Vibration Tests for Failure Mode Analysis of Piping System Toward Natech Risk Assessment

Abstract Natech refers to natural hazard-triggered technological accidents. The importance of assessing Natech and implementing appropriate measures has been underscored by incidents such as the Fukushima Daiichi nuclear power plant (NPP) disaster and the gas tank fire triggered by an earthquake in Japan. However, there are insufficient data regarding structural failures and their impact on system functions, which hinders the ability to effectively assess Natech risks in industrial plants, beyond just NPPs. Therefore, the authors have examined piping failure modes under seismic input to explore the relation between these failure modes and the assurance of piping function. Additionally, the authors have performed elbow and tee loading tests, as well as shaking-table tests on a simple piping system to observe the identified failure modes of small-bore carbon steel piping. The results revealed two primary failure modes: elbow collapse and axial crack growth in the elbow. Furthermore, it was observed that failure modes bifurcate into collapse and low-cycle fatigue modes, influenced by the interaction between the dead load and input acceleration level. To better observe failure modes under more realistic configurations, the authors fabricated a three-dimensional pipe specimen with multiple elbows and performed vibration tests using a shaking-table in this study. The test specimen was designed to monitor elbow collapse and axial crack growth in the elbows under test conditions. This paper presents an overview of the study and the results of the vibration tests, highlighting two distinct failure modes: axial and circumferential cracking, as well as a more comprehensive failure involving the collapse of multiple elements, including deformation, beyond just elbow collapse under the shaking-table tests.