Starting in 2021, this large-scale project combines technological prowess with the expertise of numerous trades.
Our goal: a safe and sustainable gas supply!
The Givisiez (Canton of Fribourg) project originates back in 2010 with the revision of the Ordinance on Protection against Major Accidents. This new ordinance has brought about major changes for the gas industry, including the reduction of risks related to the crossing of an industrial zone by a high-pressure gas pipeline.
Against this background, Gaznat started the Givisiez worksite in February 2021 with, first of all, a temporary clearing to allow the laying of the 720 meters of pipes. For 20 months, the work consisted of welding 86 tubes, then burying the existing pipeline at great depth and ensuring its safety by means of a tube that acted as a protective envelope. The pipe-in-pipe connection took place in May 2022, the commissioning in July 2022. Reforestation with native trees adapted to the local ecosystem and future climatic conditions is planned for the end of September 2022.
The pipe-in-pipe technique designed by Gaznat, in partnership with Unigaz and Gasverbund Mittelland AG (GVM), represents a technological first in Europe. The construction of such a structure is complex. Numerous technical requirements mean that the pipes have to be laid one after the other in a single block. Over a length of 720 meters, a 60 cm diameter pipe was built and pulled into a borehole under the industrial zone of Givisiez, while another 720-meter pipe with a 40 cm diameter was pulled into the first one, ensuring that the pipes were perfectly concentric and watertight.
The pipe-in-pipe system was developed for safety reasons. The principle is based on the idea that for a leak to reach the environment and cause a safety problem, both concentric tubes must be simultaneously damaged (punctured).
The double safety is achieved by monitoring the pressure of the annular space between both tubes. Indeed, an inert gas (nitrogen) is introduced between both tubes and its pressure is monitored from Gaznat control and monitoring center.
If the inner tube shows a defect (a leak), then the pressurized gas would fill the outer protection tube. This would be immediately detected by an increase in the annular space pressure. In the case of a defect in the outer pipe, the nitrogen (which is a neutral gas) would escape to the outside and the pressure in the annulus would be reduced. This would also be detected by the sensor which would trigger an alarm at Gaznat. In this second case, the nitrogen leak would also be detected vertically in the field. If the source of this accident turns out to be a construction site (e.g. drilling), it would be immediately stopped.
The Role of Nitrogen
Nitrogen is used after the construction and water pressure test to dry the pipes. Indeed, pure nitrogen allows to expel moisture until a hygrometric level is reached. Moreover, since nitrogen is neutral, it does not react with steel and thus prevents corrosion, which is a phenomenon requiring humidity and oxygen (oxidation).
Gaznat has successfully completed the first directional drilling project in Europe for Unigaz - owned by Gaznat and Gasverbund Mittelland AG (GVM) - between February 2021 and July 2022.
To complete the directional drilling project in Givisiez, many different trades had been working together on the site for 15 months. The complexity of the work and the technical requirements imposed by the pipe-in-pipe installation involved specialists in engineering, environmental monitoring, mechatronics, infrastructure and forestry work. A pipe expert and a team of welders from Italy were specifically involved in the preparation of the 720-metre pipe, to be connected to the existing one.
Schenk AG, a company specialized in drilling, has largely contributed to the smooth running of the work, in close partnership with Gaznat, which, for its part, has ensured the management of the site, communication and legal aspects.
Nearly fifteen specialized trades each contributed their skills to the success of the first pipe-in-pipe installation in Europe.