54225323 Recommended Hot Spot Analysis Procedure for Structural Details of FPSOs

7/26/2019 54225323 Recommended Hot Spot Analysis Procedure for Structural Details of FPSOs

http://slidepdf.com/reader/full/54225323-recommended-hot-spot-analysis-procedure-for-structural-details-of 1/8

Procee~ngs of the Eleventh 2001) International Offshore and Polar Engineering Conference

Stavanger, Norway, June 17-22, 2001

Copyright © 2001 by The International Society o f Offshore an d Polar Engineers

I S N 1-880653-51-6 Set); IS BN 1-880653-55-9 VoL IV); IS SN 1098-6189 Set)

R e c o m m e n d e d Ho t S p o t A n a l y s i s P r o c e d u r e f o r S t r u ct u r a l D e ta i ls o f F P S O s

a n d S h i p s B a s e d o n R o u n d R o b i n F E A n a l y s e s

W o l f g a n g F r i c k e

T e c h n i c a l U n i v e r s it y H a m b u r g

H a r b u r g , G e r m a n y

A B S T R A C T

A s p a r t o f t h e J o i n t I n d u s t r y P r o j e c t F P S O F a t i g u e a n d F r a c -

t u r e C a p a c i t y , a S p e c i a l T a s k G r o u p w i t h n i n e p a r t i c ip a n t s p e r -

f o r m e d i n v e s t i g a t i o n s r e g a r d i n g f i n i t e e l e m e n t ( F E ) m o d e l l i n g a n d

a n a l y s i s o f t y p i c a l s t r u c tu r a l d e t a i ls i n F P S O s ( F l o a t i n g P r o d u c -

t i o n , S to r a g e a n d O f f i o a d i n g U n i t s ) a s w e l l a s i n sh i p s . T h e p u r p o s e

o f t h i s s p e c i a l e ff o r t w a s t o d e v e l o p r e c o m m e n d a t i o n s o n a p p r o p r i -

a t e h o t s p o t s t r e s s m e t h o d s a n d S - N d a t a f o r f a t i g u e s t r e n g t h

d e s i g n . I n t o t a l , f iv e d e t a i l s w i t h d i f f e r e n t c h a r a c t e r i s t i c s , f r o m b o t h

g e o m e t r y a n d f a t i g u e l o a d i n g p e r s p e c t i v e s , w e r e s e l e c t e d f o r w h i c h

s t r e s s m e a s u r e m e n t s a n d f a t i g u e t e s t s a r e a v a i l a b l e . V a r i o u s f i n i t e

e l e m e n t m o d e l s w e r e d e v e l o p e d b y p a r t i c i p a n t s , u s i n g d i f f e r e n t

t y p e s a n d s i z e s o f e l e m e n t s , m o d e l l i n g a n d s t r e s s e v a l u a t i o n t e c h -

n i q u e s a s w e l l a s F E p r o g r a m s . C o m p a r i s o n s b e t w e e n t h e a n a l y s i s

r e s u l t s a n d m e a s u r e d s t r e s s e s n e a r t h e w e l d t o e s a l l o w c o n c l u s i o n s

t o b e d r a w n . T h r e e d i f f e r e n t s t r e s s e x t r a p o l a t i o n t e c h n i q u e s f o r

p r e d i c t i n g h o t s p o t s t r e s s e s a t t h e w e l d t o e s w e r e i n v e s t i g a t e d . T h e

r e s u l ti n g h o t s p o t s t r e s s e s , t o g e t h e r w i t h t h e e s t i m a t e d f a t ig u e l i v e s ,

a r e c o m p a r e d a g a i n s t t h e e x i s t i n g d e s i g n S - N c u r v e s p u b l i s h e d b y

t h e I n t e r n a t io n a l I n s t i t u t e o f W e l d i n g ( I I W ) . I t w a s c o n c l u d e d t h a t

t h e h o t s p o t s t r e s s e s p r e d i c t e d u s i n g t h e t h r e e s t r e s s e x t r a p o l a t i o n

t e c h n i q u e s , w h e r e t h e e l e m e n t s i z e s a n d s t r e s s e v a l u a ti o n p o i n t s a r e

d e t e r m i n e d b y t h e p l a t e t h i c k n e s s , c a n b e u s e d w i t h t h e c u r r e n t

d e s i g n S - N c u r v e s . M o s t s i g n i fi c a n t ly , o n e o f th e r e c o m m e n d e d

m e t h o d s r e q u i r e s n o s t r e s s e x t r a p o l a t i o n , w h i c h i s c o n s i d e r e d a n

a t t r a c t i v e a n d p r a c t i c a l a l t e r n a t i v e t o t h e e x i s t i n g p r a c t i c e s d e v e l -

o p e d b y c l a s s s o c i e t i e s . T h i s m e t h o d w i l l p r o v i d e s i g n i f i c a n t s a v i n g

o n a n a l y s i s e f f o r t s d u r i n g d e s i g n .

K E Y W O R D S : S t r e s s a n a ly s i s , h o t s p o t s t re s s , f i n i t e e l e m e n t m e t h o d ,

f a t ig u e , w e l d e d j o i n t

I N T R O D U C T I O N

T h e h o t s p o t s t r e ss a p p r o a c h f o r t h e f a t i g u e s t r en g t h a s s e s s m e n t o f

w e l d e d j o i n t s i s b a s e d o n t h e a s s u m p t i o n t h a t t h e l o c a l s t r e ss i n c r e a s e a t

t h e w e l d t o e c a n b e s u b d i v i d e d i n t o t w o p a r t s , o n e g o v e r n e d b y t h e

s t ruc tura l ( i . e . m a c ro-ge om e t r i c a l ) s t r e s s inc re a se a nd the o the r c re a te d

b y t h e l o c a l i z e d n o t c h s t r es s d u e t o t h e w e l d t o e i t s e l f w h i c h i s r e st r ic t e d

t o a r e g i o n o f a p p r o x . 2 - 3 m m a r o u n d t h e t o e . F u r t h e r m o r e , i t i s

a s s u m e d t h a t t h e f a t i g u e a s s e s s m e n t c a n b e b a s e d o n t h e f i rs t p a r t, i . e .

t h e s t r u c t u r a l s t re s s a t t h e h o t s p o t , t o g e t h e r w i t h a n a p p r o p r i a t e S - N

c u r v e , w h i c h i m p l i c i t l y c o n s i d e r s t h e e f f e ct s o f t h e l o c a l i z e d

fur the rm ore , i s va l id for a c e r ta in c la s s of we ld sha pe s a nd

R e g a r d i n g t h e h o t s p o t s t r e ss , e x p e r i m e n t a l a n d a n a l y t i c a l

h a v e b e e n d e r i v e d f o r i t s d e t e r m i n a t i o n b y e x t r a p o l a t i n g t h

s t r e s s out s ide the loc a l i z e d notc h-a f fe c te d z one to the we

a p p r o a c h w a s f i r st l y a p p l i e d i n t h e 1 9 7 0 s t o t u b u l a r j o i n t s

s t ruc ture s , whe re the inc re a se in s t ruc tura l s t r e s s c a n be ve ry

l o c a l b e n d i n g o f t h e t u b u l a r s h e l l c lo s e t o t h e c o n n e c t i o n b e

and chord (Almar-N~ess , 1 985) .

T h e h o t s p o t s t r e ss a p p r o a c h w a s l a t e r a p p l i e d a l s o t o w

s t ru c t u r es ( R a d a j , 1 9 9 0 ) b e i n g t y p i c a l f o r F P S O s ( F l o a t i n g

S t o r a g e a n d O f f l o a d i n g U n i t s ) a n d s h i p s. H e r e , t h r e e d i f f er

h o t s p o t s a t w e l d t o e s c a n b e i d e n t i f i e d w h i c h a r e e x e m p l i f i e

a

b

c

a t t h e w e l d t o e o n t h e p l a t e s u r fa c e a t a n e n d i n g a t t a c h m

a t t h e w e l d t o e a r o u n d t h e p l a t e ed g e o f a n e n d i n g a t t a c h

a l o n g t h e w d d o f a n a t t a ch e d p l a t e ( w e l d t o es o n b o t h t h

a t t a c hm e n t sur fa c e ) .

