R Gaucher

O Dolladille
email: olivier.dolladille@ineris.fr

Conventions on multi-operator sites : an efficient risk management tool

Until the end of the 20th century, single user sites were the norm in Europe, but in the last 10 years, the situation has changed. Today, especially in the chemical industry, sites are operated by several companies with facilities close to one another.

Beyond business interests, multi-operator sites can offer great advantages to deal with environment, safety, security and emergency management. But, this change generates new risks, because of the interdependence between different companies, and calls for a new organization of risk management at site scale.

For managing overall risk, it is necessary to define a minimum of common rules. In particular, companies should be encouraged to conclude clear and efficient contracts between themselves. These contracts under private law are very flexible and can be easily updated to accompany the development of sites.

The aim of this paper is to highlight the main issues brought by this new kind of organization and to present INERIS ' conclusions on appropriate contracts between operators (conventions) and efficient risk management on each site.

INTRODUCTION

Over the last decade, many industrial sites in Europe, which had been owned and operated by a single company, became multi-operator sites : several companies operate on a same site with a certain degree of interactions between their activities. This trend, which first concerned huge chemical parks, can now be seen in all industries. In the context of globalization, such changes, which are driven by economic strategies, can be explained by successive sales and purchases, focus on core businesses, outsourcing of utilities or services, integration on site of contractors' activities, and so on.

In many cases, multi-operator sites were the result of the voluntary implantation of some companies close to other ones to benefit from the links they could establish between their activities. This could lead, when framework conditions were favorable, to the formation of clusters of industrial sites more or less closely integrated. In a few cases, the development of a multi-operator site can be decided, planned and coordinated. It is for instance the case of shared warehouses or industrial harbors or sometimes chemical parks.

The economic interests of multi-operator sites are obvious : shared infrastructure and facilities, easiness for an operator to set up, change or transfer its activities... But these sites raise different questions about environment, occupational health and safety (EHS) issues. Installations operated by different companies close to one another and/or interconnected by pipe networks generate new risks on interfaces.

Moreover, multi-operator sites constitute a new phenomenon for the implementation of European and French EHS regulation, in particular the Seveso II Directive. These regulations do not answer all the questions raised by these new forms of industrial organization. On the contrary, their implementation to the letter on such sites could purely and simply lead to the closure of some of them.

In 2005, the French Environment Ministry asked INERIS to look into the issue. A first set of observations was made and INERIS identified key in-depth studies to be carried out. The main ones are presented in the first part of this paper. Among them, the ministry asked INERIS to study more specifically private agreements between operators in the field of EHS risk control.

KEY ISSUES AT STAKE ON MULTI-OPERATOR SITES

To identify the questions raised by this new situation, INERIS decided on the one hand to examine how French and European law takes into account multi-operator sites, and, on the other hand, to interview a large panel of those involved in several French regions and different industrial sectors. The graph below represents the various stakeholders involved.

French and European Legislation

First of all, both legislation only talk about individual operator, not multi-operator. Also, in the French legislation, there is no definition of what a "site" is. The environmental legislation only talks about "installation", and "establishment" for operators concerned by the Seveso II Directive.

There is no clear legal definition of a multi-operator site. It seems that, in view of the great variety of such sites, it would be difficult to find an unambiguous definition. In some cases (huge chemical sites for instance), operators share infrastructure, fence and access control. Their activities are also closely linked as far as their use of materials and energy is concerned. The complex overlapping of their activities makes it difficult to identify the boundaries of each company.

Consequently, the difficulty lies in identifying individual responsibilities at interface level, from the new risks generated by the interdependence between companies to overall risk at site scale.

The second main issue is the problem of industrial neighbors. In the eye of the law, there is no difference between neighbors, whether they are industrial or not, or whether they are inside or outside a fence. This means that an operator must respect environmental regulation on the protection of its industrial neighbors ; in terms of emission limits for hazardous chemicals and noise, in terms of exposure to accidental risks of fire, explosion and toxic dispersion, etc ... These requirements are much more restricting than those of OHS regulation which the operator must abide regarding its employees. But, on sites where activities are closely linked, employees of different operators may be exposed to the same emissions or risks. Some should be protected by OHS regulation, others by environmental one (more restricting). As said in the introduction, a strict implementation of the law could lead to the closure of some sites because of the prohibitive costs to ensure that employees on a site get the same level of protection as the one required for neighbors.

Conclusions for further in-depth analysis

The interviews confirmed the observations above : there are as many definitions of what a multi-operator site is as they are interviewees, ranging from closed site by common fence and access (called "closed site" in the paper) to side by side companies without linked activities. Another interesting observation is that the interviewees were not often aware of all the issues raised by multi-operator sites.

On the basis of these interviews, INERIS formulated the following recommendations : • To provide a definition of multi-operator sites and typical multi-operator site models ; • To find the origins of multi-operator sites and trends ; • To define a methodology to guarantee a systematic identification and control of boundaries of each company on site ;

• To address administrative issues : for instance, several operators without addition rules of chemical quantities to compare with "Seveso II thresholds" ; • To identify solutions to the problem of industrial neighbors from both a regulation and a risk prevention point of view : see § 2.1 ; • To look into the consequences of a change of operator in terms of EHS risk management ; • To consider waste water and waste gas emission issues: interest of shared facilities, risk assessment of cumulative emissions, identification of responsibilities... ; • To identify specific needs for public information ; • To study voluntary and compulsory management systems : interest of coherence, links or common part between management systems of each operator, especially Seveso II establishments concerned by a compulsory safety management system ; • To provide a methodology to identify, prevent and control interference EHS risks1 ; • To investigate land use planning issues deriving from the French regulation on Seveso II establishments which introduces "technological risk prevention plans" ; • To consider emergency management at site scale : common organization, shared services and resources ; • To examine conventions/contracts between operators.

In October 2007, INERIS met the chairman of the European Process Safety Center who worked on the issue focusing on German chemical parks (which are much more developed than in France). It appeared that our respective conclusions were very similar.

Among the recommendations detailed previously, the Ministry asked INERIS to study two issues in priority : emergency management and conventions between operators.

Only the second one is detailed in the next part of this paper.

CONVENTIONS

Scope of the study

For the next stage of this study, the French Environment Ministry asked INERIS to focus on : • multi-operator sites where at least one Seveso II establishment operates ; • major accident hazard control first, without forgetting other hazards and impacts ;

The question asked by the ministry were : • Why do operators establish such conventions ? • What is the scope of such conventions ? • How could they contribute to a better management of interference and overall risks on multi-operator sites ? • How the environment inspection body could take into account these conventions when it assesses how operator's activities comply with legal requirements ?

A working group was created by INERIS with industrialists, environment inspectors and fire brigades, selected on account of their knowledge and experience of conventions. About twenty multi-operator sites and associated conventions were considered. Several meetings took place to discuss with each member of the working group the reasons behind conventions, their content, their implementation and the way they work.

Definition and scope of conventions

"Convention" means every contractual document dealing directly or not with EHS risk issues and established between two or more companies operating on the same site. Contracts between a company and its subcontractors are not considered here. In France, these relationships are well covered by OHS regulation in terms of risk prevention.

Conventions may have the following objectives :

• To clarify responsibilities of each operator who owns a permit and each owner of a given installation, especially at interfaces ; • To specifying financial and operational rules on shared infrastructures, facilities and services ; • To define common EHS rules applicable at site scale ; • To defining organizational governance to deal with rules on site and disagreements between operators.

Moreover, conventions can be seen as a guarantee for an operator of : • continuity of shared infrastructures, facilities and services (called "shared resources" in the paper). ; • risk prevention and control of other operators' activities which could have an impact on its employees and installations.

Definition of responsibilities

The first goal of a convention is to clarify the role, responsibilities and commitments of each operator and owner regarding shared (or linked) infrastructures, installations, activities and associated resources. This could concern also land owners and the ones who are responsible for past pollution as far as they are involved in site issues.

Responsibilities to be defined concern the operation, maintenance and financing of shared and linked resources (for instance : pipe racks, effluent networks, fire protection devices, exchange of materials or energy, waste water treatment plant, retention pond, utilities, etc.). In particular, conventions must be clear about the financial contribution of each operator, depending on their respective needs, and the possibility of reviewing their commitments subject to potential compensation.

To ensure the continuity and efficiency of shared resources vital to EHS prevention, the convention can oblige operators to use these ones rather than to operate their own resources.

This clarification aims at preventing operators from a weakening of their responsibilities and at allowing them to act efficiently on interfaces between their activities and those of other operators.

Common EHS rules

In order to guarantee risk control at interfaces, operators should define and implement common EHS rules that apply to everyone. For instance, on a closed site, common rules will first concern access and circulation requirements (people, goods, vehicles).

Specific rules should also be added, only for certain operators, as regards interference risks. For instance, Seveso II establishments on site could define rules concerning shared internal emergency plan and services to deal with major accidents and domino effects.

