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 of hydroelectric tunnels, primarily due to various 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 and increasingly for insurance purposes.

Advances in robotic and data acquisition technologies with remote operated vehicles (ROVs) are increasing every year. They allow the capture of additional information of improved quality which enable more comprehensive condition assessments to be performed in operating hydroelectric tunnels.

COLLAPSE

The continued ageing of hydroelectric tunnels without appropriate inspections and maintenance can be expected to result in instabilities such as partial blockages and full-scale collapses. Ageing hydroelectric tunnels are also associated with various risks of poor environmental compliance, since tunnel collapses can result in the complete stoppage of flows for fish habitats located downstream of a tailrace. They may 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 power generation. 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 in new hydroelectric tunnels have occurred since 2009; some have occurred during commissioning due to design errors that resulted in repair outages 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 therefore 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 operation of hydroelectric tunnels is their stability at 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 at geological faults and a failure to recognise non-durable or scour-susceptible rock conditions.

Many geological fault zones are only supported by shotcrete for long-term operations, including peaking operations, that can be subjected to scour and deterioration. During an inspection, it is of paramount importance to confirm the status capacity of rock traps, since rock traps that are full of rock debris will cause further debris to pass over the rock trap and enter into steel-lined penstocks and the powerhouse, thereby causing potentially serious damage.

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:

  • 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.

AGEING

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 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. These are therefore more susceptible to damage during long-term operations, thereby warranting a higher frequency of tunnel inspections.

To prevent causing damage to an existing hydroelectric tunnel, ‘unwatered’ inspections are preferred whenever possible. Unwatered inspections occur when a tunnel is not emptied but remains full of water, typically without flow (zero-flow condition) – this is ideal for ROV inspections. With some hydroelectric plants, if may be necessary to maintain limited power generation, for example, to allow the essential operation of a smelter, or to maintain limited power generation for a local community. In these cases, a minimum-flow condition has to be maintained through the hydroelectric tunnel and unwatered ROV tunnel inspections can be performed during non-zero flow, with very low-velocity conditions.

(Contrast this with a ‘dewatered’ inspection which occurs when the tunnel is emptied completely to allow manual inspection by personnel).

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.

A manual or dewatered inspection should only be considered when the findings of an unwatered ROV inspection indicate serious concerns, such as large volumes of debris along the tunnel, large fallouts or 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.

Dewatering hydropower tunnels needs to be carefully planned and performed in a very controlled and slow manner in order to limit any instability. Manual inspections typically require extensive planning due to the greater time that is 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 where suspect or unstable areas are present.

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. When inspecting long hydroelectric tunnels, ROVs are typically tethered to receive power and to convey 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 hydro 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 2,000m.

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 in inspections – this should be a fundamental requirement as part of the service, as it allows for immediate identification of any locations which are of interest along a tunnel where debris may be present.

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 are no standard approaches for investigating and inspecting 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 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 associated with concrete linings in 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.Manual investigation may make use of the following methods:

  • Visual observations
  • Strength testing
  • Sonic and ultrasonic
  • Magnetic
  • Electrical
  • Thermography
  • Radar
  • Radiography
  • Endoscopy.

Technological advances are also being developed for new and hybrid methods to provide additional information during manual inspections. Robotic technologies for use in manual inspections, include:

  • Photogrammetry
  • Impact
  • Laser
  • Drilling

OBSERVATIONAL DATA

The presence of erosion and debris is of critical importance as it represents some of the most important information about the tunnel’s condition and performance. Volumes or voids formed due to the erosion of weak rock zones typically become concentrated and enlarged during normal operations. If present along the tunnel crown, they may result in an instability, a fall-out of rock blocks or even a large-scale collapse resulting in partial or total blockage of the tunnel. Increases in the volumes of erosion and debris during normal 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 necessary for numerous 3D visualisations to be prepared from inspection data in order to present relevant observations, such as the geometry of lining transitions where scour usually occurs, and any significant changes in the geometry of the tunnel profile.

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.