Materials Science Forum Vols. 475-479 (2005) pp. 3323-3326 online at http://www.scientific.net © 2005 Trans Tech Publications, Switzerland

The Prediction of Failure Pressure of Gas Pipeline with Multi Corroded Region

Y.K. Lee1•3

, Y.P. Kim2·b, M.-W. Moon1·c, W.H. Bang1

·d, K.H. Oh1'8

, W.-S. Kim2.t 1School of Materials Science and Engineering, Seoul National University

Seoul151-742, Korea 2Korea Gas Corporation, R&D, Training Center, Korea

8CUitro02@snu.ac.kr, bypkim@kogas.re.kr, cmwmoon@snu.ac.kr, dusk2@snu.ac.kr 8 kyuhwan@snu.ac.kr, 1wskim@kogas.re.kr

Keyword : Corroded pipeline, Finite element simulation, Failure

Abstract. The failure assessment for the defects in multiple corroded region has been considered with the real-size pipeline burst tests and the associated Finite Element Method. The corroded gas pipeline made ofX65 steel with various types of artificially machined pits has been used for a number of series of burst tests on gas pipeline. The failure pressures for the designed multiple corroded defect on gas pipelines have been measured by using in-situ burst testing and classified with respect to each designed defect sizes - length, width and depth - and distance between defects. The result from the experiments has been modeled and compared with Finite Element Method. As the result of FEM simulation, the failure pressure in multiple defects is lower than that in single defect, and closer to failure pressure in single defect of total length of defects in multiple defects as the distance between each defect is closer.

Introduction

Pipelines are the highest capacity and the safest means for gas or oil transmission. But with increasing their age, the probability of gas leaking or bursting of pipeline has increased, it can cause fatal destruction. Failure due to corrosion defects has been major concern in maintaining pipeline integrity. A number of solutions have been developed for the assessment of remaining strength of corroded pipeline. Method for the assessment of corroded pipes were developed and validated based on test result. Since the beginning of the 1990s, the development of new assessment methods have been based on result oflaboratory tests and finite element analysis [1, 2, 3, 4] .

Corrosion occur as individual pits, colonies of pits, general wall-thickness reduction, or in combinations. For the pipe with colonies of pits, as the distance between two corrosion pits decreases, they will begin to interact reducing the burst strength of the pipe. In burst test and according FE simulation, dimension of a pit is idealized to longitudinal length, circumferential length, and depth [ 5] . The effect of this interaction can be considered as a function ofthe dimension of a pit, their separation and gas pressure inside pipeline.

In this work, behavior of single pit, .longitudinally aligned pits, and circumferentially aligned pits has been observed which has been well modeled with FEM. To derive failure criterion on FE simulation, experimental test result has been compared with 3D-FE simulation result.

Experimental procedure

Burst test with real pipeline has been made from API 5L X65, which has been widely used for transmission of natural gas. Each length of test pipe has been 2.3m, outer diameter has been 762mm, and thickness has been 17 .5mm. Both ends of pipe were capped by circumferential welding to enclose pipe. Pits were artificially machined in a rectangular shape on outside of gas pipe. And corner edges

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of pit rounded in radius of 5mm. Experimental test model is classified with the number of pits which are longitudinally aligned in fixed length (300mm). Size of each pit is length 50mm, width 50mm, depth 8.75mm (50% of pipe thickness).

FEM Simulation

3D-elastic plastic finite element simulation has performed. A half or a quarter of full pipeline has been modeled by considering symmetry, and computing procedure has done using commercial finite element program ABAQUS. Since failure of pipe has been observed on defect area on experimental test, sufficiently many elements has been designed around defect area. The true stress-true strain curve was adopted from tensile test result which was performed on the same material as the burst test specimen.

For assessment of damaged pipe, inner burst pressure of damaged pipe in burst test and FE simulation has been discussed. To define burst pressure in FE model, failure criterion has been developed with the consideration oflocal stress of the defect area. When the von Mises stress at area in pits was over the reference stress, failure was assumed to occur at pipe. For determining reference stress, FE model, which has the same pit parameter of experimental model, has been simulated and compared with experimental result, and 90% of ultimate tensile stress has been accepted for reference stress. The feature of defect ofPSA-PSC model is listed in Table. 1. As a reference for the effect of aligned pits, single model of LA (length 50mm), and LD (length 300mm) are compared with multiple pit model. The burst pressure of multiple pits model has been estimated in Fig. 1. As the number of longitudinally aligned pit increase, the burst pressure of pipe has decreased. Burst pressure of multiple pits has been resulted to be in between that of model LA and LD.

Additional FE simulation model having two aligned pit has been performed. PSD series are defect model of longitudinally aligned two pits and PSE series circumferentially aligned two pits. For longitudinally aligned pits, when distance among pits has decrease, stress intensity at pits increased and burst pressure of pipe has decrease. And for circumferentially aligned pits model, there has been shown no significant change of burst pressure for increasing of distance between pits. Behavior of burst pressure of two aligned defects model is shown in Fig. 2

Model

PSA

PSB

PSC

Pit length [mm) Pit width [mm) Pit depth [mm)

50 50 Q5

Table 1. Burst test model of multiple pits model

Alignment of pit

Distance of each pit [mm]

N' 300

5 250 '5, 6200 ~ ~150

33.3 50.0 75.0

I • •

Number of pit

•

5 4

3

• •

(/)

~100 c.. • Experimental I ~ 50

• FEM Sirrulation oo o~~--~~~~~~~

LD PSC PS8 PSA LA (a) (b) (c)

Fig. 1 Picture of failed pipeline at (a) Experimental test and (b) FE simulation. (c) Burst pressure of pipeline

Materials Science Forum Vols. 475-479 3325

I 0 Circumt•ential l ~m • longitudinal

;§. 0

0 0 :g uo 0

• • ~ • "' ,. •• "' ., "' ~leo c 0 >

"' ~ .. " ,.

