Dynamic Effects of Wind Loads on Offshore Dec
Transcript of Dynamic Effects of Wind Loads on Offshore Dec
*Corresponding author.
Journal of Wind Engineeringand Industrial Aerodynamics 84 (2000) 345}367
Dynamic e!ects of wind loads ono!shore deck structures *
A critical evaluation of provisions and practices
S. Gomathinayagam!,*, C.P. Vendhan", J. Shanmugasundaram!
! Structural Engineering Research Centre, Madras, Chennai-113, India" Ocean Engineering Centre, IIT, Madras, Chennai-36, India
Abstract
Walk-ways, #are-outs, silos, cranes, heli-pad structures, ladders, rig-supporting derrickstructures, living quarters, worksheds, claddings of module supporting frames (MSF), and boatlandings are some of the major components of o!shore deck structures. These deck structuresare designed for operational and extreme wind and wave, and impact loads. This paper presentsa review of published literature on wind loading conditions on the structure, extreme windloading, wind tunnel tests on decks of compliant as well as "xed platforms and full-scalemeasurements on o!shore decks. The paper also presents a study of vibration modes of typicalderrick and inclined boom with respect to possible dynamic sensitivity to wind inducedexcitations. The scarcity of "eld measured data and the necessity of instrumenting o!shore deckstructures for collection of scarce data on wind and structural response characteristics are alsohighlighted. ( 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Dynamic e!ects; Wind loads; Deck structures; O!shore structures; Provisions and practices
1. Introduction
With the increase in demand for oil and gas, a large number of o!shore structureshave been constructed through out the world. O!shore structures are designed forrandom wind and wave loads. At the global level the lateral wind load in the design of"xed o!shore structures is of the order 10% of the total lateral loads and 25% in thecase of compliant and #oating platforms. Practical estimation of design dynamic windloads for complex shaped deck structures is an involved exercise. However, the e!ects
0167-6105/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 1 6 7 - 6 1 0 5 ( 9 9 ) 0 0 1 1 3 - 0
of extreme wind load, which constitutes one of the primary loads on local componentssuch as #are-outs, silos, cranes, heli-pad structures, ladders, rig-supporting structures,living quarters, worksheds and claddings, are signi"cantly more than that of normalwind. Many failures involving deck components due to extreme wind have alreadybeen reported in damage investigation [1]. This paper reviews the research contribu-tions on the dynamic e!ects of wind on o!shore deck structures under four broadareas which are multidisciplinary in nature, viz. (a) Wind environment in o!shore,(b) Fixed and #oating platforms in wind-wave environment, (c) O!shore deck modelsin wind tunnel, and (d) O!shore deck structures in natural wind "eld.
2. Wind loads on o4shore structures
From damage surveys on o!shore platforms [1], it has been clearly observed thatdamage to deck structures such as #are-outs, deck pipeline networks, storage silos andother similar process and production equipment pose potentially large risk of envir-onmental degradation due to oil spillage. Hence more than the overall lateral loads onthe platform as a whole, the deck components require careful consideration in theirdesign to resist lateral loads which are mainly due to winds. The models of o!shorewind "elds under normal and extreme wind climatic conditions and their use alongwith codes of practice are still being keenly studied, despite the practical problems ofmeasurements. Compliant platforms as well as #oating systems which have lowerfrequencies compared to "xed platforms, are more vulnerable to dynamic e!ects ofwind. A critical review of turbulence spectra for o!shore application has beenpresented by Kareem [2]. Further due to directional e!ects, and phase correlationbetween wind on deck structures and wave on the platform supporting system("xed/compliant) there could be reduction in total lateral loads as well as considerableincrease in total loads on the deck structures and components. The reasons are wellexplained by Kareem [1], and are summarised as follows:
2.1. Dynamic wind ewects during cyclones/hurricanes
(i) Wind load contribution is 10% of total lateral loads in jackets and about 25% incompliants during normal winds, and these loads tend to increase to 20% and40}50%, respectively, in jackets and in compliants, in the event of cyclonic winds.
(ii) The dynamic pressure due to wind [0.5 o!<(t)*<(t)] becomes higher in cyclones,
as the density of air is higher due to excessive moisture content and presence of micromolecules of water particles, in addition to the prevailing higher wind velocities thanin normal conditions.
(iii) Similar to topographic e!ects of hillocks and valleys on onshore structures,a high wave can cause temporal speed up of wind on o!shore structures.
(iv) Turbulent wind-induced loads (Fig. 1) dominate deck structural loading anddesign.
