Examination of 13%Cr flowlines in rledninger... · PDF fileTitle of presentation in...

Examination of 13%Cr

flowlines in service.

P-til 9 dec. 2009

2Title of presentation in Header and Footer

Subjects

► HISC sensitivity

► NDT Techniques for Inspection of Flowlines in Service

• During start up of one of the flowlines at the Aasgard field in year 2001 a leak was

detected at one of the anode pads. That gave uncertainty for safety of the remaining

flowlines and an initiative for an inspection campaign was taken by the Aasgard

management.

Weld toe

initiation

Macro fracture

IC fracture

Crack detection

K-101


13Cr Flowline history

► Gullfaks Satellites was the first project where Statoil used 13%Cr steel for

flowlines. These pipes were laid in the summer 1997 without any difficulties and

have been safely in operation since

► During laying of the first flowlines at Åsgard in Mars/April 1998 the first incident

due to hydrogen delayed cracking arose. A crack occurred in the HAZ for one of

the tie-in welds about one hour after reeling on the vessel drum.



► One of the flowlines installed at Åsgard in year 2000 was at the seabed

for some months before it could be set into production.

► That pipe was pressure tested several times due to leaks at flanges and

necessary repairs.

► After successful repair (at the flanges), a leak was observed when the

flowline should be set into production.

Fracture position and appearance



► It was at that time unknown that cathodic

protection could cause a hydrogen level in

stainless steels as duplex and 13%Cr

sufficient to initiate cracks (HISC) under

normal service operation!

► As the incidents occurred for loads below the

allowed design stresses, an increased

uncertainty for the flowlines quality arose. In

2004/05 it was therefore decided to perform an

UT-inspection of nearly all flowlines laid in the

second laying phase at the Åsgard field.

► The UT examination was done by means of a

cable based crawler with a head consisting of

TOFD probes.


Calibration of the UT tool

►Qualification of the NDT tool was

performed by means of spark eroded

defects in different position around

anode pads.

► The correlation found between the

spark eroded defects and the TOFD

measurements were excellent with

height accuracy within ± 0,5mm and

length accuracy within ± 10mm.

V

2

V

3

3

3

3

4V

1

3

234mm

14mm

238mm

1 2

1 214 2&3


Spark eroded defects

for calibration.


Calibration results

Flange Pad

Weld

No.

*Axial

Position

Length

Toe/Toe

Defect

No. **

Depth

mm

Length

mm

Adjacent PE

Thickness

Defect

Height

Clock

Pos.

File Name

Depth / Length (if different)

1 0

1.1 197 1-U 14.5 10 15.5 1.0 12:00 SCAN_13_75mm_TOFD4.scn

1.2 296 99 1-D 14.4 49 15.6 1.2 SCAN_10mm_TOFD4.scn

2.1 437 2-U 15.0 28 15.9 0.9 SCAN_22_5mm_TOFD2.scn

2.2 531 94 2-D 14.8 43 16.0 1.2 SCAN_20mm_TOFD2.scn

2 728

3.1 926 N/A N/A N/A 01:30

3.2 1020 94 3-D 14.9 30 15.9 1.0 SCAN_17_5mm_TOFD4.scn /

SCAN_17_5mm_TOFD2.scn

4.1 1163 4-U 14.2 25 15.9 1.7 07:30 SCAN_25mm_TOFD4.scn /

SCAN_25mm_TOFD1.scn

4.2 1260 97 4-D 14.1 48 16.1 2.0 SCAN_25mm_TOFD4.scn /

SCAN_20mm_TOFD1.scn

3 1453

5.1 1652 5-U 13.7 31 15.7 2.0 03:00 SCAN_17_5mm_TOFD2.scn /


5.2 1751 99 5-D 13.9 40 15.9 2.0 SCAN_20mm_TOFD2.scn /

6.1 1889 6-U 13.0 34 16.0 3.0 09:00 SCAN_22_5mm_TOFD4.scn /


6.2 1988 99 6-D 12.9 52 15.9 3.0 SCAN_22_5mm_TOFD4.scn /


Deviation Spark-UT measures: Green: L ±10%, H ±0,5mm. Yellow: L >±10%


Offshore inspection results.