F i g u r e 1 . T y p e s o f H o t S p o t s i n W e l d e d S t r u c t u

Fo r we ld toe s on a p la te or she l l sur fa c e , i . e . type s a )

s t r u c tu r a l s tr e ss c a n b e d e f i n e d a s t h e s u m o f t h e a x i a l a n d b

o f t h e s t re s s d i s t r i b u t i o n i n t h e t h i c k n e s s d i r e c t i o n ( N i e m i , 1

e ve r , a uniq ue de f in i t ion of the s t ruc tura l s t re s s a t p la te e dg

t y p e b ) , w h i c h c a n b e u s e d f o r f a t ig u e s t r e n g t h a s s e s s m e n t ,

b le . The re fore , e x t r a pola t ion of e dge s t r e s se s i s c ur re n t ly

t h e o n l y p r a c t ic a l w a y f o r t h e d e t e r m i n a t i o n o f h o t s p o t s t r es

T h e h o t s p o t s t r e ss a p p r o a c h h a s o f t e n b e e n c r i t i c iz e d . E s

p r o b l e m o f d e f i n i n g a p p r o p r i a t e r e f e r e n c e p o i n t s f o r

e x t r a p o l a t io n c r e a t ed m a n y d e b a t e s . S e v e r a l c o d e s a n d g

v a r i o u s i n d u s t r i a l se c t o rs r e c o m m e n d d i f f e re n t p r o c e d u

8 9



d e t e r m i n a t i o n o f h o t s p o t s t r e s se s a n d S - N c u r v e s . E v e n i n t h e

s h i p b u i l d i n g a n d o f f s h o r e i n d u s t r y , th e p r o c e d u r e s e s t a b l is h e d b y t h e

c la s s i f i c a t ion soc ie t i e s a nd o the r a u thor i t i e s a re d ive rg ing . Unc e r ta in t i e s

a b o u t t h e s u i t a b i l i t y o f t h e h o t s p o t s t r e ss a p p r o a c h h a v e b e e n r a i s e d b y

r o u n d - r o b i n a n a l y s e s s u c h a s I S S C ( 1 9 9 7 ) s h o w i n g l a rg e s c a t te r a n d

di f f e re nc e s be twe e n a n a lys i s r e su l t s a nd m e a su re d s t r e s se s .

T h e i n d u s t r y , h o w e v e r , c o n s i d e r s t h e h o t s p o t s t r e s s a p p r o a c h a s a

v e r y p r a c t ic a l a p p r o a c h , o f f e r i n g a b e t t e r a l t e r n a t i v e c o m p a r e d t o t h e

t r a d i t io n a l n o m i n a l s tr e s s a p p r o a c h f o r t h e a s s e s s m e n t o f i n d i v i d u a l

j o i n t g e o m e t r i e s i n s h i p s a n d F P S O ' s , w h i c h v a r y f r o m d e ta i l to d e t a i l

d u e t o d i f f e r e n t s c a n t l i n g s a n d g e o m e t r i c a l c o n f i g u r a t i o n s . A s l a r g e

di f f e re nc e s in f a t igue a s se s sm e nt proc e dure s a s we l l a s unde r -pre dic te d

f a t ig u e l i v e s c a n n o t b e t o l e r at e d , m u c h e f f o rt h a s b e e n s p e n t i n t o f u r t h e r

i n v e s t i g a t i o n s o f t h e h o t s p o t s t r e s s a p p r o a c h w i t h i n t h e J o i n t I n d u s t r y

P r o j e c t ( J IP ) F P S O F a t i g u e C a p a c i t y w i t h 1 8 p a r t n e r s, c o o r d i n a t e d b y

D e t N o r s k e V e r i t as . W i t h i n t h i s p r o j e c t , a s p e c i a l t as k g r o u p p e r f o r m e d

i n v e s t i g a t i o n s t o a n s w e r t h e f o l l o w i n g q u e s t i o n s :

1 . W h i c h w a y s o f m o d e l l i n g y i e l d h o t s p o t s t r e ss e s w i t h s u f fi c i e n t

a c c ura c y for typ ic a l s t ruc tura l de ta i l s ?

2 . H o w c o r r e l at e c e rt a i n t y p e s o f s tr e s s e x t r a p o l a t i o n w i t h S - N c u r v e s

a v a i l a b l e ?

I t w a s i n t e n d e d t o p e r f o r m r o u n d - r o b i n st r es s a n a l y s e s o n a n u m b e r

o f t y p i c a l d e t a i ls w h e r e l o c a l s t r a i n m e a s u r e m e n t s a n d f a t ig u e t e s t d a t a

a r e a v a i l a b le , a l l o w i n g c o m p a r i s o n w i t h s t r e ss a n d l i f e c a l c u l a ti o n s . I n

t o t al , n i n e p a r t n e r s h a v e p a r t i ci p a t e d i n t h e w o r k , o f f e r i n g a w i d e r a n g e

o f m o d e l l i n g a n d e v a l u a t i o n t e c h n i q u e s a s w e l l a s f i n i t e e l e m e n t

a n a l y s i s p r o g r a m s . A s c e r t a i n p r o c e d u r e s a n d i d e a s a b o u t s t r e s s

d e t e r m i n a t i o n a n d e v a l u a t i o n e x i s t , it w a s d e c i d e d t o i n c l u d e a l l o f t h e m

i n t h e a n a l y s i s r a t h e r t h a n s y s t e m a t i c a l l y v a r y i n g c e r t a i n m o d e l l i n g

pa ra m e te r s . Insofa r , the rou nd - rob in s t r e s s a na lys i s r e f l e c t s c ur re n t

t e c h n i q u e s f r o m w h i c h r e c o m m e n d a t i o n s f o r a p p r o p r i a t e p r o c e d u r e s

c a n b e d e r i v e d . F u r t h e r q u e s t i o n s a r i s i n g f r o m t h e a n a l y s i s a r e i n t e n d e d

t o b e i n v e s t i g a t ed i n P h a s e I I o f t h e p r o j ec t .

I t s h o u l d b e n o t e d t h a t t h e i n v e s t i g a t i o n i s f o c u s s e d o n t h e f a t i g u e

a s s e s s m e n t o f w e l d t o e s o n l y a n d n o t o n p o s s i b l e c r a c k s o r i g i n a t i n g

f r o m t h e r o o t o f w e l d s w i t h i n c o m p l e t e p e n e t r a t i o n w h i c h a r e u s u a l l y

a s s es s e d o n t h e b a s i s o f t h e n o m i n a l s tr e s se s i n t h e w e l d t h r o a t . A l s o t h e

a s s e s sm e n t o f s t r e s s p e a k s a t r o u n d e d p l a t e e d g e s a r e o u t s i d e t h e s c o p e

of th i s pa pe r .

S E L E C T IO N O F S T R U C T U R A L D E T A I L S

T h e s t r u c tu r a l d e t a i l s w e r e s e l e c te d c o n s i d e r i n g t h e f o l l o w i n g

cri ter ia :

• a l l t h r e e t y p e s o f h o t s p o t s s h o w n i n F i g . 1 s h o u l d b e i n c l u d e d

• p l a t e b e n d i n g c a u s e d b y t h e a tt a c h m e n t s h o u l d b e i n c lu d e d , w h i c h

i s q u i te t y p i c a l fo r d e t a i l s i n s h i p s a n d F P S O ' s

• s t r a in m e a s u r e m e n t s a n d f at i g u e t e st s s h o u l d b e a v a il a b l e ; m o d e l s

t e s te d w i t h i n t h e J I P a r e p r e f e rr e d b e c a u s e d e t a i l ed o r a d d i t i o n a l

i n f o r m a t i o n i s a v a i l a b l e

F i v e d e t a i ls h a v e b e e n s e l e c te d f o r t h e r o u n d - r o b i n h o t s p o t s t re s s

a n a l y s i s, w h i c h a r e s h o r t l y d e s c r i b e d i n t h e f o l l o w i n g :

D e t a i l l : I S S C M o d e l

T h e f i r s t d e t a i l , t h e m o d e l f o r t h e s t u d y o f I S S C ( 1 9 9 7 ) a l r e a d y

m e n t i o n e d a n d f u r t h e r d e s c r ib e d b y l w a h a s h i e t a l . ( 1 9 9 8 ) , i s t h e c o n -

n e c t i o n o f a b u c k l i n g s t i ff e n e r 1 0 0 x 1 2 t o t h e f l a n g e o f a T - s h a p e d l o n -

g i t u d i n a l (w e b 3 5 0 x 1 2 , f l a n g e 1 5 0 x 2 0 ) . T h i s m o d e l b e i n g t y p i c al fo r

F P S O a n d t a n k e r s t r u c t u r e s w a s t e s te d e x p e r i m e n t a l l y i n t h e J a p a n e s e

re se a rc h pro je c t SR 219 so tha t loc a l s t r a in m e a sure m e nts a re a va i l a b le ,

h o w e v e r n o f a t i g u e t es t d a t a , a s t u r n e d o u t l a t e r . T h e f i n i t e e le m e n t

m o d e l s h o w n i n F i g . 4 g i v e s a n i m p r e s s i o n o f t h e s t r u ct u r e . T h e l o n g i -

t u d i n a l i s s u b j e c te d t o b e n d i n g a n d s h e a r, c r e a t i n g a n i n c r e a s e d h o t s p o t

s t re s s a t t h e w e l d e d c o n n e c t i o n w i t h t h e b u c k l i n g s t i f fe n e r . I t s h o u l d b e

n o t e d t h a t t h e f i l le t w e l d h a s a n i n c r e a s e d l e g l e n g t h o f 1 3 m m t o a v o i d

root f a i lure . Th e hot spo t be lon gs to type a ) in F ig . 1 .