Since conventions are private agreements, it has to be noticed that all operators who sign them will have to fulfill common requirements, even those who have few EHS obligations. The result is that most of operators benefit from shared resources and additional competencies to prevent risks they wouldn't have otherwise.

A long term benefit could be the development of shared practices, vocabulary and culture of risk.

INERIS identifies several good practices that could make conventions more efficient. First, the convention which defines the EHS rules should be distinct from the one which deals with shared resources. The reason is that all operators are not necessary concerned by shared resources, depending on their needs, whereas they are all involved in the compliance with EHS rules.

Conventions should also state the obligation for an operator who wants to sale its company to inform the purchaser of the obligation to sign the convention and comply with common EHS rules. Thus, rules are linked to installations and not to operators. It is a guarantee of continuity for operators on the site.

Interest of conventions from authorities' point of view

Many benefits can be derived from the definition of conventions. As said previously, the authorities can only address individual operators. The clarification of responsibilities at interfaces, which allows to know well boundaries of each company, fits in with authorities' goals.

The French law gives few opportunities to the environment inspection body to control changes of operators and their potential consequences in term of risks. As a result, conventions also appear as an interesting tool to manage changes of operator both for industrialists and the inspection body.

When it deliver the permit required to operate installations, the environment inspection body may impose the establishment of a convention in certain conditions. In that case, it is preferable to impose only the objectives of the convention. The objectives should primarily concern, risk control on interfaces and the definition of responsibilities.

On the contrary, imposing content could restrict the flexibility of conventions which is necessary in a fast moving industrial environment and could weaken the responsibilities of operators.

Moreover, if the permit specifies that operators must inform the inspection body before any change in the convention, the inspection body will be assured to have its word on private contracts when conventions are drawn up and updated.

Finally, from an operator point of view, a convention can be a way of showing to the inspection body that some legal requirements are met with the assistance of a third party (e.g., shared fire-fighting capacities).

INERIS' observations

During this study, INERIS highlighted that most conventions are limited to commercial issues (contracts on services between operators). Risk prevention and control on interfaces are often forgotten. This situation led INERIS to make another observation. When a company gives birth to a multi-operator site by selling some business units, there are only few changes on the site : people, culture of risk, habits, relationships remain the same. The need to define contractual rules to manage EHS risks on interfaces doesn't appear as a necessity : "we come from the same family !". But, within a few years, companies follow different paths, people move and relationships are more and more business oriented. It is then much more difficult to elaborate EHS rules than to change rules defined at the creation of the multioperator site.

The last observation is that, among multi-operator sites, chemical parks are the most advanced in the implementation of conventions taking into account EHS risk management.

DEFINING EFFECTIVE CONVENTIONS

Infrastructure company

Among operators, an infrastructure company can operate and maintain shared resources : pipe networks, roads, sewers, power supplies... It can also provide services to other operators such as authorizations and general training of subcontractors before they enter the site (chemical hazards on the site, traffic rules, emergency procedures... ).

Most of the activities of an infrastructure company are concerned by interfaces with operators' activities. This implies that the infrastructure company is especially involved in interference risk control. Consequently, the more an infrastructure company has activities, the easier it is for it to act as a site manager and to organize cooperation between operators in the field of EHS management.

The experience of chemical parks shows us that an infrastructure company is in a stronger position to manage overall risk as long as :

• it shares the same industrial risk culture, because operators often originate from a single historical company ; • its status specify that it is a non-profit company (less sensitive to free market forces) ; • it imposes some compulsory services and shared resources to all operators, preventing them from setting up some less effective installations than a centralized one for instance ; • it has the power to check how operators comply with common EHS rules.

It can be assumed that when all these conditions are met, the infrastructure company can really manage the global interest of the site. The global interest of the site may go against individual interest. For instance, it may be relevant, when an accident occurs, to protect sensitive installations of a particular operator rather than those more threatened of another operator with a view to limiting consequences outside.

Governance

An efficient convention needs an efficient governance at site scale. The convention must refer to specific decision-making bodies (committees) dealing with :

• shared resources : short-/long-term financing, implementation, operation and maintenance ; • common EHS rules : definition, implementation, control and update.

These committees can eventually merge into a single one. But it should be taken into account that issues at stake are not the same for each operator.

On the one hand, for instance, only the main users of shared resources (often shareholders of the infrastructure company when it exists) have voting rights, which are proportional to their needs. In some cases, the convention dealing with shared resources can be included into the status of the infrastructure company.

On the other hand, the weight of each operator in decision-making regarding common EHS rules should not only be based on financial aspects and payroll. Seveso II establishments, as regard to the potential hazards of their activities, should see their position strengthened in decision-making, according to the consequences on risk control.

It must be kept in mind that a weakening of responsibilities must be prevented. The balance between all these constraints should be found after several amendments to of the convention, on condition that the convention and the governance bodies are defined at the creation of the multi-operator site.

Finally, the rules concerning committees which are defined in the convention should provide for the resolution of disagreements or conflicts. In particular, role of corporate level and how operators waive any claims between themselves in specific cases should be included.

It has been observed that strong infrastructure companies (when they exist) have often a driving role in committees because of their knowledge of activities and interfaces on sites.

The figure 2 shows how the governance could be define at site scale and beyond. INERIS believes conventions should concentrate on the definition and rules of the exercise of individual and collective responsibilities (committees described above). It should refer to operational documents defining common EHS rules rather than trying to include them in the convention. Otherwise, operational rules cannot be updated as fast as changes on site require. Indeed, it is obvious that conventions must define rules on the way they should be updated. It is also clear that regarding what is at stake for operators, the update process of a convention takes a long time. This is why the update frequency of conventions is 5 years on average.

Checking process

The checking process aims at assessing how operators met their commitments as defined in convention(s). Each step of the checking process, including how to deal with nonconformities, must be described in the convention.

The assessment can be made at different level : • Direct checks : this concerns one-off verifications of compliance with some common EHS rules (speed limit, loading/unloading procedures... ) ; • Internal audits : their scope is the organization at operator and site levels to implement and comply with the convention.

These two types of checking are complementary. Direct checks make it possible to verify the day-to-day observance of common rules by people on site. Internal audits are essential to highlight progress of convention' requirements necessary to improve EHS performance and the convention itself.

On a multi-operator site, direct checks can be carried out by the infrastructure company. Internal audits can be conducted by the infrastructure company, accompanied or not by some operators, an external expert or by operators from other multi-operator sites to encourage exchanges of views. In any case, the authority of the infrastructure company to conduct such checks must be stated in the convention. This is necessary to ensure that results of the checks will be followed by corrective actions.

On the basis of the conventions it studied, INERIS observed that the checking process is the most critical step regarding the methodology known as Plan-Do-Check-Act (PDCA) applied to the management of the convention. The interviews showed that, if some direct checks are made when an infrastructure company operates on a site, there is no audit decided and planned in common by operators to check the application of conventions. Moreover, audits conducted by operators are rarely carried out at interfaces. In addition to internal checking process, it can be noted that, in France, the inspection body can verify some or all the requirements of the convention when it is quoted in the permit of an operator.

Conclusion

Changes in the European economic environment have accelerated over the last 10 years the development of multi-operator sites. If many environmental and economic benefits are already clearly identified, this new situation raises questions about EHS and overall risks at site scale that are not handled by European and especially French regulations.

Following an identification of interfaces between the different companies and an indepth analysis of interference risks, the establishment of convention(s) between operators can provide the conditions of the continuity of the site. Conventions must clarify the individual responsibilities of each operator and intend to create the necessary governance to discuss and improve risk management at site scale and to guarantee the protection of industrial neighbors equal to non industrial ones.

On an economic point of view, conventions under private law are flexible enough to react to fast changes in company's strategies. On an EHS risk control point of view, they can contribute to increase the effectiveness of shared resources and rules and to develop or maintain a common culture of risk, aiming at reaching a high level of performance.

INERIS has shown through its observations on field that an important key to success is the checking step which is essential to allow a real continuous improvement of the convention. The following figure proposes the application of the PDCA methodology to convention :

Inspection body

Analysis of the Independent expert

Inspection body

operator Specific inspection of the convention Independent expert opinion On multi-operator sites, conventions can be seen as a useful tool in the service of operators but also authorities. Their development should be promoted, insisting on the quality of their content and on their real implementation and check on field.

An important issue remains : if conventions allow to handle responsibilities between companies, they do not solve the acceptance by individuals of interference risks and the consequences in term of employer's responsibility.

