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Proceedings o f the Ninth (1999) International Offshore and Polar Engineering ConferenceBrest, France, May 30-June 4, 1999

Copyright © 1999 by The International Society of Offshore and Polar Engineers

ISBN 1-880653-39-7 (Set); ISBN 1-880653-40-0 (Vol. I); ISS N 1098-6189 (Set)

FPSO-Turre t Sys tem Stabi l i ty and Wave HeadingA n d r ~ J a c q u e s d e P a i v a L e i t e

Petrobras, S . A .

R i o d e J a n e i r o , B r a z i l

A l e x a n d r e N i c o l a o s S i m o s, E d u a r d o A o u n T a n n u r i a n d C e l s o P u p o P e s c e

U n i v e r s i t y o f S ~ o P a u l o

S ~ o P a u l o , B r a z i l

A B S T R A C T

T ur r e t sys t e m pos i t i on i ng i s a c r uc i al t a sk i n t he de s i gn o f an F P S O .

A c o m m o n a p p r o a c h i s t o a v o i d e q u i l i b r iu m b i f u rc a t i o n w h e n t h e

sys t e m i s sub j e c t t o c u r r e n t a c t i on , i n o r de r t o a s su r e l ow l e ve l s o f

m oor i ng f o r c e s a s w e l l a s unde s i r e d dyna m i c be ha v i our t ha t c ou l d l e a d

t o e xc e s s i ve l oa d i ng bo t h on t he be a r i ngs a nd on t he r i s e r s .

I ns t a l l i ng t he t u r r e t ne a r t he m i dsh i p s e c t i on i s a de s i r e d t r e nd a nd

do i ng i t w h i l s t p r e se r v i ng t he sh i p ' s d i r e c t i ona l s t a b i l i t y c a n be m a de

poss i b l e w i t h t he use o f pa s s i ve c u r r e n t s t a b i l i s e r s . T h i s m ov e , on t he

o t he r ha nd , ha s t o be i nve s t i ga t e d f o r t he c a se w he n w a ve s a nd w i nd a r e

p r e se n t , a s , i n th i s c a se , t he s t a b i l i s e r m a y p r od uc e a n a dve r se e f f e c t .

T he p r e se n t w or k a ddr e s se s t h i s po i n t t h r ough a c a se s t udy w he r e

t h e d y n a m i c s o f a ty p i c al V L C C c o n v e r t e d i n t o a F P S O a n d m o o r e d t o

t he s e a be d t h r ough a t u r r e t sys t e m i s num e r i c a l l y s i m ul a t e d w i t h a

m ode l w he r e w a ve - c u r r e n t i n t e r a c t i on i s t a ke n i n t o a c c oun t .

I t ha s be e n f ound t ha t a sm a l l de g r e e o f i n s t a b i l i t y w i t h r e spe c t t o

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

sys t e m ove r a l l be ha v i our , a s f a r a s m oor i ng f o r c e s a r e c onc e r ne d . T he

e f f e c t o f r udde r - t yp e s t a b i l i s e rs on e qu i l i b r i um b i f u r c a t i on a nd o n w a ve

he a d i ng i s d i s c us se d a nd e x e m pl i f i e d . A b r i e f d i s c us s i on on o t he r

a spe c t s i nvo l ve d i n m ov i ng t he t u r r e t t ow a r ds m i dsh i ps o r i n s t a l l i ng

stabi l i se r s i s car r ied out .

KEYWORDS

F P S O , m oor i ng sys t e m , s t a b i l i t y , t u r r e t , r udde r .

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

S e ve r a l c ha r a c t e r i st i c s o f t a nke r s suc h a s l a r ge de c k a r e a s a nd

s t o r a ge c a pa c i t y , a s w e l l a s m a r ke t a va i l a b i l i t y , m a ke t he m a good

op t i on f o r use i n o i l p r oduc t i on a t s e a . T he se un i t s a r e ge ne r a l l y na m e d

F P S O ' s ( a c r o n y m f o r Floating Production Storage and Offloading) an dt he i r u se , be i t ne w l y bu i l t o r c onve r t e d , i s ve r y w e l l su i t e d f o r

p r oduc t i on i n p r o j e c t s w he r e l a r ge o i l p r oc e s s i ng a nd s t o r a ge c a pa c i t i e s

a r e r e qu i r e d .

