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8/12/2019 Overview Environmental Processes and Environmental Loads
1/27
Classification: Sta 1
3.5 WIND LOADS
3.5.1 Wind phenomenology
Wind speed is experienced essentially at two different time
scales:
1: A slowly varying mean wind level; Vm. !is wind component can
often "e considered as constant for a s!ort term period# say $ !ours.
%: A &rapidly && fluctuating wind component# Vt# riding on t!e
mean wind speed. !e period of fluctuations will "e from
second to some few minutes.
Year 'ew days (our 1 min. 1 se)
Storm spectrum
*ower
spectru
m
Speed
ime
ur"ulence
+ean wind speed
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta %
At a given point# t!e resulting wind speed can "e written:
V,t- Vm,t- / Vt,t-
!e mean wind speed is typically t!e largest. errain
roug!ness govern t!e ratio. !e ratio "etween standard
deviation of Vt and Vm is called tur"ulence intencity.
ypical tur"ulence intencity over ocean wit! storm
waves is a"out 0.1%.
!e wind speed varies wit! !eig!t# see e,1a 2 1d- in
Statoil metocean report.
'or engineering purposes# t!e mean wind speed is
descri"ed "y a distri"ution function often close to a 3ayleig!
distri"ution. !e mean direction is descri"ed "y a pro"a"ilitymass function for direction sectors ,often- of $0 deg. widt!.
!e mean wind speed corresponds to a given lengt! of
averaging. Standard meteorological averaging is 10 min..#
in design t!e lengt! of averaging is often ta)en to "e 1 !our.
Wind speed will increase wit! decreasing lengt! of averaging.
!e ration "etween a 14se) average and a 15!our average
10m a"ove sea level is 1.$6# see ta"le in Statoil report for
ot!er examples.
7xample of a wind description for design purposes is s!own"y Statoil +etocean report.
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta $
'or structures or structural components w!ere t!e tur"ulent
wind may cause a dynamic "e!aviour# t!e freuency spectrum
for wind speed is given "y 7. ,%a and %"- of Statoil 3eport or8orso) 8500$.
!is wind spectrum is deduced from wind measurements at
'r9ya.
!e tur"ulent wind is not fully correlated over t!e sie ofstructures. Co!erence spectrum "etween two points are
given in Statoil report or 8orso) 8500$.
!e loads on structures not exposed to dynamic "e!aviour
can "e calculated considering t!e wind as static:
f structural dimensions are less t!an 40m# $s gust s!ould "e
used.
f structures are larger# 14s gust can "e used.
'or structures w!ic! are exposed to simultaneous actions
from wind and waves# and w!ere t!e wave loading isdominating# t!e lengt! of averaging of wind gust may "e ta)en
to "e 1 minute. C!ec) wit! coming editions of 8orso) for
possi"le c!anges.
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta C A ,Vm,-%/ %= Vm,-=Vt,#t-- = sin
t is seen t!at load is linear wit! respect to wind speed ,since
Vt%term is neglected-. f t!e wind induced response is linear
function of load# t!e wind response may "e o"tained using
freuency domain analysis# i.e. t!e cross spectral densityfor t!e dynamic wind load is multiplied "y response transfer
function in order to o"tain response spectra for dynamic
wind induced response.
Alternatively# wind !istories for a num"er of load points may
"e simulated from wind spectrum and corresponding time
!istories for t!e response found "y solving t!e euation of
motion in time domain.
!e total extreme wind induced response can "e found "y:
'$!5max,Vm- 'm/ g = ,Vm-
g 2 extreme value factor for $5!our maximum dyn. response
2 standard deviation of wind response under consideration
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta 6
#ore$ Ind%ced #i!raions &!rief inrod%cion'
Vortex s!edding freuency in steady flow is given "y:
f St = V?D
St is t!e Strou!al num"er# V is wind speed and D is structural
diameter.
A critical velocity is defined as t!e velocity giving vortex
s!edding freuencies eual to t!e natural freuency oft!e structural mem"er:
VC 1?St = f8= D
f82 natural freuency of structural mem"er.
St is a function of t!e 3eynolds num"er# 3e VD?# w!ere
is t!e )inematic viscosity of air# , 1.
