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Proceedings of the Eleventh 2001) International O ffshore and Po lar Engineering Conference

Stavanger, Norway, June 17-22, 2001

Copyright 2001 by The International Society of O ffshore a nd Polar Engineers

ISB N 1-880653-51-6 SeO; ISB N 1-880653-52-4 VoL I); ISS N 1098-6189 SeO

Use of Light W eight Substructures for Oi l and Gas Ma rginal F ie ld eve lopmen t

W a n M a h m o o d W a n A b . M a j i d

M A R A U n i v er s it y o f T e c h n o l o g y

K u a l a L u m p u r , M a l a y s ia

M o h a m a d Bi n E m b o n g

J a c o b s - P ro t e k E n g i n e e r s S d n B h d

K u a l a L u m p u r , M a l a y s ia

A B S T R A C T

As the world o i l p r i ce i s cyc l i c i n na ture , t he economic v i ab i l i t y of

the o i l and gas development s i s of t en subjec t ed t o a cer t a in r i sk and

somet ime unforeseeable .

The dev elopm ent p lans , which w ere econo mical a t cer ta in t ime,

ma y no longer be t rue a t o ther t ime, especi a l l y when the o i l p r i ce h i t

t he bot tom l evel.

Typica l example of t h i s t ype of development i s t he margina l f i e lds ,

which requi re minimal i nves tment and fas t recovery i n order t o

remain v i ab l e . Th i s i s due t o t he shor t l i fe span of reservoir , minimal

crude reserve and a l so most ly sca t t e red i n var ious l oca t ions . In som e

cases t he development wi l l be put on-hold for t he next cyc l e or

when the oi l price at the peak or at t ract ive level .

Whi l s t t he margina l f i l ed development i s commonly s imple i n t e rm

of process fac i l i ti es , bu t t he s t ruc tures wi l l remain com plex i f t he

des ign i s t o adopt a s t andard or convent ional approach . The

complexi t i es of t he s t ruc tures a re main ly dep enden t on severa l

fac tors such as t ops ide l oads , envi ronmenta l condi t i ons , opera t i ng

phi losophy, so i l condi ti ons and the w ater depth .

Wi th t he s t ruc ture i s re l a t i ve ly more complex than o ther fac i l i t i es ,

t he s t ruc ture cos t may out weighted o ther cos t and thus put t he t o t a l

pro j ec t cos t as non-vi abl e t o proceed wi th t he development .

Thi s paper br i e f ly d i scussed ho w the use of t he L ight Weight

St ruc tures may reduce the overa l l p ro j ec t cos t t hus pot ent i a l l y save

the pro j ect . The m ain poin t of i n t eres t wi ll be : what a re t he t echnica l

chal l enges of t he cos t sav ing ideas , such as devia t i on to Codes and

Standard and com mo n prac t i ces, u se of s t a te of t he art s computer

analys i s, f i t for purpose pr inc ipa l and use o f new ly develope d or non

proven t echnologies .

Key Words: L ight Weight S t ruc tures , devi a t i on , Cost Saving Ideas ,

v i ab i l i t y .

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

Various t ypes of L ight Weig ht S t ruc tures had been des igned

ins t a ll ed world wide t oday .

Genera l l y , t hese d i f ferent t ypes of s t ruc ture may be c l ass i f i ed

the fo l l owing broad ca t egor i es :

Jacket Steel Tem plate, Leg Driv en Pi le or Skirt Pi les.

, Free S t anding St ruc ture /C ai sson .

[] Brac ed Caisso n

Figu re 1) to 3) i llust rate a typical samp le of the structures.

The su i t ab il i ty o f t he t ype o f S t ruc ture t o be considered m a

sel ec t ed depending on the des ign cr i te r i a or obj ec t ive .

Regardl ess of t he des ign cr i t e r ia or t ypes o f s t ruc tures , mos t o

weight s sav ing ideas a re norm al ly com mo n and appl i cabl e t o a ll

e l o c a t a b l e a n d e u s a b i l i t y

Relocat abl e and reusabi l i t y of t he s t ruc ture may be considered

very impor t an t aspect of t he s t ruc ture.

