Juan Antonio

Llanos Gascón
email: llgascon@mat.upv.es

Elías Cueto
email: ecueto@unizar.es

Fernando Sánchez
email: .sanchezf@uch.ceu.es

Francisco Chinesta
email: francisco.chinesta@paris.ensam.fr

J A García
email: jugarcia@mcm.upv.es

Meshless Methods with Application to Resin Transfer Molding Simulation

Keywords: Resin Transfer Molding, Liquid Injection Molding, Meshless, Natural Element Method, α-shape PACS: 81.05.Qk

published or not. The documents may come

L'archive ouverte

INTRODUCTION

Injection strategy is a practical issue in RTM in which the resin can be injected into the mold using multiple gates. Traditionally the selection of gate and vent locations in mold design has been based on the designer knowledge and trial and error attempts, but many recent research studies have been conducted to optimize the process by using computer simulation. The most important task in simulation of the mold filling process is the determination of the location of the moving free surface defined by the liquid resin flow front. Methods employed to solve such problems can be classified into two groups: Lagrangian and Eulerian schemes. The Eulerian methods use a fixed grid system, which is generated over the entire domain and is not changed until the completion of calculation. However, a special treatment is neccesary to track the moving free surface because the motion of fluid does not coincide with the calculation mesh. Fluid flows in porous media have been succesfully solved by an approach based on finite element method. In general, two alternative Eulerian numerical techniques using a fixed mesh have been developed for accounting moving interfaces in RTM: the Volume of Fluid techniques [START_REF] Hirt | Volume-Of-Fluid (VOF) Method for the Dynamics of Free Boundaries[END_REF] and the level set methods [START_REF] Sethian | Level Set Methods for Fluid Interfaces[END_REF]. However, the shape of the flow front is not necessarilly accurate by these numerical methods. Mesh adaptivity techniques have been impemented to overcome this issues in RTM modeling [START_REF] Bechet | Remeshing algorithms applied to mould filling simulations in resin transfer moulding[END_REF], but in these problems accurate finite element solutions require usually remeshing for each new position of the flow front, which makes the simulation quite expensive in terms of computer time.

Lagrangian schemes are characterized by the mesh system, which is moved or deformed as the calculation proceeds. In the last years, a wide family of computacional methods has emerged in a Lagrangian framework. These methods have been used by many researchers for treating a large variety of engineering problems. All of them share the characteristic of no need of explicit connectivity information, which is built by the method in a process transparent to the user. Many of these meshless methods do not interpolate nodal values and, as a consequence, do not exactly reproduce essential boundary conditions. This and other problems, make their use not widely applicable and have demanded the attention of many researchers in the last years. The Natural Element Method (NEM) [START_REF] Sukumar | The natural element method in solid mechanics[END_REF] is one of the methods based on a Galerkin implementation, built upon the so-called natural neighbor interpolation. This interpolation scheme is based on the concepts of Voronoi diagrams and Delaunay triangulations, and it has very interesting features, like its strictly interpolant character and its ability to exactly interpolate piece-wise linear boundary conditions.

In this paper, we propose the use of the NEM to model the flow front position of the injected resin in the RTM process. First, the NEM is reviewed. Then, the governing equations and boundary conditions for the RTM flow model are presented. Finally, we present some numerical results which show the interest of the NEM to simulate the filling process in the context of a simple RTM case. In this case, the position of the flow front and the geometry of the fluid saturated domain in the mold cavity are handled by invoking the geometrical concept of α-shape for a cloud of nodes [START_REF] Martínez | Updated Lagrangian free surface flow simulations with natural neighbour Galerkin methods[END_REF][START_REF] Alfaro | Meshless methods with applications to metal forming[END_REF], which allows to extract the shape of the domain being simulated by employing a cloud of nodes only.

NUMERICAL IMPLEMENTATION

In this section we describe the numerical model of the RTM process in the context of a natural neighbour Galerkin method. We review the governing equations and the boundary conditions. Then we describe the technique used to deal with the flow front and the geometry of the fluid saturated domain.

