Lake Constance is Europe's largest reservoir of drinking water, with a volume of around 50 billion cubic metres. Some regions in Switzerland also benefit from the excellent quality of this water. They include the commune of Kesswil in the canton of Thurgau, where the lakewater hydropower plant operated by Regio Energie Amriswil (REA) has supplied the region with drinking water since 1952. To guarantee the supply for the future, it was necessary to increase capacity by installing two new catchment pipes in Lake Constance. Josef Muff AG was invited to carry out the entire pipe construction work as a sub-contractor to Willy Stäubli AG.
The two new pipes have lengths of 1.38 and 1.44 km. The new cross section is 600 mm (DN 600). The individual 16 m sections were delivered to the welding site in Güttingen on Lake Constance and prepared there. When choosing the material, the client opted for steel pipes with an inner epoxy coating and an outer polyethylene (PE) coating. The coating should protect the pipes against damage and corrosion.
The individual sections were assembled into two 512 m pipe strings for the directional bores and various other strings with lengths of up to 64 m. These are equipped with flanges and elbows on both sides so the pipeline matches the profile of the lake bed. All of these components were prepared, welded together and treated on the welding site by JMAG. After this, they were transported to the shore by Willy Stäubli AG, immersed in the water and then placed in position and bolted together at depths of up to about 60 m by divers specially trained for this work.
The pipes were installed at the Kesswil lakewater hydropower plant operated by Regio Energie Amriswil (REA) on the shore of Lake Constance using a trenchless pipe-laying method so as not to damage the valuable shore zone. (Directional boring from the shore; about the first 500 metres of steel pipe were pulled back from the lake.) To prepare the directional bores, a construction pit had to be dug before the pullback phase. After immersion, the two 512 m single strings were transported to the installation location on pontoons, lowered near the borehole prepared for them, and then drawn into the borehole under water by a pullback device. At the lakeside end, the other individual strings with lengths of up to 64 m were bolted together using flanged connections.
The connecting pipes running from the directional bores into the new operational building were dimensioned in advance by JMAG with the help of a 3D scanner. This meant that the pipe fittings could be prefabricated in the workshop so they only had to be bolted together on site – thus saving considerable time.
Not only the logistics on the ground but also the weather conditions presented major challenges. For example, the pipe could only be pulled back when the lake was calm. Wind and waves prevented work. Precise coordination of the participating companies was therefore essential, and they also had to be flexible. However, the collaboration among the specialists was an outstanding success. Josef Muff AG can look back on a successful project and is grateful for this exciting commission.
Quagga mussels
Quagga mussels actually originate from the Black Sea region, but were also proven to be present in Switzerland in 2015. Since then, they have proliferated rapidly, pose a threat to the entire ecosystem and are inflicting damage on drinking water supply systems. Quagga mussels have also become established in the depths of Lake Constance. Although the new catchment pipes were installed at a considerable depth (60 m), it is nevertheless necessary to perform annual cleaning (known as pigging) to remove the stubborn mussels from the pipe walls at regular intervals.