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Hazardex 2017 Conference - Multiphysics simulations applied to industrial hazard assessment

Author : Amita Tripathi - Fluidyn France

18 November 2016

The increasing capabilities of computers and the development of efficient solvers have finally allowed numerical simulations, and specifically multiphysics simulations (fluids, structures, and heat coupling) to show its full potential in risk assessment and mitigation strategies on industrial sites. 

Indeed, 3D modelling based on Computational Fluid Dynamics (CFD) tools can provide a valuable input both in the design phase to ensure that the layout does not favour any avoidable hazard and in the risk assessment phase to provide inputs to the quantitative risk and emergency system survivability assessment.

Hazard consequences need to be studied for the distance at risks, for the resistance and design analysis of structures submitted to thermal and pressure loads, for the inclusion of mitigations solutions and ultimately for the preparation of emergency plans (evacuation, confinement).

Very often, the sequence of hazardous events leading to a major accident can be described as:
•  Gaseous emission and/or pool formation and evaporation
•  Dispersion of toxic and/or flammable gas
•  Jet or pool fire
•  Deflagration and/or detonation of the flammable gas
•  Structural breakdown due to the overpressure / heat wave

Indeed, to model the transport of the toxic and/or flammable gas by convection (both forced and natural) and diffusion with a detailed analysis of the flammable cloud (concentrations between the flammability limits), toxicity levels and doses, or even visibility inside the plume can be carried out, provided the model includes a good atmospheric boundary layer, a good evaluation of the local mechanical and thermal turbulence induced by the structures and processes, an accurate description of high-momentum jets, if any.

In a worst-case chain of events, the flammable gas, once released, can be ignited. The difference between a jet fire and a deflagration is mainly due to the ignition. If the ignition is immediate and occurs right at emission, a jet fire will occur and the gas will be depleted at the origin of its emission. If the ignition is delayed and the flammable cloud has the time to drift, a deflagration, or even a detonation (depending on the ignition strength and the confinement of the cloud) will occur.

These congested environments are also the primary location where an explosion could occur, first by providing ignition points and secondly, by accelerating the flame until it reaches deflagration speed. When assessing the risk associated with the accidental dispersion of a flammable gas, the consequences of a deflagration on the platform and its effects on the integrity of the entire structure have to be faced.

The need of numerical tool lies in the simulation of the mechanical and thermal interaction between fluid (in which the pressure wave will propagate) and the structure (in which the stresses will be built at the wave passage). Fast, transient phenomena involving compressibility of the air are efficiently solved by the innovative strong coupling between the finite element method for the structural computation and the finite volume method for the fluid dynamics method.

Examples of CFD modelling in design and QRA assessments will be provided for the precise analysis of effects of leaks and consequences (toxicity, fire, and explosion) on both structural design and distances at risk.

About the author:

Amita Tripathi is Technical Director at FLUIDYN France, which offers modelling software and consultancy services in Computational Fluid Dynamics and related areas such as stress analysis, heat transfer, acoustics, magneto-hydrodynamics, electro-chemistry and rarified gas dynamics.

These are used to optimise industrial processes and provide environmental risk solutions such as accidental leak modelling, air quality modelling, fire & explosion modelling and real-time monitoring and forecasting for accidental leaks.

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