Tubulars made with Corrosion Resistant Alloys (CRA) exhibit a moderate degree of transverse anisotropy. This results in a lower yield strength in the plane perpendicular to the rolling axis of the tubular. In addition, cold work during mill finishing introduces yield strength asymmetry along the axis.
The majority of CRA tubulars are low do/t (~ 10 to 15) production strings and their design is usually driven by pressure and tension. Given the reduced yield strength in the hoop direction (as compared to axial yield strength), a rigorous method to assess the effect of yield strength anisotropy on the triaxial limits of the pipe is needed. Though there is a substantial body of mature literature on anisotropy in metals and manufactured composites, anisotropy of tubulars has received little attention by tubular designers.
In this context, we use Hill's celebrated criterion (Hill 1950) for the yielding of anisotropic metals to develop an equation for the triaxial limits of a hollow cylinder and show that the currently used isotropic triaxial ellipse is a special case of the general limit. Also, by recognizing that purely elastic load excursions do not alter the yield surface, we extend the aforementioned triaxial limit to tubulars with tension-compression asymmetry along the tube axis. Finally, based on the notion of a strength utilization factor, we propose a method to determine the triaxial safety factor for anisotropic CRA tubulars. This method is consistent with the definition of the triaxial safety factor for isotropic tubulars.