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Hiring Science

This article explores how HSL can help establish the reasons behind major incidents.

Picture perfect

If a picture is worth a thousand words the laser scans produced by the Health and Safety Laboratory (HSL) must be worth several billion.  This is especially so when their purpose is to help determine the causes of major disasters.  

 "The first step is usually a telephone call from the HSE principal inspector at the scene of the incident," says Steve Graham, head of HSL's Visual Presentation Services Unit.  "We have to move quickly to get a photographer to the scene."

"Photographer" is perhaps misleading.  It brings to mind the silver halide cameras used by Steve Graham when he first joined HSL.  HSL's use of imaging is way beyond that.  Their pioneering use of laser scanners is just one example of the ways in which HSL puts cutting-edge science into accident investigation.   

Laser scans taken at different points in the accident scene are combined to produce "wire frame"    3-D image of the scene.

"We can also put texture onto this model to produce something that looks more like a conventional picture," says Steve. 

Of course it is more than just a "picture".   This becomes obvious as one of Steve's team demonstrates its power on his computer screen.  The way he zooms around the scene is reminiscent of "Minecraft" but with infinitely superior graphics.  

He can use the computer model to generate pictures from any point and angle, something that would take, quite literally, an infinite number of conventional digital photographs.     

Nor could digital photos be used to provide an accurate measure of the distance between any two points at the scene of the accident.

"Cheaper, quicker and less labour intensive than using theodolites," says Steve Graham. "And if we subsequently need a measurement we did not make a note of at the time we can simply take it from the model.

HSL can apply data from other disciplines to the scans to help elucidate the cause of the incident.  If the incident results in a court case they can also rotate and animate the image to give judges, barristers and jurors a clear picture of exactly what happened.

But this does not mean HSL have consigned the more conventional imaging techniques to the garbage heap of technological history.  These still have their uses.  For example they are light enough to mount in drones and  beam back pictures from, say, a burning factory.

"From imaging data we can often get a good idea of how the incident started," says Steve Graham

Occupational Hygiene

Chris Keen is a principal occupational hygienist.  He is based at HSL's Buxton site deep in the Pennines.  Most of his team however are located in HSE offices around the UK where they work with specialist HSE Inspectors.

"We can thus get occupational hygienists to the site of an incident very quickly," says Chris.

He identifies three main incident response services provided by his team.

"Firstly we collect samples from the site.  These can include substances such as asbestos, chemicals present initially and chemicals produced by the incident.  Now we can use drones to collect airborne chemicals."

"Secondly, we collect articles.  For example we may recover pieces of ruptured tank scattered across the site by an explosion.  We may also recover pieces of machinery whose failure may have resulted in the incident."

But their role is not confined to sample retrieval.  Their third function is to help elucidate causes. Even today, all too many incidents investigated by Chris's team arise from work in confined spaces.

"The majority of these fall into three groups," he explains, "oxygen depletion (the largest), carbon monoxide poisoning and hydrogen sulphide poisoning.

Sometimes the cause of death is not immediately obvious.

"We investigated a triple fatality that occurred on a ship.  The bodies were found in the ship's anchor chain locker.  Not an obvious high risk confined space. Our first reaction was 'what could possibly be the risk in that kind of environment?'  The culprit actually turned out to be the anchor chain.  It had been hauled from the sea dripping with salt water and left in the locker room for days on end.  During that time it had rusted. Rusting is a chemical reaction that takes oxygen from the air.  As a result oxygen levels in the locker room had reduced to, as it turned out, a fatally low level."

Sometimes the oxygen depletion results from displacement by inert gases such as carbon dioxide:

"A worker accidentally dropped his mobile phone into a tank containing pig feed. He climbed down a ladder into the tank to retrieve it.  He was overcome and died.  It turned out that the pig feed consisted of fruit cake supplied by a local baker.  The feed had fermented to produce an atmosphere of carbon dioxide at the bottom of the tank." 

Tragically, confined spaces all too frequently lead to multiple deaths when fellow workers rush to help a collapsed colleague and are themselves overcome. 

Traditional instruments such as electrochemical sensors are used to measure atmospheres in confined spaces.  Chris and his team have access to newer techniques.  In addition to the microdrone mentioned above they can use infra-red cameras to identify gas clouds.