Detai l 2: Gusse t s on Plat e Edge

T h e s e c o n d m o d e l , g u s s e t s o n p l a t e e d g e s s h o w n i n F i g . 2 a ,

t a k e n f r o m t e s t s p e r f o r m e d b y H H I i n t h e I I P ( K i m , 1 9 9

1 5 0 m m l o n g a n d I 0 m m t h i c k g u s s e t s, r e p r e s e n t i n g f a c e b

ta pe re d e nd s , a r e f i ll e t -we lde d to the e dge s o f a p la te s t r ip 60

whic h i s subje c te d to t e ns i l e s t r e s se s . The c r i t i c a l we ld toe b

type b) in F ig . 1 .

Detai l 3 Doub l ing Plat e

T h e t h i r d d e t a i l i s t h e d o u b l i n g p l a t e s h o w n i n F i g . 2 b , w

c r i t i c a l we ld toe on the p la te sur fa c e be longs to type c ) in F i

m o d e l c o n s i d e r e d h e r e w a s i n v e s t i g a t e d i n t h e J a p a n e s e r e s e ar c

S R 2 0 2 ( Y a g i e t a l . , 1 9 9 1 ) , s o t h a t s t r a i n m e a s u r e m e n t s a n d f a t

a r e a v a i l a b l e . W h e n t h e p a r e n t p l a t e i s s u b j e c t e d t o t e n s i o n ,

s i d e d d o u b l i n g p l a t e c a u s e s s e c o n d a r y b e n d i n g . T h i s h a s t o b

e r e d i n t h e a n a l y s i s , c r e a t i n g s o m e m o d e l l i n g p r o b l e m s i n c

wi th she l l e l e m e nts .

In the c ou r se of the a na lys i s , r e la t ive ly l a rge d i f f e re nc e s be

J a p a n e s e m e a s u r e m e n t s a n d t h e a n a l y t i c a l r e s u l ts w e r e f o u n d .

m o s t l i k e l y c a u s e d b y a n g u l a r d i s t o r t i o n d u r i n g t h e w e l d i n

d o u b l i n g p l a t e o n t h e p l a t e s tr i p . A s p r o n o u n c e d s t r es s m a g

d u e t o m i s a l i g n m e n t s h a l l b e c o n s i d e r e d i n t h e a p p l i e d s t r es s r a

i n t h e d e s i g n S - N c u r v e , a s t r es s m a g n i f i c a t i o n f a ct o r K m =

c e r ta in s t r e s s gra die nt we re e s t im a te d for the loc a l s t r e s s on th

a d d i t i o n a l t e s ts a n d c a l c u l a t io n s p e r f o r m e d .

D e t a i l 4 : H o p p e r C o r n e r M o d e l

T h e f o u r t h d e t a i l i s th e h o p p e r c o m e r s h o w n i n F i g . 2 c w

a l s o t e st e d i n t h e J I P . T h e c r i ti c a l w e l d i s l o c a t e d a t t h e t r a n s i

t h e 1 0 m m t h i c k f l a n g e t o t h e s l o p e d h o p p e r p l a te . T h e s t

s u b j e c t e d t o a v e r t i c a l f o r ce , p r o d u c i n g b e n d i n g a n d s h e a r i n

T h e s t re s s i n c r e a s e w i l l b e r e f e r r e d t o t h e n o m i n a l b e n d i n g s t r

k n u c k l e u n d e r t h e a s s u m p t i o n o f f u l l e f f e c t i v e b r e a d t h . T h e

be lo ngs to type c ) in F ig . 1 .

Detai l 5: Load Carry ing Fi l l e t Welds

Th e la s t de ta i l i s a ga in a m o de l t e s te d in the J IP , i . e .

w e l d e d c o n n e c t i o n b e t w e e n a v e r t i c a l I - b e a m a n d a v e r t ic a l p l a

r e p r e s en t s a s t i f f e n e r c o n n e c t i o n b e i n g s u b j e c t e d t o s h e a r a n d

se e F ig . 2d . Re fe re nc e s t r e s s i s a ga in the nom ina l be n din g s tr

u p p e r e d g e o f t h e v e r t i c a l p l a te . T h e w e l d t o e a t t h e h o t s p o t b

t y p e b ) i n F i g . 1 .

c D e t a i l 4

d O

F i g u r e 2 . D e t a i l s 2 - 5 s e l e c t e d f o r R o u n d R o b i n A n a

( D e t a i l 1 is s h o w n i n F i g . 4 )

9 0



F I N I T E E L E M E N T M O D E L L I N G A N D S T R ES S E V A L U A T I O N

I n o r d e r t o f i n d a p p r o p r i a t e t e c h n i q u e s f o r m o d e l l i n g w e l d e d s t r u c -

t u r e s a s w e l l a s e v a l u a t i n g t h e h o t s p o t s t r e ss i n c r e a s e d u e t o s t r u c tu r a l

e f fe c ts , a g r e a t v a r i e t y o f m o d e l s a n d w a y s o f e v a l u a t i o n w a s t a k e n i n t o

c o n s i d e r a t i o n . I n p r i n c i p l e , t h e d i f f e r e n t t y p e s o f m o d e l l i n g c a n b e

d i v i d e d i n t o t w o g r o u p s , i l l u s t r a t e d i n F i g . 3 :

* u s i n g p l a t e o r s h e l l d e m e n t s w i t h o u t w e l d r ep r e s e n t a t i o n ; t h e

e l e m e n t s a r e l o c a t e d i n t h e m i d - p l a n e o f t h e a s s o c i a t e d p l a t e; i n

I n m o s t c a s e s, t h e e l e m e n t l e n g t h a n d b r e a d t h s c l o s e t o t h

we re c hose n a s t x t t = p la te th ic kne ss ) . F ig . 4 e xe m pl i f i e s

l i n g b y D e t a i l 1 . S m a l l e r e le m e n t d i m e n s i o n s t y p i c a l l y t / 2 o

s e l e ct e d f o r m e s h e s i n d i c a t e d b y

2Solid2Ow(f) and 4Solid8w .

r e m a r k s o n s p e c i f i c m o d e l l i n g a s p e c ts a r e g i v e n i n t h e f o l l o w i

She l l e lem ents

t w e l d s F / 4 = 9 8 k N - - -

. o 0 e

i i i i

w = at tachmentwidth w

• S y m m e t r y

F i g u r e 3 . T y p i c a l F i n i t e E l e m e n t M o d e l s f o r th e W e l d e d J o i n t

She l l

odel

s o m e c a s e s t h e w e l d i s i n c l u d e d i n a s i m p l i f i e d w a y

u s i n g s o l i d e l e m e n t s w i t h t h e p o s s i b i l i t y to m o d e l t h e f i l l e t w e l d s

Detail

S h o w n i n F ig . 1

V a r i a t i o n s a r e s e e n i n d i f f e r e n t e l e m e n t t y p e s o f f e r e d b y t h e f i n i t e

e l e m e n t p r o g r a m s a s w e l l a s e l e m e n t s i z e s c h o s e n b y t h e a n a l y s t . T h e

f o l lo w i n g d e m e n t t y p e s w e r e c o n s i d e r e d i n t h e i n v e s t i g a t io n , u s i n g

s p e c ia l s h o r t n a m e s a s m e n t i o n e d b e l o w :

1Sol id20w and 2So l id20w

2 0 - n o d e d i s o p a r a m e t r i c s o l i d e l e m e n t , u s e d w i t h e i t h e r o n e o r t w o

e l e m e n t l a y e rs o v e r f u l l o r h a l f p l a t e t h i c k n e s s m o d e l l e d p r e f ix 1 a n d 2 ,

r e s p e ct i v e ly ) . T h e w e l d s a r e g e n e r a l l y i n c l u d e d s u f f ix w ) , i n s o m e

m o d e l s v e r y s i m p l y d i s r e g a r d i n g t h e r o o t g a p s e e F i g . 3 ) .

4Sol id8w

8 - n o d e d s o l i d e l e m e n t , n o r m a l l y u s e d w i t h f o u r l a y e r s o v e r t h e p l at e

t h i c k n e s s. T h e w e l d s a r e i n c l u d e d i n t h e m o d e l s s u f f ix w ) .

olidpw

H i g h e r - o r d e r s o l i d e le m e n t s g e o m e t r i c p - e l e m e n t s ) w i th r e f i n e d

m e s h a n d o p t i m i s e d s h a p e f u n c t i o n i n t h e c r i t i c a l a r e a . T h e w e l d s a r e

i n c l u d e d i n t h e m o d e l s s u f f i x w ) .

