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Abstract

Failure of coiled tubing, due to the repeated bending and plastic deformation of coiled tubing on and off the reel and gooseneck, is of great concern in coiled tubing operations. This paper discusses the coiled tubing working life based on one of the coiled tubing life models published in the literature, and compares the results with other models. Certain agreements are found among these models.

A group of curves is presented to illustrate the coiled tubing working life affected by coiled tubing size and wall thickness, internal pressure, yield strength, reel diameter, gooseneck radius, operation condition (corrosion) and butt- welded connection (stress concentration). The results show that coiled tubing life can be greatly increased by increasing CT wall thickness and CT strength, while the coiled tubing working life decreases under high internal pressure, corrosion, and butt-weld conditions. These curves can be easily used in estimating coiled tubing life for the field use.

Introduction

Use of coiled tubing (CT) in the oil industry is increasing rapidly for drilling, completion, and production applications. Advantages of using coiled tubing include reduced costs, time saving, and increased safety. However, the fatigue failure of coiled tubing is a serious problem due to the repeated bending and plastic deformation of coiled tubing on and off the reel and gooseneck. Although a few papers have been presented to describe the complicated failure mechanism of coiled tubing, the factors affecting the coiled tubing working life are not discussed in detail, which include the coiled tubing size and wall thickness, internal pressure, yield strength, reel diameter, gooseneck radius, operation condition (corrosion) and butt-welded connection (stress concentration). This paper is to discuss these factors and present some comparison among the published coiled tubing failure models.

COILED TUBING FATIGUE

When tripping in and out of the well, coiled tubing experiences repeated bending on and off the reel and gooseneck. This repeated bending results in alternating bending stresses (tensile and compressive) beyond the CT yield strength, and causes plastic deformation and fatigue failure to the coiled tubing. The coiled tubing internal working pressure also produces a hoop stress and accelerates the coiled tubing plastic deformation. As the coiled tubing working strokes (one stroke means one round trip of coiled tubing in and out of the well, including one bending cycle on and off the reel and two bending cycle on and off the gooseneck) increase for engineering operations, the coiled tubing diameter grows gradually, and small cracks develop on the coiled tubing surface, finally resulting in the failure of coiled tubing.

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