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Underwater construction work in the Gepatsch reservoir in Tyrol’s Kauner valley

In order to ensure that in the future, as a result of the sediment and siltation problem, the Gepatsch reservoir in Tyrol‘s Kauner valley no longer has to be drained for inspection and maintenance work ....

.... in the headrace tunnel and bottom outlet tunnel upstream of their locking devices, steel guide frames and stoplogs which can be moved within them were designed and manufactured, and were installed with the assistance of divers using the saturation diving method at a depth of around 110 m in front of the bottom outlet intake structure and the lower headrace intake. The stoplogs and their guide frames were designed in such a way that the stoplogs can be moved in the guide frames either into the open position or into the closed position. In addition, the lower part of the bottom outlet, which was required during the construction of the dam for water retention, was finally sealed off with a concrete plug that was likewise encased in concrete at a depth of 110 m. Furthermore, the lower headrace intake was increased by a few metres using what are known as trash rack towers which were simply installed by divers at a depth of 110 m in order to produce a greater operational safeguard against any build-up of silt and sediment in the future.

Given that full drainage of Gepatsch reservoir in the winter of 2015 resulted in unwanted silting up of the works water inlet with sediment, the powers-that-be decided that the outstanding remediation work should be done using an alternative method: Specialist divers were to perform the challenging concrete and steel construction works at a depth of up to 110 m in extremely murky water with poor visibility. The contract to manufacture the components subject to high loads was awarded to Muhr GmbH from Brannenburg in Bavaria, a company which is globally renowned in the hydromechanical sector. All of the planning and basic design of the hydraulic steel components was undertaken by TIWAG on its own. The works in the reservoir were completed successfully at the end of 2017.

Austria’s tallest rock-fill dam
With a capacity of around 139 million m³, the Gepatsch reservoir in Tyrol is the water reservoir for the Kauner valley power plant. When it was completed in 1964, its rock-fill dam’s length of around 600 m and height of up to 153 m made it the tenth-tallest structure of this type in the world. It is still the tallest filled dam in Austria today. The Gepatsch reservoir is fed by the melt water from the surrounding glaciers in the Kauner valley and the streams fed via waterways from the neighbouring Pitz and Radurschl valleys. The utilisable drop between the reservoir and the powerhouse in Prutz varies, according to the water level in the reservoir, between 793 and 895 m. Depending on what the level is, the maximum possible power plant output from the total of five twin Pelton turbines in the powerhouse is between 325 and 392 MW. In an average standard year, 661 GWh of electrical energy are generated, which equates to covering the annual electricity demand of 188,800 households.

Remediation required diving deployment
As part of an officially prescribed inspection and the planned implementation of maintenance works at the same time, TIWAG completely drained the reservoir in December 2015. As a result of this draining, large amounts of sediment were moved from the back of the reservoir to the front and this led to unexpected silting up of the inlet structures and, as a further consequence, to disruption to operations for several months. After the expensive cleaning of penstocks and turbines – fortunately the equipment did not sustain any damage – the hydro power plant was able to start operating again in May 2016. To ensure that the silting up of extraction and drainage valves was not repeated by any future draining of the reservoir, TIWAG decided to have the outstanding works to the works water inlet and the bottom outlet carried out by deploying divers under water. There were several good reasons for this costly option. The deployment of divers made it possible to conserve the environment because sediment movements were prevented by not lowering the level of the reservoir. At the same time, this prevented any hydro-ecological effects on the River Inn, into which the water exiting the turbines is ultimately fed. In addition, the use of divers guaranteed almost consensus-based management of the reservoir, thus reducing the energy losses.

Divers under water for weeks
Traditional dives require decompression owing to the length of the dive or depth of the dive. The general principle is: the longer the dive and the higher the pressure, the more time has to be spent on decompression. With what is known as “saturation diving”, the divers are constantly subjected to the same pressure so that there is no need for the regular decompression periods. The divers thus spend several weeks in special pressure chambers under water, with alternating teams being deployed in a shift pattern. The 3 to 4-day decompression time is thus only required at the conclusion of the 3-week diving cycle. A total of four of these 3-week diving cycles with three teams of two divers in each case were required at the Gepatsch reservoir. This enabled work to take place around the clock. The complex underwater assignment was undertaken by the Dutch company “DCN Diving”, whose professionals have been able to prove their proficiency in the past working on oil platforms or the expansion of the Suez Canal.

