weld metal properties of reeled pipelines

8/2/2019 Weld Metal Properties of Reeled Pipelines

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W e l d M e t a l P r o p e r t i e so f R e e le d P ip e lin e sC o n s t ru c t io n m e t h o d o f o f fs h o r e p i p e l in e s d e t e r m i n e s th e

c h a r a c t e r is t ic s o f th e w e l d e d j o in t s

B Y J . R . S T I L L

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iF ig . 1 - - O n s h o r e p i p e - t o - p i p e w e l d i n g u s i n gp u l s e g a s m e t a l a r c w e M i ng .

M e t h o d s f o r l a y in g p i p e o f f s h o r e i n v o l v e c o n v e n t io n a l l a y b a r g e s ,r e e l s h i ps , a n d t o w i n g o f p i p e b u n d l e s . A l l o f t h e s e m e t h o d s r e l y o nw e l d i n g t o j o i n t h e p i p e s , e i t h e r o n s h o r e o r o f f s h o r e . P i p e l a y i n g u s i n ga r e e l s h i p i s a n a t t r a c t i v e o p t i o n , a s a ll p i p e j o i n t s , w i t h t h e e x c e p -t i o n o f t h e o f f s h o r e t i e- i n s , a r e w e l d e d o n s h o r e . T h i s t e c h n i q u e i sc o n s i d e r e d t o b e o n e o f t h e m o s t r e l i a b l e m e t h o d s f o r l ay i n g a p i p e l in eo f f s h o r e . W e l d i n g t r ia l s a r e a n e s s e n t i a l p a r t o f t h e a p p r o v a l p r o c e s sw h e r e t h e w e ld jo i n t p r o p e r t i e s m u s t m e e t t h e r e q u i r e m e n t s o f th en a t i o n a l s t a n d a r d , i . e ., B S 4 5 1 5 - 1 ( R e f . 1 ) , A P I 1 1 0 4 ( R e f . 2 ) , p l u sa n y a d d i t io n a l r e q u i r e m e n t s r e q u e s t e d b y th e c l i en t . T h i s a rt i cl e d e -s c r i b e s t h e o n s h o r e p r o d u c t i o n f a c i l i t i e s , w e l d i n g p r o c e s s e s , a n d t h ep r o p e r t i e s a c h i e v e d d u r i n g w e l d i n g p r o c e d u r e t r i a l s t h a t i n c l u d es t r a in - a g e d C h a r p y i m p a c t s.Construc t ion of P ipe l ines for Ree l ing

T h e o n s h o r e f a b r i c a t io n o f p i p e li n e s f o r r e e l in g i n v o lv e s h a v in g ap u r p o s e - b u i l t w o r k s h o p , l a i d o u t i n s u c h a w a y t o e n s u r e t h a t t h e w e l d -i n g o f p i p e m a t e r i a l a n d b u t t j o in t s i s c a r r i e d o u t w i t h t h e m i n i m u mo f d i s r u p t i o n . A l s o , c o n t a i n e d w i t h i n t h e s e f a c i l i t i e s w i l l b e a r a d i -o g r a p h i c a n d c o a t i n g u n i t . F a b r i c a t e d p i p e l i n e s e c t i o n s u p t o 1 0 0 0 m

F ig . 2 - - A n i n t e r n a lc l a m p w i t h b a c k i n g s h o e s

i s u s e d f o r t h e r o o t r u n .

J. R. S TI L L is a consultant w elding and m aterials engineer, Aberdeen, Scotland.

E d ~ l J U L Y 2 0 04


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(3280 ft) in length, referred to as pipestalks, are transferre d from the fabricatio nshop to an open holding area. Within theholding area, all weld repairs, tie-ins be-tween pipe stalks, a nd coating and coatingrepairs are carried out, which are in-spected on comple tion to ensure that thequality meets the client's requirements.The pipe stalks are ev entual ly coiledonto the vessel reel. This can involveseveral pipe stalks joined by butt joi ntwelding.On loca tion offshore, the reeled pipeis uncoiled and laid on the seabed. De-pending on the length of the pipelinebeing laid, it may be necessary to tie-inthe laid pipe with a section of the re eledpipe by butt joint welding. This operationis carried out on the reel vessel and, oncomple tion of welding, nondestr uctive ex-amina tion (NDE), and coating, pipe lay-ing continue s.Pipeline Welding

