Final report for WP4.3: Enhancement of design methods ... - Upwind

damaging if the frequency of the excitation coincides with a natural frequency of the structure. In order to
avoid the range of conditions at which resonance can occur, it is important to predict the frequencies at
which large amplitudes of oscillation may be expected.
Section D.4 addresses the issue of vortex induced vibrations, giving a methodology for calculating the
critical velocities at which resonances will occur for a given structure. However this theory is only valid for
steady currents and cannot be used for waves unless the period is very long, i.e. if the KC number is high.
In the light of this it is recommended that this whole section should be revised in any future revisions of the
standard.
Annex D: Calculation of hydrodynamic loads: Appurtenances
For relatively small appurtenances it is sufficient to account for the additional hydrodynamic forces in the
dimensions and/or force coefficients assumed for the modelled elements. Section D.5 describes a method
for calculating equivalent hydrodynamic coefficients Cdeq and Cmeq taking into account appurtenances and
marine growth on a monopile support structure. The appurtenances are approximated by vertical circular
cylinders, characterized by an equivalent diameter di. Using this method the equivalent hydrodynamic coefficients
are calculated by:
C
⎪⎧
'
D
= ⎨ C
⎪⎩
D
⎡d
N '
deq ds
φ
and
C
meq
i
( R,
e)
⋅WAFd(
K ) + ∑ ⎢ ⋅Cdsi(
Ri
, e)
⋅WAFdi(
Ki
) ⋅ IFdi
( i , Ki
) ⎥⎬
i=
1 D
⎪⎭
⎢⎣
' 2
⎡ 2
( ) N ⎛ '
D
⎞
( ) ⎢⎜
di
C K , C + ⎟ ⋅ 1+ ( C ( K , C ) −1)
⋅ IF ( φ , K )
⎧
⎪
= ⎨
⎪ D
⎩
2
m
ds
∑
⎢⎜
⎟ i=
1
⎣⎝
D ⎠
( ) ⎥
mi i dsi
mi i i ⎬
⎥⎪⎭
In the above equations IFdi and IFmi are interference factors expressing the variation in hydrodynamic coefficient
for the appurtenance due to the presence of the monopile. In order to calculate these factors the
designer is referred to the reference documents [25] and [26].
An improvement to the method currently stated in the standard has been proposed [27], using Cd and Cm
values from [28] and ignoring the wake amplification factor WAFd. The method for calculating IFdi and IFmi
is also expressed in more detail in the proposed method, including separate calculations for blocking and
shielding regimes for both factors. It is recommended that the maintenance team consider this proposed
method in future revisions of the standard.
It has also been proposed that in future revisions of the standard indicative values should be given for
secondary loads and loads on secondary structures. In some instances these may be higher than loads on
primary structures, so it is important they are well defined. It is recommended that this proposal be discussed
by the Maintenance Team.
Annex E: Ice loading
Annex E of the standard provides guidance with regard to ice load calculations The standard states that
the following ice loads should be assessed:
• horizontal load due to temperature fluctuation in a fast ice cover (thermal ice pressure);
• horizontal load from a fast ice cover subject to water level fluctuations and in terms of arch effect;
• horizontal load from moving ice floes;
• pressure from hummocked ice and ice ridges due to both subduction and ridging processes;
• vertical force from fast ice covers subject to water level fluctuations.
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⎤⎫
⎪
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