Tunnel focus

While it is may not be obvious at first, the strategic prospects for Norwegian hydro to become the “battery of Europe”, in support of the Continent ensuring the dynamic balance of the electricity system as it introduces more renewables in its generation mix, brings the prospect of significantly more tunnelling activity.

At this stage, it is envisaged that much of the tunnelling would be excavations for new projects, such as power and transfer tunnels. However, there might also be a need for underground works to strengthen or alter existing assets at long established plants, which would be functioning under altered operating regimes as a consequence of the link-up to part of the Continental grid.

The challenge for the existing infrastructure comes from the introduction of a new flow regime. To bring older, established facilities under the requirements, if not the duress, of another operating regime may present challenges for owners, operators and engineers to address before the link-up to the Continent takes hold.

Existing power stations and their underground infrastructure will need to be checked through hydraulic, sedimentation, geotechnical and engineering assessments to determine what, if any, changes might be needed to cope with flows that differ in patterns, frequencies and intensities of change from those of the original design.

Of potential benefit to help inform the discussion on water hammer and flow fluctuation at existing power tunnels are studies being undertaken by the Norwegian University of Science and Technology (NTNU). Much of the work, being undertaken as part of PhD research, involves work at the tunnels of the Tonstad and Tyin schemes.

The research fits under the HydroPEAK programme, which is part of the Norwegian hydro sector’s investigation into the information and knowledge needed for its assets to safely play a balancing role in large electricity grids with significant intermittent renewables generation, such as from wind power.

New tunnels

Norway has already identified the technical potential to build up to 20GW more hydroelectric capacity in the south of the country, nearest the European market, and do so without constructing any new reservoirs.

Already having an approach of operating many plants in river basin-wide, cascade systems, and able to restrict changes in water levels to less than notable rates under the potential new operations, the main changes that would be seen would be construction of power and pumped storage plants, and the associated new tunnels, to plants and also for transfers.

NTNU worked with Sintef to investigate the challenges of such large-scale, strategic hydro development on top of an existing asset base, and for the scheme to be constructed over a period of approximately 15 years. The client was the Centre for Environmental Design of Renewable Energy (Cedren) – a research initiative in which they are both involved and is also jointly funded by government, industry, research institutes and universities. The findings were reported in March 2011.

The focus was on constructing pumped storage plants ‘to balance the European wind production’, says the cover letter to the report on the work.

From the analyses, it was envisaged that 1GW-2GW could be added annually with a peak of 6GW on one year. The required tunnelling rates are estimated, on average, to be 3 million m3 per year with a maximum of 10 million m3 – all of which would be in addition to the industry’s existing underground infrastructure activities.

‘Such development will have a significant impact on the consulting business as well as the construction business in Norway,’ the cover letter says. It adds: ‘Consequently, as the situation is today, in this industry it is hardly believed that the current parties are able to absorb this amount of work with the current manning and equipment. It would be required to increase the capacity of the industry with significant resources to enable such a development to take place.’

Existing Tunnels

The majority of hydropower plants in Norway were built before 1990 and the advent of the liberated energy market. Before then, the operational regimes of the plants were designed to be relatively steady, and afterwards some stations have been used for peak load purposes. There was 3500km of hydropower tunnels built between 1950 and 1990, and investigations by Tidemann and Bruland showed the general stability was good but there were some problems.

In preparation for the proposed strategic, long-term changes for Norwegian hydro operations, early studies are underway to generally investigate how altered flow regimes can affect power tunnel networks and associated underground structures at existing plants.

The research is being undertaken, over 2011-15, for HydroPEAK’s Work Package 6 (WP6) – Effects of Load Fluctuations on Tunnels and Associated Hydraulic Structures – by PhD student Kari Bratveit, at NTNU. The supervisor is Professor Leif Lia.

The aims to create tools to help identify, characterise and get the scale of any altered risk, and therefore be useful in prevention planning as well as assisting plant owners to examine how fast production/discharge can be changed without causing undue stress and possible failures or uneconomical wear on equipment. In addition to pressure changes, some altered risks may be generated from the development of different air entrainment patterns and also shifts in sedimentation.

“There have been incidents of turbine damage due to increased sediment transportation and emptying of sand traps,” says Bratveit.

For the new research, the initial focus of the research is the 960MW Tonstad plant in Knvitnes, southern Norway, which generates approximately 3.8TWh of electricity per year – among the highest production in the country. The New Tyin scheme (currently seeing separate tunnel expansion as part of a rehabilitation of the dormant original underground powerhouse) is also a candidate site for the research work.

Other plants may be considered as part of the research ‘after the results for Tonstad and Tyin have been evaluated,’ says Bratveit.

Tonstad has five generation units and a 23.4km long main power tunnel that was excavated by drill and blast and is unlined. The plant – and New Tyin, too – experience large sediment loads passing through their turbines. Currently, an advanced control and protection system for the waterways is under development.

“This system can provide additional data that we can utilise in our research,” she says.

The Tonstad waterways also have three sand traps, and taking advantage of planned shutdown periods the researchers have surveyed and scanned one of the chambers – a 190m long, 12.5m high and 11m wide – and installed instrumentation. Bratveit has been assisted in the field work by master students Kjersti Ittelin and Simen Bjoringsoy.

The next stage of work is to create a CFD model “to simulate the hydraulics in critical parts of a waterway during hydro peaking,” she says, adding that the most relevant software for the work is OpenFOAM and STAR-CCM+. The critical part for mass oscillations is the surge chamber while the sand trap is crucial for sediment handling.

“Our goal is to use onsite measurements to verify the results obtained from our numerical simulations,” she adds.

Once the CFD simulations have been completed the research will move to physical modelling in the NTNU hydraulic laboratory.

Choices ahead

In addition to gaining timely, and sufficient, access to hydro tunnels, the main challenges in the research work are to simulate realistic load regimes and select correct hydraulic fluctuations.

“Our current knowledge about fluctuation is from the liberated existing market,” says Bratveit. “This market has little experience in balancing large amounts of wind power. We therefore need to test different ranges, scaled beyond today’s ranges.”

More generally, the risks from altered flow regimes that the future macro-plans for Norwegian hydro might present to plant owners might include: falling rock or collapses at zones of weakness with varying degrees of blockage; increased sediment transport which would impact the wear on turbines; and; increased head loss due to air pockets which would also lead to more cases of blow outs.

The current focus of methods to deal with such risks include redesigning critical parts of plants and strengthening support in structurally weak zones. The addition of new excavations at key points as part of re-engineering of waterway systems, and their hydraulic patterns, might also be considered.

Should sufficient hydraulic, sedimentation or air entrainment risks be identified at a plant in future, the cost-benefit of different upgrade scenarios will feature heavily, not least because of interruptions needed to generation and availability. The WP6 research aims to help and inform that process, and enable engineers in future to, as far as possible, get reliable answers without first requiring shutdowns and tunnels to be drained for initial investigations only. But, depending on available accurate data on waterways such choices might not be rules out by owners when assessing the longer-term life of the asset under the changing operational regime.