dnv engineering critical assessement

Challenges in Engineering Critical Assessment of CRA Pipelines2nd Welding Technology Asia 2012 Conference17 April 2012

© Det Norske Veritas AS. All rights reserved.

Challenges

Tensile properties of the weld metal and base/parent metal

- As per DNV-OS-F101, the weld metal tensile properties shall over-match

or at least even-match the base metal properties at all conditions and

strain level under consideration.

- May not be the case for CRA pipelines.

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© Det Norske Veritas AS. All rights reserved. 3

Engineering Stress-Strain curves at room temperature

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30 35 40 45 50

Strain (%)

Stre

ss (M

Pa)

BM1, full WT including Clad



PW1, all weld

PW2, all weld

PW3, all weld

RW1, all weld

RW2, all weld

RW3, all weld

Engineering Stress-Strain curves at design temperature

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35 40 45

Strain (%)




PW1, all weld

PW2, all weld

PW3, all weld

RW1, all weld

RW2, all weld

RW3, all weld

Upper bound BM curve used for ECA

Lower bound PW curve used for ECA

lower bound RW curve used for ECA

- Material strength de-rating for

the weld metal is greater than

the parent metal !

- Yield strength of weld

metal is under-matching

the base metal.

- Severe under-matching

at design temperature

(>120°C) !

© Det Norske Veritas AS. All rights reserved. 4

- Selection of weld consumable is

very important.

- Different weld consumable,

different strength de-rating at

high temperature.

True stress strain curves of 622 AUTO Weld Metal used for ECA

0

100

200

300

400

500

600

700

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Strain (%)

Stre

ss (M

Pa)

622 AUTO at 140°C, S3

622 AUTO at 120°C, S2

622 AUTO at 100°C, S1

True stress strain curves of 625 AUTO Weld Metal used for ECA

0

100

200

300

400

500

600

700

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Strain (%)

Stre

ss (M

Pa)

625 AUTO at 140°C, S3

625 AUTO at 120°C, S2

625 AUTO at 100°C, S1


- Testing of 309Mo shop weld.

- At RT, ultimate tensile strength of weld metal is higher than the parent metal >> join transverse tensile specimen shows specimen broke at parent metal.

At DT, the UTS of weld metal is lower than that of the parent metal due to the de-rating of the weld metal strength and for this situation we have serious under-matching and almost no defect can be tolerated for the large strain condition.

True Stress-Strain curves for Crackwise (23°C)

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30 35

Strain (%)

Stre

ss (M

Pa)

BM, upper bound true stress strain curve

WM, lower bound true stress strain curve

True Stress-Strain curves for Crackwise (110°C)

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30

Strain (%)

BM, upper bound true stress strain curve

WM, lower bound true stress strain curve

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Wide Plate Test of 309LMo Weld- Wide plate test was performed by DNV to obtain information about under-

matching weld metal and conservatism inherent in the ECA.- The EDM notch was prepared either at the weld centre line (WCL) or

fusion line/ HAZ location from cap side. The EDM notch size was 7 mm in length and 1 mm in height, estimated on the basis of acceptance criteria of the manual UT.

350 350

140

R=25

250

200 200

Strain gauges (two each side)

EDM notch

100

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Wide Plate Test of 309Mo WeldsEDM Notch

Damaged specimen with notch at weld centre, broken at the notch location. (test temperature: 110°C)

Damaged specimen with notch at weld centre, broken at the notch location. (test temperature: 110°C)

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Damaged specimen WP-6 with notch at weld fusion line,broken in parent (test temperature: 50°C)

EDM Notch

Damaged specimen WP-3 with notch at weld centre, broken in the notch location (test temperature: 70°C)


Wide Plate Test (continued)Due to the good fracture resistance of the weld, the failure mode of the wide plate specimen is plastic collapse of weld metal or parent metal, and the crack ductile tearing is more than 1.0 mm in all specimens.

J R- curves, JSM G7 (Design Temp)

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Stable crack extension, ∆a [mm]

Frac

ture

toug

hnes

s, J

[N/m

m]

G7 JSW WM G7 JSW WM Curve

G7 JSW FL G7 JSW FL Curve

J=980∆a0.75

J=710∆a0.75

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Wide Plate Test (continued)

All the four segment specimens at 110°C broke at the centre of the weld metal independent of the flaw location. Whether at the WCL or at the FL / HAZ, the failure mode of the four specimens was plastic collapse of the weld metal.

The specimens tested at 70°C broke in the weld metal, and the failure mode was the same as for the specimens tested at 110°C. However, the strain increased to 8.1%, which is much higher than for the specimens tested at 110°C i.e. 2.4%.

The specimen tested at 50°C was broken in parent metal and the strain level is up to 10.3% that is close to the parent metal strain corresponding to ultimate strength of parent metal.

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Wide Plate Test (continued)

SS 309LMo tensile properties and fracture resistance properties is very sensitive to the test temperature.

Because of the difference between the de-rating of the weld metal and parent metal strengths at higher temperatures, PQR test must be performed at the design temperature to obtain the actual stress-strain behavior of the weld metal compared to that of the parent metal.

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Challenges (continued)

High Strain, HP/HT (High Pressure High Temperature)

- Effect of temperature on the tensile properties of the materials.

- High strain above yield (from 0.5% up to 2%) especially at buckle trigger

portion.

- According to DNV-OS-F101, combination of internal pressure and

longitudinal load (e.g. bi-axial loading condition) will lead to reduction of

strain capacity.

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- 3D finite element analysis to study the influence of bi-axial loading

condition on the integrity of the pipeline girth welds.

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0

100

200

300

400

500

600

700

800

0.00% 0.50% 1.00% 1.50% 2.00% 2.50% 3.00%Applied strain

J-in

tegr

al (N

/mm

)

BS7910 WMCL BS7910 FL

WMCL without pressure WMCL with pressure

FL without pressure FL with pressure

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ECA results: critical flaw size curve

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Critical Flaw Length [mm]

Crit

ical

Fla

w H

eigh

t [m

m]

PW, Surface Flaw in Field Production Weld Metal

Weld metal:Design Temperature (DT)E = 209000 MPa, Weld Metal, YS = 357 MPa, UT = 589 MPa otal peak strain = 2.2 % + welding strain = 0.5% => Pm = 556 MPa from parent metal1.6 mm Misalignment => SCF = 1.3 =>Pb = 4 MPa Lr cut-off = UT/YS = 1.75Cyclic Stress Range, ∆Pm = 346 MPa for month no. 1, SCF = 1.3 =>Pb = 77.7 MPaStress ranges for other months as per Report No.: 4211136-PL-ET-014, Rev. AFatigue Crack Growth Rate : CP -850 mV Ag/ AgClFatigue life factor (FLF) = 2

Critical Flaw Size Curves for 10" x 20.9 mm WT Pipeline Field Production Girth Welds (i.e. Zone-2)

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ECA results: critical flaw size curve

- Critical flaw size curve which is smaller than workmanship criteria.

- If adopted, high repair rate is anticipated.

- Slow installation is expected.

- High cost!

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Summary

Workmanship weld flaw acceptance criteria may not be always conservative and may not be applicable for all situation as perceived generally.

Tensile testing at design temperatures to check the actual stress strain behaviour.

Proper consideration of parameters used in the ECA in getting workable flaw acceptance criteria.

Validation of ECA results by testing of segment/wide plate specimens.

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