These fea tures ab l e t o reduce t o t a l cos t of ownership espec

when the serv i ce l i fe of a par t i cu l ar reservoi r i s shor t and

s t ruc ture m ay be eas i l y re loca t ed and reused a t the new s it e.

Al l necessary a t t achment s or mechani sms may be proper ly des i

t o su i t the s t ruc ture t ype i n order t o ease t he rem oval o f

st ructures later date.

For re loca t ion purpose , t he des igner may use gener i c da t a t h

l i ke ly t o be su i t ab l e for major i t y a reas and checked for t he

spec i f ic as t o co nf i rm the adequ acy.

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In some ins tances , the weight saving ideas may contradic t to the

relocatable and reusable concept . In this case , the designer and the

owner shal l balance between the weight savings and the importance

of the features.

The subsequent sect ions wi l l br ief ly discussed the weight saving

ideas , individual ly or a combinat ion of them may produced a

significant weight saving.

N o n c o n v e n t i o n a l D e s i g n A p p r o a c h

Design using convent ional codes and s tandards ensures safe and

sound structures. However, various safety factors built in the code

checking and conservat ive assumpt ions somehow resul t ing the

structures are well exceede d fit for purpo se during their entire

service life.

Use of state of the arts computer analysis such as finite element

based program; non- l inear analysis may contr ibute to a s ignif icant

weight saving. Non-linear analysis util ised the benefit or the reserve

strength in the plastic l imit zone ins tead o f elastic l imit. The analysis

do not check the structure in compliance to the codes and standards

but wi l l be checked for buckl ing or yie lding and col lapse behavior

of the structure. This is to ensure that there is no danger of platform

progress ing col lapse dur ing the ant ic ipated extreme design loading.

Often this approach raise a concern on safety and reliability issues

by certain parties, espec ially Platform Operator. Hazard scenarios

and effect to the pla t form shal l be invest igated to weigh the

acceptable r i sk taken.

This approach requires an ordered and systemat ic approach by

exper ience designers who can interpret the resul t accurate ly and to

be reviewed by specialist third party.

C o d e s a n d S t a n d a r d s

Select ive deviat ion to the codes and s tandards or recommended

pract ices , i f proper ly ra t ional ized based on i t s real needs m ay resul ts

in considerable weight saving w hi le mainta ining the integr i ty of the

structures.

Some typical examples where the codes and s tandards may be

rationalized:

Relax slend erness ratio limitation to 180 instead 12 0. Mo st of the

members are governed by s lenderness ra t io l imi ta t ion not s t ress

level. By increasing slenderness ratio, considerable saving may be

achieved. However members has to be designed and checked for

stresses, fatigue and vo rtex shed ding due to current.

API RP 2A joint check caps the yield stress to 2/3 of tensile (Fyc,

clause 4.1). The JIP asses sme nt criteria, reliability and reserve

st rength tubular joints by MSL, U K recom mend ed this could be

relaxed to 0.8.

U s e o f

3D C A D

Use of 3D-CAD model of ten help the designer in the design pro

to moni tor and ensure the cost and w eight saving ideas impleme

are practical and do not contribute to unacceptable safety risks.

model may be developed to ident i fy c lashes , fabr icat ion

installation related problem and operational constraints at the e

s tage of the design. Cont inuing use of 3D CAD in fabr ication

result in considerable saving, i .e. less re-work, shorter sched

reduce surplus and wastage.

D E S I G N C O N S I D E R A T I O N S

Design concept and approach is remains as the important factor

determining the final result , which is meeting the design objec

i.e. Light W eight Structure.

The subsequent sect ions br ief ly discussed some of the de

approach or concept , which may be considered in designing L

Weight Structures.

