Cyclic Mechanical and Fatigue Properties for OCTG Materials

摘要

Much of the design basis for well structures subjected to highrnamplitude cyclic loading is based on material assumptions thatrnextrapolate strength properties from uniaxial, monotonic testsrnto conditions where cyclic, multiaxial stresses are imposed.rnThis paper shows results from cyclic testing on common OilrnCountry Tubular Goods (OCTG) materials and demonstratesrnthe difference between physical behavior measured underrncyclic loading conditions and theoretical behavior extrapolatedrnby numerical modeling of uniaxial, unidirectional test data.rnModeling theories for plastic deformation are discussed, alongrnwith their limitations and relevance in a cyclic loadingrnenvironment. The implications of these limitations for designrnchoices in thermal wells also are discussed.rnFatigue properties for the high-amplitude, low cyclernapplication of thermal operations have not previously beenrninvestigated in much depth, in particular for OCTG. Alongrnwith characterizing cyclic mechanical properties, the testsrndiscussed here also were used to assess the low-cycle fatiguernproperties of steel commonly used for in thermal well casings.rnConsistent fatigue results were produced, which, applied in therncontext of analysis results using representative cyclicrnmechanical properties, provide a basis for estimating fatiguernlife for specific cyclic deformations. Depending on scenariornassumptions, substantial statistical variation in fatigue life canrnbe expected, so exact fatigue life predictions are notrnanticipated. The primary value in such modeling capability isrnthe assessment of mitigation options for extending well lifernwhen casing deformations are indicated.rnThe paper also discusses some practical implications of therndifference between actual material behavior and idealizationsrnused for modeling purposes. An example application of therncyclic material behavior and fatigue life prediction is alsornincluded.