Study on the minimum transverse impact toughness required to resist cracking of non-API high grade steel casing

In Tarim oilfield of China, the non-API high grade steel casing (HGSC) is frequently used as the intermediate and production casing in ultra-deep well with true vertical depth (TVD) more than 6000 m. So far, no internationally accepted standard has been established for non-API high grade steel casing (HGSC) presently, although some obvious shortcomings exist, such as sensitivity to crack defects and service environment, so that the fracture failure accidents of non-API HGSC happens frequently in Tarim oilfield. Here, firstly, this paper deduces the unified algorithm of burst pressure of non-API HGSC based on twin shear unified strength theory and bilinear hardening model, which takes strain-hardening, Bauschinger effect of material and effect of intermediate principal stress into consideration. Secondly, the burst pressure formula of non-API HGSC has been presented based on weighting twin shear stress yield criterion. Thirdly, the calculation method which can calculate the minimum transverse impact toughness (TITmin) required to resist cracking of non-API HGSC, has been established based on the presented burst pressure formula and API/ISO standard, and the effects of four kinds of defects, factor of effective thickness and non-destructive examination (NDE) level on the TITmin required to resist cracking of pipe body under diffenrent internal pressures have been analyzed. Finally, the full-scale ductile burst test of non-API 140 casing have been conducted, by which the accuracy and reliability of burst pressure formula and calculation method of TITmin have been validated. Research results can provide an important reference for non-API HGSC design in ultra-deep well.