(44m) Using Explicit Finite Element Analysis to Simulate the Effects of External Chemical Explosions On Single and Double-Walled Storage Tanks

Accurately simulating the overpressure or shock wave associated with a given far-field chemical explosion is extremely valuable in assessing the structural response and possible failure modes of critical process equipment such as storage tanks, piping, or pressure vessels.  Furthermore, assessing the potential damage from an overpressure wave can provide valuable information about protecting structures from external explosions and improving designs that advocate blast damage mitigation.  Such analyses become all the more important should the contents of the tank or vessel exposed to such a potentially catastrophic event pose a risk to humans or the surrounding environment.  This paper discusses the underlying theory and examines the practical application of multiple finite element based explicit computational techniques for simulating the load acting on a structure due to an external blast.  Explicit three-dimensional blast analysis of single and double-walled storage tanks that carry an extremely high consequence of failure is performed.   The structural response of the tanks due to postulated accidental explosions is investigated and likely failure modes are discussed.  

The two explicit computational blast loading methods discussed in this paper are the incident wave loading model and the Conventional Weapons Effects Blast Loading Model or CONWEP.  The CONWEP model is appropriate for detonations of conventional explosions, and in this case automates many crucial features of analysis, such as developing the overpressure time history (positive and negative phases), defining overpressure spatial decay, and accounting for spatially varying reflection effects.  Large vapor cloud explosions are known to behave very differently than conventional explosives, and for these cases, the more manual, but also general, incident wave loading approach permits the user to define their own time-varying overpressure amplitude.  Additionally, this method also allows for coupled structural-acoustic analysis (should reflection effects need to be considered in detail).  While it is not necessary to model the fluid medium using acoustic elements when using the incident wave loading model, it does becomes essential if reflection effects dominate the response of the given problem.  If the fluid medium is modeled (air in this case), the acoustic elements can be coupled to the structural elements and the loading surface becomes the boundary of the acoustic mesh closest to the blast source.  In the case of large, three-dimensional models, utilizing acoustic elements becomes impractical due to computational efficiency limitations.  Three-dimensional examples of all of the above blast loading approaches are presented in this paper; simple incident wave, incident wave with coupled structural-acoustic analysis, and CONWEP.  Comparisons are made between these methods and the corresponding computational results. 

Furthermore, commentary on the structural response and likely failure modes of the storage tanks is provided and a discussion regarding design features that supplement blast damage mitigation and process safety is rendered.  The advanced computational methods discussed herein permit realistic evaluation of complex, three-dimensional process equipment subjected to accidental explosions.  Simulating and understanding the possible failure modes due to blast loading of critical process equipment in the petrochemical and related industries is very beneficial in not only improving process safety, but also providing plant operators and safety personnel with crucial information regarding the possibility of equipment failure should a catastrophic explosion occur.