Sotiris Vardoulakis
email: sotiris.vardoulakis@ineris.fr

Norbert Gonzalez-Flesca

Bernard Fisher

Koulis Pericleous

Diffusive sampling for the validation of urban dispersion models

Keywords: Model validation, Diffusive sampling, Street canyon, Benzene

Although widely used in workplace, indoor and ambient air quality assessments, diffusive sampling has not yet been established as a common tool for dispersion model validation. In this study, three mathematical models (STREET, OSPM and AEOLIUS) that are likely to be used for regulatory purposes were validated against experimental data obtained in two street canyons in Paris. Diffusive tubes were used to sample a wide range of traffic-related organic compounds at different heights and distances from the kerb. Model input information (site geometry, meteorological and traffic data) was obtained from the competent authorities and compared with on site observations. An algorithm describing vertical pollutant dispersion and an empirical relationship between CO and benzene were used. Diffusive sampling might be seen as a practical and cost-effective method for creating data sets for dispersion model validation.

Introduction

Nowadays, several street canyon models of different levels of sophistication are commonly used by local authorities, air quality networks, and research institutions in Europe. Most of these models have been parameterised and validated against real time measurements obtained from roadside monitoring stations.

However, these on-site continuous measurements do not reflect the strong spatial variability of traffic pollution revealed by a number of recent studies [1,[START_REF] Monn | Exposure assessment of air pollutants: a review on spatial heterogeneity and indoor/outdoor/personal exposure to suspended particulate matter, nitrogen dioxide and ozone[END_REF]. This variability, which might have serious implications in terms of population exposure, can be efficiently monitored using diffusive sampling. Furthermore, it may be predicted reliably by adequately validated dispersion models.

Diffusive sampling has become a popular method for assessing air quality due to a number of practical advantages (e.g. no need for power supply, portability of samplers, etc.) Although widely used in workplace, indoor and ambient air quality studies [START_REF] Gonzalez-Flesca | Benzene exposure assessment at indoor, outdoor and personal levels. The French contribution to the LIFE MACBETH programme[END_REF][START_REF] Gonzalez-Flesca | BTX concentrations near a Stage II implemented petrol station[END_REF], it has not yet been established as a common tool for the validation of dispersion models.

The objective of this paper is to present a model validation method involving multisite diffusive sampling. This method was applied to three street canyon models (STREET, OSPM and AEOLIUS) that were validated against experimental data obtained at two different urban sites in Paris, France.

Experimental

Monitoring sites and equipment

Two air quality monitoring campaigns were conducted in street canyons in Paris during winter (Bd. Voltaire, December 1998) and summer (Rue de Rennes, July 1999). The two sites were busy four-lane streets with large pavements and uniform buildings lining up continuously on both sides. The height-to-width (H/W) ratios for Bd. Voltaire and Rue de Rennes were approximately equal to 0.8 and 1.1, and the average traffic volumes during measurements were 30,000 and 23,000 veh/day, respectively. Active (i.e. pumped) and "passive" (i.e. diffusive) tubes were used in both the canyons to sample benzene, toluene, xylenes (BTX) and other volatile organic compounds (VOC). CO, NO X and O 3 were continuously monitored using infrared, chemiluminescence and ultra-violet analysers, respectively. Local meteorological parameters were measured at street level and compared with synoptic weather information obtained from a permanent monitoring station located in Montsouris park, within a few km distance of the experimental sites. Hourly traffic volumes and average vehicle speeds were obtained from automatic counters operating in both the streets. Finally, the vehicle fleet composition was estimated from visual observation.

Diffusive sampling and analysis

Whilst active sampling was conducted only at one kerbside location in each canyon (height of inlet: 3.7 and 2.9 m in Bd. Voltaire and Rue de Rennes respectively), diffusive samplers were placed at several roadside and background locations, at different heights and distances from the kerb.

In Bd. Voltaire, diffusive tubes were located at two different heights (1st and 5th floor) near the walls of the canyon, and at one background site (Fig. 1). The devices remained exposed to ambient concentrations for five days. In Rue de Rennes, two different sets of diffusive tubes were used to examine separately BTX levels during weekend and working weekdays. A more detailed spatial resolution was obtained by increasing the number of sampling locations. In this case, apart from measurements near the walls of the canyon, samples were also taken on the kerbside (h=1.5 m), and at two different background sites (Fig. 2).

The diffusive sampling was carried out using combination Radiello/Perkin-Elmer tubes filled with Carbotrap-B and sheltered in aluminium boxes [START_REF] Bates | Ambient volatile organic compound monitoring by diffusive sampling. Compatibility of high uptake rate samplers with thermal desorption[END_REF]. After recovery from the tubes with thermal desorption, VOCs were analysed in the laboratory using gas chromatography (column type: CP-SIL 5CB, 50 mX 0.32 mm, 1.2 um) and FID. A quality assurance programme, including sampling duplicates and blanks was followed during sampling and analysis.

3.

Modelling

Three mathematical models, STREET, OSPM, and AEOLIUS, were used for the simulation of pollutant dispersion within the canyons. These relatively simple models (or variations of them) are likely to be involved ùi a variety of applications including air quality and traffic management, urban planning, population exposure studies, etc. STREET [START_REF] Johnson | An urban diffusion simulation model for carbon monoxide[END_REF][START_REF] Dabberdt | Validation and applications of an urban diffusion model for vehicular pollutants[END_REF] is a box model that uses two different empirical algorithms to reproduce CO concentrations on either side (i.e. leeward and windward) of a street canyon. When the wind direction is parallel or near-parallel to the axis of the canyon, concentrations on the two opposite sides of the street become equal and they are calculated by averaging the results from the two algorithms. The final CO values are obtained by adding the urban background contribution to the kerbside concentrations.

OSPM [START_REF] Hertel | Modelling pollution from traffic in a street canyon[END_REF] is a semi-empirical code, which was evolved from the CPBM model [START_REF] Yamartino | Development and evaluation of simple models for the flow, turbulence and pollutant concentration fields within an urban street canyon[END_REF]. It is designed to produce series of hourly pollutant concentrations at a single receptor location on either side of a street canyon. It assumes three different contributions to the kerbside levels: (a) the contribution from the direct flow of pollutants from the source to the receptor, (b) the recirculation contribution due to the flow of pollutants around an horizontal wind vortex generated within the so called recirculation zone of the canyon, and (c) the urban background contribution. The direct component is calculated applying Gaussian dispersion theory, while a box model algorithm gives the recirculation component. On the leeward side of the street, concentrations are calculated as the sum of the direct and recirculation contributions, while on the windward side, only the direct contribution of emissions generated outside the recirculation zone are taken into account. If the recirculation zone extends throughout the whole canyon, then the windward concentrations are calculated from only the recirculation component. When the wind speed is near zero or parallel to the street axis, the concentrations on both sides of the canyon become equal. These two models, STREET and OSPM, have been used in many scientific and engineering applications [START_REF] Qin | Dispersion of vehicular emission in street canyons, Guangzhou city, South China (P.R.C[END_REF]11]. AEOLIUS is a more recent model based on the same formulation as OSPM and mainly used in the U.K. [START_REF] Buckland | Validation of a street canyon model in two cities[END_REF].

In STREET, the user externally defines the height (z) of the receptor and its distance from the kerb. By contrast, OSPM and AEOLIUS produce pollutant concentrations only at street level (= 2 m), without giving the user the possibility of choosing the height of the receptors. This limitation was overcome by introducing an algorithm that enables the user to establish vertical pollution profiles in the street [START_REF] Vardoulakis | Assessment of traffic-related air pollution in two street canyons in Paris: implications for exposure studies[END_REF]:

(1) where C r is the concentration of the pollutant at a reference height z r on either side of the canyon (H: height of the canyon). Furthermore, an empirical relationship was introduced, so as to allow the calculation of benzene concentrations from CO predictions:

Benzene (ppb) = a . CO (ppm) (2)
The proportionality constant, a, was derived experimentally from simultaneous BTX and CO measurements in both the canyons (3.8 and 3.7 in Bd. Voltaire and Rue de Rennes, respectively).

As inputs, all three models required synoptic wind information, traffic and emission data, as well as the dimensions of the canyons. The rate of release of CO in the street was calculated from hourly traffic volumes and emission factors, which were derived from the site-specific vehicle fleet composition [START_REF] Buckland | Nomograms for calculating pollution within street canyons[END_REF]. The relative pollutant contributions from the street and the background were derived from diffusive benzene measurements.

4.

Results and discussion

STREET, OSPM and AEOLIUS were initially used to simulate hourly CO averages in Bd.

Voltaire and Rue de Rennes. The results showed a good agreement with the continuous CO measurements in both the streets [START_REF] Vardoulakis | Assessment of traffic-related air pollution in two street canyons in Paris: implications for exposure studies[END_REF][START_REF] Vardoulakis | Air quality monitoring and modelling techniques for street canyons: the Paris experience[END_REF]. In the present study, the three models were further validated against diffusive benzene measurements.

Applying the empirical relationship (2) to the CO concentrations calculated with STREET, average benzene values were produced for different receptor locations in the street over the diffusive sampling periods (i.e. 2 to 5 day averages) and added to the observed background concentrations. The comparison of the total calculated values with the observations showed a very good general agreement for Rue de Rennes (Fig. 3a), although the model seemed to slightly underpredict the low concentrations observed near the top of the canyon. For Bd. Voltaire, the model under-predicted all measured values (Fig. 3b).

