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Reliability Engineering and System Safety 20 (1988) 127-146
Assessment of Production Regularity for Subsea Oil /Gas
Production Systems
P . H o k s t a d
Division of Safety and Reliability, SINTE F, N-7034 Trondheim, Norway
(Received 11 July 1987)
A B S T R A C T
A mo del f o r subsea o i l/ gas product ion sys t em s , be ing use fu l f o r carry ing ou t
product ion regu lar i ty s tud ies , i s p resen ted . Form ulas f o r ca l cu la t ing
app roxim ate as ym pto t ic) mea n values o f interest ing reliabil ity param eters
wi th in this mode l are derived. The use o f these form ula s i s exem pl i f ied an d
d i scussed fo r a ra ther s imp le exam ple . I t i s a rgued tha t t he approx imate
form ula s sho uld be used in an in teract ion wi th M on te Carlo s imulat ion, g iving
a pow erful approach to the prob lem o f assessing the availabil ity produ ct ion
regu lar i t y ) o f a subsea product ion sys t em.
1 I N T R O D U C T I O N
B y p l a n n i n g o f s u b s e a p r o d u c t i o n s y s t em s f o r o i l/ ga s , t h e re w ill b e a m a i n
o b j e c t i v e t o p r e d i c t t h e o p e r a t i o n a l f e a t u r e s o f a g i v e n c o n c e p t . I n p a r t i c u l a r ,
i t w i l l b e e s s e n t i a l t o e v a l u a t e / c o m p a r e v a r i o u s
- - l a y o u t s
- - m a i n t e n a n c e a n d i n t e r v e n t i o n s t r a t e g i e s
i n o r d e r t o f i n d .th e e c o n o m i c a l l y o p t i m a l s o l u t i o n ( a m o n g t h o s e s a t i s f y i n g
t h e s a f e t y r e q u i r e m e n t s ) .
F o r a s u b s e a o i l / g a s p r o d u c t i o n s y s t e m , i t w i ll b e o f m a j o r i n t e r e s t t o
A version of this paper was presented at Reliability 87, 14-16 A pril 1987, Birmingham , U K,
and is reproduced by k ind perm ission of the organisers.
127
Reliability Engineering and System Safety 0951-8320/88/$03 50 © Elsevier Ap plied Science
Publishers Ltd, England, 1988. Printed in G reat Britain
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128
P Hokstad
p r e d i c t th e p r o d u c t i o n r e g u la r it y . V a r i o u s a p p r o a c h e s m a y b e u s e d t o a ss e ss
t h i s p a r a m e t e r , a s
- - a s y m p t o t i c ( a p p r o x i m a t e ) a r g u m e n t s
- - M a r k o v t h e o r y
-- ~ M o n t e C a r l o ' s i m u l a t i o n
I n t h e p r e s e n t p a p e r a r a t h e r s i m p l e f a il u r e m o d e l f o r o i l/ g a s p r o d u c t i o n
s y s t e m s is p r e s e n t e d . A s et o f a s y m p t o t i c f o r m u l a s f o r a p p r o x i m a t e
c a l c u l a t i o n o f p r o d u c t i o n r e g u l a r i t y is d e r i v e d . T h e s e p r o v i d e a n a l t e r n a t iv e
t o th e ra t h e r t i m e c o n s u m i n g M o n t e C a r l o s i m u l a t i o n a p p r o a c h , a n d a
c o m b i n e d u s e o f th e s e t e c h n i q u e s i s d i s c u ss e d . I t is a r g u e d t h a t a
c o m p u t e r i z e d t o o l , a l l o w i n g t h e u se o f v a r i o u s a p p r o a c h e s , ( b o t h a n a l y t ic a l
a n d s i m u l a t i o n t e c h n iq u e s ) , w ill r e p r e s e n t a v a l u a b l e t o o l i n th e p r o c e s s o f
f i n d i n g a n 'o p t im a l s u b s e a c o n c e p t .
A d i s c u s s io n o f t h e a s y m p t o t i c f o r m u l a s i s t h e m a j o r t o p i c o f t h is p a p e r .
T h e p r e c is e c o n d i t i o n s f o r t h e v a l id i ty o f t h e f o r m u l a s w i ll b e gi v en , a n d a n
e v a l u a t i o n o f t h e d e g r e e o f a c c u r a c y is i n c l u d e d . T h i s e v a l u a t i o n is c a r r i e d
o u t b y p e r f o r m i n g n u m e r i c a l c o m p a r i s o n s w i t h r e su l ts o f t he s i m u l a t i o n
p r o g r a m ' S U B S I M ' , d e v e lo p e d at S I N T E F (se e H o k s t a d l) .
E x p l i c i t f o r m u l a s , a s w i l l b e p r e s e n t e d h e r e , a r e o f c o u r s e h e l p f u l ( e ve n
w h e n t h e y a r e o n l y a p p r o x i m a t e l y v a l i d ) :
- - -T h e y g i v e a p p r o x i m a t e m e a n v a l u e s fo r i n t e r e s ti n g r e li a b il i t y
p a r a m e t e r s , a n d t h e se a r e u n d e r c e r t a in a s s u m p t i o n s a ls o e xa c t.
F u r t h e r , e v e n w h e n t h e g i v e n a s s u m p t i o n s a r e n o t s a t i s f i e d , t h e
f o r m u l a s w i l l u s u a l l y g i v e l o w e r ( o r u p p e r ) ounds f o r t h e a c t u a l
p a r a m e t e r s .
- - T h e orderh~g b y p r o d u c t i o n r e g u l ar i ty ) o f d if f e re n t a l te r n a t i v e s ( as t w o
l a y o u t s ) w i l l t y p i c a l l y b e t h e s a m e , w h e t h e r t h e a p p r o x i m a t e o r t h e
e x a c t v a l u e s a r e u s e d . T h i s m a k e s i t e a s y t o p i c k o u t t h o s e o p t i o n s t h a t
s h o u l d b e s u b j e c t t o f u r t h e r s t u d y ( f o r i n s t a n c e b y s i m u l a t i o n ) .
- - T h e f o r m u l a s m a k e it e a s y t o m a k e a
quick
e v a l ua t io n a n d c o m p a r i s o n
o f v a r i o u s a l t e rn a t iv e s . T h i s l ea v e s m o r e t i m e t o p e r f o r m a r e a ll y
d e t a i le d s t u d y o f th o s e c o n c e p t s t h a t a r e in t h is w a y s o r t e d o u t t o b e t h e
m o s t p r o m i s i n g .
- - T h e f o r m u l a s r e p r e s e n t a n i n v a l u a b l e t o o l i n t e s t i n g o u t ( v e r i f y i n g )
s i m u l a t io n p r o g r a m s . F o r n e w c o m p u t e r co d e s , t h e re s h o u l d b e a ' m u s t '
t o t e s t t he r e su l t s aga i ns t e xac t r e su l t s , a t l eas t fo r so m e l i m i t i ng cases .
T h o s e f a c t o r s ( p a r a m e t e r s ) w i t h in t h e m o d e l , h a v i n g t h e h i g h e s t i n fl u e n c e o n
t h e a v a i l a b i li t y ( p r o d u c t i o n r e g u l a r i ty ) o f th e s y s t e m w i ll b e i d e n t if i e d .
F u r t h e r , a s a c o n s e q u e n c e o f th e d i s c u s s io n s , c o n s i d e r a b l e i n s ig h t is a c h i e v e d
a s to w h e n t h e q u i c k a n d a p p r o x i m a t e a p p r o a c h c o u l d b e u se d . S i m i l ar l y i t
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Subsea oi l gas product ion sys tem s 129
w i l l b e q u i t e e a s y t o d e c i d e a t w h i c h p o i n t i n a n a v a i l a b i l i t y s t u d y
s i m u l a t i o n s s h o u l d p r e f e r a b l y b e c a r r i e d o u t , i n o r d e r t o a r r i v e a t v a l i d
c o n c l u s io n s . O b s e r v e t h a t a c o m p a r i s o n o f a M o n t e C a r l o s i m u l a t i o n
m e t h o d a n d a n a n a l y t ic a l m e t h o d h a s r e ce n t ly b ee n p e r f o r m e d a l s o b y
W i n d e b a n k . 2
T h r o u g h o u t t h e p r e s e n t p a p e r , a r a t h e r s i m p l e v er s io n o f th e m o d e l is
c o n s i d e r e d . H o w e v e r , t h e a p p r o a c h a n d t h e a p p r o x i m a t e f o r m u l a s
p r e s e n t e d h e r e c o u l d e a s il y b e e x t e n d e d t o c o v e r f a r m o r e g e n e r a l m o d e l s , se e
S e c t i o n 7 . I n f a c t s u c h g e n e r a l i z a t i o n s h a v e a l r e a d y b e e n c a r r i e d o u t , a n d w i l l
be r ep o r t ed l a t e r , s ee Hu lba~kda l . 3
2 T H E M O D E L F O R A S U B S E A P R O D U C T I O N S Y S T E M
T h e a p p r o x i m a t e a v a i la b i li ty f o r m u l a s w i l l b e o b t a i n e d f o r a q u i te s i m p l e
m o d e l f o r r e l i a b i l i t y i n v e s t i g a t i o n s o f a s u b s e a o i l / g a s p r o d u c t i o n s y s t e m .
