Norway has a long history in hydropower and has a yearly production of 135 terawatt hours (TWh) distributed across more than 1,600 hydroelectric power plants. This production capacity covers more than 94 percent of the total electricity usage in the country. The vast majority of power plants in Norway were built before 1990, and more than 200 km (124 miles) of associated tunnels were excavated by tunnel-boring machines (TBM) during Norway’s big hydropower era stretching from the late 1960s to the early 1990s.
As a larger degree of Norwegian rivers, streams and waterfalls were tamed for hydropower, public resistance grew against hydropower projects. In the mid-1990s, then-Norwegian Prime Minister Jens Stoltenberg declared that the era of big hydropower construction was over.
Nevertheless, the Norwegian topography and water resources still represented a major potential for hydropower, especially if a solution with less impact on the environment could be found. One of these solutions included small hydropower projects, defined as hydropower plants with an installed capacity of less than 10 MW.
This article addresses why small hydropower projects are a great way of generating electrical energy and why mechanized tunneling is a beneficial way of making them.
Small hydropower projects and why they matter. There are currently more than 1,300 small hydropower plants operating in Norway with an installed yearly production of 11 TWh. The small hydro share of the total power production is currently around 11 percent (Fig.1).
The local impact on nature of these small projects is generally lower than larger hydropower projects, construction is cost efficient and quicker, and the initial investment required is lower. The widespread availability of locations where these projects can be built also offers a great distribution of value generation across all parts of the country (Smakraftforeninga, 2016).
Construction of small hydropower projects. A significant number of the existing small hydroelectric power projects (SHEPPs) had been constructed either with pipes on the surface or by trenching. In recent years it has been a general trend that larger parts of these SHEPPs are built in tunnels, either due to the topography or to reduce the environmental impact even more.
In small hydroelectric projects that require an underground waterway, the tunnel is usually built by one of the following methods:
- Drill and blast tunneling.
- Raise drilling.
- Directional drilling.
- TBM boring.
As a rule, trenching is the most cost-efficient solution for such projects. However, in many cases the topography and nature of the projects do not allow for trenching. If a tunnel is needed, the other options have historically been between drill and blast tunneling, raise drilling or directional drilling or a combination of those methods.
The SHEPPs that consist of a tunnel often have some physical constraints that limit the construction method:
- There is naturally a big elevation difference between the tunnel portals.
- There is generally as much overburden as is practically possible toward the downstream portal to avoid challenging geology, hydraulic fracking and hydraulic jacking, and to lower costs.
These limitations mean that the vertical profile of a SHEPP tunnel is often similar to Fig. 2, with limited inclination in the downstream portal and high inclination toward the upstream portal.
The traditional way of constructing such projects has been to drill and blast the flat part and raise bore the incline. A concrete plug is installed where hydraulic jacking forces are lower than the minor principle stress in the surrounding rock and further through a pipe in the tunnel toward the powerhouse. The most common blasted cross section is between 16 and 25 m2 (175 and 269 sq ft) due to limitations in the available equipment as well as the challenges of excavating efficiently with drill and blast at diameters smaller than 16 m2 (175 sq ft). If the tunnel was to be excavated with other methods, a profile like Fig. 3 would be typical.
The alternative to the conventional method has been directional drilling performed with a heavily customized directional drilling rig such as that devised by Norwegian company Norhard AS. The Norhard drilling rig consists of a pilot tri-con bit for drilling with carbide raise drill cutters to ream up the diameter of about 0.7 m (2.2 ft). The hole can then be reamed up with several drillings up to a diameter of 1.5 m (5 ft). The drill string is powered by a nonrotational drill string from the outside (Fig. 4).
As the SHEPPs have become increasingly complex, TBMs have been introduced on several projects in Norway in recent years as an excavation method that has its own unique benefits.
Benefits of mechanized tunneling for hydropower. Mechanized tunneling offers some significant advantages on unlined hydroelectric power projects:
- Reduction of needed cross section due to lower surface roughness.
- Better tunnel quality, resulting in less rock support and lower lifecycle costs.
- Less impact on the environment.
- Reduction of tunnel construction time.
Due to the lower surface roughness of the tunnel wall in a mechanically excavated tunnel, the water flows better and the needed theoretical cross section can be reduced by 40-60 percent. A more detailed graph is given in Fig. 5.
