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Proceedings of the Eleventh 2001) International Offshore and Polar Engineering Conference Stavanger, Norway, June 17-22, 2001 Copyright © 2001 by The Internat ional Society of Offsh ore and Polar Engineers IS N 1-880653-51-6 Set); 1SBN 1-880653-54-0 Vol. Ill); ISSN 1098-6189 SeO Effect of Coupling of Mooring Lines and Risers on the Design Values for a Turret Moored FPSO in Deep W ater of the Gulf of Mexico J.E. W. W ichers MARIN USA Inc., Houston, TX, USA P. V. Devlin Texaco Inc., Houston, TX, USA ABSTRACT In the framework of the DeepStar CTR 4401A Theme structures coupled computations were carded out on a turret moored tanker based FPSO in 3,000 ft, 6000 ft and 10, 000 fi water depth for the Gulf of Mexico. The FPSO was exposed to both Hurricane and Loop-current conditions. The size of the FPSO corresponds to a 200 kDWT tanker moored by means of 12 lines. For the production, water injection, oil mooring system in 3,000 ft exists of a chain-jacketed spiral strand- ground chain, while the mooring system in 6000 ft was both chain- polyester-ground chain and chain-spring buoys-jacketed spiral strand- ground chain and for 10,000 ft water depth it consists of a chain- polyester-ground chain combined line. Considering the design values the question may arise on the value of the coupled analysis and the effects of the Cd-values on the dynamics on the mooring legs and risers at large water depth. To understand the questions a fully coupled mathematical model was developed. Using the DeepStar FPSO system a new set of computations in the time-domain has been carried out. By changing systematically the Cd- resistance coefficients on the risers and the mooring lines the effect of the coupling on the global motions and the mooring forces can be distinguished. The com putations were applied for the system in 3,000 ft water depth exposed to the Hurricane and Loop-current condition and in 10,000 ft water depth during Hurricane condition. The results show that the complete coupling has to be taken into account to obtain realistic design values. KEYWORDS: Coupled, FPSO, mooring forces, riser tension, global motions, deepwater NOMENCLATURE Can : drag coefficient norm al Cdt : drag coefficient tangential C ~n : added inertia coefficient norm al Cit : added inertia coefficient tangential C~ : coulomb friction seabed normal Cpr : coulom b friction seabed tangential INTRODUCTION A turret moored FPSO system generally consists of a tanker (hull, process equipment and a superstructure), a mooring system and a risers system. For the design of the system n one of the parts can be neglected. The current loads and the associated damping forces on the mooring lines and the risers will influence not only the watch circle of the turret but also the loads on the risers and the mooring forces. To design a FPSO the complete system has to be taken into account. This is called a fully coupled dynamic system. To formulate coupling effects several studies has been performed in the past (e.g. Wichers and Huijsmans, 1990, van den Boo m, 1985, Wichers and Dercksen, 1994). The forces acting on the system are: first order wave forces and hydrodynamic reaction forces with or w/o current on the hull second order wave drift forces with or w/o current on the hull wave drift dam ping with or w/o current on the hull current loads on the hull low frequency hydrodynamic reaction forces on the hull first order wave forces, current loads and hydrodynamic reaction forces on the mooring lines and risers soil friction reaction forces on the mooring lines and risers. aerodynamic exciting and reaction forces; for the excitation e.g. a constant 1-hour mean wind force and wind spectrum forces acting on the above water part of the FPSO. The sources of these forces w ill be described in more detail in the associated sections. In order to demonstrate the coupling effects on the DeepStar system computations have been carried out with a fully coupled dynamic m athematical model. The computations concern the complete FPSO system in 3,000 and 10,000 ft water depth exposed to the Hurricane weather condition and in 3,000 ft exposed to the Loop- current weather condition. For this purpose the resistance coefficient Cd of the risers and the steel spiral strand (3,000 ft) and polyester ropes (10,000 ft) were varied. The variation concerns the Cd=0, Cd=l and Cd=2 of the mooring lines and risers. Finally, to demonstrate clearly the coupling effects, computations were carried out with the same varied Cd-values in Loop-current only. 4 8

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Page 1: Effect of Coupling of Mooring Lines and Risers on the Design PDF

8/10/2019 Effect of Coupling of Mooring Lines and Risers on the Design PDF

http://slidepdf.com/reader/full/effect-of-coupling-of-mooring-lines-and-risers-on-the-design-pdf 1/1

Proceedings of the Eleventh 2001) International O ffshor e and Polar Engineering C onference

Stavanger, Norway, June 17-22, 2001

Copyright © 2001 by The International Society of Offsh ore and Polar Engineers

I S N

1-880653-51-6 Set); 1SB N 1-880653-54-0 Vol . I l l ) ; ISS N 1098-6189 SeO

E f f e c t o f C o u p l i n g o f M o o r i n g L i n e s a n d R i s e rs o n t h e D e s i g n V a l u e s f o r a T u r r e t M o o r e d

F P S O i n D e e p W a t e r o f t h e G u l f o f M e x i c o

J .E . W . W i c h e r s

M A R I N U S A I nc ., H o u s to n , T X , U S A

P. V. Dev l i n

T e x a c o I n c ., H o u s t o n , T X , U S A

A B S T R A C T

I n t h e f r a m e w o r k o f t h e D e e p S t a r C T R 4 4 0 1 A T h e m e s tr u c t u re s

c o u p l e d c o m p u t a t i o n s w e r e c a r d e d o u t o n a t u r r et m o o r e d t a n k e r b a s e d

F P S O i n 3 , 0 0 0 f t , 6 0 0 0 f t a n d 1 0 , 0 0 0 f i w a t e r d e p t h f o r t h e G u l f o f