T h r o u g h o u t i t w i ll b e a s s u m e d t h a t t h e s y s te m i s b u i lt u p o f v a r io u s
' m o d u l e s ' . T h e s e w i l l b e g r o u p e d i n a h i e r a r c h i c a l w a y , a c c o r d i n g t o t h e i r
c o n s e q u e n c e o n t h e s y s t e m , w h e n t h e y f a il . T h e m o d e l is s i m i l a r to t h e o n e
u s e d i n t h e s i m u l a t i o n p r o g r a m , S U B S I M ( re f. 1), a n d is b a s e d o n t h e
fo l lowing as sumpt ions ( s ee F ig . 1 ) .
C e n t r a l s t a t i o n C S )
C S m o d u l e , c r i t i c a l t o a l l p r o d u c t i o n )
P r o d u c t i o n
s t a t i o n s P S )
P S m o d u l e
c r i t i c a l t o a l l
p r o d u c t i o n o n template
W e l l s
~ W e l l module,
c r i t ic a l f o r t h e p r o d u c t i o n
of a well only)
Fig . 1 . M odel for t he phys i ca l l ayout .
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130
P ttokstad
M odeling the physical layout
- - O n e g r o u p o f c o m p o n e n t s ( m o d u l e s ) a r e cr it ic a l f o r t h e w h o l e o il /g a s
f ie ld (i f o n e o f th e s e f a il t h e r e c a n b e n o p r o d u c t i o n ) . T h e s e c o m p o n e n t s
m a k e u p t h e
Central Station CS).
- - T h e r e c a n b e a n a r b i t r a ry n u m b e r o f
Production Stations PS)
l i n k e d t o
t h e C S . T h u s a P S i s a ' c o l l e c t i o n o f w e l ls ', a n d w o u l d u s u a l l y b e a
t e m p l a t e .
- - F o r e a c h P S t h e re is o n e g r o u p o f c o m p o n e n t s t h a t a r e c ri ti c al f o r t h e
w h o l e P S ( i f o n e o f th e s e f ai l t h e r e c a n b e n o p r o d u c t i o n o n t h is P S ).
T h e s e c o m m o n m o d u l e s a r e i n t h e p r e s e n t m o d e l a s s u m e d t o b e
i d e n t i c a l f o r d i f f e r e n t P S .
- - T h e r e c a n b e a n a r b i t r a r y n u m b e r o f w e ll s t ie d u p t o e ac h P S ( a n d th is
n u m b e r c a n b e d i f f e r e n t f o r t h e v a r i o u s P S ) .
- - E a c h w e l l h a s s o m e m o d u l e s t h a t a r e c r i t i c a l f o r t h a t p a r t i c u l a r w e l l
o n l y . A l l w e l l s h a v e i d e n t i c a l m o d u l e s .
M odeling the m aintenance strategy
T w o t y p e s o f r e p a i r s a r e p o s s ib l e, h e r e r e f e r r e d t o a s r e p a i r b y r i g a n d
r e p a i r b y D P S ( d y n a m i c a l l y p o s i t i o n e d s h ip ) r es p e ct iv e ly . T h u s t w o
d i f fe r e n t m o b i l i z a t i o n t i m e s m a y b e a s s ig n e d t o t h e v a r i o u s m o d u l e s .
- - A r i g / D P S i s m o b i l i z e d o n t h e f ir s t o c c u r r e n c e o f a 'c r i t ic a l fa i lu r e (i.e. a
f a i lu r e th a t c a u s e s t h e s h u t d o w n o f p r o d u c t i o n i n a t l e as t o n e w ell .
- - D u r i n g t h e re p a i r o f a C S m o d u l e , n o p r o d u c t i o n is p o s si b le a t t h e f ie ld .
- - D u r i n g t h e r e p a i r o f a n y o t h e r t y p e o f m o d u l e , all p r o d u c t i o n is s h u t
d o w n o n t h e P S b e i n g a f f e c t e d .
3 T H E A S Y M P T O T I C F O R M U L A S
T h e p r o d u c t io n
availability
o f a w e ll is ty p i c a l l y d e f i n e d a s t h e p r o p o r t i o n o f
t i m e t h a t t h e w e ll is p r o d u c i n g . I n t h i s p a p e r t h r e e m e a s u r e s o f a v a i la b i l it y
a r e i n t r o d u c e d , c o r r e s p o n d i n g t o t h e t h r e e l ev e ls i n t h e l a y o u t o f t h e f ie ld
( F ig . 1 ) . T he se a r e de f ine d a s f o l l ow s :
A w = W e l l a v ai l a b il i ty . ( A v e r a g e p r o p o r t i o n o f ti m e t h a t a n a r b i t r a r i l y
c h o s e n w e l l i s p r o d u c i n g . )
A ps = P r o d u c t i o n s t a ti o n a v a il a bi li ty . ( A v e r a g e p r o p o r t i o n o f ti m e w h e n
a t l e a s t s o m e p r o d u c t i o n i s g o i n g o n a t a n a r b i t r a r y P S . )
A c s = C e n t r a l s t a t i o n a v a i la b i l it y . ( P r o p o r t i o n o f t i m e w i t h a t le a s t s o m e
p r o d u c t i o n i n t h e f i e l d . )
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Subsea oil gas produ ction systems 131
T h e P S a n d w e l l a v a i la b i l it i es w i ll d e p e n d o n t h e n u m b e r o f w e l ls a t t h e
a c t u a l P S. T h u s t h e f o l lo w i n g p a r a m e t e r s a r e al s o in t r o d u c e d :
A w ( n ) = A v a i l a b i l i t y o f a w e l l t h a t i s l i n k e d t o a P S , h a v i n g n w e l l s in
to t a l .
A ps(n ) - - A va i l a b i l i t y o f a P S , i n t o t a l ha v in g n w e l ls .
T h u s A w a n d A ps w i ll b e o b t a i n e d b y a n a p p r o p r i a t e w e i g h t i n g o f t h e v a r i o u s
A w ( n ) va lu e s a n d A ps(n ) va lu e s r e spe c t ive ly .
Note
T h e
production regularity
w o u l d u s u a l l y b e d e f i n e d a s t h e a c t u a l
p r o d u c t i o n d i v i d e d b y t h e m a x i m u m p o s s i b l e p r o d u c t i o n ( i n t h e r e l e v a n t
p e r i o d o f ti m e ). In t h i s p a p e r it w il l f o r s i m p l i c i ty b e a s s u m e d t h a t t h e a c t u a l
p r o d u c t i o n o f t h e fi el d is p r o p o r t i o n a l t o t h e n u m b e r o f w e ll s p r o d u c i n g . I n
t h a t c a s e t h e p r o d u c t i o n r e g u l a r i t y w i ll b e i d e n t ic a l t o t h e w e ll a v a i la b i l it y , a s
d e f i n e d a b o v e . I f t h is a s s u m p t i o n is n o t s a ti sf ie d , t h e p r o d u c t i o n r e g u l a r i t y
c a n n o t b e a s s e s s e d , u n l e s s t h e a c t u a l p r o d u c t i o n a t a n y t i m e i n s t a n t i s
s p ec if ie d as a f u n c t i o n o f th e n u m b e r o f p r o d u c i n g w e lls .
O b s e r v e t h a t a l l p a r a m e t e r s e n t e r i n g t h e f o r m u l a s s h o u l d b e p r e s e n t e d
h a v i n g t h e s a m e d i m e n s i o n , a n d t h r o u g h o u t t hi s p a p e r a ll t i m e u n it s a r e in
y e a r s. T h e f o l l o w i n g p a r a m e t e r s a r e n e e d e d in o r d e r t o a r ri v e a t t h e d e s i re d
f o r m u l a s :
W i = M e a n w a i t i n g t i m e ( i.e. t h e a v e r a g e r i g / D P S m o b i l i z a t i o n t im e ) f o r
a ' t yp i c a l ' r e p a i r a t t h e CS , a P S a n d a w e l l r e sp e c t iv e ly ( i = 1 , 2 , 3 ).
T h u s W i is t h e a v e r a g e f o r b o t h t y p e s o f m o b i l i z a t io n s , t a k i n g t h e i r
f r e q u e n c i e s i n t o c o n s i d e r a t i o n .