Figure 1 :

 1 Figure 1 : Distribution of interviewees

  To take decisions under their responsibilities concerning evolution of conventions To decide major investments related to shared means To resolve disagreements without solution at site scale Committee dealing with shared-means To establish, verify and revise rules concerning the status, organisation, implementation and financing of shared means To resolve disagreements between operators related to shared means Committee dealing common EHS rules EHS Department of each operator To guarantee the implementation and the compliance with : their own EHS rules ; the common EHS rules inside their establishment To define the general interest at site scale To establish, verify and revise EHS rules To define the missions and the organisation of common EHS staff and associated technical devices To approve shared EHS action plans (training, emergency exercises...) To resolve disagreements between operators related to EHS rules EHS Department of the infrastructure company To look after the general interest To co-ordinate the implementation and to verify the compliance with common EHS rules and related documents To propose and report to the committee

Figure 2 :

 2 Figure 2 : Committees and responsibilities to be defined in conventions

Figure 3 :

 3 Figure 3 : Plan-Do-Check-Act (PDCA) methodology applied to the management of the convention

When two or more companies operate close or related installations, new risks can appear due to simultaneous activities (co-activity). These new risks are called interference risks.


S Mauger

L Perrette
email: lionel.perrette@ineris.fr

Y Ollier

C Petitfrère

M Boudalaa

A NEW TESTING FACILITY TO CHARACTERIZE ESD HAZARDS IN INDUSTRIAL BAGHOUSE FILTERS

Baghouse filters are common dust collection systems for pneumatic transport processes involving finely divided powder materials. However, safe use of these systems requires appropriate control of electrostatic charge and discharge phenomena. Numerous parameters may contribute to accidental inflammation due to uncontrolled ESDs. This is the reason why the experimental approach keeps the best way to investigate these complex hazards.

The facility has been instrumented to measure appropriate electrostatic fields, and charge patterns within the filter cabinet according to operating conditions and to the type of filtering bags mounted.

Preliminary results obtained in experiments illustrate how severe electrostatic charging and electrostatic accumulation can be in such a dynamic process involving finely divided powder material. It shows that worse electrostatic cases require high velocity and moderate powder concentration. Preliminary results also show that electrostatic potentials rise nearly instantaneously to their maximum and drop as soon as powder material runs out.

The on-going testing program as well as future research opportunities offered by the new facility will permit to test other powders and bags in order to build a safety electrostatic concept for baghouse filters.

CONTEXT & OBJECTIVES

Dynamic pneumatic transport and dust collection processes operating with powder materials are responsible for almost 20% of dust explosions [1][2]. This prevalence to explode can be explained, on the one hand, by the dispersion of finely divided combustible solids into the air and, on the other, by the electrostatic charge generation mechanisms inherent to material flows, likely to lead to incendiary electrostatic discharges. The more the powder materials in operation are resistive, the more the electrostatic manifestations are severe.

Baghouse systems are amongst the most commonly used of the industrial dust collecting techniques. The principle consists of placing fabric bags filtering solid particles in the dustfilled air flow. Their very function means that the bags can be the seat of a phenomenon due to electrostatic charge accumulation. The accumulation of said charges can go as far as generating incendiary electrostatic discharges in the event of accidental contact with an earthed extraneous conductive part.

Provisions as regards controlling ignition sources and, more especially, as regards preventing electrostatic discharges are governed by standardised reference manuals [5] [6] [7]. They classify electrostatic discharges by type according to the nature of the materials used and the electrification mechanisms. We have, therefore: spark discharges involving unearthed metal surfaces; -brush discharges involving insulating surfaces; -propagating brush discharges involving the association of an insulating cladding covering an earthed conductor in combination with a high charge generation mechanism. The spark discharges as well as the propagating brush discharges are considered to be incendiary for all types of explosive atmospheres whilst the brush discharges are only incendiary for explosive atmospheres in which the minimum ignition energy is less than 3 mJ.

All these types of discharge can be expected in baghouse dust collection systems because of the different variations possible in installation and use configurations. Consequently, brush discharges can be expected in the event that the bags are insulating. Propagating brush discharges can also be expected if said insulating bags are supported by an earthed metal cabinet. And lastly, spark discharges can be expected if the bags are conductive and insulated against earthing.

Current methods used for assessing non-electrical equipment intended to be used in explosive atmosphere under European ATEX 94/9/EC directive [3] can not be applied to filtering bags because they are not "apparatus capable of causing an explosion through its own potential sources of ignition". However, explosion hazards should be under control with respect to the 1999/92/EC directive [4] on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres.

These technical and legal aspects prove the need to be able to conduct a specific electrostatic assessment for the filtering bags in their actual operating conditions in order to advise on safe conditions of use.

In this context, INERIS has launched the "BESP" (Baghouse ElectroStatic Phenomena facility) project with the aim of assessing the nature and intensity of electrostatic phenomena in dust collection systems according to installation and operating parameters as well as the incendiary potential of the expected electrostatic discharges.

This project is based on a "semi-industrial scale" test bench designed for this study.

TEST BENCH

The test bench aims to reproduce as realist an industrial baghouse dust collection system as possible. It has been specifically designed and instrumented to carry out the investigations required to assess the risks of electrostatic ignitions.

Operating principle

Air is sucked towards the dust collector via a pneumatic transport process, 100 mm in diameter and 4 meters long. The air flow rate is between 300 and 1,400 m 3 /h. The powder material is stored in a hopper before being introduced into the pneumatic transport process via a distribution unit composed of a micro-metering device and a cellular air-lock. The material flow rate is between 25 and 75 kg/h.

The biphasic mixture then goes into the dust collector where it initially encounters an empty chamber. Here, the air is slowed down and directed towards the base of the filtering bags. The heaviest particles settle and are recovered in a tray at the bottom of the chamber. The finest and lightest particles, however, follow the air flow which rises and comes into contact with the filtering bags. The filter cake that forms on the bags in this way is regularly removed via jet pulses injected against the air flow through a set of two lines, situated on the clean air side. The dust-free air comes out via the upper casing of the dust collector and is then released outside. The filter comprises a total of 6 filtering bags. Each has a diameter of 160 mm and is 2,000 mm high.

General view of the test bench and a view of the inside of the filter

Controlling the test bench

The air flow rate, therefore the speed of the flow, and the material flow rate, therefore the powder material dilution rate, are controlled by the researcher. The table below shows the operating range for the test bench. Temperature and humidity conditions are not checked; they are, however, systematically measured. The measurement principles were validated using a pilot test bench (cf. photograph below).

Pilot test bench for validating the measurement instruments

Test powder material

Talc was selected following a campaign measuring the characteristics of different powder materials likely to have an influence on electrostatic charges (resistivity, volume density, granulometry).

In addition to its incombustibility and its non-toxicity, talc has the advantage of highlighting the severest of electrostatic phenomena that it is possible to encounter in industry. This representativeness can be attributed to its low granulometry (median value < 50 um) and its high resistivity (between 10 10 and 10 11 ohm.m). Talc is notably representative of the following powder materials: Maltodextrin, Whey, Corn Starch

PRELIMINARY RESULTS & DISCUSSIONS

The strictest operating conditions

A series of preliminary tests was used to determine the conditions which enable maximum electrification of the powder material to be obtained. The results show that the electrostatic phenomena in the installation's operating range are severest when the air flow rate is set to its maximum (approx. 1,400 m 3 /h) and the material flow rate to its minimum (approx. 25 kg/h). The electric potential measured using the reference electrode can thus reach 70 to 80 kV for air speeds of approx. 30 to 40 m/s in the air lines. They vary more widely between 10 and 80 kV throughout the operating range of the test bench.

Bag tests

Initial tests were carried out using 6 bags with resistivity between 10 3 and 10 11 ohm. m under the severest of previously defined operating conditions. The conductive bags were deliberately insulated against earthing to enable the charges to accumulate.

Measurements taken inside the filter show that the field rapidly reaches its maximum and remains stable throughout the testing until the talc runs out. The values recorded also show that the potentials are much lower than the reference electrode potential (cf. graph below). This drop in potential as well as the observed change in polarity can most probably be explained by the presence of the deflector at the entrance to the dust collector. It results in a significant dissipation to earth of the charge accumulated by the dust. The measurement results also show a variation in the charge build-up for the bags according to their position in the dust collector (cf. figure and graph below). Thus, the bags furthest from the air intake are those for which the load current is highest whilst those situated at the entrance, directly behind the deflector, are those for which the load current is lowest. These observations can probably be explained by the differences in the aeraulics encouraging the accumulation of dust on the bags to a greater or lesser extent according to their position. Load current measurements (20 m/s; 50 g/m3)

These results also show that the effect of bag resistivity is not, in principle, significant. No interpretation is put forward at this stage of the project.

4-PRINCIPAL LESSONS AND PROSPECTS

The initial tests on conductive and insulating filtering bags have enabled the following principal lessons to be learned: -the severest electrostatic phenomena are observed for maximum air speeds and for minimum dust concentrations in the test bench operating range; -the electric fields measured are rapidly created once the installation starts-up and drop just as rapidly when the stock of talc runs out; -the charge accumulated by the bags seems to depend more on their installation and operating conditions than on their electric properties; -the further the bags are from the dust intake in the dust collector, the bigger the charge accumulated by the bags; -the potential of the dust-covered bags is significantly lower than that recorded in the pneumatic transport line because of the presence of a deflector at the entrance to the dust collector. This, therefore, proves that physical and instrumental adaptations are essential for the next experiments in order to assess the power of the electrostatic discharges under the most unfavourable operating conditions;

The load current measurements only make it possible to compare the bags with one another. This, therefore, proves that direct measurements of the potential of the bags are necessary in order to estimate by calculation the maximum power that an electrostatic discharge can dissipate for conductive bags insulated against earthing. A direct measurement of this power has to be envisaged when concerned with insulating bags.

