Simplified design and integrity assessment of pressure components and structures

Hossain, Md. Mosharraf (2009) Simplified design and integrity assessment of pressure components and structures. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Standard analysis methods of mechanical components and structures are based on elastic analysis, elastic-plastic analysis and limit analysis. The determination of limit load using a simplified method is considered to be an attractive alternative over the conventional limit analysis methods i.e., analytical methods, experimental methods and numerical methods. Simplified methods are considered to be effective if they are able to estimate the lower bound limit load of a general class of mechanical components and structures within a minimum number of linear elastic analysis iteration without compromising with the quality of the result. -- In this thesis, a simplified method is proposed in order to estimate the limit load of a general class of mechanical components and structures. The proposed mα-tangent method makes use of statically admissible stress fields based on a single linear elastic analysis or on an assumed distribution to estimate the limit load. The method is applied to a number of mechanical components and structures ranging from standard example problems to typical pressure vessel components. The results are in good agreement with the corresponding analytical and inelastic finite element analysis results. The underlying features of the mα-tangent method enabled its application into three major areas: analysis of cracked components, stress categorization and fitness-for-service assessment. -- The determination of load carrying capacity is an important step in the integrity assessment of mechanical components and structures containing crack-like flaws. The mα-tangent method is extended in order to estimate the limit load of components and structures with cracks. The proposed method enables the determination of limit load using a single linear elastic analysis. The method is applied to a number of cracked component configurations and the results compare well with those obtained from the corresponding analytical and inelastic finite element analysis results. -- The ASME Boiler and Pressure Vessel Code can be applied to design pressure vessels and piping systems by using the design by analysis (DBA) approach. It provides guidelines for the classification of linear elastic stresses into primary, secondary and peak stress. Although these guidelines cover a wide range of pressure containing components, the guidelines are sometimes difficult to employ for three dimensional components with complex geometry. In this thesis, a simplified method is proposed in order to categorize the elastic stresses in pressure vessel components and structures using a single linear elastic finite element analysis. It uses the mα-tangent method, an assessment of constraint in the component based on limit load multiplier estimates, as a stress classification tool. The proposed method is applicable to both mechanical and thermal loads and is able to partition the elastic stresses into primary (P), primary plus secondary (P+Q) and peak (F) stress. The proposed method is a direct and alternative approach over conventional approaches i.e., stress linearization and interaction I discontinuity analysis. The method is applied to several practical pressure vessel components from simple to relatively complex geometric configurations and the results compare favorably with those obtained by the conventional techniques. -- Thermal hot spot and corrosion are the typical of damages occurring in operational pressurized components and structures. Fitness-for-service (FFS) assessment of these components and structures need to be performed periodically in order to demonstrate the operational safety and structural integrity. In this thesis, a simplified method, based on the mα-tangent method, is proposed in order to perform Level 2 FFS assessment of aging pressure vessel components and piping systems containing thermal hot spot and corrosion damage. The method is demonstrated through a number of examples and the results are verified by Level 3 inelastic finite element analysis. -- The potential benefits of using the above mentioned simplified methods over the conventional methods is that the simplified methods are applicable to a wide range of mechanical components and structures; they require minimum expertise from the analyst to perform the analysis; they are economically viable to use on a daily or regular basis; they are computationally effective as they do not require any iterative procedure; and they are very rapid and easy to implement in practice.

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