R~ = M e a n r e p a i r t i m e ( th e t i m e f o r r e t r i e v a l o f a d e f e c t m o d u l e ) a t t h e
CS , a P S a n d a w e l l r e spe c t iv e ly ( i = 1, 2 , 3 ). T hu s R~ i s t he a ve r a g e
r e p a i r t i m e f o r t h e m o d u l e s o f t h e C S , P S o r w e ll .
2~ = T o t a l f a i l u r e r a t e o f t he C S , a P S a nd a w e l l r e sp e c t iv e ly ( i = 1, 2 , 3 ).
( M e a n n o . o f f a i lu r e s p e r y e a r .)
s = N u m b e r o f PS .
nk = N u m b e r o f w e l l s a t P S n o . k , k = 1, 2 . . . , s .
s
N = y , nk = T o t a l n u m b e r o f w e ll s.
k = l
N o w w e c a n p r o c e e d t o d e r i v e t h e a s y m p t o t i c f o r m u l a s . W i t h a n
' a s y m p t o t i c ' a r g u m e n t , w e h e r e m e a n t h a t a n ' in f in i te l y' l o n g t i m e p e r i o d w ill
b e c o n s i d e r e d . I n t h a t c a s e t h e p r o p o r t i o n o f ti m e t h a t t h e C S i s i n a f a il e d
s t a t e e q u a l s
1 - - A c s - - 21 . A c s . ( W 1 + R1)
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132
P ttokstad
T h e a r g u m e n t l e a d i n g t o t h i s r e s u l t i s b a s e d o n t h e o b s e r v a t i o n t h a t
1. F a i l u r e s o c c u r o n l y w h e n t h e C S is o p e r a t i n g , (p r o b a b i l i t y Acs ).
2. T h e n u m b e r o f f a il u r es p e r t i m e u n it t h e n e q u a l s 2 1 .A c s .
3. T h e a v e r a g e s h u t d o w n t i m e p e r f a i l u r e e q u a l s W t + R 1.
F r o m t h e a b o v e e q u a t i o n i t d i r e c t l y f o l l o w s t h a t
A c s = [ 1 + ) . 1 (W 1 + R 1 ) ] -
w h i c h i s a w e l l - k n o w n r e su lt . S im i l a r a r g u m e n t s f o r t h e s h u t d o w n t i m e o f P S
a n d w e l l s r e s p e c t i v e l y ( r e s t r i c t i n g n o w t o a P S h a v i n g n w e l ls ), g i v e
A cs - Aps(n) = 2 2 . A p s n ) . W 2 + R2) + n . 2 3 . A w n ) . R 3
A p s ( n ) - A w ( n ) - )-3 . A w ( n) . W 3
N o t e t h a t i n t h e s i m p l e m o d e l a p p l i e d h e r e , a ll p r o d u c t i o n is s h u t d o w n o n
t h e w h o l e a f f e ct e d te m p l a t e , (P S ), d u r i n g t h e r e p a i r o f t h e P S o r a n y w e l l
u n d e r t h a t P S . T h i s e x p l a i n s w h y R 2 a n d R 3 a p p e a r i n t h e f ir st o f t h e s e t w o
e q u a t i o n s o n l y . N o w s o l v i n g t h e s e t w o l a s t e q u a t i o n s , i t f o l l o w s t h a t
Aw (n) = Ac s. {1 + 2 2 .( W 2 71- R2) + 23.( W3 + n. R3) } -1
Aps(n ) = (1 + 23. W 3 ) . A w n )
I n t h e s e t w o f o r m u l a s i t h a s b e e n c h o s e n t o o m i t t h e t e r m c o n t a i n i n g 2 2.2 3
( w h i c h i s u s u a l l y v e r y s m a l l ) .
N e x t , t o s i m p l i f y t h e f o r m u l a s , w e i n t r o d u c e
B~ = 21 . ( W 1 + R1) ( l a )
/32 = 22 ( W 2 ~-
R2)
( l b )
B3(n) = )~3.(W3 + n . R 3 ) ( l c )
( T h e i n t e r p r e t a t i o n o f t h e s e e x p r e s s i o n s is r a t h e r s t r a i g h t f o r w a r d : t h e
a v e r a g e s h u t d o w n t i m e o f a w e ll c a u s e d b y C S , P S a n d w e ll fa i l u re s
r e s p e c t i v e l y ) . I t f o l l o w s t h a t
Aw (n) = [1
+ B1]
1.[1 + B e + B3 (n)} -~ ~ [1 + B~ + B 2 + B3(n)] -~ (2)
I n g e n e r a l A w ( n) is t h e a v a i l a b i l i t y f o r a w e l l, l o c a t e d a t a P S h a v i n g n w e l ls in
t o t a l . F u r t h e r , t h e s P S i n t h e f ie l d h a v e n l , n 2 . . . . . n~ w e l l s r e s p e c ti v e l y , a n d
N = ~ nk is t h e t o t a l n u m b e r o f w e l ls f o r t h e p r o d u c t i o n s y s t e m . T h e a v e r a g e
a v a i l a b i l it y o f a n a r b i t r a r y w e l l t h e n e q u a l s
2
w = n k / N ) . A w n k ) (3)
k = l
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Subsea oi l gas product ion sys tem s 133
F u r t h e r , t h e a v a i l a b i li t y o f a ' t y p ic a l ' P S e q u a l s
2 2
ps = s - 1 A ps (n k ) [1 + B3(0)] s - 1 Aw(nk)
k = l k = l
F i n a l l y
4 )
Ac s = [-1 + B 1 ]- 1 (5)
T h e s e a v a i l a b i l it y m e a s u r e s , g i v e n b y ( 3) , (4 ) a n d ( 5 ), a r e r a t h e r f u n d a m e n t a l ,
a n d s e v e r a l o t h e r r e l i a b i l i t y p a r a m e t e r s e a s i l y f o l l o w f r o m t h e s e ( s e e
A p p e n d i x 2 ). A d i s c u s s i o n c o n c e r n i n g t h e v a li d i ty o f t h e a b o v e f o r m u l a s w i ll
b e g iv e n in t h e n e x t S e c t io n . H o w e v e r , t h e f o r m u l a s d i r e c tl y i n f o r m t h a t t h e
r e a ll y f u n d a m e n t a l f a c t o r s t h a t i n f l u e n c e t h e a v a i l a b il i ty a r e t h e f o l lo w i n g :
- - T h e p r o d u c t o f t h e t o ta l f a i lu r e r a te a n d t h e m e a n m o b i l i z a ti o n t im e (i.e.
2 i . W i ) f o r t h e m o d u l e s o f
* well ,
* P S , a n d
* C S
respec t i ve l y .
- - T h e p r o d u c t o f th e t o t a l f a i lu r e r a te a n d t h e m e a n r e p a i r t i m e (2 i. R i) f o r
* well ,
* P S , a n d
*
C S
respec t i ve l y .
- - T h e t o ta l n u m b e r o f w e lls ( N ) a n d t h e w a y t h e se a r e a ll o c a t e d to v a r i o u s
P S . ( T h u s t h e n u m b e r o f P S ( t h a t is s) o f c o u r s e a l s o p l a y s a n i m p o r t a n t
role.)
T h i s l is t s h o u l d b e n o s u r p ri s e. H o w e v e r , i t s h o u l d b e s t re s s e d t h a t t h e a b o v e
lis t s h o u l d b e c o n s i d e r e d to b e t h e m o s t f u n d a m e n t a l f a c t o r t o b e t a k e n i n t o
c o n s i d e r a t i o n f o r a f ir st r o u g h r e li a b il it y e v a l u a t i o n o f th e c o n c e p t . ( O b s e r v e
t h a t t h e r e i s a t l e a s t o n e o t h e r c a n d i d a t e t o t h i s li st , t h a t is p = a v e r a g e
p r o p o r t i o n o f t i m e th a t t h e w h o l e P S is s h u t d u r i n g t h e r e p a i r o f a w e ll
m o d u l e . In t h e p r e v i o u s d i s c u s s i o n i t is a s s u m e d t h a t p = 1 .0 . O t h e r w i s e t h e p
v a l u e w o u l d e n t e r t h e f o r m u l a s . )
4 D I S C U S S I O N O F T H E A S Y M P T O T I C F O R M U L A S
T h e a s s u m p t i o n s l e a d i n g t o t h e f o r m u l a s ( 3 )- (5 ) w ill n o w b e d i s c us s e d . T h i s
d i s c u s s i o n t o g e t h e r w i t h t h e n u m e r i c a l i n v e s t i g a t i o n s o f t h e n e x t S e c t i o n w il l
r a t h e r c l e a rl y e x p l a i n w h e n t h e s e f o r m u l a s s h o u l d ( o r s h o u l d n o t ) b e u se d .
T h e r e a r e t w o m a i n f a c t o r s r e s t ri c t in g t h e a p p l i c a b i li t y o f t h e s e f o r m u l a s .