The more efficient water flow, and the capability of using the tunnel as the water-carrying pipe, reduces the need for excavated material significantly. This means less excavated material needs to be removed and stored and is also economically advantageous.
Less rock support is required in general in mechanically excavated tunnels, and because of the better tunnel quality, there are lower lifecycle costs to maintain the tunnel. Mechanized tunneling also disturbs the environment far less than drill and blast operations. The empirical data from TBM-excavated hydropower projects in Norway support these points. Results show that there is a reduction in installed rock support of between 40 and 90 percent when boring a tunnel with a TBM instead of blasting it. The theory behind this result is that a lot of the rock support in blasted tunnels with small cross sections is installed to stabilize rock that has been damaged by the blasting. The TBMbored tunnel walls are less damaged, which also increases tunnel quality, ultimately leading to lower maintenance cost of the tunnels and longer tunnel life. Also, the smaller tunnel dimension and the circularity of the hole increases the stability of the rock and decreases the need for rock support.
Excavation with TBMs also offers several environmental advantages. The TBM and muck haulage are typically run on 100 percent electric power from the grid, which in Norway consists of 94 percent renewable energy. In addition to the already-mentioned environmental aspects that include reduced excavated material, mechanized tunneling eliminates the risk of nitrous run-off and plastic waste that are present in drill and blast material deposits.
TBMs for small hydropower
For the past 10 years there has been interest from owners, contractors and the government to develop TBM solutions for some of the upcoming SHEPPs in Norway. From a TBM design perspective, there are some special challenges of the Norwegian SHEPPs:
- The theoretically needed cross section is usually very small and requires TBMs smaller than 3 m (10 ft).
- Norwegian rock is often extremely hard.
- The geometry of the projects is often challenging, with the tunnels often containing high inclines, combined with vertical and horizontal curvature.
- Some of the projects have limited space on site and no road access at the upstream portal.
- The length of the tunnel is typically between 500 and 3,000 m (1,640 and 9,800 ft).
- The budgets for these projects are often extremely limited.
These challenges require a unique TBM design:
- The TBM needs to be small but still equipped with sufficient cutter sizes to efficiently break the rock.
- The TBM cutter diameters must be as large as possible and the highest quality disc ring material must be used to reduce cutter changes due to the limited space.
- The machine needs to be able to negotiate steep inclines and the transitions between inclines.
- The TBM needs to be able to backtrack through the tunnel.
- The TBM must be optimized to be used on several projects with limited service time in between.
- The TBM package needs to be economically viable.
- The TBM might need to be able to launch from a limited space area.
This led Robbins to develop two separate solutions for the Norwegian hydro projects. One was based on small boring units (SBU), a line of trenchless boring equipment and machinery typically 2 m (6.6 ft) or smaller in diameter, while the other used more standard TBM technology.
Small-diameter design: Holen hydropower. The first tunneling machine for SHEPPs was ordered by Hardanger Maskin AS, for the project Holen Hydropower owned by Smaakraft AS in early 2018.
Robbins developed a new solution for the project using well-proven SBU technology. The Double Shield Rockhead (SBU-RHDS) provided for the tunnel includes 360 cm (14 in.)-diameter cutters and is capable of selfpropelled excavation using a gripper system.
The novel 2 m (78 in.)-diameter machine is equipped with unique features that allow it to drill at a steep incline, including electric power, modified oil and lubrication systems and a fail-safe safety gripper (secondary gripper), as well as a water-based spoil removal system, developed by the contractor (Fig. 6).
Due to local terrain, the tunnels had a small launch area of 4 × 10 m (13 × 33 ft) and the tunnel slope on the first 640 m (2,100 ft)-long drive ranged from a slight upward tilt to 45 degrees at the breakthrough.
The Rockhead launched in July 2018 with Robbins Field Service onsite assisting Hardanger Maskin AS with assembly, setup and launch of the equipment. As tunneling began, the slope was near horizontal, but as the tunnel got steeper, the special safety gripper system came into use. The safety gripper system was designed with interlocks to ensure primary grippers were never released while the safety grippers were engaged and with an additional safety mechanism that allowed for mechanical locking in the event that hydraulic pressure was lost.
