Monitoring of steel-lined pressure shafts considering water-hammer wave signals and fluid-structure interaction

High head pressure tunnels and shafts of hydropower plants with low rock overburden have to be steel-lined. Since the water can reach in an uncontrolled way the rock surface in case of failure of these water-conveying systems, high damages due to landslides and debris flow can occur. Furthermore high strength steel is used nowadays for such steel liners, which have an increased risk brittle and fatigue failure. Storage hydropower plants and especially pumped-storage power plants are operating today more and more under rough conditions in order to satisfy the highly volatile peak energy demand. Therefore, an enhancement of the existing theoretical design model for steel-lined pressure shafts and tunnels is necessary. Finally due to the considerable risk of these hydraulic structures also new, non-intrusive monitoring methods have to be developed. Dr. Fadi Hachem addressed these issues during his thesis research. Regarding the question of appropriate design methods he applied for the first time the fluid-structure interaction which results in different wave speeds during transient operation and water-hammer formation compared to the normally used quasi-static approach. Furthermore he developed successfully a new monitoring approach, which allows to detect, to locate and to quantify the formation of week zones along steel lined pressure shafts and tunnels. The method is based on the analysis of water hammer signals produced by transient operation of the hydropower plant. Thus the monitoring method is non-intrusive and continuously. Dr. Fadi Hachem validated the new proposed monitoring method with systematic tests at a sophisticated experimental set-up in the laboratory. Dr. Fadi Hachem tested for the first time the performance of geophones for the assessment of water hammer signals. For the analysis of the highly dynamic and high frequency data he used advanced statistical and mathematical methods. He showed the application and implementation with in-situ measurements at the Grimsel II pumped-storage power plant in Switzerland.