Eric Casale

Guy Marlair

Scetauroute Dtts

HEPTANE FIRE TESTS WITH FORCED VENTILATION

Keywords: Heptane fires, steady states, temperature fields

This part of the EUREKA 499 program deals with fundamental experiments in the steady state obtained by both well defined fire sources and controlled Ventilation conditions. A large number of sensors were used to describe the thermo-mechanic fields associated with these fires. These good quality results may be used for the füture qualification of Simulation tools.

l Introduction

Heptane fires used in EUREKA 499 were designed by France and Austria. They were managed by the French group, with an active participation from Austria.

One of the main objectives of France contribution in EU 499 program was the thermo-mechanic measurements related to well characterized fires. Compared to the initial program which was based on realistic vehicie fires, these specific experiments required some additional means:

-a control ofair motion; -a complete measurement field; -a controlled fire source. The air motion control was perfbrmed with a big fan (2.6 m diameter) installed by France. This Ventilation unit allowed air flow velocities up to 3 m/s in the tunnel cross section. The accuracy ofthe control was about 0.1 m/s.

The basic measurement network installed by iBMB was reorganized and completed with the French instrumentation. In the final configuration, the German sensors were located in cross sections between -200 m and +100 m (relative to the fire). The French measurement System was set in the sections located between +100 m and +1500 m. An additional section was placed at -100 m. These sensors were similar to the German ones, except for the smoke detectors (which do not provide quantitative absorption information) and a differential pressure measurement (giving coupled data conceming the hot layer thickness and its density).

In the previous experiments ofthe program, the performed fires did not present steady states ofthe heat releases, even for calibrated loads (wood cribs). A fundamental result ofthe French participation was obtaining steady state heat releases. The proposed fire source consisted oftubs filled with heptane with a remote controlled level.

One of the most important objectives of these experiments was the constitution ofbasic results allowing a comparison with all kinds ofsimulations.

The technical choices

The proposal to complement the final experimental program in the Reppar Fjord tunnel by adding some heptane fire tests in a pool configuration was clearly coming from a common wish to improve the scientific approach. Further, a heptane pool fire can be considered to be a dose approximation to a real fire scenario occuring in a road tunnel, involving petrol combustion from one or several petrol tanks after a crash.

The scientific background

Pool buming has been studied extensively for many years at the laboratory scale or at large scale in the open air (i.e. in a free ventilated mode). Early work was undertaken in the 50s by the Russians Blinov and Khudiakov [l] and later reviewed by Hottel [START_REF] Hottel | Certain laws governing Diffüsive Burning of liquids" by Blinov and Khudiakov[END_REF]. Complementary research was carried out later by Koseki and co-workers [START_REF] Koseki | Combustion properties of large liquid pool fires[END_REF], Babrauskas [START_REF] Babrauskas | Estimating large pool fire burning rates[END_REF] and an excellent review was written by Hall [START_REF] Hall | Pool Buming[END_REF].

Thus a large amount ofvaluable information was considered available for füture detailed interpretation ofthe results to be achieved with the EU 499 program.

Choice of the chemical

As regards the specific choice of heptane (see table l gathering some ofthe main characteristics of the product), a pure chemical, detailed reasons were reported previously [START_REF] Casäle | Projet EUREKA 499 FIRETUN[END_REF]. n-Heptane main characteristics [START_REF] Weiss | Hazardous Chemical Data Book[END_REF]

2-

Here too, relevant literature sources may help äs a support in characterising the real experimental fires, chiefly inside or near the burning zone and the fire plume.

As an example, Yumoto and Koseki reported some detailed temperature measurements inside the reactive zone of rather large heptane pool fires [START_REF] Yumoto | Gross structure ofheptane pool fire: some experimental results[END_REF]

[l l].
Some other works are usefül to qualify heptane buming rates, air entrainment laws in the plume, soot production, combustion efficiency, total heat release rates and radiativefraction[ll] [START_REF] Köylü | Carbon Monoxide and Soot Einissions from liquid -Fueled Buoyant Turbulent Diffusion Flames[END_REF] [START_REF] Hamins | Estimate of Flame Radiance via a single location measurement in liquid pool fires[END_REF].

Indeed due to local constraints encountered in the Reppar Fjord tunnel, it was not possible to install all the instrumentation required for a comprehensive study ofthat kind.

Figure l illustrates one ofthose reference works.

Equipement design and features

An agreement has been found around the following basic ideas or technical necessities:

-fires should develop in the optically thick, radiative mode of buming (i.e. surface ofthe pan larger than Im 2 ); -fires should induce a significant heat release rate (several megawatts); -steady-state, time controllable conditions had to be looked for (scientific purposes); -the fire test procedure should be fülly initiated and monitored at a fairly good distance (about 350 m far from the fire), without any direct possibility for the operators to witness the ignition process (safety concern). In order to fülfil all these requirements and meet all safety aspects, INERIS proposed a technical solution comprising :

-steel circular pans of l m 2 , 2 m 2 and 3 m 2 with adequate ducts for the various connexions; -a feeding line made ofreheated coppertube, entirely sealed on site; -a 2 m 3 capacity heptane tank equipped with a level meter; -two pneumatic pumps for heptane transfer through the feeding line; -all other relevant instruments (thermocouples, transducers, valves ...) providing an easy way of monitoring the fire test in füll safety. A simple regulation loop was used to keep the desired constant level of heptane in the pan [START_REF] Hamins | Estimate of Flame Radiance via a single location measurement in liquid pool fires[END_REF]. The remote controlled ignition device had been designed by IBS engineers (Austria). Due to the intensive cold, it had been decided to ignite first a spongy material filled with a solvent, and located inside the heptane tub. This disposition proved to be highiy efficient.

Figure 2 gives a schematic view ofthe operating System set in the Reppar Fjord tunnel.

Figure 2: Experimentell Installation for heptane tubßres

INERIS preparatory test

Experimental set-up and procedure

The equipment necessary for these tests were first stored at INERIS. It was checked out in a realistic Situation during a preliminary fire in the experimental gallery of INERIS (Figure 3). This test was carried out in September 1992, before the packaging ofthe equipement and its expedition to Reppar Fjord.

Figure 3: INERIS fire test gallery

This large scale fire testing facility has already been described elsewhere [START_REF] Mariair | Large scale testing in the INERIS fire gallery : a major tool for both assessment and scaling up of industrial fires involving chemicals[END_REF], [START_REF] Mariair | The behaviour of commereialy Important Diisocyanates in fire conditions. Part l : Toluene Di-isocyanate (TDI)[END_REF], [START_REF] Casäle | Puymorens Tunnel Refüges Firetests[END_REF]. Notably it provides well controlled fire conditions. Another result of this preliminary test has been to provide valuable information dealing with the heptane burning rate in physical conditions dose to those expected in the Reppar Fjord tunnel.

Results and comments

Two drums of 200 l each of heptane were used. The overall test duration was just less than one hour.

BUKNING HEPTAM MASS MONITOlaNO

THERMALPOWERASMEASUREDTOTHETOWERCrOP) The reliability of the whole System proved to be quite satisfactory with the local conditions. Particularly, the regulation of the "constant level" revealed to be fairly accurate, since only a more or less 2 kg of heptane mass fluctuation was observed, once steady state conditions were reached (Figure 4). The gap in the middle ofthe diagram is the consequence ofa manual change ofa heptane drum.

^
Frorn an estimation of the heptane consumption, one may caiculate an average buming rate to be 100 g/s, which was rather higher than expected in the open air (65 to 5-75 g/s) (Babrauskas [START_REF] Babrauskas | Estimating large pool fire burning rates[END_REF], Tewarson [17]). Significant re-radiation from the heated walls (refractory lined) ofthe gallery to the heptane pool could be a possible explanation.

Classical stratification of the fire gases along the gallery downstream from the pool was also observed (Figure 4). The total heat release rate averaged 4.0 to 4.5 MW in steady state conditions (Figure 4) showing a high overall combustion efficiency for a Im 2 pool configuration.

Other complementary information about heat flux levels, opacity, chemical composition ofthe gases, temperature in the tower at the sampling port, (50 m far from the fire place) were also recorded (Figure 4).

Reppar Fjord tests

These tests were perfönned in severe conditions, during an unusual cold wave. The external temperature reached -20 °C. As a consequence, some incidents occurred during these experiments. They took a relative importance because these tests were calibrated. In these conditions, a small difference between the planned test and the perfönned experiment appears immediately in the results.

This obliged the French managers to modify the experiments considering the limited time available for these tests (less than one week) and the limited quantity of heptane stored. The initial proposed tests and the perfönned ones are indicated in table 2.

Test number 1:H11 2:H21 Beyond the füture modelling aspects, the influence of the Ventilation conditions on the general Organisation of the velocity and temperature fields was evaluated. The description of these physical boundary conditions is provided by a anemometer located upstream the fire. The instrument gave a local measurement of the air flow velocity, Thus it is only a simple indication of the Ventilation intensity produced inside the tunnel by the fan.

For each experiment, one or two steady states were characterised. They largely depend on the equilibrium that exists between the thennal source and the Ventilation conditions. Particularly, two incidents that occurred during experiments H 11 and H21 showed the great importance ofthe stability ofthe fire source in this equilibrium .

