An increasing trend in fire safety in tunnels is for active intervention using water mists, such as retro fitted on the Channel Tunnel undersea rail link between UK and France, and the Tyne Tunnels, and refurbished Dartford road tunnel, and is under development for the Stockholm ring road.
But water mist is only one of the key types of fixed fire fighting systems (FFFS), the others being deluge water spray, and foam.
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FFFS is increasingly part of the discussion, and debate, to provide enhanced fire safety solutions in transport tunnels, notes Haukur Ingason, underground fire safety researcher with SP Technical Research Institute of Sweden. He adds that the take-up of FFFS has only really started to take hold in the last decade or so in Europe, most especially, and also North America, while it has a longer pedigree in Asia and Australasia in major road tunnels,
There is an observable tendency towards FFFS, says Armin Feltmann, sales engineer-tunnel systems with Fogtec "especially high pressure water mist systems".
He says there are increasing numbers of papers and presentations on this aspect of FFFS at conferences, and adds: "Some standards are now also acknowledging the compensation potential when installing FFFS into a tunnel."
FFFS in Channel Tunnel
The 53km long Channel Tunnel suffered serious fire damage to stretches of its tunnel lining in 1996 and 2008. There were no fatalities or injuries, and everyone escaped by cross passage into the sealed Service Tunnel, the repairs needed to the running tunnels were both extensive and expensive. Transport capacity on the rail service was reduced for months, and so revenues too.
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By GlobalData"While the Service Tunnel has shown itself to be extremely effective in terms of supporting human safety in fires, the running tunnel damage and insurance cost was huge," says John Keefe, spokesman for Eurotunnel, the owner and operator of the UK-France fixed rail link. "Firefighters were pretty powerless to intervene in the confined space once the fire took hold."
Insurance paid for all repair costs – EUR 250M (USD 334M) to recover from the 2008 fire. Annual premiums then trebled from the previous level of EUR 10M (USD 13M). "A significant increase," says Keefe.
He says the challenge was to prove to insurers that the system could protect infrastructure, and so reduce risk and premiums. After the 1996 fire, the emergency rules changed to make trains stop in the tunnel for evacuation at a cross passage to the Service Tunnel. But the question remained of how to overcome the potentially costly problem of infrastructure damage in any future incident.
Eurotunnel’s solution was the ‘SAFE’ (Station Attaque FEu – or, fire attack station) concept, which involved building insitu FFFS "stations" in the tunnel for suspect freight trains to reach and stop. In effect, a train would go to the fire fighting resource rather than vice versa – and be able to reach the SAFE station within 15 minutes.
"The SAFE stations are in place. They work," says Keefe. "We’ve never had to use them – which is good as their purpose is against an eventuality we don’t want to happen."
In each running tunnel are two SAFE stations, built one third of the way from the portals. The symmetry of the layout in the twin tunnels means they are effectively pairs, one each side of the Service Tunnel. The retro-fitting work to install the system was undertaken from within the Service Tunnel, which holds the power and pump installations, and from where a series of water pipes link through to the running tunnels to feed the main distributor pipelines.
Patented Fogtec equipment is part of the solution employed in the SAFE stations within the overall Eurotunnel design.
Others involved were Fogtec’s subsidiary Acis, consultancy IFAB Institute for Applied Fire Safety Research, and contractor Spie.
Keefe says the concept is to produce a high pressure, micromist that starves the fire of oxygen. The choke off is achieved by the thickening steam cloud as water evaporates, and in the process helps reduce heat radiation from the blaze. The fire is doused until emergency services get in and put out a blaze or investigate.
Specific nozzles come are activated in response to monitoring data from infrared detectors, and therefore only a few of the 29 sections – each 30m long – that make up the full length of a SAFE station are switched on.
"Most of the train is left untouched," says Keefe.
Research and proving tests were undertaken in a special fire tunnel at San Pedro des Anes, in Spain, and then a water-mist only test in the Channel Tunnel, all of which helped to convince Eurotunnel and insurers of the value of the SAFE system. The tests involved a pipeline high in the tunnel with nozzles 3m apart to deliver water under 115 bar pressure.
Development work was done over 2009-10, and installation got underway in 2010. Completed, Eurotunnel saw its annual insurance premiums drop to about EUR 11M (USD 14.7M). As a result, one year’s saving in premium paid for the EUR 20M (USD 26.7M) design and build of the SAFE system, and will keep down future costs.
No one else has taken up its SAFE concept, he says, although many organisations visit the Channel Tunnel. Keefe notes that the infrastructure is "unique in the rules that govern it," and that the third tunnel, the Service Tunnel, was "vital to retrofitting".
Technology, reliability and Trade-offs
FFFS has a history of controversy over its application in tunnels, notes SP’s Haukur Ingason, who led a cross-board Swedish underground transport fire safety study, called ‘Metro’, over 2009-12.
Speaking recently at the 7th International Conference on Tunnel Safety and Ventilation 2014, in Austria, Ingason discussed what he sees as an "upcoming controversy", in the question of technical trade-offs.
The question surrounds whether employing FFFS would allow for offsetting, in effect – allowing some reduction in other types of protective factors in tunnel, such as the design fire load for the ventilation system, protection for the structural lining, or even perhaps wider spacing for escape exits, according to a conference paper co-authored by Ingason (Ingason and Li, 2014).
Ingason notes the absence of guidelines and standards for evaluating the effects of the technical trade-offs. He recommends that a reliability analysis of the FFFS and other systems be done before trade-offs are made.
