STUDY ON THE YIELD BEHAVIOR OF OIL AND GAS PIPE IN TYPICAL STRESS STATES

The yield strength of oil and gas pipe has significant influence on pipeline design pressure. The axial stress which has a noticeable impact on the yield strength was studied in four typical stress states, including small specimen tensile test, mill test, hydrostatic burst test and buried in service state. It is shown that the axial stress is below zero in mill test, one half of hoop stress in hydrostatic burst test, 30 percent of hoop stress in service state, and zero in small specimen tensile test. The differences between specimen yield strength and true yield strength of pipe body were also studied by both theoretical research and experiments. The theoretical research shows that among the four stress states, on basis of von Mises criterion,the hoop stress is larger than that of Tresca criterion. And in hydrostatic burst test,the yield hoop stress of von Mises criterion is 1.15 times of specimen yield strength while 1 time of Tresca criterion. The results of six hydrostatic burst tests and specimen tensile tests show the hoop stress at yielding is 1.18 times of the specimen yield strength. This shows von Mises criterion is more suitable for oil and gas pipe design than Tresca criterion.