UTC Conference 2004Revolution or evolution? Reliable subsea technology

CONCEQUENCES OF DEFECTS IN STAINLESS STEEL WELDMENTS IN

SUBSEA PIPELINES

byChristian Thaulow, Trond Rogne, Bård Nyhus, Vigdis

Olden, Hans Iver Lange and Erling ØstbyNTNU/SINTEF

Bergen17-18 March 2004

When will cracks become dangerous?Which cracks can be accepted?

*Environmental assisted fracture mechanics*Case examples*Direct calculations

• Materialproperties

• Welding procedure

• Serviceconditions

• Fabrication• Inspection

•Construction• Loadingconditions

GeometryCrack size

K = σ ( π a) FIC

0.5

Fracture Toughness Loading

ENVIRONMENTAL ASSISTED FRACTURE MECHANICS

Effect of test conditions and environment

Effect of*loading*test specimen geometry*constraint*temperature*cathodic protection*water depth/pressure*welding*coating/coating failures

⇓

Slow Strain Rate tensile testingA smooth tensile specimen is tested at low strain rateThe failure criterion is the reduction of area at fracture

85

4 C, -800 mV SCE

4 C, -1050 mV SCE

SideEdgeNotchBend testThe specimen is preloaded to a selected CTOD-valueThe aim is to determine the CTOD threshold for initiation of fracture

Result after 40 hours exposureat -1050mV SCE

Preloaded to CTOD=0.079

RES

ISTA

NC

E [

J,C

TOD

]

CONSTRAINT [Q,T,M]

SENB (a/W = 0.5)

SENT (a/W=0.2)

SENB (a/W = 0.2)

FRACTURE TOUGHNESSMATERIALS RESISTANCE DEPENDS

ON SPECIMEN GEOMETRY

Relevant test conditions

SENT testingThe testing is performed at selected rates of loadingThe aim is to determine the ”resistance-curve”, CTOD-∆a

Fracture mechanics tests on 13Cr welds, FL

y = 0,31x0,66

y = 1,47x0,78

y = 0,03x0,14

0,01

0,1

1

10

0 0,5 1 1,5 2 2,5 3∆a [mm]

CT

OD

[mm

]

Normal SENT test in airSSR SENT, CP -1050mVConst.load SENT, CP -1050mV

13% Cr Superduplex Superduplex WM

Case: Subsea bimaterial joint

Kl.9

KL09:50 KL.11

KL.11:45

KL.12

0.85X

Through thickness fracture

The fracture has initiatedat 12 o’clock at the weld toe,and propagatedstepwise as a 5-10 mm surfacecrack along the HAZ tothe 9 o’clock position. The final through thickness crack wasa cleavage fracture

123

Phase III

Phase II

WM

Phase I

Phase II

Phase III

Initiation 12 o’clock, Phase I WM

Engineering Critical Assessment (ECA)-BS 7910-CrackWise

Elliptical cracks with length and depth close to the size of the actual crackswere evaluated:Phase 1 (crack length 2c=35mm, crack depth a=9mm) Phase 2 (crack length 2c=230mm, crack depth a=6 -10mm)

The highest tensile stresses were 323MPa (shut in), 314MPa (operation) and 287MPa (pressure testing)

Residual stresses were set equal to yield stress, and reduced in accordancewith BS 7910 for increasing load

ECA resultsPhase 1 min CTOD>0.15mm to avoid fracturePhase 2 min CTOD>0.2-0.6mm to avoid fracture

Case REELING:Acceptable critical crack size calculatedaccording to BS7910-CrackWise

• For reeling operations the acceptance criteria for flaws are usually set at max 2x50 mm for an embedded defect (NDT accuracy criteria)

• With lower bound CTOD=0.015mm (operationphase) and surface crack:– critical crack size of 1.6x50 mm with residual

stresses equal the yield stress and with noexternal loading

– critical crack size of 0.9x50 mm with an externalload of 220 MPa

Questions

• 1.Is the calculation procedure (BS 7910-CrackWise) overdue conservative?

• Need for reliable and more accurateprocedures...

• 2.Is the measured fracture toughness toolow and not representative?

• Need for relevant test methods/procedures...

ENVIRONMENTAL ASSISTED FRACTURE MECHANICS

TEST METHODS/PROCEDURESParameters*Loading: static, pre-loading, rate ofdeformation, cyclic local, plasticdeformation*Test specimen geometry: tensile, ”horse shoe”, SENB, SENT, CT...

*Constraint: smooth, notch, crack*Temperature: ambient, 4°C,-2°C*Cathodic protection: CP potential -1050mV, -800mV...*Water depth/pressure: 1atm, 100atm, 200atm.......*Welding: residual stresses, mismatch, misalignment*Coating/coating failures

3D FE

calculations

Shell elements FE

calculations

with line spring

Analytical

equations

(CrackWise)

Acc

urac

y

Costs

Accuracy

Cos

ts

Analytical+high speed, standarised-low accuracy, limited approach, must be skilled

3D+high accuracy, realisticresults-very time consuming, must be an expert

LINKpipe

3D with crack

Line Springsconnecting theshell elements

Shell element with linespring

3D model

Shell-model+linespring

a

2c

Calculation time3D 60.000 sec (cpu)LINKpipe 100 sec (cpu)

LINKPIPE FEATURES

1.The calculations are performed directly on the structurewith realistic:-size and location of defect-geometry-tensile/bending loading-internal/external pressure

2.LINKPIPE combines structural analysis (plastic collapse, buckling) and local defect analysis (fracture)

3.LINKPIPE is merging the competence of materials- and structural engineering in a user-friendly way. No in-depth expert knowledge is needed

4.The speed of calculation is very high, close to real time.The defect size and dimensions of the structure can be changedwithin a few seconds

LINKPIPE FEATURES....

5.LINKPIPE opens up for statistical evaluations

-no analytical equations are needed-scatter in material data, loads, dimensions etccan be examined-sensitivity analysis-Monte Carlo simulations-partial safety factors for a given system or project

OPTIMISATION OF COST AND SAFETY

6. Real time processing of data opens up for ”eField” applications