A 16-in. (406.4-mm) -diameter, API5L X60 (Ref. 3) pipeline, manufacturedthrough a quenched and tempered route,was recently laid in the U.K. sector of theNorth Sea. Chemical composition andmechanical properties are outlin ed in Ta-bles 1 and 2.Const ructi on of the 16-in. (406.4-mm)-diamete r pipeline involved the follow-ing welding procedu res:Pipe-to-Pipe Mainline Onshore. Th emethod used for the mainline butt joint

welds consisted of an automatic pulsedgas metal arc welding (GMAW-P), an ex-ample of which is illust rated in Fig. 1. De-tails of the weld ing procedure, which wascarried out in the 5G position, is outlin edin Table 3. Weld prepar atio n consisted ofa narrow gap bevel with zero gap betwe enroot faces. An in terna l clamp with copperbacking shoes (Fig. 2) is used for the rootrun and hot pass. Preheat temperaturewas 100C with interpass temperaturecontrolle d at 300C maximum.Complete Penetration Repair to Main-

line Weld Onshore. Repairs to the main-line welds are carried out in the pipestalks' holding area, and wel ding is car-ried out in an all-weather habitat. Theprocess used for repairing original main-line welds consists of shielded metal arcwelding (SMAW). Details o f the weld pro-cedure carried out in the 5G position areoutlined in Table 4. Welding in this in-stance is carried out in the uphill positionusing a prehea t te mper atur e of 150C andan interpass temperature controlled at300C maximum .

T a b l e 1 - - P i p e M e t a l C h e m i c a l A n a l y s i sC S i S P M n Ni0.11 0.25 0.003 0.12 1.10 0.11V Cu AI Nb N B0.05 0.14 0.025 0.022 0.0082 0.002Carbon Equivalent 0.36 Steel Making Process

Cr Mo0.08 0.11Ca Ti0.0017 0.003Electric Arc

T a b l e 2 - - P i p e M e t a l M e c h a n i c a l P r o p e r t i e s

Tensile TestsTensile Strength N/mm2567Test Temperature

Elongation (%)24Impact Properties Joules

Hardness HV 10Results

Charpy Impact Tests-40CInsideMid wallOutside

252, 252, 258, Av 254180, 179,177, Av 178184, 182,179, Av 181196, 194, 199, Av 196

T a b l e 3 - - P i p e - t o - P ip e M a i n l i n e W e l d P r o c e d u r e

Welding Position 5G FillerPass No. Process Size(mm)1 GMAW-P 0.92 GMAW-P 0.9Fill GM AW -P 0.9Last 2 GM AW -P 0.9Cap GMAW-P 0.9

Filler Metal SpecificationClassificationElectrical CharacteristicsPreheat CInterpass Temp C

16-in. Pipe-to-PipeMainline Onshore

tDiameter 406.4 mm

tMetal API 5L X60

Thickness 23.8 mmAmps Volts R.O.L. Time Speed Heat Input(mm) (s) (mm/s) (kJ/mm)

213-231 21-25 630-670 42-46 14.6/15 0.3208-225 21-25 660-670 96-100 6.6/7 0.7/0.8201-228 21-25 660-670 93-98 6.7/7.1 0.7/0.8195-219 21-25 100 12-20 5.0/8.3 0.6/1.0159-178 21-15 100 19-26 3.8/5.3 0.8/1.1

A5.28 ER80S-GDC + VE100255

Flux/ShieldingGasType Ar/CO2Classification 85% Ar/15% CO2Flow Rate 20-27 L/min.Tungsten Electrode NA

N o t e : G M A W - P ( P u l s e d g a s m e t a l a r c w e l d i n g )

WELDING JOURNAL ~gl


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T a b l e 4 - - R e p a i r t o M a i n l i n e

W e l d i n g P o s i ti o n 5 GF i l l e r

C o m p l e t e P e n e t r a t i o n R e p a i r t oM a i n l i n e O n s h o r e

D i a m e t e r 4 0 6 .4 m mM e t a l A P I 5 [ . X 6 0

T h i c k n e s s 2 3 . 8 m mP a s s N o P r o c e s s S i ze A m p s V o l ts R . O . L . T i m e S p e e d H e a t I n p u t