The Natural Element Method

The NEM interpolant is constructed on the basis of the Voronoi diagrams of a nodal set and its dual, the Delaunay triangulation. A first-order Voronoi diagram for a set of nodes in , with coordinates is a subdivision of the space in regions such that any point in is closer to the node than to any other in N. Two nodes whose associated Voronoi cells share an edge are called natural neighbours. The Sibson coordinates of a point x with respect to a natural neighbor is defined as the ratio of the overlap area of T i to the total area of the Voronoi cell related to point x:

with (1)
When the point x coincides with the node , i.e., x=x i , , and all other shape functions are zero, i.e., . In consequence, the natural neighbor interpolant has the important properties of positivity, interpolation and partition of unity [START_REF] Sukumar | The natural element method in solid mechanics[END_REF]. The linear consistency of the interpolant is derived after the local coordinate property

in conjunction with the partition of unity property [START_REF] Sukumar | The natural element method in solid mechanics[END_REF]. Another property of this interpolants is the ability to reproduce linear functions over the boundary of convex domains [START_REF] Sukumar | The natural element method in solid mechanics[END_REF].This is not true in the case of non-convex boundaries, then a suitable method to enforce essential boundary conditions is necessary. Among different possibilities, we use the geometrical concept of α-shape for a cloud of nodes [START_REF] Martínez | Updated Lagrangian free surface flow simulations with natural neighbour Galerkin methods[END_REF][START_REF] Alfaro | Meshless methods with applications to metal forming[END_REF], which allows to extract the shape of the fluid saturated domain in the mold cavity being simulated and the position of the flow front by employing a cloud of nodes only. For a more discussion on the concept of α-shape and its application for modelling nonconvex domains, the interested reader can refer to [START_REF] Alfaro | Meshless methods with applications to metal forming[END_REF] and the references therein.

Finally, Natural neighbor shape functions are at any point except at the nodes and across the interface, where it is , which allows an accurate treatment of models involving moving interfaces, as the calculation of flow front advance in RTM. The above properties make natural neighbour Galerkin methods an attractive choice among the wide field of meshless methods. The key difference between the formulation of the NEM method upon FEM techniques is the computation of shape functions. For the NEM method, the trial and test functions are constructed using the before natural neighbor coordinates. Then, a standard Galerkin procedure is used to obtain the discrete system which is solved for the unknown vector.

Governing Equations

The governing equations which describe the RTM process are given by Darcy´s law [START_REF] Martínez | Updated Lagrangian free surface flow simulations with natural neighbour Galerkin methods[END_REF] and the incompressibility equation, expressed as (4) where v is the velocity, k is the preform permeability tensor, µ is the fluid viscosity and p is the pressure. The fluid flow problem is defined in a volume Ω, [START_REF] Hirt | Volume-Of-Fluid (VOF) Method for the Dynamics of Free Boundaries[END_REF] where the fluid at time t occupies the volume . The boundary conditions to impose in Ω f (t) are:

• The pressure gradient in the normal direction to the mold walls is zero.

• The pressure or the flow rate is specified at the injection nozzle.

• Zero pressure is applied on the flow front.

Assuming constant permeability and viscosity, orthotropic preform and the fluid incompressibility, then the variational formulation of Darcy´s law extended to the fluid domain yields (

The numerical resolution of the governing equations will be performed by means of a NEM Galerkin technique. Then the resolution scheme is based in solving until the complete filling of the mold the next steps:

1. Obtain the pressure field using a NEM discretisation of the variational formulation given by Eq.( 6). The velocity vectors are interpolated using the Sibson shape functions, and can be written in the form

2. Compute the velocity field from Darcy´s law.

3. With the flow kinematics known at time t n , i.e., being velocities and pressures at time t n known , and p n respectively, the position of nodes can be updated at the same time that the flow front position , (8) 4. Introduce new nodes. 5. Update the fluid domain.

Numerical Results

In order to validate the proposed NEM technique for RTM process, a flat geometry mold with an obstacle has been simulated, as is shown in Figure 1. The mold is 1 meter in long and 0.5 in width. The injection pressure is 2 bars, the perform permeability is 1e-8m 2 and the resin viscosity is 0.1 pa*s. At the beginning of the filling process, a uniform node distribution in the injection nozzle has been considered. These nodes will be moved during the injection and define the filled domain of the mold. Due to these nodes will be more separated at each time step, it is necessary to introduce new nodes in the domain. In this work have been used two different strategies: add nodes in the injection nozzle and on the melt front. The mold geometry is defined by a set of nodes located at the mold boundary. Obviously, all fluid domains nodes must be enclosed by the contour defined by the boundaries nodes. In the case of considering that the nodes are introduced through the injection nozzle, the position of each one is exactly the node path inside the mold, and then we can track easily any property of the fluid. Usually these kinds of properties require an accurate treatment of the advection equation governing these evolutions. The incubation time constitutes an example of those kinds of properties. This strategy has the disadvantage that the position of all the nodes must be updated at each time step, if we consider a constant time step, we will obtain a greater nodes density near the injection nozzle. Also it is possible that the nodes remain accumulated in certain parts of the mold, depending on its geometry.