M e x i c o . T h e F P S O w a s e x p o s e d t o b o t h H u r r i c a n e a n d L o o p - c u r r e n t

c o n d i ti o n s . T h e s i z e o f t h e F P S O c o r r e s p o n d s t o a 2 0 0 k D W T t a n k e r

m o o r e d b y m e a n s o f 1 2 l i n e s . F o r t h e p r o d u c t i o n , w a t e r i n j e c t i o n , o i l

a n d g a s t r a n s p o r t , t h e t u r r e t w a s p r o v i d e d o f a 1 3 S C R s y s t e m . T h e

m o o r i n g s y s t e m i n 3 , 0 0 0 f t e x i s t s o f a c h a i n - j a c k e t e d s p i r a l s t r a n d -

g r o u n d c h a i n , w h i l e t h e m o o r i n g s y s t e m i n 6 0 0 0 f t w a s b o t h c h a i n -

p o l y e s t e r - g r o u n d c h a i n a n d c h a i n - s p r i n g b u o y s - j a c k e t e d s p i r a l s t r a n d -

g r o u n d c h a i n a n d f o r 1 0 , 0 0 0 f t w a t e r d e p t h i t c o n s i s t s o f a c h a i n -

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

C o n s i d e r i n g t h e d e s i g n v a l u e s t h e q u e s ti o n m a y a r i s e o n t h e v a l u e

o f t h e c o u p l e d a n a l y s i s a n d t h e e f f e c t s o f t h e C d - v a l u e s o n t h e

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

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

d e v e l o p e d .

U s i n g t h e D e e p S t a r F P S O s y s t e m a n e w s e t o f c o m p u t a t i o n s i n t h e

t i m e - d o m a i n h a s b e e n c a r r i e d o u t . B y c h a n g i n g s y s t e m a t i c a l l y t h e C d -

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

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

d i s t i n g u i s h e d . T h e c o m p u t a t i o n s w e r e a p p l i e d f o r t h e s y s t e m i n 3 , 0 0 0 f t

w a t e r d e p t h e x p o s e d t o t h e H u r r i c a n e a n d L o o p - c u r r e n t c o n d i t i o n a n d

i n 1 0 , 0 0 0 f t w a t e r d e p t h d u r i n g H u r r i c a n e c o n d i t i o n . T h e r e s u l t s s h o w

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

r e a l i s ti c d e s i g n v a l u e s .

K E Y W O R D S : C o u p l e d , F P S O , m o o r i n g fo r c e s, r i se r t e n s io n , g lo b a l

m o t i o n s , d e e p w a t e r

N O M E N C L A T U R E

C an : d r a g c o e f f i c i e n t n o r m a l

C d t : d r a g c o e f f i c i e n t t a n g e n t i a l

C ~n : a d d e d i n e r t i a c o e f f i c i e n t n o rm a l

C it : a d d e d i n e r t i a c o e f f i c i e n t t a n g e n t i a l

C ~ : c o u l o m b f r i c ti o n s e a b e d n o r m a l

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

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

A t u r r e t m o o r e d F P S O s y s t e m g e n e r a l l y c o n s i s t s o f a t a n k e

p r o c e s s e q u i p m e n t a n d a s u p e r s t r u c t u r e ) , a m o o r i n g s y s t e m a n d

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

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

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

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

F P S O t h e c o m p l e t e s y s t e m h a s t o b e t a k e n i n t o a c c o u n t. T h i s i s

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

s t u d i e s h a s b e e n p e r f o r m e d i n t h e p a s t ( e . g . W i c h e r s a n d H u i

1 9 9 0 , v a n d e n B o o m , 1 9 8 5 , W i c h e r s a n d D e r c k s e n , 1 9 9 4 ) .

T h e f o r c e s a c t i n g o n t h e s y s t e m a r e :

f i r s t o r d e r w a v e f o r c e s a n d h y d r o d y n a m i c r e a c t i o n f o r c

o r w / o c u r r e n t o n t h e h u l l

s e c o n d o r d e r w a v e d r i f t f o r c e s w i t h o r w / o c u r r e n t o n t h

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

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

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

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

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

s o i l f r i c ti o n r e a c t i o n f o r c e s o n t h e m o o r i n g l i n e s a n d r i

a e r o d y n a m i c e x c i t i n g a n d r e a c t i o n f o r c e s ; f o r t h e e x

e . g . a c o n s t a n t 1 - h o u r m e a n w i n d f o r c e a n d w i n d s

f o r c e s a c t i n g o n t h e a b o v e w a t e r p a r t o f t h e F P S O .

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

a s s o c i a t e d s e c t i o n s .

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

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

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

F P S O s y s t e m i n 3 , 0 0 0 a n d 1 0 , 0 0 0 f t w a t e r d e p t h e x p o s e d

H u r r i c a n e w e a t h e r c o n d i t i o n a n d i n 3 , 0 0 0 f t e x p o s e d t o t h e

c u r r e n t w e a t h e r c o n d i t i o n . F o r t h i s p u r p o s e t h e r e s i s t a n c e c o e

C d o f t h e r i s e r s a n d t h e s t e e l s p i r a l s t r a n d ( 3 , 0 0 0 f t ) a n d p o l y e s t

( 1 0 , 0 0 0 f t) w e r e v a r i e d . T h e v a r i a t i o n c o n c e r n s t h e C d = 0 , C d

C d = 2 o f t h e m o o r i n g l i n e s a n d r i s e r s . F i n a l l y , t o d e m o n s t r a t e

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

v a r i e d C d - v a l u e s i n L o o p - c u r r e n t o n l y .

4 8


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