44 | NewScientist | 31 March 2012

MAKE sure you’ve put in your ear plugs,” warns Vincent Tam as we prepare for one of the largest experimental

explosions in the UK’s history. We’re at a remote test site at RAF Spadeadam in Cumbria and in front of us is a long tunnel of plastic sheeting with a line of fir trees inside. It is also filled with flammable propane gas and the fuse is about to be lit. “Don’t blink,” says Tam, a fuel explosions specialist at energy company BP. “Watch the ground. You should see the grass rippling towards you with the shock wave. Then brace yourself for the impact…”

I’m here for what promises to be the culmination of an investigation into the UK’s biggest ever peacetime explosion. Just before dawn on Sunday 11 December 2005, a large fuel storage depot at Buncefield in

Until now, the cause of one of Europe’s biggest blasts has been a mystery. Will Gray unpicks the surprising truth behind the Buncefield event, and explains why hedgerows can be lethal

The impossible explosion

Hertfordshire was ripped apart by a mammoth blast. It damaged buildings up to 8 kilometres away and was audible in France and Belgium. Amazingly there were no deaths, but the explosion and resulting fire left 43 people injured, thousands more had to be evacuated and the damage totalled around £1.5 billion. “The amount of debris scattered everywhere was amazing,” recalls David Painter, an inspector for the UK’s Health and Safety Executive (HSE), who visited the site a few days later.

The immediate cause was clear: about 180 tonnes of petrol had leaked from a storage tank and the resulting vapour cloud had caught fire. However, the patterns of damage around the fuel depot were puzzling. And though the blast squashed many metal

structures flat, calculations suggested that the vapour cloud should not have been capable of wreaking such havoc. Things didn’t add up.

After seven years of meticulous detective work, the investigation team now think they have some answers. Understanding this disaster is more than an academic question. Leaks from pipelines, refineries and fuel depots are not uncommon, and a major explosion occurs somewhere in the world every two years or so. Finding out what happened should help prevent them in future. And that is why I’m now watching a line of small trees; if the investigators are correct, this overgrown hedgerow is the key to the Buncefield blast.

In the days following the explosion, HSE officials scoured the site, interviewed

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31 March 2012 | NewScientist | 45

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less than 10 metres from the explosives, where the pressure was about 16 bars,” says Tam.

They concluded that the only way to create this kind of pressure was if the whole cloud had exploded. For that to occur, the flame front would have had to spread through the vapour at supersonic speeds of about 1800 metres per second. At this speed the fuel ahead of the flame is compressed and heated to the point at which it ignites spontaneously – a process called autoignition. Suddenly the reaction becomes self-sustaining and the cloud explodes. But how could the flame accelerate to such speeds?

Clues came from a catastrophic event, also in the UK, more than 30 years earlier. In 1974 a pipe ruptured at a chemical plant at Flixborough, Lincolnshire, spilling about 40 tonnes of cyclohexane gas. The vapour spread in a layer across the site, ignited and exploded, wrecking the plant and killing 28 people.

Just as at Buncefield, the cloud should not have exploded. Researchers eventually decided that the culprit, bizarrely, was the plant’s jumble of metal tubes and pipe work. They suggested that when the flame reached these structures it wrapped around them. This increased the flame’s surface area and helped it burn faster. The pipe work also created turbulence that mixed flame and vapour, increasing the combustion rate and helping the flame accelerate.

Yet there were few buildings and almost no pipe work at the Buncefield site. Besides, despite evidence from Flixborough, no one had ever proved that these kinds of structures could accelerate a flame to detonation. >

witnesses and replayed footage from CCTV cameras around the depot. They soon worked out that faulty switches in a storage tank had allowed the petrol to overflow. With no wind, the volatile fuel’s vapour cloud had blanketed the site. At 6.01 am, an electrical circuit in a pump house sparked and the vapour ignited.

Beyond that the picture was confusing. In particular, no one was sure why the fuel had exploded with such force. When lit, flammable vapour will usually burn rather than explode. The exception is vapour in a confined space such as a building, where pressure can rise so rapidly that the structure will suddenly burst like a balloon. But at Buncefield the vapour cloud was out in the open.

