Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 574

account by adding twice the thickness of marine growth to the diameter of the
member under consideration, without an increase in mass.

4.2 Currents

Sea currents may originate from a variety of sources. Friction of the wind with the
water surface may lead to wind-driven currents. Tides also contribute to currents.
Further sources of currents are density differences, due to temperature or salinity
gradients, wind surge and waves.
Depending on the origin of the currents, the current is most pronounced at dif-
ferent depths. Wind-driven currents, for instance are felt strongest near the surface,
while tidal currents may be stronger over the entire depth. Friction with the sea bed
will result in a near-zero current velocity at the bottom. These effects require the
use of different current profi les in different circumstances. While measurements
may lead to accurate descriptions of the local current profi le, in the absence of data
standard current profi le expressions can be used.
For subsurface currents the profi le can be described by an exponential profi le,
which describes the decrease of the current velocity with increasing depth d from
the current velocity U c,sub at the surface to zero at the seabed [2]:

1/7

dz

+

⎛ ⎞

Uz () = U ( 7 )

⎜ ⎟

c,sub c,sub

⎝ ⎠

For wind induced currents the following description can be adopted:

⎛ dz + ⎞

0

Uz () = U

( 8)

⎜ ⎟

c,wind c,wind

⎝ d ⎠

0

In this equation U c,wind is the wind induced current at the still water line and d 0
is a fi xed depth at which the current is zero. If the local water depth is less than d 0
the current profi le is cut off at the seabed. For water depths larger than d 0 the wind
induced current is assumed to be zero for depths larger than d 0 . Commonly used
values for d 0 are 20 [2] and 50 [3].
For evaluation of the current loads only the drag term of the Morison equation
is relevant, as the accelerations due to the variations in current velocity over can
be neglected. Due to the non-linearity of the drag term, the current load cannot
be evaluated separately from the wave load. For a correct evaluation of the total
hydrodynamic load on a structure, the current velocity must be added to the
wave particle velocity. As the direction of the wave particle velocity and the cur-
rent velocity is opposite for half the wave cycle it is important to calculate the
term u 2 as the velocity ( u wave + u current ) times its absolute value as shown in the
following equation:

2

Ft ()=

1

pr × × CDut × × ()+

1

r × DCut × ×( ()+ u )×( ut ()+ u )

( 9 )

M wave D wave current wave current

4 2