Numerical Evaluation of Plastic Strains in Coiled Tubing Surface Flaws for Comparison to Analytical and Emperical Models

Abstract

Abstract Coiled tubing (CT) endures cyclic bending strains on the order of 2 – 3% during its normal operation. Consequently, fatigue is the major factor limiting its useful life. Furthermore, CT is routinely subjected to gouges or defects due to harsh oil field environments. Assessing the stress and strain at the root of the defects, is essential to estimate the influence of flaws on CT fatigue life. This is physically difficult to accomplish experimentally due to the small defect size and limited access to the flaw during testing. Therefore, FEA is conducted on CT samples with and without flaws of realistic size and shape. These results are used to assess the validity of conventional notch strain analysis techniques. Conventional notch strain analysis models have long been used to estimate localized notch root strains. This paper investigates applicability of conventional notch strain analysis under bulk plasticity situations for CT. This paper presents a comparison of Neuber's rule and Glinka's strain energy density approaches, along with an empirical model developed to estimate notch root strain for CT applications.