Analysis

Chris's team and other HSL scientists provide a vital role in gathering samples and data at the scene of an incident.  Underpinning this front line work are the resources back at Buxton

These include the Analytical Sciences Unit.  Scientists such as Dr Duncan Rimmer (head of the unit) and team leader Ian Pengelly painstakingly work to characterise samples retrieved from site of the incident.

Sources of samples analysed by the unit read like a who's who of high profile UK health and safety incidents. They include Buncefield, ICL Plastics, Ladbroke Grove, , the King's Cross fire and the Marks and Spencer asbestos exposure case.

This work requires state of the art analytical equipment.  The workhorse is gas chromatography (which separate substances in a mixture) coupled to mass spectrometry (which identifies them). GC-MS is highly sensitive and quantitative.  Automated thermal desorption (ATD) is often used for volatile organic compounds. A relatively new feature of ATD is the ability to split off and retain a portion of the sample during the analysis for later corroboration.

"We retain samples for as long as required, which may be five years or more," says Dr Duncan Rimmer. "They can then be made available for verification should, for example, our original analytical result be challenged prior to court proceedings."

Examples of samples analysed by the unit include:

  •   Pieces of machinery from a fairground accident analysed for the presence, or in this case the absence of lubricant (maintenance failure is a cause of many fairground rise collapses).
  •   Plastic from failed guarding on machinery (to check that cheap plastic has not been substituted for the tough polycarbonate which should have been used).

The results of the analyses can sometimes be surprising.  In one case a tank containing formic acid leaked through a tap fitted to the tank.  Analysis confirmed that the tank and tap were made from polypropylene, the right material as it is resistant to formic acid.  However, on dismantling the tap HSL analysts discovered that the washer was apparently missing.  At first this seemed an obvious cause of the leak.  However, sharp analytical eyes noticed a thin smear of pale residue on the tap.  Further analytical work established that this was the remains of a washer made not of polypropylene but nylon.  Normally this should not have mattered because nylon is very resistant to most common organic compounds.  Unfortunately, formic acid is one of the exceptions.  Over time it had slowly dissolved the nylon washer.  This example illustrates the importance of meticulous attention to detail and looking beyond the obvious.

Big bang theories

Explosions and fires feature among the more spectacular and high profile disasters investigated by HSL.  John Hodges is a team leader in the Explosives Atmospheres Section.  Elucidation of the cause of an explosion can often involve a certain amount of detective work.

"In 2004 an explosion and fire in the ICL plastics factory (commonly referred to as the Stockline plastics factory explosion) near Glasgow killed nine people.  It took over 300 fire and rescue personnel to extinguish the blaze."

"When we arrived we had no idea what had led to the collapse.  A building had collapsed.  But was this the result or cause of the explosion? And was the explosion itself a plastic dust or gas explosion?"

A possible source of gas was an underground pipeline leading from a liquid petroleum gas (LPG) tank. 

"We forced smoke with pressurised air into the underground pipeline. This revealed leaks in the pipework.  One particular leak occurred at the point where the pipe went into the building.  What had happened over the years was that the ground level had been gradually built up over the pipe.  A concrete slab was placed over where the pipe was to provide a pathway for fork lift trucks.  Corrosion, together with the resulting pressure caused cracks in the pipe.  Our smoke test demonstrated that escaping gas was drawn into the basement through the hole in the wall where the gas pipe itself entered the building."

It was maintenance failures that caused the ICL plastics fire.  But sometimes the risks can be increased by "improvements".  For example, over the past twenty years or so intermediate bulk containers (IBCs) have steadily replaced steel drums for transporting liquids.  The advantages of these cubic plastic containers include resistance to corrosion, ease of storage and ease of recycling. 

Unfortunately a series of incidents in the UK indicated very poor performance when exposed to fire.  In some cases it was possible to ignite a container with a single match. 

"This was surprising," says Dr Graham Atkinson of HSE's Fire Engineering Section.   "Tests carried out in the USA indicated good performance when the containers were exposed to fire.  However, when we investigated we discovered that these tests had been carried on containers filled with water.  When filled with other liquids fire performance significantly deteriorated." 

Research by Graham Atkinson and his team confirmed this effect.  They not only showed that resistance to fire was dependent on the nature of the liquids contained;  they were able to show which liquids were most likely to cause problems.  These included hydrocarbons such as fuel oils, edible oils and lubricants.   In 2006 Dr Atkinson co-authored a paper1 on this work which was awarded the Frank Lees Medal. 

