Influence of a complex stress-strain state on physical and mechanical properties of pipe steels

UDK: 622.692.4-034.14
DOI: 10.24887/0028-2448-2020-9-133-135
Key words: underground pipelines, biaxial stress-strain state, cyclic fracture resistance, tensile strain, cleavage failure, torsional strain, shear fracture, ring welds, complex stress-strain state
Authors: E.E. Zorin (The Pipeline Transport Institute LLC, RF, Moscow), E.P. Studenov (The Pipeline Transport Institute LLC, RF, Moscow

The analysis of failures in underground pipelines indicates a significant difference in the causes and mechanisms of such failures depending on the pipeline deformation pattern after the soil movement. Thus, in the pipelines of Siberia, the main cause of failure is corrosion processes on the outer surface of the pipe after its insulation destruction, whereas for pipelines laid in permafrost soils accidents are due to the destruction arising from various structural and process defects under conditions of significant strains. We show that cyclic movements of the pipeline bottom deflection point in the area of flooded thermokarst with its uneven freezing in the autumn-winter period can cause the pipe wall torsion effect, combined with loading by internal pressure and a significant bending of the pipeline axis.

The paper considers fatigue failure mechanisms of a main pipeline wall under conditions of a complex stress-strain state (SSS) with a detected torsion element in the zone of ring assembly weld at the main pipeline exits from thermokarst into design position. Fractures in the pipe wall arise from the fusion area of the ring assembly weld and has all the signs of fatigue failure. The fractographic analysis of the fracture surface established the presence of three stages of fracture development: the initiation, stable growth, and surface fracture transition into a through fracture with areas of high local plastic deformation. In all development areas of the above fatigue failure of a pipe wall made of steel of the K56 strength grade, significant differences are recorded in the structure of the fatigue fracture surface, when compared with fractures obtained on these steels under the conditions of cleavage failure. All stages of failure include cleavage fragments together with quasi-brittle and brittle fracture areas. This indicates a significant contribution to the destruction of torsional strain in the form of micro-shear deformations, against the background of a biaxial SSS of the pipe wall loaded with internal pressure. On impact samples with a KCV notch, we conducted a comparative analysis of the influence exerted by plastic tensile and torsion strains on the fracture resistance parameters of K56 strength grade pipe steels under the conditions of a biaxial SSS.

References

1. Zorin E.E., Razrabotka osnov prognozirovaniya rabotosposobnosti svarnykh truboprovodov iz ferrito-perlitnykh staley s uchetom usloviy ekspluatatsii (Development of the basis for predicting the performance of welded pipelines made of ferritic-pearlitic steels, taking into account operating conditions): thesis of doctor of technical science, Moscow, 1993.

2. Rodionova S.G., Revel'-Muroz P.A., Lisin Yu.V. et al., Scientific-technical, socio-economic and legal aspects of oil and oil products transport reliability (In Russ.), Nauka i tehnologii truboprovodnogo transporta nefti i nefteproduktov = Science & Technologies: Oil and Oil Products Pipeline Transportation, 2016, no. 5 (25), pp. 20–31.

3. Neganov D.A., Makhutov N.A., Zorin N.E., Formation of requirements to reliability and security of the exploited sections of the linear part of trunk pipelines transportation of oil and oil products (In Russ.), Neftyanoe khozyaystvo = Oil Industry, 2019, no. 6, pp. 106–112.

4. Geyt A.V., Zorin E.E., Mikhaylov I.I., Application of automated ultrasonic inspection systems in assessing the quality of girth welds of main pipelines (In Russ.), Nauka i tehnologii truboprovodnogo transporta nefti i nefteproduktov = Science & Technologies: Oil and Oil Products Pipeline Transportation, 2018, V. 8, no. 3, pp. 92–101.

5. Lanchakov G.A., Zorin E.E., Stepanenko A.I., Corrosion-mechanical strength and statistics of pipeline failures (In Russ.), Gazovaya promyshlennost', 1991, no. 10, pp. 14–19.

6. Zorin E.E., Efimov V.M., Tolstov A.E., Stress-strain state of subsurface pipeline in cryolithic zone (In Russ.), Neft', gaz i biznes, 2015, no. 9, pp. 9–12.

7. Larionov V.I., Gumerov A.K., Novikov P.A., Analysis of stress and strain state of a pipeline in karst areas (In Russ.), Vestnik MGTU im. N. E. Baumana. Ser. Mashinostroenie, 2012, no. 3(38), pp. 60–67.