Generally, each experiment has shown at least one steady state (Figure 5). This Situation may require about 20 min to be established. The criterion of stationary state is the temperature recorded downstream from the fire.

6-

Test Hll

The tub surface is Im 2 . The initial operations were repeated for each experiment. Most ofthem are directiy linked to safety aspects. They were:

-Ventilation is established along the tunnel; -the tub is filled with heptane through the feeding line;

-when the level of liquid is correct (regulation level), the pump is stopped; -the tub is covered with a plastic film (this is necessary to avoid diffüsion of explosive vapors); -operators leave the fire location; -at t = 0, the ignition is performed with the remote controlled ignition System; -the plastic film bums and the fire develops slowly. The longitudinal Ventilation is controlled with the big fan located at the entrance portal of the tunnel. The velocity is fixed at 0.5 m/s during the first hour of the experiment. As a consequence, this test led to a clearly stabilised Situation.

The incident that occurred about 30 min after ignition was the projection of concrete blocks due to the Vaporisation of water contained in the ground lining. Some visual observations were made inside the tunnel during the test. They only dealt with that part of the tunnel where the access was possible in acceptable safety conditions, that means, upstream from the fire.

The backlayering phenomenon began äs soon äs the complete cross section was filled with smoke. After 20 min, a Situation that appeared to be steady was established. The backlayering limit was stabilised at -150 m but it has been observed that this Situation was, linked to the ceiling roughness (Figure 7).

Figure 7: Inßuence ofceiling conßguration on backlayermg limils

After the increase in Ventilation (Figure 6), all the smoke was blown downstream of the fire. This motion does not look like a conflict between cold and hot air masses. The observations show that the fümes are progressively diluted by fresh air. The visibility is restaured slowly.

0

6W 1200 1800 2400 3000 3600 4200 4800 5400 6000 6600 7200 TlnK(l) The incident that occured about 30 min after the ignition appears clearly in the temperature evolution (Figure 8). The equilibrium which resulted frorn the balance between Ventilation and transient characteristics ofthe fire required first about 12 min to be locally established. Between this moment and 27 min, the steady state was slowly reached: in the fire area, the other parameters do not present any significant evolution.

About 27 min after ignition, a chaotic behaviour appeared. This is the consequence ofthe loss ofcontrol ofthe fire due to the concrete projections. However, a cyclic evolution was found (the period is 10 min for the four last cycles). This may be explained by the following evolution:

-the concrete projections create an overflooding of heptane out of the tub (pieces of concrete falling inside the tub); -the increase ofburning surfäce induces temperature peaks and protects also the concrete from direct radiations (this energy is used for the Vaporisation of heptane); -the concrete temperature decreases and, äs a consequence, concrete projections stop; -when all the heptane on the ground is bumt, the air temperature decreases and tends to reach the steady state level (asymptotic evolution); -the concrete of the ground is exposed to radiations and new explosions happen, so that the cycle may begin again. In the following tests the ground lining was insulated from the radiation in the fire area. The rockwool was installed on the floor, downstream from the tub. It is to be noted that this device did not prevent "concrete explosions" in other tests (particularly in the 3 m 2 tub fires). These "explosions" were located beyond the lining limits so that the fire was no longer disturbed.

Time

"-200 m -+300 m" field "-20 m -+20 m" field

• J.:. Some temperature field evaluations (at various times) are presented in figure 9. A large width field (with regards to the distance parameter) is selected to show the evolution of temperature in the far field. In addition, a shorter width field is presented to allow comparisons in the fire area. Discrepancies seem to appear in the location of some isothennal curves between the two modes of representation. This is simply due to the principle of the Software used to construct temperature fields in a 3D mode from the actual measurments. Nonetheless, valuable results have already been achieved by this procedure.

If the steady state corresponds to the Situation described by the fields at t = 20 min, the chaotic behaviour provides a mean increase of temperature levels (see also figure 8), and the cyclic evolution of the fire induces cyclic expansions of energy. The situations described in figure 9 show the thermal fields during the last heat wave (t = 80 nun corresponds to the minimum heat spread, and t = 85 min to the maximum). At this moment, the air velocity is higher than at t = 20 min (about l m/s).

Test H21

In this test, the size ofthe tub was kept equal to l m 2 . The initial air velocity was increased, in comparaison to the Hl l test. Two different steady states were obtained. They correspond to longitudinal velocities of l .5 m/s and 2.0 m/s.

The incident that occured was the deformation of one of the metallic beams that supported the tub, so that burning heptane spread on the ground, increasing the fire power. In the next tests, this structure was reinforced. --,----------^. 90 120

3,0- 2,5- 2,0- 1,5- 1,0- 0,5- 0 30 60 . Air velocity (m/s) [~| overflooding J 0,0-1-- , -, - l T Tub l Time (min)

Figure 10: Ventilation conditions for heptane test H21

The increase in the Ventilation was performed about 50 min after ignition. At this moment, the thermal field was not completely stabilised (Figure 11, section 0 m). This increase of air velocity induced a heat dilution effect downstream fi-om the fire, so that the temperature generally decreased (Figure 11 section +10 m). On the contrary, the increase in the Ventilation led to local increases near the fire of about 10°C (section 0 m). In the two tests using a l m 2 tub, the heptane buming rate was about 80 g.s-'.m-2 determined from the level ofthe füel in the storage tank (Figure 2).

Test H31

This test was performed with a 3 m 2 tub. An apparent stationary state was noted 20 min after ignition. At this moment, it was difficult to precise whether this observation corresponded to a real steady state because the fire power began to decrease slowly. The H32 test eventually showed that the asymptotic Situation had actually been reached in this experiment-

The backlayering developed up to 80 m upstream from the fire (visual observations, figure 12). Some ofthe mechanisms leading to this state are described in figure 7.

Figure 12: Observed Situation of test H31

The very low temperature conditions of heptane storage outside the tunnel (about -20°C) explained that a control valve failure occurred, cutting off the heptane feed to the tub. From this moment, the fire power output decreased slowly to final extinction.

Test H32

This fourth test was not planned in the initial program. Experiment H32 could be performed because the failure that happened during H31 test was rapidly located and repaired. This second fire was ignited about three hours after the first one. This test was performed without any incident.

The initial conditions were identical to the H31 test. Particularly, air velocity was maintained at 1.5 m/s. The development of physical conditions inside the tunnel was identical to H31 test. This demonstrates a remarkable reproducibility of phenomena relative to fires in tunnels.

About 40 min after ignition, the Ventilation was increased to 3.0 m/s. This Operation took 10 min (Figure 14). ----i------i----------------0 30 60 90 120

Figure 14: Ventilation conditions for heptane testH32

Under the longitudinal wind effect, flames were bended. Their development reached about 5 meters (Figure 15). Buoyancy effects were observed downstream from the fire. A short study shows that the thermal convective effects are fülly developed in the fire zone. This Situation appears clearly in the very first minutes ofthe fire. In the central vertical plane, the buoyancy efFects induce a massive motion of air toward the ceiling (Figure 16). The impact zone ofthe plume is located about 8 m downstream from the fire. This is the place where the wall temperature was maintained at a high level during the experiment. A the end ofthe test, when the fire is extinguished, a big amount ofheat comes from this zone (thennal restitution effects).

In the lateral vertical planes, the maximum of temperature is located at the ceiling, between 8 m and 10 m. This is coherent with the observation dealing with the central plane. The isothermal curves show that, in these lateral planes, heat convection happens from this point down to the ground where a new relative temperature maximum is found between 16m and 18m.

Time

Left side (east) Right side (west) This Situation suggests that two large symmetrical eddies ofopposite rotation are responsible for heat convection downstream from the fire (Figure 18). They are the result of the interaction between natural convection (Benard eddies) and the forced convection due to controlled Ventilation.

The development of these flow structures is linked to the fact that the fire does not represent any material blockage in the tunnel cross section like previous fires (rail vetucles may have important blockage coefficient, larger than 60%). The motions in relation with buoyancy effect may be fülly developed (Figure 18).

Figure 18 : Scheine ofthe eddies linked to buoyancy effects downstream from the fire zone

The representation ofthe cross section thermal field downstream from the fire shows that the development ofthe eddies is actually not symmetrical (Figure 19). The right eddy convects more energy than the left one. This induces a dissymmetry of the temperature levels. In regions located far downstream from the fire, the motion looses its three dimensional character. Velocities measured in the central vertical plane show great movements ofair coming backward to the fire. These situations are quite classical in this kind ofexperiment and they are linked to the importance ofbuoyancy effects.

In the two tests using a 3 m 2 tub, the measured heptane buming rate is in order of 75 g.s-'.m-2 . This value is in agreement with Babrauskas [START_REF] Babrauskas | Estimating large pool fire burning rates[END_REF] and Tewarson [17] estimations.

Conclusions

The heptane fire experiments performed by France with the active participation of Austria in EUREKA 499 encountered some incidents. However, the tests are to be considered successfüll for the following reasons:

-each experiment has been characterized by at least one steady state; ' -a great amount ofdata were collected during these tests; -their quality is compatible with a füture comparison with Simulation works; -two tests proved a remarkable reproducibility of the phenomena, giving more credibility to Simulation processes. Some phenomenological descriptions presented in this paper show that the analysis drawn from these experiments may provide valuable information. The qualitative content of this information may be considered äs an excellent basis tö a technical orientation for modelling purposes.

