In January 2005, GRTgaz, the European market leader in natural gas transmission, took over the work previously done by Gaz de France’s transmission department. GRTgaz is a 75%-owned subsidiary of the ENGIE Group and operates over 32,450km of underground pipelines and 28 compressor substations to transport gas between suppliers and consumers. The network covers all of France, except for the south-west region, where the pipelines are operated by Téréga, the second largest gas transmission network operator in France.
L-gas, also known as Groningen gas, comes from a natural gas field that was discovered in the Netherlands in 1959. The field was originally one of the world’s largest reserves of L-gas: natural gas with a low calorific value. France’s other main suppliers of natural gas, such as Norway, Russia and Algeria, supply H-gas, which has a high calorific value.
Following an announcement from the Netherlands, due to the seismic risks encountered during the Groningen gas field’s exploitation, and to redress its decline in production, its gas supply contract will not be renewed when it expires in 2029.
This decision means that in order to transport gas to areas in France currently receiving L-gas, GRTgaz must convert its network to be able to transmit H-gas. Around 1.3 million people are still supplied with L-gas.
In accordance with Decree No. 2016-348 of 23 March 2016, the gas transition is to take place in each sector separately, one after another, with the whole area being converted by 2029 at the latest. To ensure this major project goes smoothly, GRTgaz is working alongside the public authorities and in consultation with the various local energy suppliers (GRDF, Gazélec Péronne, SICAE Somme et Cambraisis and Storengy).
The first phase, intended to ensure the secure supply and transmission of H-gas, involves getting the Doullens, Gravelines, Grande-Synthe and Dunkirk sectors ready for conversion between 2018 and 2020.
For the Doullens sector, a new interconnection substation was built in the Valhuon municipality. The new site is located where H-gas and L-gas pipelines intersect, and has been supplying 10 towns (5600 customers) in the area with H-gas since 9th April 2019.
The next part of the conversion plan involved the existing site in Gravelines. Work was carried out so that the facility could connect the H-gas and L-gas networks. The shift from L-gas to H-gas in this sector was completed successfully on 17th–18th September 2019.
A new DN200 pipeline around 2km long needed to be built between the Brouckerque and Spycker substations to upgrade the Grande-Synthe and Dunkirk sectors. The Brouckerque substation has now been expanded with a pressure reduction substation for the gas passing through. The substation will, in time, be able to reheat and odorise gas.
Credit Photo© GRTgaz
Domestic gas in its natural state has no smell. For safety reasons it is odorised by adding tetrahydrothiophene (THT) when it enters the French gas transmission network. This organic compound gives domestic gas its distinctive smell.
In order to reach its end users, gas has to travel along a network made up of pipelines, compressor substations and interconnections. Gas is sent through pipelines at high pressure because natural gas fields are located a long distance away from French delivery points. For example, Maximum Allowable Working Pressure (MAWP) is set at 85 bar for the section that connects the Hauts-de-France region’s supply artery to the new substation in Brouckerque. This is well above the pressure that pipes at consumer delivery points can handle, which is between 4 and 20 bar.
It is therefore essential to decrease the gas pressure gradually during its journey. This is done by the pressure reduction stations.
Transmitted natural gas is a stable element: it is neither toxic nor corrosive. However, its processing and the connected infrastructure are subject to thorough testing for thermal and explosion risks, as these sites are classed as ICPEs (Installations Classified for the Protection of the Environment).
During the research phase before the building and conversion of the Valhuon and Brouckerque substations, the team responsible for the smooth running of the projects had to find innovative safety solutions suitable for the pressure changes.
To prevent explosion risks and to control the drop in gas pressure safely, the client, GRTgaz, insisted on Explovent® panels.
Explovent® patented, stand-alone safety devices make it possible to relieve any build-up in gas volume, at the point where an explosion might occur. Excess pressure caused by an increase in the gas flow volume is therefore released before the walls of a structure fail and the structure explodes.
This device is the only system on the market that can be tested on site and then be reset. This feature proves that buildings fitted with Explovent® vents have a reliable and effective system in place, in the event of an incident on their premises. It also guarantees sustainable design and does not require any site repair after use.
The Valhuon substation was fitted with two 1200mm square XRV-IC Explovent® panels on the roof and 24 2000mm x 1000mm ERP-T-IC Explovent® explosion vents designed for wall installation. The Brouckerque substation has 36 ERP-T-IC Explovent® panels (32 1500mm x 900mm, and 4 1500mm x 1000mm panels) fitted on the building’s walls.
The sizes were specified by the client after calculating their excess pressure evacuation requirements, and the panels in the Valhuon and Brouckerque substations were installed by the construction company Eiffage Génie Civil, based in Templemars.