The use of Coiled Tubing (CT) for servicing oil and gas wells is increasing rapidly. Larger CT sizes are being developed to meet new service requirements. The CT is being pushed closer to its performance limits for many of these services. To avoid CT failures in the field, it is important that these limits are well understood.

There are three major types of CT limits:

  • Life limits due to fatigue and corrosion [1].

  • Pressure and tension limits

  • Diameter and ovality limits

These limits are interrelated in some ways but can be analyzed separately. The corrosion effect on the CT must be considered for the first two limitations, but, the fatigue life of the CT does not significantly affect the pressure and tension limits.

The diameter and ovality limits are determined based on present surface hardware such as the stripper bushings and BOP inserts. Real-time monitoring of the CT diameter is necessary to locate mechanical damage, such as dents and ballooning, which can not be predicted by computer modelling.

This paper addresses the pressure and tension limits. It describes a mathematical model which is used to determine these limits. Test data used to validate the model is also presented.

Selectipn of Limiting Criteria

A criterion must be chosen as the limiting criteria for determining the pressure and tension limits. For this model, the initiation of yielding for a straight section CT with no residual internal stresses was chosen as the limiting criterion. This criterion is discussed further in the Incipient Yield Criteria section of this paper.

The incipient yield criterion was chosen because it is well accepted in the industry; it is relatively simple to calculate; and it agrees well with test data. In fact, this criterion is not entirely accurate in representing CT.

CT is plastically deformed when it is "bent" onto and off of the reel and over the gooseneck of the injector head [1]. This plastic deformation causes significant residual stresses which are not considered by this yield criterion. These residual stresses cause yielding to begin much earlier than this yield criterion would predict. This yielding due to the residual stresses acts to relieve the residual stresses; however, it is not this yielding that is being considered for determining the CT limits but the yielding that would occur if the CT did not have any residual stresses.

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