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P Hokstad
Stationarity
For deriving (3)-(5) the system is assumed to be in a stationary (not time
dependent) state. This means that all variations during time should be
random, and thus that there is no systematic 'trend'. However, in an actual
situation, the system should probably start with no failures at time 0
(availability = I'0), and it will take some time before the availability
decreases to the asymptot ic value that is provided by the formulas. Thus the
validity will depend both on how fast the system reaches stationarity, and on
the length of the life cycle of the field.
- -T he 'speed' of convergence (towards the stationary state o f the process)
will mainly depend on the failure distributions (the statistical
distributions of the time until failure for the various modules). If the
exponential distribution is used, stationarity will be reached rather
quickly. (But for the asymptotic formulas to be correct, there should in
fact be a certain probability for having failed modules also at time 0.)
Using a Weibull distribution with increasing failure rate instead, there
will usually be a longer time until failures start to occur and stationar ity
is reached. The Weibull model being referred to here assumes that the
failure rate of the time until first failure is of the form
~ 1 ) = ~ / ~ . / ~ I ) ~ - 1
Here ~ is the sh pe parameter, and/~ is the sc le parameter. For ~ = 1
this reduces to the exponential model.
- -The life time, T o of the field will be another important factor. For the
formulas to be accurate, the time period until the field reaches the
asympto tic state should be short compared with the life time of the field.
Thus for a field with a very short life time the asymptotic formulas
could actually give a rather inaccurate (too low) asympto tic availability
value.
'Multiple failures'
Occasionally two (or more) failures will occur close in time, and the repair
can be carried out during the same intervention. The effect of such events are
not accounted for in the formulas (3)-(5). Quite similarly the total repair time
for a 'multiple repair' on a template (PS) should typically be less than the sum
of the two repair times.
Thus, since actually some of the mobilization times and repair times will
be common for two or more failures, the availability obtained by the
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Subsea oi l gas product ion sys tem s
35
a p p r o x i m a t e f o r m u l a w ill b e t o o lo w . T h r e e f a c to r s a r e i m p o r t a n t f o r t hi s
p h e n o m e n o n :
F a i l u r e r a t e s . I f t h e f a i lu r e r a t e s a r e h i g h , t h e r e w i ll b e m a n y m u l t i p l e
e r ro r s , a n d t h e f o r m u l a s b e c o m e i n a c c u ra t e .
M o b i l i z a t i o n a n d r e p a i r t i m e s . L o n g m o b i l i z a t i o n / r e p a i r t i m e s w i ll a l s o
t e n d t o i n c r e a s e th e n u m b e r o f m u l t i p l e fa i lu r es .
N u m b e r o f we lls. A l s o i f t h e r e is a v e r y l a r g e n u m b e r o f u n i t s (w e lls),
t h e r e w i ll b e a t e n d e n c y t o e x p e r i e n c e m u l t i p l e e r ro r s.
O b s e r v e t h a t t h e i n f l u e n c e o f t h e f a c t o r s l i st e d a b o v e a ll im p l y t h a t t h e
a v a i l a b i l i t y f o r m u l a s r e p r e s e n t l o wer l im i t s o f th e a c t u a l v a l ue s . H o w e v e r ,
t h e r e i s o n e e x c e p t i o n : if t h e f a i lu r e t i m e d i s t r i b u t i o n h a s a d e c r e a s i n g f a i lu r e
r a t e , t h i s m i g h t in t h e i n i t i a l p h a s e o f t h e l if e c y c l e g i v e a n u m b e r o f f a i lu r e s
t h a t is h i g h e r t h a n t h e a s y m p t o t i c v a l u e ( a n d t h u s g i v e a n a v a i la b i li ty t h a t i s
a c t u a l l y l o w e r t h a n t h e a s y m p t o t i c v a l u e ) .
5 N U M E R I C A L E X A M P L E ' B A SE C A S E '
T h e u s e o f t h e f o r m u l a s d e r i v e d i n S e c t i o n 3 w i ll n o w b e i l lu s t r a t e d b y s o m e
n u m e r i c a l e x a m p l e s . A n e x p o n e n t i a l m o d e l w i l l f i r s t b e a s s u m e d f o r t h e
f a il u r e t im e s . F u r t h e r t h e f o l l o w i n g n u m e r i c v a l u e s a r e s p e ci fi e d a s t h e ' B a s e
C a s e ' o f t h e n u m e r i c a l i n v e s t i g a ti o n s :
- - T o = t o t a l l if e t i m e o f t h e f ie l d = 1 0 y e a r s .
- - T h e r e a r e t w o p r o d u c t i o n s t a t io n s w i t h t w o a n d f o u r w e ll s r e sp e c ti v el y
( th us s = 2 , n l = 2 , / /2 = 4).
- - T h e m e a n m o b i l i z a t i o n t i m e e q u a l s 9 0 d a y s f o r r i g , a n d 2 4 d a y s f o r
D P S .
- - T h e t o t a l f a i lu r e r a te s ( s u m m e d o v e r all m o d u l e s ) o f C S , P S a n d w e ll s
r e s p e c t i v e l y a re g i v e n i n T a b l e 1. I t w i ll b e s e e n t h a t a ll r e p a i r o f C S is
c a r r i e d o u t b y a r ig , g i v in g a m e a n w a i t i n g t im e o f 9 0 da y s . F o r P S
m o d u l e s a ll r e p a ir s a r e b y D P S , g i v i n g a w a i t in g t i m e o f 2 4 d a y s . F o r t h e
w e l l s t h e r e i s a s s u m e d t o b e o n e f a i l u r e r a te o f 0 .1 ( y r - 1 ) , r e q u i r i n g
r e p a i r b y r ig , a n d o n e f a i l u r e r a t e o f 0 .2 f o r f a il u r e s t h a t c a n b e h a n d l e d
b y D P S ( g i v in g a n a v e r a g e m e a n w a i t i n g t i m e o f ( 90 × 0-1 + 2 4 × 0 .2 ) /
(0 .1 + 0 . 2) = 4 6 d a y s ). S o t h i s v a l u e i s o b t a i n e d b y t a k i n g a n a v e r a g e o v e r
t h e v a r i o u s w e ll m o d u l e s . T a b l e A 1 i n A p p e n d i x 1 p r e s e n t s th e i n p u t
v a l u e s t o t h e a s y m p t o t i c f o r m u l a s , b a s e d o n t h e d a t a i n T a b l e 1 .
S o m e (2 00 ) s i m u l a t i o n s f o r t h e B a s e C a s e w e r e c a r r i e d o u t . I t is s e e n in T a b l e
2 t h a t t h e r e i s i n f a c t a v e r y g o o d a g r e e m e n t b e t w e e n t h e v a l u e s o b t a i n e d
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136 P. Hokstad
T A B L E I
Input Parameters to the Ba se C ase . Failure Rate
(yr 1). M obilization and Repair Tim es (days)
) 365. PP 365. R 365( W +
R)
c s 0.06 90 10 1oo
PS 0-20 24 6 30
Well 0.30 46 14 60
f r o m t h e s i m u l a t i o n s a n d t h e a p p r o x i m a t e f o r m u l a s . T h e s i m u l a t e d ( i . e .
' a c tu a l ' ) v a l u e s o f A w a n d A ps a r e a s e x p e c t e d s l i g h t ly h i g h e r t h a n t h o s e
o b t a i n e d f r o m (3 ) a n d (4 ). B u t i t is re a l iz e d t h a t s t a t i o n a r i t y m u s t b e r e a c h e d
v e r y f as t ( c o m p a r e d w i t h t h e l if e t i m e o f 1 0 y e a rs ) , a n d a l s o t h e t o t a l
f r e q u e n c y o f f a il u re s m u s t b e r a t h e r s m a ll , to g i v e a s m a ll n u m b e r o f m u l t i p le
r e pa i r s .
I t s h o u l d b e o b s e r v e d t h a t t h e s i m u l a t i o n s h a v e b e e n c a r r i ed o u t u n d e r t h e
f o l l o w i n g t w o s i m p l i f y i n g a s s u m p t i o n s c o n c e r n i n g t h e m a i n t e n a n c e
s t r a t e g y .
- - I n t h e ca s e o f a 'm u l t i p l e r e p ai r ' at a P S ( th a t is m o r e t h a n o n e m o d u l e
r e p a i r e d i n t h e s a m e i n t e r v e n t i o n ) , t h e t o t a l r e p a i r t i m e i s r e d u c e d t o
e q u a l t h e r e p a i r t i m e o f t h a t c o m p o n e n t r e q u i r in g t h e l o n g es t re p a ir .
- - T h e D P S is d e m o b i l i z e d w h e n e v e r a f a il u re o c c u r s t h a t r e q u i r e s th e u s e
o f a ri g. ( T h u s t h e r e a r e n e v e r t w o v e ss e ls m o b i l i z e d a t t h e s a m e t i m e .)