(v) Increased dynamic forces on deck (Fig. 1).(vi) Increased uplift forces on heli-deck or similar lifting surfaces on the deck (Fig. 1).
346 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
Fig. 1. Schematic of wind action on o!shore deck structures.
(vii) Turbulent wind on lattice structures (the complexities of the loading duringcyclonic winds are yet not known fully even from full-scale measurements on onshorestructures) and on cylindrical #are-outs and storage tanks.
(viii) Down burst loads on deck structures.(ix) Wind #ow trained by wave crest and trough under the platform causing
reversal of forces of uplift and drag.(x) Turbulent wakes of one structure over the other on the deck, as well as one
platform over the other.(xi) Unsteady turbulent wind over crane booms and cantilever girders causing
torsional dynamic loads.(xii) Impact of debris of cladding, berthed ships and boats on the platform
structures.
3. Wind environment in o4shore
The parameters that concern the structural loading are wind velocity, direc-tion and terrain characteristics/wave "elds. Realising the devastating e!ects of
S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 347
hurricanes/tropical cyclones on o!shore structures, many studies mainly concentratedon the evaluation of extreme wind characteristics. The research papers cover invest-igations on hurricane wind models [3}6,12}14,21], study of wind pro"le along heightunder various sea states [4,7,8,14], design load speci"cations [15,16], measurement ofcombined wind, wave and current loads [17}19], and joint probability description ofwind and waves [20]. The problems associated in this grey area are not fully solveddue to practical di$culties associated with uninterrupted operability of wind sensorsand acquisition of noise-free signals [8}11]. Di$culties in obtaining good samplingrates for accurate measurement of turbulence had been one of the problems in the1970 s [10]. Even with the availability of satellite data for numerical ocean surfacewind predictions, improvements in models of simulation and in accuracy of predic-tions were limited by computer speeds [11]. A brief review of individual researchcontributions is presented in the Table 1.
4. Fixed and 6oating platforms in wind-wave environment
Scarcity of data has not been the barrier in the design of complex o!shorestructures, built for the exploitation of o!shore oil, which is clearly seen in the study ofvarious "xed and #oating/compliant o!shore platforms [22}48]. The need for dy-namic analysis of o!shore structures to gusty wind has been realised even threedecades ago [22]. Simple dual mass model [22] and use of codes of practice [29] havepaved the way for safer designs. Structural analyses of jackets [22}26,34}36], tensionleg platforms (TLP) [31,32,37,40,41,45,48], semi-submersibles [27,29,30,34,47], guyedtowers [38], articulated towers [28], jackups [44] and moored vessels [33] have beencarried out either in the time domain or in the frequency domain for appropriateloading of wind and wave. However, mention can be made of speci"c investigationson two-dimensional and steady state/static responses [23,34,36], three-dimensionaland dynamic responses [25,26,36,46] and combined responses due to wind and waves[24,28,32,38,40,45]. Wind-induced dynamic responses of deck structures such asderricks [34,43], #are-outs [42] and other deck structures [36,39,46] have also beeninvestigated with measured or simulated data. From the results for compliant struc-tures, it has been well established [37,40,41,45,48] that wind excites su$cient numberof higher modes also. Apart from di$culties in wind tunnel modelling of latticestructures, a recent computational #uid dynamic (CFD) study [47] has presentedpractical problems of analytical modelling for studying the #ow around lattices. Thisfact and the results of Kareem [1,2] explain the need for further improvements ofexisting design provisions based on full scale testing and analysis. Table 2 providesa comparison of historical developments and published results in this area.
5. O4shore deck models in wind tunnel
To capture the aerodynamic admittance function or the forces on a complexplatform deck, many scaled model studies of the deck, with lattice and cladded
348 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
Tab
le1
Stu
dies
on
win
den
viro
nm
entin
o!sh
ore
Ref
.No.
Yea
rLea
d-A
utho
rExp
erim
enta
lA
nal
ytic
alSpec
i"c
win
de!
ect
Obse
rvat
ion
119
93A
.K
aree
m*
Dam
age
surv
eys
on
o!sh
ore
stru
ctur
esH
urr
ican
eA
ndre
w-ind
uce
dda
mag
esto
o!sh
ore
pla
tform
so!
the
Flo
rida
coas
t
Rea
soni
ng
offa
ilur
eofdec
kst
ruct
ures
and
plat
form
sar
epre
sente
dan
ddi
scuss
edth
eco
mple
xna
ture
ofdyn
amic
win
de!
ects
219
85A
.K
aree
m*
Ten
sion
leg
pla
tform
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d-induc
edre
spons
eofT
LPs
tova
riou
sw
ind
spec
tra
Fea
ture
sof
win
dtu
rbule
nce
spec
tra
ofK
aim
al,H
arris
and
Dav
enpor
tdiscu
ssed
and
ades
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trum
for
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Psu
gges
ted
with
mor
een
ergy
nea
rth
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oun
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z3
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Col
lins
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pli"
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S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 349
Tab
le1.
