Natural gas, once extracted from the subsoil, is transported to places where its calorific potential is used, often thousands of kilometers away.
Transport is carried out through terrestrial and marine gas pipelines of hundreds of kilometers in length, when the deposit and the place of destination are connected by means of this network of conduits, or large methane tankers that transport it, in liquid form, in the case that there are no pipes that communicate both points. Some marine gas pipelines even connect continents, such as those linking Africa and Europe by crossing the Strait of Messina -from Algeria to Italy-, the Strait of Sicily -between Tunisia and Sicily-, and the Strait of Gibraltar -between Morocco and Spain.
When the gas circulates through the pipelines, it does so at a very high pressure –between 36 and 70 atmospheres–, and it is propelled every hundred kilometers by means of stations that compress it and send it back to the pipeline. The pipes are made of steel and have a diameter of more than 1 meter. The welds that join the pipes are subjected to rigorous control, through x-rays of the pieces, to prevent the possibility of gas leaks and the danger of explosion. These pipes, when they have to be buried or have to cross water courses, are protected with special coatings and even with electrical protection to avoid corrosion -chemical, electrochemical, biological…- and environmental risk and for people that can lead to the emission of methane into the atmosphere. In populated areas, aerial reconnaissance and routes over the lines are essential to prevent agricultural or urban activities from threatening the physical integrity of the pipelines.
In the case of methane tankers –also called cryogenic, because they are adapted to transport products at very low temperatures–, the gas is liquefied at a temperature of about 160 degrees below zero to reduce its volume in the order of 600 times, which makes transportation much easier. Maritime natural gas traffic in the world is carried out by more than 70 ships, some of which have a capacity of up to 136,000 m3, although even larger ships are being planned.
Once the methane tankers arrive at the port, the liquid gas is stored in large tanks where it remains waiting to be regasified and introduced into the distribution networks, when the increase in demand so requires. In some cases, the gas is stored in a gaseous state in geological formations similar to natural deposits, injecting it into layers of aquiferous terrain -in which the gas is trapped taking the place of water-, in salt mines, or in old deposits of natural gas. This procedure allows the gas stored during the summer to be used in winter, a time when consumption is lower.
In the population centers that are not connected to the gas pipeline network nor are they reached by methane tankers, satellite plants are built that receive the gas by tanker trucks, store it and inject it into the local distribution network.
In the year 2000, Gas Natural SDG imported and supplied approximately 12,000 million m3 of gas, 6,000 million m3 of which were imported as liquefied natural gas (LNG) and regasified at the three terminals in Barcelona, Huelva and Cartagena. During the LNG unloading process; from methane tankers to tanks and also during storage, vaporization of part of the liquefied natural gas (boil-off) occurs. This boil-off gas is recovered and treated to prevent its emission into the atmosphere.
The process is carried out by compressing and reliquefying the vaporized gas, and returning it to the LNG tanks.
La cantidad de gas tratado de esta forma es aproximadamente el 1% de la totalidad del gas procesado en las plantas, es decir, unos 60 millones de m3, una cantidad nada despreciable.
The first methane tanker built in Spain was the “Laietano”, which still makes its way between Libya, Algeria and the Barcelona plant. It has a capacity of 40,000 m3, a volume that once regasified represents more than 20,000,000 m3.
Spain currently has three liquefied natural gas (LNG) storage and regasification plants, located in Barcelona, Huelva and Cartagena. The Barcelona plant, which was inaugurated in 1969, has a storage capacity of 240,000 m3 of LNG, and a possibility of regasifying 950,000 m3/hour of natural gas. The Huelva plant, which came into service in 1988, has a storage capacity of 165,000 m3 of LNG, and a possibility of regasifying 400,000 m3/hour of natural gas. Finally, the Cartagena plant, in operation since 1989, has a storage capacity of 55,000 m3 of LNG, and a possibility of regasifying 150,000 m3/hour of natural gas.
The length of the Spanish gas pipeline network is more than 5,000 km. When the gas arrives in Spain, coming from Europe or the Maghreb, it is transported through 4 gas pipeline axes and a link:
- Eastern Axis: Barcelona-Valencia-Alicante-Murcia-Cartagena;
- Central Axis: Huelva-Córdoba-Madrid-Cantabria-Basque Country;
- Western Axis-Ruta de la Plata: Almendralejo-Cáceres-Salamanca-Zamora-León-Oviedo;
- Spanish-Portuguese Western Axis: Córdoba-Badajoz-Portugal-Pontevedra-La Coruña-Oviedo;
- Ebro link: Tivissa-Zaragoza-Calahorra-Burgos-Santander-Oviedo.
This infrastructure has five entry points for natural gas in Spain: two gas pipelines (the Lacq-Calahorra to the north and the Maghreb-Europe to the south) and three regasification plants (Barcelona, Cartagena and Huelva).
The Maghreb-Europe gas pipeline, which links the Algerian fields of Hassi R'Mel with Córdoba, was inaugurated in 1996. It is 1,400 km long and more than 1.5 million hours of engineering work were invested in its construction, involving more than 8,000 people and more than 120,000 pipes were welded.