On the other hand, the strategy of introducing the nodes in the melt front presents the advantage that only it is necessary update the positions of the new nodes, then it is easier to control the nodes density. However, the physical interpretation of the results is sometimes more complex since the position of the nodes indicates the position of the melt front at different time instants. At each time step the position of the nodes located on the melt front is checked. The time step must be small enough to avoid that nodes move out of the mold. When the distance between the boundaries nodes ant the melt front nodes is small enough, these nodes are activated as bodes belonging to the fluid domain, as is represented in Figure 2. The boundary nodes belong either to the melt front or to the mold boundary. At the nodes located on the melt front the null pressure condition applies. Figure 3 shows a sequence of a mold filling.

FIGURE 1 .

 1 FIGURE 1. Mold geometry and injection conditions.

FIGURE 2 .

 2 FIGURE 2. Boundary domain adjustment during the injection

FIGURE 3 .

 3 FIGURE 3. Mold filling simulation

ACKNOWLEDGMENTS

This research work is supported by a grant from Ministerio de Ciencia y Tecnología (Spain), Proyect DPI2004-03152 and Generalitat Valenciana under GVA05/209.


Annabelle Larmure

Maud Benezit

Ronan Trépos

Christophe Lecomte

Marie-Helene Jeuffroy

Azodyn-Pea, a crop

Azodyn-Pea will be used to :

►Assess the performance of existing varieties of the three pea types over ten past years in different regions, in order to identify which type best suits to each environment (soil x climate) ► Explore novel combinations of plant traits in order to design ideotypes adapted to future climate and contribute to the development of new varieties within each type.

  , a dynamic crop model to study the current and future performance of the pea crop cultivars in various French regions with contrasted climates 1 UMR Agroécologie1347, AgroSup-INRA-Université de Bourgogne, Dijon -France 2 UMR Agronomie INRA, AgroParisTech, Université Paris Saclay, Thiverval-Grignon -France 3 UR INRA MIAT Mathématiques et Informatique Appliquées, Equipe RECORD, Toulouse -France sativum L) is an important annual legume crop grown in temperate regions for its high seed nitrogen concentration and environmental benefits Emergence of new varieties to face the high abiotic stress sensitivity of pea Context are a major cause of recent yield stagnation and projected decline. Climate change in Europe increases the unpredictability of both frequency and impact of abiotic stresses on crops while they few data are available to adapt the choice of the pea type to local abiotic stress… However Azodyn-Pea simulates, at a daily step and at the plot scale, the effect of different abiotic environmental variables on yield and grain protein content formation. It integrates relevant and measurable varietal parameters representative of the three pea types Main varietal parameters representative of the three pea types -Durations of the development stages -Maximal seed weight of the genotype -Frost resistance and acclimation rate Azodyn-Pea was implemented into the INRA modelling platform RECORD using the VLE software in order to optimize its use. It was developed in the line of the family of model "Azodyn" (including wheat, barley and oilseed rape crops).


Mohamed-Habib Mazouni
email: mohamed-habib.mazouni@inrets.fr

Jean-François Aubry
email: jean-francois.aubry@isi.u-nancy.fr

A PHA

Keywords: Ontology, Methodology, Systemic, PHA, Hazard, Risk, Dependability, Safety, Railway

This communication addresses the "Service Design, Engineering, Operations, & Innovations" topic. It is oriented towards the "Security & safety-related services and management" and deals more precisely with the Preliminary Hazard Analysis (PHA) practiced in high risk industrial fields: nuclear, chemical, military, and among others, guided transport.

The PHA was developed at the beginning of the sixties in the aeronautical and military fields. Almost five decades later, the practice of the PHA remains not less problematic and completely far from any harmonization optics. Moreover, the European lawful and international normative contexts remain very pragmatic in that sense.