" " "" w(cm)

(a) (b) (c) Fig. 2 Illustration of(a) longitudinally aligned pits model (PSD) and (b) circumferentially aligned

pits half model (PSE). (c) Burst pressure ofPSD and PSE model

Result and Discussion

For multiple pits model, an equation for capture the global behavior oflongitudinally aligned two pits has been derived. For two longitudinally aligned two pits having length L, as distance between two pits w is too small, it would acts as a single pit of length 2L. When w is too large, it would act as a single pit of length L. The relationship of burst pressure may be expressed fitting procedure of FE simulation result. On comparison with experimental test result, estimation of PCORRC equation proved to be conservative and the closest when using 95% 'ofUTS of tensile test, a •. rest as a •. The

burst pressure of damaged pipe of API 5L X65 with a same equation has been used as such.

Where the value of C is a curve fit constant for capture the global behavior of the defect failure relationship and the value is 0.157 for PCORRC criterion. Eq. 1 can be changed as follows

With the result of FE simulation of this work, fitting procedure ofEq. 1 has been performed for the determination of the C value. When pit depth is constant, behavior of burst pressure with increasing defect length L can generate C value by fitting procedure. The resulted C value varies from 0.142~0.224 with the change of pit depth. However for conservative prediction of damaged pipe, we can choose maximum value of 0.224 as curve fit constant for whole range of depth. Then, limit solution about single pit model is as follows.

And to include single result of length L and 2L, distance w has redefined as w = w I( w +a), where w is re-defined distance between two pits. Parameter a is a constant to normalize w. When w =0, distance of multiple pits is zero, so the burst pressure can be replaced with a single defect having distance of 2L, on the contrary, w = 1, multiple pits model considered having unlimited distance.

If relationship of burst pressure change due to superposition of stress between pits and distance between two pits expressed asf(w ), burst pressure of two pits model can be expressed as follows.

~.Multi= f(w)(~.L - ~.2L) + ~.L

Where, P d, L, P d,2L means burst pressure of single pit model oflength L, and length 2L, respectively. By fitting procedure of FE simulation result, f(w) can be expressed as f(w )=1-Exp(- w ltJ). The behavior of longitudinally aligned pit model is derived as follows,

(1)

(2)

(3)

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~.Multi = (1- Exp( -w' / ti))(~.L- ~.2L) + ~.L

Fig. 3 shows FE simulation result and pridiction by Eq. 4 for two longitudinally aligned pits. When depth of each pit is lower than 62.5%of pipe thickness, equation express the global behavior of burst pressure well.

~, ; . . I 0 0 0 r2so s"" o ___ _ E Q-~- --- --~ _ Q~ ~ 6 ~ 200 j Q "' :_, ~ _.:-:. _ •. - ·· -·-- ·-· ·-· -·

i 150i1: .. ~ ... .. -··- d = 0.3251 c FEM

<n -Eq. 4 ~ 100 d=0.6251 o FEM

Q:: --· Eq.4 d = 0.875 I 6 FEM

-·-- Eq. 4

w'

(a) (b) Fig. 3 (a) Schemetric illustration of reduced PSD model by 4-fold symmetry. (b) Result of FE

simulation and Eq. 4 for increasing modified distance w'

Conclusion

In this paper, a series of burst tests to investigate effect of weld region and multiple pits on burst pressure of pipes and the results has been compared to finite element predictions. For longitudinally aligned pits in fixed area, as number of pits increase, burst pressure has decreased. On further FE simulation for two longitudinally aligned pits, burst pressure of corroded pipeline has been decreased as distance between pits, and expressed as a function of distance between two pits and limit solution for single defect. For circurnferentially aligned two pits, the burst pressure has shown no significant change as distance between of pits change.

Acknowledgements

The authors are grateful for the support provided by a grant from Korea Gas Corporation (KOGAS).

References

[1] B. A. Chouchaoui, R. J . Pick, and D. B. Yost, 11 1h Conference on Offshore Mechanics and Arctic Engineering (OMAE'92), Part A, Pipeline Technology, Vol. 5, (1992) p. 203.

[2] T. A. Bubenik, R. J. Olson, D. R. Stephens, and R. B. Francini, 11th Conference on Offshore Mechanics and Arctic Engineering (OMAE'92), Part A, Pipeline Technology, Vol5, (1992) p. 225.

[3] B. A. Chouchaoui, and R . J. Pick, International Journal of Pressure Vessels and Piping, Vol. 67 (1996)p.17

[4] X. W. Zhao, J. H. Luo, M. Zheng, M. X. Lu, and H. L. Li Metals and Materials International, Vol. 8, No. 5 p. 479

[5] J . B. Choi, B. K. Goo, J. C. Kim, Y. J. Kim, W. S. Kim, International Journal ofPressure Vessel and Piping Vol. 80 (2003) p.121

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