Furthermore, relationship (1) was used to calculate CO concentrations at receptor heights corresponding to the diffusive sampling locations in Bd. Voltaire and Rue de Rennes, using the street level OSPM and AEOLIUS outputs as reference values. Applying relationship (2), average benzene values were obtained and added to the observed background concentrations. Despite some slight under-predictions in the case of Bd. Voltaire, OSPM ! reproduced successfully the concentration profiles detected in the two canyons (Fig. 3 c and3d). AEOLIUS1 gave also very good predictions for Rue de Rennes (Fig. 3e), but under-predicted the values observed in Bd. Voltaire (Fig. 3f).

The tendency of all three models 1 to under-predict pollutant concentrations in the case of Bd. Voltaire (i.e. the models under-predicted the CO concentrations from which the benzene averages were calculated) might be attributed to under-estimated CO emissions in this street. More diffusive benzene measurements from other urban canyons are needed for further validating the models as well as the empirical relationship (1). It should be also emphasised that this expression is not applicable to traffic-related pollutants with very short chemical lifetime, like NO 2 , which can be also sampled with diffusive tubes. It has been experimentally demonstrated [START_REF] Vâkevâ | Street level versus rooftop concentrations of submicron aerosol particles and gaseous pollutants in an urban street canyon[END_REF] that NO 2 concentrations may even increase along with height within a street canyon, when the weather conditions favour photochemical activity.

The present study showed how diffusive sampling can be used to test the performance of urban dispersion models at different receptor locations within a street. While high spatial resolution might be achieved using diffusive tubes, the temporal resolution obtained from these measurements is relatively low (e.g. weekly averages). Therefore, the "spatial" validation of the models using diffusive sampling results should be coupled with traditional "temporal" validation using continuous monitoring data (e.g. hourly CO averages).

Conclusions

This study demonstrated that diffusive sampling can be a useful tool for validating urban dispersion models. Following a relatively simple methodology, three mathematical models (STREET, OSPM and AEOLIUS) were validated against multisite benzene measurements obtained in two street canyons in Paris. This methodology involved the use of two empirical relationships: the first one for reproducing vertical pollution profiles using street level concentrations, and the second one for calculating benzene values from CO predictions. All three models gave very satisfactory benzene estimates for Rue de Rennes, but under-predicted (especially STREET and AEOLIUS) the concentrations measured in Bd. Voltaire.

The three models, STREET, OSPM and AEOLIUS, had already been validated in the past against continuous measurements from urban air quality monitoring stations. In the present study, the main advantage of using diffusive sampling was that it enabled us to test the models for different receptor locations within the same street. Most importantly, that was achieved without investing a great amount of resources (e.g. sophisticated instrumentation, power supply, etc.) For this reason, it is believed that diffusive sampling will be increasingly used in the future as an alternative technique for creating data sets for model validation.

Fig. 1 :Fig. 2 :

 12 Fig.1: Diffusive sampling in Bd. Voltaire(14-18 December1998)

Fig. 3 :

 3 Fig. 3: STREET, OSPM and AEOLIUS predictions vs. observed benzene concentrations obtained with diffusive sampling in Rue de Rennes and Bd. Voltaire.

It refers to the model as it was applied in this occasion, i.e. together with expression (1)

Acknowledgements

The authors would like to thank Armelle FREZDSR (INERIS) for the chemical analysis of the samples, Jean-Claude PINARD (INERIS) for his technical assistance, Mairie de Paris (Direction de la Voirie) for the traffic data furnished, and Dr. R. BERKOWICZ (NERI) for providing and discussing OSPM.


L Fournier

A Carrau

P Russo

C N R S Lcd

N S M A Upr 9028 -E

ROOF CHARACTERISATION RELATED TO FIRE PROPAGATION RISK BY A NUMERICAL APPROACH

The expérience shows that roofmg Systems are to be considered as a major factor of thé fire propagation in industrial buildings like warehouses. Currently in France, régulations require that two main measures be implemented to limit thé propagation of thé fïre by thé roof: -one is thé use of intumescent strips on thé roof, -thé other consists of extending thé fire walls (typically 70 cm or 1 m) above thé roof levé1 in order to prevent thé flame from being blown down onto rooftop. A set of normalised-like live tests is used to study thé behaviour of roofing Systems submitted to interior or exterior fire. Unfortunately, thé aforementioned measures are not easily modelled through live tests, and this prompted thé need to develop an accurate computer modelling method. Therefore, this paper présents a computer modelling approach aiming at studying thé influence of such intumescent strips or wall upstands on thé heat propagation properties of current roofing materials. In this paper, two kinds of roofing Systems are studied: -one containing stone wool as insulating material, -thé other containing polyurethane as insulating material. The numerical code used is Fire Dynamic Simulator 12 (FDS), developed by thé NIST, with a spécial additional heat conduction module.

INTRODUCTION

During fires, thé structure stability and thé fire propagation behaviour are major information for safety services. The framework of industrial fire is currently being given a spécial attention in European countries régulation. The conception of industrial premises like for example warehouses that requires a set of tests and spécifications are considered adequately. The underlying technical constraints are more or less spécifie to each country, but are usually based on "large scale" tests. The présent work points out thé roofing System characterisation in thé case of external fires. It focuses on thé détermination of thé roof behaviour of a building adjacent to a large fire. The roof is in that case only exposed to thé fire heat flux. This kind of characterisation is usually done by standardised life tests of thé roofing System composite material. Thèse tests are developed for usual size of fires. In thé case of an actual industrial accident, thé size and thé power of fires are not of thé same order of magnitude. Consequently, in thé one hand, standardised life tests cannot be satisfying for such a configuration. In thé other hand, thèse tests cannot realistically be extended to représentative real size buildings. Computational simulation can be an adéquate way to treat thèse real scale problems. The aim of this paper is to présent a numerical way of work and some preliminary éléments. The paper comprises two main parts. The one is dedicated to a short présentation of standardised European life tests, and several points of thé French régulation in thé field of industrial buildings. The second one is dedicated to thé numerical approach présentation and more precisely to thé models used and some preliminary results.

PROBLEMATIC OF WAREHOUSE ROOF

In thé European market, there is a lot of différent kind of roofs. Generally, a warehouse roof is composed by a support, an insulating material and a covering material. The combination of thèse three components gives more than one hundred possibilities to construct a roof. In France [START_REF] Cwiklinski | Comportement au feu des toitures-Synthèse bibliographique[END_REF] , roofs with a slight slope (less than 5%) are very popular for industrial buildings. Approximately, 80% of thé warehouses are constnicted with a steel support. Because of a restrictive régulation, thé insulating material is mainly minerai wool (92%).

3.1

Large-scale fîre tests for thé assessment of an external fire exposure to roofs

The aggression of thé roof by an external fire concerns first thé covering membrane and then thé insulating material. It can be due to a drop of a burning brand, to a flame or to a radiant heat impact. Ail three factors may be modified by meteorological conditions like thé wind. If thé covering material is ignited, two problems hâve to be analysed: thé fire propagation on thé roof surface and thé in depth pénétration of thé fîre and potential propagation through thé roof on thé internai side of thé building. In Europe, différent countries hâve developed large-scale fire tests to study thèse problems. The European commission is currently trying to harmonise them with no success at thé time. A draft standard pr EN 1187 [START_REF] Cwiklinski | Comportement au feu des toitures-Synthèse bibliographique[END_REF] Also, in France, a roof can be classified according to nine différent FR classes: T30/1, T30/2, T30/3, T15/l, T15/2, T15/l, T5/l,T5/2, T5/3. The difficulty to harmonise such a large-scale tests can be explained by thé diversity of thé roofing in thé Systems existing in thé différent European countries.

Thèse différent large-scale tests allow comparing thé fire behaviour of thé différent roofs but they are not necessary représentatives of a real industrial fire such as warehouse fire. Indeed, thé analysis of différent warehouse accidents shows that thé actual fire propagation by thé roof can be significantly faster than thé propagation observed in a large-scale test.

In addition, a modelling approach could be an interesting alternative to try to characterise thé roof behaviour with an industrial fïre.

The French régulation requirement

In France, a new régulation concerning thé warehouses was published on thé l st January of 2003. In particular, this régulation enforces rules to limit thé fire propagation by thé roof. For instance, thé roof has to fulfïl thé following characteristics: -thé roof support has to be realised with materials of combustibility class MO, -thé insulating material, if any, shall be made of materials of combustibility class MO or materials of combustibility class Ml with a gross heating value lower than 8,4 MJ/kg, -thé global roof FR rating is at least T30/1 (see previous paragraph).

Another point to limit thé fire propagation concerns thé use of fire wall barriers between two cells of thé warehouse. This wall is defined as follow:

-thé wall has to stop and resist to a fire during at least 2 hours, -thé wall overtops thé roof of 1 meter. The roof has to be covered by an incombustible layer of 5 meters wide on both side of thé fire wall.

NUMERICAL SIMULATION APPROACH

Physical and numerical model

The simulation of a real industrial fire is a diffïcult scientific and technical work. It involves complex phenomena (combustion, heat transfer, turbulent flow...) with complex geometries (large building clutter up numerous small objects). The study of thé roofing System stability during an extemal fïre needs to represent realistically thé incident heat flux and thé coupling between thé heat transfer in thé roof and its surrounding. This requires a good modelling of: -thé fluid flow, -thé adjacent fïre, -thé heat transfers in thé fluid, -thé heat transfer in composite solids (thé roof) - The présent chapter aims at described thé key point chosen for this modelling based on thé use of thé computational fluid dynamics code named FDS 1-: from NIST.

