Assessment of Risk for the Transportation of Dangerous Goods. D.G. Leeming and S.A. Gadd, Health and Safety Laboratory, HSE, Broad Lane, Sheffield, UK; and T.N.K. Riley, CHD, HSE, St. Anne's House, Bootle, UK
INTRODUCTION
Recommendations made in the Advisory Committee on Dangerous Substances
(ACDS) report of 1991, led the HSE's Major Hazards Assessment
Unit (MHAU) to commission the Risk Assessment Section of the Health
and Safety Laboratory (HSL) to develop Transport RISKAT, a computerised
tool for the assessment of risks to people arising from the transport
of dangerous substances.
Transport RISKAT (Transport Risk Assessment Tool)
is designed to estimate the risks to people arising from potential
releases of toxic and flammable substances during transport by
road and rail. The program is based on a generalisation of the
Quantified Risk Assessment (QRA) methodology developed in the
ACDS report (1991). On a selected transport route, the risks
to both the on and off-route populations are estimated and the
total risk is expressed in terms of 'route societal risk'. Societal
risk is used because of the potential for harming a significant
number of people in a single incident. The risk to any one person,
on the other hand, is likely to be low, since individuals along
the route will only be exposed briefly to the potential hazard.
Transport RISKAT enables comparisons to be made between the risks
arising from different routes or modes of transport, or from different
sections of the same route. In addition, it can be used to identify
risk 'hot-spots' along a particular route and the lowest risk
option from a number of alternatives.
In this paper, the discussion of Transport RISKAT is restricted
to consideration of the risks arising from the transport of liquefied
toxic gases which form dense clouds when released into the atmosphere
(e.g. chlorine and ammonia). The methodology is described, and
a case study outlined in which the implications of changing the
mode of delivery to a site from rail to road were assessed.
METHODOLOGY OF TRANSPORT RISKAT
Route Definition
In order to define the route, the selected rail or road route
is first divided into a number of sections, of variable length,
but with similar characteristics. The sections are characterised
by: road type (road routes only); travel direction; type and quantity
of substance transported; density and distribution of surrounding
resident population. The residential population is defined by
three uniform bands of average density, parallel to the route.
The width of these bands, and the appropriate population density
within them is, at present, determined from visual inspection
of 1:50,000 scale O.S. maps for each route section. A single
set of representative weather conditions is chosen for the whole
route.
Frequency Analysis
Once the route has been defined, the release frequencies for each
potential release of hazardous material from road tankers and
rail wagons, for each route section, are estimated. In the ACDS
report (1991), two main causes of spillage from dangerous goods
road tankers and rail tank wagons were identified; puncture or
damage to the tanker/wagon following an accident; and failure
or mal-operation of the tanker/wagon equipment. In the case of
rail transport. collision and derailment were the two main causes
of accidents.
A high degree of uncertainty is recognised in the estimation of
release frequencies for toxic gases following an accident. At
the time of the work for the ACDS report (1991), there were no
recorded incidents in the UK where properly designed road or rail
tankers for pressurised liquefied toxic gases had been punctured
as the result of an accident, however, such an event remains foreseeable.
Consequently, for the ACDS report (1991), an approach was adopted
using an analysis by ICI Transport Engineering Section to estimate
appropriate puncture probabilities. Data from incidents involving
punctures of 'thin' walled tank wagons following accidents were
analysed. Expert engineering judgement was then used to estimate
the probability of failure had the vessel concerned been a 'thick'
walled ammonia or chlorine vessel. The results of this analysis
were checked using a statistical technique which gave consistent
results. Equipment failure frequencies were quantified by analysing
historical data. Event trees were constructed to determine release
frequencies for each route section for each potential release
scenario, combining accident and equipment failure frequencies
with the probability of vessel puncture. Two additional sources
of release frequency information which can be used are: Saccomanno
et al. (1989) and Saccomanno et al. (1993).
Consequence Analysis
The consequences of a release of toxic gas depend upon the nature
of the release (i.e. whether it is instantaneous or continuous),
and on the mass released. In an instantaneous release, the entire
contents of the tanker are released at once, whereas in a continuous
release, the tanker contents are released over a period of time.
Dense gas dispersion models (Fryer and Kaiser, 1979; Jagger,
1983) are used to describe how the gas cloud spreads, for each
potential release scenario, for each route section. The dispersion
of the toxic cloud is influenced by the prevailing atmospheric
conditions.
The impact of the toxic cloud on the population is calculated
using a 'probability of fatality' approach based on a probit relationship
for the dose-response relationship (toxicology). The 90%, 50%
and 1% fatality hazard areas are calculated for both the outdoor
and indoor population. The percentage of the resident population
indoors is dependent on the weather conditions, and it is assumed
that a percentage of those outdoors can 'escape' indoors depending
on the level of exposure.
