Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 369

by far the most important factor governing foundation geometry and shape. This
ratio is several orders of magnitude higher in wind energy structures than in the oil
and gas structures.
Hence the foundation design to break the 25-m barrier needs signifi cant research
and development efforts pertinent to the wind industry-specifi c situations on top of
leveraging the oil and gas experience.
4.4 Design envelope challenge
Wind turbines operate in uncertain environments. There is currently an insuffi cient
understanding of the design envelope that accurately and probabilistically char-
acterizes the extreme conditions off the United States northeast and mid-Atlantic
coasts. Because of this, several challenges must be considered when developing
new wind turbines.
4.4.1 Wind speed assumptions
The currently accepted wind turbine and foundation design envelope, such as the
one used for the GE 3.6 platform, was developed by the IEC and adopted by
Germanischer Lloyd AG in Hamburg, Germany. This design envelope specifi es
Class I wind turbine plants be designed for 50-year recurring, 10-min average
wind speeds of 50 m/s and 5-s wind gusts of 70 m/s. However, it is not clear how
closely these extreme design assumptions, developed based on European onshore
experiences, will apply to U.S. regions that host offshore plants.
4.4.2 Turbulence ratio
A common wind industry practice calls for utilizing a 12% turbulence intensity for
offshore sites [5]. This is based, again, on European experiences and it is not yet
clear whether this is truly representative of potential U.S. sites.
4.4.3 Hurricanes
U.S. east coast is susceptible to hurricanes. The presence of hurricanes or typhoon
is unique to the U.S. and Asian-Pacifi c areas because there is no similar scale
storm hazard in Western Europe where the offshore wind technologies were pio-
neered. Although U.S. offshore construction and foundation design codes of prac-
tice have acquired signifi cant expertise in understanding the impact of hurricanes
in the Gulf of Mexico, this expertise may not be all transferable to the wind indus-
try along the upper east coast.
In the northern hemisphere and because of their counter-clockwise rotation,
hurricanes generally have lower wind speed at their western or left side compared
with their eastern or right side as shown in Fig. 3. In the Gulf of Mexico where
landmass largely extends east to west, oil and gas installation must be designed for
the maximum wind speed of the hurricanes on its eastern side. For the wind indus-
try, which focuses mainly on the east coast of the United States currently, land-
mass extends roughly south to north. It is less likely that the eastern side of a
hurricane will affect a wind power plant unless the hurricane makes prior landfall
and hence losing half or more of its energy (Fig. 4).