Inclusion Of Crack Face Pressure - 2H Offshore

Inclusion Of Crack Face Pressure In Reference Stress For Thick Walled Risers And Flowlines

OMAE2013-11021

12 June 2012

Mark Cerkovnik -2H Offshore

Learn more at www.2hoffshore.com

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Outline

Background and Motivation

Where this fits in the general FFS problem

Reference Stress Method for Fracture of Ductile Steel

Method for Accounting for Crack Face Pressure:

Results for a Practical Application


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The evaluation of defects in fatigue loaded structure

Risers and other parts of subsea architecture face the double threats of fatigue and corrosion

Corrosion pits in fatigue loaded risers are areas of stress concentration and hydrogen charging can increase crack growth rates and lower toughness.

Irregular morphology of the pits makes calculation of the SCFs problematic.

Production Risers are subject to wellhead pressure which can be quite high .


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Fitness for Service Methods for Evaluating Remaining Fatigue Life

SN Method

SN curves in corrosive environments are difficult to establish because of frequency effects

SCFs depend upon a good measurement of the radius of curvature which is nearly impossible to obtain.

Load histograms

Fatigue Crack Growth Method (FCG)

Requires an initial flaw size

Requires knowledge of crack growth rates in the corrosion regime

Required knowledge of toughness

Load histograms

Extreme load to evaluate the possibility of unstable fracture or plastic collapse.

BS7910 or API 579


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Use of FEA to refine stress intensity solutions

In applying the FCG method the pit can be treated as a planar flaw.

Less conservatively FEA can model a very small crack at the bottom of the pit.

Depending upon the pit shape this can make a small or larger difference in the computed stress intensity


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Failure Assessment Diagram

Fracture and plastic collapse is judged using the FAD approach

𝐾𝑟 =𝐾

𝐾𝑚𝑎𝑡

𝐿𝑟 =𝜎𝑟𝑒𝑓

𝜎𝑦

yL

y

refaP

P

,

L

yref

PP

PP

21

21

,

,


Non Linear Response of CTODf at high plasticity

The shape of the FAD reflects the non linear increase in crack tip opening as the plastic region ahead of the crack expands.

The behavior is very dependent upon the stress strain curve and particularly the hardening exponent.

Typically seamless pipe, is different than DSAW pipe and the weld metal and heat affected zones also have their own characteristics.

* [1] Kim, et al, Elasto Plastic Estimates Of J and CTOD for axial through wall cracked pipe… Learn more at www.2hoffshore.com

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Reference Stress Approaches


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Thick Pipe and Crack Face Pressure

High Pressure 70 MPa to 100 MPa

Corrosion allowance 4 mm to 8mm

Result is thick pipe with D/t of 6 to 8.

No BS 7910 solutions for CFP for K or reference stress

API 579 has CFP K solution but ignores CFP reference stress.

Solution is needed to fill the

gap.


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Stress caused by crack face pressure for internal surface flaws

Pressure is applied to crack face in FEA model. Reference stress component is taken as the stress normal to the crack, half way between the crack tip and the outer surface.

FEA is compared against closed form solutions

𝜎𝑟𝑒𝑓 =𝑃𝑎

𝑡

𝜎𝑟𝑒𝑓 =𝑃𝑎

𝑡−𝑎

a is the crack depth

Far field stress (as calculated by classical equations)


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y = 4.389x3 - 3.0481x2 + 1.4422x - 0.0579 (Equation 8)R² = 0.9991

y = 5.7461x3 - 3.8116x2 + 1.7206x - 0.0823 (Equation 7)R² = 0.9997

0.00

0.20

0.40

0.60

0.80

1.00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

No

rma

lize

d A

ve

rag

e N

et

Se

cti

on

Str

ess

Crack Depth/Wall Thickness, a/t

FEA Results for Internal Flaws (Circumferential and Axial Orientation)Average Net Section Stress due to Crack Face Pressure

FEA, Circ. Flaw FEA, Axial Flaw


0

5

10

15

20

25

30

35

10 210 410 610 810 1010 1210 1410 1610

Fla

w D

ep

th (

mm

)

Pit Length (mm)

UNSTABLE FRACTURE LIMIT (WITH AND WITHOUT CRACK FACE PRESSURE)Pipe OD = 323.85 mm, Wall Thickness = 40 mm,

Pm = 325 MPa, Pb = -55 MPa, Based On API-579-FFS

No Crack Face Pressure With Crack Face PressureLearn more at www.2hoffshore.com

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Conclusions

The crack face pressure has a significant effect in the evaluation of flaws for FFS and for ECA in thick walled risers and flowlines in high pressure service.

API gives good guidance for accounting for CFP in calculation of K for thick pipes; BS7910 does not.

Neither code addresses the effect of crack face pressure directly in the determination of reference stress.

A formulation for reference stress due to CFP must account for the proximity of the load to the ligament.

As a practical matter, the local ligament stress formulations should provide sufficient conservatism to cover the effects of crack face pressure.

A rational way to estimate the effect of crack face pressure on the reference stress is developed in this paper.


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