Tales of the Unexpected – When the party ended with a bang!: 3rd in a series of articles by Bill Campbell (right), retired HSE Group Auditor, Shell International, about safety issues relating to the Shell Prelude FLNG project
Thus methane-air explosions are unpredictable, and by definition unpredictable events take you by surprise and can occur when you least expect them, and often when you are least prepared. And unfortunately, from time to time, these unpredictable events can have catastrophic consequences as history tells us.
By Bill Campbell
LNG is natural gas (methane) refrigerated, the chilling process eventually turning the gas into a liquid shrinking its volume by 600 times. As we are aware from elementary physics, energy cannot be created or destroyed, so the whole economic model of the use and transportation of LNG worldwide, which really started in the 50′s and is due to exponentially expand in the next decade, is that the heat energy contained in one metre cubed of the liquid equates to six hundred metres cubed of methane. So the physical characteristics of liquified natural gas is what makes it economically viable in its transportation over in some cases many thousands of miles from its source to where it will be used when converted again into its natural state. But it’s this conversion that can make it so dangerous should it spill or leak into the atmosphere accidentally.
In the US, the use and transportation of LNG was curtailed and did not take off again till the early 70′s. It was during the war, and this may have been a factor, but in 1944 an LNG tank ruptured in Cleveland (inadequate nickel content in the stainless steel tank). The resulting explosion and fire affected 47 surrounding acres, the black and white photographs are reminiscent of pictures of post bomb Hiroshima. The resulting fire and explosion killed 128 people.
Fortunately, spills of LNG in volumes that can have catastrophic consequence are rare but in recent years are increasing in frequency. Human error as ever, seems a key causal factor. In 2004 at Skikda, Algeria, an explosion and fire caused 27 deaths, 74 injuries and material damage well outside the plant boundary. In the same year a pipe carrying LNG was accidentally damaged in Belgium, 23 known fatalities and in 2005 in Nigeria an LNG underground pipe exploded and the resulting fire engulfed an estimated 27square kilometres.
So what?
Well the good news is that many other LNG spills occurred with no serious consequence, it would appear that it is the physics of LNG and how it vaporises back into its gaseous state that saves the day. When spilled, being heavier than air it sinks until it contacts the ground or the sea. The dispersion of the vapour cloud then depends on many variables, mainly volume spilled, wind speed and direction, contact with water or seawater etc. If large volumes are spilled, the expansion ratio can lead to vapour clouds measured in cubic kilometres rather than cubic metres but much of the bubble is either lean, below the flammability limits, or rich, above the flammability limits of methane. God, or whichever God you believe in, wisely made methane a gas with a restricted flammability range, from circa 5 to 15 per cent by volume in air. Nevertheless it is still the major industrial killer, and causes significant deaths domestically as well.
Concerned about the potential consequences of LNG spillage, accidental or due to terrorist activities,much research was carried out worldwide. Shell also involved itself in trials with LNG spillage at sea, videos were taken but restricted in distribution. The bottom line was that LNG spills were difficult, or nigh impossible to ignite. Within the large and expanding vapour cloud you need to have a subset bubble within the flammable range to come into contact with a source of ignition.
Getting back to our vested interest in a Prelude, and offshore installations in general.
There are many hundreds of significant gas releases with no consequential damage. From UK North Sea official statistics over the two year period 2008/9 for example, there were 155 gas leaks of significance or one every 5 days or so. This data covers some 200 installations. So the probability of gas leaks on offshore installations is ever present, and historic data means that in the formulation of an offshore installation risk matrix for Prelude FLNG say, such a leak, whether they be raw gas, gas condensate, LPG, or LNG must be considered as a likely if infrequent event. The 155 North Sea leaks used in the example above, defined in The UK legislation as dangerous occurrences because of the potential of these events, turned out to be of little consequence as no ignition of the flammable atmosphere occurred or if it did ignite there was insufficient fuel to cause a damaging explosion
So in summary, explosions of methane air mixtures appear to be randomly distributed, that is the probability of an explosion today, is the same as it was yesterday, and the same as it will be tomorrow. If this gives you false hope it should not. The fatal explosion on Cormorant Alpha in 1982 was the first time gas had entered a non hazardous area on this facility, the explosion on the same facility in 1989 was the first time gas in any quantity had been released in a concrete column, in 1988 the explosion on Piper A was preceded by a significant gas release a year or so earlier which did not ignite, the explosion on Deepwater Horizon appears to have occurred after only two previous occasions when drill kicks caused significant volumes of gas in the atmosphere, and the minor ignition of gas within the accommodation on Fulmar was the first known time that gas had entered such a building.
Thus methane-air explosions are unpredictable, and by definition unpredictable events take you by surprise and can occur when you least expect them, and often when you are least prepared. And unfortunately, from time to time, these unpredictable events can have catastrophic consequences as history tells us.
This can be best demonstrated in the short but true tale below.
The party was a dull affair until unexpectedly things went off with a bang
This is a true story, a story of how unpredictable but constantly dangerous LNG spillages can be. At a safe and well designed test facility located for from the madding crowd, at Mercury in the Nevada Desert, in 1987, the US Dept of Energy huffed and puffed, day after day, doing large scale tests to get LNG spills to ignite. Frustrated, no doubt with their lack of success the boffins were ready to pack bags to go home when a vapour cloud accidentally ignited and damaged and propelled polyurethane pipe and insulation outside the test site boundary fence. It is not known or reported that anyone was injured, so hopefully the only cost was an increased laundry bill.
Bill Campbell
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