Investigation of the post-expansion collapse strength of solid expandable tubulars made of different steels

Abstract

At present, there are different ideas about on the main factor influencing post-expansion collapse strength of solid expandable tubulars (SETs). In this study, three SETs with the same dimensions, which were made of 20 (Chinese grade), a twinning-induced plasticity (TWIP) and a transformation-induced plasticity (TRIP) steels, were expanded so the inner diameter increase by about 14%. The test results showed that the SETs made from high-strength steels (a TWIP and a TRIP) had poor resistance to collapsing after expansion. Through a combination of finite element method (FEM) simulations and experimental tests, the post-expansion collapse strength of three SETs were comparatively analyzed to find out the main influencing factor. After isolating the influences of geometric factors on the collapse strength, the influences of the Bauschinger effect and the residual stress on the collapse strength were studied. There was no correlation between the obvious degree of the Bauschinger effect of the three steels and the post-expansion collapse strength of the three SETs, which suggested that the Bauschinger effect was not the main factor influencing the post-expansion collapse strength. This is obviously different from the conclusion for SETs made from only one steel. The post-expansion collapse strength calculated by FEM simulations differed greatly from that of the actual measurement if the residual stress after expansion was ignored, though both were very close when the residual stress after expansion was considered. This clearly indicates that the residual stress after expansion has a significant effect on the post-expansion collapse strength and it is the main factor influencing the post-expansion collapse strength.