En - Blasting Simulation Drives Plants Design for Safer Workplaces

8/18/2019 En - Blasting Simulation Drives Plants Design for Safer Workplaces

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lasting Simulation drives plantsdesign for safer workplaces

IntroductionLS-DYNA(originated from the 3D FEA program DYNA3D),developed by Dr. John 0. Hallquist at LawrenceLivermore NationalLaboratory LLNL)in 1976. DYNA3D wascreated in order to simulatethe impact of theFull Fusing Option FUFO)or Dial-a-yield nuclearbomb for lowaltitude release (impact velocity of - 40 m/s). At thetime, no 3D software was available for simulating impact, and 20software was inadequate. Though the FUFO bomb was eventuallycanceled, development of DYNA3Dcontinued. DYNA3Dusedexplicit time integration to study nonlinear dynamic problems, withthe original applications being mostly stress analysis of structuresundergoing various types of impacts.In the recent decades , LS-DYNA isuniversally knownas a referencesolution to solve blast, vehicles IED and mines), and home landsecurity problems.

Inthe last fewyears,EnginSoft has extended the use of thecode to civiland industrial applications; blasting simulation is a key solution in

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Fig - L Eoverpressure validation

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order to improve the safety of plants,workshops, off-shore platforms andin general, all buildings/rooms withexplosion risks.New features are continuouslyimplemented in LS-DYNAfor fasterand reliable blasting simulation. Inthis article an application examplewillbe illustrated.

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1. Blasting modelling methods Fg 2 - L Epress ure dstributionDifferentmethods are implemented on the t rget surfacein LS-DYNA.These methods can be applied in different domainsand coupled during the solving process. Turin Polytechnic and

EnginSoft have validated all the approaches by comparing thenumerical solutions with the analytical/experimental ones.

1.1. Load _Blast_ Enhanced LBE)The first method used for simulating a blast wave with LS-DYNAis called Load_Blast_Enhanced LBE).It is based on the CONWEPfunction and allows the user to simulate bursts using an analyticalformulation to include the distance from the center of the burstand the amount of the explosive used, through a parameter calledreduced distance . This parameter is the ratio betweenthe distance

from the explosive charge and the TNTweight (inkg), the latter beingraised to thepower of 1/3. The algorithm is based on the equivalent

TNTmethod; indeed, several kinds of explosive can be simulatedby using an equivalent amount of TNT andappropriate conversionfactors. This method allows the simulation of differentkind of bursts:

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which is not modeled by using finite elements, the shock wave issimulated with the LBEmethod, while for the modeled domain , theALE_MULTl_MATERIAL methodis employed .To better understand this technique, consider the configurationshownin the figure 7: the explosive is situated at a ong distance from thetarget; furthermore the target is not placed within the direct field ofview of thedetonation point. Thepressure pulses reach thetarget after

reflecting on the surrounding structures. If thesimulation were justbased on the MM_ALEsolver, the huge external environment, whichalso includes a quarter of the explosive domain, should have beentaken into account within the Euler-type mesh modeling process.But proceeding according to the presented mixed method, the twohighlighted surfaces of the smaller Euler-type domain are processedby the LBEsolver, while the reflected wavesare processed by theMM ALEsolver.To simulate the air flow moved by the shock wave within the ALEdomain, some receptor solid elements have been employed ; theyhave thetask ofmeasuring the shock wave load which is computedby the LBEnumeric method . At a ater stage, they propagate within

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Fig8 - LBEvtM_ LETimesequence ofpressure contour

the rest ofthe air domain which, in turn, is also modeled with solidelements . The receptor elements and the air domain make uptheMulti-MaterialDomain.As mentioned before, EnginSoft is using numerical modelsextensively in investigating the blast effects for the assessment of

industrial and civilstructures.The numerical approach allows:

• to determine the blasting effects on the surrounding structureswithout experimental tests that are difficult or impossible toperform;

• to evaluate different planVlaboratory map from the safetypoint ofview spending mainly CPU time.

• to evaluatedifferentmitigationstrategies includingreinforcements,openings, collapsible windows, absorbers, etc etc

It is important to note that the physical phenomena is very complex

and it is almost impossible to define an effective mitigationstrategywithout the contribution of the numerical simulation.For confidential reasons, a simplifiedmodel that represent the real

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MM -ALE

Fig 7 - ouplingschemeof the mixed methodscenario

OVERP RESSURE

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Fig9 - Typical trend ofthe overpressure

by analogy has been defined. The simulation methodologydevelopment and application involved EnginSoft, the TurinPolytechnic and other customers that wantto remain anonymous .