The fluid flow modelling

The fluid flow is modelled by thé numerical résolution of conservation équations (Navier Stokes équations) for total mass, momentum, energy, and species mass fractions (fuel and oxygen), formulated as,

dp dp U j = 0 [1]
a Hère, u is thé velocity, p thé perturbation pressure from ambient, p thé density, g thé accélération of gravity, T y thé standard viscosity stress tensor, q c thé heat release rate per unit volume, q r thé radiant energy flux, and Pr t (=0.5) thé turbulent Prandtl number.

As large tires take place in turbulent flow, a Large Eddy Simulation (LES) approach is used with implementation of thé Smagorinsky sub-scale model 2 which introduces thé sub-grid scale (SGS) Reynolds stresses, ÏIJ.SGS » associated with thé local large scale rate of strain. To compute thé pressure, an idéal gas is considered by thé use of its équation of state.

Fire m odelling

The global one-step irréversible chemical réaction for full combustion of a hydrocarbon fuel is assumed,

C,HyO, + VOXOI + 3.75N 2 ) - oe oe Oi H , o2 [41 + v s C+v O2 3.75N 2
where v i is thé stochiometric coefficient of thé substance /. Combustion process is assumed to be diffusion controlled, permitting a mixture-fraction-based modelling approach,

dt d Xj

where Sc t ( = 0 -5) dénotes thé turbulent Schmidt number and / is thé mixture fraction.

Modelling of heat transfer in thé fluid

A fraction of thé energy released by thé combustion is scattered by radiation and thé rest by directly transfer to thé fluid. This fraction is arbitrarily taken at 35%. This value is known to decrease while thé fîre size increases and can hâve values varying for 50% to 5%.

The heat release rate is directly proportional to thé rate of oxygen consumption as illustrated by thé next équation:

q c = -AH o m" 0 [6]
Hère, A// o is tne neat release rate per unit mass of oxygen consumed and m 0 is thé mass of oxygen consumed by unit time and volume which is calculated from thé local mixture fraction [5] and a state relation.

The radiant flux vector in thé energy équation [3] is calculated by integrating thé radiation intensuy over ail directions. The radiation intensity is found by solving thé following radiant transfer équation without scattering,

V.Q/ + *-/ = *-- [7]
where K is thé absorption coefficient, / thé radiation intensity and a thé Stefan-Boltzmann constant. The absorption coefficient, K , is a function of thé mixture fraction and independent of thé wavelength.

Conduction of heat in thé roof

FDS provides a 1D homogeneous semi-infinité conduction model. Consequently, a new conduction model has been implemented in thé FDS code to serve thé purpose of this study. The heat conduction in thé roof is modelled by use of thé Fourrier équation for an heterogeneous 3 dimensional material:

div(& • grad T ) + q = p c [8] dt
where p dénotes thé solid density, T its température, k its thermal conductivity, c its spécifie heat and fïnally q represents a volumic heat source. This équation is discretised using a fïnite volume method for thé 3 spatial dimensions, and a fully implicit scheme for the time dependency. This kind of approach is a usual technique described by Patankar [START_REF] Patankar | Numerical Heat Transfer and Fluid Flow, séries in computational methods in mechanics and thermal sciences[END_REF] . Validation has been performed by comparison with Holman 5 's examples. The coupling between the fluid flow, the radiation heat transfer and the solid heat transfer is proposed in the FDS technical documentation 2 and leads to use the following equation:

) [9]
where T" and T" + respectively dénotes thé wall température at time step n and n +1.

Consequently, thé boundary condition for conduction is a linear function in température and heat. The coherency of results with FDS own conduction module has been checked.

Computation hypothesis

General description

The simulated scénario is a large open fire bordering a warehouse. The side of thé warehouse is made of incombustible brick wall. The résistance of this wall is not in thé scope of thé study. It is consequently considered adiabatic. The wall is 6m high and 0.2m depth. The roof composed of Sandwich panels is placed horizontally on thé save side of thé wall at 0.7m meter from its top. The related case study background is synthesised in thé followins figure:

Sandwich panel représentation

The next figure represents thé composite structure of thé considered sandwich panel. The steel bottom provides thé mechanical stability. The outside protection of thé insulating layer is fulfîlled by thé bituminous felt. The shape of thé steel structure is represented for thé conduction computation. The mesh used to solve conduction has 20 nodes across thé panel and a corresponding mesh with thé fluid flow problem on thé surface side.

Composition of sandwich panel

First numerical results

The first modelling trials hâve been made considering only heat conduction in roof. The pyrolysis phenomenon is not yet considered.

Two computations hâve been done: one for each insulating material. Figures 7 and8 show a cross view plan of thé roof. The altitude (depth in material) and thé distance to thé wall and thé fire are used to define thé related plan view. Readers should take care of thé scale that is not same in both directions.

Taking into account only thé heat conduction process, a slower thermal wave in thé roof is observed for thé Polyurethane foam because of its better insulating properties. Moreover, thé steel structure reaches only 31 0K in température and keeps its mechanical stability in case of stone wool insulation after an exposure of 2 hours. Nevertheless, this conclusion will be différent by considering a secondary fire due to ignition of thé covering material. It should be noticed that thé ignition température of bitumen is around 300°C and its self-inflammation température around 500°C. Figure 9 shows that a bitumen zone has reached its ignition température after only 2 minutes. The self-inflammation température is locally reached after 5 minutes. Thèse two observations mean that a fire at thé roof surface will likely start no more then 5 minutes after thé main fire. Also, thé propagation of thé fire to thé rest of thé roof cannot be modelled only by considering thé radiation impact of thé main fire and thé heat conduction inside thé roof. Its appears from thé computation, that thé formation of thèse zones is not significantly dépendent on thé insulating material.

The ignition of thé covering membrane at thé roof surface would significantly change thé level of heat received by thé roof. The radiant heat emitted by thé main fire might be partially absorbed by thé secondary fire smoke plume. By contrast, thé radiant heat produced by thé secondary fire would be added to thé previous one. In addition to those considérations, thé secondary fire would drive thé convection motion pattems in thé surroundings. The heat exchange with thé roof surface would be completely modifïed. The production of bitumen puddle and its vaporisation would resuit in a loss of energy received by thé solid. One other important phenomenon is thé material ablation due thé to combustion réaction.

At this stage, we can guess that thé propagation is fairly différent between roofmg Systems according thé type of insulating material in considération as far as thé secondary fire actually reaches thé insulating material itself. Thèse more détails considérations can be partially looked at by use of thé current code (FDS + 3D conduction module). The main difficulties resuit in thé energetic distribution between thé three phases of thé combustible (solid, liquid and gas) and thé rate of fuel production. Thèse aspects are essential to evaluate thé fire properties and thé ablation phenomena. It will be thé future way of investigation to get a realistic and usable model 5 CONCLUSION Fire propagation by thé roof is an important phenomenon in industrial accidents such as warehouse fires. The hazard prévention régulations in Europe impose a set of spécifications for such roofmg System. Thèse spécifications are based on roofmg System characterisation by thé use of large-scale live tests. However, thèse tests are not necessarily représentative of industrial size buildings and of industrial fîres and still suffer today from lack of harmonisation.

The numerical modelling can be an alternative approach at thé standardised live tests. It aims at considering thé real dimensions as well as thé actual characteristics of large industrial fires. The original CFD code named FDS is specially designed for such kind of fire, but has not reached a status of development allowing detailed interaction phenomena between fire and materials. Consequently, a new 3D-conduction model has been implemented to evaluate heat conduction inside thé roof. This paper présents preliminary results where only heat transfer by conduction has been considered. Thèse results show that thé roof surface, composed of bitumen, will start to burn rapidly in thé event of our test case study (a major fire adjacent to thé pièce of roofmg material under considération). Also, thé future developments of this work will be focusing on thé implementation of other physical models such as liquéfaction and vaporisation models suitable for bitumen and insulating materials and a fire ablation model.

  treated in this study are polyurethane foam and stone wool.6. Thermophysical data used for roofing components

  7. Stone wool insulated roof at 0.7m from thé top of thé wall température (foam insulated roof at 0.7m from thé top of thé wall température (K) itîngth (ir.) tune

9.

  Time évolution of roof surface température (stone wool insulating material)

J. P. Holman, Heat Transfer, 8 * édition, McGraw-Hill, ISBN 0-07-114320-3, 1997.


P Hourtolou
email: david.hourtolou@ineris.fr

O Salvi

ARAMIS Project: development of an integrated Accidental Risk Assessment Methodology for industries in the framework of SEVESO II directive

ARAMIS is a three-year European project that started in January 2002 and aims at developing a new integrated risk assessment methodology for high-risk industrial sites in combining the strengths of the two approaches most currently used in Europe: the risk-based and deterministic methods. In this respect, the methodology could be used as a supportive tool to speed up the harmonised implementation of the Council Directive 96/82/CE also called SEVESO II Directive. The development of ARAMIS meets the need for a methodology giving consistent rules for the identification of scenarios that take into account some safety devices and some aspects of safety management, and that is recognised by risk experts from Competent Authorities and Industry. The paper presents the overall frame of ARAMIS project. The proposed method results in an integrated risk index composed itself of three independent indexes. Index S assesses the consequence severity of first defined reference scenarios. Index M evaluates Safety Management effectiveness and accounts thus for the scenario probability. Index V estimates the environment vulnerability. Efforts have been made to disseminate work progress and results from the start. A dedicated web-site has been created and a review committee gathering industry risk experts and EU competent authority representatives periodically monitors the project.

