Complex engineering challenge11 February 2021
Numerous hydroelectric plants include headrace tunnels that are approaching elevated ages, and in some cases, have been operating beyond their originally envisaged design life. Dean Brox from Dean Brox Consulting in Canada discusses the importance of inspections and maintenance for hydroelectric tunnels.
Headrace tunnels represent key components of hydroelectric plants and some facilities are rapidly increasing in age beyond 40 years. While routine maintenance and some major upgrades or repairs can be easily performed during short outages for powerhouse and hydraulic intake components, much more effort is typically required for longer outages to undertake inspections and maintenance for hydroelectric tunnels, primarily due to access constraints.
The linear nature of hydroelectric tunnels, and in particular, for single configuration tunnels, is associated with elevated risks where there is no redundancy in the event of a serious problem. Many ageing hydroelectric tunnels have experienced serious problems, including some collapses, simply due to their extended age and the absence of maintenance and repairs. New hydroelectric tunnels are also required to be inspected after a period of initial operations in order to confirm the adequacy of their original design.
Advances in robotic and data acquisition technologies with remote operated vehicles (ROVs) are increasing every year to capture additional information of improved quality to enable more comprehensive condition assessments to be performed of operating hydroelectric tunnels.
Figure 1: Rock fallout observed on tunnel floor during a robotic inspection
The continued ageing of hydroelectric tunnels without appropriate inspections and maintenance can be expected to result in additional partial blockages and full-scale collapses. Ageing hydroelectric tunnels are also associated with risks of poor environmental compliance since tunnel collapses can result in the complete stoppage of flows for fish habitats downstream of a tail-race or can even require excessive flow releases from the intake structure, which may be detrimental and cause significant erosion/scour and/or flooding of vulnerable areas.
Consequences of tunnel problems
The structural integrity of hydroelectric tunnels is of paramount importance to safeguard long-term operations for the generation of power. The occurrence of partial or full collapses in hydroelectric tunnels poses serious risks for overall operations and typically results in extended shutdowns for major repairs. Several major collapses of new hydroelectric tunnels have occurred since 2009 and some occurred during commissioning due to errors in design that resulted in outages for repairs of more than 24 months and total costs of more than US$250 million.
Similar to other engineering infrastructure, hydroelectric tunnels have a finite life of integrity, and maintenance and repairs should be included as part of normal operations. While some hydroelectric tunnels continue to operate without problems after several decades, the life of most tunnels is finite and serious problems can be expected after 30 to 40 years.
The greatest risk posed to the safe long-term operations of hydroelectric tunnels is the stability at the locations of geological faults and/or other weak geological conditions, such as non-durable rock conditions that were encountered during construction. Most of the recent collapses of hydroelectric tunnels that took place during commissioning occurred because of inadequate tunnel support of geological faults and the non-recognition of non-durable or scour-susceptible rock conditions.
Many geological fault zones are only supported with shotcrete for long-term operations that can be subjected to scour and deterioration. The status capacity of rock traps is also of paramount importance to confirm during an inspection, since rock traps that are full of rock debris will cause further debris to pass over the rock trap and enter into steel lining penstocks and the powerhouse and possibly cause serious damage.
Figure 2: International database summary of the collapses in unlined hydroelectric tunnels after years of operations. This provides an indication of the maximum life of unlined hydroelectric tunnels and the importance of inspections
Technical criteria for inspections
One of the main challenges for most hydroelectric operators is to decide when it is appropriate or necessary to perform a tunnel inspection. The requirement for a first or subsequent inspection and the frequency of follow-up inspections should be based on consideration of all relevant technical criteria including the following:
- Original tunnel lining design and distribution
- Original construction quality
- Existing age of tunnel
- Hydraulic operations (internal head/velocity)
- Hydraulic operations (peaking/non-peaking
- Historical construction problems
- Historical operational problems
- Historical erosion and debris accumulation
- Historical repairs and performance
- Status capacity and performance of rock traps
- Findings/defects of previous inspections
- Inferred tunnel performance and integrity
Figure 3: Recommendations for consideration for the frequency of inspections of aging hydroelectric tunnels.
A first inspection is typically performed as a result of a detected or inferred problem after many years of operation. The most important aspects to be considered are the hydraulic operations and the occurrence of historical problems. Hydroelectric tunnels that operate under peaking conditions are subjected to highly variable internal operating pressures with associated cyclic loading of the tunnel support and linings, and are therefore more susceptible to damage during long-term operations and warrant a higher frequency of tunnel inspections.
Unwatered inspections of hydroelectric tunnels are the preferred type of inspection whenever possible to prevent causing possible damage to an existing tunnel. Unwatered tunnel inspections using remote operated vehicles (ROVs) are typically performed during a total outage of the hydroelectric plant with zero flow conditions in order to obtain optimal data. However, unwatered ROV tunnel inspections can also be performed during non-zero flow, very low velocity, conditions, if absolutely necessary to maintain some limited power generation.
The execution of a manual or dewatered inspection should only be considered in the event that the findings of an unwatered (ROV) inspection indicate serious concerns, such as large volumes of debris along the tunnel, or large fallouts have occurred, the occurrence of substantial leakage, and associated head losses. The dewatering of hydroelectric tunnels for manual inspections can be expected to impose risks to the tunnel by causing additional instabilities.