Risk estimation
The number of people exposed to each representative release is
derived from the calculated hazard areas and the population information.
For each release scenario, the mean number of fatalities is combined
with the estimated release frequency. Thus, an estimate of societal
risk for each route section, and for the route as a whole, is
calculated. Route societal risk is expressed using F-N curves,
which are plots of the number of fatalities, N, against the cumulative
frequency per year of N or more fatalities.
ONGOING RESEARCH
Background
The case study involved the use of Transport RISKAT as part of
an assessment of the overall risks to population from a major
industrial facility in the UK (henceforth referred to as 'the
site'). The site uses chlorine delivered from elsewhere in the
UK in the manufacture of various products. The assessment was
prompted by a proposal by the site owners to switch from rail
delivery of chlorine by one route, to road delivery by another.
Frequent "just in time" road deliveries would reduce
the amount of chlorine being stored on the site and, in turn reduce
the risk to population in the vicinity of the site.
The proposal generated two questions which were addressed in this
study: is the risk to population along the route greater from
the proposed road route than from the existing rail route, and
if so, are the overall risks reduced as a result of the change
in the delivery system? Transport RISKAT was used to carry
out a comparative assessment of the en-route risks for transporting
chlorine by rail and road to the site. A comparative assessment
of the risks from on-site operations before and after the switch
in transport mode was performed using HSE's tool RISKAT (Clay
et al. 1994). The total risks of site and delivery system with
each mode of delivery were then considered.
Methodology
The two routes were sub-divided into sections possessing similar
characteristics. For the rail route, all chlorine transport took
place at night when passenger traffic on the rail network was
negligible, and consequently no on-route population was included
in the study. For the road route, the motorist population was
assumed to be constant along the route, and to be concentrated
around an accident due to congestion behind the accident, and
as a result of motorists on the opposite carriageway slowing down
to view the accident. Motorists were also considered to be effectively
outdoors. In the case study, to ensure that the results of the
comparison of the supply options took account of variability in
estimation, release frequency estimates from three independent
sources were considered. As the same models and assumptions were
applied to the risk assessment for each delivery route, the inherent
uncertainty was similar in each case. This is significant when
comparing the rail and road risks since then it is the relative
rather than the absolute risk which is important.
Results and Outcome
Within the uncertainties of the risk estimation, no significant
difference between the risks from the road and rail routes was
found. The different sources of release frequencies led to conflicting
conclusions regarding the route societal risk. In all cases however,
the motorist population contributed on average about two thirds
of the societal risk due to the road delivery system, implying
that the model is very sensitive to assumptions made about the
on-road population. The main uncertainties arose from the uncertainty
of the release frequency information, and from the population
data. The risk estimate is highly sensitive to changes in the
release frequency and to the variation of population density within
1 km of the route. This effect is more pronounced for rail than
road due to the larger contribution to the overall risk from the
off-route population. The results indicate that the societal
risk estimates are more sensitive to assumptions made about the
variation of population density with distance than to variations
in the frequency analysis.
The change in delivery mode significantly reduced the risks from
the site itself The result, within the uncertainties of the risk
estimates, did not show that a significant risk was being transferred
to the population along the road route, and indicated that overall
the risk was reduced. Today, chlorine is delivered to the site
by road.
CONCLUSIONS
The methodology of Transport RISKAT for toxic substances has been
described. The estimates of societal risk are sensitive to the
assumptions which are made about the population. The density
and distribution of population along possible routes may be a
deciding factor in the comparison of hazardous materials transport
options. The results of the application of Transport RISKAT to
a case study have been presented.
REFERENCES
1) Advisory Committee on Dangerous Substances (1991). Major hazard
aspects of the transport of dangerous substances. HSC, HMSO,
ISBN 011 885676 6.
2) Clay, GA, Nussey, C and Corlett, TC (1994). The basic methodology
of toxic RISKAT. HSL Internal Report IR/L/RAW94/1.
3) Fryer, LS and Kaiser, GD (1979). DENZ - a computer program
for the calculation of the dispersion of dense toxic or explosive gases in the atmosphere.
UKAEA Report SRD R152.
4) Jagger, SF (1983). Development of CRUNCH: A dispersion model
for continuous releases of a denser than air vapour into the atmosphere.
UKAEA Report SRD R229.
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6) Saccomanno, FF, Shortreed, JH and Van Aerde, M (1989). Assessing
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Goods by Road and Rail. A review of the Benchmark Corridor Exercise,
part of the International Consensus Conference on the Risks of
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