Environmental Engineering Reference
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methods, including hub-height masts equipped with anemometers, stand-alone lidars,
and, in some cases, no on-site measurements at all.
This mixed experience notwithstanding, a fixed, instrumented tall tower must still
be regarded as the approach offering the highest confidence and accuracy. For reasons
that will become clear in the next section, such towers are extraordinarily expensive,
with costs ranging upward of $2-3 million, and some as high as $10 million. They
can also take many months to be designed, permitted, and built. Consequently, few
project developers choose to install more than one such tower, and there is always
keen interest in alternatives and complementary approaches. One possible alternative
is to install the tower on an existing structure, if there is one not too far from the
project area. Other options include floating or fixed remote sensing systems (mainly
lidar and sodar), which can be deployed far more cheaply than tall towers, as well as
satellite-based radar measurements. For the time being, these options are best regarded
as complementing rather than replacing purpose-built towers, but in the future, lidar,
in particular, may become a primary instrument for offshore resource assessment.
14.2.1 Purpose-Built Meteorological Towers
To date, the majority of European offshore meteorological towers for resource assess-
ment have been purpose-built, self-supporting lattice structures, such as that shown
in Figure 14-6. To understand why such a tower should be so expensive, consider
that the tower height is the sum of the heights above and below water. Thus, a tower
deployed in water 30 m deep and extending 80 m above the water is actually 110 m
tall from the seabed to the top. Moreover, the part in contact with the water must
be able to withstand powerful ocean currents and waves. Water is about 800 times
denser than air, and so a typical tidal current of, say, 2 m/s exerts a force equivalent
to a wind speed of 57 m/s—a class 3 hurricane. During storms, waves can exert still
larger forces, and in cold climates, the impact of ice must also be accounted for in
the structural design. Add to that the potential wind and ice loads on the parts of
the structure that are above the water, and the result is that offshore masts must be
massive and their foundations attached securely to the sea floor.
Offshore wind monitoring towers are typically equipped with instruments very
similar to those deployed on land, including anemometers, direction vanes, and air
temperature and pressure sensors, although marine-grade models may be called for. In
addition, the towers and their platforms may have motion sensors to detect deflections
caused by wind and waves, which can affect speed measurements, and a variety of
instruments to measure ocean currents, wave heights, water temperature, and other
parameters-even bird activity. These other elements can transform an offshore mast
into a complete ocean-atmosphere-wildlife monitoring system, providing data critical
to the design and permitting of a wind project.
The anemometer configuration on an offshore mast generally differs somewhat
from that recommended for land-based towers. Typically three or more anemometers
(at least one per tower face) are installed at each monitoring height, rather than only
two. This added redundancy aims to achieve a high overall data recovery with fewer
visits for maintenance and repair. Furthermore, having three anemometers (installed
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