PSI - Issue 59

Roman Hrabovskyy et al. / Procedia Structural Integrity 59 (2024) 112–119 Roman Hrabovskyy et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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The data in Table 1 characterize the fracture conditions of gas pipelines made of 17H1S and 10H2BТ steels, which were operated at the maximum working gas pressure, respectively p max =5.0 MPa and p max =7.4 MPa, that is, under the given circumstances, the working pressure is destructive. It should also be taken into account that when assessing the defect depth influence, the norm stipulated in the Standard (N B V.2.3.-21:2008, 2008) was used, according to which the pipe of the main gas pipeline must be rejected if the depth of the crack-like defect is a = 0.8 t , where t is the thickness of the pipe wall. The analysis of the obtained results (Table 1) confirms that the depths of crack-like defects such as corrosion pit and corrosion line reach critical values under the condition a cr < 0.8 t , regardless of the operation time. At the same time, when a semi-elliptical defect of the corrosion pit type reaches a critical depth ( a / t ) cr , the formation of a fistula (through hole) is possible in gas pipelines. In this case, the catastrophic destruction of the pipe will not occur ( Kryzhanivs’kyi et al. (2013)), since the length of the surface crack in this case satisfies the condition 2 c < 70 mm for the pipe D = 1 220 mm and the condition 2 c < 90 mm for the pipe D = 1 420 mm (Table 1). When semi-elliptical defect of the corrosion line type reaches a critical depth ( a / t ) cr , gas pipelines may be destroyed over the full cross-section (Kryzhan ivs’kyi et al. (2013)) , since the length of the surface crack for the considered pipes significantly exceeds the given conditions for the fracture impossibility of the considered pipes. An alternative estimation of fracture conditions of the investigated gas pipe was carried out using the engineering criterion for assessing the strength of the material of the gas pipe, namely its ultimate strength ( σ UTS ), which was determined experimentally. During experimental research, load forces were recorded using the data of the machine's built in dynamometer, and elongation recorded from the strain gauge extensometer using a PC. A set of ten samples was tested and the average values of yield strength ( σ YS ) and ultimate strength ( σ UTS ) were determined (according to ISO 80000-1, the results in MPa were rounded to the nearest integer), as well as the strain ( ε ) after rupture was recorded (according to ISO 80000-1, the results are given with an error of up to 0.5%). The data of experimental studies on the tension of cylindrical samples made of steel of the main gas pipeline pipes under study are given in Tables 2 and 3.

Table 2. Change of 17H1S and 10H2BТ steels strength under the long-time operation.

Operation time, years

σ UTS , М P а

σ YS , М P а

σ UTS , МPа

σ YS , МPа

Operation time, years

Steel

Steel

σ YS /σ UTS

σ YS /σ UTS

0

578 565 525

365 360 330

0.63 0.64 0.63

0

618 602 581

483 471 451

0.78 0.78 0.78

17H1S

30 45

28 42

10H2BТ

Table 3. Change of mechanical characteristics of plasticity of 17Г1С та 10Г2БТ steels from the main pipeline under the long -time operation.

Operation time, years

Operation time, years

ε, %

Δε, %

ψ, % 78 69 68

Δψ, %

ε, %

Δε, %

ψ, % 46 43 42

Δψ, %

Steel

Steel

0

23.0 21.5 19.0

0

21.0 20.0 19.5









17H1S

30 45

6.5

11.5 12.8

28 42

4.8 7.1

6.5 8.7

10H2BТ

17.4

The estimation of the conditions of pipe fracture of the main gas pipeline under study was carried out by determining the failure pressure. For this, the Standard (DNV-RP-F1001 (1999)), which is a comprehensive and generalized method of calculating the internal failure pressure for pipelines with crack-like defects was used. It is based on the data on the outer diameter of the pipe ( D ), the thickness of its wall ( t ), the ultimate strength of the pipe material ( σ UTS ), the length ( L = 2 c ) and the depth of the defect ( a ), but the shape of the defect itself is not specified. According to this standard, failure pressure is         2 1 1 2 UTS f t p а t а t q D t           , where   2 1 0.31 q L Dt   . (3) The results of the calculation of the critical values of the failure pressure for pipes of main gas pipelines with crack-like defects are shown in Table 4.