( m m ) ( r a m ) ( s ) ( r a m / s ) ( k J / m m )1 G T A W 2 . 4 9 11 -1 61 1 g - 1 1 6 5 9 0 3 5 - 8 8 1 . 0 - 2 . 0 0 . 6 - 1 . 22 G T A W 2 . 4 1 6 0 - 2 4 0 g 1 1 7 0 - 9 0 2 7 - 5 8 1 . 6 - 2 . 6 0 . 8 - 1 . 1F ill S M A W 3 .2 t)O 1 4 0 2 0 2 4 8 0 - 1 5 0 4 5 - 6 0 1 . 4 - 2 . 6 1 . 0 - 1 . 8C a p S M A W 3 .2 9 0 - 1 3 0 2 0 - 2 4 6 0 - 1 1 0 4 3 - 6 3 1 . 4 -2 . 0 1 . 2 -1 . 8

C l a s s i f i c a t i o nF i l le r M e t a l S p e c i f i c a t io n

A 5 . 2 8 E R 8 0 S - GA 5 . 28 E R g 0 S - GA 5 . 5 E 9 0 1 8 - G1 . E R g 0 S - G

2 . E R 9 0 S - GF i l l a n d C a p E g 0 1 8 - GD C - V ED C - V ED C + V E

15 03 0 0

P a s s N o .

E l e c t r i c a l C h a r a c t e r i s t i c s

P r e h e a t Cl n t e rp a s s T e m p C

T y p eC l a s s i f i c a t i o n

F l u x / S h ie l d i n g G a sH i g h - P u r i t y A r g o n9 9 . 9 9 5 % A r g o n

F l o w R a t e

T u n g s t e n E l e c t r o d e

1 2 - 2 0 L / r a in

2 .4 m m , 2 % T h o r i a t e d

T a b l e 5 - - P i p e - t o - P i p e C u t - o u t / T i e - in O n s h o r e

W e l d i n g P o s i ti o n 5 G1 6 - in . P i p e - t o - P i p e M a i n l i n e

C u t - O u t / O n s h o r e T i e - inD i a m e t e r 4 0 6 .4 m m

M e t a l A P I 5 L X 6 0T h i c k n e s s 2 3 .8 m m

F i l l e rP a s s N o . P r o c e s s S i ze A m p s V o l ts R . O . L . T i m e S p e e d H e a t I n p u t

( m m ) ( m m ) ( s) ( r am / s ) ( k J / m m )1 GT AW 2.4 90-2 00 8 11 90 235 60-1 60 1 .0-2 .1 0 .7-1 .12 G T A W 2 . 4 1 5 0 - 2 8 0 8 - 1 1 1 0 11 -3 31 / 5 1 - 1 2 0 1 . 6 - 2 . 8 0 . 8 - 1 . 0F i ll F C A W - G 1 .2 1 4 0 - 2 2 0 2 4 - 2 6 9 1 /- 32 1 / 3 2 - 1 4 1 2 . 0 - 4 . 0 1 .1 2 . 4C a p F C A W - G 1 .2 1 4 11 -2 1 5 2 4 - 2 6 1 0 0 - 3 8 0 3 2 - 8 6 2 . 8 - 4 . 4 1 . 0 - 1 .5

F i l le r M e t a l S p e c i f i c a t io nC l a s s if i c a ti o n A 5 . 2 8 E R g 0 S - G

A 5 . 28 E R g 0 S - GA 5 . 2 9 E 8 0 T - K 2P a s s N o . 1 . E R 8 0 S - G

2 . E R 8 0 S - GF i l l a n d C a p E 8 0 T - K 2E l e c t ri c a l C h a r a c t e r i s t i c s D C - V ED C - V ED C + V EP r e h e a t C 1 0 0

I n t e r p a s s T e m p C 3 0 0

T y p eC l a s s i f i c a t i o n

F l o w R a t e

F l u x / S h ie l d i n g G a sH i g h - P u r i t y A r g o n

A r g o n 2 59 9 . 9 9 5 % A r g o n7 5 % A r / 2 5 % C O 2

G T A W 1 2 - 2 1 / L / r a i nF C A W - G 1 5 - 2 5 L / ra i nT u n g s t e n E l e c t ro d e 2 . 4 m m , 2 % T h o r i a t e d

l t H I J U L Y 2 0 0 4


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T a b l e 6 - - P i p e - t o - P i p e O f f s h o r e T i e - i n

