TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe API design rules for casing collapse 1 give widely varying predicted failure probabilities over the D/t range for well tubulars. For low D/t, they give unconservative designs (excessive risk); and for moderate D/t, over-conservative designs (excessive safety).This paper describes the development of improved design equations, calibrated using structural reliability methods to obtain a uniform predicted failure probability. The new design rules are simpler to use than the existing API equations, and give either safer or more economic design, depending on D/t ratio.