Finally, it is important to state that numerous other available data have not been presented in this analysis.

Figure I :

 I Figure I:Temperature profile in a heptane ßame[START_REF] Yumoto | Gross structure ofheptane pool fire: some experimental results[END_REF]

-Figure 4 :

 4 Figure 4: Main characteristics recorded during the l m 2 heptane test m the INERISfire gallery (Sl: cross section located 2.5 m downwindfrom thefire; S2: 5 m, S3: 10 m; S4: 20 m; S5: 40 m)

Figure 5 :

 5 Figure 5: Heptane tubßre in Reppar Fjord tunnel

  ---i----r fc-

Figure 6 :

 6 Figure 6: Ventilation conditionsfor heptane test Hll

Figure 8 :

 8 Figure 8: Temperature at the ceiling, above the fire

Figure 9 :

 9 Figure 9: Temperature ßelds for steady state and chaotic states

Figure 11 :

 11 Figure 11: Temperatures recorded at the ceilmg above the fire and 10 m dcwnstream frorn the fire

Figure 13 :

 13 Figure 13: Ventilation conditions for heptane test H31

Figure 15 :Figure 16 :

 1516 Figure 15: Picture off he 3 m 2 heptane tub

Figure 17 :

 17 Figure 17: Temperature fields at t = 5 min in the lateral vertical planes

Figure 19 :

 19 Figure 19 : Temperature ßelds in fhe vertical cross section 20 m downstreamfrom the ßre at t = 70 min

Table I :

 I Main characteristics of n-Heptane

		(7] [8]
	Molecular weight	100.2 g
	Flash point	-4°C
	Freezing point	-90° C
	Boiling point	98.4° C
	Flammability limits (in air)	l.l%to6.7%
	Auto-ignition temperature	215°C
	Density	0.68
	Immediately Dangerous for life and Health	4250 ppm
	AH°]^ (gross) Standard heat combustion (total)	48.07 MJ/kg
	AH°(; (low) Standard heat combustion (net)	44.56 MJ/kg

Table 2 :

 2 Heptane tests programDespite this unfavourable Situation, the general results obtained are to be considered äs positive.

	3:H31 4:H32	Planned test Im 2 2m 2 3m 2	Perfönned test Im 2 Im 2 3m 2 3m 2	Date 27 Oct. 92 28 Oct. 92 29 Oct. 92 29 Oct. 92

Aknowledgements

The authors gratefülly thank D. CARSON for his technical assistance.


INERIS, FRANCE J P Pineau

PRESENTATION OF WORK PERFORMED BY ESReDA "ACCIDENT ANALYSIS" WORKING GROUP AND GOALS FOR THE FUTURE

and United Kingdom. Interests of participants were divided between two major areas : validation of physical modeis and softwares for consequences assessment of major accidents and a more statistical approach of data collected on accidents to define a safety policy in relationship with regulations, insurance, economic and social costs.

A first step was to prepare a questionnaire for assessing strengths and deficiencies of existing databases. The answers to the questionnaire distributed in 1994 in the above mentioned countries were analysed.

1/10

industrial or transport nature in which hazardous substances were involved. By reference to this limitation, the work would be directiy related to the application of the Seveso Directive dealing with specially hazardous plants and to the regulations for power plants and transportation. It could also be assumed that any database System on accidents would probably have similar requirements äs regards data collection, validation and storage for easy retrieval, verification of dependability and convenient use of data. The investigation performed by the working group which will be described can readily be extended to other types of database.

For the purpose of the present project, a database is defined äs any collection of accident data independent of the storage medium (e.g. card index or CD ROM).

As a final objective, members of the working group accepted the promotion of networking of current databases for more efficient use by end users.

In this paper, a summary of individual interests of involved members is first given by reference to different possible goals and final objectives. A further section is devoted to reasons why, in a first stage, a questionnaire for assessing the strengths and deficiencies of databases on industrial accidents was prepared. The findings from the analysis of returned questionnaires (93 from 13 different countries) are described in another paper.

In a final paragraph suggested future work for the "Accident Analysis" working group is presented. For further development, provide criteria to define probabilities of occurrence of accidents are also to be dealt with (e.g. using European Gravity Scale, Bradford Disaster Scale...). Some members were also involved occasionnally in investigations of accidents and required access to databases on accidents for comparison purposes.

II -INDIVIDUAL INTERESTS OF MEMBERS AND POSSIBLE GOALS

In addition to these goals, a final objective is to promote transnational cooperation and networking.

11-3 Organisation of the work

III -QUESTIONNAIRE ON STRENGTHS AND DEFICIENCIES OF EXISTING DATA BASES (INCLUDING TRANSPORT OF DANGEROUS GOODS) -

111-1 Aims of the questionnaire

It is currently known that the Seveso Directive (82/50/EEC) related to major accident hazards of certain industrial activities is under extensive revision. In the revised version, emphasis will be given to Safety Management and thus to reporting and investigating incidents and accidents. Recently a proposal of a Directive was issued (94/C 26/08), related to the transportation of dangerous goods. More specific directives deal with emissions, explosions and fires.

In European countries, there are now mandatory requirements for industries to prepare safety cases to comply with statutory regulations of hazardous plants and for competent authorities to assess these safety cases betöre authorizing the Operation of such hazardous plants. Through regulär inspections these competent authorities are also involved in identifying the deficiencies in industrial plants.

Insurance companies have to assess plausible accident scenarios, their probabilities of occurrence and the maximum possible effects (worst case scenarios) in order to assess premiums. They require reliable statistics on accidents and reliable data of involved plants and equipment.

Industrial organisations are more concerned with the general policy related to hazard assessment, risk reduction and emergency planning and response.

Universities, consultants, research organisations have generally developed and used physical modeis and Software dealing with emissions, fire and explosions. For the validation of these modeis and Software, they need accident data (accident initiation, sequence and effects).

Considering all these requirements, a proposal for a questionnaire was discussed at the tirst and second meetings and finalized at the end of 1993. The questionnaire is of a very broad scope and contains a large number of questions distributed in four main areas :

• Details of the responding Organisation (8 items, mainly for identification of the general type of business)

• Use of dedicated in house databases (for each database, items were divided into six subsections : general field of application, technical information, main users, practical information, type of data stored, Organisation of databases)

• Use of generic databases ; for each database, items were divided into four subsections : general information, frequency of use, aims of the user and evaluation of the performance • Suggestion for possible improvements for future databases (features to be introduced, areas to be improved, possible participation in a new database).

111-3 Prooress of work on assessment of strenaths and deficiencies of current accident databases

This questionnaire together with an explanatory letter was distributed by members of the working group in 13 different countries mainly in Western Europe from February to July 1994.

In some countries, the national sample was relatively large :

• In France, the questionnaire was sent to about 300 companies from whom 41 answers were received. These answers were received from consultants, government bodies, industrial firms mainly in the chemical and petroleum sectors, insurance companies and research organisations. Only relevant answers (29) were analyzed.

In other countries and particularly the Netherlands, 113 questionnaires were sent to FACTS users or others ; 31 answers were received In other countries a limited sample was chosen according to the national contacts of the member of the working group. For example in Italy the questionnaire was distributed through the 3 ASI association and, in Finland, to a group of 20 experts in industrial risks.

The questionnaire was also sent to international organisations (CONCAWE, European Process Safety Centre, JRC, Ispra, OECD, UNEP).

Considering this global sample, caution must be taken when looking at the applicability of the conclusions to a given country or to a particular group of users.

A preliminary assessment of questionnaires received by TNO, Environmental and Energy Research was presented by M. BRASCAMP at the third meeting in June 1994.

The general methodology proposed with some amendments was then applied to the 93 questionnaires at the DPLU in August-September 1994.

The initial assessment of these questionnaires will be reported on another paper.

It must be emphasized that, owing to limited allocation of time and funding, only the most salient aspects were investigated and more in-depth work is required.

IV -FUTURE WORK -

The above mentioned initial assessment of questionnaires demonstrates the existence of more than 85 databases but can give only broad ideas for the future development of

  of potentially hazardous plants and transportation Systems (design and Operation) • help to identify whether current emergency procedures are appropriate assist national and international competent authorities and financial and insurance companies to formulate proactive policies develop quality aspects for data and Software improve total quality management of safety collect reliability data and failure rates look at compliance with regulations, codes of practice and Standards • develop training of operators and management encourage the incorporation and use of databases in the curricula of universities and other academic courses.

  Three meetings have been organized until now : • at INERIS, Verneuil-en-Halatte, France on March 10, 1993 at Disaster Prevention and Limitation Unit (DPLU), Bradford, UK on June 2-3, 1993 at TNO, Environmental and Energy Research, Apeldoorn, N.L., on June 20-21, 1994.

•

  In United Kingdom about 40 questionnaires were sent and 19 valid answers received.

11-1 Main interests of members

The members came from organisations directiy linked with industrial groups (EdF Cli, Gesellschaft für Anlagen und Reaktorsicherheit -GRS-, Norsk Hydro), from consultant organisations (Det Norske Veritas, Four Elements Ltd, Nukem), competent The interests of the members can be differentiated into two main areas :

• validation of physical modeis and Software to be used for assessment of consequences arising from major accidents (emission, explosion, fires)

• a more statistical approach to accident data, taking into account economic, environmental and social costs in relationship to the probabilities of occurrence.