T h e f i rs t a s s u m p t i o n i m p l i e s t h a t t h e d i f f e re n c e b e t w e e n t h e r e s u l t o f t h e
f o r m u l a a n d o f t h e s i m u l a t i o n is ' m a x i m i z e d ' (a m o r e r e al i st ic r u le
c o n c e r n i n g m u l t i p l e r ep a i rs w o u l d g i v e t h a t t h e re s u lt s f o r t h e f o r m u l a a n d
t h e s i m u l a t i o n b e c o m e e v e n c l o s e r ) .
I t s h o u l d , h o w e v e r , b e s tr e s s e d t h a t f o r t h i s e x a m p l e t h e r e i s a r a t h e r l o w
n u m b e r o f m u l t ip l e r e pa ir s , a n d t h e a b o v e t w o a s s u m p t i o n s d o i n fa c t h a v e a
v e r y l i m i t e d i n f l u e n c e o n t h e s i m u l a t i o n r e su l ts .
T A B L E 2
Availability Results for the Base Case
Aw Avs
Form ula 0'900 0 938
Sim ulation 0.904 0.940
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Subsea o i l gas produc t ion sys tems 137
6 N U M E R I C A L I N V E S T IG A T I O N S . V A R I A T I O N S O F T H E
'B A S E C A S E '
The stationarity condition
T h e f a c t o r s t h a t i n f l u e n c e t h e s t a t i o n a r i t y c o n d i t i o n a r e i n v e s t i g a t e d f i r s t .
Ta b le 3 g ives s im u la t ion r e su l t s fo r l if e t imes , T o , o f 2 , 10 an d 50 yea r s , and
f a il u re t im e s h a v i n g a W e i b u l l d i s t r i b u t i o n w i t h t h e s h a p e p a r a m e t e r e q u a l
to 0 .75 , 1-0 , 1 25 , 1 .50 an d 1-75 respec t ive ly . (Al l o th er a ssu m pt ion s are as in
t h e B a s e C a s e , a n d t h e s c a l e p a r a m e t e r o f t h e W e i b u l l d i s t r i b u t i o n i s
a d j u s t e d t o g i v e t h e s a m e m e a n t i m e t o f a i l u re i n a ll ca se s .) T h e s e r e s u l ts a r e
g i v e n a g r a p h i c a l p r e s e n t a t i o n i n F i g . 2 . ( R e m e m b e r t h a t a l l r e s u lt s s h o u l d b e
c o m p a r e d w i t h t h e s a m e a p p r o x i m a t e v a l u e , 0 . 9 0 0 . ) T h e r e s u l t s a r e v e r y
m u c h a s s h o u l d b e e x p e c t e d . T h e a c c u r a c y o f t h e f o r m u l a i n c r e a s e s w i t h
i n c r e a s i n g l i f e t i m e . A l s o t h e w e l l a v a i l a b i l i t y i n c r e a s e s a s t h e s h a p e
p a r a m e t e r i n c re a s es , t h e f o r m u l a g i v i n g a m o s t a c c u r a t e r e su l t f o r th e s h a p e
p a r a m e t e r c l o s e t o 1 .0 . I n p a r t i c u l a r :
- - F o r f ie ld s w i t h q u i te lon l if e t i m e s ( a n d n o t t o o m a n y m u l t i p l e r e p ai rs ),
t h e a v a i l a b i l i t y f o r m u l a g i v es a v e r y a c c u r a t e r e s u l t ir r e s p e c t iv e o f o t h e r
p a r a m e t e r s n o t e n t e r i n g t h e f o r m u l a ( s u c h a s t h e s h a p e p a r a m e t e r ) .
- - T h e s h a p e p a r a m e t e r o f t h e fa i lu r e t i m e d is t r i b u t io n i s a p a r a m e t e r o f
c o n s i d e r a b l e i m p o r t a n c e f o r f i e l d s w i t h a
short
l i fe t ime. ( In fac t th is
i n fl u e nc e is s u rp r i si n g l y h ig h , a p h e n o m e n o n t h a t h a s p e r h a p s n o t b e e n
fu l ly rea l ized . )
S i n c e a l if e t i m e o f a t l e a s t 1 0 y e a r s is m o s t c o m m o n , a n d t h e s h a p e
p a r a m e t e r o f f a i lu r e t im e s ( f o r s u b s e a i n s t a l la t i o n s ) is u s u a l l y a s s u m e d t o l ie
i n t h e i n t e r v a l 1 . 0 - 1 . 2 ( t a k i n g m a i n t e n a n c e a n d r e s t r i c t e d l i f e t i m e i n t o
c o n s i d e r a ti o n ) , i t c a n b e c o n c l u d e d t h a t t h e ' s ta t i o n a r i t y ' a s s u m p t i o n s f o r
t h e f o r m u l a s a r e m o s t o f t e n s a t i s f i e d .
TABLE 3
Simulated Values for the Well Availability for
Varying Life Time of the Field and Shape
Parameter of the Failure Rates
0 75
1
00
1 25
1 5 0
1 75
2 10 50
0 848 0'882 0 897
0 906 0'904 0 904
0 940 0'920 0 906
0-960 0-927 0'907
0'975 0-932 0 908
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138
P Hokstad
>- 1
k-
98
r •
tI
_J .96
t~ 94
5_1 •
n,,
. 9 2
z
o 9
H-
U .88
7
8 6
0
o_ 84
Fig. 2.
2 Y E A R S
i Y E A R S
5 Y E A R S
. 5 t i . ' 2 s i 5 i . b s '
S H R P E P A R A M E T E R
Production regularity as a function of shape parameter of failure time.
' M u l t i p l e r e p a i r s '
Secondl y the effect of multiple repairs is investigated. First the failure rates
are varied. Since the accuracy is quite good for the Base Case, there is no
reaso n to co nsider cases with lower failure rates. Thus in total the following
four cases are investigated:
Case 1: Base Case.
Case 2: All failure rates are twice the values of the Base Case.
Case 3: All failure rates are three times the values of the Base Case.
Case 4: All failure rates are four times the values of the Base Case.
Table 4 and Fig. 3 give the well availabilities obta ined both from the f ormu la
and the simulations for these four cases. It is observed that the accuracy is
surprisingly 'resistant' to increase in the failure rates. Even for Case 4 (having
unrealistically high failure rates), the value ob tain ed from the fo rmul a is very
T A B L E 4
Well Availability as a Function of the Failure Rates
Case 1 Case 2 Case 3 Case 4
Formula 0.900 0 817 0-746 0 685
Simulation 0 904 0 828 0'761 0702
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P r o d u c t i o n r e g u l a r it y a s a f u n c t i o n o f t o t a l f a il u r e r a te .
a ccu ra te i n th i s exa m p l e . ( In crea s i n g W i a n d Ri w i l l h a ve a s im i l a r e ff ec t a s
i n crea s i n g 2 i a n d i s n o t i n ves t i g a ted h ere . )
T a b l e 5 a n d F i g . 4 i l lu s t ra t e t h e b e h a v i o u r f o r a n in c r e a s in g n u m b e r o f
w e l l s . F o u r c a s e s a r e s t u d i e d i n a d d i t i o n t o t h e B a s e C a s e , i n t o t a l h a v i n g
3 , 6 , 1 2 , 18 a n d 2 4 w e l l s r e sp ec t i ve l y. In a l l ca s e s th e w e l l s a re s p l it b e tw een
t w P S . T h e n u m b e r o f w e l l s at s ta t i o n 2 i s a l w a y s t w o t i m e s th e n u m b e r a t
s ta t i o n 1 . A l l o th er p a ra m eters a re a s s p ec i f i ed i n th e B a s e Ca s e .
A s s h o u l d b e e x p e c t e d , t h e a p p r o x i m a t e f o r m u l a g iv e s d i st i n c tl y to o l o w a
r e su l t as t h e n u m b e r o f w e l ls b e c o m e s l ar g er ( d u e to t h e i n c r e a se d n u m b e r o f
m u l t i p l e r ep a ir s) . B y e s t i m a t i n g t h e n u m b e r o f m u l t i p l e r e p ai rs i t s h o u l d b e
p o s s i b le b y m o r e r e f in e d a r g u m e n t s t o t a k e t h is f a c t o r a l s o i n t o a c c o u n t , a n d
th u s i n crea s e th e a ccu ra cy . T h i s w i l l , h o wever , n o t b e i n co rp o ra ted i n th e
p res en t d i s cu s s i o n , a n d i t i s co n c l u d ed th a t th e fo rm u l a i s ra th er i n a ccu ra te
h a v i n g a t o o l a r g e n u m b e r o f w e l l s .
T A B L E 5
W e l l A v a i la b i l it y a s a F u n c t i o n o f th e N u m b e r o f W e l l s
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Fig. 4.