Shell Shell8r and Shell8p

8 - n o d e d s h e l l e l e m e n t w i t h o u t w e l d r e p r e s e n t a t i o n . I n c a s e o f o f f -

s e ts , e it h e r r i g i d l i n k s s u f f i x r ) o r a p l a t e c o n n e c t i o n b e t w e e n t h e m i d -

p l a n e s o f t h e p la t e s s u f f i x p ) a r e a r r a n g e d .

Shell4, Shell4r and Shell4p

S a m e a s a b o v e , b u t w i t h 4 - n o d e d s h e l l e l e m e n t s , p a rt l y h a v i n g c o n -

s t a n t s t r es s s t at e c s s ) a n d p a r t ly i m p r o v e d i n - p l a n e b e n d i n g b e h a v i o u r .

She l l8w

8 - n o d e d s h e l l m o d e l w i t h s i m p l i f i e d w e l d r e p r e s e n t a t i o n b y a r r a n g -

i n g a r e i n f o r c e d p l a t e s t r i p a t t h e f o o t o f t h e a t t a c h e d p l a t e , h a v i n g t h e

t h i c k n e s s i n c r e as e d b y o n e l e g l e n g t h a n d e x t e n d i n g t o t h e a c tu a l w e l d

t o e p o s i t io n . T h e e n d i n g s a r e s l o p e d , m e e t i n g t h e w e l d t o e p o s i t i o n o n

the pa re n t p la te .

F i g u r e 4. T y p i c a l F i n i t e E l e m e n t M o d e l l i n g

I n

Detail 1 (ISSC-Model) ,

the r e inforc e d f i l l e t we ld a t t

t i o n a l s t re s se s w h i c h i s c o n s i d e r e d i n o n e s h e l l m o d e l c o n

w e l d i n a s i m p l i f i e d w a y (ShellSw). I n a l m o s t a l l s h e l l m o d e

2 (Gusse t on P la te Edge) the gusse t s we re a r r a nge d d i r e c t ly

o f t h e l o n g i t u d i n a l p l a te e d g e e x c e p t f o r o n e m o d e l w h e r

e l e m e n t s w e r e a r r an g e d i n t h e m i d - p l a n e o f t h e g u s s e t a n d c o

a r e inforc e d p la te s t r ip w i th s lope d e nds

(She l l8w) . De ta i l 3

Doubl ing P la te ) w a s g e n e r a l l y m o d e l l e d t a k i n g t h e o f f s e t i n

In c a se of she l l e l e m e nts , e i the r r ig id l inks or ve r t i c a l p la

we re a r r a nge d , s e e F ig . 5a .

a ) D o u b l i n g F '

r i g id l i n k s o r

p l a t e

e l e m e n t s

b ) H o p p e r C o r n e r

F i g u r e 5 . M o d e l l i n g o f O f f s e ts i n S h e l l M o d e l

9 1



O n e s p e c if ic p r o b l e m o f

Detail 4 Hopper Corner Mod el)

i s t he o f f -

s e t b e t w e e n t h e i n t e r s e c t in g h o r i z o n t a l , v e r t i c a l a n d s l o p e d p l a t e a t t h e

c r i t i ca l po in t . T h i s i s au tomat i ca l l y accoun ted f o r i n so l id mode l s , wh i l e

i t i s c o n s i d e r e d o n l y i n p a r t o f t h e s h e l l m o d e l s b y a p l a t e c o n n e c t i o n

b e t w e e n t h e t w o i n t e r s e c t i o n p o i n t s , s e e F i g . 5 b . T h e s e m o d e l s a r e

d e n o t e d b y Shell8p and Shell4p. A n add i t i ona l s t udy showed tha t t h i s

o f f se t r educes t he s t resses i n t he ac tua l s t r uc tu r e by up to 10 in t he

v i c i n i ty o f t h e h o t s p o t . F o r Detail 5 Load Carrying Fillet Welds),

a g a i n o n e s h e l l m o d e l c o n t a i n s t h e w e l d i n s i m p l i f ie d f o r m Shell8w).

T h e s t re s s e s w e r e e v a l u a t e d w i t h t w o o b j e c t i v e s , (a ) t o c o m p a r e t h e

c o m p u t e d s t r es s e s w i t h m e a s u r e d v a l u e s i n t h e a r e a c l o s e t o th e h o t s p o t

a n d ( b ) t o e x t r a p o l a t e t h e s t r e s s e s t o t h e h o t s p o t u s i n g c o m m o n t e c h -

n i q u e s , f r o m w h i c h t h e f o l l o w i n g t h r e e w e r e s e l e c t e d :

1 . L inear ex t r apo la t i on ove r r e f e r enc e po in t s 0 .5 and 1.5 x p l a t e

t h i c k n e s s t a w a y f r o m t h e h o t s p o t ( t e c h n i q u e p r e f e r r e d b y m o s t

c l ass i f i ca t i on soc i e t i e s )

2 . L inear ex t r apo la t i on ove r r e f e r enc e po in t s 0 .4 and 1 .0 x p l a t e

t h i c kn e s s t a w a y f r o m t h e h o t s p o t ( t e c h n i q u e r e c o m m e n d e d b y t h e

I n t e r n a ti o n a l I n s t i tu t e o f W e l d i n g , s e e H o b b a c h e r , 1 9 9 6)

3 . No ex t r apo la t i on , bu t con s ide r in g the s t ress va lu e a t 0 .5 x p l a t e

th i ckness t a s t he r e l evan t ho t spo t s t r ess

T yp ica l s t r e ss eva lua t ion pa ths a r e i nd i ca t ed i n F ig . 3 . F i r s t compar i -

s o n o f t h e a n a l y s i s r e s u l t s s h o w e d t h a t a b e t t e r a g r e e m e n t b e t w e e n t h e

r e s ul t s f r o m t h e d i f f e r e n t m o d e l s a n d f r o m t h e m e a s u r e m e n t s i s

a c h i e v e d i f t h e d i s t a n c e o f t h e r e a d - o u t -p o i n t s ( R O P ' s ) i s m e a s u r e d t o

t h e h o t s p o t a s m o d e l l e d , i . e . t o th e w e l d t o e i f t h e w e l d i s m o d e l l e d o r ,

e l se , to t he s t r uc tu r a l i n t e r sec t ion po in t , i f t he we ld i s no t mod e l l ed . T he

s a m e a p p l i e s t o t h e p o i n t w h e r e t h e s t re s s i s e x t r a p o l a te d t o . T h e j u s t i f i -

ca t i on f o r t h i s p r ocedur e , wh ich a f f ec t s t he r esu l t s f o r sheU mode l s

wi thou t we ld r ep r esen t a t i on , i s t ha t t he s t r ess a t t he f i c t i t i ous we ld t oe

p o s i t i o n i s i n m a n y c a s e s t o o l o w d u e t o t h e r e d u c e d s t if f n es s c o m p a r e d

to t he r ea l s t r uc tu r e . T h i s means on the o the r hand , t ha t t he p r oposed

e x t r a p o la t i o n t o t h e s t r u ct u r a l i n t e r s e c t io n p o i n t m a y y i e l d c o n s e r v a t i v e

r esu l t s - a tr i bu t e t o s imple mod e l l i ng .

A g r ea t va r i ab i l i t y ex i s t s i n se l ec t i ng t he t ype o f s t r ess and in eva lu -

a t i ng t he s t r esses a t t he des i r ed l oca t ions . T he se l ec t ed de t a i l s do no t

show s ign i f i can t d i f f e r ences be tween p r inc ipa l and d i r ec t i ona l s t r e ss

( pe r pend icu l a r t o t he we ld t oe ) so t ha t t he t ype o f s t r ess does no t a f f ec t

t he r esu l t s he r e . E i the r noda l s t r esses o r e l emen t s t r esses a r e eva lua t ed ,

t h e l a t t e r n o r m a l l y e x t r a p o l a t e d l i n e a r l y f r o m t h e i n t e g ra t i o n p o i n t s t o

t h e p l a t e s u r f a c e o r e d g e a n d i n s o m e c a s e s a v e r a g e d t o o b t a i n a m i d -

s ide s t r ess . Pa r t i cu l a r ly f o r 4 - noded she l l e l emen t s , d i f f e r en t r e su l t s

w e r e o b t a in e d d e p e n d i n g o n t h e s h a p e f u n c t i o n a n d t h e c o n s i d e r a ti o n o f

i n - p l a n e s t r e s s c o m p o n e n t s . B y c o m p a r i n g a l l r e s u l t s , c o n c l u

d r a w n w i t h r e s p e c t to a p p r o p r ia t e w a y s o f d e t e r m i n i n g h o t s p o

