The Fatigue Behavior of Concentric Coiled Tubing

Abstract

This paper investigates the fatigue behavior of Concentric Coiled Tubing (CCT) and specifically the influence of internal and annular pressures on fatigue life and diametral growth rate. This behavior is assessed relative to that of conventional Coiled Tubing (CT) using samples of identical material, with software modelling predictions providing an additional basis for comparison. Samples of 1.25" CT were situated concentrically within samples of 2.375" CT, and both were repeatedly bent and straightened on a standard CT fatigue testing system, with separate pressures in the inner volume and the annular volume between the CT samples. Current SPE literature does not include prior art for any type of concentric coiled tubing fatigue testing or analysis. The testing apparatus used in this study is the industry standard for the assessment of conventional coiled tubing, however this paper presents the world-first experimental study of concentric coiled tubing fatigue. Since the testing focused on the failure of the smaller diameter samples, baseline fatigue tests were only conducted on the smaller, inner coiled tubing samples. These results are cross referenced to current fatigue models. Concentric samples were tested on the same machine and setup parameters, using special fixturing that allowed the internal sample to move axially on one end, with its pressure contained. A matrix of pressure differentials was examined, with various levels of inner and annular pressures. The exterior pressure was atmospheric for all tests. The orientation of the longitudinal Electric Resistance Welding (ERW) seams in both samples was examined in this study. The current approach to monitor CCT fatigue integrity is to use a conventional CT fatigue model, based on the differential pressures caused by the inner CT pressure, annular CCT pressure, and well pressure. While pressure may be monitored accurately, uncertainties exist with regards to the influence of radial stress in the tube wall and factors such as mechanical abrasion between the inner and outer coiled tubing strings, especially due to the presence of internal longitudinal ERW seams. Current models assume perfect concentricity; however, eccentricity varies throughout the string in real-world applications. Results from this study include an empirical derating factor, post mortem failure analysis (including assessment of the influence of ERW seam abrasion), diametral growth analysis, and recommendations for future testing.