Standards and Practices - Structural Integrity of Offshore Wind Turbines - page 53

Although turbines should always be type-certified to the expected site
wind conditions (under Class S in IEC 61400-1 and 61400-3), the cur-
rent standard does not specifically address hurricanes in the estimation
of peak wind and wave heights, duration of sustained high winds, or
extreme directional wind changes. In addition, IEC 61400-3 DLC 6.2
allows dependence on yaw system backup power for 6 hours, which
may not be sufficient to ensure safe hurricane ride-through.

• Monopile substructures for wind turbines exceed the diameters and

experience base of the oil and gas industry. Extrapolating current prac-
tice to the larger sizes can introduce unintended effects. Monopiles up
to 5 m in diameter are in use today. In 2010, hundreds of offshore wind
turbine installations were discovered to have excessive tilt due to fail-
ure of the grouting connection at the tower transition piece. This raises
issues concerning vertical tilt tolerances and transition piece grouting
practices in the current standards.

• The behavior and possible degradation of soil strength under combined

dynamic loading from the wind turbine and waves are not well described
in the current standards. Moreover, the empirical cyclic degradation
methods specified are not appropriate. [A recent paper (Andersen 2009)
provides a good description of cyclic degradation of clays under shallow
foundations.]

• Offshore wind turbines in the Great Lakes will encounter freshwater ice,

which may induce first-order loading from numerous new DLCs.
Research and specification development for ice loading in the Great
Lakes are needed, because the loads cannot be estimated from prior
wind energy experience in the Baltic Sea.

• Extreme wave loads may result from breaking waves at some shallow-

water sites. The magnitude of the loading will depend on the type of sub-
structure used and in some instances could be a controlling factor in
design. Standards require analysis of this condition to estimate ( a ) the
wave characteristics and ( b ) the turbine response to the waves, for which
models have not yet been validated for some substructure types.

• Gravity-based substructures are used frequently but are more poorly

documented in the standards than are steel substructures, which are
more commonly used by the offshore oil and gas industry. However,
design of shallow-water, steel substructures for oil and gas structures