P r o d u c t i o n r e g u la r it y a s a fu n c t i o n o f n u m b e r o f w e l ls .
T h e a l l o c a t i o n o f t h e w e l l s t o d i f f e r e n t P S
T h e t o t a l n u m b e r o f w e l ls a n d t h e w a y t h e s e a r e s p li t b e t w e e n t h e v a r i o u s P S
is o n e o f th e f u n d a m e n t a l f a c t o rs f o r th e p r o d u c t i o n r e gu l ar it y ( se e e n d o f
S e c t i o n 3 ) . T h e e ff ec t o f t h i s f a c t o r w i ll n o w b e i n v e s ti g a t ed s o m e w h a t
f u r th e r . T h e i n f l u e n c e o f t h is f a c t o r o n t h e a c c u r a c y o f t h e f o r m u l a w i ll a l s o
b e c o m m e n t e d o n .
T h e s i x w e l ls o f t h e B a s e C a s e c a n b e sp l it b e t w e e n t h e t w o P S i n t h re e
w a y s : 1 - 5 , 2 - 4 a n d 3 - 3 ( th e a l l o c a t io n 2 - 4 i s t h e B a s e C a s e) . T h e r es u lt s f o r
a l l t h r e e c a s e s a r e p r e s e n t e d i n T a b l e 6 , a n d i n d i c a t e a h i g h e r a v a i l a b i l i t y
w h e n t h e w e l ls a r e e q u a l l y d i v i d e d b e t w e e n t h e t w o P S . ( T h e r e w i l l be a l o w e r
a v a i l a b i li t y a t a P S w i t h a h i g h e r n u m b e r o f w e l l s , d u e t o f u l l s h u t in d u r i n g
w e l l r ep a i rs . T h u s f o r t h e c o n f i g u r a t i o n 1 - 5 t h e r e a r e f iv e w e l l s w i t h a ' lo w '
a v a i l a b i l i t y a n d o n l y 1 w i t h a h i g h e r a v a i l a b i li t y . )
T A B L E 6
A v a i l a b i l i t y f o r D i f f e r e n t A l l o c a t i o n s o f t h e S i x
W e l l s ( c o n n e c t e d t o t w o P S )
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S u b s e a o i l / g a s p r o d u c t i o n s y s t e m s
T A B L E 7
A v a i l a b i l i t y f o r D i f fe r e n t A l l o c a t i o n s o f t h e W e l ls . ( In t o t a l 2 0 W e l l s
d i v i d e d b e t w e e n t h e v a r i o u s P S )
141
N o . P S 2 2 2 3 4 5
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F o rm u la 0.780 0.823 0-841 0.870 0.885 0.894
Si m ula t io n 0.842 0-855 0.863 0.878 0.889 0 '895
T o g i ve a m o r e t h o r o u g h d i s c u s s i o n o f t h is a s p e c t, t h e a v a i l a b il it y is a ls o
c a l c u l a t e d f o r s e v e r a l c o n f i g u r a t i o n s , a l l h a v i n g 2 0 w e l l s i n t o t a l . A l l o t h e r
d a t a a r e s t il l a s in t h e B a s e C a s e . T h e r e s u l t s a r e p r e s e n t e d i n T a b l e 7 . F o r t h e
f ir st th r e e c a s e s th e r e a r e o n l y t w o P S , a n d a g a i n t h e r e i s a c l e a r t e n d e n c y
t o w a r d s i n c r e a s e d a v a i l a b i li ty w h e n t h e w e l ls a r e s p l it m o r e e q u a l l y b e t w e e n
t h e t w o P S . T h e a v e r a g e a v a i l a b i l it y o f t h e 2 0 w e l ls w i l l i n c r e a s e f r o m 0 . 84 2
( in t h e e x t r e m e c a s e w h e n o n e o f t h e P S h a s o n l y o n e w e ll) to 0 . 8 63 ( w h e n 1 0
w e l ls a r e a l l o c a t e d t o e a c h o f t h e t w o P S ).
T h r e e o t h e r a l t e r n a t i v e s a r e a l s o s t u d i e d , h a v i n g t h r e e , f o u r a n d f i v e P S
r e s p e c t i v e l y . S i n c e t h e w e l l a v a i l a b i l i t y i n t h e m o d e l d e c r e a s e s w i t h t h e
n u m b e r o f w e lls i n t h e P S , t h e a l te r n a t iv e 4 - 4 - 4 ~ ( h a v in g f iv e P S) o f
c o u r s e g i v es t h e ' b e s t ' r e s u lt . I n f a c t 2 0 P S w i t h 1 w e l l e a c h g i v e a v a i l a b i l i ty
0 9 2 3 ( f r o m t h e f o r m u l a , w h i c h i s p r o b a b l y v e r y c l o s e t o w h a t w o u l d b e
o b t a i n e d f r o m a s i m u l a t i o n ) .
O f c o u r s e n o d e c i s iv e c o n c l u s i o n s s h o u l d b e d r a w n f r o m t h e s e f ig u re s .
F i r s t o f a ll t h e r e a r e s e v e r al c o n s i d e r a t i o n s l e a d i n g t o t h e d e c i s i o n o n h o w
t h e w e l l s s h a l l b e s p li t b e t w e e n v a r i o u s P S , a v a i l a b i l it y b e i n g o n e f a c t o r . I t
s h o u l d a l s o be r e m e m b e r e d t h a t t h e a b o v e r e s ul ts a re b a s e d o n s o m e
a s s u m p t i o n s ( fo r in s t a n c e t h a t p r o d u c t i o n is s h u t d o w n a t l l wel l s on a P S
d u r i n g a n y r e p a i r o n t h a t t e m p l a t e ). N e v e r t h e l e s s , t h e s e f i gu r e s i n d i c a t e t h a t
t h i s ' w e ll a l l o c a t i o n ' i s a f a c t o r o f h i g h i m p o r t a n c e , a n d s h o u l d s e r v e a s a
r a t h e r g o o d i l l u s t r a t i o n o f t h e w e ll k n o w n f a c t t h a t a v a i l a b i l i ty ( re l ia b i li ty )
c o n s i d e r a t i o n s s h o u l d b e i n c l u d e d i n th e d e v e l o p i n g p l a n s o f a c o n c e p t f r o m
t h e v e r y b e g i n n i n g .
F i n al ly , a c o m m e n t o n h o w t h e a c c u r a c y o f th e f o r m u l a i s a ff ec te d by t h e
n u m b e r o f w e l ls is i n o rd e r . I t h a s b e e n o b s e r v e d t h a t t h e a c c u r a c y d e c r e a s e d
w i t h a n i n c r e a s i n g n u m b e r o f w e l ls ( T a b l e 5 a n d F i g . 4 ). H o w e v e r , T a b l e 7 is
a r a t h e r c l e a r i n d i c a t i o n t h a t i t is n o t t h e t o t a l n u m b e r o f w e l ls t h a t is a
c r it ic a l f a c t o r h e r e , b u t r a t h e r t h e n u m b e r o f w e ll s a t t h e i n d i v i d u a l P S . T h e
c o n f i g u r a t i o n 1 - 1 9 g iv e s a r a t h e r b a d r e s u l t f o r t h e c o m p a r i s o n , w h i l st t h e
c o n c e p t s w i t h 4 a n d 5 w e l ls a t e a c h t e m p l a t e g iv e v e ry c l os e r e s u lt s f o r t h e
f o r m u l a a n d t h e s im u l a t i o n . F u r t h e r i t sh o u l d b e n o t e d h e r e th a t t h e r a t h e r
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142
P Hokstad
h i g h a v a i l a b i l it y f i g u re s o f t h e s i m u l a t i o n s w h e n a P S w i t h m a n y w e ll s is
i n v o l v e d t o s o m e e x t e n t i s e x p l a i n e d b y t h e a s s u m p t i o n ( se e e n d o f S e c t i o n 5 ),
t h a t t h e r e p a i r t im e f o r a ' m u l t i p l e ' r e p a i r o n a t e m p l a t e e q u a l s t h e lo n g e s t
r e p a i r ti m e a m o n g t h e f ai le d m o d u l e s ( th u s n o t im e is a d d e d f o r t he m o d u l e s
h a v i n g s h o r t e r r e p a i r t im e ) . I n f a c t t h e r e s h o u l d b e a r e d u c t i o n i n r e p a i r t i m e
f o r m u l t i p l e r e p a i r s , b u t t h e a b o v e a s s u m p t i o n i s e x t r e m e a n d w i t h a m o r e
r e a l is t ic m o d e l f o r t h i s e v e n t , t h e r e w o u l d n o t b e s u c h a b i g d i f fe r e n c e i n t h e
f o r m u l a a n d t h e s i m u l a t i o n r e s u l t s .