P R E S E N T A T I O N A N D D I S C U S S I O N O F T H E R E S U L T S

T he eva lua t ed s t r esses a r e sum mar i z ed in F igs . 6 - 10 , w

t h e m e a s u r e d s t r e s s e s a r e s h o w n . T h e n o m i n a l s t r e s s h a s b e

un i ty f o r de t a i l s 2 - 5 , so t ha t t he r esu l t s a r e ac tua l ly ho t sp

c o n c e n t r a t i o n f a c t o r s ( S C F ' s ) . I t s h o u l d b e k e p t i n m i n d t h a t

m e a s u r e d s t re s s e s c o n t a i n s o m e u n c e r t a i n t i e s w h i c h w a s r e v e a

t es t s by sca t t e r ing s t r a ins ob t a ined f r om d i f f e r en t compar ab le

and t e s t mode l s . T he r i gh t pa r t o f the f i gu r es show s the s t r esse

o u t - p o i n t s ( R O P ' s ) s e l e c t e d b y t h e p a r t i c i p a n t s . T h e s e R

loca t ed a t ce r t a in d i s t ances f r om the ho t spo t mode l l ed ( i . e . we

s t r u ct u r al i n t e r s e c t i o n p o i n t , i f t h e w e l d i s n o t m o d e l l e d , s e e

chap te r ) and can b e i den t i f i ed as noda l o r e l emen t s t resses ( a t

o r i n t e g r a ti o n p o i n t s ) i n t h o s e c a s e s w h e r e t h e e l e m e n t l e n g t h e

p l a t e t h i c k n e s s t ( e x c e p t i o n s a r e f i n e r m e s h e s d e n o t e d b y 2S

4Solid8w and Solidpw). The l e f t pa r t g ives t he s t r esses ex t r ap

t h e h o t s p o t a c c o r d i n g t o t h e t h r e e m e t h o d s m e n t i o n e d a b

s t r esses a t t he r e f e r ence po in t s wer e i n t e r po la t ed by the pa r t i c

cu r ve f i t ti ng .

I n o r der t o f ac i l i t a t e t he i n t e r p r e t a t i on o f t he r esu l t s , d i f f e r

o f s y m b o l s w e r e c h o s e n f o r t h e r e su l t s f r o m s o l i d a n d s h e l l m o

s y m b o l s h a v e b e e n c o n n e c t e d b y s t r a i g h t d o t t e d l i n e s w i t h s m

f or she l l mode l s . T hese s t r a igh t l i nes appear t o i nc r ease t h

h o w e v e r , r e l e v a n t a r e o n l y t h e s y m b o l s . T h e m e a s u r e m e n t r e

p l o t t e d u s i n g c o n t i n u o u s l i n e s . T h e y h a v e n o t b e e n e x t r a p o l a

m e s h s o l i d m o d e l s w i t h h i g h e r - o r d e r e l e m e n t s , w h i c h p l a y

r o l e , c a n b e r e c o g n i z e d b y :

* o p e n s q u a r e s ( p - e l e m e n t s Solidpw)

• f u l l squar es 2Solid20w 39)

I n t he f o l l owing , t he r esu l t s a r e d i scussed in de t a i l t b r t he

m o d e l s .

Detail I ISSC-M odel), Fig . 6

T h e m e a s u r e m e n t s h o w s a s t r e ss o f a p p r o x . 1 4 0 M P a a t a

1 0 m m a w a y f r o m t h e h o t s p o t ( I S S C , 1 9 9 7 ). O n l y o n e c o a

m o d e l i s i n c l u d e d 1Solid2Ow), y ie ld ing s l i gh t ly h igher s t r ess

the r esu l t s a r e somewhat a f f ec t ed by the s ingu la r i t y a t t he

a l t h o u g h t h e s t r es s w a s a v e r a g e d o v e r t h e a t ta c h m e n t w i d t h

s t i ff e n e r p l us w e l d s ) b y a r r a n g i n g r e l a t i v e l y w i d e s o l i d e l e m e n

con t r a r y , t he ve r y f i ne so l id mes h

2Solid2Ow f))

show s smal l e

wi th a s t eep s t r ess i nc r ease t o t he ho t spo t .

2 2 0 , 2 2 0

2 0 0 2 0 0

~ 1 8 0 ~ 1 8 0

~ 1 6 0

r 6 o

~ 1 4 0 ~ E 1 4 c

~ 1 0 0 ~ 1 0 ¢

80 - 8C

Re~ P~ . (t=2Omm)

i i i r r

0 1 0 2 0 3 0

~ E x p e r i m e n t

• , , - I -- ,S he l l4

• ~ - . S h e l l 8

- . 0 - . S h e l l 8 w

- ~ - . S h e l l 8

- - I - - . S h e l l 4

- ~ - . S h e l l 8

- , o - . S h e l l 4 ( c s s )

- - ( - - . S h e U 4

1 S o l i d 2 0 w

~ 2 S o l i d 2 O w ( f l

4 0 5 0 6 0

distance ROP s to hot spot [mm ]

F i g u r e 6 . S t r es s R e s u l t s f o r D e t a il 1 ( I S S C - M o d e l )

9



T h e r e s u l ts o f t h e s h e l l m o d e l s a r e f a i r ly s c a t te r e d . L o o k i n g a g a i n a t

t h e l o c a t i o n 1 0 m m a w a y f r o m t h e h o t s p o t , t h r e e r e s u lt s a r e r e l a ti v e l y

c l o s e t o t h e m e a s u r e d o n e s , w h i l e f o u r o t h e r s a r e r e m a r k a b l y l o w e r . T h e

r e a s o n f o r t h i s c o u l d n o t b e c l a r i fi e d a s t h e s t r e s s e v a l u a t i o n p o i n t s a n d

methods a r e qu i t e d i f f e r en t ( a t nodes , a t e l emen t cen t r es , and ex t r apo-

l a t e d f r o m i n t e g r a t i o n p o i n t s t o t h e e l e m e n t e d g e ) .

T h e g e n e r a l t r e nd , t h a t s o l i d e l e m e n t s o v e r e s t i m a t e a n d s h e l l e l e -

men t s under es t ima te t he s t r esses , has been obse r ved a l so i n o the r s tud -

i es (e . g . I SSC, 2000) . P ou t i a in en and N iemi ( 2000) show ed tha t t h i s

h a p p e n s i n t h o s e c a s e s w h e r e a w e b i s a r r a n g e d b e l o w t h e p l a t e w i t h a

l o n g i t u d i n a l a t t a c h m e n t . I n o r d e r t o l i m i t t h e u n d e r - e s t i m a t i o n o f

s t re s s e s, t h e i n - p l a n e b e n d i n g b e h a v i o u r o f p la t e s s h o u l d b e i m p r o v e d

( t h e 4 - n o d e d e l e m e n t s s h o w i n g t h e l o w e s t r e s u l t s h a v e o n l y c o n s t a n t

s t r ess s t a t e ( css ) ) and the e l emen t wid th migh t have t o be r es t r i c t ed .

N i e m i ( 1 9 9 5 ) p r o p o s e d t h e ' a t t a c h m e n t w i d t h ' ( t h ic k n e s s o f a t t a c h m e n t

p l u s t w i c e w e l d l e g l e n g t h ) f o r t h e b r e a d t h o v e r t h e t w o e l e m e n t s i n

f r o n t o f t h e w e l d . F u r t h e r m o r e , t h e c o n s i d e r a t i o n o f t h e r e l a t i v e l y t h i c k

W h e n l o o k i n g a t t h e h o t s p o t s t r es s e s e x t r a p o l a t e d o v e r 0 .

s c a t te r i s m o d e r a t e w i t h i n ± 1 0 ( 1 5 2 M P a ± 1 5 M P a ) . I t i

t he ex t r apo la t i on i s pe r f o r med over 0 .4 t / 1 .0 t . T h i s i s obv ious

o f t h e r e l a t i v e l y c o a r s e m e s h e s , m a t c h i n g b e t t e r t h e f i r s t e x

t e c h n i q u e .

Detail 2 (Gussets on Pla te Edge), Fig. 7

T h e m e a s u r e m e n t s h o w s a m e a n s t r u c t u r al s t r e s s i n c r e a s e

l o c a t io n 5 m m a w a y f r o m t h e h o t s p o t , h o w e v e r w i t h a l a rg e s

I f t he two sm al l es t s t r e sses r esu l t i ng f r om the f i ne me

h i g h e r - o r d e r s o l i d e l e m e n t s

(2Solid2Ow(j ) and Solidpw )

a n d

the 4 - noded she l l e l emen t s wi th cons t an t s t r e ss s t a t e ( css ) a r e

the r esu l t s a r e f a i r l y c lose t oge the r . T h i s can a l so be seen in t

s t r e sses ex t r apo la t ed over 0 .S t /1 .5 t , wh ich a r e be tw een [ .85

i . e . _+6 . T he d i f f e r en t i dea l i sa t i ons o f t he r oo t gap an d s t r

t i o ns i n s o l i d m o d e l s ( a v e r a g e d o v e r t h i c k n e ss o r t a k e n a t c o

l i t t l e e f f ec t on t he r esu l t s . Ag a in , t he sca t t e r i s m uch h ighe r i

i s ex t r apo la t ed ove r 0 .4 t / 1 .0 t , wh ich i s due t o t he s t eep s t r ess

we ld s l igh t ly i nc r eases t he s t r ess ( by appr ox . 2 ) .

