Actions on wind turbines - Concrete Structures for Wind Turbines - page 32

Table 2.10 Wave elevation coef cient d

d/(g T D ) H D /(g T D )

0.02 0.01 0.005 0.001 0.0005 0.0001

0.20 0.60 0.55 0.50 0.50 0.50 0.50

0.02 — 0.68 0.58 0.52 0.50 0.50

0.002 — — — 0.87 0.80 0.68

H D design wave height ( ¼ H max,50 , see Section 2.5.8)
T D design wave period (see Section 2.5.8)
d depth of water

The highest wave elevation above still water level (HSWL, MSL or LSWL) is specified
in [11] as follows:

x ¼ d H D

where
d wave elevation coefficient (see Table 2.10)

Intermediate values may be obtained by linear interpolation provided the waves do not
break (H D < 0.7 d).
Estimates of the water depths must be based on measurements carried out at the site.
The levels of tides and storm tides must be derived from statistics based on long-term
measurements and numerical models.

2.5 Hydrodynamic environmental conditions
The foundations of offshore wind turbines are exposed to the sea state and the
currents in addition to the aforementioned actions. The principles for calculating
the hydrodynamic actions plus their interaction with the foundation anchored to the
seabed are briefly outlined below. In doing so, reference is made to the classic work
by Kokkinowrachos, Hydrodynamik der Seebauwerke [17], which is recommended
for a more in-depth study of the material and the answers to specific questions.
Further textbooks for a more thorough introduction to the theory of water waves are
given in [18].

2.5.1 Sea currents
The velocity potential of a sea current is mostly determined from the superposition of
the tidal current (U c,sub ) and the wind-generated current (U c,wind ) plus, if applicable, a
current induced by waves, especially breaking waves (U surf ) [11].