Collapse Pressure of Coiled Tubing Under Axial Tension

Abstract

As the coiled tubing is cycled on and off the reel and over the gooseneck, initially nearly perfectly round CT becomes oval. This ovality significantly decreases the collapse failure pressure as compared to perfectly round tubing. An analytical model of collapse pressure for oval tubing under tension load is developed based on elastic instability theory and the von Mises criterion. The theoretical model shows satisfactory agreement with experimental data for 3.5" OD and 0.190" wall thickness tubes with small ovality. Introduction The collapse pressure is a measure of an external force required to collapse a tube in the absence of internal pressure. It is defined as the minimum pressure required to yield the tube in the absence of internal pressure. From this definition, "collapse pressure" should be "collapse yield". In coiled tubing, the ratio of outer diameter to wall thickness is typically around 15. Thin-walled theory is adequate to apply to coiled tubing and "collapse pressure" or "collapse yield" can be used without distinction. Coiled tubing is sometimes used in high-pressure wells. If the pressure becomes too high, the coiled tubing will collapse. This could not only lead to serious well-control problems, but may result in extensive fishing operations. A reliable safety criterion of collapse pressure is needed by the coiled tubing operators. Theoretical models of collapse pressure are well developed for perfectly round coiled tubing but not for oval coiled tubing. Coiled tubing is initially manufactured with nearly perfect roundness, sometimes having a small ovality (typically = 0.5%). Perfectly round CT can become oval owing to the plastic mechanical deformation of the coiled tubing as it is spooled on and off the reel and over the gooseneck. As the cycling continues, the ovality may increase. This ovality can significantly decrease the collapse failure pressure as compared to perfectly round CT. An analytical model of collapse failure pressure for oval CT is then developed based on elastic instability theory and the von Mises criterion.

Design and Qualification of an Ultra-High Pressure Perforating System

“…The effect of thickness variations up to 10% of mean wall thickness on the collapse strength (under external pressure) has been shown to be insignificant (Yeh, M. K., and Kyriakides, S., 1986), but collapse strength was found to be highly influenced by the initial ovality of the tube. Several simplified models for collapse prediction with ovality have been developed in recent years (Yeh, M. K., and Kyriakides, S., 1986;Yang, Y.S., 1997;Avakov, V., May 1996), but these models do not account for geometric imperfections (scallops) on the tube as is true with HCs in the perforating industry.…”

“…One of the issues in the numerical simulation of collapse is the determination of an adequate collapse criterion. The inception of yield in the tubing may not necessarily mean that collapse occurs (Yang, Y.S., 1997;Avakov, V., May 1996). Furthermore, the region of plastic deformation may not extend all the way through the wall thickness when collapse occurs (Newman, N.N., 1992).…”

Buckling load of bending coiled tubing under an injector

“…If the compressive load on the CT increases too much, it causes catastrophically buckle in this unsupported interval. 1 Y Yang 2 studied the failure of straight tubing under the outer diameter pressure and found a relationship between the collapsed compressive stress and the tensile force. K Newman et al 3 used the equation from Gordon-Rankine and obtained the buckling load of the CT, but he assumed that the CT is straight.…”

High-Pressure Coiled-Tubing-String Design

“…It has been shown that collapse resistance of coiled tubing is reduced not only by increased axial load, but also by increased ovality. [2][3][4][5][6] Therefore, the likelihood of HPCT collapse increases when it is used in a deep well, or when its ovality increases. In normal tubing operation, high wellhead pressure entails high circulating pressure, as it is required for pumping.…”