A m o n g s t m a n y m o o r i n g o p t i o n s f o r t h e i r s t a t i o n k e e p i n g , t h e S P M

( S i ng l e P o i n t M oo r i ng) o f t u r r e t t ype s t a nds ou t due t o t he ve s se l ' s

w e a t he r va n i ng a b i l i ty ( f ig . 1 ) .

V -

tu

m o o r i n g l i n e s

Figure 1 - T u r r e t m o o r e d F P S O

H o w e v e r , d e s i g n i n g b o t h t h e r i s e r a n d t h e m o o r i n g s y s t e m s o f a

t u r re t t y p e F P S O i s n o e a s y t a s k a n d s t u d y i n g t h e i r d y n a m i c b e h a v i o u r

bo t h qua l i t a t i ve a nd qua n t i t a t i ve l y ha s be e n t he c onc e r n o f m a ny

e ng i ne e r s a nd r e se a r c he r s i n r e c e n t ye a r s . T h i s w or k ha s r e c e i ve d

i n t e ns i ve suppor t f r om o i l c om pa n i e s s e e k i ng t o a dd r e l i a b i l i t y t o t he i r

sys t e m s a nd t o e x t e nd t he r a n ge o f a pp l i c a b i l i t y o f the i r de s i gn t oo l s .

S e e , f o r i n s t a nc e , P a pou l i a s a nd Be r n i t s a s ( 1988) , G a r z a - R i os a nd

Be r n i t s a s ( 1996) , Be r n i t s a s a nd G a r z a - R i os ( 1996) , N i sh i m ot o , Br i na t i

a n d F u c a t u ( 1 9 9 6 ), F e r n a n d e s an d A r a t a n h a ( 1 9 9 6 ) o r P e s c e a n d

Y a nur i ( 1997) .

O ne o f t he a spe c t s t ha t s t r ong l y i n f l ue nc e t he r i s e r be ha v i ou r on a

t u r r e t i s i t s l oc a t i on w i t h r e spe c t t o t he m i dsh i p s e c t i on . T he c l ose r t o

t he bow t he t u r r e t i s l oc a t e d , t he h i ghe r t he f i r s t o r de r e xc i t a t i on

e xpe r i e nc e d by t he r i s e r . T he r e f o r e , b r i ng i ng t he t u r r e t a f t w a r ds i s a

de s i r e d t r e nd . U sua l l y w he n t he t u r r e t i s a t t he m i dsh i p s e c t i on o r ne a r

i t , bow a nd / o r s t e r n t h r us t e r s a r e i n s t a l l e d i n o r de r t o ke e p t he ve s se l ' s

h e a d i n g c o n v e n i e n t l y o r i e n t ed .

P e t r o b r a s s t a n d a r d o p t i o n f o r t u r r e t m o o r e d F P S O ' s d e s i g n i s t o

a dop t a pa s s i ve ve s se l . Conse que n t l y , t u r r e t l oc a t i on i s ge ne r a l l yr e qu i r e d t o be ne a r t he bow i n o r de r t o a s su r e t ha t d i r e c t i ona l s t a b i l i t y i s

p r e se r ve d .

I n a r e c e n t w or k L e i t e e t a l . ( 1998) i nve s t i ga t e d t he r o l e o f r udde r

t ype s t a b i l i s e r s in t he d i r e c t iona l s t a b i l i t y o f F P S O ' s b y t ow t e s t i ng sh i p

hu l l s i n a t a nk f o r d i f f e r e n t r udde r sha pe s a nd t u r r e t l oc a t i ons . A ba r e

hu l l c a se w a s a l so pe r f o r m e d . I n t he se t e s t s , t he t u r r e t w a s s i m ul a t e d by

a ve r t i c a l t ow i ng ba r w i t h a r o l l e r be a r i ng c onne c t i ng i t t o t he sh i p

m ode l ( f i g . 2 ) . H e r e on l y c u r r e n t a c t i on w a s s i m ul a t e d a nd t he r e su l t s

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