Continue
d
Ref
.No.
Yea
rLea
d-A
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rExp
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enta
lA
nal
ytic
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ion
819
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oor
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1119
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entional
ship
mea
sure
men
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ratr
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ied
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rmw
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ds
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i"ed
from
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ate
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ratu
re14
1980
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rist
all
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dan
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ts20
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sea
leve
l
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sure
men
tson
adrilli
ng
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tform
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dve
loci
tyan
ddi
rect
ion
mea
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ican
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quen
cies
inth
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nge
of
0.25}0.
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zse
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be
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load
ing
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s
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edon
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for
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1719
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alys
isof
storm
trac
ks
Sta
tist
ical
anal
ysis
ofex
trem
esfo
rw
ind,w
ave
and
curr
ent"el
ds
350 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
1819
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uan
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erim
enta
lst
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ith
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for
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ised
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iction
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and
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S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 351
Tab
le2
Stu
dies
on"xe
dan
d#oa
ting
pla
tfor
ms
inw
ind-
wav
een
viro
nmen
t
Ref
.No.
Yea
rLea
d-A
utho
rExp
erim
enta
lA
nal
ytic
alSpec
i"c
win
de!
ect
Obse
rvat
ion
2219
69R
.Blu
mber
g*
Singl
ean
ddua
lm
ass
dyn
amic
mode
lsofpla
tform
and
deck
stru
ctur
es
Sust
aine
das
wel
las
gust
win
de!
ects
Scar
city
ofo!sh
ore
win
ddat
a,th
ene
edfo
rdyn
amic
anal
ysis
repo
rted
2319
70G
.H.W
ork
man*
Norm
alm
ode
met
hod
;tim
e-an
dsp
ace-
depen
dantfo
rces
Pla
nefram
em
odel
Discu
ssed
stea
dyst
ate
forc
es,
de#
ection
san
dst
ress
es24
1974
A.E
.M
anso
or*
Spec
tral
and
lum
ped
mas
sm
ode
lling
Win
d,w
ave
and
curr
ent,
and
e!ec
tof
com
bin
edlo
adin
gV
ort
exsh
eddi
ng
e!ec
tne
glec
ted
and
Ave
.C
d"0.
925
1975
N.J.H
eaf
*C
onc
epts
ofw
ind
load
ing
on
o!sh
ore
stru
ctur
esD
ynam
ice!
ects
ofw
ind
Prim
ary
and
seco
ndar
ye!
ects
of
win
d26
1976
F.H
olter
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esig
nco
nce
pts
for
inte
grat
edpla
tfor
man
dde
ckde
sign
Win
don
the
dec
kan
dW
ave
on
plat
form
stru
cture
sC
apital
cost
can
be
min
imised
by
inte
grat
eddec
kan
dpla
tfor
mde
sign
2719
77M
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chi
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i-su
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est
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ure
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dsp
eeds
from
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hse
alo
cation
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dsp
eeds
and
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cyof
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ceuse
dfo
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mpu
ting
resp
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2819
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il*
Art
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ated
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te!
