Abstract

An experimental study has been conducted to investigate the effect of tool joints on the buckling/post-buckling behavior of drill pipes constrained in straight horizontal wellbores. Buckling/post-buckling behavior of drill pipes has been traditionally investigated using continuous pipes. To our knowledge, this is the first time that the effect of tool joints is included in such a study.

The University of Tulsa Drilling Research Projects experimental buckling facility has been used to carry out the desired experiments.

Axial loads at the both ends of the pipe and contact forces at the tool joints were measured. Change in drillpipe configuration was also investigated visually as the axial load increased.

Some of the new findings of this study can be summarized as follows:

  1. Sequential occurrence of buckling/post buckling configuration of jointed pipe is similar to that of continuous pipes that have been reported previously by various investigators. In other words, in both cases, the pipes buckle first laterally, and then helically as the axial compressive load increases.

  2. The presence of tool joints does not affect the critical lateral (sinusoidal) buckling load significantly. However, it increases the critical load causing helical pipe configuration (helical buckling) by about 20 %.

  3. Efficiency of the axial load transfer is improved by about 40 % with the tool joints.

Results of this study will help to improve the design of operational parameters for drilling with jointed pipes as well as using coiled tubing. In particular, improved axial load transfer performance would allow drillers to use higher weight on bit and consequently faster and possibly less costly drilling.

Introduction

It is well recognized by the drilling industry that the occurrence of pipe buckling is of great practical importance. Slower rate of drilling (due to poor axial force transfer to the drillbit), and pipe overstressing (that may result in its mechanical failure) are some of the major factors contributing to high cost of drilling.

There are a number of technical papers focused on the issues of drillpipe/CT buckling. Most of the papers are theoretical in nature and subject to a number of simplifying assumptions. Most of investigators assume that the pipe is static (no rotation, no fluid flow etc.), continuous and the system is frictionless.

In his classical paper, Lubinski et al (1962) showed that the axial force-pitch relationship for helical pipe configuration (the so called helical buckling) is as follows,

  • Equation (1)

Eq.1 gives the pitch length, p, as a function of the axial force, F, and pipe bending stiffness EI. The main assumptions made by Lubinski were that the system is frictionless and the pipe is weightless. Based on these assumptions the helix pitch length is constant along the string.

Cheatham and Pattillo (1984) investigated the force pitch-relationship for loading (compression is increasing) and unloading (compression is decreasing) cases. Their solution was the same as Lubinski's solution for the case of loading.

The authors stated that a radially inward virtual displacement is permissible during the unloading. Therefore, Eq.1 cannot be used for this case.

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