Session: DA-08-03 Developments in FFS Assessment

Paper Number: 121682

121682 - On the Required Distance to Major Structural Discontinuity From Local Metal Loss Area 

API 579-1/ASME FFS-1 Fitness for Service Part 5 local metal loss Level 2 assessment requires a sufficient distance　between local metal loss area and neighboring major structural discontinuity based on S.Timoshenko's theory, and it often limits the application of assessment. The original model assumed a concentrated force and moments on a cylindrical shell, and evaluated a sufficient distance that could settle down the bending moment from the point. The model would be well applicable for piping nozzles. However, some of "Major Structural Discontinuity", such as stiffeners and heads, would be different. Therefore, those stiffeners and heads might affect the local metal loss area in a different manner than piping nozzles. Recently, S.Bouse et al. discussed the effect of ellipsoidal head and hemisphere head in their paper ; PVP 2023-106371 Re-Evaluating the Minimum Required Distance Between Local Thin Area In Cylinder and an Ellipsoidal Head. It was concluded that hemisphere head would be beneficial for local metal loss rather than harmful based on FEM model data. The outcome is incorporated in 2021 version of API579-1/ASME FFS-1 Fitness-For-Service, table 4.2.

 

While this, an analytical equation of plastic zone size is derived and reorganized in conjunction with the step-by-step derivation of Mt and Ms in our previous PVP paper ; PVP 2022 Re-organize Significance of Mt, Ms and Plastic Zone Size Against Lmsd Under Plastic Collapse Regime. The results indicated current required distance to major structural discontinuities could be optimized if there was no significant external force and moment working on them except for pressure stress.

 

As the continuous work of our previous paper above, authors developed a concept of required distance to a major structural discontinuity from the viewpoint of load carrying capacity of surrounding area that is based on the cross-sectional area to satisfy the analytical model assumption of Mt and Ms. The load carrying capacity model concept is also confirmed by FEM for typical cases. In the paper, authors will discuss criteria to tell if the structural discontinuity is beneficial one or non-beneficial one. While our discussion is still limited to hoop stress dominated flaw size model to some extent, some of the major structural discontinuity can be explained as “beneficial discontinuity” so that those may be exempted from the restriction. Autors hoping that our discussion would enhance the discussion of required distance for each major structural discontinuity and will lead to increase the application of level 2 local metal loss assessment in the future. 

Presenting Author: Yoichi Ishizaki Idemitsu Kosan Co.Ltd.

Presenting Author Biography: Yoichi Ishizaki is a senior chief engineering associate of Idemitsu Kosan Co.Ltd, Tokyo, Japan. He has been working for oil industry since 1992. After working for ExxonMobil Research and Engineering, he is now serving to Idemitsu Kosan Co.Lt’d, Japan, since 2014. He earned Master of Science in Material Engineering, NAGOYA UNIVERSITY, NAGOYA, JAPAN, 1992. He is a licensed P.E., TX, USA, and has been served as a member of API/ASME joint committee on fitness for service since 2019.

Authors:

Yoichi Ishizaki Idemitsu Kosan Co.Ltd.
Hiroyasu Ameya Idemitsu Kosan Co.Ltd.