The main aspect is safety policy related to regulation, insurance and safety management. Quantitative risk assessment could also be an area of major relevance.

11-2 -Possible aoals and final objectives

When considering such broad areas it must be realized that current databases have wide implications directed to various goals : databases. Such improvements äs the means for collection of data, the definition of accident scenarios and users friendliness were frequently pointed out.

Provided this assembly approves that we proceed, it is proposed that the next step of our working group "AA" should be to produce a directory containing information on pertinent databases such äs their nature, special features, usage, availability and cost.

All interested parties will be invited to send this information according to a defined protocol.

This step should assist the harmonization of existing databases and would facilitate the final goal of networking of these databases which would also need a choice of compatible Computer Systems.

Arising from previous contacts of members with organisations of Central and Eastern

Europe and Russia, the working group is ready to cooperate with representatives of these countries ; this will require an additional work. However, available funding from CCE would facilitate such an extension äs it would help promoting the development of a harmonized safety policy throughout Europe. (20.6.1994) Working group "Accident analysis" Tel. 44.71.488.63.31 Fax. 44.71,488.63 FRANCE Tel. 33.72.82.44.44 or 72.82.42.23 Fax. 33.72.82.45 Tel. 39.35.48.38.98 Fax. 39.35.48.40.10 MESTERW. GESELLSCHAFT FÜR ANLAGEN UND REAKTORSICHERHEIT (GRS) Schwertnergasse 1 50667 KÖLN Tel. 49.221.20.68.501 Fax : 49.221.20.68.442


A C Van Den Berg

H G The

W P M Mercx

C J Hajrhurst

AutoReaGas -A CFD-TOOL FOR GAS EXPLOSION HAZARD ANALYSIS

INTRODUCTION

Gas explosions constitute a major hazard for offshore gas and oil producing installations. A gas explosion is the consequence of an accidental release of a flammable gas, the mixing with air and a subsequent ignition. Under appropriate boundary conditions the resulting flame propagation process may develop explosive combustion and damaging blast loadings. In spaces containing a lot of equipment, this is a particular problem and a small quantity of fuel may be sufficient to give rise to the development of high explosion overpressures. If such overpressures are not anticipated in the design they may have fatal consequences for both crew and rig.

The hazard of gas explosions offshore was demonstrated by the incident with the Piper Alpha rig in 1988 (Petrie^). A small-scale gas explosion caused the failure of vital control and communication functions on board. In consequence of this, the incident escalated to unforeseen circumstances leading to the total loss of the rig and the death of 167 people. However, gas explosion effects can be controlled by a proper design of the installation.

Modern offshore installations consist of a number of separate modules of limited size.

Present understanding of the phenomena indicates that the module shape, the positioning of the equipment inside the module and the positioning and the size of vents largely affect the 5.8.1 development of an internal gas explosion. So far, simple venting guidelines (e.g. Cubbage and Simmonds 9 ; Bradley and Mitcheson 4 ' 5 ) are widely used to assess the consequences of possible gas explosions on board of offshore installations. Venting guidelines are empirical correlations based on experimental data. Most of the experimental data have been obtained in small-scale tests using near cubical, empty vessels. These guidelines, however, do not allow application beyond the experimental conditions they were derived from. Application to larger volumes of more complex geometries which contain many objects may lead to substantial underestimation of effects.

For the design of adequate gas explosion control provisions in the offshore, more sophisticated methods are essential.

AutoReaGas is a software package capable of userfriendly, interactive, 3-D numerical simulation of any aspect of gas explosion phenomena. AutoReaGas contains both a gas explosion simulator and a blast simulator, each tailored to specific problem features.

After a general description of the phenomena and how they are modelled, in this paper the software is demonstrated in a practical offshore case study.

PHENOMENA

Gas Explosion

In a gas explosion a flammable gas mixture is consumed by a combustion process which propagates through the mixture in the form of a flame front. The flame front is the interface between cold reactants and hot combustion products. Because combustion products are of high temperature, the cold flammable medium expands strongly on combustion. The expansion induces a flow field whose structure is fully determined by the nature of its rigid boundaries. In this flow field the combustion process is carried along. The rate of combustion is strongly affected by the flow structure (velocity gradients and turbulence) met. Flow velocity gradients stretch the flame front, enlarge its interface and increase the effective combustion rate. Low intensity turbulence wrinkles the flame front with a similar effect on the combustion rate. Higher combustion rates intensify the expansion. Higher flow velocities go hand in hand with more intense turbulence levels. Higher turbulence levels speed up the combustion, etc. etc

In other words: under the appropriate (turbulence generative) boundary conditions, a positive feedback mechanism is triggered by which a gas explosion develops exponentially both in speed and overpressure.

5.8.2

Blast

During the explosion process, the rapidly expanding combustion products do work on the surrounding medium. In this way, the chemical energy (heat of combustion) of the flammable mixture is partly converted into mechanical energy (expansion). Such a process is characterized by a thermodynamic efficiency with a maximum of approximately 40%. The mechanical energy is transmitted from the explosion into the surrounding atmosphere in the form of a blast wave. Such a blast wave may do damage on structures a large distance from the explosion.

An object struck by a blast wave experiences a blast loading which is a combination of two effects. On the one hand, a blast wave is experienced as a transient change in the static overpressure (a pressure wave) and on the other hand as a transient change in the medium velocity (a gust of wind). The pressure wave character induces a static pressure distribution while the medium velocity wave induces a fluid dynamic drag force on an object struck.

MODELLING

Gas Explosion

As outlined in Section 2, the essence of a gas explosion consists of the interaction of a premixed combustion process with its self-induced expansion flow field. The development of this process is predominantly controlled by the turbulence induced in the flow field by the boundary conditions. Modelling of a gas explosion requires careful modelling of all aspects of this complicated process. The model underlying the AutoReaGas gas explosion simulator can be characterized as follows:

The gas dynamics is modelled as a perfect gas which expands as a consequence of energy addition. This is mathematically formulated in conservation equations for mass, momentum and energy, i.e.: the Navier-Stokes equations.

The energy addition is supplied by combustion which is modelled as a one step conversion process of flammable mixture into combustion products. This is formulated in conservation equations for the fuel mass fraction and the composition.

The combustion rate is a source term in the fuel mass fraction conservation equation.

Turbulence is modelled by a two parameter model (k-s) which consists of conservation equations for the turbulence kinetic energy k and its dissipation rate s (Launder and Spalding* 1).

5.8.3

Turbulent combustion is modelled by an expression which relates the combustion rate to turbulence. Several options are available varying from theoretical relations such as the Eddy Break Up model (Spalding^) and the Eddy Dissipation model (Magnussen and Hjertager [START_REF] Magnussen | On the mathematical modelling of turbulent combustion with special emphasis on soot formation and combustion[END_REF] and Hjertager et al. [START_REF] Hjertager | Computer modelling of gas explosion propagation in offshore modules[END_REF] ) up to experimental correlations between turbulence and combustion (Bray^). Because the applied cell size is often too large to fully resolve a turbulent combustion zone, the combustion rate is corrected.

The initial stage of combustion upon ignition is modelled by a process of laminar flame propagation whose speed is controlled on the basis of experimental data.

Objects too small to be represented by solid boundaries in the computational mesh, are modelled by a subgrid representation. The presence of a subgrid object is modelled by the specification of appropriate flow conditions: i.e.: a fluid dynamic drag and a source of turbulence.

Numerical solution of the set of equations is accomplished by means of the "power law" scheme applied within a finite volume approach (Patankar^).

Blast

As long as objects with large cross-flow dimensions are considered, the interaction with gas explosion blast is predominantly governed by the pressure wave character of the blast. The In case of a gas explosion the internal overpressure can be controlled by venting any expanding gases. Therefore, modern modules are constructed so that they are as open as possible. Outer walls often consist of light-weight windcladding or windscreens which are attached to the main structure in such a way that they may easily fail and are blown off at a low internal overpressure.

A vented gas explosion gives rise to an external explosion. As soon as the combustion process in the module is initiated, the flammable mixture inside the module starts venting in the form of a turbulent flammable jet. This jet explodes when it is ignited at the time the combusting gas mixture vents. The resulting blast may do damage to, for instance, nearby equipment and structures.

A vented gas explosion is the subject in the present analysis carried out with AutoReaGas. covers the module as well as the space between module and living quarters. Within this domain, the software allows the specification of the physical layout of any system of rigid boundaries e.g. boxes, beams, vessels and tubes by means of a CAD-like interface. The specified configuration of objects in the domain is automatically converted by the software into the proper input for the explosion simulator. Large objects are represented by rigid boundaries while the presence of small objects is modelled by the subgrid formulation.

The software allows the specification of any distribution of fuel in the domain which can be ignited in any desired location. However, to approach worst case conditions in this problem, the module is assumed to be filled with a stoichiometric propane-air mixture and ignited in the centre of the back wall.

The AutoReaGas software allows fixlly interactive simulation, showing the distributions of any specified process parameter on the screen, any wanted number of time steps again. During the simulation, process parameters can be monitored throughout the domain.