In an unconfined cloud of fuel, a flame will typically travel at about 10 metres per second,

The mammoth fire that followed the Buncefield blast burned for three days

” You should see the grass rippling towards you with the shock wave. Then brace yourself for impact...”

which is far too slow to create a shock wave. But what if a flame moves much faster? Air and fuel vapour in front would be unable to move aside to allow the hot gases to escape, so pressure would build up along the flame front, creating a strong pressure wave that could damage anything in its path. Could this have happened at Buncefield?

To investigate, a team of researchers modelled a flame moving at speeds of several hundred metres per second. Their analysis suggested a burning cloud could generate a

pressure peak of about 1.5 bars. This is not enough to account for the devastation seen on-site: vehicles, including a brand-new Porsche, looked as if they had been through a crusher, and oil drums and metal cabinets were crumpled like paper. “Just from the car tyres which came off the wheels, you can see the numbers don’t match up,” says Tam. “Tyres are inflated to 2 bars so you’re talking peak pressures of at least 4 or 5 bars.”

So the team tried to recreate this level of damage . They sealed metal boxes and drums like those on the site in a chamber and raised the pressure to 10 bars. This still wasn’t enough. It wasn’t until they detonated 170 kilograms of TNT that they were able to replicate the damage. “To crush a car to a similar level as in Buncefield we had to place it

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46 | NewScientist | 31 March 2012

” In one case a massive gas blast threw some trees towards the epicentre rather than away from it”

The unexpected force of the blast wasn’t the only puzzle. Some lamp posts and metal signs were bent in one direction, but others close by were bent the opposite way. Scratches and damage on trees and walls showed similar patterns. Cars, too, had been blown about in different ways depending on where they were parked. With something like a bent lamp post, it seemed safe to assume the blast was moving in the direction it was leaning, says Mike Johnson from oil and gas consultants GL Noble Denton and one of the lead investigators.

When the team examined videos of test explosions they noticed that a detonation moving through a cloud of gas can also create a force in the opposite direction. They realised that the high pressure zone just ahead of the

It turned out that this effect has been seen before, though no one had investigated it in any detail. In one case in 1989, a massive liquefied petroleum gas blast at Ufa in what is now Russia threw trees in some areas towards the epicentre rather than away from it.

With the blast patterns making more sense, the investigators were finally able to pinpoint where they believed the detonation had begun. The evidence indicated a spot near the junction of two roads that ran alongside the depot (see map). Intriguingly, both roads were lined with shrubs and small trees; could their twigs and branches have behaved like the pipe work at Flixborough and created a supersonic flame capable of triggering an explosion?

When the team tested this in simulations,

100 m

Anatomy of a disasterOn 11 December 2005, a leak at the Buncefield fuel storage depot ignited 250,000 litres of fuel, setting off an explosion that caused £1.5 billion of damage

How the explosion occured

Pressure builds up ahead of detonation wave, creating a “cap” which forces hot gases to expand backwards

OBJECT BENT BACKWARDS BY EXHAUST

OBJECT BENT FORWARDS BY DETONATION

Fuel vapour from tank ignites atthe pump house

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Flame accelerates along hedgerows by Three Cherry Trees Lane

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Detonation occurs near the junction of Buncefield Lane and Three Cherry Trees Lane as flame reaches supersonic speed

BUNCEFIELD TERMINAL

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Tree branches and twigs create turbulence and increase mixing of fuel and oxygen. This accelerates the combustion wave

SUBSONIC

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Passing around branches increases the surface area of the flame (orange), creates turbulence and accelerates combustion

Maximum capacity of Bunce�eld site

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detonation acts like a cap, preventing the hot gases behind from expanding. The only way they can escape is in the opposite direction – in other words, a detonation wave can send out a powerful flow of hot gas behind it, rather like the exhaust of a jet engine. Simulations by Tam and a team of engineers at Kingston University in West London, UK, confirmed this mechanism (Journal of Loss Prevention in the Process Industries, vol 24, p 187).

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31 March 2012 | NewScientist | 47

Lessons from Buncefield might yet help us build novel jet engines

One tiny part of the £1.5 billion damage caused by the Hertfordshire blast

their digital hedgerow created enough turbulence to accelerate a flame close to the speed of sound. This is near the point at which detonation should occur, Johnson says.

