overview environmental processes and environmental loads

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


2/27

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.


3/27

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.


4/27

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


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#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.


8/27

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-


9/27

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?


10/27


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.


11/27


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


12/27

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.


13/27

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


14/27


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