In order to meet the ceaseless needs of generic methodologies allowing to harmonize the PHA, we consider that the divergences between the various actors are rather of a semantic nature then syntactic and that it wouldn't be enough to write and than distribute a glossary of concepts, but to model them in a dedicated ontology.

Indeed, our objective is to conceive a "systemic ontology based generic methodology" in order to exploit efficiently the exchange of PHA know-how, of course without concession on results' coherence and completeness .

Introduction

The PHA is usually used first and very early in the design process of a Safety Management System. It aims at identifying the various hazardous elements which could be internal or external for the studied system. Then, each element shall be studied in order to know how it could lead to an incident or to a more or less serious accident, further to an event causing a potentially hazardous situation.

The objectives of the safety evaluator can be to [START_REF] Nf En | Railway applications: the specification and demonstration of Reliability, Availability, Maintainability and safety (RAMS)[END_REF]:

Classify the functions according to their severity, then to be able to process them according to the acceptable risk level, Define the system specifications, Refine the hazard covering methods.

The various actors involved in the development and the acceptance of industrial systems (project superintendents, customers, appraisers and administrations) still have currently a problem of vocabulary divergence [START_REF] Mazouni | Proposal of a generic methodology to harmonize Preliminary Hazard Analyses for guided transport[END_REF]. Consequently, that last handicap slows down seriously the European harmonization processes of common safety methods engaged by the European Union Commission. Concerning the railway field, the transposition of the directives 2004/49/EC and 2001/16/EC related respectively to safety and to interoperability is considered as a significant step [1], [2], [START_REF]Directive 2004/49/EC on safety on the Community's railways, L164[END_REF].

State of the art of PHA practice

According to the multitude of "confidential" Preliminary Hazard Analysis files we studied, we had immediately noticed that the PHA is practiced in various ways by the different concerned actors (e.g. Siemens Transportation System, Alstom, Faiveley, SNCF, RATP, Bombardier, etc.).

In any case, we have noticed that the whole of the studied PHAs allows mainly to highlight: A list of potential accidents, A list of hazardous events, A risk calculation, A list of risk reduction measures.

Nevertheless, many important points haven't been taken into account. For example:

The allowance of the responsibilities to various interveners, The covering of the risks through an a posteriori assessment of severity and frequency.

3 A proposal of a generic system of systems for PHA

The collocation of the term system might seem inappropriate, since this term has a very generic meaning. However, the definition of system we adopted is quite specific: "A system is a set of interacting components for achieving common objectives".

The systemic approach applied to a complex system takes into account the internal interactions between the whole of its components. In other words, the total system is, in fact, not equal to the sum of its components, because quite simply, each one of them is indissociable from the whole [START_REF] Gallou | Systémique : Théorie & Application[END_REF]. doesn't guarantee the achievement of the safety and dependability targets. Consequently, a complete analysis of the interaction's trip-type between these entities must be performed (fig. 1).

Figure 1: Human/System/Environment boundaries System vs. Environment:

The expansion of the system on the environment's space (notice the new border beyond the limit of the two arrows on the Figure 1) lead to consider new humans and new subsystems being, indeed, inside the new system boundaries. For example, the infrastructure or the agents of maintenance intervening on its rail track can either be regarded as subsystems or parts of the environment.

Socio-technical system

A socio-technical system is an organized total unit of interrelationships between elements, actions or individuals [START_REF] Morrin | La Méthode, 1 : la nature de la nature ; 2 : la vie de la vie[END_REF].

Technological System

It concerns rather the equipments entities than the functional aspects allowing to manage them.

However, defining a generic system means being able to describe the following characteristics:

The general description of the system and its boundaries, The description of its various mission profiles, The functional description including a structural breakdown of the system into lower level subsystems and components. In order to be consistent with European Railway directives [1] [2] [START_REF]D 2.1.1 report: Analysis of existing approaches[END_REF], a generic railway system must be separated into two main subsystems: Structural subsystem: is the physical railway transport system. This system basically comprises infrastructure and trains:

Track infrastructure: track bed, tracks, points and crossings, civil works (bridges, tunnels, viaducts, level crossings, etc.), station infrastructure such as platforms, Energy: power supply, overhead catenary supply, pantographs, etc, Signalling and train control systems, Rolling stock, Operation and regulation.

Functional subsystem: Maintenance (procedures and mandatory corrective and preventive tasks),

Operations (normal and degraded modes),

Human -workforce

This entity includes the whole of the management and technical staff and training facilities implicated in the project life cycle between the specification and the dismantling phases.

