Forensic Investigation of Submerged Draft Tube Outlet Gate Failure: A Case Study of Incident at Upper Trishuli-3A

Abstract

The paper deals with a Forensic Engineering Study on the Draft Tube Outlet Gate (DTOG) collapse at Upper Trishuli-3A Hydropower Station. The aim of this research was to find the probable cause of the gate failure using a technique that incorporates scenario recreation, observed operation, as well as Finite Element (FE) Modal Analysis. Data extracted from the SCADA system and observations taken from CCTV footage provided the scenario development with evidence for the collapse sequence. The design document, notes gathered from the post - accident inspection and statements collected from involved authorities supplied supplementary useful information. A 3D FE model was prepared of the DTOG and hoisting machine using relevant design drawings and a mode shape analysis was undertaken to extract inherent structural modes. Findings show that the gate was affected by erratic vibrations prior to failure of its hoist - rod - sleeve bolt connection, then finally it completely fell. After evaluating vibration characteristics, the 5th mode stood out because it corresponded to the hoist rod vibrating along its load path. Its non - wetted (dry) frequency was found to be 11.603 Hertz, but the runner, whirling at 428.57 rpm, was generating vibrations at a frequency of 7.143 Hz. The presence of water will add mass effects, potentially bringing the mode frequency down and increasing vibration. Accounting for this added-mass effect (an estimated 30-33% reduction based on the increase in effective modal mass), the submerged (wet) natural frequency of Mode 5 is estimated at approximately 7.77-8.12 Hz, close to the runner excitation frequency of 7.143 Hz.So the failure resulted in flow - induced vibration of the DTOG with its fatigue sensitive connections.