► Shortly summarized, the offshore UT inspection detected minor defects at

more than 50% of all inspected anode pads. (Shown in a later view)

► However, the most demanding defects were observed around the spare

electrical heating connection plates.

• As some of these defects were reported to run 360° around the pipe, a guillotine fracture

become possible, close to the Åsgard FPSO!


UT inspection results, pads

Line Year Inspected Cat 1

<2mm

deep

Cat 2

(<25%WT)

Cat 3

(<50%WT)

Cat 4

(>50%WT)

J-101 2005 112 18 0 2

J-102 2005 110 11 0 0

N-101 2005 108 6 1 0

N-102 2005 106 1 0 2

H-101 2005 83 5 0 0

H-102 2005 85 12 1 0

K-101 re-inspection 2005 12 2 0 0

M-101 2005 62 7 0 0

AB-101 2005 55 0 0 0

J-101 re-inspection 113 18 0 2 re-insp

J-102 re-inspection 2006 110 15 0 0

P-101 2006 90 12 0 0

P-102 2006 96 6 0 0

Total number of indications 920 83 2 4


UT inspection results, external DEH connection plates

Pipeline Ext. Features

Nos

Defect size

EF1

Defect size

EF2

B-scan

performed

Defect

propagation

J-101 2 1,3mm 2,0mm Yes 360°

J-102 1 1,1mm NA Yes 360°

H-101 2 0,5mm 0,6mm Yes 360°

H-101 2 1,7mm 1,5mm Yes Partly

N-101 2 1,7mm 3,0mm Partly

N-102 2 1,8mm 1,4mm Partly


Onshore studies

►As the offshore situation was confusing it become necessary to establish a

better correlation between the pipes service conditions and life time

assuming a crack was initiated.

►An important observation is that only six defects with height of more than

50% of the wall thickness were observed.

►Additionally, the re-inspection of the J-101 performed after a time lapse of

about 10 months did not give any crack propagation!

• That was unexpected, and caused doubt about the UT-TOFD accuracy.

►A research programme was therefore established to study the effect of

stress alteration, pressure and temperature on life time and crack growth.

► The R&D programme should therefore re-qualify the UT (TOFD) technique

used offshore.

• That should be performed by means of fatigue initiated defects, believed to be more

realistic than the spark eroded failure.


Onshore studies

► The R&D programme was therefore divided into different phases.

• Welding of test pipes

• Initiation of minor fatigue cracks

• NDT test spool trials

• Fatigue testing.

►Retrieving of different pipe segment from flowline J-101 where the

inspection had given more than 100 defect indications.

• Metallographic examination both for the retrieved pipe segments and the test pipes.


Onshore studies

► Welding of test pipes

• Welding of pads and external features were performed using the same WPS as used for

through the project.

– However, the weld bead sequence were logged, as it could influence the HAZ mechanical properties,

through sensitization.

Sensitiv?

Ref. JFE.


Initiation of minor fatigue cracks

► To establish realistic defects for recheck of the UT system 6 m long pipes were

loaded in a resonance machine at NTNU (Technical University in Trondheim)

until cracks were initiated at the fillet weld toes, aiming 0,5-1,0mm deep.

► These test pipes should later be used for crack growth analyses under cathodic

protection and different service conditions.


Typical set-up parameters for crack initiation.

Crack initiation parameters:

•f ≤ 44Hz

•R = -1; Δσ≤260MPa

Dye pen for detection

of cracks


Initiation of minor fatigue cracks

► After a few initial trials we succeeded in making shallow fatigue cracks.