Structural measures on the lower headrace intake and on the bottom outlet intake
The structural measures on the lower headrace intake comprised the installation of a total of three intake rack towers and one guide frame with stoplog. As the inlet area increases by several metres, this enhances the operational reliability in the event of a possible increase in sedimentation deposits in the future. The lower bottom drainage inlet (previous build workflow) was finally plugged shut with a reinforced concrete seal made from around 400 m³ of concrete which was produced at a water depth of 110 m (world record). For the upper bottom outlet intake, three guide frames together with movable stoplogs were fitted. “These structural designs now make it possible to carry out inspection and maintenance work in the headrace and bottom outlet tunnel upstream of their locking devices in the reservoir with drops of around 20 m above the minimum operating level,” explains Richard Obendorfer, the technical project manager from TIWAG. To produce the concrete plug seal, the divers first had to remove the sediment which was up to 15 m deep. This was done using the “air-lift procedure”. This involves lowering a steel pipe which has a diameter of 25 cm and is wider at the bottom end vertically down as far as the sediment. This steel pipe has an air line attached to it whose end is inserted into the expanded section at the bottom of the pipe. A compressor is used to press air downwards in this line, and the buoyant force causes a mixture of water and sediment to be transported up in the pipe. After this, this water/sediment mixture was pumped by a booster pump via a floating pipeline to the storage point around 300 m away and transported down via a down pipe with a depth of 75 m to just above the bottom of the reservoir.

High-quality components made from black steel and stainless steel in Bavaria
Given the challenging nature of doing the construction works under water with extremely difficult visibility conditions (almost no visibility due to the suspended sediment) and with water temperatures of around 5°C, TIWAG conducted extensive planning work and studies into different options in advance of the project. As well as the static load-bearing capacity with a water column of up to 35 m, the leaktightness for these high water pressures also had to be considered. When designing the hydraulic steel components, consideration had to be given to making the installation work under water as easy as possible so that it was actually possible to do the installation under water with no visibility. The production planning and manufacturing of the components were done using the designs drawn up by TIWAG by Muhr Gmbh, a company from Brannenburg in neighbouring Bavaria which has demonstrated its proficiency in hydraulic steel construction on numerous occasions around the world. “As the steel components can no longer be accessed under water and are therefore almost impossible to inspect, and given the long operating life for which they are designed, TIWAG required the steelwork to be executed in accordance with DIN EN 1090 EXC3. The standard places extremely high requirements on the manufacturing technology and the associated quality checks and documentation thereof. This meant that all the relevant stages of production were monitored directly on site by an accredited inspection company that was commissioned by TIWAG,” says Muhr technical editor Florian Kufner in explaining the project requirements, adding with even more detail: “The existing, uneven contour of the bottom outlet intake demanded, for example, an appropriate configuration of the guides and stoplogs, i.e. the steel structures and seals had to be manufactured so that they could still be fitted even with the existing irregularities.” Specifically the assignment involved supplying three identical inlet towers made from round stainless steel pipes with a triangular outline, with a height of 9.16 m, a width of 2.92 m and a depth of 4.93 m. The 5.65 m high and 4.12 m wide stoplog at the works water inlet was produced with a seal on four sides. The assignment was completed with three further stoplogs including guides for the bottom drainage outlet. Kufner also mentions that the working relationship with TIWAG and specifically with Mr Obendorfer was extremely cooperative and efficient.

Operation of the plant safeguarded
Following the completion of the underwater assignment at the end of December of last year, the first practical test of the installed components finally followed in the spring of 2018. “With the reservoir at a low level, the new stoplogs were employed for the first time in April and the bottom outlet and headrace tunnels upstream of the locking devices were drained. On the one hand, it was demonstrated that both the stoplogs and their guide frames and the reinforced concrete seal plug fulfil their purpose completely and at the same time are easy to operate. On the other hand, the checks revealed that the two tunnel structures and their plant components are in a very good state of repair. The successful implementation of the concrete and installation works described here safeguards the operation of the Kauner valley hydropower plant for the future,” states Obendorfer. The project, which has been ongoing since 2013, has amassed total costs of around 16 million euros.

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