3 . 2

3 . 0

2 . 8

~ 2 . 6

2 . 4

~ 2.2

~ 2 . 0

~ 1 . 8

~ 1.4

1.2

1.(1

0.8

, , 3 . 2

3 . 0

2 . 8

( n 2 ' 6

~ 2 . 4

0 2 . 2

~ 2 . 0

1.4

1.2

1.0

0.8

R e f . P t s. ( t = 1 0 m m )

f r on t o f t he ho t spo t .

' ~ E x p e r i m e n t

1 S o l i d 2 0 w

- - ¢ ] - - S ol id p w

: 4 S o l id 8 w

0 4 S o l id 8 w

~ a ~ . - 2 S o l id 2 0 w

- - J k .- - 2 S o l i d 2 0 w

= 2 S o l i d 2 O w ( f )

- . 0 - . S h e l l 8 w

- . - t. . - .Sh el l4

I I - - I - - , S h e l l 4

:

1 0 5 2 0 2 5 3 0

d i s t a n c e R O P ' s t o h o t s p o t [ m m ]

Figur e 7 . S t r ess Resu l t s f o r De ta i l 2 ( Gusse t s on P l a t e E ~dge)

2 ,2 . , 2 .2

2 . 0 I 2 . 0

~ 1 . 8 ~ ) 1 . 8

'-- I::lZ

1 . 2 1 . 2

1 . 0 - - 1 . 0

Ref . Ots . ( t= 1gram)

I J - - O - - E x p e r i m e n t I

I i - - s o 2 o w j

I E ~ o l i d p w I

i ~ I - - e " 1 S ° l id 2 0 w

~ - )< - . S h e l l 4 r

\ . ' . / I . )K - 'Sh e l l 8 r I

• : ~ ' ~ ~ ~ , . o . ' S h e 4 r ( c s s )

~ t ~ ; . = . / I

- X S h e l l 4 r /

= . w ~ - - ~ ~ j - .

. . . . 5 . . . . 1 0 . . . . 1 5 . . . . 2 0 2 5 3 0 3 5 4 0

d i s t an c e R O P s t o h o t s p o t [ m m ]

Fig ur e 8 . S t r ess Resu l t s f o r De ta i l 3 ( Doub l in g P l a t e )

93



Detail 3 One-Sided Doubling Plate), Fig. 8

T h e r e s u l t s a r e f a i r l y c l o s e t o g e t h e r a t t h e h o t s p o t a n d m a t c h t h e

m e a s u r e d v a l u e s c o n t a i n i n g t h e s t r e ss m a g n i f i c a t i o n f a ct o r K m

e x p l a i n e d e a rl i er . T h e s c a t te r i n t h e r e s u l ts i s m a i n l y d u e t o t h e p r o b l e m

o f c o r r e c t l y m o d e l l i n g t h e o f f s e t i n t h e s h e l l e l e m e n t m o d e l s . M o s t o f

t h e r e s u l ts o r i g i n a t i n g f r o m s h e l l m o d e l s w i t h c o n n e c t i o n s b y p l a t e s o r

r i g i d l i n k s a re s l i g h t l y a b o v e t h e o t h e r o n e s .

T h e r e m a i n i n g r e s u l t s f o r t h e c o a r s e s o l i d m o d e l s a r e v e r y c l o s e

toge the r , a l so whe n lo ok ing a t the e x t r a pola te d s t r e s se s.

Detail 4 Hopper Corner Model) , Fig. 9

At a f i r s t g la nc e , the r e su l t s a ppe a r r a the r s c a t t e re d . Only the m e a n

v a l u e f r o m t h e m e a s u r e m e n t , w h i c h a l s o s h o w e d a l a r g e s c a t te r , h a s

b e e n p l o t te d .

F i v e m o d e l s , w h e r e t h e m i s a l i g n m e n t i l l u s tr a t e d i n F ig . 5 b h a s n o t

b e e n c o n s i d e r e d Shell4 and Shell8) s h o w t h e h i g h e s t r e s u lt s a t 5 m m

dis ta nc e f rom the ho t spot . I f the se r e su l t s a s we l l a s those f rom the

m o d e l s w i t h h i g h e r - o r d e r s o l i d e l e m e n t s 2SoIid2Ow09 and So l

e xc lude d , the s c a t t e r i s f a i r ly sm a l l , r e su l t ing in a hot sp

i n c r e a s e b e t w e e n 1 . 9 6 a n d 2 . 1 6 i f e x t r a p o l a t ed o v e r 0 . 5 t / 1 . 5 t

f r o m t h e m e a n .

Detail 5 Load C arrying Fillet Welds), Fig . 10

Also for th i s m o de l , the r e su l t s r e ve a l a h igh sc a t t e r . As for

the s t r e s s s ingula r i ty p la ys a pa r t , inc re a s ing a dd i t iona l ly t

- v a l u es c l o s e to t h e h o t s p o t . I t is u n c l e a r w h y t h e m e a s u r e m e

ra the r sm a l l va lue s .

T h e s c a tt e r is m u c h r e d u c e d , i f t h e r e s u l ts f o r t h e f i n e m e s

h i g h e r - o r d e r s o l i d e l e m e n t s

2Solid2Ow D and Solidpw)

a s w

l o w e s t r e s u l t s f o r t h e 4 - n o d e d s h e l l e l e m e n t s w i t h c o n s t a n t s t

( c s s ) a re a ga in e xc lude d . The n the hot spot s t r e s s inc re a s

b e t w e e n 1 . 6 4 a n d 2 . 1 8 ( _+ 1 0 ) i f e x tr a p o l a t e d o v e r 0 . 5 t /1 . 5 t , w

l o w e s t r e s u l t re f er s t o a m o d e l w i t h s i m p l i f i e d w e l d r e p r e

Sh e l l Sw ) .

2 . 8 ~

2 . 6

2 . 4

2 . 2

2 . 0

~ 1 6

~ 1.4

~ 1 2

1 0

0 8

R e f . P t s . ( t= l O m m )

4

0 5

- - ( > - E x p e r i m e n t

-

- I - - , S h e l l 4

-

- I - - , S h e l l 4

-

~ . - , S h e l l S p

-

SK- ,She l l8

-

- X - . S h e l l 4 p

-

- & - . S h e l l S p

.

. o . , S h e l l 4 ( c s s )

-

. -P - ,She l l4

1 S o l i d 2 0 w

,

. - - E l -. S o l i d p w

' . ¢ 4 S o l i d S w

~ k - - 2 S o l id 2 0 w

' ~ ¢ : ] ~ 1 ~ ~ - • : ~ . . - - - = - - 2 S o l i d 2 O w ( f )

~ i I i i I i I i ,,

0 1 5 2 0 2 5 3 0

distance ROP s to hot spot [mm]

F i g u r e 9 . S t r es s R e s u l ts f o r D e t a i l 4 ( H o p p e r C o m e r M o d e l )

3 . 2 •

2 8

2 6

1.0

0.8

u2. ~. m.

Ref. Pts. (t=lOmm)

\ \~ \ \ ,1

~ , ~ L

~ - x p e r l m e n l

- -+ - ,SheU4

-

- I - - , S h e l l 4

-

~ - . S h e l l 8

-

- ~ - , S h e l l 8

-

. o . , S h e l l 4 ( c s s )

, , , , , ~

5

- N.-,, 'Sh el l4

-

< > . S h e l l 8 w

~ IS o l i d 2 0 w

' - - 0 - - S o l i d p w

- - A - - 2 S o l i d 2 0 w

• - @ = - 4 S o l i d S w

~ 4 S o l i d 8 w

• ' = - 2 S o l i d 2 O w ( t )

t : = k

i i / i i i i

1 0 1 5 2 0 2 5 3 0

distance ROP s to hot spot [mm]

F i g u r e 1 0 . S t r e ss R e s u l t s f o r D e t a i l 5 ( L o a d - C a r r y i n g F i l l e t W e l d s )

9 4



D E S I G N S - N C U R V E S

A v a i l a b l e f a t i g u e t e s t d a ta , i . e. f a t i g u e l i v e s o b t a i n e d f o r g i v e n l o a d

l e v e l s , c a n b e u s e d t o v e r i f y i f t h e c o m p u t e d h o t s p o t s t r e s s es c o r r e l a t e

wi th s t andar d S - N cu r ves . Fa t igue t e s t da t a a r e ava i l ab l e f o r de t a i l s 2 , 4

and 5 ( Kim , 1999) and a l so f o r de t a i l 3 ( Yag i e t a l . , 1991). I n add i t i on ,

m o d e l s s i m i l a r t o d e t a i ls 1 a n d 3 h a v e b e e n a n a l y s e d a n d t e s t e d b y H H I ,

w h i c h a r e i n c l u d e d i n t h e p r e s e n t a n a l y s i s a s

Mo del 1 Longi tudinal

Gusset s ) and Mo del 3 Two -Sided Doubl ing Plates ) . I t s h o u l d b e n o t e d

t h a t o n l y t es t s w i t h h i g h R - r a t io a r e t a k e n f r o m t h e d a t a b y K i m ( 1 9 9 9 ) .