U s e o f F y 4 5 0 v s F y 3 5 0 M P a s t e e l

Use of Extra High strength steel (450 MPa) could contribute

s ignif icant weight saving, provided that the components are

governed by buckling (slenderness ratio), fatigue or deflection.

The cost penal ty i s approximately 10% o f High S trength S teel

MPa) . However this i s normal ly out weighted by the benef i t

weight, weld and fabrication saving.

U s e o f G R P o r A l u m i n u m

Some of the component of the s t ructures , especia l ly non-

bear ing may adopt to use G RP or A luminum instead of carbon s

Typical example is us ing of G RP/Aluminium grat ings for ja

walkway, thus e l iminat ing maintenance. However GRP

deter iorate in UV unless i s proper ly UV protected.

Mudmat , which is served as a temporary foundat ion pr ior to

piling may adopt to use aluminum. This will provide a lig

s t ructure and reduce cathodic protect ion requirement for

structures.

Est imated saving of 30kg/m^2 when using aluminum and fabrica

saving, also avoiding re-g alvanisin g after cutting, banding etc.

I n t e r n a l r i n g s t i f f e n e r

Internal R ing s t i f feners to s t rengthen joints are much m

expensive than increasing can s izes , provided PWHT can

avoided.

C a s t i n g J o i n t s

Use o f cas t ing joints may improve fa t igue and weight , how

since the joints has to be spe cially tailored, i t will becom e long

i tems and expensive comp ares to normal joints .

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O v e r l a p p e d J o i n t s

A l t h o u g h o v e r l a p p e d j o i n t s m a r g i n a l l y s t r o n g e r t h an s i m p l e j o i n t s ,

s im ple tubula r jo in t s w i th g a ps a re m uc h e a s ie r to f a br ic a te a nd c os t

e f f ec t iv e . O v e r l a p p e d j o i n t s o f t e n c o m p l i c a t e u n d e r w a t e r i n s p e c t i o n

dur ing the s e rv ic e l i f e ; thus i t i s no t a dvi sa ble to a dopt the use of

o v e r l a p p e d j o i n t s.

C a t h o d i c P r o t e c t i o n

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

a ga ins t c or ros ion . Prov id ing suf f i c ie n t a node s w i l l e nsure the re i s

no c or ros ion w i l l t a ke p la c e in the s t ruc ture dur ing the s e rv ic e l i fe .

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

o f d e s i g n l i fe b y d o u b l e .

The de s igne d of a node to a spe c i f i c ope ra t iona l l i f e w i thout s a fe ty

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

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

pr ior to r e use o f the s t ruc ture s .

T h e r m a l S p r a y e d A l u m i n u m C o a t i n g

The s t ruc ture s in the sp la sh z one , norm a l ly de f ine d in the r a nge of

+ ) 4 m a b o v e a n d b e l o w s e a w a t e r l e v e l a r e s u b j e c t t o h e a v y

c or ros ions .

N o r m a l p r a c t ic e i s t o p r o v i d e d a s p l a s h z o n e c o a t i n g a n d a t t h e s a m e

t i m e t o p r o v i d e a c o r r o s i o n a l l o w a n c e u p t o 1 2 m m . T h i s a l l o w a n c e

provide s no s t ruc tura l s t r e ngth but c ont r ibute to a s igni f i c a nt we ight

pe na l ty .

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

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

T o p o f J a c k e t O u t s i d e S p l a s h Z o n e .

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

a n a l t e rna t ive so lu t ion to the c or ros ion a l lowa n c e ; i . e . top of j a c ke t

f r a m i n g a b o v e s p l a s h z o n e

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

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

U s i n g o f th i s d e v i c e m a y r e d u c e w a v e a n d c u r r e n t l o ad i n g d u e t o

e x i s t e n c e o f m a r i n e g r o w t h . H o w e v e r t h e m a i n t e n a n c e

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

J a c k e t T e m p l a t e S t r u c t u r e U s e o f S k i rt P il e s v s . L e g D r

P i l es

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

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

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

use fu l f e a ture , r e loc a ta b le .

