Cyclic Mechanical and Fatigue Properties for OCTG Materials

Abstract

Abstract Much of the design basis for well structures subjected to high amplitude cyclic loading is based on material assumptions that extrapolate strength properties from uniaxial, monotonic tests to conditions where cyclic, multiaxial stresses are imposed. This paper shows results from cyclic testing on common Oil Country Tubular Goods (OCTG) materials and demonstrates the difference between physical behavior measured under cyclic loading conditions and theoretical behavior extrapolated by numerical modeling of uniaxial, unidirectional test data. Modeling theories for plastic deformation are discussed, along with their limitations and relevance in a cyclic loading environment. The implications of these limitations for design choices in thermal wells also are discussed. Fatigue properties for the high-amplitude, low cycle application of thermal operations have not previously been investigated in much depth, in particular for OCTG. Along with characterizing cyclic mechanical properties, the tests discussed here also were used to assess the low-cycle fatigue properties of steel commonly used for in thermal well casings. Consistent fatigue results were produced, which, applied in the context of analysis results using representative cyclic mechanical properties, provide a basis for estimating fatigue life for specific cyclic deformations. Depending on scenario assumptions, substantial statistical variation in fatigue life can be expected, so exact fatigue life predictions are not anticipated. The primary value in such modeling capability is the assessment of mitigation options for extending well life when casing deformations are indicated. The paper also discusses some practical implications of the difference between actual material behavior and idealizations used for modeling purposes. An example application of the cyclic material behavior and fatigue life prediction is also included.