T h e v e r i f i c a ti o n i s p e r f o r m e d o n t h e b a s i s o f l o w e r - b o u n d h o t s p o t

S C F s f r o m t h e a n a l y s i s. I n t h i s w a y t h e c o m p a r i s o n w i t h s ta n d a r d S - N

c u r v e s i s c o n s e r v a t i v e . A s d i s c u s s e d b e f o r e , t h e r e s u l t s f r o m t h e f i n e

m e s h e s w i t h h i g h e r - o r d e r s o l i d e l e m e n t s a s w e l l a s t h o s e f r o m t h e 4 -

n o d e d s h e l l s w i t h c o n s t a n t i n - p l a n e s t r e s s s t at e h a v e b e e n e x c l u d e d

w h i c h m e a n s t h a t S - N c u r v e s d e r i v e d a r e a s s o c i a t e d t o a n a l y s e s w i t h

c e r t ai n m o d e l l i n g a n d e x t r a p o l a t i o n t e c h n i q u e s .

T h e l o w e s t S C F - v a l u e s a r e m o s t l y d e t e r m i n e d b y s o l i d m o d e l s u s i n g

8 - n o d e d e l e m e n t s w i t h d i m e n s i o n s t / 4 x t / 4 x t /4 . T h e a d d i t i o n a l m o d e l s

1 an d 3 w e r e a l s o a n a l y s e d b y t h is t e c h n i q u e a n d a l s o b y s h e l l e l e m e n t s .

I t i s i n t e r es t i ng t o no t e t ha t t he r esu l t s f r om the ve r y f i ne so l id mod-

els

2Solid2Ow f)),

ex t r apo la t ed ov er 0 .4 t and 1 .0t , f i t qu i t e we l l w i th

t h e o t h e r v a l u e s e x t r a p o l a t e d o v e r 0 . 5 t / 1 . 5 t . H o w e v e r , t h e r e s u l t s

o b t a i n e d f r o m t h e p - e l e m e n t s a r e b e l o w t h e s e v a l u e s .

T he S - N p lo t i n F ig . 11 show s the f a t i gue l i ves i n r e l a t ion t o t he ho t

spo t s t r ess r ange f o r t he f i r s t ex t r apo la t i on t echn ique , i . e . nomina l

s t re s s r a n g e m u l t i p l ie d b y t h e l o w e s t c a l c u l a t e d h o t s p o t S C F s e x t r a p o -

l a te d o v e r 0 . 5 t/ 1 .5 t . T h e d e t a i l s 4 a n d 5 s h o w g e n e r a l l y h i g h e r li v e s

which a r e assumed to be due t o benef i c i a l r e s idua l s t r e sses f ound in

de t a i l 5 and to a f a i l u r e c r i t e r ion de f ined by a r e l a t i ve ly l ong c r ack .

Fur the r mo r e , t he r esu l t s f o r mod e l 1 and de t a i l s 2 - 3 a r e c lose r t o t he

l o w e r b o u n d d u e t o t h e f a c t th a t o n l y t h e w o r s t o f fo u r c o m p e t i n g h o t

spo t s i n t he mode l s a r e r ep r esen t ed .

I n a d d i ti o n , t h e S - N c u r v e F A T 1 00 , c h a r a c t e r i z e d b y a s lo p e e x p o -

n e n t m = 3 a n d a r e f e r e n c e v a l u e o f 1 0 0 M P a a t 2 m i l l i o n c y c l e s , is

p lo t t ed i n t he f i gu r e , r ep r esen t ing th e l owe r boun d o f a l l r e su l t s . T he

s a m e e v a l u a t i o n f o r t h e o t h e r e x t r a p o l a t io n t e c h n i q u e s , w h i c h a r e o m i t -

t e d h e re , s h o w s a l s o F A T 1 0 0 a s l o w e r b o u n d f o r th e r e s u l ts e x t r a p o -

l a te d o v e r 0 . 4 t / 1. 0 t , w h i l e F A T 9 0 i s t h e l o w e r b o u n d f o r t h e re s u l ts a t

0 .5 t . I t shou ld be no t ed t ha t a l l t e s t da t a u sed h e r e i s r e l a t ed to j o in t s

w i t h p l a t e th i c k n e s s r a n g i n g f r o m 1 0 to 1 5 m m .

R E C O M M E N D A T I O N S F O R H O T S P O T ST R E S S A N A L Y S I S

B a s e d o n t h e r e s u l t s o f t h e r o u n d - r o b i n f i n i t e e l e m e n t a n a l y s i s

d e s c r i b e d a b o v e , t h e f o l l o w i n g r e c o m m e n d a t i o n s c a n b e g i v e n .

H o t s p o t s t r e s s e s a r e c a l c u l a t e d a s s u m i n g l i n e a r m a t e r i a l b e h a v i o u r

u s i n g a n i d e a l i s e d s t r u c t u r a l m o d e l w i t h n o f a b r i c a t i o n - r e l a t e d m i s -

l

I 0 0

10

a l i g nm e n t . T h e l a t t e r h a v e t o b e c o n s i d e r e d i n c e r t a i n j o i n ts

p r o n o u n c e d m i s a l i g n m e n t e f f e c t s a r e p o s s i b le , s u c h a s p l a t e

c r uc i f o r m jo in t s an d t r ansv er se f i l l e t we lds on f r ee p l a t es , .b

a t e Kin- fac to r s d e f ined e . g . by H obb ache r ( 1996) . T he ex

l o c a l f i n i t e e l e m e n t m o d e l h a s t o b e c h o s e n s u c h t h a t e f f e

s t r uc tu r a l de t a i l cons ide r ed a r e su f f i c i en t ly smal l and r easona

a r y c o n d i t i o n s c a n b e f o r m u l a t e d .

I n p l a t e s t r uc tu res , t h r ee t ypes o f we ld t oes can b e i den t i

a r e exempl i f i ed i n F ig . I . T he r e l evan t ho t spo t s t r ess i s t h

s t r e s s o n t h e s u r f a c e o r a t t h e e d g e o f t h e p l a t e a c t i n g a p p

per pend icu l a r t o t he we ld t oe ( 45 to 90° ) .

M o d e l s w i t h t h i n p l a t e o r s h e l l e l e m e n t s o r a l t e r n a t i v e l y

e l e m e n t s m a y b e u s e d . I t s h o u l d b e n o t e d t h a t o n t h e o n e

ar r angemen t and type o f e l emen t s have t o a l l ow f o r s t eep s t

e n t s a s w e l l a s f o r t h e f o r m a t i o n o f p l a t e b e n d i n g , a n d o n

hand , on ly t he l i nea r s t r ess d i s t r i bu t ion in t he p l a t e t h i cknes

needs t o be eva lua t ed wi th r espec t t o t he de f in i t i on o f s t r uc t

T h e f o l l o w i n g m e t h o d s o f m o d e l l i n g a r e r e c o m m e n d e d , s e e a

Plate and Shel l Model s .

A s i m p l e m o d e l l i n g i s o f f e re d b y t h i n p l a t e a n d s h e l l e l e m

h a v e t o b e a r r a n g e d i n t h e m i d - p l a n e o f t h e s t ru c t u ra l c o m p

n o d e d e l e m e n t s a r e r e c o m m e n d e d p a r t i c u l a r ly i n c a s e o f s

g r ad i en t s . Car e shou ld be g iven to poss ib l e s t r ess unde

e s p e c i a l l y a t t y p e b ) w e l d t o e s i n c o n n e c t i o n w i t h 4 - n o d e d

w h i c h s h o u l d c o n t a i n a t l e a s t i m p r o v e d i n - p l a n e b e n d i n g m o d

T h e w e l d s a r e u s u a l l y n o t m o d e l l e d e x c e p t f o r s p e c i a l c

the r esu l t s a r e a f f ec t ed by h igh loca l bend ing , e . g . due t o

b e t w e e n p l a t e s o r d u e t o a s m a l l f r e e p l a t e l e n g t h b e t w e e

w e l d s . H e r e , t h e w e l d m a y b e i n c l u d e d b y v e r t i c a l o r i n c

e l e m e n t s h a v i n g a p p r o p r i a t e s t i f f n e s s o r b y i n t r o d u c i n g c

e q u a t i o n s t o c o u p l e t h e n o d e d i s p l a ce m e n t s . P a r t i c u la r l y f o

o f t ypes b ) and c ) , a s imp le a l t e r na t ive f o r we ld mode l l i ng i s

t he a r r angem en t o f a r e in f o r ced p l a t e s t r i p a t t he f oo t o f t he a

h a v i n g a t h i c k n e s s i n c r e a s e d b y o n e l e g l e n g t h a n d s l

ex t end ing to t he ac tua l we ld t oe pos i t i ons .