S o m e o f t h e a d v a n t a g e o f u s i n g s k i rt p i l e s a re :

W e i g h t s a v i n g o n p i l es a b o v e m u d l i n e .

L e g s i ze s a r e i n d e p e n d e n t o f p i le d i a m e t e r w h i c h i s a f u n c

o f s o il p a r a m e t er s a n d p e n e t r a t i o n l i m i ta t io n . M a y o p t i m

le g s iz e s to r e s i s t s e rv ic e loa d o nly .

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

a c hie ving the obje c t ive s , i . e . l igh t we ight s t ruc ture s a nd r e loc a t

fea ture .

T y p i c a l e x a m p l e o f d i s a d v a n t a g e s o f a d o p t i n g s k i rt p i l es a r

f o l l o w s :

[] Re qu i re s c ha se r p i l e . P i l e dr iv ing ne e ds e x t r a pre c a ut

a ga ins t p i l e buc kl ing d ue to a long s t i c k-up .

[] Grou t ing ope ra t ion s for sk i r t p i l e .

[] J a c ke t l e ve l l ing ne e ds unde rw a te r gr ippe r or su i t a b le de vi

i . e . pa de ye s . Re la t ive ly d i f f i c u l t ope ra t ions , unde rwa te r

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

w i th a ppropr ia te de s ign a nd ins ta l l a t ion m e a sure s a s fo l lows :

U n d e r w a t e r h a m m e r o r c o n v e n t i o n a l h a m m e r t o g e t h e r

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

[] App ropr ia te ly de s ign e d a nd c he c ke d the c ha se r for buc k

a nd ins ta l l a t ion m e a sure s m a y be provide d , i . e . p i l e s gui

etc.

[] Grou t ing ope ra t ions i s m inim a l c om pa re s to the

ins ta l l a t ion t im e .

[] L e v e l l i n g m e c h a n i s m o r p a d e y e s n e a r m u d l i n e c a n b e d e s i g

a n d p r o v i d e d f o r j a c k e t l e v e l l in g p u r p o s e .

[] Spe c i f i c r e qui re m e n t of of f shore ins ta l l a t ion c ont ra c tor m a

inc orpora te d a t the e a r ly s t a ge s of the de s ign .

U s e o f S u r p l u s M a t e r i a l s

M a k e u s e o f s u r p l u s m a t e r i a l s m a y c o n t r ib u t e a s i g n i f i c an t c o s t

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

c ons t r a in t due to the l im i te d s i z e s a va i l a b le a nd thus ha m pe r the

d e s i g n o b j e c t i v e ; L i g h t W e i g h t S t r u c tu r e .

U s e o f M a r i n e G r o w t h R e m o v a l D e v i c e

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

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

forces .

A P P U R T E N A N C E S

For the sm a l l s t ruc ture , the we igh t of the a ppur te na n c e s

se c onda ry s t ruc ture s would c ont r ibute to the s igni f i c a nt we igh

the s t ruc ture . The im por ta n c e of the se i t e m s m a y b e r a t iona l i z e d

b a r e m i n i m u m r e q u i r e d f o r s a f e o p e r at i o n s . S o m e o f t h e t y p

sa m ple o f the i t e m s a re d i sc usse d in the fo l lowing :

U s e o f t o p o f j a c k e t e l e v a t i o n a s s u m p p u m p d e c k

I n s t e a d o f d e s i g n i n g a n a d d i t i o n a l i n t e r m e d i a t e l e v el , s u m p p

d e c k t o u s e t h e t o p o f j a c k e t e l e v a ti o n .

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E l i m i n a t e t o p o f j a c k e t p e r i m e t e r w a l k w a y a n d h a n d r a i l i n g

E l i m i n a t i n g t o p o f j a c k e t w a l k w a y a n d h a n d r a i l in g . T h i s w i l l r e d u c e

w a v e l o a d i n g o n t h e j a c k e t a n d m a i n t e n a n c e c o s ts . H o w e v e r , i f

node s inspe c t ion i s r e qui re d or poss ib le a c c e s s to c la m ps ,

sc a f fo ld ing i s to be e re c te d .