To meet reliability needs by ensure equipment has high availability, Feltmann emphasises that performance targets are meet by studies in the initial stages of design; the studies are Reliability, Availability, Maintainability and Safety (RAMS), he says. Feltmann adds that high availabilities are founded upon selecting proper components – both technology and materials.
With respect to Fogtec’s involvement in the Channel Tunnel’s SAFE system, Feltmann describes the need to ensure components can deliver the lowest maintenance needs – being "basically maintenance free" – and are such the likes of welded stainless steel pipes, and passive stainless steel nozzles without moving parts. Redundancy in pump units also can be sought, and spares for normal consumables should be stocked on site.
Section valves are the ‘most critical’ components in the FFFS and would be the only ‘active’ part needing regular testing, says Feltmann. He adds that some have been developed to be tested remotely – and so a tunnel closure or part closure is not needed, and traffic is not interrupted. Further advantages, he points out, include speed of testing and lower costs in doing so, and that economic benefits come from the regular testing regime, leading to high availability of the component.
Although there is no specific and universally guidance for FFFS, then based on research done so far, given a satisfactory reliability analysis during design development on a transport tunnel project, Ingason and colleagues propose some suggestions around fire load and ventilation, lining protection, and exit intervals.
In looking at fire loading for inputs to the technical tradeoffs analysis, Ingason also discusses full-scale tests carried out by SP and Trafikverket (Swedish Transport Administration) at Runehamar tunnel, in late 2013. The tests are described (Ingason, Li, Appel and Lundstrum, 2014) given to another conference, the 6th International Symposium on Tunnel Safety and Security, held earlier this year, in France.
The tests were developed by the two organisations working in collaboration with Swedish consultancy Brandskyddslaget. They were undertaken to examine water deluge systems for limiting the size and spread of a fire, and so allow for evacuation in congested traffic, and were based on simple, low pressure dispersion from pairs of nozzles on a pipeline.
The system for the full-scale tests in the 9m wide Runehamar tunnel was established in preparatory lab tests. The water supply for emergency services is integrated to FFFS. Estimated costs, including maintenance, are about half that of traditional deluge systems, the researchers said.
Among the conclusions from the Runehamar tunnel fire tests, the researchers say that early activation of the FFFS is vital, although state in their paper that the system has "a sufficient safety margin to allow delayed response while retaining the ability to fight the more severe fires produced by such a delay." But further research is advised, not least to look at the extra smokes and toxic fumes released by not having the FFFS activated early enough.
Trafikverket, which is responsible for both road and rail infrastructure in Sweden, has the fire safety tests and planning underway as part of developing Stockholm’s new ring road system, much of which will be underground – in tunnels up to an average of 15m wide. The fire safety tests were part of a strategic EU transport initiative, which provided a grant towards the work as part of developing the Stockholm bypass.
Also on possible trade-offs – or compensation potential, as called by Feltmann – the recent Safety of Life in Tunnels II (SOLIT-2) research project had this as one of its points of study for road tunnels. The SOLIT-2 latest research was reported in November 2012, and includes reference to earlier guidance, such as UPTUN, and also references Eurotunnel’s SAFE system.
The SOLIT-2 project says: "Practically, this means that investing in water mist fire fighting systems may possibly allow for a higher safety level and savings in conventional equipment at the same time. In the overall balance over the life cycle, costs can thus be reduced."
Feltmann explains the further importance of SOLIT-2 in fire safety design by explaining that it is used – notably Annex 3 – in many tender documents, such as recent projects like City Tunnel Bregenz, Arlbergtunnel, Heathrow, and fire tests for the Mont Blanc Tunnel.
SOLIT-2 guidance also include Annex 7 to provide an overview of how fire tests should be carried out, and this helps to determine the FFFS layout to be tested, including flow rates, pressure, distance, and type of nozzles, he adds.
The SOLIT research was founded, and is funded, by Germany’s Ministry of Economics and Technology. Those involved in the research include Fogtec, research association Stuva, consultancy Bung, Ruhr University Bochum, the state of Saxony-Anhalt’s fire institute, IFAB, and TUV Sud Rail
Before the SAFE system
The two fires that damaged the channel tunnel left Eurotunnel with major, and expensive, infrastructure repair works to execute, and do so against the clock as each day the rail link was left to operate under reduced capacity. Its approach to handling the tunnel repair works was markedly different in each case, and they took notably less time to complete after the second fire incident, all due to lessons learned and applied.
Following the first fire, in 1996, Eurotunnel found severe damage to the concrete lining along a stretch of the central section of the south tunnel. Temperatures in the fire had reached 700 to 800oc, and over hundreds of metres rebar was exposed – with an average loss of up to 200mm thickness of ring thickness. There were spots where the concrete was totally gone, leaving only the steel cage and sight of the grouted annulus.
The fire damage in 2008 happened in the north tunnel, approximately 11km from the French portal. The lining once again suffered significant damage. in both cases, the freight passengers – in a special car near the front of the train – were evacuated safely along with the crew into the service tunnel via a cross passage, and sealed doors.
In both cases, the repair works removed parts of the lining too weak to remain, and then employed some steel mesh placement along with sprayed shotcrete. However, whereas the first repair works took place over many months (January-May 1997), on the second occasion they were completed in a fraction of the time.
On the first repair project, the civil works were performed using a single train parked insitu and the section of tunnel sealed by bulkheads; access was through the service tunnel. However, on the second project a portal was out of operation and could be used for direct access using works shuttle trains. In the damaged section, gantries and crews were deployed in a more intricate, phased series of rolling works, allowing multiple activities to be underway.
Eurotunnel said at the time that the almost ‘industrial’ approach to the repair the second time round resulted, in part, from its team having far more experience by then in maintaining the infrastructure.
