PSI - Issue 22

Yaorong Feng et al. / Procedia Structural Integrity 22 (2019) 219–228 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3.5. Effect of crack propagation length and crack arrest probability on the prediction results of crack arrest toughness The allowable crack propagation length is different, and the requirement of crack arrest toughness is also different. In general, the shorter the acceptable crack propagation length, the higher the required crack arrest toughness. The probability of crack arrest is related to the required value of crack arrest toughness and the distribution of toughness (mean and standard deviation) of the supplied steel pipe. The higher the required crack arrest toughness, the larger the mean value of the toughness distribution and the smaller the standard deviation, the greater the probability of crack arrest (as shown in Figure 4).

Dividing line

0.2

Toughness distribution

Propagation zone

0.1

arrest zone

0

Toughness distribution

Charpy impact energy

Fig. 4 Effect of crack arrest toughness and distribution on the probability of crack arrest in gas pipeline Determination Based on EPRG database, TGRI proposes that the average minimum C V for crack arrest is 220 J and 170J for a single minimum C V in the 2 nd WEGP. If the average value of steel pipe supply is 220J and the standard deviation is less than 55J, the probability of crack arrest in two pipes (i.e. within 12 meters of unilateral propagation) is 95%, and in three pipes (i.e., within 18 meters of unilateral propagation) the probability of crack arrest is 99%. For example, the correction coefficients of 1.71 and 1.43 are taken for the class 1 area of the 2 nd WEGP conveying Tarim Gas, respectively. Under the condition of unilateral crack arrest within 18 meters, the probability of crack arrest is 99% when the correction coefficient is 1.71 and 95% when the correction coefficient is 1.43. 3.6. Determination of Toughness Test Temperature For the buried pipeline of the 2 nd WEGP, the minimum design temperature is not less than 0 ℃ . DWTT is mainly used to prevent brittle fracture of steel pipe at the lowest operating temperature, so the test temperature is 0 ℃ . For Charpy test, because of the size effect, the test temperature is reduced by 10 C, that is - 10 ℃ . 3.7. Calculation results of crack toughness of the 2 nd WEGP For Tarim Gas, the predicted value of BTC is 126J, and if 1.71 correction coefficient is used, the average minimum 220J is required for crack arrest impact energy after correction. For the transportation of Central Asian natural gas, the methane content decreases relative to Tarim gas, but the heavy hydrocarbon content increases, which has exceeded the lean gas category and belongs to the neutral gas. According to the previous analysis, the pressure and temperature at the outlet are the highest, and the required crack arrest toughness is the highest. According to the data provided by the Joint Design Body of the 2 nd WEGP, the minimum temperature of 15 ℃ for the pipeline is calculated by reference to the Central Asian Pipeline and Alliance Pipeline. By using the BTC method, the transmission pressure is calculated as 12MPa and 10MPa respectively, and the requirement of crack arrest impact energy in the class Ⅰ area of the 2 nd WEGP are shown in table 2 and table 3 respectively.