As far as the study prospects prior to the formulation of pre-standardised recommendations are concerned, we plan to assess the influence of the nature of the powder materials (granulometry, nature) and the filtering bags (new or used) on the intensity of the electrostatic phenomena. We also have to be able, in principle, to estimate electrostatic phenomena intensity via computerised modelling.

5-THANKS

We would particularly like to thank TTL and Mortelecque for their financial and technical contribution to this project.

6-REFERENCES

  The test bench has been instrumented in the aim of measuring the following electrostatic quantities:Measurement of the electric field radiated by the air line (reference electrode) Measurement of the electric fields radiated in the dust collector Measurement of the load currents specific to each bag Measurement of the leakage currents specific to each bag alternately polls the load current and the leakage current for each of the 6 bags automatically. The currents are measured by means of two electrometers; one electrometer measures the load current via capacitors and the other electrometer measures the leakage current via resistance. A program has been developed under Labview for data acquisition and processing.

  measurement (40 m/s, 65 g/m 3 )

-

  Aerial view of the dust collector and position of the bags -05 Non conductive bag filter 3

  Dokumentation Staubexplosionen, Analyse und Einzelfalldarstellung, 1997. 3. Directive 94/9/EC of the European Parliament and the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres, Official Journal of the European Communities L1 00/1, 1994. 4. Directive 1999/92/CE of the European Parliament and the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres, Official Journal of the European Communities L 23/57, 2000. 5. CEN/TC305, European standard EN 13463-1, Non-electrical equipment for potentially explosive atmospheres -Part 5: Protection by constructional safety "c", 2003. 6. CEN/TC305, European standard EN 13463-1, Non-electrical equipment for potentially explosive atmospheres -Part 1 : Basic method and requirements, 2001 7. CENELEC, Technical Report CLC/TR 50404, Code of practice for the avoidance of hazards due to static electricity, 2003.

	1.	Going, E. John, and Lombardo, Tony, Process Safety Progress, Dust collector Explosion Prevention and
		Control, 2007.
	2.	HVBG, BIA Report 11/97,


ls phytoextraction a suitable green treatment for metal-contaminated sédiments?

Keywords: Arabidopsis halleri, cadmium (Cd), hyperaccumulation, phytoextraction, sédiment, speciation Lqual participation

Àbstract. The cleaning of waterways by rcgular dredging générâtes great volumes of sédiments and, owing to human activities, thèse sédiments often contain large amounts of metals. Thèse materials are usually spread on landiïll sites. Phytoremediation couid be a stategy for the réclamation of thèse polluted sédiments. To our knowledge, phytoextraction with hyperaccumulating plants has been few tested on contaminated sédiment. This work focuses on the mechanisms of Cd accumulation in Arabidopsis halleri, a Cd and Zn hyperaccumulator, and the effects of this species on a métal polluted sédiment.

Introduction

In the Northern part of France, the maintenance of waterways by regular dredging générâtes large volumes of metal-polluted sédiments, which are often rich in métal contaminants and présent a risk to the local environment. Thèse materials are usually deposited on landfill sites (180 sites in the Nord-Pas de Calais région). The use of métal hyperaccumulating higher plants to extract metals accumulated in polluted soils (phytoextraction) might represent an alternative low cost rernediation strategy 11, 2|. However, to our knowledge, there are few investigations into phytoextraction using hyperaccumulating plants on contaminated sédiments [3,4,[START_REF] Bert | [END_REF]6].

Plants called hyperaccumulators are defined by their ability to accumulate very high amount of metals in their aerial parts: more than 100, 1000 and 10000 mg kg ' dry weight (DW) for Cd, Ni and /n, respectively |7, 8]. Second to this définition, A. halleri is considered as a hyperaccumulator, and. as a conséquence, might be used in phytoextraction. Previous studies hâve shown that métal tolérance and accumulation traits vary as a function of the plant origin [9,10]. In récent years, Arabidopsis halleri has been intensively studied for the genetics and physiology of métal tolérance and hyperaccumulation [START_REF] Bert | [END_REF]11,12,13,14,15, I6J. Moreover, this species develops populations on contaminated and uncontaminated soil [9, 17|. Despite thèse numerous studies, the mechanisms of cadmium accumulation in plants are still poorly understood. Besides, data concerning plant impact on Cd remaining in the substrate are lacking. Métal uptake by plants is often limited by low métal bioavailability in soil [18]. This parameter is mostly controlled by the pH and the composition of the soil (organic and amorphous oxîdes content, etc..) 119, [START_REF] Flyun | [END_REF]21 ].

In thîs study, a 5-month pot experiment was conducted on a Zn-and Cd-contaminated dredged sédiment with A. halleri of two origins, metallicolous and non-metallicolous. The sédiment in its initia] state was characterized and particularly the chemical form(s) of cadmium was determined using Scanning Electron Microscopy / Energy Dispersive Spectroscopy (SEM-EDS), micro X-Ray Fluorescence (u.XRF) and, X-ray Absorption Fine Structure (XAFS) spectroscopy. Because Cd concentration is generally very low in the sédiment, the sensitivity and micrometric resolution of synchrotron techniques are appropriate tools to identify and quantify Cd chemical species in heterogeneous Systems. During the culture, plant growth parameters and métal accumulation were monitored. Métal concentrations and pH in the substrate and leachates as well as métal phytoavailability were also studied.

Materials and methods

Origin of the sédiment. The sampling area is located in the North of France, about 30 km south of Lille. This région is among the oldest and largest industrial areas in Europe. Past and présent contamination by non-ferrous métal processing has released huge amounts of ore particles and slag into the environment [22]. Five hundred kg of sédiment were collected by dredging the Scarpe River in an area known to be heavily polluted with Cd (around 140 mg kg"'). The sédiment was mixed and stored for draining under cover for seven months in order to reduce its water content. Thereafter, the sédiment was mechanically homogenized several times. Macroscopic characterics of the bulk sédiment including granulometry, physico-chemical parameters and major and trace élément concentrations were determined.

Plant material. A. halleri develop natural populations on both métal contaminated and uncontaminated soils in Europe [9]. Seeds of A. halleri were collected in a smelter-impacted site (-Auby, Fr, pH= 6.80) and in an uncontaminated site (Hautes Fagnes, Be, pH = 6.94) Pot experiment. A. halleri seeds were germinated on compost for 4 weeks. Seedlings were transferred to the sédiment pots after roots had been washed carefully with deionised water. Seedlings were grown in 4 L pots containing 2,8 kg of sieved (<5mm) sédiment (Photo 1). Non vegetated pots were used as control. Each pot contained one seedling and was triplicated. Soil and sédiment moisture was maintained at 80% field water capacity by regular addition of deionised water. Plants were grown for five months in controlled conditions (12h photo period, 20/16°C day/night température, 80% relative humidity -photo 1).

Before the start of the experiment, total métal concentrations, total carbon and pH in the sédiment were measured. Health status and plant growth were regularly monitored by counting new leaves and measuring the size of the rosette. Once a month, leachates were collected at the bottom of the pots after the substrate (cultivated and non-cultivated) had been sprinkled with 1 L of deionised water. Besides, on monthly sacrificed pots, three plants were harvested to analyze their aboveground parts; Cd and Zn phytoavailability in sédiment was performed using diffusive gradient in thin film (DGT) probes [23] and Ca(NC>3)2 extraction. Total Cd and Zn concentration were measured in sédiment, leachates and plants.

Chemical analyses. Sédiment samples were oven-dried at 50°C, ground and sieved (<100 um). Aliquots of 0,5 g of sédiment was digested in close vessels using a mixture of HC1 (6ml), HNO3 (2 ml) and H2O (lml) heated in a microwave oven. The filtered extract was adjusted to 100 ml with Milli-Q® water, and stored at 4°C before analysis. Leachates were filtered (0.45 p.m), then aliquots of 10 mL were acidified with HNO3 (suprapur®) and stored at 4°C before analysis.

At harvest, shoots and leaves were washed with deionised water. Leaves were oven-dried at 50°C for 3 days, then leaf biomass was weighed and finally ground and homogenised to prépare one aliquot per pot. Total concentrations of Cd and Zn in plant tissues were determined after digestion of 1 g dry matter in 10 ml of UNO.; and 3 ml of H2O, in a microwave oven. Solutions were filtercd, adjusted to 100 ml with Milli-Q® water and stored at 4°C before analysis.