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta F
3.( Wa)e and *%rren loads
Water levels:
+ean sill ,aer le)el
Lo,es asronomical ide &LA-'
ighes asronomical ide &A-'
idalrange
+a$im%m sill ,aer le)el
*ositive
storm surge
+inimum still water level
8egative
storm surge
Classification of structures
= !ydrodynamic transparent ,slender structures# +orison
loading-
= !ydrodynamic compact ,large volume structures#
diffraction analysis-
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta G
Diffra/s0onsanalyse +orisons ligning
.asighespoensial"
=
++=6
1k
kdi
Inn
/ommende
2e)egelses
ind%ser
3rafinensie"
udcuudcf MD 2
4||
2
1
+=
Drag
ledd
+asse
ledd
Dragledde
dominerer
+asseledde
dominerer
Linearisere
dragledd 4
ig%r"
alinsen&1665'
9lger og str9m vi)tig9lger vi)tigst
When to account for which effects?
8/12/2019 Overview Environmental Processes and Environmental Loads
10/27
Classification: Sta 10
3.6.2 Current velocity field
'or most design wor)# current profiles ,speed versus dept!-
are esta"lis!ed from current measurements.
+easurements are typically made at a num"er of dept!s. Hp
to now extremes are typically estimated for eac! dept!
separately. Iinear interpolation "etween dept!s.
t is li)ely t!at suc! profiles are concervative for most cases#
"ut not necessarily for all.
10512 design profile
*resently wor) is going on regarding developing more
adeuat design profile:
= Current is descri"ed as a sum of empirical ort!ogonal
functions.
= 'amily of profiles wit! 105%speed at one dept! and
associated values at ot!er dept!s.
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta 11
Current components:
= idal current.
= ac)ground current.
= Wind driven current
= +eanders or vortex current
f data are not avvaila"le# current field may "e ta)en as t!e
sum of t!e tidal current ,constant t!roug! water column-
and t!e wind driven current , 15%J of mean wind speed att!e surface decaying linearly to ero at a"out 40m.
n connection wit! loads on structures# t!e current is
considered as a slowly varying p!enomenon# i.e. t!e
current speed is )ept constant for a s!ort term sea state.
ypical surface current speed 8ort! Sea ,no eddies present-:
1051 5current: 1m?s
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta 1%
3.(.3 Wa)es and descriion of ,a)es
!e sea surface is of an irregular nature# "ut it can to a
first appoximation "e written as a sum of sinusoidal wit!different amplitude# different freuency# ,different direction-
and different p!ase.
'or practical application# t!e long term variation of t!e sea
surface elevation process is consider as i piecewise
stationary ,and !omogeneous- stoc!astic process ,field-.
f t!e sea surface elevation can "e modelled as a Kaussian
process# eac! stationary sea state is in a statistical sense
completely c!aracteried "y t!e directional wave spectrum:
S!,f# - s!,f-=d,-
'reuency SpectrumSpreading function
Several models are proposed for t!e freuency spectrum:
= SSC ,Keneralied *ierson +os)owit# fully developed
wind sea-
= LM8SWA* ,pure wind sea# may "e growing-
= orset!augen ,com"ined sea# wind sea / swell-
Common for all models is t!at t!ey are parameteried in
terms of significant wave !eig!t# !s# and spectral pea)period# tp.
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta 1$
Long term modelling of sea states
n view of w!at is said a"ove# one can conclude t!at a
s!ort term sea state is for practical purposes descri"edin terms of significant wave !eig!t# spectral pea) period
and direction of propagation.
!e long term description of wave conditions can "e done
"y esta"lis!ing a @oint pro"a"ility density function for
(s# pand :
f(s#p#,!#t#- f (s#pN,!#tN-=f,-
w!ere: approximated
"y pro". mass function
f(s#p,!#t- f(s,!- = fpN(s,tN!-
$5p Wei"ull
or
IonoWe
'itted to availa"le data
Iog5normal orWei"ull
fitted to data for eac!
!s 5 class
8/12/2019 Overview Environmental Processes and Environmental Loads
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Classification: Sta 1