Predicting Coiled-Tubing Failure Below Injector

Abstract

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.