Hydrotest of Retired Vessel With Local Thin Area and Effect of Structural Discontinuity

Abstract API 579-1/ASME FFS-1 Fitness for Service Part 5 local metal loss Level 2 assessment requires a sufficient distance between local thin area (LTA) and neighboring major structural discontinuities. To some extent, the concept is based on S. Timoshenko’s model (1) that assumed a concentrated external load on a cylindrical shell. While the model is well applicable for piping nozzles and supporting attachments that takes external force and moment on cylindrical shell with local thin area, some of structural discontinuities, such as stiffeners, weld neck type flanges, vessel heads and manway nozzles in kind, would show a different effect on cylindrical shell with local thin area since there are no significant external loads. In addition, experiences of Level 3 finite element analysis (FEA) indicated those seems to reinforce LTA because of increased cross section to sustain pressure load. Often, LTA has been observed around a major structural discontinuity in our industry, however, application of Lv.2 assessment was limited due to the current required distance between LTA and structural discontinuity. For those cases, Lv.3 assessment or repair is the only option. If the current distance to major structural discontinuity criteria was exempted or relaxed, it would lead to enhance the usage of Lv.2 assessment that is relatively easier compared to Lv.3 assessment. From above viewpoint, S. Bouse et al. discussed the effect of ellipsoidal head and hemisphere head on LTA assessment. Their outcome is incorporated in 2021 version of API579-1/ASME FFS-1 Fitness-For-Service table 4.2 (2). Later, they wrote up the detail of discussion in their paper; PVP 2023-106371 Re-Evaluating the Minimum Required Distance Between Local Thin Area In Cylinder and an Ellipsoidal Head (3). The conclusion is that ellipsoidal head was beneficial for local metal loss assessment based on numerous finite element analysis (FEA) models data. Following their work, authors developed our previous paper, PVP2024-121682 On the Required Distance to Major Structural Discontinuity from Local Metal Loss Area (4). In the paper, it was concluded that some structural discontinuity such as weld neck flanges, stiffening rings and integral type manway nozzles would be also a beneficial structural discontinuity in addition to 2:1 ellipsoidal head through several finite element analysis (FEA) and theoretical discussion. As a continuation of our previous paper above, authors conducted hydrotest of retired vessel to examine the effect of structural discontinuity on LTA. In this paper, the detail of hydrotest and comparison with FEA model based on strain data will be discussed.