Unravelling the cause of an accident is useful for apportioning responsibility.  More importantly it also provides understanding which can help prevent a recurrence. 

An example is the recent work prompted by the 2005 Buncefield incident.  Buncefield occurred when vapour from an overflowing fuel tank ignited.  Graham Atkinson applied the science of fluid dynamics (which describes fluids, e.g. gases, liquids and vapours in motion) to see what lessons could be learned.

An overflowing fuel tank results in a cascade of fuel droplets falling through the air.  The droplets drive a flow of fuel-contaminated air downwards and the result is a heavy, explosible vapour cloud that rolls away from the tank.

 Using fluid dynamics Graham Atkinson and his team demonstrated how such a heavy vapour cloud would move through a fuel storage depot.  They showed how factors such as the nature and height of the tank and the rate and volume of overfill combine to determine the rate and extent of the vapour cloud's movement.  This information now helps the HSE advice local councils on planning applications.   The councils can now more confidently specify how close property developments can be allowed to approach fuel storage depots.

Dr Mat Ivings, HSL's head of Computational Fluid Dynamics (CFD) section aims to take fluid dynamics one step further.   CFD draws on vast amounts of computer processing power to solve the partial differential equations governing fluid flow. The maths is incredibly complex and not surprisingly, all seven members of Mat's team have PhDs in fluid dynamics.  But the end result is an animated and highly visual picture of the flow.

"We can use this to understand which factors control the flow behaviour."

In 2007, Mat and his team used CFD to understand how leaks from the Pirbright Institute of Animal Health facility carried foot and mouth disease (FMD) virus into the surrounding countryside leading to the 2007 FMD outbreak. 

"The drains had been inspected and found to be in poor condition.  In places tree roots had penetrated them.  It was likely that the spread of the virus originated from a leak of effluent from a manhole.  The source of the contamination was a laboratory within the facility which handled the FMD virus.  We ran our model of the drainage system for different effluent pumping rates and were able to demonstrate that the overflow occurred with the higher flow rate."

CFD has also been used to model smoke movement from a fire in a London underground station and predict the dispersion pattern of a gas release in an enclosure.

And more recently it has thrown further light on Buncefield.  CFD modelling showed that obstacles such as hedges and buildings, and the slope of the  ground have significant effects on the movement of heavy flammable vapour clouds.  To assess the consequences of a Buncefield-type tank overfilling release at other fuel depots, the CFD team have been running simulations, using a complex three-dimensional model that incorporates terrain data supplied by the Ordnance Survey. Their results have provided valuable information to help design mitigation measures and reduce the risks arising from such spills.

Engineering

Virtual modelling initiatives such as CFD are providing valuable information already and promise even more for the future.  But HSL still make use of hard physical evidence.  Examples include the use of tower cranes to investigate collapses and study the effect of wind on crane components (Richard Isherwood of HSL's Engineering and Personal Safety Section) and sections of rail to determine train accidents (Bill Geary of the metallurgy).   Bill Geary points to a section of rail retrieved from the Hatfield train derailment.   "The cracks visible in the top of the rail are caused by the localised very high stresses between the train wheel and the rail. Initially, the cracks grow at a shallow angle but the cracks continue downwards into the rail until a critical size is reached, at which point catastrophic failure of the rail occurs. This is what derailed the train at Hatfield.

Over time harder rails have been used to reduce the wear rate and thus costs, however, this has led to an increase in incidences of rolling contact fatigue cracking. Since Hatfield sophisticated inspection techniques have been developed to detect rail head cracking and rail grinding is used to remove cracks before they reach a critical size."

Synergy

HSL have a sustained track (no pun intended) record in accident investigation.  What is their secret? Obviously the quality of staff is an important factor.  A great number of them are recognised as world experts in their fields.

But equally important is the synergy resulting from the presence of all these scientists on one 550 acre site.    They are able to bring a multi-disciplined approach to accident investigation.  These range from "hard" sciences such as chemistry, mechanical engineering, physics and mathematics to the so called "soft" sciences such as psychology and the social sciences.  This means that when HSE and other organisations engage HSL they are not just buying into a single expertise.  They are hiring science.

References

1 CONTROLLING THE FIRE RISKS FROM COMPOSITE IBCS, G. Atkinson and N. Riley, Symposium Series No 151.

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