Defining Coiled Tubing Limits- A New Approach

Abstract

Abstract The burst, collapse and axial load operating limits for Coiled Tubing (CT) are currently established using the Von Mises incipient yield criterion. This criterion has historically been used to calculate the limits for oil country tubular goods (OCTG), The limits according to this criterion are based on the point at which the pipe material reaches a load state in which it begins to yield. Because of the bending that occurs when the CT is spooled on and off the reel, and when it is bent over the guide arch, the CT is already far beyond the yield point before it enters a well. Thus, this criterion does not really apply to CT. This paper describes a research project currently in progress, The purpose of this project is to define a new set of CT limits based on criteria other than 'incipient yield'. This new approach to setting CT operating limits takes into account the internal residual stresses in the CT which are a consequence of repeated bending cycles and the accompanying change in material properties. Introduction The use of the Von Mises incipient yield criterion to establish CT limits is discussed in detail in Refs. I through 4. This criterion calculates the tri-axial stresses (axial, tangential and radial) caused by the forces applied on the CT due to the following:axial force (tensile or compressive),internal pressure,external pressure,bending due to helical buckling. The tri-axial stresses caused by these externally applied forces and pressures are combined using the Von Mises criterion to calculate the Von Mises stress, ?vme. This combined stress, ?vmeis then compared to the yield stress, ?ym CT, P, which is determined from a uni-axial pull test on a sample of the CT. When ?vme reaches ?ym, it is assumed that the CT material will begin to yield. This point of 'incipient yield' is currently used by the CT industry to determine the burst. collapse, tensile and compressive limits that the CT can be subjected to. However, this method ignores the internal residual stresses in the CT caused by the bending that occurs when the CT is bent on and off the reel and over the guide arch. These esidual stresses cause yielding to begin much earlier than predicted by this method. The material used to make CT has a well-defined yield point in the axial direction. However, beyond the yield point ?yp, the Bauschinger effect and work softening change ?yp and make it less well defined. Also, in the tangential or hoop direction may not be the same as ?yp in the axial direction. The Gas Research Institute (GRI) has funded a research project to understand and model the material characteristics and to develop an improved method for setting CT limits.