| dc.description.abstract | There is change in operational regime of hydropower power plants in Norway after deregulation of energy markets and recent integration of renewable and unregulated energy sources such as wind and solar in an energy mix. Hydropower being a regulated system, their operators adjust production level based on demand and supply trends to get benefit from variable power prices. This causes an increase in the number of load changes in hydropower plants causing frequent pressure transients in the waterway system. The frequent pressure pulsations induce cyclic loading on rock mass resulting in rock mass fatigue over the long run and may contribute to increased instances of block falls.
This study is focused on understanding the effect of frequent start and stop sequences of hydropower in unlined pressure tunnels. For this purpose, it utilizes data from pore water pressure monitoring system which was installed at the downstream end of headrace tunnel at Roskrepp hydropower plant. This study describes a framework for accessing raw pressure data, evaluating Hydraulic Impact (HI) and Maximum Pressure Difference (MPD), and quantifying the impact of transients based on HI and MPD. HI was introduced earlier, whereas MPD is a new term proposed in this study to quantify the hydraulic stress exerted on the surrounding rock mass due to pressure transients in unlined hydropower tunnels. The monitoring of pressure data over the years clearly shows that the frequent load changes due to transient in the waterway could cause a considerable effect in the rock mass and constituent joint system. A delayed response to pressure from boreholes with respect to the tunnel is seen in all start and stop sequences and is considered as the main reason for instability arising due to transients. The response of pore pressure in boreholes is greatly influenced by the condition and properties of joint, its geometry and wall properties.
Results show that start sequence induces more HI and MPD than stop sequence. Numerically, the start sequence produces three times as much as HI than the stop sequence over the study period. It is seen that there are changes in operation pattern of hydropower and decrease in shutdown duration after late 2019 which has increased both HI and MPD in all boreholes. The study indicates that a faster shutdown duration induces more stress in the rock mass surrounding the tunnel. The HI can be as high as five times when the shutdown duration is decreased by 50 %.
A Machine Learning approach is attempted to predict the borehole pressure using LSTM method. The model works satisfactorily with MSE of 0.0055 and R2 of 0.82. However, this approach cannot be used to evaluate the impact of transient at the moment and is potential prospect for future research work.
Analysis in UDEC program is carried out to evaluate the block movement due to change of roughness parameters of rock joints by transient induced cyclic loadings. Analysis indicates that there is increase in total and shear displacement of block when the roughness characteristics of rock joints are degraded due to cyclic fatigue of rock joints.
Finally, this study recommends minimizing the shutdown duration so that the impact caused by start and stop of hydropower in rock joints of unlined tunnels and shaft could be minimized, enhancing their lifetime. It is recommended to increase the number of instrumentation programs in other projects particularly where block falls are expected to better improve the understanding of transient induced instabilities in unlined pressure tunnels. This study also recommends implementing a more conservative design approach, particularly in tunnels with weak rock masses and projects that involve frequent load changes. | |