E l i m i n a t e s t a i r ca s e

Adopt ve r t i c a l (GRP) l a dde r or e l im ina te s t a i rwa ys a l toge the r .

P r o v i d e a c c e s s t o w a t e r l i n e t h r o u g h e s c a p e c h u t e . M i n i m a l o n e

s t a ir c a se t o b e p r o v i d e d f r o m d e c k t o t h e j a c k e t

w a l k w a y / b o a t l a n d i n g .

E l i m i n a t e s u m p c a i s s o n

C o n s i d e r u s e o f d ra i n v e s s e l h u n g u n d e r n e a t h d e c k . R e d u c e w a v e

loa ding to the subs t ruc ture . Proc e s s c ons ide ra t ion i s r e qui re d to

jus t i fy the su i t a b i l i ty of the s c he m e .

E l i m i n a t e r i s e r g u a r d

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

Sa fe ty c a se s m a y c onf i rm e d the r i sk a s soc ia te d w i th th i s a pproa c h .

S U M M A R Y

T h e L i g h t W e i g h t S t r u c t u r e i s c o n s i d e r e d m e t i ts u l t i m a t e o b j e

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

s igni f i c a nt we ight r e duc t ion whi le m a in ta in ing i t s s a fe ope ra t ion

Fi t for Purp ose de s ign sha l l be c ons ide re d a s m a in c r i t e r i

de s igning l igh t we ight s t ruc ture s .

One poss ib i l i ty to fur the r m a xim ise c os t s a v ing i s to p la n

ins ta l l a t ion o f the s t ruc ture to be c o inc id ing w i th the a v a i l

ba rge s or r igs for o the r a c t iv i t i e s a t ne a rb y s i t e , suc h a s dr i l l ing ,

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

T h e s e b a r g e s a n d j a c k - u p d r i ll i n g r i g s a r e n o r m a l l y e q u i p p e d w

l i f t ing c ra ne w i th a l im i te d c a pa c i ty in the r a nge of 300 to

t o n n e s , w h i c h m a y b e u s e d s u f f i c i e n t t o i n s t a l l L i g h t W e

St ruc ture .

O P E R A T I O N A L C O N D I T I O N

R e l a x t h e e n v i r o n m e n t a l f r o m 1 y e a r t o 1 m o n t h d u e t o s h o r t

ope ra t iona l l i f e . Sa ving on p i l e a nd j a c ke t . Re q ui re s ope ra t ing

p h i l o s o p h y c h a n g e .

I N S T A L L A T I O N

T w o h o o k u p e n d i n g

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

f looding sys te m .

O I C l i a is o n

E a r l y d i s c u s s i o n w i t h O f f s h o r e I n s t a l l a t i o n C o n t r a c t o r o n l o a d -

out / f loa ta t ion a nd ins ta l l a t ion m a y c ont r ibute to m ore e f f e c t ive

we ight s a v ing ide a s .

L o a d o u t b y l i f t in g

C o n s i d e r l o a d o u t b y l i f t in g i n s t e a d o f s k i d d in g . E l i m i n a t e j a c k e t t o

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

l im i t the de f le c t ion of the los s of suppor t . E l im ina te loa dou t s t e e l

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

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American Petro leum Institute - API RP 2A

American Institute Steel Construction

Structural Analysis Computer System Manual (SACS),

Engineering D ynamic Inc.

1999 Worldwide Survey of Minimal O ffshore Fixed

Platform & Decks f or Marginal Fields, January, 1999.

The JIP assessmen t criteria, reliability and reserve

strength tubular joints by MSL, UK

Figure (2) Typical sample- Free Standing Structure/Caisson

Figure (1) Typical Sa mp le- Jacket Steel Template Stru cture - 2D

Figure (3) Typical Sam ple -Br aced Caisson

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