ects
Win
dve
loci
tyan
dse
ast
ate
are
assu
med
indep
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t29
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P.M
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esouza
*Sem
i-su
bm
ersibl
esw
ith
win
dan
dw
ave
load
ing
Win
dsp
eed
with
grad
ientan
ddra
gan
dlif
tco
e$ci
ents
used
with
clas
si"ca
tion
rule
s
Cons
erva
tism
incl
assi"ca
tion
rule
sto
be
pres
erve
dunt
ilfu
rthe
rre
sults
are
obta
ined
3019
79H
.Boon
stra
Win
dfo
rces
and
mea
sure
dan
chor
chai
nfo
rces
Sem
i-su
bm
ersibl
epla
tform
and
DN
Vru
les
discu
ssed
Mea
sure
dan
chor
chai
nfo
rces
due
tow
ind
Conc
luded
that
win
dtu
nnel
test
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lts;
full-
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easu
red
forc
es
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ynam
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dtu
rbul
ence
and
wav
ech
arac
terisitics
expl
ained
Dav
enpo
rt,H
arris,
and
Kai
mal
spec
tra
com
pare
d.Forth
ede
sign
ofTL
Ps,
Kai
mal
spec
trum's
super
iority
due
toth
epre
sence
of
high
eren
ergy
inth
em
eso
scal
eof
0.01
Hz
(ran
gely
ing
bet
wee
nsy
nop
tic
(low
)an
dm
icro
(hig
h)
scal
esof
freq
uen
cies
)hig
hllig
hte
d
352 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
3219
83N
.Sp
idoe
Mea
sure
dm
otio
ns
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icul
ated
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ing
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tfor
mW
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and
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e-in
duce
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otions
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umin
gin
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cyofr
andom
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ess,
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dan
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spec
tral
resp
onse
sar
esu
mm
edup
3319
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o*
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ady
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rren
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ads
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entiona
lcoe$
cien
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rlo
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mput
atio
n34
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man
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ive
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tion
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kon
Sem
i-su
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ersible
Step
ped
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esin
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itic
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idoe
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em
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ents
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rum
ente
dpla
tform
Jack
etpl
atfo
rmin
the
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hSe
aTheo
retica
lm
odel
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ral
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dad
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ighe
rle
ngt
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ales
oftu
rbule
nce
for
o!sh
ore
appl
icat
ions.
Com
monly
used
aero
dyn
amic
tran
sfer
func
tion
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gera
tes
the
spat
ialco
her
ence
3619
85J.W
.Bun
ce*
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grity
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cture
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udin
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ures
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ule
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ign
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ysis
asper
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3719
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uat
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ined
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tral
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ysis
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nst
atio
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bin
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and
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ean
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rren
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g
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OF
freq
uen
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ain
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ula
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sses
3919
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ctio
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tform
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cture
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ject
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rw
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ing
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and
6m
odule
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tform
Win
dsp
eed
@10
mab
ove
MSL
and
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i*
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spon
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chas
tic
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onse
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bin
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ean
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rren
t"el
d41
1990
A.K
aree
m*
TLP
resp
onse
inth
efreq
uen
cydom
ain
Sim
ultan
eous
lyac
ting
win
dw
ith
wav
ean
dcu
rren
tIn
crea
sed
drag
with
adde
dm
odu
les,
for
various
win
dan
dm
ean
dire
ctio
n.M
ean
Cd,
Cl,
and
Cm
valu
espro
vided
4219
92R
.H.K
irkvi
k*
Fla
rebo
om
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ded
and
brac
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peC
ritica
lw
ind
spee
ds
and
vort
exsh
eddi
ng
e!ec
tsst
udi
edFour
plat
form#ar
eboo
ms
exa-
min
edan
dsu
gges
ted
stre
ssra
nge
sdue
tow
ind-indu
ced
vibra
tions
beco
mbin
edw
ith
nor
mal
fatigu
e,du
eto
along
win
d#uc
tuat
ions
S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 353
Tab
le2.
Continue
d
Ref
.No.
Yea
rLea
d-A
utho
rExp
erim
enta
lA
nal
ytic
alSpec
i"c
win
de!
ect
Obse
rvat
ion
4319
92V
.G
use
lla*
Dyn
amic
beh
avio
ur
ofdrilli
ngder
rick
on
stee
lja
cket
plat
form
Win
d,w
ave
and
drilli
ng
load
sar
eha
ndle
dM
easu
red
acce
lero
met
ertr
aces
com
par
edw
ith
num
eric
alre
sults
4419
95T.O
.Wea
ver
Full-
scal
em
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rem
ents
Tim
e-do
mai
nan
alys
is,
dyn
amic
anal
ysis
ofJa
ckup
pla
tform
Win
dsp
eed
range
:7.9}22
.9m
/sW
ind,w
ave
and
curr
entfo
rces
and
Leg
forc
esco
mpar
ed
4519
96A
.K.Ja
in*
Ste
p-b
y-st
eptim
e-do
mai
nap
pro
ach
toTLP
Win
dan
dw
ave
load
ing
Win
dex
cite
ssu$
cien
tnu
mbe
rof
high
erm
ode
s46
1996
K.V
andiv
er*
Fat
igue
dam
age
of#ar
eboom
inun
stea
dy
win
ds
Nort
hse
a"el
dda
taofw
ind
use
dC
onc
luded
that
know
ledg
eof
turb
ule
nce
inte
nsitie
sin
o!sh
ore
islim
ited
4719
97C
.A
age
Win
dtu
nne
l,Full-
scal
ete
stin
harb
our
Sem
i-su
bm
ersibl
eby
com
put
atio
nal#uid
dyna
mic
s(C
FD
)
Win
dlo
adin
aho
rizo
nta
lpla
ne
studie
dusing
CFD
and
exper
imen
talre
sults
com
par
ed
Exp
ress
eddi$
cultie
sofm
odel
ling
latt
ice
stru
ctur
essu
chas
cran
es,
boom
s,an
dder
rick
usin
gC
FD
4819
97P.Tei
gen
Full-
scal
em
easu
rem
ents
Ext
rem
eva
lues
ofre
spons
eofTL
PW
ind
and
wav
ew
ith
dire
ctio
ne!
