Wind Energy

2
Wave Loads on Wind-Power Plants in Deep
and Shallow Water
Lars Bergdahl, Jenny Trumars and Claes Eskilsson
Summary. A concept for describing design waves for a near-shore site of a
wind-power plant and ultimately the wave loads is to transform the off-coast
wave spectrum to the target site by a model for wave transformation. At the site
second order, irregular, non-linear, shallow-water waves are subsequently realized in
the time domain. Alternatively a Boussinesq model is used. Finally in the examples
here Morison’s equation is used for the wave load and overturning moment.
2.1 A Concept of Wave Design in Shallow Areas
Usually there is little knowledge of long-term wave conditions at prospective
sites for wind-power plants, while the deep-water or open sea conditions may
be more known and geographically less varying. Then a concept for assessing
design waves for the site and ultimately wave loads would be to transform
the off-coast waves to the target near-coast site or shallow offshore shoal by
some model for the wave transformation. Such models can be divided into
two general classes: phase-resolving models, which model the progression of
the physical “wave train”, predicting both amplitudes and phases of individual
waves, and phase-averaging models, which model the progression of
average quantities such as the wave spectrum or its integral properties (e.g.
Hs, Tz). Here examples of using phase average models (WAM and SWAN)
and a phase resolving model (Boussinesq) will be demonstrated. Using e.g.
the phase-averaging model SWAN for the transformation to the site, it is
subsequently necessary to make a time realization of the transformed wave
spectrum into the time domain as the loads on a slender structure is due to
non-linear drag forces, the instantaneous elevation of the water surface and –
for high waves – the skewness of the elevation. For a phase resolving method
the transformed wave is already in the time domain and can thus be used
“directly” in the load modelling.
M = 1
2 ρCDD
�
η
−h
u |u| (z + h)dz + ρ πD2
4 CM
�η
−h
ut(z + h)dz (2.1)