Three-dimensional well tubular design improves margins in critical wells

One consequence of the evolution of long and deep wells is that the design margins become increasingly smaller. Casing and tubing are pushed towards their yield strengths to meet the requirements of hot and highly stressed conditions. Also, the pipe used during drilling and intervention is subjected to elevated pressures and axial loads. These factors motivated the present study to develop an exact well design model for burst and collapse under axial load. In critical wells, the tubing designs are commonly based on triaxial software packages. It will be shown here that results from the conventional approach are accurate only under certain conditions.This paper presents an exact solution to the three-dimensional well tubular design problem. The term well tubular design, used in this paper, refers to the calculation of burst, collapse and axial loading limitations of drill pipe, snubbing pipe, tubing and casing. The new model calculates design factors that are exact, thereby offering a tool for improved well tubular design. The presented theory is based on the von Mises yield criterion and the Lamé thick-walled solution for the pipe.The new model is dimensionless, so the same graph can be used for all pipe materials and pipe dimensions. The model is also valid for cases with applied torque, bending or buckling. The general model is based on incipient yielding at the inside pipe wall, and the collapse predictions from the model for diameter-to-thickness ratios less than 14 are in agreement with experimental evidence. For large casing strings, the model should be used with caution for collapse predictions. The model is valid for burst calculations for all pipe sizes. Bending and torque effects are modeled by considering both the inside and outside pipe walls.