2.1 Phenomena descriptionFrom the theoretical viewpoint, the explosion is an unexpected andaggressive emission of mechanical, chemical or nuclear energyusually with production of high-temperature and high-pressuregases.

Fg O Pressure distributionin the erased room

Such gases spreadin thesurrounding environment as a shock wavewhich in the absence of obstacles, expands like a spherical surfacecentered in the explosion point.Whena shock wave encounters an obstacle, the force is greater ifmore surface is exposed and ifthe distance from the explosion point isshorter. In fact, the pressure peak decreases whenthe distance from

the explosion center increases. Considering the variation of pressurein time, fixed in a point in space, it changes with an exponential lawachieving two load stage: he firstone is positive due to overpressure

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Fig 11 - Pressuredistributionin the room withmitiga/ionwindows

while the second one is negative due to the depression caused bythe explosion winds (figure9 . It is mandatory for the realization ofamodel to investigate the behavior of the structures under the effectsof dynamics loads with high intensity and short duration, which arethe effects generated by the explosions.

Fig 12 - Steel safety wails

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Fig 13 - Comparison of the pressure peak for different configurations

The problem is complex and can be interpreted in three basicprospective:

• Dynamics load evaluation due to the explosion and applicationon a structural element;

• Mechanical characterization of thestructure behavior when it isexposed to high-intensitydynamics loads;

• Identification and implementation, through the use of thecomputer, of a numerical analysis method for the description

and solution of the physical and mechanical problem.

The first investigates the scenario when an explosion occurs in a

41 - Newsletter EnginSoftYear 12 n°3

restricted, closed environment (figure 10).This model represents several industrial incidents: explosion of atank containing gas and/or chemical reagents, hydraulic systemrupture under the effect of extreme pressure, storehouse explosion,etc etc.The simulation has been done with a spherical TNTcharge explosionin a restricted surrounding, to study which methods can reduce the

pressure wave intensity and preserve the boundary walls integrity.The numerical method used to perform this simulation has been:Multi-Material-Arbitrary-Lagrange-Eulerian.

The pictures show the pressure contour as function of thedistancefrom detonation center. In this first run, the walls of the box are rigid.Thewalls strength (reinforced concrete) is inadequate to resist to thepressure peaks.

Thefirst investigated strategy to decrease the pressure level has beenthe realization of some openings on oneside of the room (figure 11 .Subsequently, the pressure level with mitigation windows andsteel safety walls have been compared. In a first time these wallsare considered as rigid in order to verify the contribution and, in asecond time, as deformable with steel rupture modelling (figure12 .

2 2 Results discussion and conclusion

The figure 13 shows the pressure peak on the room wall for thedifferent configurations. As can be noticed, the mitigationstrategiesdetermine a ower wall stresses due to the wall pressure reduction:

B- Closed Box: This run corresponds to the higher measuredpressure . The wall room that, in the realscenario are in reinforcedconcrete. are not able to contain the explosion and the structurecollapses .A- Box with openings: This model correspond to the installationof collapsible windows on the wall and on the roof of the room.Obviously, it is possible to insert windows only on the buildingperimeter the blast wavescan escape incircumscribed external area).D-Rigid mitigation walls: this case has not a real correspondence,but is important to preliminarilyunderstand the internal wall effectand investigate different architectures (number of walls, shapes,distances, heights, materials, etc etc) .C- Deformable mitigation walls: Simulating different model, it has

been learnthat ifthe mitigationwalIs strength is too high, the blastingwave overload the room wall. In agreement with the customer, thefinal protective structure has been designed to absorb energy despiteof its integrity.The priorityis to preserve the externalwalI n order toprotect workers in adjacent rooms.In this case, it doesn't exists a protective structure, made oftraditional materials (concrete and/or steel) able to absorb energy

without rupture and collapse risks; this means, from the safety rulespoint of view, that has to be forbidden to stay in the room if thesystem works.

E Cestino G FruitaTurinPolitecnico

G. Bolla A OrtaldaEnginSoft

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En - Blasting Simulation Drives Plants Design for Safer Workplaces