INTRODUCTION

The ARAMIS project was accepted for funding in February 2001 by the European Commission, in the 5 th Framework Programme for Research and Technological Development. This three-year project started in January 2002 and gathers fifteen organisations expert in the field of risk analysis. Among these organisations, three research institutes represent Pre-Accession countries from Eastern Europe. The project has been built in particular on the conclusions and results of ASSURANCE and I-RISK, two other European projects funded in the 4 th Framework Programme. ASSURANCE stands for ASSessment of the Uncertainties in Risk Analysis of Chemical Establishments. This project was a benchmark exercise, which aimed at improving the understanding of the sources and types of the uncertainties connected with risk analyses. As a rough conclusion, the project stated that the benchmark exercise revealed noteworthy variation in the final results. Discrepancies were present both in the assessment of frequencies and in the assessment of consequences. The different results would have obviously affected the relevant risk-informed decisions, mainly land-use planning, emergency planning and acceptability of risk. The initial statement of I-RISK project -I for integrated-was the idea that Quantitative Risk Assessments (QRA) and safety management audits were so far two separate tools and that would be valuable to integrate both to address major hazard management. In this respect, the main objective was first to develop a management model for risk control and monitoring, then to implement this model into a dynamic QRA tool. Conclusions of the project point out that the integrated technical and management model was very robust and helped audit organisations in a new way. However it also turned out that a full-scale site integrated QRA was too time and detail demanding, so not currently practical or relevant. From both projects, but also from everyone's experience in his own country, it emerges the need for a methodology giving consistent rules to identify accident scenarios and taking into account both prevention and mitigation measures peculiar to each plant operator. Those safety measures are obviously controlled in a safety management system. There is also an underlying need for a risk assessment method that could reach a consensus amongst risk experts from both Industry and Competent Authorities and then be used with reduced uncertainty to make risk-informed decisions.

The ARAMIS project has been set up to propose solutions to both latter requirements. The paper first recalls the context of major accident hazards prevention in the EU. ARAMIS overall objectives are then expressed and the work contents are described in detail. As a conclusion, the expected impacts of such a methodology are addressed.

CONTEXT OF INDUSTRIAL SAFETY AND THE SEVESO II DIRECTIVE

The annual report from the European Environment Agency (1999) indicates, among others, that the trend in notified accidents has been constant over the last twenty years. This statement alone shows that many of the often seemingly simple "lessons learned" from accidents have not been yet enough implemented in industry's standards. There is unfortunately no doubt that disasters will continue to occur throughout the EU. Some will be due to technology, some to the hazards of nature. The problem of low-probability, high-consequence accidents is likely to remain a key issue in terms of risk management. Nevertheless, hazards have to be managed and risks can be reduced. The most significant EU Directive to help protect people and the environment from major accident hazards is the SEVESO II Directive. This applies to industries that use "significant amounts of hazardous substances". Their operators must demonstrate in particular they apply a major accident prevention policy and they implemented appropriate prevention and mitigation measures controlled and mon i tored in a safety management system. The SEVESO II Directive sets very clear objectives relating to major hazard management, but the remaining question is: how to reach them and to control they are reached? For instance, there is no harmonised definition of the scenarios to be considered for risk assessment. The ASSURANCE project showed in this way that the chosen scenarios (BLEVE, full bore rupture or small leakage, amount of substance caught in an explosion, etc.) are different according to the experts judgement and experience, and according to the deterministic or risk-based approach of the Member State applying the Directive. Moreover, constraints in land-use planning [START_REF] Cassidy | Special issue : The SEVESO II Directive (96/82/EC) on the control of major accident hazards involving dangerous substances[END_REF] sometimes urge the operators to consider reduction of the safety zones by choosing "realistic" scenarios and accounting for the effectiveness of dedicated safety devices. Actually the lack of rules for identifying scenarios and carrying out risk assessment makes often the expert's job tricky and largely too subjective to base transparent risk-informed decisions on it. In addition to uncertainties in risk analyses, differences of culture among the Member States result in a multiplicity of methods and approaches [START_REF] Kirchsteiger | Special Issue on International Trends in Major Accidents and Activities by the European Commission towards Accident Prevention[END_REF]. At a recent JRC International Workshop [START_REF] Kirchsteiger | Promotion of technical harmonisation on risk-based decision-making[END_REF], most participants agreed that comparative risk assessment along harmonised procedures would significantly help the decision understanding. A harmonised risk assessment methodology would thus ensure that risk-based decision making provides the necessary transparency and the right balance between scientific understanding and principle of precaution. Proposing a harmonised methodology for risk assessment is a tough and tricky task. However, deterministic and probabilistic approaches should not be opposed since they are often complementary [START_REF] Libmann | MIRIAM: an integrated approach to organise major risk control in hazardous chemical establishments[END_REF]. From a historical point of view, deterministic methods first allow to check the safe design of an installation. Probabilistic methods help then evaluate the residual risk of the installation. Both approaches have their strengths and weaknesses [START_REF] Hourtolou | ASSURANCE -Assessment of the Uncertainties in Risk ANalysis of Chemical Establishments[END_REF]. A first basic idea of ARAMIS is to take advantage of each approach's strengths and to develop upon it an alternative semiquantitative method based on the evaluation of the safety barriers -lines of defence-peculiar to each site.

OBJECTIVES

The main objective of the ARAMIS project is to create a new integrated risk assessment methodology by combining the strengths of both probabilistic and consequence-based approaches. Accordingly, the method should be flexible enough to account eventually for the different national cultures, in order to become a recommended tool used by risk experts and endorsed by risk decision makers in the whole EU. The proposed method in ARAMIS should allow to characterise an integrated risk index composed it-self of three distinct and independent indexes. Index S is to assess the consequence severity of first de-fined reference scenarios. Index M is to evaluate prevention management effectiveness, which allows thus to account for the reference scenarios probability in a semi-quantitative manner. Index V is to estimate the environment vulnerability by evaluating the sensitivity of potential targets located in the vicinity of a SEVESO plant. The project has been set up (Figure 1) to reflect the logical construction of the risk index and has been divided accordingly into work packages:

1. First goal is to develop a method to identify "reference" accident scenarios. These scenarios are consensual "realistic" scenarios to be used in SEVESO II safety report and taking account of some prevention and mitigation measures of the site according to their effectiveness. 2. Second task is to build up independently each of the three previously defined indexes.

Spatial

DESCRIPTION OF WORK

The work plan of the project has been built according to the logical construction of the final resulting risk index and it has been presented the same way in this article. Halfway through the life of the project, the newly built methodology is to be tested on three SEVESO industrial sites in Europe. At this stage, two new partners from eastern-European countries will join the consortium and will also test the full method as totally unbiased end-users, each in one additional test site of their own country. Moreover and since ARAMIS is intended to be a supportive tool to promote harmonised risk assessments throughout Europe, the project leaves from the start an extensive part to large exchanges with potential end-users of the method. Identified end-users are both industrial companies and Competent Authorities in charge of enforcing SEVESO II Directive. For that purpose, a dedicated Work Package deals with valorisation and dissemination of project progress and results. Industrial partners have also been directly included in the consortium and a Parallel Review Team gathering potential end-users has been constituted.

Scenario identification

Identification of the possible accident scenarios is a key-point in risk assessment [START_REF] Lauridsen | The ASSURANCE project[END_REF]. However, especially in a deterministic approach, worst case scenarios are considered, often without taking into account existing safety devices and implemented safety policy. This approach can lead to an overestimation of the risk level and does not promote the implementation of safety systems. The aim of this Work Package (Delvosalle & al., 2003) is first to identify major accidents without considering safety systems. A second step is then to study in depth safety device effectiveness and safety management efficiency, which will allow to assess -qualitative-probabilities, in order to identify finally Reference Accident Scenarios taking into account some of the implemented safety systems. The first objective is to define a Methodology for the Identification of Major Accident Hazards (MIMAH). On the basis of considered equipment and properties of handled chemicals, the methodology must be able to predict which major accidents are likely to occur. Properties of substances are found out thanks to Directive 67/548/EEC (substance classification and labelling) and their own conditions of use (pressure, temperature, flow, etc.). The work has been divided in several parts. Firstly, it was necessary to select a general approach. The bow-tie method was chosen [START_REF] Bellamy | Major Hazard Management : Technical-Management Links and the AVRIM2 Method, in Seveso 2000 -Risk Management in the[END_REF] because it is a highly structured tool and it is considered as a very good way to establish links with other parts of the project and especially Management Efficiency (Figure 2). Secondly, a special effort was made to develop a common typology of equipment and hazardous substances. Thirdly, event trees and fault trees centred on critical events have been built, and above all a methodology able to build generic trees was created. Critical events are defined as "Loss of Containment" or "Loss of Physical Integrity" events.

PREVENTION Safety Barriers MITIGATION C

O N

ORGANISATIONAL ACTIVITIES FOR BARRIER ASSURANCE

Figure 2. Bow-tie approach for scenario identification

At this stage, the MDVIAH methodology is able to predict which major accidents are likely to occur on a given equipment.

With the help of a deep study of safety systems, causes of accidents and a historical analysis of known accidents, the objective of the work to be done is now to place lines of defence -safety functions, safety barriers-on the different branches of the trees. This will lead to a second Methodology of Identification of Reference Accident Scenarios (MIRAS) which has to take into account some of the safety systems according to their effectiveness. Therefore, the Reference Accident Scenarios will use results from the work performed on the prevention management effectiveness. These scenarios are used afterwards as an input to evaluate the severity index, i.e. the hazard potential.