For example, the following workforce groups could be affected by the operation and maintenance of a railway system:

Railway workers on trains, at stations, on or near the lines, Railway companies subcontractors.

The human error is defined in standard IEC 50(191) as: "a human action that produces an unintended result". The standard gives a generic definition of the concept of "Error": "(an error is) a discrepancy between a computed, observed or measured value or condition and the true, specified or theoretically correct value or condition" [START_REF]International Electrotechnical Vocabulary, Chapter 191: Dependability and quality of service[END_REF].

J. Reason has classified human errors in three categories [START_REF] Reason | Managing the Human Factor in Road Safety[END_REF]:

Error at the behavioural level: the error is imputed directly to the individual, Error at the contextual level: "to err is human", but it is necessary to investigate and deduce the related causality, Error at the conceptual level: here, the assumptions on the cognitive mechanisms are explored by distinguishing error types (Intentional, Unintentional) and forms (Violation/Fault, Mistake/Slip).

Environment

L. GOFFIN [START_REF] Goffin | Environnement et évolution des mentalités[END_REF] defines the environment as a dynamic system characterized by the physicochemical, biological and cultural interactions, perceived or not, between the man, the other alive beings and all the surroundings elements, which can be natural, transformed or created by the man.

The environment of the major industrial systems (nuclear, chemical, military, transport, etc.) is primarily defined through three types of risk: the first one could be grouped as a part of the public risks, the second one as a part of the technological risks and the last one as a part of the natural risks.

In the Railway field, the environment is perceived as a composition of a whole of entities, which are not under the control of any railway organization, but could be targets or suppliers of hazards.

Human -members of the public

It can be approached as members of the public behaving in either legitimate or illegitimate manner and not being under the organization control. In the railway system, the following groups can be distinguished:

Technological environment

The technological environment can be composed of:

Neighbouring technological systems providing services with potentially to be either hazardous or vulnerable entities towards the organization's global system, Building and factories with potentially hazardous processes, Roads, airports, harbours, Electromagnetic environment.

Natural environment

The natural environment can include the meteorological, hydrological, geological, seismic conditions, etc.

4 Proposal of a generic systemic ontology for PHA

Introduction

The ontology we propose takes into account the possibility of several Vulnerable Target Entities (VTE) to be in implication with one Hazard Supplier Entity (HSE) in the same accident scenario.

Ontology's elementary entities

The standard IEC 50 (191) defines the concept of entity as follow: "Any part, component, device, subsystem, functional unit, equipment or system that can be individually considered. It may consist of hardware, software or both, and may also in particular cases, include people" [START_REF]International Electrotechnical Vocabulary, Chapter 191: Dependability and quality of service[END_REF].

Hazard Supplier Entity (HSE)

Any entity (natural or created by man) or adopted provision comprising one or several hazards; in the field of the technological risks, a "hazard potential" corresponds to a technical unit necessary to the envisaged operation of a given process.

Vulnerable Target Entity (VTE)

Any entity such as people, goods or various environment components likely to be threatened in certain circumstances of exposure in the hazard space.

Threat of HSE on VTE

Refers to implicit consequences in a hazardous situation, but more precisely describes its realization circumstances and its imminent propagation into an accident situation.

Ontology's elementary events

An event is the dual concept associated with the concept of state of an entity; in other words it can produce or be produced by a change of that state. An event is characterized by its occurrence (date, frequency, number, etc.) and it can be deterministic or stochastic (random).

Exposure Event (EEv)

Event, running or abnormal, internal or external for the system allowing to expose a VTE in a hazard space. In other words, to put it in a hazardous situation.

Initiating Event (IEv)

Event, running or abnormal, internal or external for the system, located upstream the hazardous event in the kinetic sequence, having the capacity to excite a HSE which becomes generator of a hazardous phenomenon.

Hazardous Event (HEv)

An incidence of a hazardous phenomenon in a context of exposure situation. Conventionally defined, within the framework of a hazard analysis, as in the heart of the accidental process. Generally, it acts as a loss of containment for the fluids or as a physical loss of integrity for the solids.

4.4 Ontology's elementary situations

Initial situation

It is the phase considered to be normal where the whole of the entities belongs to their specification. The VTEs are beyond the range of the hazard space, then protected from any HSE's threat.