Métal concentrations in sédiment, leachates and plant aboveground parts of A. halle ri of two origins were deterrnined by ICP-AES (inductively coupled plasma atomie émission spectroscopy). Quality control for sédiment and plant samples was based on the use of certified standard samples (sédiment : NWRI II1-2 ; spinach leaves : SRM 1570a ) and internai control samples.

Statistics. Statistic analysis was performed using one-way ANOVA followed by post hoc comparison using Tukey's test for comparison of individual means, using the statistical program SPSS 15.0(2007).

Physical methods. Cd speciation was studied in the sédiment before the phytoextraction treatment. The sédiment was freeze-dried, ground and pressed as pellets for EXAFS measurements. Cd Kedge EXAFS spectra of powder samples were recorded on the beamline FAME (ESRF, Grenoble. France) at 15K using a Fie cryostat. One part of the freeze-dried sédiment was embedded in epoxy resin. eut to 30 um-thick thin sections, micro-polished and mounted on glass slides. Thèse sections were examined by environmental SEM-EDS and by fiXRF, Cd Em-edge uXANE.S and S K-edge (iXANES spectroscopy on the beamline LUCIA (SES, Villigen. Switzerland) with a spot of 3x3 fini. Sample spectra were compared to model compound spectra, including Cd minerais. Cd-sorbed minerais and Cd-complexed organic compounds, fltted by linear combinations of thèse référence spectra and shell simulated using IFEFFIT suite of programs.

Results & Discussion

Sédiment characterisation.

The sédiment under study présents a neutral pH (7.48 ± 0.01) and is relatively rich in organic carbon ( I 1.65 t 0.05). After seven months, its water content is still of about 20 %. Cd and /n contents are around 140 and 4000 DW mg.kg , respectively (table I), Le., clearly higher than the geochemical background (around 0.5 and 75 mg.kg' respectively, data from Agence de F Eau Arlois-picardie). Moreover, fine particles represent about 70 % of total weight sédiment and Cd is mainly associated with thèse fine particles 124]. The speciation of Cd in the sédiment before culture was examîned by EXAFS spectroscopy. I.inear combination fits of the sédiment spectrum showed that greenockite was the closest référence, but did not match correctly the sédiment spectrum. Adding a second component did not improve the fit. suggesting that our database of Cd référence compounds did not contain the species présent in the sédiment. The sédiment spectrum was then treated by FEFF simulations. The first shell was simulated with 3.9 S at 2.52 Â. The second shell was not correctly simulated with Cd only (Fig. la), but by a mixture of Cd and Zn (not shown) or of Zn, Cd and Fe (7.2 Zn, 2.6 Cd and 1.4 Fe at 2.51, 3.88À, 4.18Â and 3.69À, respectively, Fig. lb). This suggests that Cd occurs as a mixed Cd,Zn,Fe-sulphide. This resuit is consistent with the identification of pyrite (FeS) and sphalerite (ZnS) in similar sédiments [25,26]. CdS was also identified in marine sédiments [27J.

Normalized absorption . CdS

Photon energy (KeV) Normalized absorption

""""" ""• "'" • • -• • • • • • • • • -.. . . . .

Photon energy (KeV)

Sediment : Cd and S rich région Sédiment : S rich région Micro-scale investigations were achieved to précise Cd speciation and identify possible minor forms of Cd. SEM-EDS enabled the localization of Cd-rich régions in sédiment thin sections and provided geochemical associations between the éléments. The grains on the backscattered électron image shown in Fig. 2a contain associations of Cd, Fe, Zn and S (Fig. 2c-f), which is consistent with the EXAFS results.

The comparison of ( i XANES spectra recorded on Cd-rich régions with référence spectra supports the hypothesis of metallic sulphide as major Cd speeies (Fig. 3). Sulphur K-edge XANHS spectroscopy is sensitive to the various forms of sulphur, whose oxidation state ranges from -II to 4 VI Sulphur is présent in reduced form in the metals-and S-rich grains, and as a mixture of reduced and oxidi/ed speeies in other régions, (Fig. 3b).

To sum up, Cd was présent as a mixed Cd,Zn,Fe-sulphide in the sédiment. The sédiment after the 5-month culture with A. halleri is currently being studied. Panfili et al. [25] showed that plants favour the oxidation of ZnS and the formation of Zn-phosphate, so a similar phenomenon might take place for Cd. Open questions include the stability of the Cd,Zn,Fe-sulphide and the nature of the distribution of a possible secondary speeies. Thèse processes may change the bioavailability of Cd, and thus the efficiency of the phytoextraction.

Plant growth parameters

Recall the experiment design, health of plants, rosette sizes and biomass were measured each month. Flach pot contained one plant. Bach month, three différent individuals were measured that means that one plant is not monitored during the experiment. At the beginning of the experiment. when plants were transferred on sédiment, plants of both origins presented similar biomass, i.e. 3.0 ± 1.1 g and 3.2 ± 1.7 g (average for n-3 ± standard déviation (SD), Fig. 4-A). For metallicolous origin, aboveground part biomass was increased after one month, up to 1 5.3 ± 3.1 g (Fig. 4-A). Ihe increase of the biomass production for plants of nonmetallicolous origin (7.4 ± 4.3 g at one month, Fig. 4-A) was not signifieant. Standard déviations were important; for instance after one month of growth, one plant produced 3.4 g of biomass vvhereas another plant produced 1 1.9 g of biomass. Regardless of the origin of the plants, plants looked healthy with no visual symptoms of toxicity..

From II to 14, plants from metallicolous origin produced more biomass than plants from non-metallicolous origin (Fig. 4-A). Ihe maximum différence in biomass is observed the third month of the experiment where plants of metallicolous origin reached 33.1 ± 4.6 gwhereas plants of non-metallicolous origin reached the average value of 9.1± 8,0 g. Ilere again standard déviations for non-metallicolous origin were important..No signifieant increase in biomass was observed for plants from non-metallicolous origin between the first and fourth month (Fig. 4-A; p>0.05). This différence between the two origins of A. halleri was highlighted by the corrélation diagram between plant biomass and rosette diameter (Fig. 4-B). Indeed, the two origins appeared clearly in two distinct groups, with higher biomass and rosette diameter for metallicolous plants (( 1 3 to 38.5 g and 29.5 lo 44.5 cm) than for non-metallicolous plants (3.4 to I 8.4 g and 1 8.5 to 34.5 cm).

From biomass results, A. halleri from metallicolous origin appeared more suitable for phytoextraction than A. halleri from non-metallicolous origin. Because of the high interindividual variability (this work was not done with clones), it was difficult to conclude about the tolérance of the two studied origins of A. halleri.

Evolution of Cd and Zn concentrations in aboveground parts of A halleri

The average of Cd concentrations were measured each month on différent plant (Fig. 5-A). It meant twelve values of Cd concentration for each population of A. halleri. For each origin tested separately, no significant différence was observed (p>0.05) (Fig. 5-A). Maximum average Cd concentrations were 145 ± 90 mg.kg" 1 (n=3 ± SD) and 185 ± 18 mg.kg" 1 for, respectively, metallicolous origin at T4 and non-metallicolous origin at Tl (Fig. 5-A). At T4, the Cd concentration non metallicolous plants was 129 ± 40 mg.kg" (n=3 ± SD). There was not significant différence between origins (p>0,05). Standard déviations were important, suggesting a high interindividual variability in Cd accumulation. ] measured in aboveground parts of A. halleri from metallicolous (white) and non-metallicolous (dark grey) origins during the first four months of monitoring (averages and standard déviations over 3 samples). Fig. 5 B: A letter means a significant différence at 5% (oneway ANOVA followed by a Tukey test). The oneway ANOVA is not significant for metallicolous plants.

To sum up, Cd concentrations did not increase significantly with time regardless the origin of A. halleri. Plants seemed to hâve reached a maximum value after one month only. This species, growing in hydroponic solutions reached Cd concentration in their leaves about 6000 mg.kg"' after 14 weeks [16]. In order to explain this "plateau response" two hypothèses can be proposed. Cd accumulation could be limited by Cd phytoavailability [18,28,[START_REF] Chaney | Science and engineering of composting : Design environmental, micobiological and utilization aspect[END_REF][START_REF] Christensen | Processing and Use of sewage Sludge[END_REF] or the plant itself could limit the entry of the métal to prevent toxicity [9,[START_REF] Mcbride | [END_REF]32].

For Zn accumulation, A. halleri from metallicolous origin showed the same behaviour than for Cd accumulation. For metallicolous plants, no significant différence was observed at the various times (p>0.05) (Fig. 5-B). Again, standard déviations were very high, suggesting a high individual variability of plants, particularly at T4. Indeed, the three replicate plants had accumulated 24577, 4578 and 7091 mg.kg" (Fig. 5-B). On the contrary, Zn concentration in aboveground parts of A. halleri from non-metallicolous origin increased significantly between the first and the four month of culture. At T2 and T3, plants from non-metallicolous origin accumulated more Zn than plants from metallicolous origin (Fig. 5-B), but interindividual variability was important at T3 (22835, 12281 and 11578 mg.kg"). To conclude on Zn accumulation, A. halleri from non-metallicolous origin show a différent behaviour than for Cd accumulation. This resuit is consistent with that of Bert et al. [17].