S index : Severity of the consequences

The objective of this task is to define a severity index S characterising the possible consequences of accident scenarios (Casal & al., 2003). In this respect, only the physical characteristics of the phenomena involved in accidents are studied in order to evaluate the severity of both major scenarios and reference scenarios identified through MIMAH and MIRAS. First task in this respect was to select the most suitable models for the calculation of the effects of the various dangerous accidental phenomena. Thus, a survey of the existing models for the calculation of the effects due to explosions (overpressure and missiles), fires (radiation), toxic clouds (concentration), BLEVE-Fireballs (overpressure, missiles and radiation), pollutant plumes into the water (concentration), soil pollution and domino effects is now available at this stage of the project. The Severity Index must be independent of the other two indexes. It is thus constructed in such a way that every dangerous phenomenon has a corresponding specific sub-index. The contribution of each dangerous phenomenon to the global index S is strongly related to the probability of occurrence of the phenomenon associated to each critical event (e.g. probability of ignition) and identified in the WP1 event trees (Figure 2). Each specific sub-index associated to the various physical phenomena takes into account in its construction the following parameters:

• the effect area of one phenomenon, e.g. a disc in case of an explosion, a plume surface for gas dispersion ; • the kinetic of the phenomenon: fast for an explosion, much slower for a fire ; • the ability to generate domino effect: fragment emission, delayed phenomena triggered off.

The severity index S is therefore a function of parameters only associated with physical phenomena. All the identified scenarios should then be evaluated and ranked in this way according to the calculation of S o for Major Accident Hazards and S re f for Reference Accident Scenarios.

M index : Prevention management effectiveness

This operation deals with the assessment of safety management efficiency and its effect on the calculation of external risks for SEVESO plants [START_REF] Duijm | ARAMIS project : Assessing the effect of safety management efficiency on industrial risk[END_REF]. The methodology is based on the identification of initiating events and direct causes of the accident scenarios (bow-tie approach). Safety barriers are then related to generic fault and event trees representing all possible accident scenarios leading to critical events (Figure 2). The safety organisation includes both the adequacy and completeness of technical and managerial barriers (lines of defence) that are implemented to prevent these accidents and the management system that ensures that these barriers are maintained and adjusted properly. The methodology recognises a number of dimensions of safety management (delivery systems), derived from previous work on safety management modelling, notably the I-Risk [START_REF] Hale | An audit method for the modification of technical risk assessment with management weighting factors[END_REF] and MIRIAM (Plot, 2002) models. These are made explicit in specific functions that need to be executed to maintain a safety barrier. Examples of these delivery systems are: ensuring good competence and commitment of employees, manpower availability, communication, procedures, plans, hardware and human-machine interfaces.

Major Hazard Identification

Major Accident Hazards

Identification of Safety Critical Functions

Reference Accident Scenarios

Generic lines of defense (LOD)

Currently, the focus is on developing instruments to measure the set of dimensions, using a combination of audit, questionnaire, interview and observation techniques. The combination of measurements ensures that not only the implementation of functions, but also its conditions and outcome (e.g. good safety commitment of the employees) are taken into account. The measurement techniques address in particular the specific safety functions (M LO DI and M LO D2> figure 3) found in a given establishment. However evaluation of the management will also be carried out in a general way onsite (M SM s, figure 3), by assuming that measurement quality represents a common mode for the quality of safety barrier maintenance. Barrier effectiveness can then be "adjusted" according to the measurement scores to select the final set of Reference Accident Scenarios.

The assessment of technical barriers effectiveness follows the principles described in the standards IEC61508 and IEC61511 (Functional safety : safety instrumented systems for the process sector). Among these principles, effectiveness is analysed through the definition of "Safety Integrity Levels" linked to device characteristics (design, reliability, maintainability, testability...) and also through criteria upon the activities in charge to maintain them. Challenges in the development of the methodology lie the need to calibrate the scores in the measurement of dimensions, as well as the need to determine the efficiency of technical barriers as a function of these scores.

V index: Vulnerability of the surroundmgs of a plant

An installation handling dangerous substances is hazardous only according to the potential of vulnerable targets liable to be affected. In assessing the overall risk level of a plant, it is therefore quite relevant to characterise the spatial vulnerability of the environment surrounding the plant. Such is the aim of this operation [START_REF] Tixier | ARAMIS project : Assessment of the environment vulnerability in the surroundings of an industrial site[END_REF]. Vulnerability of the surroundings depends on the features of the environment that are potential targets (human, environmental and material) and on the type of impact due to hazards (fire, explosion, and toxic release). It also strongly depends on the considered target area. To address this issue, the area of interest in the vicinity of a plant has been first divided into meshes and the potential targets have been identified and localised with the support of Geographic Information Systems (GIS). The major difficulty is then to rank and prioritise the various sensitivities of the targets according to the various expected impacts.

A suitable solution has been found out by applying a multi-criteria ranking approach, such as SA-ATY methodology which allows to define priorities from complex situations. At this stage of the project, SAATY method has been extensively applied to the concern of vulnerability estimation. First step was to describe and classify potential targets, hazards and impacts in adequate typologies. Following step involved expert judgement. Through experts' answers to specific questionnaires, SAATY method helped build up mathematically the generic coefficients of target vulnerability. Final step will be to test and validate the calculated coefficients and resulting index through full-scale case studies. Thanks to the combined use of SAATY method and GIS, index V should be represented through vulnerability maps of one plant's surroundmgs, and should become in this way a powerful tool for risk-informed decision making.

VALIDATION AND VALORISATION OF THE ARAMIS METHODOLOGY

Case studies

Halfway through the life of the project, five case studies will be carried out with the collaboration of five different SEVESO establishments throughout Europe in order to test and validate or improve the new methodology.

To select the test sites, it has been assured that both countries with consequence-based and probabilistic approaches would be represented. Moreover two case studies out of five will take place in Slovenia and Czech Republic. Two institutes from these countries will indeed join the consortium at this stage and test the full method with the test sites. Both of them will thus act as totally unbiased end-users since they were not involved in the method development. After these full-scale exercises, the ARAMIS methodology will be improved again and will give rise to its last version in the project.

In the meantime, a Parallel Review Team gathering risk experts from industry and EU competent authorities has also been constituted. This review team gathers every six months to monitor the project and thus to ensure needs from end-users are indeed fulfilled and the final approach will be widely accepted. In this respect their main comments concern the applicability and usefulness of achieved results.

Dissemination actions

Since ARAMIS is intended to be a supportive tool to promote harmonised risk assessments throughout Europe, the project leaves from the start an extensive part to large exchanges with potential end-users. Identified end-users are both industrial companies and Competent Authorities in charge of enforcing SEVESO II Directive. Industrial end-users are directly represented in the consortium through the European Process Safety Centre. This helps the consortium to relay information about the project progress and results, and to convince plants for the case studies.

In the dissemination process, a dedicated web site has been set up first to help promote the project towards the public: please visit http://aramis.jrc.it. An electronic newsletter is also released every six month on the web-site, in order to get the public informed of work progress. Two workshops were also planned during the project. The first public workshop was held during the ESREL 2003 conference, 16-18 June in Maastricht, The Netherlands. A final workshop will also take place at the end of the project to disseminate main achievements to all relevant stakeholders. Proceedings of both workshops will be made available on the web-site.

CONCLUSIONS

ARAMIS project supports the European Research Area concerning knowledge improvement, encouragement of the Science-Industry dialogue and harmonisation in decision-making process related to hazardous establishments. The resulting method should indeed be proposed as a recommended and harmonised tool used by risk experts and recognised by risk-informed decision-makers of EU competent authorities. Harmonising industrial risk assessment in Europe would significantly contribute to the European Commission's overall efforts to establish harmonised policies following the Seveso II Directive.

For both Competent Authorities and Industry, such a harmonised risk assessment procedure would constitute first a useful comparison tool for industrial sites, which integrates the strengths of both probabilistic and deterministic approaches. The risk assessment procedure would at last be linked to the set-up of progress plans within the framework of a safety management system. It would also allow to reach a consensus in the selection of accident scenarios that takes into account plant-specific safety devices and safety management effectiveness, i.e. suitable frequencies for the scenarios as required in a safety report for risk control demonstration.

The participation of potential end-users in the project from the start, in particular through the Parallel Review Team ensures that the ARAMIS project will contribute on a very practical level to EC research objectives and to consistent implementation of European policies in major hazard prevention.

  Figure 1. ARAMIS methodology representation

Figure 3 .

 3 Figure 3. Structure of index M

ACKNOWLEDGEMENTS

The ideas presented in this publication are developed in the frame of the EU project ARAMIS "Accidental Risk Assessment Methodology for Industries", contract n° EVG1- CT-2001-00036, co-ordinated by INERIS (F) and with the effective contribution of EC-JRC-IPSC-MAHB (I), Faculté Polytechnique de Mons -Major Risk Research Centre (B), Universitat Politecnica de Catalunya -CERTEC (SP), ARMINES (F), Ris0 National Laboratory (DK), Universita di Roma -Dipartimento Ingegneria Chimica (I), CMI -Safety Management and Technical Hazards (PL), Delft University of Technology -Safety Science Group (NL), European Process Safety Centre (UK), Ecole des Mines de Paris -Pôle Cindyniques (F), Ecole des Mines de Saint Etienne -SITE (F), Ecole des Mines d'Aies -LGEI (F), Jozef Stefan Institute (SL) and Technical University of Ostrava (CZ). The programme is organised within the Energy, Environment and Sustainable Development Programme in the 5 th Framework Programme for Science Research and Technological Development of the European Commission. The Parallel Review Team represents also fifteen organisations, which are also kindly acknowledged for their advice and contribution during the periodic reviews.


