Buckling of slender string in cylindrical tube under axial load: Experiments and theoretical analysis

Abstract

Buckling of drill and tubing strings in drill or casing holes will affect the life of the string and cause difficulties in drilling and in oil production. A number of theoretical models have been developed to study the buckling load and the post-buckling configuration of the strings under some idealized conditions. However, verification of these theories in practice or in laboratory has not been extensively reported.

In this paper work on laboratory buckling tests of strings is presented and the results are compared with theoretical formulas. The results indicate that the process of string buckling can be divided into two stages, namely plane buckling and spatial buckling. In the spatial buckling the string-buckling configuration can roughly be represented by a helix. The relation between the buckling force and the helical pitch, obtained from the laboratory tests, is similar to that predicted by helical buckling theories. The friction between the string and the wall of the confining tube causes a nonuniform buckling shape and prohibits further buckling at some critical loads. The buckling theories do not cover these effects and may thus induce significant errors in predicting the string-post-buckling configuration. An approximate formula for estimating the friction force is proposed, which provides more accurate results. Further studies of the friction effect are suggested.