Consequence assessment tool - how significant for risk assessment

There are a number of tools and techniques available for process hazards consequence assessment which includes models for manual use and computer codes. Many of the software programs use the Gaussian dispersion or similar algorithms to evaluate gas dispersion. These tools are often developed in order to assess a particular application (passive gas, dense gas, or specific chemical); some of them are further developed and modified to extend the range of usability.

Mostly, commercially marketed software models are validated using experimental field test, but often the results are far from the real scenarios experienced. The main reason being the limited range of application; in most process hazard cases, the materials involved are not a single, pure material and the effects of obstructions close to the release or incident site are not included in the model.

In general, for consequence assessment, using software tools some of the challenges are:
- Modelling the effects of multi-component material with varying physical & chemical properties
- Incorporating the effects of terrain, barriers, slops, structures and buildings
- Changes in weather condition during the release
- Near and far field effects

In order to overcome these challenges, the computational fluid dynamics technique which uses the physical formula was considered as an alternative to more conventional models. This paper looks into the modelling of dispersion characteristics using computational fluid dynamics around a built up area of a chemical plant and compares the results obtained using Gaussian algorithm. This is based on a consequence assessment study performed for a loss of containment event within a tank bund.

The CFD tool used is fluidyn-PANEPR and the tool used with gas dispersion algorithm is DNV PHAST.


The poster based on the above abstract was shown for ChemEng08 and is viewable from the following link:
http://myaker.net/_upl/icheme_poster_sreeraj.pdf

Acknowledgements: To co-author/contributor Mr.Ken Norrie

Risk perception

The term ‘risk’ is a term used quite often and in day to day life. Risk is defined and represented in different ways based on the associated activity and its nature. Risk consists of a combination of the probability of a perceived threat or opportunity occurring and the magnitude of its impact on objectives.

In process and manufacturing industries (not exclusive), most commonly, risk is assessed, analysed and expressed in terms of individual risk and societal risk. The risk estimation and analysis is to ensure that the risk an industrial activity is not resulting in intolerable levels.

Risk is perceived in terms of the severity from an event and its frequency. The fact to be noted is that those events with minimal severity but frequent posses similar risk to those events with maximum severity but not that frequent. The everyday activities like smoking (including passive) and driving (including being on road or on a means of transport) also could result in fatality and keep public at higher risk. These events may not be often classified as severe but happen quite frequently.

Then there are near-to-known or unknown treats like dust explosions and exposure to asbestos. The risk from such activities is seldom assessed due to lack of knowledge on the potential consequence. This raises the question on perceiving risk from potential major incidents. Even though the severity is estimated and well understood, it is often neglected considering as not likely.

An example: Catastrophic failure of a hazardous material storage tank could result in multiple fatalities and the recorded failure frequency is one in a million years (1 x 10-6 /yr). There are more than a million tanks which could result in similar catastrophe; which means, there will be a catastrophic failure of tank every year. The tank could be the tank on your way to work, which supply the fuel to the garage from where you re-fill, the site where you or dear one works, the near to school or hospital. And this is happening, yes every year there is a major incident associated with storage tanks.