Mr Jacques

BP Chaineaux Ineris

Implementation of Directive 1999/92/EC: some concerns for the definition of ATEX zones

Directive 1999/92/EC 1 is to be implemented within a year. It requires the determination of hazardous zones where an explosive atmosphere (ATEX) may occur. This zoning is expected to be accurate. So is to be the evaluation of effects associated with an ignition of the so defined ATEX zones. Hence, physical phenomena which are involved in the formation of ATEX, namely leak and vaporization of a flammable liquid, leak of a flammable gas, dispersion of a layer of combustible dust, must be taken into account and quantified. It is not recommended to oversize a zone, nor to design a zone having such a small volume that the effects of an ignition would be negligible.

Introduction

Directive 1999/92/EC defines minimum requirements, which every employer must apply to improve the safety and health protection of his workers potentially at risk from explosive atmospheres (ATEX).

Every employer shall assess specific risks arising from an ATEX, by taking into account (article 4):

• the likelihood that an ATEX will occur and its persistence; • the likelihood that ignition sources will be present and become active and effective; • the installations, substances used, processes, and their possible interactions; • the extent of the foreseeable effects.

He must classify places where ATEX may occur into zones, on the basis of the frequency and duration of the ATEX (article 7), in accordance with the following classification (annex I):

• Zone 0 or 20: place where an ATEX is present continuously, for long periods or frequently;

• Zone 1 or 21: place where an ATEX is likely to occur occasionally in normal operation; • Zone 2 or 22: place where an ATEX is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

How to define ATEX zones

Effects resulting from an "ATEX ignition must be evaluated. These effects strongly depend on the volume of this ATEX (especially when the volume of the ATEX is large in comparison with the volume of the containment). Thus it is important to evaluate as precisely as possible the dimension of each zone where an ATEX is expected to occur.

To do so, physical phenomena which are involved in the formation of the ATEX must be understood and quantified.

Evaluation of the volume of ATEX zones

Case of an ATEX generated in free air from a pool of flammable liquid

Phenomena involved in the generation of an ATEX are as follows:

• vaporization of the liquid; • mixing of the vapour with air.

The vaporization of the liquid produces a flow of molecules evolving from the liquid surface; this flow mainly depends on:

• the liquid temperature; • the area of the pool; • the heat transferred from the ground; • the air movement above the liquid surface.

If the temperature of the liquid is not sufficient, an ATEX cannot be formed; typically, an ATEX can only occur above a liquid if its temperature is higher than its flash point.

If the air above the liquid is perfectly calm, the mixing of the vapour with air is only due to diffusion which is a very slow phenomenon. Theoretically, an equilibrium between liquid and vapour could be reached, producing a homogeneous ATEX in the whole containment. The ATEX vapour content relies on the liquid temperature. The larger the volume, the longer the time it would take to reach such equilibrium.

Actually, equilibrium conditions can hardly be met. Indeed, the air movement above the liquid (due to forced or natural ventilation) would drag vapours and dilute them in such a way that no more than an inhomogenous ATEX could be formed at the liquid surface.

Mathematical models can calculate the following parameters as a function of liquid temperature, pool area and air speed above the liquid surface:

• the liquid vaporization rate; • the ATEX volume and shape.

Moreover, by taking into account the value of the vapour lower explosibility limit (LEL), it is possible to calculate the air flow conditions for which a forced ventilation of the containment is able to:

• prevent the presence of any ATEX in the ventilation network;

• limit the volume of the ATEX above the liquid surface at such a level that this place would not be considered hazardous anymore.

Case of an ATEX generated from a leak of flammable gas

Phenomena involved in the generation of an ATEX from a leak of flammable gas into free air are as follows:

• discharge of the gas as a jet;

• mixing of gas with air.

The mixing of gas with air depends on the nature of the jet; according to its low or high speed, the jet may be respectively laminar or turbulent.

Jet speed mainly depends on the absolute pressure in the containment out of which the gas is leaking. If this pressure is higher than 2 bar, the jet is hypercritical and surely turbulent.

If the jet is laminar, the mixing of gas with air is only due to the diffusion, which is driven by buoyancy forces depending on the difference between air and gas densities. A light gas (e.g. hydrogen, methane) will disperse upwards, while a heavy gas (e.g. propane, butane) will disperse downwards. The movement of air close to the leak can also influence the dispersion of the gas.

If the jet is turbulent, the mixing of gas with air occurs within the jet itself. Mathematical models can calculate, as a function of the nature and the pressure of the gas and of the cross section of the leaking orifice, the followingparameters: • the gas flow; i • the distance from the leaking orifice where the LEL is reached on the jet axis; • the volume of the ATEX.

Generally, in process buildings, a flammable gas is fed to the equipment (e.g. burner, oven) by a low pressure pipe network (e.g. pressure slightly higher than atmospheric). Gas leaks must be considered possible. However, if the network is only made with rigid elements and soldered fittings, the leak orifice is more likely to have a very small cross section, lower than 1 mm (a leak cross section equal to that of the pipe would correspond to an unlikely catastrophic rupture of the pipe).

In such a case, if the jet does not impinge any obstacle, the LEL is reached on the jet axis at a distance less than 20 cm from the leak orifice and the volume of the ATEX is smaller than 1 dm 3 .

Moreover, it must be pointed out that this volume remains constant, as long as the leaking gas is mixed with plain air. Indeed, a mixing with air progressively enriched with combustible gas would steadily lead to an ATEX volume increase.

Finally, the volume of an ATEX generated from a slightly pressurized source discharged into free air through a small orifice is small enough so as to be considered non-hazardous.

Case of an ATEX generated from a layer of combustible dust

The only phenomenon involved in the generation of an ATEX from the dispersion of a layer of dust is the blowing of the dust by an air flow. This flow must be strong enough to lift the dust and mix it with air.

A very thin layer is able to give rise to a large volume of ATEX. For example, if the LEL and specific gravity of the dust are supposed to be 40 g m and 1 g cm" respectively, a layer having a mean thickness of 1 mm and covering an area of 1 m 2 is sufficient to form an homogeneous ATEX of 25 m 3 at LEL. It appears to be awkward to evaluate precisely the volume of the ATEX which can be generated by blowing a layer of dust.

Some concerns for the classification of ATEX zones

It is clear that the classification of ATEX zones (or zoning) is an important stage of the implementation of the Directive 1999/92/EC. Indeed, the extent of measures to be taken in accordance with the minimum requirements for improving the safety and health of workers potentially at risk from ATEX is bound to this classification.

One may think that it is safer when:

• a zone is classified at a level higher than the real one (e.g. Zone 1 or 21 instead of 2 or 22, or Zone 2 or 22 instead of non-hazardous place); or

• the volume of a zone is oversized.

On the contrary, some examples show that such a zoning is neither safe nor recommended.

Example 1 : case of a spray booth for powder coating

In the draft standard prEN 12981: 2000 2 , entitled 'Coating plants -Spray booths for application of organic powder coating material -Safety requirements', a schematic diagram (Figure 1) related to hazardous zones for an enclosed recovery system is given in normative annex A.

Figure 1 shows that the whole volume of the spray booth is suggested to be Zone 21. However, this proposal does not match the definition of Zone 21 given in the Directive.

Indeed, in normal operation, there is a forced ventilation inside the booth which drags and dilutes unused powder particles. It is therefore only in the case of a ventilation malfunction that there could be an ATEX throughout the cabin. This failure should lead one to classify the inner volume of the booth as a Zone 22 rather than 21.

Another possibility consists of fitting the booth with a secured ventilation or with an automatic safety device which switches the powder gun off whenever ventilation stops. In this case, the inner volume of the booth would then become a non-hazardous zone. The decision to classify a place as a hazardous Zone 21 or 22 or as a nonhazardous zone is paramount since the zoning determines 'the extent of measures to be taken in accordance with annex II part A' (i.e. minimum requirements for improving the safety and health protection of workers potentially at risk from ATEX). This particular matter is stated in the preliminary note of annex I of the Directive.

r-j

If the inner volume of the booth is classified as a Zone 21 or 22, it means that, if an ignition source is present, an explosion could occur within it; then, it would develop an overpressure high enough to produce damaging effects, not only within the booth itself, but in its surroundings as well. Thus, if ignition sources cannot be under control, it would be necessary to fit the booth with a protective device (e.g. explosion venting).

Another example would be the conical spaces shaped by the powder sprays. ATEX are likely to be found within these spaces. Hence, each cone volume would be entitled to a Zone 20 or 21 classification.

The classification should be decided after taking into account the first preliminary note of annex I of Directive 1999/92/EC, which states that 'a place in which an ATEX is not expected to occur in such quantities as to require special precautions is deemed to be non-hazardous within the meaning of this Directive'. Hence, the following question has to be answered: what could be the effects of an ignition of the ATEX contained within a cone?

The volume of cones should then be compared with the volume of the whole booth in order to evaluate whether the explosion could have a significant or negligible effect so as to decide if cones should be classified as a hazardous zone (20 or 21) or a non-hazardous zone.