The dewatering of hydropower tunnels needs to be carefully planned and performed in a very controlled and slow manner in order to limit instability. Manual inspections typically require extensive planning due to the greater time required for emptying the tunnel and an extended outage. ROV inspections are therefore recommended before a manual inspection to provide useful preliminary information. Drones can also be used for a preliminary manual inspection if suspect or unstable areas may be present.
Figure 4: Saab Seaeye Sabertooth ROV and Sub-Atlantic Mohican ROV
ROV technology and advances
ROVs have been used for underwater inspections of dams and other hydraulic structures for decades and more recently have been used for the inspection of long hydroelectric tunnels. ROVs are typically tethered for the inspection of long hydroelectric tunnels to provide power and for the transfer of the collected survey data. The maneuverability of ROVs enables them to access complex geometries of surge shafts and intake gate slots to enter long hydroelectric tunnels.
The longest single pass inspection of a hydroelectric tunnel was 12km at the Snowy Mountain scheme in Australia and the longest total inspection completed was for the 120km Paijanne drinking water supply tunnel for Helsinki, Finland. ROVs have operated at depths of more than 600m and some are capable of operating up to 2000m.
In low turbidity clear water ROVs can provide high-resolution photographs and video imagery. In high turbidity conditions, profiling sonars are used to provide continuous 360° data for a high resolution 3D point cloud model and associated visualisations.
Lastly, ROV contractors have developed and used visualisation software to aid in the observations during an inspection and this should be a fundamental requirement as part of the services, as it allows for immediate identification of locations of interest where debris may be present along a tunnel.
The integrity of aged tunnel linings is paramount as part of the overall condition assessment for a hydroelectric tunnel and to understand if there is any deterioration or void forming. To date there does not exist any standard approaches for the investigation and inspection of the integrity of shotcrete, concrete or steel tunnel linings within hydroelectric tunnels including the presence of voids behind linings. The use of modified forms of ground penetrating radar, seismic reflection, and acoustic emission techniques may be developed in the future to be used with an ROV inspection to provide this important information.
Technology for manual inspections
Dewatered manual tunnel inspections benefit from having direct access along the tunnel for detailed observations and identification of possible defects, scour and debris. Another important benefit is that samples can be obtained from shotcrete and concrete lining sections and tested for durability and strength.
One of the greatest uncertainties with concrete linings within ageing hydroelectric tunnels is the integrity behind the concrete lining since many tunnels were historically supported using wooden beams which commonly undergo rotting over time and result in the formation of voids behind the lining. Several methods of investigation are available in the industry for manual inspections as follows:
- Visual observations
- Strength testing
- Sonic and ultrasonic methods
- Magnetic methods
- Electrical methods
- Thermography methods
- Radar methods
- Radiography methods, and
- Endoscopy methods.
Montero et al (2015) also present a comprehensive list of robotic technologies for use in manual inspections along with their advantages and limitations including:
- Photogrammetry methods
- Impact methods
- Laser methods, and
- Drilling methods.
Technological advances are also being developed for new and hybrid methods to provide additional information during manual inspections.
Figure 5 illustrates an example of an external view of a long drill and blast excavated hydroelectric tunnel showing adjacent unlined and concrete lined sections that was created using specialised software.
The presence of erosion and debris along a tunnel is of critical importance as it represents some of the most important information about the condition and performance of the tunnel. Volumes or voids formed due to erosion of weak rock zones typically become concentrated and enlarged ongoing operations. If present along the tunnel crown they may manifest into an instability or with the fall out of rock blocks or even a large-scale collapse with partial or total blockage of the tunnel. Increases in the volumes of erosion and debris after ongoing operations are critically important to identify and compare from previous inspections as part of a condition assessment of a tunnel.
The conditions of concrete and shotcrete tunnel linings are typically of most interest, It is also necessary that numerous 3D visualisations are prepared from the inspection data to present relevant observations such as the geometry of the transitions of linings where scour usually occurs and any significant changes in the geometry of the tunnel profile.
Figure 6 presents an example of high-resolution data of a transition of a concrete lining section. These examples of visualizations are not typically prepared as part of the services of a ROV contractor and therefore require additional services as part of the post-processing of the point cloud data from a ROV inspection. Typical specialty software that can be used to prepare these images are Polyworks and Geomagic which can handle very large point cloud data files.
Tunnel asset risk assessment
The risk assessment for a hydroelectric tunnel represents a complex engineering challenge. The most important aspects to be considered are age, hydraulic operations, and the type of tunnel lining.
Following the completion of a tunnel inspection, an updated or new risk assessment should be performed based on the information obtained from the inspection, and both qualitative and quantitative risk assessments should be completed. A qualitative risk assessment should be based on all historical and recent observations made during the inspection, historical problems including leakage failures or instabilities, and historical and normal hydraulic operations.
A quantitative risk evaluation should be based on optimistic and pessimistic assumptions in order to provide an indication of the remaining life of the tunnel before a major collapse. Hydroelectric operators are recommended to perform a comprehensive risk assessment via a formal risk workshop with hydroelectric tunnel experts.