W el d i n g Po s i t i o n 6 G P i p e - t o - P i p e O f f s h o r e T i e - i nD i a m e t e r 4 0 6. 4 m mF i l l e rPass No . Pro cess S i z e A m p s Vo l ts

( m m )1 G T A W 2 . 4 9 0 - 2 0 0 8 - I 12 GT A W 2 .4 1 5 0 -2 8 0 8-1 1Fi l l FC A W -G 1 .2 1 4 0 -2 2 0 2 4 -2 6 .5C ap FC A W -G 1 .2 1 4 ( /-2 1 5 2 4 -2 6 .5

F i l l e r M e t a l S p e c i f i c a t io nClassification

Pass No .

E l e c t r ic a l C h a r a c t e r i s t i c s

P r e h e a t Cl n t e r p a s s T e m p C

A 5 . 2 8 E R 8 0 S - GA 5 . 2 8 E R 9 0 S - GA 5 .2 9 E8 0 T-K2I . E R 8 0 S - G2 . E R 9 0 S - GF i l l a n d C a p E 8 0 T - K 2D C - V ED C - V ED C + V E

1 0 03 00

86

aLL- JM e t a l A P I 5 L X 6 0

T h i c k n e s s 2 3 . 8 m mR . O . L T i m e S p e e d H e a t I n p u t( r a m ) ( s) ( m m / s ) ( k J / m m )90-235 60-1 60 1 .0-2 .1 0 .7-1 .1100-33 0 51 - 120 1 .6-2 .8 0 .8-1 .09 0 -3 2 0 3 2 -1 4 1 2 .0 -4 .0 1 .1 -2 .41 0 0 -3 8 0 3 2 -8 6 2 .8 -4 .4 1 .0 -1 .5

F l u x / S h i e l d i n g G a sT y p e H i g h - P u r i t y A r g o nC l a ss if i ca t io n A r g o n 2 59 9 .9 9 5 % A rg o n

7 5 % A r g o n / 2 5 % C O ,F l o w R a t e G T A W 1 2 -2 0 L / ra inF C A W - G 1 5 - 2 5 L / r a i n

T u n g st en E l e c t r o d e 2 . 4 m m , 2 % T h o r i a t e d

T a b l e 7 - - W e l d M e t a l C h e m i c a l A n a l y s i sW e k l P r o c e d u r e C S i S PPi p e - t o -P i p e M a i n l i n e 0 .1 0 0 .5 2 0 .0 0 8 0 .01R ep a i r t o M a i n l i n e 0 .1 0 0 .5 2 0 .0 0 8 0 .0 1Pi p e - t o -P i p e C u t Ou t 0 .1 0 0 .5 5 0 .0 0 7 0 .0 1a n d P i p e - t o - P i p e O n s h o r e T i e - i nP i p e - t o - P i p e O f f s h o r e T i e - i n 0,10 0.44 0 .008 0 .01

M n C r M o N i1.24 0.04 0.02 0.031.17 0.05 0.07 0.621.28 (1.04 0.09 0 .651.17 0.04 0.14 0.68

Pipe-to-Pipe Mainl ine Cut Out~Tie-InOnshore. T h e w e l d p r o c e d u r e i s u s e d f o rw e l d i n g b u t t j o in t s b e t w e e n p i p e s , w h i c hc a n c o n s i s t o f m a i n l i n e b u t t j o i n t w e l d c u t -o u t s a n d b u t t j o i n t w e k t i n g o f p i p e s t a lk st o p i p e s t a l k s, o r p i p e s t a lk s t o r e e l e d p i p e .W e l d p r e p a r a t i o n c o n s i s t s o f a 5 0 to 60 -d e g b e v e l w h e r e t h e r o o t r u n i s d e p o s i t e du s i n g g a s t u n g s t e n a r c w e l d i n g ( G T A W )a n d f i l l e d w i t h S M A W . B r i d g e t a c k s a r eu s e d t o m a i n t a i n t h e r o o t o p e n i n g , w h i c ha r e r e m o v e d a n d r e p o s i t i o n e d t o p r e v e n tt h e o p e n i n g f r o m s h r i n k i n g a n d r e s t r i c t -i n g a c c e s s . D e t a i l s o f t h e w e l d i n g p r o c e -d u r e 5 G p o s i t i o n a r e i l l u s t r a t e d i n T a b l e5 . P r e h e a t a n d i n t e r p a s s te m p e r a t u r e s a r ei d e n t i c a l to t h a t a p p l i e d f o r t h e m a i n l i n eb u t t j o i n t w e l d s .Pipe-to-Pipe Tie-In Q~khore. T h e w e l dp r o c e d u r e u s e d f o r o f f s h o r e t ie - i n s is s i m -i l a r t o th a t u s e d f o r p i p e - t o - p i p e t i e - i n so n s h o r e , w i th t h e e x c e p t i o n t h a t t h e w e l d -i n g p r o c e d u r e i s q u a l i f i e d i n th e 6 G p o s i -

