A Hierarchical Finite Element Framework for the Assessment of Pressure Vessels to the ASME III Code

Advances in the design of nuclear power plant pressure vessels require parallel development of associated numerical assessment methods to ensure that the opportunity to optimise plant safety, availability and cost are realised. In the UK, such pressure vessels are generally assessed against Section III, Subsection NB of the ASME Boiler and Pressure Vessel Code. The design by analysis (DBA) concept embedded in ASME III provides a set of design limits that, if achieved, provide assurance against possible pressure vessel failure modes such as ductile burst and ratcheting. The methodology on which DBA is based pre-dates the routine use of FE analysis and as a result can be difficult to apply to the assessment of complex 3D FE stress results. Generally, these traditional methods rely on through-wall stress classification line and stress linearisation techniques, the application of which can be subjective in certain circumstances. This paper describes a hierarchical framework for the assessment of pressure vessels to the ASME III code primary and secondary stress limits using a range of techniques commensurate with 3D FE stress analysis. FE based limit load analysis is used to assess against plastic collapse, in conjunction with automated FE direct shakedown techniques and strain based fatigue post-processing to assess the structural response to complex thermo-mechanical cyclic loading. This approach enables the analyst to obtain expediently the shakedown response and fatigue life of the structure. A demonstration of the hierarchical assessment framework and comparison to a traditional linearisation based assessment is presented in this paper for an example that is representative of nuclear power plant.