Last winter, a team from the Swedish tunneling company BAB Rörtryckning AB was struggling through centimeter-thick ice about 70 km north of Stockholm. The mission was to install two sewage and drinking water pipelines under the Länna Kyrksjö lake.
They are used to supply the nearby community of Norrtälje, whose pipeline system is also to be supplied in future with water from Mälaren lake to the west, because the town has grown so fast in recent years. The drilling tool used by BAB Rörtryckning was a horizontal directional drilling rig type HK150C made by Herrenknecht with a pulling force of 150 tonnes.
The first hole for the drinking water pipeline led from the shore to the bottom of the lake. There, the pipeline had to be connected to a conventionally laid pipeline, thus creating a link to the opposite shore. After only two days of drilling, the 700-m long pilot was completed at the bottom of the lake. Then, the weather changed dramatically. The temperature fell by more than 25 degrees and the general contractor SVEAB was suddenly faced with a frozen lake with a decimeter thick ice layer.
“We fought against the ice for a total of two weeks, 24 hours a day. Day and night up to four boats were working on the lake as ice-breakers,” recalled Magnus Tingstrand from BAB Rörtryckning AB. The boats had to break through the ice to be able to lift the pilot head, change it for a reamer and to connect the pipe which also was frozen into the ice. The effort was worth it – the 28-in. pipeline was installed safely in time for Christmas.
After a well-deserved Christmas break, BAB started installing the sewage ducts in January 2013. The plan was to install this in the form of a single “Double Pack” pipeline consisting of two polyethylene tubes (2 x 355 mm). This runs parallel to the previously created drinking water pipeline, which in turn, was to be produced in one step from shore to shore deep under the seabed. The weather was constantly icy.
“We first had to clear the jobsite of ice and snow, which took us one full day with temperatures of minus 27 Celsius (-16.6 F),” says Tingstrand.
Nevertheless, the drill team was able to achieve record-breaking performances with the HDD rig. The pilot drill of the 1,385-m long crossing under the lake was mastered with a record of 457 m per day within an absolutely sho rt period of time. No HDD project with a comparable length had ever been realized successfully before in Scandinavia. The pipeline installation itself took just under 48 hours.
A result that is pretty impressive. The core part of the new supply and disposal lines for the Norrtälje community was completed after only three months. The completion of the entire project is scheduled for 2015.