A Consequence Based Assessment Method/Software Model To Facilitate The Safe Location and Design Of Occupied Buildings On Or In Close Proximity To Process Facilities

Major accidents such as Flixborough (UK) and Union Carbide Plant (India) in the early 70’s/80’s, to Hickson & Welsh (UK), Phillips Pasadena plant (USA) and TOTAL La Mede Refinery (France) in the 90’s and more recently BP Texas City in the 2000’s have and continue to highlight the importance of ensuring the safe location and design of buildings on or in close proximity to process facilities as they often represent a key source of harm (often fatal) to occupants when things go wrong (for example following building collapse due to blast overpressure). The vast range of technical standards (e.g. API 752/753), technical papers, guidelines and scientific research available on this issue underscores the importance of this activity.

Various methods exist to inform the overarching goal of ensuring the safety of people within buildings. These range from “Simplified methods” (e.g. lookup tables that codify industry experience e.g. Dow’s Fire & Explosion Index) to “Consequence based techniques” (i.e. consideration of the magnitude of the consequences with no explicit quantification of the likelihood of the hazardous event) to “Risk based approaches” (full consideration of both the likelihood and impact of an event e.g. using exceedance curves/hazard frequency contours). It should be noted that the above methods are not mutually exclusive and are sometimes used in conjunction with each other or in succession.

“Consequence based assessment methods” are the focus of this paper which examines recent advances in a consequence analysis software tool developed by DNV Software—namely “Phast”. The key advancement to the Phast Software package centres on the addition of advanced explosion modelling capability that considers the interaction of a flammable cloud profile with congested regions of process plants in all three dimensions (length, width and height) according to wind conditions to characterize explosion potential (using the “Baker Strehlow Tang” or “TNO Multi-Energy” explosion models) and derive blast loads (overpressure, impulse, pulse duration etc.). A second key innovation is the capability to plot combined hazard contours to a defined threat level (e.g. 0.1 barg overpressure) for a range of scenarios on a facility layout diagram. This feature allows for clearer understanding and communication of hazards ranges and their impact footprint as well as makes for more efficient and comprehensive analysis - the full range of flammable release scenarios (as opposed to a limited subset) can be modelled simultaneously. Together, these advances allow for a more detailed assessment of hazards to occupied buildings from a consequence based perspective and thus enables a more robust and rigorous consequence based assessment.

A case study of the developments as applied to a real-life occupied building siting challenge is presented and the value of using the tool discussed. An extended discourse on how the software package can be used to aid the safe location and design of occupied buildings is also outlined.