Phytoextration yield of Arabidopsis halleri.

From results of biomass, Cd and Zn accumulation in aerial part, in leachates and in sédiment, the amount Cd présent in the sédiment and extracted by A. halleri was calculated. Put under optimal conditions in the growth chamber, for both origins, after 1 -month growth, plants extracted around 1.5 mg pot". On the other hand, after 4-month growth, plants Irom mctallicolous origin extracted about 3.7 ± 0.3 mg pot " whereas plant frorn non-metallicolous extracted 1.8 ± 0.1 mg. For plants from mctallicolous origin, the extraction yield seemed to increase with time (table 2). Indeed. after one month of growth, the plant extracted 0.41 ± 0.1 % of the Cd présent in the sédiment pot, and 0.98 ± 0.7 % after four months (table 2). The maximum Cd extraction was 1,8 % and was obtaîned after four months. By contrast, Cd extraction by A. halleri from non-metallicolous origin werc significantly lowcr (0.7 % maximum, obtained after three and four months). for both origins. A. halleri extracted more Zn than Cd in their aboveground parts. The maximum Zn extraction yield was 6.3 % after three month for the metallicolous origin, and 2.9, % after four months of growth for the other origin.

l.ess than 0.05 % of Cd présent in the sédiment pot at the beginning of the experiment was lost in [cachâtes. In spite of the Cd extraction by A. halleri and Cd lost in leachates. the Cd majority still in the sédiment (table 2). The same trends were observed for Zn (table 2).

Fable 2 : Material balance of Cd, in percent, between plant, leachates and sédiment comparîments, after one and four months of culture bv A. halleri from a metallicolous and a non-metallicolous origin.

Conclusion et perspectives

Second Cd and Zn concentrations in leaves, A. halleri regardless the plant origin, was really a Cd and Zn hyperaccumulator [7]. For phytoremediation, the idéal plant should possess multiple characteristics including fast growing, high biomass, deep roots, easeto harvest and high métal tolérance nd accumulation in their aerial part and harvestable parts [33J. Klang-Westin and Eriksson |34| postulated that biomass production is the crucial parameter for phytoextraction. In our study, the biomass production and métal phytoextraction was higher for the metallicolous origin. But the success of phytoextraction also dépends on the substrate [35]. A very précise characterization of the substrate and Cd chcmical form and phytoavailability is needed. Thèse aspects are being currently studied.

Figure 1 :

 1 Figure 1: (a) Cd K-edge EXAFS spectrum for the bulk sédiment and its and Fourier transform (blue line) and FEFF simulation with a S first shell and a Cd second shell (red line). (b) Same spectrum simulated with a S first shell and a Cd. Zn. Fe second shell.

Figure 2 :

 2 Figure 2: (a) backscattered électron SEM image and (b) EDS spectrum of a Cd-rich spot of the sédiment thin section, (c-f) Elemental maps for Cd, Fe, S and Zn of this région.

Figure 3 :

 3 Figure 3: (a) Cd L nl -edge uXANF.S spectra of a Cdrich région of sédiment and référence compounds and (b) S K-edge U.XANES spectra recorded on two sédiment samples (one Cd and S rich région and one S rich région)

  (A) Average values of biomass (n 3 ~ SD) measured in aboveground part of -/. halleri from a metallicolous (white) and non-metallicolous origins (dark grey) before plant transfer on sédiment (T-0) and during the first four months of monitoring (T~~l, T~2, To and T-4). l.etter means a signifieant différence at 5% (oneway ANOVA followed by a Tukey test). The oneway ANOVA is not signifieant for non metallicolous plants. (B) Relationship between plant biomass and rosette diameter for metallicolous (white) and non-metallicolous plants (dark grey) at each time of monitoring (rectangles represent T~1, triangles T 2, circles f'-3 and diamond T-4).

Figure 5 :

 5 Figure5: Average of Cd (A) and Zn (B) concentrations [mg.kg-1] measured in aboveground parts of A. halleri from metallicolous (white) and non-metallicolous (dark grey) origins during the first four months of monitoring (averages and standard déviations over 3 samples). Fig.5 B:A letter means a significant différence at 5% (oneway ANOVA followed by a Tukey test). The oneway ANOVA is not significant for metallicolous plants.

Table 1 .

 1 Sediment metal

		concentrations	| mg.kg ]	]. average on	ten samples
			As	Cd	Cu	Pb	Zn
	métal concentrations	[mg.kg 1 ]	57	141	233	600	4074
	standard déviations		4	24	9	2	280


Jean Christophe Le Coze

Kenneth Pettersen
email: kenneth.a.pettersen@itis.no

Is resilience engineering realist or constructivist?

This exploratory paper questions whether resilience engineering is a realist or constructivist approach of safety. After a présentation of two positions: critical realism and radical constructivism, the authors explain how the frameworks provided by thèse two positions were used in différent researches, answering différent questions that arose during the work. For one, critical realism offered an answer to the problem of the relationship between agency and structure. For the other, radical constructivism afforded a good définition of what models can be thought as. Both developments show the relevance of taking into account of philosophical antécédents for applied research and how this is valuable for resilience engineering and the directions that could be pursued for future development.

INTRODUCTION

In the process of writing papers for a book gathering young researchers from the field of accident and safety research [START_REF] Svedung | Anthology on Learning from Accidents, Swedish Rescue Services Agency[END_REF], we found ourselves asking similar type of questions regarding human and social science. With expérience from independent research projects conducted in, among other, civil aviation and chemical industry, we hâve arrived at some similar questions regarding the status of models applied in safety science. In this paper, we suggest to introduce some of thèse questions and to see how they can provide an interesting point of view on resilience engineering. The focus will be on the distinction between realist positions and constructivist epistemologies. Before discussing the relevance of thèse questions for resilience engineering, we will, in a first part, introduce the context in which they emerged.

Resilience Engineering (Hollnagel 2006;2008) has relatively rapidly emerged as an approach to be reckoned with in relation to how we understand and manage safety and vulnerability in socio-technical Systems. What makes the approach innovative is, as described by Hollnagel in the introduction to the latter of the two resilience books, not advances in applied methodology within the field but more the basic System model it explicates related to the production of failures and successes in socio-technical Systems. But can we however separate models from méthodologies as it is asserted? Does a model not shape methodology? Does a methodology not shape models? This is the type of concrète question that the philosophical discussions (asking whether resilience engineering is realist or constructivist) hâve led us to.

BACKGROUND

For one author, his expérience is mainly in the chemical and petrochemical industry, and his empirical cases range from studies for research or consulting either following accidents or for diagnosis of safety. His empirical works and subséquent theoretical developments led him to deeper philosophical issues when questioning the use of models in gênerai and their relationship with reality (Le [START_REF] Le Coze | Are organisations too complex to be introduced in technical risk assessment and current safety auditing[END_REF], 2008a[START_REF] Coze | Complexity and learning from expérience[END_REF], Le Coze and Dupré, 2006). One of thèse questions, in relation to assessing the level of reliability or resilience of organisations, was "how does the past teach us lessons for the future""] Can models constructed from past expérience be useful for predicting the future? Also, other philosophical issues were raised when questioning the necessary interdisciplinary nature of safety research and the difficulty of embracing the whole given the variety of disciplines that need to be articulated. How do we get a global picture? Can we do so? Can we do it by reducing phenomena to simple model, formula or principle (issue of reductionism in science)? Ultimately, ail thèse questions led to a core one regarding the value of the models for safety. In trying to find éléments of answers to thèse différent problems, theoretical development on philosophy of complexity [START_REF] Morin | La méthode -tome I, La nature de la nature[END_REF][START_REF] Morin | Restricted complexity, gênerai complexity. Worldviews, science and us. Philosophy and complexity[END_REF] and constructivist theory of knowledge (Le Moigne, 1995, 2003[START_REF] Glasersfeld | Radical constructivism. A way of knowing and learaing[END_REF], 2001) provided viable frameworks in which thèse issues fitted ail together.

For the other author, empirical research on safety and aircraft maintenance has led to several questions regarding theory of social Systems and the relevance of currently applied models for managing system performance (Pettersen 2008;[START_REF] Pettersen | Interactions between social reality and the physical -A causal nexus for operational safety and vulnerability[END_REF]. Among thèse questions is how to understand human action as part of sociotechnical system performance (McDonald & Morrison 2006a[START_REF] Mcdonald | Modelling the human rôle in operational Systems -theory and practice[END_REF]. Approaching a possible answer has involved digging into philosophical positions concerning the rôle of, and relationship between, human agency and social structure. What are the alternative theoretical models of this relationship? How can theory be applied to model how actions of aviation personnel are enabled and constrained by the social system they are part of, also taking account of the rôle of human intentionality in producing human actions? Within the available philosophical discussions on to thèse topic, critical realism developed by [START_REF] Bhaskar | The Possibility of Naturalism -A Philosophical critique of the contemporary human sciences[END_REF] and further advanced by among other [START_REF] Archer | Realist social theory : the morphogenetic approach[END_REF], is proving to be a relevant framework for developing his research into a more consistent articulation between philosophical assumptions, methodology and applied theory.