Example 2: case of premises

When combustible dusts, gases or vapours are processed within a premises, one has to choose what the zoning of this premises has to be. This choice is based on a thorough study of normal use or malfunctions, leading for instance to leaks of combustible material into air. Such a study is likely to split the premises into different zones with regards to the risk of creating and igniting an ATEX.

A rapid assessment could lead to a single excessive zoning throughout the premises. Such an approach does not meet the Directives expectations. Indeed, it does not highlight health and safety at work issues. It only presents a broad picture that tends to overestimate risks without pointing out the main concerns for workers.

Such an adverse implementation of the Directive is likely to be seen in industries where electrical equipment standards are beyond practices in usual industries. Petroleum industries are an example. In this particular case, it could be tempting to match zoning with existing electrical equipment categories. In doing such, one would fail in assessing workers exposure to potential ATEX.

This unwanted situation could also be seen in places where no or very few electrical/mechanical equipments are in use. In such places, a rough assessment could be favoured since it has no impact or a limited impact on new and appropriate equipment investment.

Main differences between the control of major-accident hazard involving dangerous substances Directive 96/82/EC and the 1999/92/EC Directive

The major-accident hazard control Directive 96/82/EC 3 (Seveso Directive) focuses on the environment (e.g. people, properties) whereas the ATEX Directive focuses on workers. Both are concerned with effects of explosions.

A risk assessment carried out within the scope of the Seveso Directive would consider and evaluate the effects (e.g. thermal, overpressure, fragments) of an explosion on the surroundings of the industrial site. If these effects turned out to be unacceptable, mitigation measures (organisational and technical barriers) will be amended. Among those, explosion venting of a premises can be decided so as to control missile effects.

Such a conclusion would neither be suitable nor sufficient for an ATEX Directive approach. Indeed, this measure does not evolve from a health and safety at work risk assessment. The venting of the premises does not in any way prevent workers from being injured or killed.

A more appropriate approach in line with the ATEX 1999/92/EC Directive philosophy would rather lead to the reduction of ATEX volume and therefore potential damages.

There are real differences between the objectives of Directive 1999/92/EC and Directive 96/82/EC. Employers must not be mistaken. Implementing the first one does not mean that expectations of the second one are satisfied. Both approaches have to be implemented.

Conclusion

For implementation of Directive 1999/92/EC, employers have to evaluate the volume of places where ATEX can occur as well as effects triggered by the ATEX ignition.

In order to perform an evaluation as accurately as possible, it is necessary to take into account real phenomena, which are involved in the generation of ATEX. By doing so, it makes it easier to take advantage of the first preliminary note of annex I of the Directive, which states that 'a place in which an ATEX is not expected to occur in such quantities as to require special precautions is deemed to be non-hazardous within the meaning of this Directive'.

Finally it may not be safe nor appropriate to oversize ATEX zones.

Figure 1

 1 Figure 1 Powder spray booth with enclosed recovery system

Directive 96/82/EC of the European Parliament and the Council of 9 December 1996, on the control of major-accident hazards involving dangerous substances.

What seems to be safe and relevant consists of:

• first, taking prevention measures, in order to minimize or clear ATEX zones; • then, taking mitigation measures, dedicated to prevent ignition sources and to protect workers from harmful effects of explosions occurring in ATEX zones.


Auscultation et surveillance des perturbations hydromécaniques d'ouvrages souterrains par la mesure et l'analyse de la propagation d'ondes dans les roches

Monitoring and survey of the hydromechanical perturbations in underground works using measurement and analysis of ultrasonic wave propagation in rock

Résumé : Lorsqu'une galerie est excavée, quelle que soit la méthode d'excavation, la roche encaissante subit des perturbations dans une zone dite EDZ (Excavation Disturbed or Damaged Zone). Ces perturbations peuvent se traduire par différents effets allant d'une faible variation de la pression interstitielle à un endommagement important, voire la rupture de l'ouvrage et peuvent mettre en danger la stabilité d'une partie ou de l'ensemble de l'ouvrage souterrain concerné. L'étude des caractéristiques mécaniques de cette zone « EDZ » est essentielle pour estimer la stabilité de l'ouvrage à court, moyen ou long terme. La propagation des ondes ultrasoniques est utilisée comme moyen d'analyse en mécanique des roches depuis les années 1960. Initialement, elle a été mise au point pour déterminer les modules élastiques dynamiques des roches. Ensuite, elle a progressivement été adaptée à l'étude d'autres propriétés du matériau telles que : fissuration, porosité, saturation, etc. Avec les outils informatiques sans cesse plus puissants, un traitement des signaux plus élaboré est devenu possible, ce qui permet d'envisager la caractérisation de I' « EDZ » par cette méthode. Des essais en laboratoire à la fois acoustiques et mécaniques, nous ont permis de relier qualitativement les paramètres de propagation des ondes et les caractéristiques mécaniques du milieu étudié pendant les phases élastique et plastique. Par ailleurs, la modélisation mécanique de l'EDZ autour d'une galerie permet de comprendre les phénomènes mis en jeux à l'échelle de l'ouvrage. Enfin, les résultats d'une expérimentation in situ ont permis de caractériser la distribution de la zone endommagée autour d'un pilier de mine, ce qui a contribué à la vérification des modèles mécaniques existants et à l'estimation de la stabilité de l'ouvrage.

Abstract: The excavation of a drift creates a disturbed zone, which is called EDZ (Excavation Disturbed or Damaged Zone). The study of the mechanical characteristics of this zone is essential to estimate the stability of such a drift at short, mean or long term. The propagation of ultrasonic waves is used as a mean of analysis in rock mechanics since the 1960s. Initially, it was used to determine the dynamic elastic properties. Then, it was gradually adapted to the study of the other properties of the material such as cracking, porosity, saturation, etc. With the computer tools continuously more powerful, a more elaborated treatment of the signals became possible, which allows characterization of the "EDZ". The Tests carried out in laboratory, allowed us to correlate P-wave and mechanical parameters of the studied media in elastic and plastic phases. Besides, the mechanical modeling of the EDZ around a drift allows understanding the mechanical phenomena on the scale of the work. Finally, the results of an in-site experiment allowed us to locate the EDZ around an underground mine pillar. It contributed to have a better understanding of the mechanical models available and to estimate the pillar stability.

Introduction

Les méthodes d'imagerie et de détection au sein de milieux rocheux, basées sur ï propagation d'ondes élastiques, ont beaucoup progressé ces dernières années, grâce à « nouveaux concepts et grâce aussi au perfectionnement des techniques de traitement d signal (Ngugen and Tuong Truong, 2002), lequel est généralement masqué par un bruit dU fond parasite qu'il faut réduire (Pettitt, 2001), éliminer ou parfois, ce qui est plus étonnant utiliser pour la construction de l'image. Leurs applications couvrent des domaines tr variés. Les plus connues sont celles de la médecine et du contrôle industriel. Une autre application concerne l'exploration des couches profondes de la terre, par la détection et l e traitement très élaboré des signaux engendrés au cours des phénomènes sismiques. En ce qui nous concerne, nous avons utilisé cette technique pour connaître la distribution vitesses de propagation d'ondes P dans un pilier de mine et dans le plan radial d' échantillon de roche argileuse (argilite).

Cet article s'articule autour de deux parties. Dans la première, la méthode de mesure des vitesses dans un massif rocheux ainsi que les principes généraux de la tomographie sont présentés. La deuxième partie présente une application de la tomographie à un pilier de mine et à une éprouvette d'argilite. Celle-ci a permis d'une part d'évaluer les structures du pilier et de l'éprouvette et d'autre part de poursuivre le développement du logiciel de tomographie sismique.

Tomographie sismique

Généralités

La tomographie est basée sur l'analyse des caractéristiques de propagation des ondes dans le milieu se trouvant entre les sources d'où elles sont émises et les capteurs où elles sont enregistrées. Il existe plusieurs types de tomographie sismique (Côte, 1988) qui sont : -les tomographies en émission : principalement utilisées dans le domaine médical; -les tomographies en réflexion : principalement utilisées dans le domaine pétrolier; -les tomographies en transmission : les plus diversement utilisées (de la microscopie à la physique du globe). Les sources et les récepteurs sont disposés autour de l'objet. Les ondes physiques interagissent avec les matériaux avant d'être enregistrées sur les capteurs (Figure 1). L'analyse de l'ensemble des modifications apportées aux ondes permet d'étudier la distribution spatiale de certaines propriétés du matériau. C'est dans cette catégorie de méthodes tomographiques que se situe le travail présenté dans cet article.

Inversion des vitesses des ondes

L'objectif de la tomographie sismique de vitesse est de calculer le champ de vitesse qui explique le mieux les temps de trajet mesurés. Pour atteindre cet objectif, il faut tout d'abord discrétiser les rais et l'espace traversé par ces rais (Figure 2). La surface d'auscultation est donc divisée en cellules (Ivansson, 1992) auxquelles est affectée une lenteur u (inverse de la vitesse). Les rais sont découpés en éléments de petite longueur [START_REF] Ballivy | Auscultation des structures hydrauliques en béton par tomographie sonique[END_REF]. Le temps de trajet total F (pour un rai entre la source sismique et le récepteur) est donc la somme des temps nécessaires pour traverser chaque cellule [START_REF] Bishop | Seismic tomographie imaging of a buried concrete target[END_REF]. La relation entre le vecteur de temps de trajet et la matrice des lenteurs s'écrit [START_REF]Seismic P-wave velocity imaging in underground mines : observations on thé relationship between velocity structurband mining-induced seismicity[END_REF]:

= Au (1)
Avec Y : vecteur des temps de trajets de dimension M (nombre de rais = nombre d'émetteurs x I nombre de récepteurs u : vecteur des lenteurs de dimension N (nombre de cellules) et A : matrice de dimension MxJV.

