Evaluation system for corrosion defects in pipelines

1. Evaluation system for corrosion defects in pipelines Dr. Gyöngyvér B. Lenkey, Dr. László Tóth, Zsolt Balogh

2. Objectives of the work • Evaluation of the applicability of FEM for predicting the failure pressure and the safe operation pressure for corroded pipelines • Development of safety diagrams • Development of evaluation system for corrosion defects

3. Previous projects • Development of FEM model with real defect geometry • Development of simplified defect geometries and comparative assessment • Comparison of FEM results with pressure tests and with engineering methods • Development of failure criteria for failure pressure

4. Mapping the real 3D defect geometry Laser distance measurment Sample (negative)

5. Boundary conditions for FEM model Modelling the pressure test: • Quarter modell • Increasing internal pressure • Increasing axial tension (proportional with the pressure)

6. Parameters of FEM calculations • Elastic-plastic material law (determined from tensile tests, ReH=350 MPa, Rm=480 MPa) • Von-Mises yield criteria, isotropic hardening • Large deformation option

7. Transfer the real defect geometry into the FEM modell

8. Development of simplified defect geometries Parabolic modell Rectangular modell 6th order surface modell

9. Predicting the failure pressure

10. Validation of failure criterion and applicability of simplified geometries s Criterion: = R ' eq m 25 20 15 Pressure, MPa 10 Real defect Rectengular modell 5 Parabolic modell 0 Measured failure pres. 0 1 2 3 4 5 6 7 6th order modell Defect depth, mm

11. Comparison of measured and predicted failure pressure values – with engineering methods Pressure, MPa Defect depth, mm Meas. fail. pres.

12. Objectives of the present project • Performing large number of FEM calculations with simplified defect geometry (parabolic) • Development of safety diagrams and defect evaluation system

13. Basic data for the FEM calculations • Different pipe geometry (diameter, wall thickness) • Different materials • Different defect sizes (d, L, b)

14. Definition of critical pressure values (pys, pyf, pF)

15. Development of safety diagrams • Different representation possibilities (as a function of L/D, d/t, L or d) • E.g. normalisation of critical pressure values: • for pF-flawless=2.Rm'.t/(D-t), • 1.      norm - pys= pys/ pF- flawless, • 2.      norm - pyf= pyf/ pF- flawless, • 3.      norm - pF= pF/ pF- flawless, • 4.      norm - pü= pü/ pF- flawless.

16. Normalised pressure values vs. L/D –comparison with the operation pressure

17. Final safety diagrams For Pys

18. For Pyf

19. For PF

20. Definition of safety factors • For the critical pressure values: • n1=Pys/Pü for the beginning of plastic deformation • n2=Pyf/Pü for the localisation of the plastic deformation (contraction) • n3=PF/Pü for the failure (plastic instability) • Operational safety? – combination of n1, n2, n3 – application possibility of risk based approaches – owner’s responsibility!

21. Summary and conclusion • FEM calculations gave more accurate prediction for the failure pressure than engineering methods. • The predicted failure pressure (based on FEM calculations) were in good agreement with the pressure test results. • Simplified defect geometries could be applied for predicting the failure pressure, so it gives opportunity to perform large number of FEM calculations and development of safety diagrams. • With the application of the safety diagrams a proper safety evaluation system can be developed together with the owner. • Possibility for more complex safety assessment system and application of risk based principles.

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