Offshore Wind Technology and Status - Structural Integrity of Offshore Wind Turbines - page 19

Filled with
Water

Free Water
Evacuation

Apply
Suction/
Pumping

pumps

pumps

Vertical Flotation Before Touchdown Self-Weight Penetration

Suction Penetration

FIGURE 2-5 Installation of a suction caisson foundation. Suction caissons
are inverted buckets that initially are settled partially into the seabed by the
weight of the platform and then are pulled deeper by suction created when
water is pumped out of the top of the caisson. (S OURCE : http://www.
power-technology.com/projects/hk-windfarm/hk-windfarm2.html.)

special installation vessels and equipment for driving the pile into the
seabed and lifting the turbine and tower sections into place.
Suction caissons can be alternatives to driven piles, eliminating the
intense underwater hammering noise that is a concern for marine mam-
mals. Large-diameter suction caissons can be welded to the base of a
monopile, in which case they often are referred to as “mono-bucket” foun-
dations. Smaller-diameter suction caissons can be used in place of slender
piles to pin jacket substructures to the sea floor. Medium-diameter suc-
tion caissons can be used in place of piles to pin tripods to the sea floor, as
shown in Figure 2.5.
Approximately 20 percent of offshore installed wind turbines are on
reinforced concrete gravity-base foundations, which avoid the need to use
a large pile-driving hammer and instead rely on mass and a larger base
dimension to provide stability and resist overturning. Gravity-base systems
require a significant amount of bottom preparation before installation and
are compatible only with firm soil substrates in relatively shallow waters.
For water depths of 30 m to 60 m (98 ft to 197 ft), which are considered
“transitional depths” between fixed and floating substructures, monopile