A la matrice ci-dessous correspond par exemple à la discrétisation de la longueur des rais dans chaque cellule du plan de tomographie pour l'exemple de la figure 2. En généralisant, la matrice A a donc autant de lignes que de rais et autant de colonnes que de cellules. Le problème inverse [START_REF] Lagabrielle | Bilan du thème Auscultation et imagerie des Journées Physique des Arcs[END_REF] est d'estimer u en résolvant l'équation (1). Cette opération est effectuée en utilisant une des méthodes d'inversion existantes.

Méthode d'inversion

Plusieurs méthodes d'inversion ont été développées pour résoudre le problème inverse.

Elles sont soit purement mathématiques (inversions matricielles) comme la méthode des moindres carrés, soit géométriques (inversions géométriques) comme la méthode en transformée de Fourier, la méthode de reconstruction algébrique (Gordon etal, 1970), ou la méthode de reconstruction itérative simultanée (Gilbert, 1972)etc.... Dans le logiciel de tomographie « TOMI », développé par l'INERIS dans l'environnement Mathematica [START_REF] Verdel | Logiciel TOMI, version1.2.0. Documentation électronique Laego-lneris[END_REF], nous avons utilisé la méthode d'inversion des moindres carrés amortis qui consiste à trouver le vecteur u qui minimise à la fois le carré des écarts aux données et le carré des écarts à un modèle défini a priori u 0 [START_REF] Ballivy | Auscultation des structures hydrauliques en béton par tomographie sonique[END_REF], on cherche ainsi u tel que l'équation (2) soit minimum. Cette minimisation (moindres carrés amortis) conduit à la solution qui se présente par l'équation (3).

Y-A

u-u (2) ) (3)
Avec A 2 , facteur d'amortissement servant à quantifier l'importance relative accordée à l'une ou l'autre de deux minimisations qui se trouvent dans l'équation 2. La valeur de ce facteur est déterminée par le « point genou » de la courbe de la variance de la vitesse de en fonction de la variance des temps d'arrivée de l'onde [START_REF] Mcgaughey | Expand abstracts of the technical program with authors' biographies[END_REF]) (Figure 3).

Applications

Pilier G de la mine de Tressange

Le site pilote de Tressange appartenant aux anciennes mines de fer de Lorraine (France) a été choisi pour étudier la structure et le comportement des ouvrages miniers sous l'influence de l'eau. Ce site est constitué de 10 piliers (Figure 4) sur lesquels ont été construits des murs étanches de manière à permettre un ennoyage contrôlé du site (Al Heib, 2001). L'un des objectifs de cette expérimentation est de réaliser une étude tomographique sur un des piliers d'environ 18 m de large et 30 m de long (pilier G), pour estimer la distribution des vitesses de propagation des ondes P et en déduire certains caractères mécaniques. La réalisation de cette expérience a été conduite avec une distribution de 70 forages autour du pilier (Figure 5). Les sources sismiques (explosives) ont été reliées à un dispositif de déclenchement qui permet de mesurer le temps de départ (TO). Les récepteurs ont été distribués dans plusieurs forages autour du pilier et enregistrent les signaux produits par la source sismique. Ces récepteurs sont reliés à un ordinateur équipé du logiciel d'acquisition SYTMIS (SYstème de Traitement et d'acquisition des données MlcroSismiques) développé par l'INERIS. L'introduction des données dans le logiciel de tomographie TOMI permet d'inverser ces dernières pour obtenir la carte des vitesses de propagation des ondes P dans le plan étudié du pilier G. Plusieurs inversions ont été réalisées avec des maillages de tailles différentes afin d'étudier la sensibilité des résultats à ces paramètres. L'étude paramétrique a mis en évidence une divergence à partir de 225 cellules. La cause de cette divergence tient à l'existence de trop nombreuses cellules qui ne sont pas (ou qui sont peu) traversées par des rais, introduisant de nombreux zéros dans la matrice à inverser. La carte des vitesses du pilier G présentée sous la figure 6, correspond aux résultats obtenus avec une grille (maillage) de 200 cellules. L'image tomographique ainsi obtenue montre que le pilier est constitué d'un coeur sain où les vitesses sont assez élevées, et d'une couronne endommagée où les vitesses sont nettement plus faibles. Les hautes vitesses (le coeur de pilier), indiquent une zone saine avec des propriétés élastiques d'origine. Tandis que les faibles vitesses (la couronne sur le pourtour du pilier), indique l'extension de l'endommagement dans le parement du pilier où les propriétés élastiques sont dégradées. La perspective de ce travail est de traiter les données de la tomographie du pilier G après l'ennoyage de ce dernier. Ce traitement nous permettra de connaître l'influence de l'ennoyage sur certaines propriétés mécaniques du pilier G pour estimer l'influence de l'ennoyage sur les piliers des mines.

Tomographie en laboratoire

La deuxième expérience concerne un essai de laboratoire sur un matériau différent du cas précédent. Elle a porté sur une éprouvette d'argilite cylindrique de 70 mm de diamètre et de 140 mm de hauteur. Cet échantillon a été extrait à une profondeur d'environ 460 m sur le site du laboratoire souterrain de l'ANDRA à Bure qui se trouve à la frontière de la Meuse et de la Haute-Marne en France. Un plan d'auscultation tomographique horizontal a été installé à 50 mm de la tête de l'échantillon. Ce plan a été équipé de 12 capteurs (Figure 7) comprenant 4 sources et 8 récepteurs (Balland, 2003). Les mesures ultrasoniques et mécaniques ont été réalisées lors d'un essai de compression uniaxial en laboratoire [START_REF] Wassermann | Protocole expérimental des essais mécaniques -acoustiquesultrasoniques (essais « MAU »)en compression uniaxiale[END_REF]. La figure 8 montre la variation de la vitesse moyenne des ondes P enregistrées en fonction de la contrainte axiale appliquée à l'échantillon. La vitesse moyenne diminue avec l'augmentation de la contrainte appliquée. Les cartes tomographiques confirment cette diminution de la vitesse de l'onde P dans la direction horizontale (Figure 9). Ces cartes montrent que les vitesses au coeur de l'éprouvette ne sont pas influencées restent voisine de 2950 m/s. Par contre, en périphérie de l'éprouvette, la vitesse des ondes P a diminué en passant d'une valeur supérieure à 3200 (zones sombres sur l'image de gauche) jusqu'à moins de 2700 m/s (zones sombre sur l'image de droite). Le résultat de la tomographie réalisée sur l'échantillon d'argilite indique donc une diminution de la vitesse de propagation des ondes P localisée dans des zones de la périphérie de l'éprouvette en fonction de l'augmentation de la contrainte appliquée. Cette diminution de vitesse est reliée à la réduction des caractères élastiques de l'échantillon liée à l'initialisation et à la propagation et l'apparition de fissures suivant la direction axiale de l'échantillon.

Conclusions

Le logiciel de tomographie TOMI développé à l'INERIS permet aujourd'hui de réaliser un traitement complet de données de tomographie acquises sur le terrain ou en laboratoire.

Figure 10 : Localisation des émissions acoustiques

D'une part l'application du logiciel TOMI sur des ouvrages in situ de grandes dimensions comme le pilier étudié a montré que le coeur de ce pilier est sain et sa couronne endommagée. L'étude a permis d'estimer la zone sur laquelle le pilier est porteur et ainsi mieux connaître sa stabilité. D'autre part, son application à l'étude du comportement d'éprouvette d'argilite a permis de mettre en évidence l'effet de la contrainte axiale sur le champ de vitesse et de localiser les zones d'initialisation de l'endommagement. Ce résultat est corrélé à l'étude des émissions acoustiques dont a été montré qu'elles étaient localisées dans la périphérie de l'éprouvette et traduisaient d'un endommagement et de la coalescence des fractures sous l'effet de la contrainte appliquée (Figure 10).

  Jamil DAMA J: CNRS Libanais, LAEGO -INERIS -INPL, iamH.damai&mines.inol nancv.fr Cyrille BALLAND : LAEGO -INERIS, cyrille.balland@ineris.fr Thierry VERDEL, DavidAMITRANO, Françoise HOMAND : LAEGO -INPL, thierrv.verdeKSumines.inpl-nancv.fr .david.amitrano0jnines.inDl-nancy.fr. francoise.homandfàensg Jnpl~nancv.fr Laego, INERIS -Ecole des Mines, Parc Saurupt,

Figure 1 :

 1 Figure 1: Schéma de principe des tomographies sismiques en transmission (• récepteurs et t sour ces)

Figure 2 :

 2 Figure 2 : représentation d'une grille de 9 cellules et 2 rais (une source et deux récepteurs) à gauche et de la matrice A correspondante à droite

Figure 3 :

 3 Figure 3 : Variance des vitesses et variance des temps de trajet pour différentes valeurs de À2 (dans ce cas la valeur optimale de A 2 est À 2 = 4)

Figure 4 Figure 6 :

 46 Figure 4 : Zone d'expérimentation

Figure 7 :Figure 8 :Figure 9 :

 789 Figure 7 : Echantillon d'argilite équipé de 12

Remerciements

Nous tenons à remercier l'Andra qui nous a fourni les échantillons de roche et la société ARBED chez qui nous avons effectué l'expérimentation in situ.