The hazard ranking procedure is a methodology which was developed by Kenexis for internal use and has been made available publicly to assist Effigy users in performance target selection. This is the same hazard ranking procedure that Kenexis uses for the majority of the projects we execute. On occasion we are asked to comply with end user FGS philosophy documents which require performance target selection to be performed using some other method but these projects are rare. Many of our clients have adopting the hazard ranking procedure as their preferred method for performing Fire and Gas Mapping studies.
The following is a background of the hazard ranking procedure:
Currently the most comprehensive standard/guidance on Fire and Gas Mapping is the ISA 84.00.07 Technical Report – Guidance of the Evaluation of Fire and Gas System Effectiveness. This technical report, as it is currently written, promotes the use a performance based (risk based) method for determining performance targets but does not specify how this should be done. The technical report is currently being revised by the TR committee as the latest release was in December 2010 and there have been may advances in fire and gas mapping since then, one of these advances being the development of the hazard ranking procedure. We are expecting the next release of the TR will contain an appendix with a recommended semi-quantitative method for performance target selection. This appendix will contain the hazard ranking procedure.
With regards to the basis of the hazard ranking procedure. In the early days of fire and gas mapping Kenexis was performing all studies using a fully quantitative method for selecting performance targets. Performing the analysis this way requires the development of consequence event trees for each potential leak source similar to figure 1 below.
Figure 1: Consequence Event Trees
While performing the analysis in this way will result in a very complete understanding of the hazards and the risk, it is extremely time consuming. The event tree above is for a collection of production separators. One or more of these event trees would need to be developed for each major equipment item (or collection of identical equipment items). For each equipment item the following work would need to be performed:
- Three Ignition Probabilities (early ignition, late ignition & late ignition w/ activation of the gas detection system)
- The consequence severity of up to 11 unique outcomes (Estimating the consequence severity requires that dispersion and/or fire modeling be performed – and in many cases will require a facilitated meeting similar to LOPA to discuss the consequence severity of the event).
- Personnel Occupancy for each of the 11 unique outcomes.
Performing this type of analysis over an entire offshore platform gets extremely time consuming and costly. However, from experience performing projects in this manner, we were able to start predicting the performance targets that a fully quantitative method would produce before we began developing the event trees. This realization is what lead to the development of the hazard ranking procedure. What was found is that for the vast majority of applications the performance targets could be predicted based on a number of inputs. These inputs are listed below:
- Equipment Type (Which effects the frequency of a leak)
- Occupancy in the Immediate Area
- Potential Ignition Sources in the Area
- Stability of the process fluid (gas, volatile liquid, stable liquid)
- Process Pressure
- Level of Confinement and Congestion in the area (this impacts the likelihood or a VCE if a gas cloud ignited)
- Toxic Concentration (if process contains H2S or other toxics)
The items listed above are all of the inputs to the hazard ranking method. After identifying these as the key inputs that impact the performance requirements for the fire and gas system, the process of calibrating the ranking criteria such that tolerable risk goals could be achieved formed. The process of calibration involved performing the fully quantitate method for a large population of “typical” environments found in a chemical process. The performance targets required from the fully quantitate method were then compared against the performance targets produced by the hazard ranking procedure. If the fully quantitative method produced higher performance targets, the hazard ranking procedure was modified (or “Calibrated”) to ensure that the results will always error on the conservative side (i.e. hazard ranking procedure will produce performance targets >= the fully quantitative method).