CONCLUSION

AutoReaGas is a CFD-tool for analysis of gas explosion problems. AutoReaGas consists of a gas explosion simulator and a blast simulator, placed in a user friendly environment.

Several possibilities of the software were demonstrated in a practical case study. Problems can be defined in a userfriendly CAD-like environment. Computational results indicate that the software is capable of realistic simulation of (vented) gas explosions. The exercise in this paper demonstrated the possibility of detailed computation of the blast loading of objects specified in the computational domain.

Offshore oil and gas production platform

\-\mm

AutoReaGas process equipments representation (cont). Pressure differential between front and back area of vessel A on deck.

5 . 8 . 4 4.

 584 drag component can be neglected. The pressure wave character of blast flow fields can be accurately represented by the assumption of inviscid flow. Often, blast flow fields are characterized by the presence of gas dynamic discontinuities such as shocks. Modelling of blast-object interaction requires careful description of such phenomena. Therefore, the blast simulator in AutoReaGas models blast-object interaction as follows:The gas dynamics is modelled as inviscid compressible flow of a perfect gaseous fluid which can be formulated in the conservation equations for mass, momentum and energy for inviscid flow, i.e. the Euler-equations. Description of shock phenomena requires a sophisticated numerical technique tailored to proper representation of steep gradients. To this end, the blast simulator utilizes Flux-Corrected Transport (FCT) (Boris and Book 2 and Boris-^). FCT makes an optimized use of numerical diffusion so that steep gradients present in shocks are retained. Numerical diffusion is added only where it is required for numerical stability. ANALYSIS OF A GAS EXPLOSION ON AN OFFSHORE PLATFORM 4.1 Problem Modern offshore installations are characterized by a modular structure. The various aspects of the oil and gas production process take place in different areas separated by fire/blast resistant walls. The intention is to keep the consequences of a possible incident within bounds -the module.

Figure 1

 1 Figure1shows a highly simplified, made up representation of an offshore production platform. The platform consists of a main deck and a cellar deck. The main deck consists of several modules. One of these modules is almost completely built in. The only possibility for venting for this module is the space on deck between the modules and the living quarters. At this side the module is left completely open as a vent. The consequences of a gas explosion in this module are analyzed by applying the AutoReaGas software.

Figure 2

 2 Figure 2 represents an AutoReaGas configuration of the domain showing only the larger pieces of equipment. The module (left) is filled with a number of horizontal and vertical vessels, interconnected with a lot of piping and appendages. A 3 m diameter, 8 m long vessel (A) as well as a 0.3 m diameter tube are defined in the space on deck between module and living quarters (right).

Figure 3

 3 Figure 3 shows a compilation of such a series of pictures. The pictures show the temperature field in both a horizontal and vertical cross-section at a number of consecutive points of time. The temperature is visualized by means of a suggestive colour gradation. The timing of the pictures indicates how the flame propagation process develops. After a slow laminar start, it speeds up under the influence of the equipment in the module. The combustion process vents in the form of a mushroom-like shaped flame front, which is fully in line with experimental observations (Catlin^ ^nd Bimson et al.l).

Figure 4 represents

 4 the overpressure traces recorded inside the module in the gauges 1 (ignition point) and 2 (vent opening). The traces show the characteristic behaviour of a gas explosion: a relatively long initial phase of slow development and low overpressure progressing into a more violent development characterized by a sudden pressure pulse. A maximum internal overpressure of approximately 70 kPa is observed at the back wall of the module. The Figure 5 represents five overpressure traces (3 -7) recorded at the living quarterns wall in front of vent opening (Figure 2). All traces are more or less similar showing a double-peak 5.8.6 shape with a maximum of approximately 40 kPa.The blast loading of the 3 m diameter vessel on deck is monitored by recording the pressure differential between the front and back area of the vessel. This pressure differential is represented in Figure6. Multiplication of this pressure differential with the cross-sectional area of the vessel immediately results in the horizontal force induced by the vent flow.The blast loading of the 0.3 m diameter tube is recorded by monitoring both the density and gas velocity components in three different gauges in the row of cells in which the tube is specified as a subgrid object. The force on the tube per meter length is calculated from these parameters assuming a drag coefficient equal to 1. The drag force on the tube per meter length as a function of time is represented in Figure7. The double-peak shape, amplitude and duration of the tube load are in line with experimental observations (Catlin°).The double-peak shape of the various loading traces seems to be characteristic and can be explained considering the process parameters in more detail. Stagnation pressures are proportional to both the density and the square of the flow velocity. Initially, stagnation pressures are the result of relatively low-velocity/high-density flow, i.e. the blast from the internal and external explosion. At the instant the gauges are reached by low-density combustion products, stagnation pressures drop. Stagnation pressures rise again in the growing burned gas vent velocities. Gas vent velocities tend to increase strongly at the instant combustion products start venting.

Figure 2

 2 Figure 2AutoReaGas process equipments representation.

Figure 3

 3 Figure 3Compilation of AutoReaGas process monitoring.

Figure 5

 5 Figure 4Overpressure-time traces recorded at gauges 1 and 2 inside the module

Figure 7

 7 Figure 7Blast loading at three gauges along a pipe above vessel A.


Les sites et monuments historiques, comme tous les ouvrages naturels et humains, évoluent avec le temps du fait de l'action des éléments naturels ou de l'activité anthropique. Le caractère unique et l'intérêt général d'un monument ou d'un site amène donc la collectivité à se poser la question des moyens à mettre en oeuvre pour assurer la pérennité des ouvrages qu'elle souhaite conserver dans son patrimoine et la sécurité des personnes susceptibles die les visiter.

En effet le risque principal auquel il faut faire face est celui de la "ruine" du site ou monument. Cette ruine résultat d'une instabilité peut prendre des formes lentes d'évolution mais qui peuvent s'accélérer et mettre en péril brusquement l'édifice et ses visiteurs.

Protéger le patrimoine historique contre ce risque de ruine consiste à entreprendre des actions à trois niveaux successifs : (Fig. 1) Diagnostiquer l'évolution probable en identifiant les points faibles de l'ouvrage et les facteurs d'influence externes constituant une menace pour celui-ci.

Surveiller les points sensibles détectés lors de la phase de diagnostic et décider le cas échéant d'une intervention.

-Traiter pour agir sur le processus de ruine ou dans les situations d'urgence pour prendre les mesures conservatoires nécessaires et le cas échéant éviter d'exposer les visiteurs ou populations voisines à un risque de ruine soudaine.

La pratique des études de protection du patrimoine historique met en évidence les difficultés à prendre des décisions compte tenu des incertitudes techniques du diagnostic, du coût et des imperfections des moyens de surveillance, des aspects administratifs, culturels et économiques des actions à entreprendre.

-LE DIAGNOSTIC II s'agit ici par analogie avec la médecine de préciser et de quantifier des symptômes d'instabilité qui sont le plus souvent des déformations ou des fissurations au sein de l'ouvrage ou de la structure géologique sur laquelle ou au sein de laquelle il est implanté. Le diagnostic peut également permettre, en corrélation avec l'observation de l'environnement de l'ouvrage, de remonter aux causes de l'évolution constatée.

Nouvelles Technologies et Monuments Historiques, Le Caire, 4-8 décembre 1994

INERIS

De même qu'un médecin examine un malade avant de porter son diagnostic avec une méthodologie spécifique pour laquelle il a été formé, il est nécessaire d'entreprendre le diagnostic d'un ouvrage avec une méthodologie qui permette d'avoir un diagnostic le plus sûr possible. L'INERIS a une expérience de cette méthodologie de diagnostic des ouvrages miniers ou de génie civil et des sites industriels ; celle-ci peut être transposée au cas des ouvrages de sites historiques selon un schéma présenté dans la figure 2 (a,b).

Dans un premier temps il faut considérer le système représenté par le monument ou site considéré et son environnement. Seule cette analyse globale de système permettra de ne pas passer à côté de facteurs importants d'instabilité et non forcément évidents à déceler.

Ensuite on procédera à l'identification systématique des facteurs potentiels d'instabilité en ne négligeant aucun d'eux a priori dans cette phase du diagnostic.

Cette identification sera suivi alors d'une première analyse de risques qui consistera pour chaque facteur à évaluer sa probabilité d'intervention et l'impact de son action ou la gravité de ses conséquences selon les modalités qu'il prend.

Cette phase d'analyse des risques peut permettre un classement des facteurs d'instabilité et une identification des points sensibles de l'ouvrage ou du site sur lesquels ils sont susceptibles a priori d'avoir une influence .

Ces points sensibles étant a priori décelés on entreprendra alors une auscultation qui peut permettre d'apprécier l'évolution d'éléments de certains ouvrages ou facteurs de risques. Elle peut aussi permettre d'étudier la sensibilité de l'ouvrage à des sollicitations : variation du niveau de la nappe phréatique, vibrations produites par la circulation, pluie, vent... Une auscultation doit toujours être une opération limitée dans le temps et bien ciblée suite à l'analyse de l'ouvrage. Pour être efficace et bien acceptée une auscultation doit aussi mettre en oeuvre des techniques simples de mesures (inclinaison, écartement de fissures, mouvements de blocs) en faisant appel dans cette phase à des moyens de mesures à grande sensibilité plutôt qu'à des moyens fiables à long terme.