While the excavation rate of the machine was good, the novel design experienced some reliability issues during tunneling in the hard granite. Despite the challenges, the machine completed a daring hole-through at a steep 45-degree incline on Jan. 1, 2019. It has since been relaunched to bore its second 640 m (2,100 ft)-long tunnel, where the design continues to be fine-tuned (Fig. 7).
Unique TBM and conveyor solutions: Salvasskardelva SHEPP. The other solution, based on more standard TBM technology, came into use in the summer of 2019. Robbins supplied the 2.8 m (10 ft)-diameter specialized main-beam TBM, Snøhvit, to Norsk Grønnkraft to use on several of their hydroelectric tunnels. In addition to investing in a TBM, Norsk Grønnkraft also started a specialized contracting company, NGK Boring, that entered into a cooperation with Entreprenørservice AS to construct the tunnels.
The first tunnel, the 2.8 km (1.7 mile)-long Salvasskardelva SHEPP located in Bardu, Norway, has a modest positive gradient of 5.2 percent. To combat boring on a grade, the small main-beam TBM was designed for adaptability with an option to add a safety gripper on future tunnels for boring at high inclines.
The TBM is equipped with nineteen 432-mm (17-in.) cutters with a load rating of 267 kN each (Fig. 8). The 2.8 m (1.7 ft)-diameter cutterhead is powered by four 210-kW variable frequency drives (VFDs).
A continuous conveyor was provided for muck removal, making it the smallest conveyor belt Robbins has ever provided. The 450 mm (18 in.)-wide conveyor belt will need to travel through curves, which begin at the 650-m (2,132-ft) mark at Salvasskardelva. The structure was designed to minimize muck spillage in curves despite its narrow width and is within its design limits. The small jobsite also required the use of a double-stack belt storage cassette standing 5 m (16 ft) tall. The unique system is planned to be reused at each of the tunnel sites.
NGK and Robbins worked together during the design period to design a launch frame instead of excavating a starter tunnel, which allowed the machine to advance until it was well enough into the tunnel to grip the tunnel walls. The launch frame is planned for reuse on subsequent tunnels.
As of December 2019, the TBM had surpassed the 1,300-m (4,265-ft) mark, and the machine was excavating well in mica gneiss and schist rock, achieving rates above 100 mm (3.9 in.) per minute.
Small-diameter hydro tunneling looks poised to continue making a big impact in Norway. A third TBM, a 2.6 m (8.5 ft)-diameter Robbins Double Shield TBM with a safety gripper, began excavation in the winter of 2019 at the Tokagjelet SHEPP. The alignment of the 2.2 km (1.4 mile)-long tunnel will increase gradually from near-horizontal to a 45-degree incline.
Hydroelectric power generation has historically required high investments and mountainous topography with water in abundance. Unfortunately, the sizes and complexity of traditional hydropower projects also tended to have a negative impact on the surrounding environment.
The small hydro project approach introduces an opportunity to construct renewable energy with limited investment and limited negative consequences on the local environment. Given the increasing interest in small hydro tunnels and the fine-tuning of effective designs for rock tunnels at steep inclines, there is a huge potential for continued projects in Norway and in other locations such as the United States. Renewable energy with a reduced initial investment and construction time could become essential wherever the terrain is hilly or mountainous and water features abound.
Infrastructure spending, particularly on water tunnels, is on the rise in the United States, and it is believed that there is a tremendous potential for small hydropower projects in North America. Small hydropower is likely to become more popular as it offers the best of several worlds. It is an environmentally friendly way of generating power, is less taxing on natural resources and is costeffective and quick to implement. It does not require the large waterfalls and high mountains that big hydropower schemes require.
SSB (2016). Kryssløpstabeller – ESA Questionnaire 1850 – Symmetric input-output table for domestic production (industry*industry). Tilgjengelig på https://ssb.no/en/nasjonalregnskap-ogkonjunkturer/tables/ supply-and-use-and-input-output
Småkraftforeninga (2016).“Samfunnsnytte av småkraft”. Thema Consulting group
NTNU (1998). NTNU-Anleggsdrift: Project Report 1A-98 HARD ROCK TUNNEL BORING Design and Construction. Norhard (2019). Pictures from Norhard.no NGK (2019). Picture courtesy of Norsk Grønnkraft