Remaining Life of a Pipeline

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This relationship is particularly appropriated for the carbon steels commonly used in pipelines By substitution of equation (vii) into equation (vi), we get the following expression: d 1 T Sp strength ( Sy 68.95) . (viii) d 1 ( TM . ) • Consideration #3 The folias factor (M) is the value, which account for the stress concentration around the defect. There have been developed several expressions for the folias factor, but the last, more exact and less conservative approximation of “M” was proposed by Kiefner and Vieth (references 7 and 8), and it is expressed as follow: where: M 1 0.6275. L2 DT . 0.003375. L 4 D 2 . T 2 (ix) T=wall thickness L=defect length D=pipe diameter By substitution of equation (ix) into equation (viii), we get the following expression: Sp strength ( Sy 68.95) . 1 d 1 T d T. 1 0.6275. L2 DT . 0.003375. L 4 D 2 . T 2 (x) Equation (x) express the pipeline strength (Sp strength ) taking into account the stress concentration around the corrosion defects, therefore it can be substituited into equation (ii) to form the failure pressure model presented by M. Ahammed, which can be express as follow: pf 2. T ( Sy 68.95) . D . 1 d 1 T d T. 1 0.6275. L2 DT . 0.003375. L 4 D 2 . T 2 (xi) 7
3.2. Corrosion Model Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. The major classifications of corrosion given by Fontana ( Reference # 9) are: Erosion corrosion Crevice corrosion Galvanic corrosion Stress corrosion cracking Uniform attack Selective leaching Pitting corrosion Intergranular corrosion Cavitation Corrosion is such a complex failure mechanism, which can damage a metal surface in many different ways. The work developed by M. Ahammed is particularly concern with those types of corrosion which produce macroscopic defects due to material losses, because these defects results in the reduction of the metal cross section with its correspondent reduction of the pipeline strength (Sp strength . Such corrosion types are erosion, pitting corrosion and uniform attack. This approach is not because of simplicity, but because of the experience in analyzing pipeline failures, which has shown that for most of the common industrial applications, pipeline failures aren’t due to other types of corrosion than the ones mentioned. Nevertheless, for complex applications, as nuclear applications or operating conditions with extremely high stress solicitude, a more extensive analysis is required. The Ahammed’s considerations about corrosion are focused in the growth rate of macro-defects. The growth of macro defects is directly related with the exposure period and depends on the characteristics of the pipeline material, properties of the fluid being transported and the surrounding environment. It has been found that this rate is high during an initial period and then gradually decrease to finally reach a steady-state rate. The researches cited by Ahammed (Southwell et al. Reference # 10) shown that the initial period of relatively high corrosion rate, (about a year in average), is not of much concern because during this period the defects are usually small and hence do not pose much threat to pipeline integrity. As the exposure period increases, the growth rate decreases, but the overall size of corrosion defect increases and becomes a greater risk for the pipeline integrity. By this time, the steady-state growth rate is a good approximation. 8
Page 1 and 2: ANALYSIS OF THE PAPER: PROBABILISTI
Page 3 and 4: Table of Content 1.- Summary ......
Page 5 and 6: see apendix # 2), which is resumed
Page 7: From the above consideration, it is
Page 11 and 12: andom variable with its own degree
Page 13 and 14: C 2 d dRd 2 . T ( Sy 68.95 ) . D .
Page 15 and 16: confidence of our estimation. In se
Page 17 and 18: Sensitivity Analysis Contribution o
Page 19 and 20: of the remaining strength (Pf) give
Page 21 and 22: 1.- Montecarlo Simulation Strength
Page 23 and 24: 2.- Simulation Stress (Pa) Pa rnorm
Page 25 and 26: 4.- Determination of the failure st
Page 27 and 28: 6.- Confidence Intervals Estimation
Page 29 and 30: estimate the radial rate of corrosi
Page 31 and 32: (10) Southwell CR, Bultman JD, Alex
Page 33 and 34: general behavior of the wall thickn
Page 35 and 36: Maximun Likelihood parameters estim
Page 37: υ Sol 1, 0 j 20.. 10000 Es( j) Eo.

Remaining Life of a Pipeline

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