Les résultats de cette phase d'auscultation permettront alors de conforter ceux de l'analyse a priori et un diagnostic global pourra alors être porté.

Ce diagnostic comportera généralement à côté de la description de l'ouvrage ou du site : la liste des facteurs à plus fort risque leur probabilité (même approximative) d'apparition et d'évolution la gravité de leurs conséquences pour l'ouvrage les indications sur l'évolution possible des dégradations dans le temps et leurs conséquences vraisemblables sur le pronostic de ruine de l'ouvrage.

Comme dans le domaine médical le diagnostic d'un site ou monument comporte des incertitudes. Le coefficient de sécurité attaché à un ouvrage rocheux ou à une construction ancienne n'est jamais connu avec la même précision que celui d'une construction moderne. Ceci est lié en particulier à la variabilité naturelle des matériaux le constituant et de la structure géologique sur laquelle il est établi. Il faut raisonner en probabilité de rupture mais d'une part ceci n'est pas encore accepté de manière courante et d'autre part cette évaluation probabiliste est très délicate à entreprendre.

La réalisation de diagnostics est un élément de choix capital pour les organismes chargés de la gestion et de la protection du patrimoine. Les budgets annuels étant toujours limités, les diagnostics réalisés sur plusieurs ouvrages peuvent permettre de faire des choix de priorité. En aboutissant à la mise sous surveillance des ouvrages à plus haut risque, le diagnostic permet d'optimiser dans le temps les dépenses en réservant surveillance, interventions et traitements aux cas les plus urgents.

Il -LA SURVEILLANCE

Le rôle de la surveillance

Quand un diagnostic a permis de mettre en existence une évolution probable de l'ouvrage du site il est nécessaire de connaître l'évolution de la dégradation en fonction du temps pour pouvoir décider du degré d'urgence des travaux à réaliser. De plus il est rare que ce traitement puisse être directement entrepris et il faut monter un dossier administratif et financier souvent complexe et les travaux peuvent s'étendre sur plusieurs années.

Pour toutes ces raisons, la mise en place d'un réseau de surveillance va permettre un suivi dans le temps du monument ou site concerné et le déclenchement éventuel de mesures de première urgence en cas de dépassement de seuils d'alarme.

De plus, la surveillance de l'ouvrage permet d'assurer la sécurité visà-vis du public. Lorsque le site devient un établissement recevant du public la sécurité doit être assurée non plus seulement vis-à-vis de l'extérieur (conséquence d'une dégradation ou ruine sur l'environnement de l'ouvrage) mais aussi vis-à-vis de l'intérieur pour pouvoir le cas échéant prendre les mesures vis-à-vis des visiteurs du site. Pour des ouvrages souterrains en particulier les risques d'instabilité doivent être surveillés au même titre que les risques liés à la ventilation et à l'incendie.

Enfin, plus tard, dans la phase de traitement (cf. III) le réseau de surveillance va aussi permettre de contrôler les travaux de traitement. Il faut effectivement vérifier pendant leur réalisation que ceux-ci n'introduisent pas de perturbations dommageables sur d'autres parties de l'ouvrage. Les travaux de traitement terminés le réseau de surveillance permettra aussi le plus souvent de vérifier leur efficacité.

Comment surveiller ? (Figure 3 (a, b) )

Une surveillance efficace ne peut être mise en place qu'après une étude fine de la stabilité qui n'a été que grossièrement approchée dans la phase de diagnostic. La première phase de cette étude fine de stabilité est l'établissement d'une carte "géotechnique qui rassemble pour l'ouvrage et son assise les données géométriques, géologiques et structurales (fracturation naturelle ou induite).

Cette phase est complétée par des mesures de caractérisation mécanique du sol, des roches et du matériau constitutif de l'ouvrage ainsi que par des mesures in situ de fissuration (auscultation ultra sonique) et de contraintes. De plus quand la géométrie de l'ouvrage est complexe (ce qui est souvent le cas), il faut recourir à des modèles de calcul dont les plus utilisés sont fondés sur la méthode des éléments finis, sachant que pour les ouvrages et sites à forte fracturation seuls les modèles dits "de blocs" permettent une représentation correcte de la complexité d'un site. En effet, ils permettent de simuler l'ouvrage et son socle pour un assemblage d'éléments discontinus ou "blocs" réagissant les uns ou les autres selon les lois de la mécanique.

Caractérisations et modèles permettent alors de tester différents scénarios de rupture et de proposer les types de mesures représentatives et l'emplacement optimum des dispositifs de mesures.

Pour la surveillance plusieurs types de mesures peuvent être utilisées : des mesures de contrainte permettant de suivre l'évolution du niveau de contrainte au voisinage des points les plus sollicités et où les ruptures sont susceptibles de s'amorcer ; des mesures de microsismicité qui permettent de suivre l'évolution de la structure rocheuse sur laquelle est implanté le site ou l'ouvrage et de constater la sensibilité aux mouvements d'ensemble des massifs rocheux voisins ; des mesures d'inclinométrie qui permettent de suivre avec grande précision l'inclinaison d'éléments sensibles du site ou de l'ouvrage.

Mais les mesures, de loin les plus utilisées en matière de surveillance sont les mesures de déformations. En effet, avant la rupture, les matériaux rocheux présentent une augmentation de volume que peut détecter la précision des capteurs utilisés couramment (1/100 mm). L'INERIS dispose de matériels suffisamment fiables pour rester en service pendant des années dans des conditions d'environnement parfois difficiles tout en restant d'un coût raisonnable.

La télémesure des capteurs a d'abord été réalisée pour répondre aux difficultés (et au danger) d'accès au site. Mais une fois résolu le problème de la transmission des mesures, il était tentant de réaliser une automatisation de la saisie des données qui présente un double intérêt :

. augmenter la fréquence des mesures à moindre coût . assurer une meilleure répétabilité des mesures.

En effet, seul un ordinateur peut continuer pendant des années sans se lasser à effectuer un contrôle sans alerte ; car heureusement on fait souvent une surveillance où il ne se passe rien.

Du point de vue technique, l'interprétation des mesures repose sur le choix d'un critère d'alarme et les méthodes de détection de dépassement du critère. Le critère peut être un seuil de déformation à ne pas dépasser, une vitesse de déformation critique ou, plus souvent, la détection d'une accélération ces déformations ; c'est en effet ce dernier type de critère qui demande le moins de connaissance sur l'historique de l'ouvrage.

L'INERIS a pu automatiser l'interprétation des mesures et mettre en place sur certains sites une télésurveillance déclenchant des alarmes. Le développement de la micro-informatique permet de réaliser des installations réduites et facilement gérables sur les sites.

Les aspects administratifs et sociaux sont très importants dès que la surveillance peut se traduire en termes d'alarme. Il y a lieu de définir très précisément qui gère le système de surveillance, qui reçoit l'alarme et ce qu'il en fait. La concertation avec les différentes autorités administratives est ici primordiale et il importe de juger des conséquences des décisions prises trop tôt (alarme à tort) ou trop tard (défaut d'alarme) lorsqu'il s'agit de barrer une route ou de faire évacuer un immeuble. La notion de "péril imminent" ne s'appuie en fait sur aucune valeur précise de vitesse d'évolution ou de seuil atteint. L'interprétation des mesures de surveillance ne saurait donc se réduire à une simple mesure automatique.

III -LE TRAITEMENT

La décision de traitement (figure 4) interviendra le plus souvent après une phase de diagnostic suivie d'une phase de surveillance. La définition du mode de traitement va amener à résoudre des problèmes à la fois techniques, administratifs et financiers.

Pour l'aspect technique il convient à nouveau de considérer l'ouvrage ou le site comme un système en interaction avec son environnement comme cela a été souligné précédemment ; en effet ce système va réagir au traitement et il faut donc prévoir, suivre et pouvoir le cas échéant contrôler cette réaction.

Si on dispose d'un modèle descriptif de l'ouvrage dans son environnement, il sera plus aisé de simuler l'effet d'un facteur exogène comme une modalité de traitement sur l'évolution du système. Dans le cas particulier des études de stabilité, le recours à des modèles de simulation numérique permet là encore de tester l'efficacité de différentes modalités possibles de renforcement ou de confortement et de faciliter les choix techniques.

A côté de l'aspect technique il convient de prendre en compte les aspects administratifs et financiers qui sont d'ailleurs souvent liés. Ils sont souvent assez complexes et différents selon les pays. Une harmonisation des procédures administratives de protection du patrimoine culturel est recherchée tant au niveau européen (c'est un des points du traité de Maastricht) qu'au niveau international (sous l'égide de l'UNESCO qui a déjà recensé les législations nationales en matière de protection des biens culturels et incite les pays les plus démunis d'un cadre législatif protecteur à faire évoluer leur système).

