Cyclic Mechanical and Fatigue Properties for Oil-Country-Tubular-Goods Materials

Abstract

Summary This paper describes differences between actual material behavior and idealizations used for modeling purposes and discusses some of the implications for interpreting model predictions. Much of the design for well structures subjected to high-amplitude cyclic loading is based on material assumptions that extrapolate strength properties from uniaxial, tensile tests to conditions where multiaxial, cyclic stresses are imposed. This paper presents results from cyclic testing on a common oil-country-tubular-goods (OCTG) material and demonstrates differences between the physical behavior measured under cyclic loading conditions and theoretical behavior extrapolated by numerical modeling. Modeling theories for plastic deformation are discussed with their limitations and relevance in a cyclic-loading environment. The implications of these limitations for design choices in thermal wells also are discussed with example applications of cyclic material behavior and fatigue-life prediction. Material fatigue properties for the high-amplitude, low-cycle application of thermal operations have not been investigated in much depth previously, particularly for OCTG. Along with characterizing cyclic mechanical properties, the tests discussed here also assessed the low-cycle fatigue properties of the sample OCTG steel. The consistent fatigue measurements, combined with analysis results using representative cyclic mechanical properties, can provide a basis for estimating fatigue life. Depending on analysis-model assumptions, substantial variation in predicted fatigue life can occur; therefore, exact fatigue-life predictions are not anticipated. The primary value in such modeling is in evaluating the relative effectiveness of mitigation options for extending well life.