Deterministic risk assessment methodology in road tunnels

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Deterministic risk assessment methodology in road
tunnels
Peter Vidmar Dr. Stojan Petelin Prof. Dr.
ISEP 2007

• Ljubljana, 9. May 2007

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INTRODUCTION

• The paper presents the use of the CFD model in
  the tunnel safety assessment process.
• The set-up of the initial and boundary conditions
  and the requirement for grid density found during
  the validation tests is the used to prepare three
  kind of fire scenarios 20MW, 50 MW and 100 MW,
  with different ventilation conditions natural,
  semi transverse, full transverse and longitudinal
  ventilation.
• The observed variables, soot density and
  temperature, are presented in minutes time steps
  trough the entire tunnel length.
• The obtained data allow the analyses of the
  ventilation conditions for different heat
  releases from fires.
• The second step is to add additional criteria of
  human behaviour inside the tunnel (evacuation)
  and human resistance to the elevated gas
  concentrations and temperature.
• What comes out is a fully deterministic risk
  matrix that is based on the calculated data where
  the risk is ranged on five levels, from the
  lowest to a very danger level.

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METHODOLOGICAL APPROACH ON TUNNEL SAFETY - Risk
criteria
Slide 7
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METHODOLOGICAL APPROACH ON TUNNEL SAFETY
Deterministic safety analyses
The deterministic approach breathe into the
analysis of the greater part of physical events
like fire source characteristic and its dynamics,
the operation of the ventilation system and other
conditions as well as their reciprocal
interactions. When the approach is used in
practice, we should define a number of safety
categories
Table 1 Deterministic safety analysis supposed
safety categories
Sl. 29
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METHODOLOGICAL APPROACH ON TUNNEL SAFETY
Deterministic safety analyses
Following the assumptions in Table 1, a risk
matrix would then be defined as a 5 x 5 matrix
with each side corresponding to one severity
category
Deterministic safety analysis example of risk
matrix
Hazards with high assessments, such as A1, B1 and
A2 in the black squares, are thought of as being
very severe and requiring immediate action to
reduce. Hazards with low assessments, such as E5,
E4 and D5 in the white squares, are considered to
require no further action. Hazards between these
two (grey squares) are considered worthy of some
improvement if a cost-effective solution can be
found.
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METHODOLOGICAL APPROACH ON TUNNEL SAFETY- Event
tree
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METHODOLOGICAL APPROACH ON TUNNEL SAFETY-
Deterministic safety analyses
The results of the event tree are several
predicted scenarios with calculated final event
frequencies. Between nine final scenarios, there
are three fire scenarios with a major frequency
G2, G5 and G8. Further work leads in two
directions, with probabilistic approach on
deterministic approach. In the following sections
the methodology and requirements of using a
deterministic approach in explained more in
detail.
Risk matrix (Fire behaviour and ventilation
effect )
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FIRE ANALYSES IN ROAD TUNNELFire scenarios in
road tunnel

• Fire sources 20 MW, 50 MW and 100 MW
• Ventilation 1 - natural, 2 - longitudinal, 3
  semi transverse 4 transverse of improved
  transverse.
• The whole cut-out of the tunnel is 650 m long, 10
  m large and 8 m or in case of lowered roof 6 m
  high.

   

a.) b.) c.) d.)
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FIRE ANALYSES IN ROAD TUNNELGeometry, initial
and boundary condition
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MODELLING FIRE WITH CFD Geometry, initial and
boundary condition
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MODELLING FIRE WITH CFDParameters and approach
of results analysis

• LC50 (Lethal Concentration 50 ) for soot
  particulates is 30 g/m3 during the 30 min
  exposure or ali 1-3 g min/m3
• Limit resist temperatures for human are 100 oC at
  30 min and 75 oC at 60 min exposure
• The choosen limit concentration of soot
  particulates is 500 mg/m3 to 1000 mg/m3,
• Limit temperature is 50 oC

The risk or consequences are divided in five
categories, these are LR low risk
smaller injury MR
medium risk serious injury with
full recovery SR
serious risk permanent injury
VHR very high risk low
casualty number (1-3), numerous injured EXR
extremely high risk numerous casualties
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MODELLING FIRE WITH CFD Risk cateories
LR ASD lt 500. MR ASDL gt 500. ? SLH gt
ASLH SR ASDgt 500. VHR ASDL gt 500. ? SLH lt
ASLH EXR ((SR ? VHR) ? AT gt 50.) ? ATL gt 50.
Where the abbreviations mean ASD- Average
smoke density value in profile mg/m3 ASDL -
Average smoke density value in layer mg/m3 SLH
Smoke layer height m ASLH Allowed smoke
layer height m AT Average temperature in
profile oC ATL Average temperature in layer
oC TLH Temperature layer height m ATLH
Allowed temperature layer height m
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MODELLING FIRE WITH CFDSpatial averaging in
height longitudinal the tunnel shaft

• In the analysis the number of spatial steps in
  the lonbitudinal direction in equal to the model
  discretusation
• The results in tunnel height are averaged in four
  layers

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MODELLING FIRE WITH CFD Evacuation model

• The model consider basic parameters of movement
  start of self rescue, walking speed, tunnel
  lenght and the condition that dirent the right
  way of escape depending on the location of the
  fire.

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MODELLING FIRE WITH CFD Results Deterministic
risk matrix for the chosen observer location (252
north)
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POSSIBILITY OF DETERMINISTIC AND PROBABILITY RISK
METHOD CONNECTION
For example the probability of event EXR-1 is
computed taking in account the higher value of
frequency from the first table, and multiply it
with the rate of 5/5.
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POSSIBILITY OF DETERMINISTIC AND PROBABILITY RISK
METHOD CONNECTION
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CONCLUSION

• Comparing twelve scenarios it is mathematically
  found which scenarios, at what time intervals and
  at which locations represent high risk.
• The presented examples of deterministic matrix
  use show how to find the efficiency of the
  ventilation system and the sutability of the
  operation with it.
• The paper beside presenting a novel approach of
  risk evaluation open a wide possibiliry of
  research of safety analyses in road tunnels. The
  application fo the methodology presents the
  guidance how to approach the problem of
  understanding and improving the safety during the
  fire in a road tunnel.