Models are one thing, but whether a real hedgerow behaves this way is another matter. That is why we are waiting at RAF Spadeadam, with all eyes fixed on a row of small trees. “Today is one of the defining moments,” says project manager Bassam Burgan from the UK Steel Construction Institute which is coordinating the research. “If this detonates we know it can happen with vegetation. If it doesn’t, there’s serious thinking to be done.”

The team had arranged trees into a 100-metre-long hedgerow and enclosed the whole lot in a steel frame covered with plastic sheeting. The tube fills slowly with propane. Finally a radio message confirms the gas has reached the correct concentration. This is it. A flash of fire lights up the tunnel at one end and a bright flame races through the trees… then fades.

No detonation. No shock wave. Just a disappointed silence from the crowd. “It’s like waiting to see the best firework in the world and it just fizzling out,” sighs one engineer.

Wrong treesAfterwards we learn that the flame accelerated through the trees for 10 metres, reaching a top speed of 140 metres per second. But it never came close to exploding. Johnson suggests that though the trees helped to accelerate the flame front, there wasn’t enough turbulence to reach detonation, “We know it is possible,” says Johnson. “We just haven’t got the right trees yet.”

Three months later, in February this year, in a second test with a different set of trees, the propane exploded. “Rather than the flexible pine trees we had last time, we had dense deciduous trees like those at Buncefield,” says Burgan. “It’s an exciting result.”

A few years ago no one would have predicted that a row of trees and shrubs could make the difference between a serious fire and a catastrophic explosion. But most of the team now back the hedgerow theory. “To explain it any other way you would have to go to some completely new form of flame propagation that generates high pressures,” says Johnson. “You would have to step outside what is known about combustion and explosions.”

It now seems that trees may also have played a key role in vapour cloud explosions like that at Port Hudson in Missouri in December 1970 and at Brenham, Texas in 1992.

More recently, in October 2009, a near-identical explosion occurred at the Catano oil refinery in San Juan, Puerto Rico. The shock wave – equivalent to a 2.8 magnitude earthquake – tore up a highway and forced the evacuation of 1500 people as the plant burned for three days. Again a leaked vapour cloud ignited and caused heavy damage and there was little pipe work or chimneys on site. Yet there were trees aplenty.

Back in the UK, spill prevention and leak-monitoring procedures at fuel depots have been updated. In the longer term, the investigation might change the way storage depots, refineries and pipelines are designed, and how the sites are landscaped. Along with conventional safety features like sensors and alarms, site operators may have to rethink the way that trees, hedges and shrubs are positioned. Even structures on nearby commercial developments could help to accelerate a flame, says Johnson.

There is still plenty to learn about the complexities of combustion, though, and the team’s experiments aren’t over. Another

hedgerow test is planned for 2012, which should help reveal what density of branches constitutes a hazard. Meanwhile, physicists at Imperial College London are investigating whether dust, leaves and dirt on the site could have contributed to the severity of the blast through a poorly understood process called episodic combustion.

According to this theory, a pressure wave created by rapid combustion kicks up any lightweight material on the ground ahead of the flame front. This stuff heats up and, if combustible, ignites, radiating more heat into the fuel vapour and making the flame jump ahead. Perhaps this process helped accelerate the flame across open areas such as car parks? “The whole site had debris and leaves everywhere, so there was plenty there to make that happen,” says Bob Simpson of the HSE.

These investigations could have consequences far beyond the oil and gas industry. Understanding the way that burning fuel can explode is important for building a novel aircraft propulsion system called a pulse detonation engine, says Elaine Oran, an engineer at the Naval Research labs in Washington DC. These engines use a detonation wave to burn fuel and air and should be far more efficient than conventional designs. In the long term, the knowledge could also help astrophysicists understand why white dwarf stars explode, she says. Known as type Ia supernovae, these explosions begin with subsonic thermonuclear flames that spread across the star and somehow trigger a supersonic detonation. It is thought that turbulence plays a role. “Everything we learn adds up,” says Oran. Who would have guessed that a hedgerow might eventually offer insights into one of the most violent events in the universe. n

Will Gray is a freelance writer based in London

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