The aim of the first part of the paper is to elaborate on one of the questions associated with thèse philosophical positions as it has not yet been treated much in the field of safety science. This situation is probably linked with the philosophical nature of the investigations required by the questions that we hâve. The aim of an applied field of research such as safety science is not to produce philosophical thoughts but to provide useful models and methods so that safety is concretely, in the reality of industrial practices, enhanced. We however believe that any applied scientific field also needs to ask itself where it stands in terms of commitments rather than only concrète results. Avoiding methodological and ontological questions is in fact a statement in itself and does not "save" an applied field of research from its ontological and methodological commitments. Wilhout giving définitive conclusions to the questions raised, we introduce the two related but disiinctly différent philosophical developments that hâve proved to be useful l'or addressing our questions concerning safety science perspectives: critical realism and constructivist epistemologies. One of their main différences will be stated and explored as support for a following discussion on resilience engineering, asking whether resilience engineering is realist or constructivist. But first of ail, hère are two présentations of critical realism and radical constructivism.

The relevance of the philosophy of science perspective for resilience engineering

As indicated in the previous section, both our empirical and practical researches led us back to theoretical questions. The type of questions philosophers, historians and sociologists of science deal with (i.e. [START_REF] Soler | Introduction à l'épistémologie[END_REF], Andler et al, 2002, Barberousse et al, 2000). Questions about induction & déduction, about causality and teleology, about laws and determinism, about empiricism and rationalism, about truth and growth of knowledge, about realism, etc are some examples. Ail of thèse are very much linked together but can also be studied separately. Moreover, they are today discussed in many scientific areas, from natural to social sciences. We are hère interested in how thèse types of questions underpin and are introduced in the resilience engineering approach.

Simultaneously, given the size of the paper many shortcuts are taken that might leave some of the outlines a bit blurry for those who are not familiar with the philosophical positions we introduce. For us, thèse are complex and evolving philosophical topics, and we will never give crédit to their depth and subtleties in few pages. Nonetheless, we however hope to make it clear to the reader, firstly, that taking a philosophy of science perspective is of fundamental relevance for the resilience engineering project and, secondly, that discussing resilience engineering as realist or constructivist provide some interesting références for understanding and developing the resilience engineering project. We are ourselves not philosophers but believe in the importance of addressing philosophical questions in the context of safety science and resilience engineering as any scientific approach is influenced by its philosophical preconceptions.

Critical realism

On realism. In A realist theory of science (1975) and The Possibility of Naturalism (1979) (PON) Roy Bhaskar distinguishes between intransitive (ontological) and transitive (epistemological) objects and argues that it is essential that thèse dimensions are clearly distinguished. The intransitive is that what is and so exists independent of identification. It is based on the ontological claim that "If men ceased to exist sound would continue to travel and heavy bodies fall to the earth in exactly the same way, though ex hypothesi there would be no-one to know it. Let us call this, in an unavoidable technical neologism, the intransitive objects of knowledge (...) they are the real things and structures, mechanisms and processes, events and possibilities of the world; and for the most part they are quite independent of us" (Bhaskar, 1975). There are thus real entities and relationships that make up the natural and social world. In contrast, the transitive dimension is a social product, such as our knowledge and perception (concepts, models etc.) of reality (e.g. social science). As Bhaskar said in an interview [START_REF] Noms | Roy Bhaskar interviewed[END_REF] " A realist theory of science re-thematised ontology, argued for its necessity and irreductibility in any account of science, and gave it a radically différent shape or context. In particular, it is argued against the epistemic fallacy, that is the idea that one can reduce or analyse knowledge in terms of being. It was argued that being was an absolutely irréductible and necessary category." As its name suggests, critical realism involves a critical dimension that acknowledges the fallibility of our knowledge. Thus, there is no unquestionable foundation for science; knowledge is a social and historical product and, as a conséquence, ail facts are theory-laden (Robson 2002). Bhaskar (archer et al. 1998 p.xii) argues that Western philosophical traditions hâve mistakenly reduced the question of what is to the question of what we can know and conceptualises this blunder as the "epistemic fallacy". Critical realism présupposes that théories, both natural and social, are alternative accounts of the same world, and that there is a rational criterion for theory choice based on the possibility that one theory can explain better or more significant mechanisms than another. On social sciences. Critical realism is a movement that in relation to the social sciences delivers a realist social ontology and addresses the epistemological implications of this ontology for social science. According to critical realism, social Systems hâve unique characteristics that must be dealt with conceptually in an analytical dualism between agency and structure because; 1) social reality dépends upon human activity and 2) social reality is transformable, but 3) as human agents, we are not immutable because we are constrained and enabled by the society in which we live and work [START_REF] Archer | Realist social theory : the morphogenetic approach[END_REF]. The ontological premise for analytical dualism is the notion of society's émergent properties, which, in terms of structure and agency, is their interplay over time and space: [The ontological premise of émergence] being so, then social realism implies a methodology based upon analytical dualism where explanation of why things social are so and not otherwise dépends upon an account of how the properties and powers of the 'people ' causally intertwine with those of the 'parts' (Archer 1995 p. 15).

Radical constructivism

Constructivist is a rather fashionable term used in many différent domains (see [START_REF] Hacking | The Social Construction of What[END_REF]. Radical constructivism is the term given by [START_REF] Glasersfeld | Introduction to radical constructivism[END_REF] to Piaget's constructivist approach of knowledge (1967,1970). Glasersfeld interprets Piaget's contribution to epistemology by radicalising the idea that there is no possibility of describing a world as it is by separating between epistemology and ontology. Ontology is not in the scope of this theory of knowledge. Ontology belongs to the realm of metaphysics, and is outside the constructivist epistemology, which describes a rational way of knowing. As Glasersefeld (1982) puts it "The difficulty in explicating the radical constructivist epistemology springs above ail from the fact that, from the very beginning of our Western philosophical traditions epistemology has been tied to ontology. "Truth" and "objectivity" hâve an unalterable meaning, once the impossible condition has been set that cognition should lead to verifiable knowledge of a pre-established ontic reality." Therefore "radical constructivism is intended as a model of rational knowing, not as a metaphysics that attempts to describe a real world" (Glasersefeld, 1995).

Based on an analogy with biology and evolutionary theory, "truth" in constructivism (following hère also ideas promoted by an instrumentalist approach of science among which pragmatism) is replaced by "viability" or "fit" instead of a "match" with the environment. Only the observer can judge of the experiential relevance of his models given his/her purposes, and therefore not an objective criterion but rather subjective ones (such as for example simplicity, generality, accuracy etc). This point view is difficult to admit for realists convinced of a world existing independently of them. To this, Glasersfeld (1995, p 52) clearly replies "Recently it has been suggested that radical constructivism is contradictory because it attacks realism and at the same time assumes a realist position by admitting that an ontological reality must constrain human action. (...) In the usual language of philosophers, "realists" are those who believe that they can obtain knowledge of a world as it is in itself. This I deny, and admitting "ontic" constraints does not contradict it, because while they may détermine what is impossible, they do not détermine the ways of acting and thinking that can be constructed within them".

The constructivist approach of knowledge considers that models and théories are viable (combining usefulness, purpose and cohérence) but not "true" in the common sensé. Famous event in history in favour of this instrumentalist position is Osiander préface of Copernicus treaty on heliocentrism. In order not to upset the religious dogma held by institutions about the earth being at the centre of the universe (geocentrism), Osiander indicated that the mathematical model of Copernicus did not need to be true. It was a way of "saving the phenomenà" rather than providing an accurate représentation of the world as it is. Within this perspective, it is impossible to step out our human way of experiencing and perceiving the world in order to compare it with an "objective" or a "true" picture of the world as it is. It is, in a sensé, "agnostic" toward this spécifie metaphysical problem of ontology [START_REF] Riegler | Towards a Radical Constructivist understanding of science[END_REF]. This statement is met with much résistance and misunderstanding from many. "The constructivist conclusion is unpopular. The most fréquent objection takes the form of the accusation that constructivism dénies reality. But this it does not. It only dénies that we can rationally know a reality beyond our expérience. (...) From my point of view, the trouble is that most critics seem to be unwilling to accept the explicit, programmatic statement that constructivism is a theory of knowing, not of being. That a model of the construction of knowledge could be designed without making ontological daims about what is known, is apparently difficult to accept"'. (Glasersfeld, 2001, p lO).

