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
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