ect
cons
ider
edC
dva
lues
ofc
olu
mns
and
ponto
ons
pre
dict
edusing
direc
tional
spre
adsp
ectr
um
354 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
structures on the deck have been carried out [49}67].While most of the experimentshave aimed at quantifying mean drag and lift force coe$cients for the deck as a whole,there has been signi"cant research in understanding the contribution of variousmodules to the overall lateral load due to wind. Mean load coe$cients [49,54,55,61],dynamic e!ects [53,66], and use of power law [50], have been studied for jackets [49],TLPs [53,58,62,66], guyed towers [57,60], semi-submersibles [51,54}56], jackups[59,64] and ships [50]. General deck [57] con"guration and heli-deck optimum[52,65] location and Reynolds number e!ects have also been investigated. Haldo [65]has studied the complex Reynolds number (Re) e!ect by modelling an o!shore derrickand cautioned that large di!erences in drag coe$cients are possible, by testing latticetower under low Re, which needs very careful interpretation. Also he concluded thatthe turbulence intensity variations have signi"cant e!ect on the drag of latticestructures. Simiu [68] has compiled the experimental results on wind force coe$-cients, viz. the drag coe$cients, and lift force coe$cients of lattice frame works as wellas plate girders, which have been one of the major sources of data for manycontemporary design procedures. The claim of conservatism of using codal recom-mendation, based on wind tunnel model experiments, has been contested by theconclusion of Boonstra [30] which states that full-scale measured forces are far morethan those predicted by the wind tunnel forces. More research has to be carried out oncomparison of wind tunnel results with that of full scale. Summary information withregards to the di$culties of wind tunnel modelling and the interpretation of laborat-ory results to the "eld are presented in the Table 3.
6. O4shore deck structures in natural wind 5eld
The design of o!shore deck structures is complex due to the non-availability offull-scale measured data. Field measured data has been compared with analyticalprocedures developed for wind [17,18], structural responses of jackups [44], semi-submersibles [47], and TLPs [48]. Spidoe and Brathaug [32] has compared full-scaleexperimental results on an articulated loading platform in natural wind with a simpli-"ed theoretical model based on structural impedance and aerodynamic admittancefunctions. Full-scale results of semi-submersibles are compared in Refs. [30,56]. Somedetails of the instrumented platform measurements are already presented in Tables1}3. However, speci"c survey of full-scale experimental investigations are given inTable 4.
7. Dynamic characteristics of deck structures and platforms
The location of a derrick on any o!shore platform is governed by functionalrequirements and it normally has a skid base. The operations on oil well are usuallysuspended under extreme wind conditions. During gusty winds the derrick/inclinedlattice boom gets a base excitation triggered by the wave, wind and current acting onthe platform structure. In addition, wind gust loading with high-frequency contents
S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 355
Tab
le3
Studi
eson
o!sh
ore
dec
km
odel
sin
win
dtu
nnel
Ref
.N
o.
Yea
rLea
d-A
uth
or
Exp
erim
enta
lSt
ruct
ure
mode
lSp
eci"
cw
ind
e!ec
tO
bse
rvat
ion
4919
69J.T.A
isto
nW
ind
tunn
elO
vera
llde
ck:sm
allan
dla
rge
model
sA
nove
rall
dra
gco
e$ci
ent,
Cd
corr
espond
ing
toth
edim
ensions
ofth
eco
mpo
nen
ts
Using
incr
emen
talav
erag
ing
of
win
dal
ong
hei
ght,
the
over
allC
dfo
rth
edec
kis
give
nas
0.90
5019
77F.A
.Ben
ham
Win
dtu
nnel
(forc
ebal
ance
)Lar
gecr
ude
carr
ier
Com
bined
Liftan
dD
rag
forc
eC
onc
luded
(1/7
)th
pow
erla
w"ts
good
for
com
pariso
nw
ith
inte
grat
edpre
ssure
s51
1978
E.T
.D.B
jerr
egaa
rdW
ind
tunn
el1
:250
Sem
i-su
bm
ersible
Win
dov
ertu
rnin
gm
om
ent
Cons
erva
tive
nes
sofem
piric
alca
lcula
tion
met
hod
s52
1979
K.H
.V.Bla
tin
Win
dtu
nnel
1:1
50H
elidec
klo
cation
and
shap
eW
ind#ow
aroun
dhel
ide
ckA
irga
pis
am
ustan
dLee
war
ded
gesh
ould
bew
ellc
lear
o!