T a b l e 8 - - C r o s s - W e l d T e n s i le T e s t sW e l d P r o c e d u r e U l t i m a t e T e n s il e F r a c t u r e

S t r e n g t h ( N / r a m 2 ) L o c a t i o nP i p e - t o - P i p e ( M a i n l i n e ) 6 0 9, 6 1 7 B a s e B r e a kPipe- to-P ipe (Cut Out) 6 2 8 B ase B rea kR e p a i r t o M a i n l i n e 5 8 9, 5 9 0 B a s e B r e a kP i p e . to - P ip e O f f s h o r e T i e - i n 6 2 1 , 6 2 8 B a s e B r e a k

t io n . D e t a i l s o f t h e w e l d p r o c e d u r e a r eo u t l i n e d i n N l b l e 6 .W e l d M e t a l P r o p e r t i e sChemical A nalyses

C h e m i c a l a n a l y s e s f o r t h e a b o v e w e l dp r o c e d u r e s a r e o u t l i n e d i n T a b l e 7. W i t ht h e e x c e p t io n o f t h e G M A W - P w e l d p r o -

c e d u r e , t h e w e l d m e t a l c o m p o s i t i o n s f o rt h e r e m a i n i n g p r o c e d u r e s a r e s i m i l a r a n dc o n t a i n 1 % n i c k e l .Mechanical and Metallurgical Properties

W e l d p r o c e d u r e m e c h a n i c a l t e s t i n gw a s c a r r i e d o u t i n a c c o r d a n c e w i t hB S 4 5 1 5 - 1 ( R e f . 1 ), p l u s a d d i t i o n a l r e -q u i r e m e n t s s p e c i f i e d b y t h e c l ie n t .

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T a b l e 9 m W e l d M e t a l H a r d n e s s S u r v e y

LocationBaseMetalHAZWeldHAZBaseMetalSpecification

Pipe-to-Pipe (Mainline) Pipe-to-Pipe Cut Out Repair to MainlineTop Middle Bottom Top Middle Bottom Top Middle Bottom188 189 207 188 178 203 201 195 203

Pipe-to-Pipe Offshore Tie-inTop Middle Bottom183 183 215

212 194 219 230 183 205 203 198 201 222 182 215197 219 247 202 188 232 220 213 222 209 215 247216 192 220 206 188 193 203 192 201 206 197 207185 183 203 174 178 195 199 189 206 183 180 206

Weld Metal Root and Mid-Thickness 250 HAZ Root and Mid-Thickness 250Cap 275 Cap 275

T a b l e 1 0 - - S t r a i n - A g e d W e l d M e t a l a n d H e a t - A f f e ct e d Z o n e C h a r p y I m p a c t R e s u l t s a t - 3 0 CPipe-to-Pipe Mainline Repair to Mainline

Location Position Joules Position JoulesWeld Cap 0 deg 73, 74, 72, Av 73 RepairWeld Cap 180 deg 83, 77, 75, Av 78HAZ Cap 0 deg 94, 172, 184, Av 150 RepairHAZ Cap 180 deg 40, 111, 162, Av 104Weld Root 0 deg 81, 80, 80, Av 80 RepairWeld Root 180 deg 79, 87, 79, Av 82HAZ Root 0 deg 88, 129, 168, Av 128 RepairHAZ Root 180 deg 220, 116, 76, Av 137Charpy Impact Dimensions 10 x 10 x2 V