Exposure situation

The Exposure event announces the entry of one or several VTEs in the hazard space. A passenger being on the rail track is regarded as a VTE in exposure to several hazard fields (electrocution, crushing by a train, legal proceeding (another dimension of the hazard space)…).

A scientist (VTE) who's working in a laboratory with radiation effects (Hazard) is considered in exposure while he's intervening inside.

Hazardous situation

It is characterized by the development of a hazardous phenomenon as soon as appears a stimulating "Initiating Event". A hazardous phenomenon is a release of whole or part of a hazard potential. This concretization produces undesirable effects (dispersion of a pollutant gas cloud, skid of a car, etc).

Accident situation

This phase succeeds like a logical continuation for the hazardous and exposure situations when the hazardous phenomenon intensity reaches one or several vulnerable

Hazardous phenomenon

An incidence due to the exposure of a HSE to an Initiating Event (IEv). It is a release of a whole or a part of a hazard's potential through the hazardous situation (see above, section 4.4.3).

Risk

The concept of risk is very badly understood and differently defined:

HMSO [5]
A combination of the probability, frequency, occurrence of a definite hazard and amplitude of the related consequences.

BARBET [6]

The risk is a contingent and detrimental event for the people, goods and organizations. Contingent: It can occur or not according to the realization of one or several causes.

INERIS Working Group : Methodology [8]

The risk is regarded as the possibility of supervening of a damage resulting from an exposure to the effects of a dangerous phenomenon. It is an expectation of losses in human lives, wounded, damage to the goods and nuisance to the economic activity during a reference period and a given area, for a particular hazard.

IEC 61508, ISO 300-3-9, ISO/CEI 51

Combination of the probability of a damage and its severity.

The risk is a measurement of the occurrence of an undesirable event and its associated effects and consequences. We can admit that the risk is the measurement of the hazard expressed by a relation between several factors (Severity, Occurrence, Exposure, Avoidance possibilities, etc).

Risk calculation a. Severity assessment

According to the systemic classification (cf. section 3), we take into account the harm caused to the system's and environment's people, the damage caused to the technology constituting the system and that localized in the environment's space, and finally, we integrate the potential threats on the organization's issues.

b. Occurrence assessment

An accident frequency is the measurement of the number of awaited events occurring in a given lapse of time under given conditions. Generally, the occurrence classes are selected in a qualitative way with giving indications on the corresponding qualitative values per a given duration:

Qualitative label

Quantitative correspondence (/hour)

Unlikely Extremely incredible to occur during the life of the system ( <= 10 -9 occurrence per hour) Rare Incredible to occur but possible during the life of the system ( > 10 -9 occurrence per hour) Occasional Probable that it occurs during the life of the system Frequent Probable that it occurs frequently during the life of the system c. Risk ranking through the criticality matrix

The approach of risk level's classification per interval allows to engage the risk reduction decisions with a priority to the protective actions aiming at reducing the severity (primary safety). Graphically, the purpose of the committed actions is to bring back the risk level towards the most possible clearest colour (i.e. the up-left case):

Negligible risk: Accepted without agreement of the Official Authority. It refers to a level of risk whose occurrence is unlikely (i.e. about 10 -6 per year in the railway field [START_REF] Nf En | Railway applications: the specification and demonstration of Reliability, Availability, Maintainability and safety (RAMS)[END_REF]) and the possibility of realization does not affect the everyday life [START_REF]A guide to Risk Assessment and Risk Management for Environmental protection, Her Majesty's Stationery Office[END_REF]. Undesirable risk: Shall only be accepted when its reduction is impracticable. It requires an agreement of the Official Authority. It's a residual risk remaining after the application of the measures of prevention and whose reduction is impossible or insufficient.

Minor

Intolerable risk: Shall be eliminated/insulated at least to obtain a tolerable level.

b. Risk control policy

A risk is either accepted if it is lower than the risk already incurred or tolerable if there is a counterpart.

It is noticed that the railway casualty data, and therefore safety targets, may not be comparable between countries adopting different safety principles [START_REF]D 2.1.1 report: Analysis of existing approaches[END_REF]:

The GAME principle: A new system acceptance depends on the proof that it reaches at least the state of the art or the same degree of safety of an existing referential system.

The ALARP principle: The ALARP principle defines a first range where the risk is so small that it appears to be broadly acceptable and a second range where the risk is so high that it cannot be accepted. Between these two intervals there is a width band where it is possible to put into relationship the additional costs for risk reduction and the benefits.

