AUSTRALIA’S LNG MEGASHIP

Oil and coal seem dead and buried, but one fossil fuel is still hot: natural gas. A fleet of huge, floating refineries are ready to extract gas from fields located deep below the ocean, which have so far been impossible to reach.

Off the coast of WA, the world’s largest floating vessel (so far) will extract and process natural gas. Here’s how.

Three towboats are working hard in the Indian Ocean. Although the powerful boats are each 75+ m long, they look like toy boats compared to the huge red vessel they are towing. The Shell Prelude, as the red behemoth is called, is not only the world's largest ships, it is also one of the first vessels to include a complete natural gas refinery.

The Shell Prelude is the jewel of the crown of a new class of floating extraction platforms that can tap undersea gas fields located far away from the coast. The fields make up an important resource, as they include the majority of Earth’s natural gas, so vessels such as the Prelude will be central in the global switch-over to greener energy.

Huge ship resists storms

The Prelude involves two major advantages as compared to a traditional gas rig. First of all, it floats instead of being bolted to the ocean floor. This means that the ship is much better at handling the threat of intensive tropical storms with wind speeds of up to 300 km/h, which occur in the south-eastern corner of the Indian Ocean. There, the Prelude will be extracting gas from the Browse Basin, which is located at a depth of 275 m 200+ km from the north-western coast of Australia. The water is regularly swept by some of the world’s most severe tropical storms. Engineers have taken this into account by installing a mooring tower, which runs all the way up through the vessel's stern. The tower is anchored to the ocean floor via 16 of the world’s most powerful cables, whereas the rest of the Prelude can rotate freely around it. The system is so robust that it can resist extreme storms, which only occur once every 10,000 years.

The second major advantage of the Prelude is that its 488-m-long hull includes a complete refinery, allowing the vessel to purify the gas, refine it into different products, and subsequently cool the pure natural gas to a temperature of -162 °C. At this temperature, natural gas becomes liquid – in the same way as water vapour condenses into liquid water, when it gets in contact with a cold surface. Liquid natural gas takes up 600 times less space than the same quantity of gas. So, it can be carried by tanker directly from the place of extraction to consumers throughout the world. The technology is known as FLNG, floating liquefied natural gas.

With its complete refining and cooling plant, the Prelude eliminates the need to establish a long and troublesome pipeline to the closest major port, which would make the gas so expensive that nobody would buy it. In practice, the FLNG technology is hence the only thing that makes it possible to extract gas from the many submarine gas deposits of the world. Geologists estimate that around 60 % of the world’s total gas reserves are located in

places where they can only be extracted profitably using the FLNG technology.

Five aircraft carriers

The Prelude is not only the first FLNG platform in the world, it is also by far the biggest. Still, it was quite a challenge for engineers to find space aboard the Prelude for al l the equiqment that normally makes up a natural gas plant on dry land.

Shell initiated the task in the mid-1990s, and in the end, engineers managed to reduce the plant to ¼ of the normal size. Still, the Prelude ended up weighing 600,000 t, corresponding to the world’s five biggest aircraft carriers, making the Prelude the world's biggest vessel. The total price is unknown, but experts estimate it to be DKK 90 billion.

The volume is also the factor that allows the Prelude to recover its huge costs. At full capacity, the vessel will annually supply 3.6 million t of liquefied natural gas, which is more than enough to cover the gas consumption of some 2 million homes. Add to this by-products resulting from purification of the gas of 0.4 million tonnes of propane gas and 1.3 million tonnes of the easy-flowing condensate, which is used to make plastic and many other things.

Improves climate and air quality

The major investment will probably take decades to break even, but there is every indication that natural gas will only be a more popular resource in the future. And although gas is a fossil fuel, it clearly beats coal and oil in two important ways.

First of all, gas emits less CO2 than other fuels, when it is burned. Energy researchers rank the climate effect of fuels according to how many g of CO2 are emitted per kWh produced. Coal is by far the worst with 350400 g of CO2 per kWh. Fuel oil emits 280 g, whereas natural gas only emits 200 g. In other words, the climate effect is only half, when the heat in the radiator comes from natural gas rather than from coal.

Secondly, natural gas also includes much fewer impurities than oil and coal. A gaspowered power plant pollutes less in the shape of harmful particles such as sulphur compounds. Sulphurous particles in the air are major killers, as they are typically the main ingredients of toxic smog, which exists in several megacities throughout the world. The poor air quality causes some 7.2 million early deaths annually.

However, the natural gas itself makes up a potential problem, if it escapes. It consists of methane, CH4. One tonne of methane is an about 30 times more efficient greenhouse gas than one tonne of CO2 measured over a period of 100 years. Consequently, it is crucial that all pipes and tanks be 100 % tight and that no gas escape the system during extraction, processing, and transportation.

Methane is also extremely inflammable. The explosive power of the gas supplies the chemical energy that power plants and engines use, but as the Prelude can contain some 436 billion l of liquefied methane, it also means that the gas poses a major safety threat. As a result, all manned areas of the Prelude are located as far away as possible from the gas treatment, and other ships must keep a distance of at least 1,500 m from the vessel.

The world demands green gas

So far, only a few FLNG vessels exist in the world, and none of them are close to being able to produce liquefied natural gas of the volumes that the Prelude can. That is a problem, according to the International Energy Agency (IEA), which has calculated that the demand for the purest of the fossil fuels will explode in the years to come.

One of the major players is China, which imports more and more gas – particularly to do something about the nation's major problems with toxic smog. The prognoses demonstrate that China will buy 60 % more gas in 2023 than it did in 2017.

Moreover, the IEA points out that natural gas forms part of ever more industrial processes – not as fuel, but as a raw material. Among other things, the gas is an important ingredient of new methods for making fertilizer, which is vital for global food manufacturing being able to keep pace with the growing numbers of people.

Consequently, the IEA also urges the other major energy companies of the world to invest billions in their own FLNG vessels, so a fleet of floating gas refineries can sail from one gas field to the next, supplying the world with climatefriendly gas in the future.