8. Zorin A.E., Malyarevskaya E.K., Muradov A.V., Influence of pipe manufacturing technology on crack resistance of plastically deformed metal (In Russ.), Neft', gaz i biznes, 2010, no. 1, pp. 79–80.

The analysis of failures in underground pipelines indicates a significant difference in the causes and mechanisms of such failures depending on the pipeline deformation pattern after the soil movement. Thus, in the pipelines of Siberia, the main cause of failure is corrosion processes on the outer surface of the pipe after its insulation destruction, whereas for pipelines laid in permafrost soils accidents are due to the destruction arising from various structural and process defects under conditions of significant strains. We show that cyclic movements of the pipeline bottom deflection point in the area of flooded thermokarst with its uneven freezing in the autumn-winter period can cause the pipe wall torsion effect, combined with loading by internal pressure and a significant bending of the pipeline axis.

The paper considers fatigue failure mechanisms of a main pipeline wall under conditions of a complex stress-strain state (SSS) with a detected torsion element in the zone of ring assembly weld at the main pipeline exits from thermokarst into design position. Fractures in the pipe wall arise from the fusion area of the ring assembly weld and has all the signs of fatigue failure. The fractographic analysis of the fracture surface established the presence of three stages of fracture development: the initiation, stable growth, and surface fracture transition into a through fracture with areas of high local plastic deformation. In all development areas of the above fatigue failure of a pipe wall made of steel of the K56 strength grade, significant differences are recorded in the structure of the fatigue fracture surface, when compared with fractures obtained on these steels under the conditions of cleavage failure. All stages of failure include cleavage fragments together with quasi-brittle and brittle fracture areas. This indicates a significant contribution to the destruction of torsional strain in the form of micro-shear deformations, against the background of a biaxial SSS of the pipe wall loaded with internal pressure. On impact samples with a KCV notch, we conducted a comparative analysis of the influence exerted by plastic tensile and torsion strains on the fracture resistance parameters of K56 strength grade pipe steels under the conditions of a biaxial SSS.

References

1. Zorin E.E., Razrabotka osnov prognozirovaniya rabotosposobnosti svarnykh truboprovodov iz ferrito-perlitnykh staley s uchetom usloviy ekspluatatsii (Development of the basis for predicting the performance of welded pipelines made of ferritic-pearlitic steels, taking into account operating conditions): thesis of doctor of technical science, Moscow, 1993.

2. Rodionova S.G., Revel'-Muroz P.A., Lisin Yu.V. et al., Scientific-technical, socio-economic and legal aspects of oil and oil products transport reliability (In Russ.), Nauka i tehnologii truboprovodnogo transporta nefti i nefteproduktov = Science & Technologies: Oil and Oil Products Pipeline Transportation, 2016, no. 5 (25), pp. 20–31.

3. Neganov D.A., Makhutov N.A., Zorin N.E., Formation of requirements to reliability and security of the exploited sections of the linear part of trunk pipelines transportation of oil and oil products (In Russ.), Neftyanoe khozyaystvo = Oil Industry, 2019, no. 6, pp. 106–112.

4. Geyt A.V., Zorin E.E., Mikhaylov I.I., Application of automated ultrasonic inspection systems in assessing the quality of girth welds of main pipelines (In Russ.), Nauka i tehnologii truboprovodnogo transporta nefti i nefteproduktov = Science & Technologies: Oil and Oil Products Pipeline Transportation, 2018, V. 8, no. 3, pp. 92–101.

5. Lanchakov G.A., Zorin E.E., Stepanenko A.I., Corrosion-mechanical strength and statistics of pipeline failures (In Russ.), Gazovaya promyshlennost', 1991, no. 10, pp. 14–19.

6. Zorin E.E., Efimov V.M., Tolstov A.E., Stress-strain state of subsurface pipeline in cryolithic zone (In Russ.), Neft', gaz i biznes, 2015, no. 9, pp. 9–12.

7. Larionov V.I., Gumerov A.K., Novikov P.A., Analysis of stress and strain state of a pipeline in karst areas (In Russ.), Vestnik MGTU im. N. E. Baumana. Ser. Mashinostroenie, 2012, no. 3(38), pp. 60–67.

8. Zorin A.E., Malyarevskaya E.K., Muradov A.V., Influence of pipe manufacturing technology on crack resistance of plastically deformed metal (In Russ.), Neft', gaz i biznes, 2010, no. 1, pp. 79–80.


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