CT100+ Bias Weld Fatigue Life Estimations - Are Adjustments Required?

Abstract

Abstract "Pure" fatigue failures (i.e. not started by corrosion or mechanical damage) are not common in coiled tubing strings because of the implementation of systems/models to manage the fatigue life. However, in recent years, a series of "pure" fatigue failures occurred on bias welds of high-strength (CT100+) strings. These strings are normally operated at high pressures (≈ 7,000 psi), and their bias welds were subjected to high levels of strain (1.7% to 2.2%). The objective of this paper is to show the approach used to establish the adjustments needed to the fatigue life model to account for these types of failures. The work presented in this paper includes a brief characterization of the failures, summary of the fatigue data from the field, lab testing (at low and high strain) and the resulting analysis of the data. Prior to analyzing the data it was thought that some sort of acceleration of bias weld fatigue, relative to pipe fatigue, as a function of strain would be required. Results of the study confirmed that, for CT100+ coiled tubing strings, the fatigue life of the bias welds, relative to pipe fatigue, was affected by the level of strain. At high level of strain (around 2.1%) the bias weld fatigue life was around 60% of that measured for the pipe base material. At low level of strain (around 1.4%) the bias weld fatigue performance was close to that of the pipe base material. Therefore, model adjustments were required in order to de-rate the fatigue life based on the level of strain at which each bias weld is subjected. Coiled tubing interventions that use CT100+ strings, 2-in and larger outside diameter [OD], that are spooled on small reels (core reel diameters in the order of 80 to 84 in.) are becoming common within industry. These interventions impose high strain on the bias welds. An effective way to control and manage the fatigue life for these bias welds is needed to avoid unexpected failures that impact well intervention costs, job safety, and customer satisfaction.