OFFSHORE RESOURCE ASSESSMENT - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 202

120 ◦ apart) at each height allows for the wind to be measured by at least two sensors
from every direction, thereby reducing the influence of the large tower on the wind
measurements.
One of the main challenges associated with self-supporting offshore masts is their
large face width. In order to minimize the effect of flow distortion on the measure-
ments, IEC guidelines recommend a minimum separation distance on lattice towers of
three to four face widths, depending on structure porosity. Achieving this standard may
be impossible at every monitoring level. Nonetheless, adequate instrument separation
requires booms that are significantly longer than those typically installed on land-based
met towers. These must be reinforced, as shown in Figure 14-6, to maintain struc-
tural stability. The booms in this illustration are designed to rotate vertically toward
the tower so that the instrumentation at the end of the boom can be accessed for
maintenance. The booms are held in place by tensioned wires connected to the tower.
The design and deployment of an offshore meteorological tower usually takes much
longer than that of a land-based tower. In general, several preconstruction studies are
required to obtain the necessary permits. A variety of meteorological, oceanographic,
and geophysical studies are needed to characterize the environmental conditions the
tower will face as well as the geological conditions below the sea or lake floor.
As shown in Figure 14-7, the installation of the FINO 1 research platform, an
offshore meteorological tower in the North Sea, required a variety of vessels and
equipment. For this particular installation, the jacket foundation structure was first
lowered onto the seabed and piles were then driven through the foundation's pile
sleeves and into the seabed using a hydraulic hammer. The piles and foundation
were bonded using grout. Next, the platform, on which the met tower had been
partially constructed, was installed by a crane barge and grouted into place. Finally,
the remaining sections of the met tower were installed by the jack-up barge's crane.
The FINO 1 tower represents just one way that offshore towers can be constructed.
Many other approaches can be taken depending on the characteristics of the platform
and tower. For example, lattice towers can be erected using a gin pole, which extends
above the top of the partially built tower and is equipped with a pulley, allowing the
next section of the tower to be lifted into place.
Offshore towers will most likely be deflected to some extent by wind and waves
during operation. Although previous studies have found that such movements have
only a small impact on wind speed measurements (5), the impacts for any particu-
lar mast depend on the station design and instrument choice. Thus, it is prudent to
measure the deflections directly to be able to quantify the impact on speed. The instal-
lation of multi-axis accelerometers at each monitoring height should be considered to
accomplish this.
14.2.2 Surface-Based Remote Sensing Systems
As on land, surface-based remote sensing technologies, mainly lidar and sodar, are a
useful complement to traditional tower-based measurement campaigns offshore. Colo-
cated with a mast, they can extend the wind resource measurements from the mast
height up to the top of the turbine rotor plane. As the use and acceptance of remote