Long-term impact on unlined tunnels of hydropower plants due to frequent start/stop sequences

Abstract

Abstract

The concept of unlined pressure tunnel design is well-tested and has a history of more than 100 years. In Norway, more than 95% of hydropower pressure tunnels are unlined and most of it was built between 1950 and 1990. It is also popular outside of Norway because of its cost-effectiveness and shorter construction time. The main design principle is to prevent hydraulic jacking, which is obtained by suitably aligning the tunnel such that the in-situ stresses are sufficient to withstand the internal water pressure, without the use of extensive rock support and lining. Minor rockfalls are accepted during operation as long as they do not develop significantly and increase the frictional loss or cause blockage in the tunnel. It is seen that the operational regime of power plants in Norway has changed after the power market de-regulation in 1991. In the demand driven market, the power prices can vary on an hourly basis and the power plants can experience multiple load changes per day to benefit from the variable power prices, causing frequent pressure transients in the waterway. Further, an increasing share of unregulated energy from solar and wind power in the energy system as seen in the recent years will demand more operational changes from regulated hydropower systems which are used to maintain the balance between supply and demand. Such an operation will lead to frequent pressure pulsations and cyclic loading on the rock mass around unlined tunnels, and may contribute to increased instances of block falls as a result of rock mass fatigue. This research is focused on understanding the effects of frequent pressure pulsations in the long-term stability of unlined water tunnels. The work is based on cases from Norway and includes observations from inspection of four dewatered tunnels, instrumentation, and monitoring of one tunnel, operational data of 10 hydropower plants and numerical modelling using the distinct element code 3DEC. Results indicate that pressure transients can have significant influence on the pore pressure variation and joint displacement in the rock mass around unlined pressure tunnels as a result of the time-lag between the pressure transient in the tunnel and the rock mass pore pressure. It is the source of hydraulic stresses in the rock mass and is dependent on their hydro-mechanical properties. Results confirm the previous knowledge that mass oscillations cause larger hydraulic stresses in the rock mass as compared to water hammer. However, exceptions are known and the effect of water hammer may not be completely ignored. It is seen that 200-400 start/stops and more than 1000 load changes of varying magnitudes occur every year per generating unit in Norwegian power plants, causing frequent pressure transients. It is envisaged that this trend will further increase in the future due to addition of larger share of unregulated power from wind and solar energy. This implies that rock mass fatigue in unlined pressure tunnels may occur at an accelerated rate. The results indicate that an increased conservatism may be needed in rock support decisions in critical areas where the rock mass permeability permits significant pore pressure changes in the rock mass during pressure transient, especially for tunnels excavated in schistose rock mass, and power plants with multiple load changes within a day. Power plant operation is seen to have a significant influence on the amount of hydraulic stress acting on the rock mass during pressure transients. The shutdown/opening duration is usually dependent on the individual operator due to lack of standard guidelines for speed of load changes. Especially for large load changes, the power is usually changed in smaller steps, where the size and number of these steps are decided by the individual power plant operator. Results show that the shutdown/opening duration during load changes directly affects the time-lag between pressure in the tunnel water and in the rock mass. It is seen that shorter shutdown/opening duration i.e., faster speed, can cause significantly high hydraulic stresses on the rock mass. Thus, slowing down the load change operation can provide significant benefit in slowing down the fatigue process. Hence, it is recommended that more emphasis should be given towards keeping the speed of load changes consistently slow. A new term called “Hydraulic impact” is proposed to quantify the hydraulic stress on the rock mass caused by pressure transients in unlined hydropower tunnels. It can also be used to define a suitable shutdown speed of the power plants in order to help slow down the fatigue process. It is recommended to instrument and monitor more tunnels in order to validate and expand the results.