While it is true that an LNG tanker’s cargo holds the same energy as that produced by some 50 atomic bombs a comparison between the two is completely inappropriate, according to a paper published by SIGTTO – The Society of International Gas Tankers and Terminal Operators.
On Saturday, engineer and PN Birzebbugia councillor Stanley Zammit said that a gas cloud emanating from the storage unit could have devastating effects if it ignites. “The gas storage aboard the FSU has energy equivalent to 50 nuclear bombs.”
But on Monday this paper was contacted by a member of SIGTTO, who referred us to a paper titled “Exploding the Myth” written by society official Roger Roue and Gordon Milne of Lloyd’s Register.
The authors confirm that 135,000 cubic metres of LNG – the amount carried in a conventional LNG carrier, and roughly the same amount that the Marsaxlokk Floating Storage Unit will carry – has a base energy of 3,280 Tera Joules – equivalent to the base energy of 40 nuclear bombs.
But the effects of an LNG and a nuclear explosion are incomparable because the way in which energy is released is totally different.
An explosion is defined as the “sudden and violent release of energy.” In terms of energy an LNG cargo has significantly more energy than a nuclear bomb. But the rate of energy release of a nuclear bomb would be four million times greater than an LNG explosion.
In the case of a nuclear bomb the explosion would be very violent as the process would happen within the space of a few short seconds, creating a powerful blast, heat and radiation wave.
But LNG is an altogether different case. Liquefied Natural Gas, as the name implies, is stored in liquid form. Apart from taking up far less space, liquid gas is not flammable. LNG becomes combustible as its temperature rises and the gas evaporates and gets mixed with oxygen. But a scenario where all the gas inside an LNG tanker leaks out into the air, creating a massive gas cloud, is very unlikely, say the authors, who insist that the gas would most probably catch fire almost instantly after the leak starts. This would mean that the ship’s cargo does not all ignite at the same time, meaning that the energy output is spread over a longer period of time and is restricted to a smaller area.
Besides, a nuclear explosion would have three main factors – a huge explosion creating a powerful blast, a heat wave and radiation. But an LNG explosion would cause none of these factors. Radiation is, obviously, out of the question. Blast is also a non-existing factor because the airborne gas would burn at a relatively slow speed. And while the fireball from a nuclear explosion would be 5 kilometres across and its effects can be felt 11 kilometres away, the fireball created by the leak of a whole cargo of LNG – a highly unlikely event – would not be bigger than 600 metres across. A 600 metre wide gas cloud would also take some 147 seconds to burn.
“There have been a number of experiments examining the consequences of an LNG vapour cloud being ignited. It is a commonly held perception outside of the LNG industry that when methane is ignited it detonates violently. However numerous historical experiments have all shown that methane vapour will not detonate in an unconfined/unenclosed area, such as the open air surrounding an LNG tanker. Instead there will be a slow moving flame front.”
The authors of “Exploding a Myth” also insist that, using the same argument, a ship full of timber would actually be carrying more energy than an LNG tanker. If its entire cargo is set on fire the amount of energy released would also be bigger than that released by an atomic bomb. But similar to LNG, the timber would release its energy at a far slower rate than a nuclear bomb.
“A ship carrying timber has more power output potential than an LNG tanker, but we do not worry that the forests surrounding our home will suddenly explode.”