Sol id Element Model s :

A n a l t e rn a t i v e p a r t ic u l a r ly f o r c o m p l e x c a s e s i s o f f e r e d b

m e n t s w h i c h n e e d t o h a v e a d i s p l a c e m e n t f u n c t i o n a l l o w i n g

g r ad i en t s a s we l l a s p l a t e bend ing wi th l i nea r s t r ess d i s t r i bu

p la t e t h i ckness d i r ec t i on . T h i s i s o f f e r ed , e . g . , by i sopar amet r

e l e m e n t s ( w i t h m i d - s i d e n o d e s a t t h e e d g e s ) w h i c h m e a n s t h

e l e m e n t i n p l a t e t h i c k n e s s d i r e c t i o n n e e d s t o b e a r r a n g e d

e v a l u a t i o n o f t h e m e m b r a n e a n d b e n d i n g s t r e s s c o m p o n e n

p o s s i b l e i f a r e d u c e d i n t e g r a t i o n o r d e r w i t h o n l y t w o i n t e g ra

i n t h e t h i c k n e ss d i r e c t i o n i s c h o s e n . M o d e l l i n g o f t h e w e l d s i

r e c o m m e n d e d a n d e a s i l y p o s s i b l e a s s h o w n i n F i g . 3.

1 . 0 E + 0 4 1 . 0 E + 0 5 1 . 0 E + 0 6 1 . 0 E + 0 7

N c y c l e s )

F ig ur e 1 1 . F a t ig ue t e s t re su l t s v s . ho t spo t s t r ess r a ng e ba se d o n lo w e s t c a l c u la t e d SCF s e x t r a po la t e d o v e r 0 . 5 t /1 . 5 t

9 5



Elemen t S i z es :

For both types o f modelling, the dimens ions of the first two or three

elements in front of the weld toe should be chosen as follows:

W e l d to e s o f t y p e s a a n d c : T h e

deme nt length should correspond

to the plate thickness. In the transverse direction, the plate thickness

may be chosen again for the breadth o f the plate elements. However, the

breadth over the first two elements should not exceed the 'attachment

width', i. e. the thickness of the attached plate plus 2 x the weld leg

length (in case of type c: the thickness of the web plate behind plus 2 x

weld leg length). This attachment width may also be taken for the width

of solid elements in front of the weld toe, see Fig. 3. Still open is the

question how the element size has to be defined e. g. for bulb profiles.

Furthermore it should be noted that a much finer mesh is needed in case

of 8-node solid elements.

Weld toes o f type b :

For weld toes at the plate edge, the plate thick-

ness is principally not a suitable parameter to determine the element

size and stress extrapolation points. A further study (Fricke and Bog-

dan, 2001) shows that fixed values can better be used to describe the

stress increase at in-p lane notches. From this s tudy and from details 2

and 5 of the current study it can be concluded that higher-order ele-

ments with element lengths of 10 x 10 mm together with a linear extra-

polation over the mid-side points yield conservative results. Therefore,

it is recommended to use element dimensions fixed to these dimensions .

St res s Eva luat ion:

The structural stress components on the plate surface should be

evaluated along the paths shown in Fig. 3 and extrapolated to the hot

spot. Recommended are stress evaluation points located at distances

0,5 t and 1.5 t away from the hot spot modelled, where t is the plate

thickness at the weld toe. I f the weld is not modelled, the hot spot is the

structural intersection point modelled. In case of type b) weld toes,

fixed distances of 5 mm and 15 mm are recommended.

If a relatively coarse mesh with the element sizes mentioned above is

chosen, the stresses may be evaluated as follows:

* In case of plate or shell elements the surface stress may be evalu-

ated at the corresponding mid-side points (i. e. taking the mean

value of the element stresses at the adjacent corners). At weld toes

of type c) the stress may be averaged over the width of the attach-

ment behind by taking the surface stress in the element centres as

also shown in Fig. 3.

* In case of solid elements the stress may be extrapolated l inearly to

the surface centre (usually after averaging the stress components at

each of the the two layers of integration points)

• At weld toes of type b), only part of plate bending obviously needs

to be considered. Further investigations are considered to be neces-

sary as such cases are outside the scope of the present analysis.

Fat igue Asses sment :

If the hot spot stress is evaluated by linear extrapolation in the way

described, the fatigue strength may be assessed with a usual design S-N

curve based on hot-spot stresses (e. g. Hobbacher, 1996).

Alternatively a simplified approach without any stress extrapolation

seems to be reasonable where the stress is taken at the location 0.5 t

(resp. 5 mm for weld toes of type b) away from the hot spot modelled

and assessed with a design S-N curve reduced by one fatigue class.

If the hot spot stress is evaluated from strain measurements or from

refined models with improved finite elements, a stress extrapolation

over reference points at distances 0.4 and 1.0 x plate thickness or a

quadratic extrapolation is recommended in line with Hobbacher (1996).

CONCLUDING R E M R K S

The present study has been used in the JIP together with other data

to derive a design hot spot S-N curve (Maddox, 2001). Here, a lower

S-N curve was found, which might be due to additional test data and in

particular to the inclusion of larger plate thickness (up to 25 ram).

The present study shows that further inves tigations are re

clarify especially the following items:

• Modelling and stress evaluation for very thick component

bulbs o f profiles

• Extent of plate bending to be considered at type b) weld toe

• Type of stress and evaluat ion in case of complex stress field

Finally it should be noted that root cracking at fillet we

covered by the hot spot stress analysis procedure. Approp

practical procedures for the analysis of the relevant stresses in

structures and their fatigue assessment are still to be developed

ACKNOWLEDGEMENTS

The JIP has been supported by 19 participants under coord

Det Norske Veritas. An overview over the project is given by

(2001). The present study has been managed by Germanisch

Hyundai Heavy Industries, as a participant of the JIP, perfo

fatigue tests with small scale specimens of typical fillet welded

ship structures, which was one of the main tasks in the JIP for

basis of the investigations described in the paper.

Nine participants contributed with great enthusiasm to th

robin analyses: Weimin Chen (Umoe Technology), Ben Fero

water), W.S. Kim (Hyundai Heavy Industries), Helena Po

(Lloyd's Register of Shipping), Philippe Rucho (Bureau

Armin S~ibel (Germanischer Lloyd), Tore Ull dand (Aker M

Richard Yee (American Bureau of Shipping) and also Y.-

(National University of Singapore). The author wishes to tha

them for their work and all participants of the JIP for their

comments and permission to publish the results.

REFERENCES

Almar-N~ss , A. (1985). Fatigue Handbook - Offshore St

Tapir Publishers, Trondheim.

Fricke, W. and Bogdan, R. (2001). Determinat ion of Hot Spot

Structural Members with In -Plane Notches Using a Coars

IIW-Doc. XIII- 1870-01, International Institu te of Welding.

Hobbacher, A. (1996). Fatigue Design of Welded Joints and

nents. Abington Publishing, Cambridge (UK).

ISSC (1997). Report of Committee ILl - Quasi-Static R

Proc. of 13 h Int. Ship and Offshore Structures Congress

Moan and S. Berge), Elsevier Science.

ISSC (2000). Report of Committee II.1 - Quasi-Static R

Proc. o f 14 h Int. Ship and Offshore Structures Congress

Ohtsubo and Y. Sumi), Elsevier Science.

Iwahashi, Y. et al. (1998). Finite Element Comparative Stud

Structural Detail. Mar ine S t ructures 11, pp. 127 - 139.

Kim, W. S (1999): Fatigue Tests of Typical Welded Joints.

Heavy Industries Co., Ltd. Ulsan (unpublished).

Lotsberg, I. (2001). Overview of the FPSO - Fatigue Capa

Proc. of OMAE'01, ASME, Rio de Janeiro.

Maddox, S. (2001). Recommended Design S-N Curves fo

Assessment of FPSOs. ISOPE' 2001, Stavanger.

Niemi, E. (1993). Recommendations Concerning Stress Dete

for Fatigue Analysis of Welded Components . IIS-IIW-

Abington Publishing , Cambridge (UK).

Poutiainen, I. and Niemi, E. (2000). The Determination of

Stress in Gusset Structures Using a Coarse Element Mesh.

XIII- 1820-2000, International Institute of Welding.

Radaj, D. (1990). Design and Analysis of Fatigue Resistant S

Abington Publishing, Cambridge (UK).

Yagi, J.; Machida, S.; Tomita, Y.; Matoba, M.; Kawasaki, T

Definition of Hot Spot Stresses in Welded Plate Type Stru

Fatigue Assessment. IIW-Document IIW-XIII-1414-91

tional Ins titute of Welding.

96

54225323 Recommended Hot Spot Analysis Procedure for Structural Details of FPSOs

Documents

Transcript of 54225323 Recommended Hot Spot Analysis Procedure for Structural Details of FPSOs