thepla
tform
edge
5319
80A
.K
aree
mW
ind
tunn
elD
ynam
ican
alys
isof
TLP
Dyn
amic
win
de!
ects
Sign
i"ca
nce
ofse
cond
ord
erfo
rces
due
tohig
hm
ean
win
dan
dtu
rbule
nt#uct
uations
emphas
ised
5419
81D
.J.N
ort
on
Win
dtu
nnel
192
:1Se
mi-su
bm
ersible
com
ponen
te!
ects
Dyn
amic
win
de!
ects
Curr
entm
ethod
sov
erpre
dict
win
dlo
ads
as75
%of
alldr
agfo
rces
during
drillin
gan
d90
%of
alldra
gfo
rces
whe
n#oat
ing
5519
81E.T
.D.B
jerr
egaa
rdW
ind
tunn
elSe
mi-su
bm
ersible
Win
din
duce
dlift/d
rag
ratio
Forva
rious
angl
esofi
nci
den
cean
dH
4/¸va
lues
5619
82H
.Bonn
stra
Win
dtu
nnel
and
full
scal
eSe
mi-su
bm
ersibl
eTota
lw
ind
load
sat
variou
sw
ind
spee
dsC
om
pariso
noffu
ll-s
cale
and
win
dtu
nne
lre
sults
5719
82P.J.Pik
eW
ind
tunn
elte
sts
Guye
dto
wer
o!sh
ore
plat
form
sW
ind
load
san
dpre
ssure
son
blu!
body
ofde
ckco
mpa
red
Win
dtu
nnel
forc
ebal
ence
win
dlo
ads
eval
uat
ed58
1983
Arm
stro
ngW
ind
tunn
elTL
PU
nst
eady
win
dfo
rces
Mea
sure
dae
rodyn
amic
adm
itta
nce
funct
ion,w
ith
freq
uency
depe
nde
ncy
dev
elope
d59
1984
A.G
.D
aven
por
tW
ind
tunn
elJa
ck-u
ppla
tform
dec
kTurb
ule
ntw
ind
load
ing
Dyn
amic
resp
onse
calc
ula
tion
s60
1985
B.J.V
icker
yW
ind
tunn
elm
ode
l1
:400
Len
aG
uyed
tow
erW
ind-indu
ced
drag
-mom
ent,
tors
ional
mom
ent
and
late
ral
mom
ents
Thre
ety
pes
ofex
posu
reca
tego
ries
studi
ed
356 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
6119
86D
.C.Bay
erW
ind
tunn
elSe
ctio
nm
ode
lst
udie
son
lattic
efram
esC
dan
dC
mofla
ttic
eto
wer
Solid
ity
ratio
vs.dra
gco
e$ci
ent
obta
ined
6219
87A
.K
aree
mW
ind
tunn
elTL
PM
ean
aero
dyn
amic
forc
eco
e$ci
ents
Incr
ease
ddra
gw
ith
added
modu
les
for
various
win
dm
ean
dire
ctio
nsC
d,C
l,C
mva
lues
prov
ided
6319
92C
.Sw
anW
ind-W
ave
Envi
ronm
ent
ofw
ind
and
wav
eC
ritica
lhe
ight
atw
hic
hw
ind
velo
city"
wav
ece
lerity
Orien
tations
for
young
and
esta
blished
win
d-w
aves
iden
ti"ed
6419
93T.S
.Lee
Win
dT
unnel
1:2
68M
obile
o!sh
ore
pla
tform
E!ec
tofre
mov
able
com
ponen
ton
deck
Appl
icat
ion
ofbui
ldin
gblo
ckm
ethod
ofw
ind
load
ing
dev
eloped
6519
93A
.E.H
old
oW
ind
tunn
elLat
tice
der
rick
on
ao!
shore
plat
form
Turb
ule
nce
inte
nsity
variat
ion
Lat
tice
mode
lsbel
owR
4"22
00m
ayha
vedr
agco
e$ci
ents
di!er
ing
inor
der
of40
%66
1995
M.T
.S.
