The effects of imperfections and nonlinearities on the failure mode and the B2 stress index of thin-walled straight pipes are investigated with finite element (FE) analyses. The analyses were performed for pipes made of an ideal elastic–plastic material and the austenitic steel X6CrNiNb18-10. The B2 index is calculated from the instability bending moments obtained by limit load analyses. The effects of initial imperfections as well as the D/t-ratio and the yield stress on the B2 stress index are studied. As a first result, it is noted that thin-walled straight pipes and imperfections fail due to local plastic buckling. Further analyses show that the type of imperfections, the ovality, the D/t-ratio, and the yield stress have significant influences on the B2 index. The obtained B2 indices for thin-walled straight pipes with D/t > 40 and possible technical imperfections are considerably higher than 1.0. The results have been compared with those of other investigations.