7 C O N C L U S I O N S A N D T O P IC S F O R F U R T H E R W O R K
Co n c l u s i o n s o f n u m er i ca l i n ves t i g a t i o n s
A s a m a i n c o n c l u s i o n o f th e l a st S e c t io n , i t w a s f o u n d t h a t t h e f o r m u l a o f t h e
w e l l a v a i l a b i l i ty i n p r a c t i c a l s i t u a t i o n s s u r p r i s i n g l y o f t e n w i l l p r o v i d e r e s u l ts
v e r y c l o s e t o e x a c t . A g a i n i t s h o u l d a l s o b e s t r e s s e d t h a t e v e n i n c a s e s w h e r e
t h e f o r m u l a s a r e s o m e w h a t i n a c c u ra t e , t h e y m a y b e v e ry h e l p f u l f o r
c o m p a r i s o n p u r p o s e s . I f t w o l a y o u t s a r e to b e c o m p a r e d , t h e e x ac t a n d t h e
a p p r o x i m a t e a v a i l a b i l i t y v a l u e s w i l l t y p i c a l l y h a v e t h e s a m e o r d e r i n g . S o
e v e n i f t h e f o r m u l a d o e s n o t p r o v i d e t h e e x a c t r e s u lt f o r t h e w e l l a v a i la b i li ty ,
i t w i ll te ll w h i c h o f t h e t w o l a y o u t s t h a t h a v e t h e h i g h e s t ( lo w e s t ) a v a i l a b il i ty .
T h i s s t a t e m e n t w i ll o f c o u r s e n o t n e c e s s a ri ly b e v a l i d if t h e t w o m o d e l s
( l a y o u t s ) a ls o d i f f er i n a f a c t o r t h a t a f fe c ts t h e a c c u r a c y o f t h e f o r m u l a ( i f s a y
t h e n u m b e r o f w e l l s i s d i f f e r e n t ) .
I n a n a c t u a l s i t u a t i o n i t s e e m s t h a t t h e r e i s o n l y o n e f a c t o r t h a t c o u l d
r e a ll y ' d e s t r o y ' t h e v a l i d i ty o f t h e f o r m u l a s : i f t h e n u m b e r o f w e ll s a t o n e o r
m o r e P S b e c o m e s q u i t e h i g h , t h e d i s c r e p a n c y b e t w e e n t h e ' e x a c t ' a n d t h e
a p p r o x i m a t e v a l u e s c o u l d b e s u b s t a n t i a l (s ee T a b l e s 5 a n d 7). T h u s i f t w o
c o n c e p t s a r e t o b e c o m p a r e d , h a v i n g a d i ff e r en t a l l o c a t i o n o f w e lls , i t s h o u l d
b e s t r e s s e d t h a t t h e a p p r o x i m a t e f o r m u l a m i g h t g i v e a m a r k e d l y d i f f e r e n t
d e g r e e o f a c c u r a c y f o r t h e t w o c a s es , p o s s i b ly d e s t r o y i n g t h e c o m p a r i s o n .
F o r o t h e r f a c t o r s i t s e e m s t h a t r e a ll y 'u n t y p i c a l ' p a r a m e t e r v a l u e s m u s t b e
u s e d f o r t h e a p p r o x i m a t e a v a i l a b i l i t y v a l u e n o t t o a p p l y ( t o a r e a s o n a b l e
degree) .
A s a c o n c l u s i o n o f th i s d i s c u s s io n i t s e e m s t h a t a n a v a i la b i li ty s t u d y
s h o u l d b e c a r r i e d o u t a t d i f f e r e n t l e v el s o f d e t a il , u t il i z in g b o t h a n a l y t i c
r e s u l t s ( a s a p p r o x i m a t i o n s ) , a n d s i m u l a t i o n s .
1. A t t h e lo w e s t l ev e l o f d e t a il t h e a p p r o x i m a t e f o r m u l a s a r e a p p l i e d , t o
e v a l u a t e t h e r e l e v a n t c o n c e p t s w i t h r e s p e c t t o t h e a b s o l u t e l y
f u n d a m e n t a l f a c t o r s , ( f a i l u r e r a t e s , m e a n w a i t a n d r e p a i r t i m e a n d
w e ll a ll o c a t i o n ; s e e t h e c o m m e n t a t t h e e n d o f S e c t i o n 3 ).
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Subsea oil gas production systems
143
2. B y s y s te m a t i c a ll y g o i n g t h r o u g h t h e a s s u m p t i o n s le a d i n g t o t h e
f o r m u l a s , w e i d e n t i f i e d s o m e r a t h e r f u n d a m e n t a l f a c t o r s ( s u c h a s
s h o r t l if e t i m e o f th e f ie ld , n o n c o n s t a n t f a i lu r e r a te , a l a r g e n u m b e r o f
w e l ls a t e a c h P S ), t h a t m a y t o s o m e e x t e n t d e s t r o y t h e v a l id i t y o f th e
a p p r o x i m a t i o n . I t h a s b e e n s e e n t h a t t h e i m p a c t o f t h e s e f a c t o r s (i.e.
t h e i r e f f e c t o n t h e a p p r o x i m a t e r e s u l t s ) , c a n t o s o m e e x t e n t b e
a s s e ss e d b y s o u n d j u d g e m e n t . A n i n v e s t i g a t i o n th a t r e al ly t a k e s t h e s e
f a c t o r s i n t o a c c o u n t s h o u l d , h o w e v e r , o n l y b e c a r r i e d o u t u s i n g
s i m u l a t i o n .
3. F i n a l l y t h e r e a r e f a c t o r s t h a t h a v e n o t b e e n t h o r o u g h l y d i s c u s s e d s o
f ar , a n d w h i c h d o n o t c o m p l y w i t h t h e a s y m p t o t i c r e a s o n i n g l e a d in g
t o t h e f o r m u l a s . I n a ' l e v e l 3 ' s t u d y ,
time dependent
i n p u t m i g h t b e
u s e d , a n d v a r i o u s
m n gem ent str tegies
e v a l u a t e d . S o m e f a c t o r s o f
t h i s t y p e a r e a s f o l l o w s :
• T h e p r o d u c t i o n p r o f il e v a r ie s w i t h t h e s e a s o n .
• T h e p r o d u c t i o n is s h u t d o w n i f i t is n o t a b o v e a c e r t a i n le ve l.
• E n v i r o n m e n t a l f a c t o r s v a r y w i t h s e a s o n , l e a d i n g f o r i n s t a n c e t o
d i f f e r e n t r e p a i r t i m e s i n w i n t e r a n d s u m m e r .
• P a r t i c u la r m a i n t e n a n c e p h i l o s o p h i e s a p p l y , a s p r e v e n t iv e m a i n -
t e n a n c e o r m o r e r e f i n e d c r i t e r i a f o r r i g / D P S m o b i l i z a t i o n .
• S p e ci fi c a s s u m p t i o n s f o r t h e l e n g t h o f ' m u l t i p l e re p a i r s ' a r e
i n c l u d e d .
• D i f f e r e n t f a i l u r e m o d e s , a l s o n o n c r i t i c a l , a r e p r e s e n t .
T h e i n f l u e n c e o f s u c h m o r e r e f i n e d f a c t o r s / a s s u m p t i o n s s h o u l d
e s s e n ti a ll y b e i n v e s t i g a t e d u s i n g s i m u l a t i o n .
T h e r e i s n o c l e a r d i s t in c t i o n b e t w e e n l e ve ls 2 a n d 3 i n t h e a b o v e p r e s e n t a t i o n .
I t s h o u l d a l s o b e st re s s e d th a t a s i m u l a t i o n w o u l d g i ve c o n s i d e r a b l y m o r e
o u t p u t i n f o r m a t i o n ( as t i m e d e p e n d e n t r es ults ).
F u r t h e r w o r k
T h e r e a r e s e v er a l w a y s o f e x t e n d i n g t h e a p p r o a c h o u t l in e d i n th i s p a p e r t o
c o v e r m u c h m o r e g e n e r a l m o d e l s .
- - T h e m o d e l o f th e p h y s ic a l la y o u t s h o u l d a l l ow f o r:
* D i f f e r e n t l a y o u t s o f t h e P S . ( T h e t o t a l n u m b e r a n d t y p e o f P S
m o d u l e s d o n o t h a v e t o b e i d e n t i c a l f o r a l l P S . )
* S a t e l l i t e we l l s ( t i ed t o a P S o r d i r ec t l y t o t he C S ) .
* W e l ls b e i n g d i f f e r e n t ( t e m p l a t e a n d s a t e ll it e w e l ls m a y h a v e
s o m e w h a t d i f f e r e n t m o d u l e s , f a i l u r e r a t e s o r r e p a i r t i m e s ) .
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144 P Hokstad
C e n t r a l s t a t i o n C S)
P S
W e l l s
Fig. 5.
P r o d u c t i o n
s t a t i o n s P S )
W e l l s
S a t e l l i t e w e l l s
G ene ra l i z ed m ode l ~ r phys i ca l l ayou t .