Dan
esw
aran
Win
dan
dW
ave
tunne
lfa
cilit
yC
ompl
iant
o!sh
ore
tow
ers
Win
don
lyan
dw
ind-w
ave
com
bin
atio
nsus
ing
tim
edo
mai
nan
dfreq
uency
dom
ain
solu
tion
s
RM
SofC
mva
lues
used
and
mea
sure
dan
dth
eore
tica
lva
lues
com
pare
d67
1995
L.H
uan
gW
ind
tunn
elw
ith
hydr
odyn
amic
test
ing
Flo
atin
gpro
duc
tion
syst
ems
Step
ped
win
dpr
o"le
give
nby
API
and
AB
SForc
esco
mput
edon
mod
els
com
par
edw
ith
codal
provi
sion
s
S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 357
Tab
le4
Full-
scal
eex
per
imen
talst
udie
son
o!sh
ore
dec
kst
ruct
ures
Ref
.No.
Exp
erim
ent
O!sh
ore
stru
cture
/Loca
tion
Spec
i"c
win
de!
ect
addre
ssed
Rem
arks
17Fie
ldw
ind,w
ave
and
curr
entdat
am
easu
rem
ent
ove
ra
long
dura
tion
South
Chin
a-se
aw
ind
stru
cture
Ext
rem
eva
lues
ofw
ind
ina
give
nw
ave
and
curr
ent
envi
ronm
entan
ddirec
tion
alan
alys
isof
storm
trac
ks
Sta
tist
ical
anal
ysis
ofex
trem
esfo
rw
ind,
wav
ean
dcu
rren
t"el
ds
inth
esite
exam
ined
with
the
hel
pofm
easu
red
and
past
dat
a18
Exp
erim
enta
lst
udy
inha
rbou
r,to
reduc
eth
ee!
ects
ofw
ave
and
curr
ent
Open
bottom
ed#oa
ting
pla
tform
,lar
gedia
met
erco
lum
ns
separ
ated
by
30m
using
truss
-wor
k
Win
dsp
eed
ofor
der
19m
/san
ddirec
tion
sw
ithin
1803
Win
d,w
ave
and
curr
ent
stud
y,in
dica
ting
the
com
ple
xin
tera
ctio
nofdirec
tion
aloc
curr
ence
softh
eth
ree
envi
ronm
enta
lra
ndo
mpro
cess
esw
hich
hav
elo
wde
gree
ofco
rrel
atio
nam
ong
them
30W
ind
forc
eson
the
dec
kst
ruct
ures
and
mea
sure
-m
entofre
sultin
gan
chor
chai
nfo
rces
Sem
i-su
bm
ersibl
epla
tform
inN
orw
ayC
oast
Mea
sure
dan
chor
chai
nfo
rces
due
tow
ind
com
pone
nts
insm
all-sc
ale
mod
elst
udy
.Exp
ress
edth
atm
utu
alin
ter-
action,s
hie
ldin
ge!
ects
,and
acco
unt
-in
glif
tfo
rces
are
di$
cult
tom
odel
inw
ind
tunne
ls
Conc
luded
that
win
dtu
nnel
test
resu
ltson
smal
l-sc
ale
mod
els
have
tobe
inte
rpre
ted
with
caution
sinc
eth
em
easu
red
model
forc
esar
ean
ord
erle
ssth
anth
atof
full-s
cale
mea
sure
dfo
rces
due
toso
me
pra
ctic
aldi$
cultie
sof
model
ling
32M
easu
red
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358 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
48Full-
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S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 359
Fig. 3. Modes of derrick with assumed partial-deck-"xture (one leg "xed).
Fig. 2. Modes of derrick with assumed rigid-deck-"xture.
excites partially "xed/damaged free standing lattice structures. While shielding, inter-ference and base #exibility may reduce the severity of extreme wind load e!ects ona derrick, local speed-up of wind, uplift e!ects, and possible resonant vibrations withhigher frequencies of platform will increase the dynamic stresses in the structure.Under extreme wind conditions we will have high stress ranges which means lowercycles to failure requiring additional caution in design and detailing of the deckcomponents. In order to draw inferences with regard to typical structural frequenciesin relation to that of frequency content of wind loading a few examples are consideredhere. The natural frequencies and modes of a derrick and an inclined boom arepresented in (Figs. 2}5). The structural data for these examples have been scaled fromwind tunnel model data as given by Vickery [60]. The results for a Bombay Highjacket platform (Fig. 6) resting on a bed of soft clay and a semi-submersible (Fig. 7)[69] are also presented.
