Flørli finds more power

In its operating lifetime Flørli has seen electricity develop from a luxury commodity to underpin our business and daily life. The oldest parts of the plant date from 1918, but in 1944-1948 it was already being expanded and at its installed capacity of 28MW it has been providing an average 168GWh to utility Lyse Kraft for the last 50 years. Now the plant has only a few good years left, and Lyse Kraft has decided to replace it, not only to meet current needs but to anticipate the next half-century of growth.

The decision to redevelop Flørli was taken in November 1995, explains Paul Johannessen, construction manager at Lyse Kraft. General ageing of the plant’s electrical equipment and machinery meant that efficiency was decreasing, while the cost of monitoring and maintenance was increasing. The point at which operation was no longer economic had not been reached, but it was certainly getting nearer.

Meanwhile, the attention of the Norwegian safety authorities had focused on the plant penstocks. Dating from 1918 and 1936, the penstocks – the steel pipelines that carry the plant’s feedwater from the reservoir above – are among the oldest structures in the plant. In each, the metal is joined along its 1400m length by forge welding, in which the metal is heated to 700-800°C and hammered. The Norwegian Water Resources Energy Administration licensed the penstocks to 2006, but decided after that date they must be replaced.

Lyse Kraft, considering the longer term, has decided that since electricity demand in the region is growing the future lies not simply in meeting safety requirements but in exceeding them, and is seizing the opportunity to make Flørli more productive and more efficient. The rebuilding is economically attractive too: investment in stage 1 is NOK300M (US$40M) and that in stage 2 (described below) is NOK120M (US$16M). Overall the new capacity is costed by Lyse Kraft at NOK1.35/kWh (US$0.18/kWh).

Johannessen says that in its new configuration Flørli’s capacity will be increased from 168GWh to 290GWh using the same catchment area. The penstocks will be replaced by a 1500m-long pressure tunnel and it is here that much of the efficiency gain is made: head loss, which was 60-70m using the original penstocks, has been reduced to 4-5m. The remainder of the extra power arises from more modern equipment and reduced losses in machinery, according to Johannessen.

As well as improving efficiency, replacing the penstocks with a tunnel will greatly reduce the visual impact of the plant – a consideration that is appreciated by local authorities, as some of the reservoirs that feed the plant are situated in one of Norway’s national parks.

A similar approach has been taken for the powerhouse, with similar benefits: the original above-ground structure is being replaced by one situated 800m deep inside the rock. This is not only the most economic option, Johannessen explains, but it enables efficiency to be improved because the new headrace tunnel reduces loss of head.

Project progress

Work on the reconstruction began in August 1996, less than a year after the November 1995 decision to go ahead. The longest part of the project has been rock work, Johannessen explains. All together some 170,000m3 has to be excavated – all of which is solid rock, and since it is similar to that falling into the Lyse Fjord every day, it too is being placed in the fjord. This excavation volume includes the power house, access tunnel, tailrace tunnel and headrace tunnel, including a vertical pressure shaft. Finally, it also includes the digging of a road access tunnel connecting the power plant area with the reservoirs that feed it.

The road tunnel, 2200m long, represents more forward planning by Lyse Kraft – it will be used to implement a project to increase the height of the dams and extend the reservoir (see below).

The excavation is due to be completed in mid-June of this year, Johannessen says, and as IWP&DC went to press at the end of February work was well up to schedule: the only parts still to be dug were the vertical shaft and a 200m headrace tunnel. Although the tunnellers had to deal with an unusually steep gradient – the tunnel drops 1m in 5m – they have been proceeding extremely fast since the turn of the year. Top speed has reached 170m per week, which Johannessen says is a new record in Norway for such a steep tunnel.

Construction is being overseen by Norwegian contractor Selmer, and the 35 tunnellers are using three Atlas Copco Rocket Boomers – 353S, H188 and H115 versions – for the work.

The excavation work hit some problems in its earlier stages, Johannessen notes. ‘There was very high stress in the rock masses’, he says, ‘and we saw ‘earthquakes’ in the area that reached level 2 on the Richter scale – especially in the main tunnel’. The tunnel has since been reinforced with shotcrete and steel bars inserted into the rock masses surrounding it, he said. Water was another problem: ‘There was water incursion at pressures up to 40bar,’ Johannessen says, ‘which was dealt with by cement injection’. He is confident that these measures have dealt successfully with the problems.

The second stage of the project, due to begin on 2 June this year, will see the power house equipment installed. In fact, some equipment is already in place: some turbine parts will be surrounded by concrete and these were installed, and the concrete poured, in mid February. Major equipment is being supplied by Kværner (turbine), ABB (generator and transformer) and GEC Alsthom (control and electrical systems).

The rebuilt plant at Flørli is due to go into operation on 1 January 1999. Lyse Kraft expects the new version of the power station to supply it with around 290GWh annually.

Flørli will contribute to maintain the Norwegian energy balance, but Lyse Kraft sees an opportunity to expand in the longer term, and it is already making plans to increase capacity.

The new configuration at Flørli can incorporate plenty of expansion. The tunnel and station have space to double capacity again – adding an extra 80MW – when it becomes economic. Further, Lyse Kraft would like to upgrade the reservoir system above the power plant and has already applied for permission to do so.

Johannessen explains that the company aims to increase the head by raising the level of the main reservoir from 763.5m to 780m, and increase the level of one of the smaller reservoirs by 8m. As well as improving the head, this will also increase the reservoir capacity from 30% to 55% (ie the reservoir will be able to store 55% of annual rainfall) and so will increase the amount of electricity that is available in winter time.

Raising the level, according to Johannessen, can be accomplished by rehabilitating one currently existing dam and building three new dams: one rockfill dam 35m high with a moraine core (which will contain 200,000m3 of rockfill) and two minor concrete dams (about 4000m3 in volume). One of the latter will be combined with a spillway.

Finally, the company plans to construct a pump station so that it can pump from today’s intake reservoir at 740m asl to the new reservoir at 780m asl.

Construction of these dams is expected to be completed in a single season, 1 May to 1 November 1999, and landscaping is planned for the summer of 2000.

Will such ambitious plans come to fruition? Johannessen admits that there are no customers willing to pay for the additional capacity yet, but says that if the Norwegian authorities grant permission work on the reservoirs will start in summer 1999. The chances are that Lyse Kraft is right to expect demand to grow: all the evidence of 80 years operation at Flørli is that history is on the side of expansion.