Pipe-to-Pipe Cut Out andPipe-to-Pipe OnshorePosition Joules64, 56, 70, Av 63 0 deg180 deg270, 230, 248, Av 249 0 deg

180 deg48, 64, 50, Av 50 0 deg180 deg210, 190, 250, Av 217 0 deg180 degCharpy Impact Test Temperature -30 C

Pipe-to-Pipe OffshorePosition Joules

62, 67, 97, Av 97 0 deg 131, 136 137 Av 135119, 85, 98, Av 101 90deg 133, 137, 135 Av 13553, 75, 116, Av 81 0deg 188,242,237 Av 222179, 234,228, Av 214 90 deg 20 1,190, 220 Av 20483, 39, 82, Av 68 0deg 115, 119, 95 Av 11057, 60, 48, Av 55 90 deg 126, 119, 108 Av 118227, 254, 247, Av 243 0 deg 163, 184, 242 Av 196194, 224, 202, Av 207 90 deg 22 8,168, 222 Av 206All Charpy Impacts Heat-Treated at100 C for 1 hour after 2.4% Strain

S T R A I N A G E D L O W E R Y I E L DE X T E N S I O N

"E"DOE'O Y /

I N I T I A L

E X T E N S I O N

C H A N G E N U T S D U Eu T O S T R A I N A G I N GZ ~ uvX

AEC H A N G E N E L O N G A T I O ND U E T O S T R A I N A G I N G

S T R A I N

Fig. 3 -- Stress~strain curve shows relation-sh ip be tween s tra in and y ie ld s treng th.

Tensile TestsTensile tests consisti ng of cross-weld

tensile specimens were carried out on allweld procedures. Tensile results variedfrom 589 to 628 N/mm2, depe nding on theweld procedure, which were well abovethe m inim um pipe tensile result of 567N/mm2. Details of the results obtai ned areoutli ned in Table 8. The ten sile specimensall fractured in the base metal.

F ig . 4 - - M i c r o st r uc t u re o f th e A P I 5 L X 6 0s e a m le s s p ip e m e ta l , a u s te n i t i z e d a t 9 2 0C,w a te r q u e n c h e d , a n d t e m p e r e d a t 6 9 0 C.

Hardness SurveyWeld joint hardness surveys were car-ried out at three locations: on the top,middle, and bottom of each macro usinga 10-kg load. The results recorded are out-lined in Table 9. Hard ness results variedfrom 174 to 222 HV, with the exception ofthe following isolated values: Pipe -to-pipe recorded a weld metal roothardness valu e of 247 HV. Pipe-to-pipe cut-out recorded a heat-affected zone (HAZ) value from the tophardne ss value of 230 HV, and a weld

metal value of 232 from the root. Pip e-to-pipe offshore recorded a weld

metal value of 247 HV from the root.The high weld metal root h ardness val-ues recor ded are difficult to explain, sinceone would have expected that the rootarea would have experie nced the effect ofdepositing the hot pass (and subsequentpasses) and where the previously de-posited microstructure would have beenrefined. It would appear that the HAZhardnes s value recorded from the top sur-vey may be due to a reduction of the pre-heat temperature.Strain-Aged Charpy Impacts

Weld metal Charpy impact tests arecarried out in the strain-aged condition tosimulate the effect that reeling on and offthe vessel has on the me chanical proper-ties of the weld metal. Charpy impactspecimens were remov ed from the 0-degand 180-deg positions and tested at -30Cin the strain-aged c ondition. To obtainstrain-aged Charpy specimens involves re-moving a section of the pipe weld fromthe longitudina l direction of the pipe andapplying a 2.4% strain an d aging at 100Cfor one hour. Toughness requirementsspecified were 40 joules minim um aver-age and 30 joules mi nimum individualvalue. Details of the Charpy impact testresults in the st rain-aged cond ition are il-lustrated in Table 10.

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F ig . 5 - - M i c r o s t r u c t u r e s o f t h e w e l d m e t a l s ( a l l p o s i t i o n 5 G ) . A - - T h y s s en K N o v a , A 5 . 2 8E R 8 O S - G ; B - - B o h l e r - B V D 9 0, A 5 . 5 E 9 0 1 8 - G , S M A W ," C - - F i la r c 9 8 S, A 5 . 5 E g O 1 8 G ,S M A W," D - - L i n c o l n O u t e r s h ie l d , A 5 .2 9 E 8 O T , F C A W - G .