360 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
Fig. 4. Modes of inclined boom with assumed rigid base.
Fig. 5. Modes of inclined boom with assumed partial (one leg) damage.
7.1. Codal provisions for wind loading and response in owshore/onshore
The available terrain-dependant wind parameter measurements, even in onshoresites are today far from being su$cient around the world, even though onshoremeasurements are comparatively cheaper and reliable than o!shore wind measure-ments. For o!shore structural designs, mostly international codes such as API-seriesare being followed in the Indian coastline as well. However, rational use of regionalwind data will be appropriate for a good design. Keeping this aspect the API(American Petroleum Institute) and BIS (Bureau of Indian Standards) codalprovisions are brie#y compared in Table 5. API-RP-2A [15] suggests design-oriented
S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 361
Fig. 6. Frequencies of a jacket platform on soft clay, in Bombay High, India.
Fig. 7. Frequencies of a Semi-submersible platform [69].
simple techniques for overall platform design subjected to wind and wave and otherrelevant forces. Rightly recognising the increased wind sensitivity of TLPs, API-RP-2T [16] vividly describes the actions for varying time scales of wind and for the designof components and systems as a whole. Fig. 8 gives excitation source spectra used ino!shore design [8].
362 S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367
Table 5Codal provisions for o!shore wind loads (for use o! the Indian Coast)
Parameter O!shore (API-series) Onshore (IS875/ISO)
Basic wind speed 1.15 <"(as recommended by
IS875 up to 200 km o! thecoast, for India)
<"
m/s (Wind zones Map, forIndia)
Power-law coe$cient for meanwind pro"le
0.125 (API-RP-2A of July 1993) 0.07 * Class A open (greatestdimension of height or width(20 m)0.14 * Class C type structures(greatest dimension of height orwidth'50 m)
Wind gust factor @ 10 m(3-s gust) G(t, z)"(<(t, z)/<(3600, z))
"1#g(t).0.15(10/z)~0.275
g(t)"3.0#ln(3/3)0.6"1.55 (API-RP-2T for TLP)
1.49 (for 3-s gust from hourly mean)
Turbulence intensity (p/;)(for #uctuating component)
At 50 m 0.15*(50/20)~0.275
"0.120Category 2+0.20
p * standard deviation; * Mean wind velocity At 10 m 0.15*(10/20)~0.275
"0.181Category 3+0.24
Note:<"* Basic wind speed (India);<(t, z), G(t, z) wind speed and wind gust factor for averaging period of
t seconds at given elevation &z'.
Fig. 8. Excitation Source spectra for o!shore structures [2].
S. Gomathinayagam et al. / J. Wind Eng. Ind. Aerodyn. 84 (2000) 345}367 363
8. Conclusion
Dynamic wind e!ects on o!shore deck structures as investigated by various authorshave been reviewed to gain an insight into the possible missing links and focus newresearch on unexplored areas. The excitation frequencies in the design wind spectraduring normal wind conditions seem to have considerable energy in the high-frequency regime. Under the extreme wind conditions the high-frequency regime islikely to extend further in the regime of design structural frequencies [70]. The reviewreveals that, only very few researchers have studied the individual module contribu-tion to the overall wind load for the design of o!shore platforms. The numericalresults of natural frequencies of typical platforms and "xed/partially "xed der-rick/lattice boom frequencies and modes illustrate the possibility of response involv-ing component modes as well as platform modes under the action of extreme windand wave, which are generally assumed to be two independent random processes withconsiderable energy in the higher frequencies near the tail end of spectra. The modesof the inclined booms having frequencies closer to extreme wind excitation frequen-cies, not only increases the risk of higher torsional loads on the platform, but also posepotential danger of impact loads on the lower deck structures. It is obvious that failureof #are booms can cause extensive damage through oil sleek as well as "re hazards onthe deck. For the design of components on the deck, the dynamic e!ects must beunderstood by conducting more elaborate full-scale measurements, by instrumentingexisting platforms and analysing both the load and response data. Comparing themeasured observations, which could be even from routine inspection and mainten-ance, with the existing design practice, more consistent analytical models for rationaldesigns can be developed.
Acknowledgements
The authors thank Dr. T.V.S.R. Appa Rao, Director, SERC, for his guidance,encouragement and permission for this publication
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