Mais devant l'importance du problème (en particulier dans les pays riches en éléments de patrimoine historique et culturel) le principal obstacle à la mise en oeuvre d'une politique de traitement des ouvrages et sites est un obstacle financier. On ne peut pas tout traiter et il faut donc faire des choix. Là encore le recours à l'analyse systémique peut permettre pour un ensemble de sites une approche coût/efficacité des traitements qui jointe à l'analyse de risques de chaque site donnera aux autorités compétentes des éléments de hiérarchisation des priorités. * * * CONCLUSION En conclusion nous avons voulu montrer que dans la description de ces trois phases d'une politique de protection du patrimoine historique (Diagnostic -Surveillance -Traitement) il était important : 1 ) de procéder à une analyse du système global constituée par le monument, l'ouvrage, le site et son contexte environnemental, 2) d'avoir recours dans la mesure du possible à des modèles déterministes de représentation du système permettant de simuler les influences des facteurs endogènes ou exogènes sur l'évolution du système, 3) de raisonner en terme de risques c'est-à-dire en évaluant de façon conjointe les probabilités d'évolution des facteurs de ruine et la gravité des conséquences qu'ils peuvent entraîner. 4) d'intégrer correctement la notion de temps à toutes les étapes (cinétique des évolutions, adaptation dans le temps de la surveillance, définition du moment opportun pour engager le traitement en fonction de l'urgence, priorités et délais de mise en oeuvre). LA SURVEILLANCE

• Quels endroits surveiller ?

• Quelles mesures faut-il mettre en oeuvre ?

• Comment faut-il les interpréter ?

• Que décider ? -Continuer la surveillance

  FIGURE n° 1

FIGURE

  FIGURE 2 (a)

  FIGURE 2b :

(

  FIGURE


Atefel Shabrawi

Thierry Verdel

Jack-Pierre Piguet

Sümmary

THE DISTINCT ELEMENT METHOD : A NEW WAY TO STUDY THE BEHAVIOUR OF ANCIENT MASONRY STRÜCTÜRES UNDER STATIC OR DYNAMIC LOADING BY THE USE OF NUMERICAL MODELLING

Numerical modelling methods are regularly used for the design of new buildings and sometimes, for the stability analysis of ancient buildings. But ancient buildings or monuments are much different from new constructions because of their blocky (stone) structure. This is why the Finite Element Method or any continuous modelling approach may be not suitable for their analysis. To represent the blocky structure of ancient monuments, we used the Disünct Element Method which allows the representation of all joints and blocks and is able to simulate their non-linear behaviour very easily. Furthermore, the method allows dynamic simulations in a very physical way, easy to understand. Consequenüy, we modelled the behaviour of a stone wall under different types of seismic loadings to demonstrate the efficiency of the method. The dynamic caiculadons we have run helped us to understand the Status of conservation of similar exisüng structures.

INTRODUCTION

Numerical modelling methods have reached a very high level of development during the last few years and gained a remarkable success in many fields. Recenüy, the Disünct Element Method (DEM) became one of the best techniques used for the analysis of fractured rock masses. Many attempts to study the stability of historical monuments such äs tombs, pylons and stone colossi, under static loads have been developed [START_REF] Verdel | Stability of the Colossi of Memnon in Egypt[END_REF]. [START_REF] Lemos | A Distinct Element Model for Dynarnic Analysis of Jointed Rock with Application to Dam Foundations and Fault Motion[END_REF] and Hart, Jing & Stephansson (1990) introduced the DEM in the field of dynamic modelling and applied it to study the dynamic behaviour of dams subjected to seismic loads äs well äs blasting in mines. In that field, the DEM is now also used in soil dynamics, specially for the analysis of non cohesive soils (represented by parücles assemblages) under dynamic excitation äs proved by the number of papers presented at the lOth International Conference on Earthquake Engineering, Madrid, 1993. Usually, the main difficulty in modelling an old masonry building is to take into account the non-linear behaviour of the masonry. Furthermore, the modeis dealing with this type of structures presently do not consider correctiy the presence of joints (mortar) between masonry blocks or cracks through the masonry. The main advantage of the DEM is to propose a representation of these discontinuities which has a physical sense.

In this paper, the authors are trying to emphasize the usefulness of the DEM for the dynamic analysis of ancient masonry structures such äs historical monuments. The main principles ofthe DEM are firstly exposed. Then, in order to quantify the influence of the mechanical characteristics of stones and mortars on the seisnüc response of an historical masonry building, nine modeis of a simple masonry wall have been tested. Each of them represents a particular Situation of the degradation of mortars and stones.

PRINCIPLES OF THE DISTINCT ELEMENT METHOD

General principles.

The DEM, firstly proposed by Cundall (1971), has been developed to studyjointed media subjected to quasi-static or dynamic loads. The medium studied by this method is simulated äs an assemblage of discrete blocks (rigid or deformable) which interact through comers and edge contacts (figure l). Disconünuiües are regarded äs boundary interactions between these blocks and the method utilizes an explicit time-stepping (dynamic) algorithm which allows large displacements, large rotations and general non-linear constitutive behaviour for both the matrix and the discontinuities. Furthermore, a complete detachment of blocks is possible and new contacts are automatically recognized.

The fundamental equations of the DEM.

The numerical formulation of the DEM was coded in a Computer program called UDEC (Universal Distinct Element Code), which takes into account the blocky System geometry, by considermg polyhedral blocks, The fundamental equations for the establishment of the calculations by this method are :

-the force-displacernent law which relates the forces developed at block contacts (or at gridpoints of deformable blocks) to the relative displacements. This constitutive relation may be linear-elastic or elastic-plastic under Coulomb failure criterion control;

Figure l. Modelling ofthe interaction between

Wo discrete blocks in the DEM.

Equation of motion

\ new forces and new stresses

-the motion equation which defines the moüon ofthe blocks. This equation allows the new kinetic quantities (accelerations, velocities and displacements of grid points) to be determined.

new velocities and new displacements \____

The solution procedure is explicit in the time domain. The two basics sets ofcaiculations corresponding to the execution of the caiculation scheme are illustrated on figure 2. The energy losses in a natural System modelled with UDEC can take two forms : in the case of quasi-static problems, an adaptive viscous (mass proportional) damping is used and automatically adjusted by the code to accelerate caiculations ; in the case of dynamic problems, damping is chosen more realistic and may be given a mass proportional and/or a stiffness proportional component. Such a two components damping is called the Rayieigh damping. The mass proportional damping has an effect similar to that of a block system drawn in a viscous fluid. The süffness proportional damping is physically equivalent to that produced by dashpots across contacts which damp the relative motion of blocks.

Constitutive relations

Representation of boundary conditions (modelling infinite and semi-infinite bodies).

In static analysis, fixed boundaries can be realistically placed at some distance from the region of interest. In dynamic problems, however, such boundary conditions cause the reflection of outward propagating waves back into the model and do not allow the necessary energy dissipation. The use of a larger model can minimize the problem, since material damping will absorb most of the energy of reflected waves, but such solution is computer-time consuming (Itasca,1991), In UDEC, two solutions are proposed to avoid this problem :

-viscous (non-reflecting) boundaries using the förmulation ofLysmer & Kuhlmeyer (1969), -free-field boundaries may be introduced in seismic analysis of surface and embedded structures where the seismic input is normally represented by a plane wave propagating upwards. In such case, the lateral boundary ofthe model must account for the free-field motion at those boundaries. The goal is to "enforce" the free-field motion in such a way that lateral boundaries retain their non-reflecting properties. In UDEC, this procedure involves the execution of one-dimensional free-field caiculations in parallel with the blocky system analysis. Furthermore, the lateral boundaries are coupled to the free-field grid by the mean ofthose viscous dashpots.

Simulation of dynamic loadings.

A dynamic loading in UDEC can be applied äs a velocity, force or stress time history. For simulating an earthquake with the code UDEC, there are two ways :

-introducing the recorded signal, -applying an harmonic loading history which can model either P-wave, S-wave or a combination of both.

Time-step optimiwtion.

The central difference scheme, used in UDEC to integrale the equations of motion, requires the use of a time-step (or time interval) which has to be chosen carefully because of the physical significance of time in dynamic analysis. UDEC provides a time-step optimizaüon depending on the stiffness of blocks andjoints, and the size ofzones (in case ofdeformable blocks). For most cases, this optimization insures the numerical stability of caiculations.

THE MAIN ADVANTAGES OF THE DEM IN MASONRY MODELLING

The main advantages of using the DEM in modelling historical structures are : -the representation of contacts, by the mean of a normal and shear stiffness, the behaviour ofwhich being controlled by available criteria (Mohr-Coulomb, tension cut-off, etc.), makes possible to model the assembly of stones bonded by mortars in a realistic manner ;

-the soil-structure interaction can be modelled in a physical way ; -the degradation of stones and mortars can be easily modelled by changing their mechanical characteristics;

-the method gives both staue and dynamic solutions ; -it is possible to study some reinforcement schemes by introducing blots elements or structural supporting System in the model;

-the method was firstly set up for 2-D problems analysis, but it has also been developed for 3-D problem analysis (Cundall, 1988) in order to describe more complicated geometries.

MASONRY STRUCTURES AND EARTHQUAKES

The observation ofearthquake damages into masonry structures shows that there are three main factors affecting the earthquake performance of masonry buildings : the Status or quality of mortars, the quality of workmanship, and the direction of the seismic waves. For the first two parameters, the modified Version of the Mercali Intensity Scale proposed by Richar (1958), classified masonry buildings into four categories, äs shown on table l.

In most cases, one or two diagonal cracks directions are also observed on masonry walls which have been subjected to earthquakes. Accordingly, the shear strength is critical for this type of buildings, äs well äs the cohesion and the tensile strength of mortars which play the role of bonding the masonry blocks together and may control sliding phenomena.

