Back in service for hydro expansion

21 February 2011

A veteran Robbins TBM has been brought back into service on the Faroe Islands to aid expansion of the Eidi hydropower station

More than 25 years after its first tunnel, a veteran Robbins TBM has embarked on its third project on the Faroe Islands. The 3.35 main beam machine is the first and only TBM to ever operate in the Faroes, and was purchased in 1984 by Streymoy, Eysturoy and Vagoy (SEV), the utility company of the Faroes. Once its third project is complete, the machine will have nearly 34km of hard rock tunnels under its belt.

“The biggest challenge so far has been the disassembly, renovation and reassembly of the old TBM and material due to the location on an isolated island. Parts were sent all over the world for renovation, then sent back to the Faroes for assembly onto the machine. Having said that, we have tried to refurbish as much as possible here on the Faroe Islands and have been very impressed by our local suppliers and contractors,” said contractor MT Hojgaard.

In October 2010, Danish and Faroese contractors MT Hojgaard and J&K Petersen launched the seasoned machine for the newest phase of the islands’ green energy initiative. The Eidi II hydropower expansion project is located on the main island of Eysturoy, and involves bored collector tunnels to transfer rainwater to the Eidi station, generating an additional 16GWh/yr, for a total of 56GWh/yr when finished.

Project background

The Faroes, an archipelago in the North Atlantic located between Iceland and Norway, consists of 17 inhabited islands and one uninhabited island. The islands have a population of just over 49,000 people. Due to its location in the Gulf Stream, the climate is very temperate and produces 250 days of precipitation throughout the year, which flows down the many rivers and streams across the steep mountainsides. These factors are optimal for hydropower, an industry that started on the Faroes back in 1921.

The Eidi II tunnels are seen as a main part of SEV’s green energy initiative. “Today approximately 8% of the energy supply in the Faroes is based on renewable resources; the rest is based on fossil fuels. The government has declared a new climate policy, calling for a 20% reduction in greenhouse gas emissions in 2020 compared with 2005. To fulfill the policy, we need several green initiatives to be executed. In the power sector, we have projects including turbines and the hydroelectric station,” says Leivur Hansen, public relations manager for SEV.

Annual production of electricity in the Faroes in 2009 was 276GWh, with 96GWh from six hydropower plants. With the exception of the Botni plant, all of the hydropower stations supply power to the same electric power grid, which serves 11 islands. Thermal power is the main source for electricity but wind energy using four wind mills on Eysturoy is also showing promise for the green energy sector.

Eidi hydropower station

The Eidi project, which started back in 1984 and was operational by 1987, consists of a reservoir with a holding capacity of 16Mm3 at an altitude of 149m. The structures are connected to multiple TBM collector tunnels totalling 24.7km in length and receiving rainwater runoff from 87 intakes. The powerhouse runs on two Francis turbines each 6.7MW, which allows the plant to produce 39.8GWh/yr.

During original construction, a Robbins Model 24RM blind raise drill was used to bore a total of 150 collector shafts from the bored tunnels, which ranged from 5-30m deep. The same Robbins blind raise drill will also be used to bore multiple collector shafts from the new Eidi II tunnels.

Eidi II, currently under construction, is scheduled to be commissioned in 2013. This new phase extends the southwestern branch by 9km of bored tunnel, as well as 2.4km of drill and blast tunnel between the old and new TBM tunnels, all of which will feed from 32 new intakes. The existing hydro station will receive a new 8MW turbine expected to increase production by an estimated 16.2GWh/yr.

Veteran TBM

The Robbins main beam TBM was first built in 1984 for the original two tunnels of the Eidi project, measuring 4 km and 6.4 km in length. The initial tunnels were completed in 1987 and during the 1990s an additional three tunnels were bored measuring 3.2km, 1.2 km and 9.9 km respectively – for a total of 24.7km of bored tunnels. The machine was then stored in a closed workshop by the hydroelectric station for ten years before it was recently brought back into commission for its latest mission.

“The TBM was well-built by Robbins, and we have done a good job maintaining it,” said Anders Nedergaard-Hansen, head of the power production department for SEV.

Due to sitting for ten years, the machine underwent some refurbishment to the gearboxes, main bearing, lube system and hydraulic hoses. “Prior to starting up this project, we contacted Robbins for an assessment. We got an immediate response, and two men were sent to inspect the machine. They concluded it was in good condition following the upgrades,” said Nedergaard-Hansen.

The 3.35m TBM has 5227 kN of thrust and 519,791 N-m of torque, making it ideal for the islands’ hard basalt rock.

Third project launch

Prior to the start of tunnelling, crews mapped the surface area around the site and conducted a thorough geological survey with probing and examination of drilled core-samples. The geological conditions surrounding the tunnel were determined to consist of 4.4km of basalt and 4km of sill. Rock strengths for the basalt range from 69-177 MPa UCS. The TBM was assembled on a construction site near the Nordskali-tunnel, an existing road tunnel. Robbins GmbH provided technical support during the assembly, and is currently providing field service, additional support, training and spare parts throughout the duration of the project.

Following assembly, the TBM began excavation of the collector tunnel, which brings the machine 100m into the mountain and within a few meters of the existing tunnel before drilling 8.4km away from the Eidi hydroelectric station. This tunnel is the third extension of the project, and when finished will provide a total of 17.5km of collector tunnel for the plant.

Excavation is being done five and a half days a week with two 12hr shifts Monday through Friday, and one shift on Saturday. Each shift employs six crew members plus supporting personnel outside the tunnel. Though the machine has only been boring for three months, tunnelling has advanced as far as 1.2km. As of December 2010, the machine is excavating around 200m a week as boring begins to ramp up. No ground support has been needed to date; however, rock bolts, wire mesh and dry shotcrete will be used if more difficult ground is encountered.

Cutter wear is minimal, which is largely attributed to the rock being suitable for excavation using a TBM. Excavated muck is either being conveyed out of the tunnel and stored at a designated location or being distributed for use at local projects (harbour expansions and road works in the mountains). The tunnel is on a 3% grade, climbing a total of 25m over the course of the project.

Technology has advanced significantly in the tunnelling world since the machine was built more than 25 years ago. Overall, the veteran machine is looking and performing well despite some minor challenges. The TBM steering is done manually by adjusting the TBM to a laser beam, which is hung from the tunnel roof and directed in a specific direction using prisms also hung from the tunnel roof. The crew is adapting the system for use around slight curves, which can cause the laser beam to disappear if not adjusted correctly.

The tunnel for Eidi II will mark the final stage for this project; however, projects throughout the island may use the TBM in the future. The added kilometers will, no doubt, make the TBM one of the world’s veteran tunnelling machines. Many such machines have become lifetime investments, boring up to 50km or more.

Nedergaard-Hansen is up to the challenge: “We are looking forward to using the machine for additional projects totaling up to 20 km in the next five to ten years.”

Article by Brian Sethman. Email: [email protected]

Crew Crew

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