T h e m i c r o s t r u c t u r e c o n s i s ts e s s e n t i a ll y o fv e i n s o f p o l y g o n a l f e r r i t e w i t h a n a c i c u l a rf e r r i t e m a t r i x .S u m m a r y

P i p e r e e l i n g i s c o n s i d e r e d o n e o f t h em o s t r e l i a b l e m e t h o d s f o r l a y in g a p i p e l in eo f f s h o r e . A l l w e l d i n g , w i t h t h e e x c e p t i o no f t h e o f f s h o r e t i e - i n s , i s c a r r i e d o u t o n -s h o r e . T h i s p r a c t i c e a l l o w s b e t t e r c o n t r o lo v e r m a t e r i a ls , w e l d i n g , r e p a i r s , a n d w e l dc o a t i n g s .I n o r d e r t o r e d u c e t h e e f f e c t o f s tr a i na g i n g o n r e e l e d p i p e w e l d s , s t e e l m a k e r sa r e n o w m a k i n g a d d i t i o n s o f n io b i u m ,v a n a d i u m , m o l y b d e n u m , o r t i ta n i u m t ob i n d t h e i n t e r s ti t ia l c a r b o n a n d n i t r o g e na t o m s t o f o r m s t a b l e c a r b i d e s , a n d a l u -m i n i u m t o f o r m n i t r i d e s . I t h a s b e e n r e -p o r t e d t h a t s t r a i n - a g e d t r i a ls c a r r i e d o u to n p i p e l i n e s t ee l s m o d i f i e d w i t h t h e a b o v ee l e m e n t s s h o w e d n o s i g ni f ic a n t d e g r a d a -t i o n o f p r o p e r t i e s ( R e f . 5 ) .

AcknowledgmentsT h e a u t h o r w o u l d l ik e to t h a n k D a v i dB a i l l ie fo r c o m m e n t i n g o n t h e p r e p a r a -t i o n o f th i s p a p e r , a n d D r . K . P r o s s e r f o rt h e u s e f u l d i s c u s s i o n o n s t r a i n a g i n g .

Strain AgingS t r a i n a g i n g i s t h e r e s u l t o f a t o m s o fc a r b o n a n d n i t r o g e n d i s s o l v e d in f e r r i te ,w h i c h s e g r e g a t e t o d i s l o c a t io n s a n d l o c kt h e i r m o v e m e n t ( R e f . 3) . T h i s i n c r e a s e st h e s t r e n g t h a n d h a r d n e s s o f a s t e e l b u tr e d u c e s i ts d u c t il i ty . T h e r a t e o f s e g r e g a -t i o n i s c o n t r o l l e d b y t h e f o l l o w i n g e s s e n -

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

a t o m s .I n o r d e r t o p r o v i d e a b r i e f d e s c r i p t i o no f s t r a i n a g i n g , t h e s t r e s s / s t r a i n c u r v e i l -l u s t r a t e d i n F i g . 3 ( R e f . 4 ) i l l u s t r a t e s t h ee f f e c t t h a t s t r a i n i n g h a s o n t h e y i e l ds t r e n g t h . W h e n t h e w e l d s p e c i m e n s a r es t r a i n e d , y i e l d in g ta k e s p l a c e a t p o i n t ' W 'u p p e r y i e l d p o i n t a n d d r o p s d r a m a t i c a l lyt o p o i n t " B " l o w e r y i e l d p o i n t . T h e h o r i -z o n t a l s e c t i o n a f t e r p o i n t B i s k n o w n a st h e " I n i t i a l L o w e r Y i e l d S t r e n g t h " ( R e f .4 ) . S t r a i n i n g o f t h e w e l d s p e c i m e n i s a l -l o w e d t o c o n t i n u e t o p o i n t " C " o n t h es t r e s s / s t ra i n c u r v e " x . " A t t h i s p o i n t , t h et e n s il e s p e c i m e n i s u n l o a d e d a n d a g e d a t1 0 0 C f o r 1 h o u r . W h e n t h e s t r a i n i s r e a p -p l i e d t o t h e w e l d s p e c i m e n , y i e l d i n g r e -t u r n s a n d c o n t i n u e s t o p o i n t " D " o n t h es t r e s s / s t r a i n c u r v e " y . " T h e h o r i z o n t a l p o r -t i o n a f t e r t h e y i e l d p o i n t i s r e f e r r e d t o a s