Table l. Classification of masonry buildings, [START_REF] Newmark | The Fundamentals ofEarthquake Engineering[END_REF]. In particular, much effort has been devoted to the definition of simplified structural modeis capable of representing the behaviour of masonry structures. The major part of this research was realized by either equivalent continuous or idealized (homogenized) modeis. On another hand, the recent tentatives for modelling the real behaviour of masonries äs an anisotropic and no tension material made by Pietruszczak 1991), have undergone many difficulties, especially in regard with the non-linearity.

Otherwise, because of the discontinuous nature of its modeis, the DEM offers a good opportunity to analyse the behaviour of blocky Systems, where mortar/blocks and blocks/ blocks interactions are simulated by spring slider Systems which allow the evaluation of shear and normal forces at contacts.

Case study l: a masonry wall.

We firstly tried to demonstrate the capacity of the DEM to model different masonry wall failure mechanisms according to the mechanical characteristics of mortars, and the type of seismic waves. A simple masonry wall subjected to dynamic loading in its plane wall modelled.

l. Construction ofthe modeis and model materials.

The geometry ofthe model represents a simple masonry wall which is 10 m high and 30 m long (figure 3). All the caiculations are carried out in plane stress conditions, and the loading is applied at the base of the wall.

As we know, the values of the elastic and shear moduli influence the dynamic properties of the masonry building. So, the choice of these moduli is very important to determine the response of structures which are known äs heterogeneous anisotropic media. The load-bearing capacity tests realized by Sheppard et al (1986), for different types of old masonry walls, including the walls in its original and strengthened state, indicate a wide ränge of expected values of mechanical properties. To accomplish our modeis in a realistic manner and because of the lack of experiments carried out on masonry buildings until now, it was decided to take the characteristics of the model tested by Tomazovic (1992) äs a reference. All chosen mechanical parameters are given in table 2.

Modal Analysis.

To test the capacity of the code UDEC to estimate the fundamental frequency and the modes of Vibration of the model, a modal analysis was realized separately using a finite element code. The fundamental frequency obtained in this analysis corresponds It must be noted that the modal analysis realized by the FEM didn't take into consideration the presence ofjoints and was carried out on a continuous model. This is why the analysis carried out with ÜDEC, have been realized with ficütious joints (joints having the same mechanical characteristics äs the blocks). The stress distribution äs well äs the state of the structure before the application of the seismic action influence the propagation of the seismic waves across the structure. Therefore, a preliminary stage of caiculation is necessary to caiculate the initial stress distribution in the model. This stage consists of a quasi-static gravity loading and uses an adaptive automatic damping. It is followed by the dynamic loading.

Modelling by UDEC : caiculation sequence.

The dynamic (or earthquake) loading consists of a harmonic wave propagating upward and applied at the base ofthe model. For this stage, viscous boundaries have been introduced at the base of the model to prevent the reflected wave to retum upward in the model.

Two modes of loading have been considered : a shear wave only and a combination of both. Amplitude, frequency and duration of the input loading are indicated on table [START_REF]UDEC 1.8 and FLAC 3.0 manuals[END_REF].

To proceed arealistic physical damping, Rayieigh damping was used and the critical damping ratio was given the value of 10 %.

Damage propagation andfailure mechanisms

The response of the model under the different types of dynamic loading is shown on figures 5 and 6. Each figure illustrates the deformation ofthe wall during the dynamic loading, at different time intervals. The deformation of blocks on figure 5 has been magnified to illustrate the movement of blocks more cleariy. These figures obviously show the performance of the DEM for modelling an assemblage of blocks under this type of loading.

Figure 5 shows the deformation of blocks under a shear wave loading. Induced relative horizontal displacements may be easily seen and the wave tends to shake the wall horizon-tally from left to right and from right to left. Because of the very high strength of the joints, there is no dislocation in the model.

Moreover, in the case of the combination of the two waves, figure 6 clearly shows the contribution of each waves. Here, both shear and compression waves have been given the same amplitude. The devastating action of this combination is characterized by two diagonal openings. This is a typical shear failure mode in which cracks or joints openings have usually a diagonal direcüon crossing the model with an angle of about 45° from the vertical direction.

The mechanical properties of the joints play a very important role regarding to the transmission of the waves through joints. To test the influence ofthese properties, a serie of caiculations has been carried out using different values for the joint cohesion and the joint tensile strength. Moreover, the deformability of blocks has also been considered for this analysis. The loading was only a shear wave applied at the base of the model. As shown on figure 7 (a) the absence of tensile strength at the joints (mortarbad adhered, masonry from category D) induces disorders in the masonry which is not moving anymore äs a single entity. Diagonal openings also appear (see the right side of figure 7 (a)).

In addition to that, for the case of no cohesion joints, the joints do not resist to any shear stress. Consequently a sliding of the first row of blocks is takirig place without affecting upper rows, figure 7 (b).

For the case where the joints have no tensile strength and no cohesion at the same time, figure 7 (c Parametric studies on dynamic discontinuous modeis carried out with UDEC, exhibit the complexity of such analysis. Many parameters have to be taken into consideration such äs the mechanical characteristics of blocks and joints, the boundary conditions, the damping parameters, the time step, the amplitude, frequency, duration and type of dynamic loading, etc.

Case study 2 : a masonry side wall.

Earthquake or ground shaking causes dynamic movements in all three dimensions ofthe wall. In considering the side cross-section ofthe wall modelled so far, an attempt to find out the effects ofthe same shaking forces in another direction has been carried out.

The geometry ofthe model is very simple (figure 8 The response of the model to the applied shear wave is illustraied on figure 9. The scenario of the loading begins with the movement ofthe base of the model towards the left.

It involves sliding because of the absence of joint cohesion. Consequently, the bottom ofthe model is becoming completely dislocated. Then the movement of the base driven to the right (after the change of the wave induced displacements) leads to the collapse of the upper blocks (äs shown at the right side of figure 9). This mode of collapse is very familiär to the tall buildings (chemineys, columns, etc.), where the horizontal forces induce a horizontal displacement relatively large compared with the width of the structure. In that case, the gravity centre of the structure is not anymore located over the base and, consequently, the toppling of the superstructure occurs.

CONCLÜDING REMARKS

On the basis of the results so far discussed, the following comments can be made : -after using the DEM in modelling masonry structures, we found that this method may considerably improve investigaüons on the stability assessment of such structures. The concept of interacting deformable or rigid blocks separated by joints which may be given realistic deformaüon properties seems to be very well adapted to the study of ancient masonry structures (wall, arches, columns) äs well äs rock block constructions crossed by mechanical cracks or fissures (such äs Memnon colossi in Egypt). The ability ofthe DEM and specially of UDEC to take into account the typical non-linear behaviour of such structures is of a very high importance in the analysis of ancient monuments. So, the DEM is supposed to answer to many engineers and researchers who always face this problem in using finite element codes ;

-the demonstrative examples showed how the responses ofthe model can differ according to many factors such äs the type of the seismic waves and the mechanical characteristics of the mortars or stones. The obtained results have emphasized also the importance of the mechanical properties of the mortar äs a significant factor determining the mode of failure of masonry structures.

Figure 2 . 1 .

 21 Figure 2. Time marching scheine for solving equations in DEM.

  During the last few years, extensive research works have been carried out on the analysis ofthe seismic response of masonry buildings (seeClough 1975[START_REF] Tomazevic | Dynamic modelling of masonry buildings : storey mechanism model äs a simple alternative[END_REF], Gentil 1991).

Figure 3 .

 3 Figure 3. The geometry ofthe model.

  perfectiy to the one caiculated by UDEC (estimated by reproducing undamped oscillations under sudden gravity loading). Furthermore, the shape ofthe fundamental modes of Vibration of the model, obtained by the FEM, are shown on the figure 4. As can be seen, the caiculated fundamental mode of Vibration reflects a monolithic behaviour of the wall.

Figure 4 ,

 4 Figure 4, Theßrst three mode shapes ofthe model detremined by afinite elemnt modal analysis.

Figure 5 .

 5 Figure 5. Effects ofa shear wave only

  ), the damages are more obvious man on previous modeis. The blocks are completely detached at the lateral sides of the model. A sliding has been observed between the first row of blocks and the upper part of model (see the vertical joints ofthe first row of blocks which became aligned). Diagonal open-y^re 6. Effects ofa combination ofS-and P-waves ings are also observed. Finally, for the case where the joints have no tensile strength, no cohesion and where the stones have a Young modulus hundred times less than the last cases, the damages are observed in the form of the opening ofvertical joints almost every where, especially at the lateral boundaries of the model where the joints have the maximum shear displacements figure 7 (d).

Figure 7 . 1 Figure 8 .

 718 Figure 7. Response ofthe model for different mechanical properties of joints.

Figure 9 .

 9 Figure 9. Scenario offailure ofthe side • wall section, arrows indicate the amplitude and the direction of the velocity.

Table 2 .

 2 Mechanical and dynamical properties.

-the shear failure modes of the wall (diagonal openings ofjoints or diagonal cracks) are obviously observed för the walls built with mortars having no tension strength and also for those having no cohesion (very degraded mortars). This is a typical case encountered in many old masonry buildings.