Ductile iron pipes with restrained joints make difficult pipe-laying work possible in the Gastein Valley

Since the beginning of July, work has been underway in Dorfgastein, Salzburg on a new small hydropower plant for ÖBf Wasserkraft GmbH, a 100 % subsidiary of Österreichische Bundesforste AG. This is a modern high-pressure power station which in future will use the energy of the Luggauerbach mountain stream. The plant is to be built in compliance with the highest ecological standards using environmentally sound construction methods. Steep mountain slopes, geological pitfalls and an extremely narrow route are the central challenges confronting the construction team when constructing the pressure pipeline for this project.

To ensure that the penstock is built to last and to be reliable, the operator, ÖBf, is placing its trust in the quality of ductile iron pipes from the TRM company. The penstock should be ready for final pressure testing by November 2017.

The challenge of constructing the pipeline

A natural gradient of 270 m in height forms the basic topographical condition for the new Luggauerbach power station in the beautiful Gastein Valley. To route the water from the mountain stream safely and effectively to the power station turbine, a 1,630 m long pressure pipeline is being constructed. Accomplishing the construction of the penstock is certainly one of the greatest challenges in the course of this project. As at the end of September 2017, the team from the contractor company had already completed around two thirds of the total length of the route.

“The ratio of 270 m height to a pipeline length of 1,630 m alone suggests that the course of the route is relatively steep here. This is good from an economical point of view, but in construction terms it can present difficulties”, explains the Project Manager of ÖBf Wasserkraft GmbH Gerhard Breitenbaumer.

Here he points out a historic scree slope directly above the steep section in the upper section of the route, for which special precautions had to be taken: the area was secured with crossbars in order to avoid accelerations in the line of steepest slope. Even this was no simple undertaking for Rumpf Bau, the construction company commissioned for the work.

“The Rumpf Bau company really had to put their abilities to the test here. Without walking excavators and without a material ropeway, the team managed to get concrete, pipe materials and other equipment up the precipice and work on it there. For mobility in the steep terrain, raised web boards were fitted to the digger caterpillars. In this way it was possible for the machines to manoeuvre in the steep slope even in dry conditions,” says the Project Manager.

Extreme loads mastered

In the light of the steepness of the terrain, the difficulties with access and the geological instability, choosing the right pipe material naturally played a decisive role.

To ensure that the penstock is built to last and to be reliable, the operator, ÖBf, is placing its trust in the quality of ductile iron pipes from the Tiroler Rohre GmbH. For the experienced hydropower operators at ÖBf there could be no doubt that the ductile iron pipe system from the Tyrol manufacturer Tiroler Rohre GmbH, known for quality, offers a high degree of reliability from this point of view.

“In the end, the cast iron pipe was the best and most economical option,” explains Gerhard Breitenbaumer about the choice which proved to be very satisfactory. Functionality and a long working life – these are the well-known plus points of iron pipes from Hall in Tirol. It is obvious that, as an operator, one is eager to avoid costly welding work and weld inspections when working in difficult terrain; above all this can be directly reflected in time saved and therefore in costs saved. Which is an important argument in favour of ductile iron pipes.

To be precise, pipe systems in dimension DN 500 with VRS^®-T push-in joints were used for the 1,630 m long pressure pipeline above Dorfgastein. This is a positive locking restrained push-in joint which also withstands stresses from extremely high forces. Depending on nominal size, therefore, operating pressures of more than 100 bar or permissible tensile forces of up to 200 kN can be absorbed.

“With this positive locking joint we can save ourselves the trouble of constructing concreted fixing points, which are not only relevant from an economic perspective but are also time-consuming to produce”, explains Gerhard Breitenbaumer.

Tight conditions

Another important point in the planning of the route for the pressure pipe was posed by nature protection requirements: in order to interfere as little as possible in the natural landscape of the Gastein valley, it was stipulated that the maximum width of the pipe route should be 6 m.

“That sounds like more than it is. Particularly in the steep sections, it was not easy for the construction company to restrict the pipeline area to 6 m. But Rumpf Bau managed even this very successfully,” are the words of praise expressed by the project manager for the construction company, which applied its broad experience to the installation of the cast iron pipes.

The pipes were installed according to the open-close method. This quite simply means that only one pipe is ever laid at a time and, as soon as it is connected, the pipe trench is closed again. This not only makes for fast progress with the construction but it also ensures that the work is essentially independent of weather conditions. Around 5 to 6 pipes with a unit length of 6 m are currently able to be installed per day by the team from Rumpf Bau.

Thanks to this tempo, to date there has also been no problem in keeping up perfectly with the project schedule. “Our schedule is very tightly controlled, but it does leave some reserves open” says Gerhard Breitenbaumer. Which means, according to the project manager, that even external factors played a central role in the schedule: an instruction by the hydrographic agency had warned that work in the water run-off area should only be started as from 1st September. Another from the mountain torrent and avalanche barrier agency again only approved construction work in the area of the Mur protection embankments close to residential areas as from 15 October. Understandably, the structures had to be able to perform their protective function to 100 % right across the period of flood hazard.

Harmonious coexistence

There are many reasons why the project schedule was kept to so well. According to Gerhard Breitenbaumer, the dry weather during this Summer was just as favourable to rapid progress as the good project management and excellent collaboration between the firms participating. Meanwhile the construction team from Rumpf Bau pulled in the empty conduit for the fibre optic cable, as well as the power cable for supplying energy to the water catchment. Basically – as at the end of September – so far the upper, and more difficult, section of the route has already been completed and the somewhat flatter area from the powerhouse to the stream crossing is on the agenda for the coming weeks.

Basically, the pipeline is planned so that everything is downhill, so one can manage without high and low points. This also relates to the stream crossing, which is constructed at considerable depth by means of culverting beneath the stream.

“Particular attention was paid by the construction company to make sure that no stones get into the pipeline during the work. To this end, before pressure testing, a camera is run through the entire pipeline to detect any debris and then remove it if necessary. Eventually it would cause damage to the nozzles or impeller when the pipeline is commissioned if it were to reach the turbine working at 27 bars”, explains the project manager.

Power for 1,000 households

But it is not only the installation work which is fully on schedule. In the water catchment area and in the machine building, progress is going well. But again, the situation in the catchment area can be described as not at all easy.

In order to enable the Tyrolean weir to be securely constructed it was first necessary to install netting for protection against falling stones. In this geologically unstable mountain gorge, stones are constantly coming loose and without stone protection netting these would be a real hazard for the workers. In order to secure the precipice next to the desander structure, shotcrete was applied for the excavation from top to bottom with numerous rock bolts in lengths of 6 to 10 m.

In future the Tyrolean weir will be extracting up to 500 l/s of water for electricity production in the power station and routing this to the pressure pipeline. This feed water will then be put through a 4-jet Pelton turbine which is designed for an installed capacity of 1.1 MW.

Once in operation, in a normal year the new Luggauerbach power station will produce around 4 GWh of clean energy. This will allow about 1,000 average households in Gastein to be supplied and around 3,400 tonnes of CO₂ emissions will be saved each year. As things stand at the moment, everything suggests that the pressure testing of the pipeline will be able to take place as early as November and the new power station will go into operation as the snow melts next Spring.

 

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