Abstract

With the increased use of coiled tubing in high-pressure wells, the collapse of coiled tubing between the injector and the stripper has received much attention recently. In high-pressure wells, the failure of this tubing section (typically less than 2 ft in length) usually occurs under the combined loadings of axial compression and internal pressure. Previous analytical models to predict failure under such loading conditions have focused mostly on the buckling behavior of the short coiled-tubing section with minor modification of yield strength to account for the effect of internal pressure. Such approaches underestimate the effect of internal pressure on the collapse failure, especially for higher internal pressure.

In this paper, a new analytical model is developed to predict the collapse of short coiled tubing under the combined loadings of axial compression and internal pressure. The analytical model first analyzes the buckling load of the short coiled-tubing section under axial compression only, and the burst pressure of the coiled tubing under internal pressure only, respectively. Then an interaction failure criterion is used to model the failure locus of the short coiled tubing under the combined loadings of axial compression and internal pressure. Experimental data are used to validate this new model.

Introduction

With the increased use of coiled tubing in high-pressure wells,1,2 the collapse of coiled tubing between the injector and the stripper has received much attention recently. To snub the coiled tubing into a high-pressure well, the injector has to exert a significant amount of compression to overcome the resistance from the wellhead pressure. Given the fact that the concerned section of coiled tubing between the injector and the stripper has an unsupported length of about 6 to 18 in., with each end supported by the injector chains and the stripper, respectively, the compression exerted by the injector could become high enough to cause catastrophic failure on this coiled-tubing section. In fact, field experiences indicate that such failures indeed happen. However, failure of this kind is not addressed in API Recommended Practice RP5C7,3 which was released almost eight years ago. To mitigate the risk of this kind of collapse failure, many high-pressure coiled-tubing units have an anti-buckling guide installed above the stripper to shorten the length of the unsupported coiled tubing between the injector chain and the stripper. However, it is still desirable to develop a model to predict the failure envelope for the concerned tubing section under the combined loadings of axial compression and internal pressure.

Considerable effort has been devoted to predicting the failure envelope of coiled tubing under the combined loading of axial force and pressure (both internal and external).4,5 Ref. 4 presented an approach to calculating the failure envelope for coiled tubing under the combined loadings of axial force and pressures. For coiled tubing under compression, the approach is more suitable for long coiled tubing inside a wellbore where helical buckling occurs prior to tubing failure. Ref. 5 presented a model to calculate the buckling load of the coiled tubing between the injector and the stripper. In this model, a short column buckling theory based on the Gordon-Rankine formula is used to calculate the buckling load. The model considers the effect of internal pressure by modifying the yield stress. Such a treatment obscures the interaction between the compression failure and burst failure modes and may not adequately account for the effect of internal pressure on failure when the pressure is high. Recent test data indicate that high internal pressure does have a significant effect on the collapse of a short coiled-tubing section, which prompted the development of this work.

In this paper, an interaction failure criterion is developed to predict the failure of short coiled tubing under the combined loadings of axial compression and internal pressure. Experimental data from existing publications and from new testing are used to validate the model. In the following sections, a recent experiment on short coiled tubing under combined loading of axial compression and internal pressure is first presented, followed by the development and validation of an interaction model for collapse failure under such loading conditions.

This content is only available via PDF.
You can access this article if you purchase or spend a download.