Spool set-up for UT (TOFD) inspection

NDT test spool trialsTEST-SPOOL

Launcher

Pipe: L3 Pipe: L4 Pipe: L5 Pipe: L6 Pipe: L1 Pipe: L2 Pipe: L7

Ring-Weld type: Ring-Weld type: Ring-Weld type: Ring-Weld type: Ring-Weld type: Ring-Weld type: Ring-Weld type:

Ikke slipt Ikke slipt Ikke slipt Ikke slipt Ikke slipt Ikke slipt Slipt

Sensitiv sveis Sensitiv sveis Sveist Sveist Sensitiv sveis Sensitiv sveis Sveist

Total lengde på test-rør ( utenom launcher/reciever ) : 61m

Pipe (1m) from Åsgard R101 - DJ 12177 - Pipe 24768.

Cut from ML Weld no.: W2.24 with anode pad / bracket

4 3 2-2 2-1 1-2 1-1

Pipe 2 ( med pad )

Pipe / Pad from Statoil Sensitiv sveis AGR Test-pipe


Reported defects size

EF No. Indication length

(mm)

Indication height

(mm)

No of indications

1 8-142 0,9-4,0 11

2 12-112 0,8-3,2 4

3 <10-54 1,3-2,4 6

4 13-92 0,8-5,6 8

5 10-75 0,9-1,8 6

6 15-150 0,9-2,1 6

7 17-118 0,9-1,5 8


Typical fatigue defects


Spare DEH cable plate, connection to pipes

~59 mm

L7-1 L7-2

No cracks at +25 mm

Weldment L7-1 and L7-2;

No crack findings (some

porosity)

Indicated defect in this

position.


Examples for accuracy for defect detection.

322 mm 276 mm 240 mm

220 mmTOFD max depth of 1.5 mm at 262 mm

195276

Puls Eccho

DP

258268TOFD

322 299


Conclusion from onshore analyses.

TOFD results Puse echo results

In practice no defect less than 2mm in depth is reliable

Estimated maximum crack depth (mm) Estimated maximum crack depth (mm)


Fatigue testing

► Full scale fatigue jig set up.


Examples for the fatigue test results

As expected increased

temperature reduce the

HISC effect


Load frequency effect

The Hydrogen effect will depend of the

Load frequency. However,

test results indicate a “low” effect

for the design life calculation.

Frequency reduction of 1/100 increase

Crack growth speed with a factor ten(?)


Results for retrieved pads and EFs

TOFD Pulse

Depth Length

No

.

Anode

pad

P.Side T.side P.side T.side

Depth

Comments

2 HP I 3 HP II

34A 1,9 2,5 33 62 0 4

34B 2,1 2,3 61 68 0 35A 2,6 2,0 68 61 0 5

35B 1,3 0,8 58 48 0 36A 2,7 2,1 60 58 0 6

36B 1,2 1,7 47 67 0 41A 0,9 1,2 44 35 0 7

41B 1,8 1,5 63 47 0 EF 0,7 2,0 800 800 0 57A 0,7 0,7 36 20 0 57B 2,0 1,1 54 68 0 58A 1,0 1,6 8 26 0 58B 1,0 1,0 4 5 0 59A 1,4 1,3 15 61 0 59B 1,2 1,0 39 58 0 60A 1,1 1,9 63 9 0 60B 1,2 1,5 41 55 0

8

CC 1,75 228 0

9 CC at-

temp.

end

1,85 215 0

Values most probably from

the outside of the weld rather

than the pipewall

1,1mm deep crack

Only one defect observe on macros!


Summary (1)

► Based on the study presented and additional information from other studies the

life time calculation is shown below.


Summary (2)

► Until to day after 9-11 years in service no leak have been observed, and through a

normal yearly inspection programme the probability for a leak is negligible.

► The study has also shown that a brittle guillotine fracture is unlikely, due to an

uneven hydrogen profile through the pipe wall. In case a crack will propagate

through the pipe wall, a minor leak will arise before a full rupture can occur.

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