.11. Another benefit of the use
of a spar buoy as basis for the platform design is that the
large hull has sufficient volume to store the collected
plastic before it is transported to shore.
The large hull construction also means that a relatively
large amount of material is required however, resulting in
higher construction costs. The wave and, especially, current
forces on the platform are also relatively large due to
the large submerged volume, resulting in large mooring
loads and thus an extensive mooring system.
Another point of concern is the transportation to location.
The large draught of the spar buoy means that it is usually
not possible to transport it in upright position near shore.
Therefore the buoy needs to be transported horizontally
and upended at location. This requires a rather complex
installation using controlled buoyancy tanks and furthermore
means that the topside of the platform either needs
to be transported on its side and in contact with water
or connected to the buoy afterwards, both options posing
some problems for the design.