t h e " s t r a i n - a g e d l o w e r y ie l d e x t e n s i o n ."T h i s i n c r e a s e i n y i e l d s t r e n g t h A y is a n i n -d i c a t i o n t h a t s t r a i n a g i n g h a s o c c u r r e d .H o w e v e r , o t h e r f a c t o r s s u ch a s i n t e r -s t it i a l a t o m s o f c a r b o n a n d n i t r o g e n p l a ys i g n i f i c a n t r o l e s i n t h e s t r a i n a g i n gp r o c e s s . W e l d m e t a l d i s l o c a t i o n i s e f f e c -t i v el y i m m o b i l i z e d d u r i n g c o l d w o r k i n g b yi n t e r s t i t i a l a t o m s o f c a r b o n a n d n i t r o g e na l o n g t h e c o r e s o f t h e d i s l o c a t i o n ( P o i n tB - C ) . F u r t h e r c o l d w o r k r e s u l t s i n th e u n -l o c k i n g o f n e w d i s l o c a t i o n , r e s u l ti n g i nb l o c k i n g a t p o i n t " D . " Y i e l d i n g c o n t i n u e su n t i l n o f u r t h e r d i s l o c a t i o n m o v e m e n t i sa v a i l a b l e . H o w e v e r , i f t h e s t r a i n i s c o n t i n -u e d f r a c t u r e w o u l d r e s u l t .S t r a i n - a g e d t e s t s a r e c a r r i e d o u t o n a l lw e l d p r o c e d u r e s w h e r e t h e s t r a i n r a t e a n da g in g t e m p e r a t u r e c a n v a ry d e p e n d i n g o nt h e d i a m e t e r a n d t h i c k n e ss .I n a d d i t i o n t o t h e a b o v e , t h e s t e e l s u p -p l i e r c a r r i e s o u t s t r a i n - a g e d t e s t s o n p i p em a t e r i a l w h e r e t e s t s p e c i m e n s a r es t r a i n e d a t 5 % a n d a g e d a t 2 50 C .Weld Metal Microstructures

F i g u r e s 4 a n d 5 i l l u s t r a t e t h e p i p e m a -t e r i a l a n d w e l d m e t a l m i c r o s t r u c t u r e s r e -s p e c t iv e l y . T h e p i p e m a t e r i a l w a sp r o c e s s e d t h r o u g h a q u e n c h e d a n d t e m -p e r e d r o u t e , p r o d u c i n g a m i c r o s t r u c t u r ec o n s is t in g o f t e m p e r e d m a r t e n s i t e . A l -t h o u g h w e l d m e t a l s w e r e d e p o s i t e d b y d if -f e r e n t c o n s u m a b l e s a n d p r o c e s s e s , th e i rt h e w e l d m e t a l c o m p o s i t i o n s w e r e s i m i -l a r , w i t h t h e e x c e p t i o n o f th e G M A W - P .

References1. BS 4515-1 2000, Specification for Weldingof Steel Pipelines on L an d a nd Offshore, Part 1:Carbon and Carbon Manganese Steel Pipelines.London : B r i t i sh S tandards .2. A PI 1104, Standard fo r Welding Pipelinesand Related Facilities. W a s h i n g to n , D . C . : A m e r -i c an P e t r o l e u m I n s t i t u t e .3 . B l i c k w e d e , D . J . 1 9 6 9 . S t r a i n a g i n g a n dits effect. Metal Progress July: 91-94.4 . B a i r d , J . D . 1 9 6 3 . S t r a i n a g i n g o f s t e e l -a c r i t ica l rev iew, pa r t 1 : p ra c t ica l a spec ts . Ironand Steel May: 186-192 .5 . E E M U A . 1 9 9 9. Guidelines for MaterialsSelect ion and Corrosion Control for Subsea Oiland Gas Equipment. P u b l i c a t i o n 1 9 4 . L o n d o n :T h e E n g i n e e r i n g E q u i p m e n t a n d M a t e r i a l sU sers A ssoc ia tion .

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