The MEM principle: MEM is based on the endogenous mortality caused by natural reasons e.g. illness or natural defects.

Risk management:

It's a risk covering phase, while engaging the appropriate risk reduction actions allowing to avoid the causes, to mitigate the effects or to limit the consequences. Also, it is suitable to reduce the exposure factor.

Modelling of the proposed ontology

The identification of the accident scenarios would be based on the state/transition based accidental process: The figure below (Figure 2) shows a sketching-out of the proposed PHA's ontology: The methodology we proposed is mainly composed of six primary and complementary phases [START_REF] Mazouni | Proposal of a generic methodology to harmonize Preliminary Hazard Analyses for guided transport[END_REF]:

1.

Conclusion

The paper intends to show the difficulties noticed in the practice of the PHA, which is the starting point of the definition of any SMS (Safety Management System).

The preoccupation with genericity led us to widen our investigation with various fields. Henceforth, we are persuaded that an accident develops semantically according to the same process, only the specificity of the circumstances and the consequences characterizes it differently.

The proposed ontology is an interesting framework for the definition of a PHA's methodology which would comply with the regulation measures, with standards recommendation and above all in harmony with various industrial contexts.

Defining the environment helps define the system interfaces. Therefore, a harmonized PHA's methodology based on a systemic ontology constitutes an interesting step towards a decision-making aid system allowing to:

Organize the defence in-depth lines by establishing barriers suitable with each elementary entity, situation or event of the ontology. Inter-connect the different subsystems PHAs (generally elaborated by subcontractors) in order to constitute the PHA of the whole system by identifying if the effects of an accident relative to a subsystem doesn't represent immediate causes of a new accident scenario implying the whole system. Inter-communicate the similar system's PHAs via experience feedback similarity mechanisms on already studied, and better known existing systems. This will undoubtedly optimize the practice of the safety principles GAME and ALARP.

Currently, we are working on the development of an Interactive System of Decision-Making Aid for drafting, editing and checking the PHAs'output documents.

Figure 2 :

 2 Figure 2: UML Class Diagram Modelling of the PHA's Ontology 4.7 Proposal of a PHA's methodology based on the systemic ontology

  INFORMS International Conference on Service Operations and Logistics, and Informatics (SOLI'2007), Loews hotel, Philadelphia, Pennsylvania. USA -August, 26-29 USA

	People legitimately on railway property (e.g. on
	level crossings),
	People living or working outside the physical
	boundaries of the railway,
	Emergency services (fire services, police,
	emergency medical services, etc.).
	E		E
	H	S	H	S

Passengers on trains or at stations, Expansion of the system's space Restriction of the system's space restriction IEEE/

  IEEE/INFORMS International Conference on Service Operations and Logistics, and Informatics (SOLI'2007), Loews hotel, Philadelphia, Pennsylvania. USA -August, 26-29 USA Tolerable risk: Accepted with the implementation of an adequate control and the agreement of the Official Authority. However, it is accepted in a certain context based on the current values of the society [4] [7] [10].

		Important Critical	Catastrophic
	Unlikely Negligible Negligible Negligible Negligible
	Rare	Negligible Tolerable Undesirable Intolerable
	Occasional Tolerable Undesirable Intolerable Intolerable
	Frequent Undesirable Intolerable Intolerable Intolerable

  IEEE/INFORMS International Conference on Service Operations and Logistics, and Informatics (SOLI'2007), Loews hotel, Philadelphia, Pennsylvania. USA -August, 26-29 USA situations. Here, the analyst would be guided by the elaborated checklists (tree) of Potential Accidents, 3. First Phase: deductive identification of the HSEs and the corresponding VTEs, 4. Second phase: inductive identification of the accident scenarios from each HSE identified through the first phase. Indeed, the analyst tries to develop the

	different ways inducing potentially an hazardous
	event,
	5. Risk calculation,
	6. Risk management:
	Pro-active measures: to eliminate HSEs.
	Preventive measures: to prevent of HEvs.
	Protective measures: to mitigate impacts reaching
	VTEs.
	Aftercare measures: to go back to the normal
	(initial) situation before a HEv's apparition.

Development of the Potential Accident Tree, 2. Hazard study: identification of the hazardous elements (VTE, HSE) and the resulting hazardous