A planned underwater hydrogen pipeline connecting Barcelona and Marseille is a risky project, but one that is key for the European Union's energy independence.

From the roadmap to the cost and the timeline, here is what we know about this ambitious initiative, which was officially launched on Friday by the leaders of Spain, France and Portugal and has won the backing of the EU.

- What is it? -

Dubbed "H2Med" or "BarMar", the pipeline will transport green hydrogen between Spain, France and the rest of Europe.

Green hydrogen is made from water via electrolysis in a process that uses renewable energy.

Announced in October, the H2Med is an alternative to the defunct 2003 MidCat project which was to have carried gas across the Pyrenees from Spain to France but was dropped in 2019 over profitability issues and objections from Paris and environmentalists.

As well as the submarine pipeline, the project includes another connection between the northeastern Portuguese town of Celorico da Beira and the northwestern Spanish town of Zamora.

- What are its goals? -

When it becomes operational, H2Med is expected to carry two million tonnes of hydrogen per year, or 10 percent of European consumption.

The idea is to boost the decarbonisation of European industry, giving it large-scale access to clean energy provided by Spain and Portugal, which are hoping to become world leaders in green hydrogen thanks to their numerous wind and solar power farms.

The three nations had said it would initially carry gas to help reduce Europe's dependence on Russian energy -- a logical assumption given Spain and Portugal's huge capacity to turn liquefied natural gas (LNG) that arrives in tankers back into gas form.

But they later said H2Med would only be used for carrying green hydrogen, a key condition for being declared a project of "common interest" which could unlock European funding for up to 50 percent of the costs.

- Why Barcelona and Marseille? -

Its backers say it is "the most direct and efficient way of linking the peninsula with central Europe".

Barcelona is an energy hub in Spain, and Marseille is a key point in the French network and a gateway to the Rhone Valley, northern Italy and Germany -- industrial regions that could become big consumers of green hydrogen.

- What route will it take? -

The route has not yet been decided but the roadmap lays out three options, with the "optimal" choice being one that stretches 455 kilometres (283 miles) at a maximum depth of 2,600 metres.

Although not the shortest route, it would benefit from having a "more gentle slope" upwards, the roadmaps says.

- When will it be ready, how much will it cost?

Operational by 2030, the pipeline will cost around 2.5 billion euros ($2.6 billion). Construction is expected to begin in 2025 and take 54 months.

The connection between Spain and Portugal should cost another 350 million euros.

- What are the obstacles? -

"An offshore hydrogen pipeline at this depth and distance has never been done before," said Gonzalo Escribano, an energy expert at Madrid's Real Instituto Elcano think tank.

The innovative project faces certain technical challenges.

One of the main problems is that hydrogen is made up of small molecules which can escape through the joints and cause corrosion, said Jose Ignacio Linares, a professor at Madrid's Pontificia Comillas University and an engineer by training.

But such problems could be overcome by "installing a membrane inside (the pipeline), a kind of plastic that prevents the hydrogen from escaping," he said.

- What's the outlook? -

The biggest risk is its economic viability, experts say.

"It is not clear when the green hydrogen market is going to take off and whether Spain will be in a position to produce enough to export it," said Escribano.

But Linares said its construction would take so long "that we can't afford to wait".

"If we do, we'll end up with a huge volume of hydrogen that we won't be able to export."