Strain-Based and Low Cycle Fatigue Design Approaches for Thermal Service Tubulars

Abstract

Strain-based approaches that allow post-yield loading are now quite common in the design of tubulars for high temperature service in cyclic steam stimulation and similar applications. The traditional notionally strain-based design approach proposed by Holliday (ASME 69- PET-10, 1969) has formed the basis of design for several decades. In this work, a modified Holliday approach, which provides a rational, easily applied basis of design for thermal service tubulars, is presented. The modifications improve upon several assumptions made in the original approach, especially that of an ideal elastic-plastic, symmetric material. Thermal effects important in design, such as temperature deration of yield, cyclic strain hardening, Bauschinger effect, thermal stress relaxation, and strain localization are incorporated in the design approach, and acceptable design factors for different tubular grades are proposed. While familiar and able to produce reliable and acceptable designs, the modified approach has several limitations, which are discussed in the paper. These limitations largely arise from the fact that cyclic plastic strain is usually unavoidable in a thermal service tubular, and as a result, mechanical failure is rooted in fatigue. In this paper, a new, Low Cycle Fatigue (LCF) approach is presented as an alternative for critical thermal applications. The approach is based on the concepts of Critical Strain, a material property, and Ductile Failure Damage Indicator (DFDI), a plastic damage parameter. The new approach accumulates the plastic damage through the parameter DFDI, and can handle both cyclic and applied monotonic strains in the plastic region. The plastic damage is correlatable to critical strain, and can incorporate the influence of principal stresses and cyclic loads in the plastic region. The paper presents the basis of these parameters, and their use in assessment of plastic damage failure in other tubular and structural design approaches. The method offers some advantages over the more traditional, Coffin-Manson based LCF models, including reduced experimental burden, ability to incorporate connections and sour service considerations, and the ability to handle non-zero mean strain. Limitations of the method, and ongoing efforts to address them, are presented. The paper also presents an example illustrating the application of the new LCF approach.