Main différence between critical realism and radical constructivism

With thèse short présentations of the two positions as a background, we direct attention to similarities and différences. Both critical realism and radical constructivism indicate that it is important to distinguish epistemology from ontology. However, their positions concerning this matter differ. Critical realism is a realist theory of science asserting that there is a world existing independently of us necessitating ontological developments to support this. Radical constructivism is a model of rational way of knowing. But it does not commit to ontological position because thèse concerns metaphysics rather than epistemology such as described in its model of knowing based on expérience. An important différence is also found in the lack of clear extension of radical constructivism in terms of social sciences, whereas critical realism has extensively developed this side of its model. Récent contributions regarding the application of radical constructivism to sociology are however now available in [START_REF] Glasersfeld | Who conceives of society? In Constructivist foundations[END_REF].

APPLICATION TO EMPIRICAL RESEARCHES

Critical realism in aviation safety

In my research on aircraft maintenance and aviation safety I hâve found that the activity of aircraft maintenance comprises an unofficial social system, documenting unofficial actions and informai social structures within the workplace as characteristic tendencies of work (Pettersen 2006;Pettersen & Aase 2008;Pettersen 2008). In this research I suggest that there is another side to what goes on in the process of ensuring aircraft safety than what is described in formai descriptions of work, which needs to be accounted for in applied théories of safety and accidents in technological Systems (Pettersen et al. 2008). Following this, I argue that studying normal functioning is essential because it encompasses the same conditions that precondition ail forms of outcomes and events. However, as argued in Pettersen (2008), data gathered from a spécifie background and made relatable to other areas can only develop from being context-validated knowledge if it is related to a more gênerai model of socio-technical functioning. This emphasises why the possibilities of system meta-theory are so important and fondamental to socio-technical analysis in the field of safety. i.e. it argues for the fundamental need to address ontology and being clear on fundamental models, independently of possibility or goodness of a spécifie set of ontological assumptions.

In order to contribute to the above, critical realist philosophy and social theory developed within the critical realist approach [START_REF] Bhaskar | The Possibility of Naturalism -A Philosophical critique of the contemporary human sciences[END_REF][START_REF] Archer | Realist social theory : the morphogenetic approach[END_REF] has been applied for analysing the relationship between individual actions and social structures in aircraft maintenance emphasising how thèse properties contribute to the production of safety (Pettersen et al. 2008;Pettersen 2008). This research has strengthened an argument for a social conception of technological Systems and provides a basis for categorising différent social science approaches to such Systems. Through this research critical realism is argued to hold a largely unused potential for meeting many of the current objectives and requirements of socio-technical safety research and management.

Radical constructivism in applied safety researches for the Chemical industry

Radical constructivism as I understood it, has proved to be very helpful the researches I hâve been involved in. I hâve tried to explain such a position in the field of accidents investigations (Le Coze, 2008a) but it is also appropriate for research in normal functioning (Le Coze & Dupré, 2006, 2008) . Because within a radical constructivist perspective a model is assessed according to its viability based on expérience, appreciating its relevance is impossible without making clear what its purpose is. How much "true" is the model is therefore a matter of how viable it is. This is important in an applied research on organisational safety. As outsider of the system that I studied, I will not in the end implement the actions required in order to transform it. Insiders will potentially do. A radical constructivist approach of this problem would therefore insists on the importance of taking into account the background and expériences of insiders and the adequacy of the model according to thèse backgrounds and expériences. If the purpose is to improve safety thanks to organisational measures implemented by individuals, then the model produced needs to be viable within the context of its use. It is what Starbuck (2006, 165) says "Social scientists who believe they hâve something valuable to say to contribute hâve to be willing to persuade others of this value; and to do that, they hâve to adapt their manuscripts to the perceptual framework of potential readers". Thereforc, with a radical constructivist point of view, describing from an outsider point of view the way safety is organisationally produced will not be sufficient (i.e. HRO descriptions), although viable within a particular community of people (i.e. researchers in social sciences). This is a point that LaPorte (2006,151) made very clear "Researchers in this field assert that identification of the characteristics of highly reliable organisations is not the same thing as knowing as to make them so". It means that making organisations safer does not consist in simply applying results from more descriptive models. This position rather implies spécifie research designs, designs allowing actionable models to be produced through interactive work between researchers with practitioners and tested in real situations (i.e. action research type of approach).

THE RELEVANCE OF THESE APPROACHES FOR RESILIENCE ENGINEERING

Resilience engineering and critical realism

It is clear, both from the basic concepts and precepts of resilience engineering (Hollnagel et al. 2006) and the work that has evolved following thèse foundations [START_REF] Hollnagel | Resilience Engineering Perspectives -Remaining Sensitive to the Possibility of Failure[END_REF], that resilience engineering differs from traditional approaches to safety that focus on failures as isolated cases. It explains instead safety and accidents as dynamic outeomes of normal functioning of socio-technical Systems. By so doing, resilience engineering shifts focus from actual events to underlying facts and mechanisms of Systems that are driving the functioning of socio-technical Systems. However, refocusing the attention on System functioning is not only about making the statement, but developing a consistent approach of how this is in fact possible. This is where critical realism does hold some substantial potential as a philosophical référence.

Currently, critical realism is not applied widely in the field of safety and sociotechnical theorising. However, two well known accident théories in the field, [START_REF] Turner | Man-Made Disasters[END_REF] theory of Man-Made Disasters and Diane Vaughan's Normalisation of Déviance (1996), can be argued as commensurable with critical realist theorising (Pettersen 2008). However, I would also ad, the socio-technical models argued for in Hollnagel et al. (2006;2008) may share this commensurability. At a fundamental level the critical realist epistemology comprising an analytical dualism between structure and agency seems to be central to the socio-technical theorising developed in resilience engineering as it delivers an explanation of the basic relationships between individuals and their social environment that addresses the relationship between stability and change in Systems [START_REF] Mcdonald | Organizational resilience and industrial risk[END_REF]. There is thus a fundamental potential in critical realism in relation to what it contributes to the possibility and potential for modelling technological Systems that hâve relevance for the Resilience Engineering project.

In summary, fundamental gains can be made by looking deeper into how critical realism may inform the resilience engineering approach to system modelling, how the critical realist conception of social causality may inform resilience and the applied theory of émergence (Hollnagel et al. 2006) as well as how resilience engineering defines it self in terms of a methodological project.

Resilience engineering and radical constructivism

Given what has been said about radical constructivism in the previous sections, it will be found that the following sentence (Hollnagel and Woods, 2006) is very close to it. "A model that is cumbersome and costly to use will from the very start be at disadvantage, even if from an académie point of view it provides a better explanation. This should, however, not lead to the conclusions that we must give up on models and try to describe reality as it is, since this is a philosophically naïve notion." And a note is added to comment on the use of "better": "Better is, of course a dangerous term to use since it implies that some objective criterion or standard available. Although there is no truth to be used as a point of référence, it is possible to show that one explanation -under given conditions -may be better than another, e.g., in providing more effective countermeasure " Hollnagel & Woods (2006 p. 353). By questioning the notion of reality as it is, and by introducing an example of purpose {"providing more effective countermeasure") against which the relevance of the model can be assessed instead of an objective external criteria, this sentence favours a radical constructivist approach of resilience engineering. It can be noted that such a relationship between resilience engineering and radical constructivism would not be surprising as "engineering" in the expression is clearly associated with the search for practical relevance. In management sciences, "actionable knowledge" (as a définition of models aiming at practical purposes), has gained strong support from philosophies of knowledge (such as pragmatism 1 or constructivism, [START_REF] Avenier | Repères pour la transformation d'expérience en science avec conscience[END_REF].

CONCLUSION: IS RESILIENCE ENGINEERING REALIST OR CONSTRUCTIVIST?

This paper resuit from the confrontation of two philosophical perspectives, based on différent expériences of researches in the field of safety. One author found critical realism to provide a good framework for his research. The second authors found radical constructivist theory of knowledge as a useful approach for the questions that came up throughout his projects. The two perspectives are however quite différent and imply différent positions with regard to ontology and epistemology. Choosing one over the other can be linked to personal conviction and beliefs. There are in fact no definite answers as both positions are still hotly debated in philosophy. However, subscribing to one or the other led to différent conclusion when applied first to our researches, then translated to resilience engineering issues. Critical realist suggested to consider the importance of social System ontology (in particular the dualism of structure and agency) and this has implications for méthodologies. Radical constructivist stressed the importance to put the viability of models (usefulness, purpose and value) into tests within their spécifie context of use, and consequently to create the proper research conditions to do so (such as action research). What we find interesting is that philosophical questions often seem as inconsequential to practical problems. On the contrary, they reveal hère to hâve concrète implications.

See for example the conférence thème of the academy of management, 2004, using as an introductive sentence James pragmatic principle 'Truth in our ideas means their power to work."

ThLs exploratory paper would need to be dcveloped further in order to better ground thèse implications, we for example think that the conséquences of the two positions might not exclude each other. But it is a first step.