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


Tab

le1

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den

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ng

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ures

and

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form

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sente

dan

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eco

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xna

ture

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amic

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219

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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


3219

83N

.Sp

idoe

Mea

sure

dm

otio

ns

Art

icul

ated

load

ing

pla

tfor

mW

ind

and

wav

e-in

duce

dm

otions

Ass

umin

gin

dep

enden

cyofr

andom

proc

ess,

win

dan

dw

ave

spec

tral

resp

onse

sar

esu

mm

edup

3319

83P.A

.Pal

o*

Moor

edve

ssel

sSte

ady

win

dan

dcu

rren

tlo

ads

Use

ofnon

-dim

entiona

lcoe$

cien

tsfo

rlo

adco

mput

atio

n34

1983

H.R

olfs

man

*A

lter

nat

ive

loca

tion

ofde

rric

kon

Sem

i-su

bm

ersible

Step

ped

win

d;se

gmen

talw

ind

pro"le

Cd

valu

esin

the

rang

eofsu

b-

critic

al0.

5}0.

6Sup

ercr

itic

al0.

7035

1985

N.Sp

idoe

Full

scal

em

easu

rem

ents

on

inst

rum

ente

dpla

tform

Jack

etpl

atfo

rmin

the

Nort

hSe

aTheo

retica

lm

odel

for

natu

ral

win

dad

opt

edH

ighe

rle

ngt

hsc

ales

oftu

rbule

nce

for

o!sh

ore

appl

icat

ions.

Com

monly

used

aero

dyn

amic

tran

sfer

func

tion

exag

gera

tes

the

spat

ialco

her

ence

3619

85J.W

.Bun

ce*

Inte

grity

ofpl

atfo

rmsu

per

stru

cture

sA

llpo

ssib

lelo

ads

incl

udin

gw

ind

load

sfo

rde

ckst

ruct

ures

MSF

mod

ule

des

ign

and

anal

ysis

asper

AP

I-R

P-2

Apr

esen

ted

along

with

typic

aleq

uipm

ent

wei

ghts

3719

87A

.K

aree

m*

Fre

quen

cydom

ain

appro

ach

Conc

eptu

alst

eps

for

eval

uat

ion

ofw

ind

induc

edre

spon

ses

Fre

quen

cydom

ain

appro

ach

toM

DO

Fex

pla

ined

3819

89S.See

thar

aman*

Guye

dto

wer

spec

tral

anal

ysis

Non-

zero

mea

nst

atio

nary

ran-

dom

pro

cess

and

com

bin

edw

ind

and

wav

ean

dcu

rren

tlo

adin

g

MD

OF

freq

uen

cydom

ain

calc

ula

tion

ofre

spon

ses

and

stre

sses

3919

89ESSO

*D

esig

nin

stru

ctio

nsfo

rja

cket

pla

tform

deck

stru

cture

sPro

ject

edar

eafo

rw

ind

load

ing

for

4,5

and

6m

odule

softh

ede

ckin

apla

tform

Win

dsp

eed

@10

mab

ove

MSL

and

the

pro"le

variat

ion

aspe

rgu

idel

ines

ofU

SG

eolo

gica

lsurv

ey,

bas

edon

ave.tim

eofw

ind

4019

90Y

.L

i*

TL

Pre

spon

ses

Sto

chas

tic

resp

onse

Com

bin

edw

ind,

wav

ean

dcu

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

wel

ded

and

brac

edty

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


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

easu

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


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


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


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


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

mot

ions

due

tow

ind

and

wav

esA

rtic

ula

ted

load

ing

pla

tform

inN

ort

hSea

MxK#

Cx5#

Kx"

q@ !(t)#

q@ )(t)

a*

aero

dyn

amic

h*

hyd

rodyn

amic

Mea

sure

dm

otion

tran

sfer

funct

ion;U

sing

indep

enden

cyofra

ndom

proce

ssofw

ind

and

wav

e,sp

ectr

alre

spons

esar

esu

perim

pose

d

35A

erody

nam

iclo

adin

gon

win

dse

nsitive

stru

cture

sSt

eelja

cket

inN

orth

Sea

Exi

stin

gW

ind

load

mod

els

exam

ined

Adeq

uac

yof

win

dsp

atia

lco

her

ency

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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.


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


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].


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].


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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