Helical Buckling of Pipe with Connectors and Torque

Abstract

Abstract It has been generally recognized that connectors should have some effect on the buckling of pipe. For instance, the connector outside diameter may be as much as 50% greater than the pipe body diameter. As a result, the radial clearance of the connector can be substantially smaller than the radial clearance of the pipe body. The analysis of buckling has received extensive attention in the last 20 years. The effect of connectors on pipe stresses has received somewhat less attention. Lubinski used the beam-column equations to analyze the effect of connectors on pipe bending stresses for a pipe in tension in a twodimensional constant curvature wellbore. Bending stresses were significantly magnified by the connector stand-off. Paslay and Cernocky completed this analysis by analyzing the pipe in compression. Mitchell extended these results to threedimensional helical buckling. Torque adds a new dimension to the buckling problem. Without torque, buckling occurs only for positive effective axial force (compressive axial force plus pressure effects). A pipe with applied torque can buckle in tension! The contact force between pipe and wellbore can be increased or decreased, depending on the direction of the applied torque. And, of course, pipe used in rotary drilling always has applied torque; so buckling analysis without torque is always questionable. This paper looks at three-dimensional buckling of pipes with connectors with applied torque. The problem formulation is similar to Lubinski's buckling analysis: the wellbore is vertical and straight. The beam-column equations considered in the plane buckling analysis are used, but now there are deflections out of the plane. A solution for helical buckling is developed that produces pipe sag, maximum dogleg angle, contact force, and bending stress magnification as a function of pipe effective axial force and torque. An application problem is solved and the relative effects of compressive axial force and torque on sag between connectors, contact loads, and maximum bending stress are examined. Applications include the analysis of bottom hole assemblies, drill pipe, casing, and tubing. The solutions are simple enough that they are suitable for spreadsheet calculations.