T h u s a l a y o u t a s i l l u s t r a t e d i n F i g . 5 s h o u l d b e p o s s i b l e w i t h i n t h e
e x t e n d e d m o d e l .
- - F o r ( s o m e ) P S m o d u l e s i t s h o u l d b e p o s s i b l e t o l e t t h e f a i l u r e r a t e
d e p e n d o n t h e n u m b e r o f w ells ti ed t o t h a t p a r t i c u l a r P S. S o m e
c o m p o n e n t s a r e t y p i c a ll y m o r e p r o n e t o f a il u re i f m o r e w e l ls a r e
p r e s e n t , a n d t h e r e s h o u l d b e a p o s s i b i l it y to s p e c i f y f o r i n s t a n c e a f a i lu r e
r a t e o f t h e f o r m 2 = 2 o + n 21 ( w h e r e n = n u m b e r o f w e ll s, a n d 2 o a n d 21
a r e g i v e n c o n s t a n t s ) .
- - F o r e a c h w e ll m o d u l e i t s h o u l d b e p o s s ib l e t o s p e c if y th e r e l at iv e t i m e , p ,
t h a t t h e w h o l e P S is s h u t d o w n w h e n t h a t m o d u l e is re p a i r e d . ( In t h e
p r e s e n t f o r m u l a a n d s i m u l a t i o n p is a l w a y s e q u a l t o 1 .0 .)
T h e a b o v e p o i n t s a r e e x a m p l e s o f f e a tu r e s t h a t a r e e a s il y i n c l u d e d i n t h e
a r g u m e n t s o f S e c t io n s 2 - 3 , g i v in g a s y m p t o t i c f o r m u l a s t h a t a r e r e a ll y u se f u l,
s ee re f. 3. H o w e v e r , t h e s e g e n e r a l i z a t i o n s c o u l d a l s o b e c o n s i d e r e d p a r t o f
t h e m o r e a m b i t i o u s g o a l o f d e v e l o p i n g a g e n e r a l c o m p u t e r i z e d a n a l y si s t o o l
f o r e v a l u a t io n a n d c o m p a r i s o n o f v a r i o u s c o n c e p t s ( to a r r iv e a t a n ' o p t i m a l '
s o l u ti o n ) . T h i s t o o l c o u l d b e c o m p o s e d o f a s er ie s o f i n d e p e n d e n t 'm o d u l e s ' :
- - O n e m o d u l e f o r in t e r ac t iv e , g r a p h i c a l s p e c i f i c a ti o n o f th e s u b s e a l a y o u t .
- - A n o t h e r m o d u l e s h o u l d a l lo w f o r s p ec i fi c at io n o f v a r io u s r e li ab il it y
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Subsea oi l gas product ion sys tems
145
a n d f ie ld d a t a . T h i s s h o u l d b e c o n n e c t e d t o a d a t a b a s e (w h i c h s h o u l d b e
a n o t h e r m o d u l e ).
- - V a r i o u s m o d u l e s f o r c a rr y i n g o u t s e v er a l a n a ly s e s s h o u l d b e a v a il ab l e.
R e l e v a n t t e c h n i q u e s a r e :
• A s y m p t o t i c , q u i c k f o r m u l a s ( as d i s c u s s e d i n t h e p r e s e n t p a p e r ) .
• M A R K O V m e t h o d s (s ee re f. 3).
• S i m u l a t i o n t e c h n i q u e s .
T h u s t h e c o m p u t e r i z e d t o o l s h o u l d b e f le xib le b o t h w i t h re s p e c t t o m o d e l
s p e c i f ic a t i o n a n d c h o i c e o f a n a ly s is . I n t h i s w a y h a v i n g s e v e ra l a n a l y si s
t e c h n i q u e s a v a i la b l e , u s in g t h e s a m e u s e r i n t e rf a c e , w o u l d g i v e a p o w e r f u l
t o o l i n t h e p r o c e ss o f f o r i n s t a n c e f in d i n g c o n c e p t s t h a t m a x i m i z e p r o d u c t i o n
r e g u l a r i t y .
A C K N O W L E D G E M E N T
T h i s w o r k is c a r r ie d o u t w i th t h e f in a n c ia l s u p p o r t o f t h e N o r w e g i a n C o u n c i l
f o r S c ie n ti fi c a n d I n d u s t r i a l R e s e a r c h ( N T N F , p r o j e c t S E R A , 9 20 4 .1 6 88 8 ).
R E F E R E N C E S
1. Hoks tad , P . SUBSIM: A Computer P rogram for Rel iab i l i ty and Avai lab i l i ty
Asses sm en t o f S ubsea Oi l / Gas P roduc t i on S ys t em s , S INTEF R epor t S TF 75
A86002, 1986.
2 . Windebank, E . Reliabi li ty E ngineering 5 (1983), pp. 73-81.
3. Hulb~ekdal, J . M.Sc. thesis , D ep ar tm en t of M athe m atical Statist ics , The
Techn i ca l Un i ve rs i ty o f Norw ay , T rondhe i m , 1987.
A P P E N D I X
1 .
I N P U T D A T A T O T H E N U M E R I C A L
C A L C U L A T I O N S
F r o m T a b l e 1 i t f o l l o w s (s e e e q n ( 1)), t h a t f o r t h e B a s e C a s e
B 1 = B a = 6 /365 ,
B3(n ) = 0.3(46 + 14n)/365
T h e c a l c u l a t e d v a l u e s f o r B3(n ) an d Aw(n) fo r n = 1 , . . . , 20 , a re a s g i ven i n
T a b l e A 1 .
F o r t h e B a s e C a s e o n l y n = 2 a n d n = 4 a re n e e d e d , b u t t o i n v e s t i g a te th e
i n f l u e n c e o f s p l i t t in g 2 0 ( o r 6) w e l l s b e t w e e n s e v e r a l P S , t h e o t h e r e n t r i e s o f
t h e t a b l e w i l l b e a l s o n e e d e d .
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146 P. Hoks tad
T A B L E A I
Input to the N um erical Calculations
n B3 n )365 Aw n ) n B3 n )365 Aw(n)
1 18-0 0'923 11 60.0 0'833
2 22-2 0.913 12 64.2 0.825
3 26.4 0 904 13 68.4 0-817
4 30.6 0.894 14 72.6 0.809
5 34.8 0 885 15 76 8 0.802
6 39.0 0'876 16 81.0 0.794
7 43-2 0-867 17 85.2 0.787
8 47-4 0.858 18 89.4 0.780
9 51.6 0.850 19 93.6 0.773
10 55-8 0.841 20 97.8 0.766
A P P E N D I X 2 . O T H E R R E L I A B I L I T Y P A R A M E T E R S
H a v i n g a s s e s se d t h e a v e r a g e a v a i l a b i l i t y / p r o d u c t i o n r e g u la r it y o f a c o n c e p t ,
a s d e s c r i b e d i n t h i s p a p e r , it i s a l s o p o s s i b l e t o e s t i m a t e s e v e r al o t h e r
p a r a m e t e r s . J u s t a c o u p l e o f e x a m p l e s a r e g i v e n h e r e .
l N um b er o f w el l a i l u r es
( t h a t is t h e t o t a l n u m b e r o f f a i lu r e s t h a t
r e s u lt s in t h e s h u t d o w n o f o n e w e l l o n l y ) . T h i s p a r a m e t e r w i ll h e r e b e
e s t i m a t e d b y
F3 = N)~3
T o A w
F o r t h e B a s e C a s e t h i s g i v e s 6 x 0 - 3 x 1 0 × 0 9 = 1 6 .2 . T h i s is a l m o s t
i d e n t i c a l t o t h e v a l u e o b t a i n e d f r o m t h e s i m u l a t i o n ( 1 6 - 4 + 0 .5 ) .
2 T o t a l r e p a i r t ime
F i r s t th e n u m b e r o f P S f a il u r es , F 2 , a n d t h e n u m b e r
o f C S f a il u r es , F t , a r e e s t i m a t e d i n t h e s a m e w a y a s F 3 . T h e n t h e t o t a l
r e p a i r t i m e i s e s t i m a t e d b y
R-t o T = F ~ R x + F 2 R z + F 3 R 3
F o r t h e B a s e C a s e i t i s f o u n d t h a t
F2 = 3.75 and F~ = 0-61, gi vi ng Rv o v = 8.4
m o n t h s . T h i s is r e a s o n a b l y c l o s e t o th e s i m u l a t e d v a l u e , 7.5 + 0 5. T h e
d i f f er e n c e is e s s e n t i a ll y e x p l a i n e d b y t h e s p e c ia l a s s u m p t i o n s u s e d i n t h e
s p e c i f ic a t i o n o f th e s i m u l a t i o n m o d e l , s e e t h e e n d o f S e c t io n 5.