Fire and Gas Mapping Study

Prior to the release of a risk-based standard for the design of Fire & Gas System (FGS) in 2010, designs were traditionally implemented using rules of thumb and engineering heuristics. However, they often suffered from the main flaw which was that FGS were often unable to detect hazards due to an insufficient number of or poorly located detectors due to the lack of rigorous methods for evaluating coverage of detector arrays.

The F&G mapping study using a commercial Modelling Software is so far a scientific way of locating the detectors adopted by Major Oil Companies, to ensure that the current F&G arrangement is good enough to perform their function assuming that associated instrument has been properly selected and installed.

FGS typically reduces the magnitude and severity of the consequence instead of eliminating it. Therefore, FGS ineffectiveness is directly related to the inability of the mitigation elements (e.g., fire water system, ventilation system) to perform their functions with a high probability of success.

F&G mapping study is usually conducted to ensure effective design and implementation of the F&G system, which actually plays an important role to prevent the severe consequences from an initial event (e.g loss of containment). If a FGS is properly designed, then:

• It helps in identifying the leakage at an early stage and acts as an optimization tool to fix up the quantity of the detector.
• With the installation of Hydrocarbon Gas Detection System, the risk is considerably reduced to As Low as Reasonably Practicable (ALARP) level.

• Normally, the F&G mapping study should be conducted in the design stage to assure that the detector coverage targets are met and the current F&G arrangement is effective to fulfill its intended designed function.Moreover, during the normal operation, the F&G mapping study should be performed when major changes or modifications are applied to the facilities. As a result, the study shall confirm whether the current arrangement remained suitable for continuous operating. If it is not the case, a set of recommendations shall be provided and considered to improve the FGS performance.

A typical F&G mapping study is conducted as per the following steps:

• Step 1: Identification of leak scenario;
• Step 2: HC dispersion modelling using in-house spreadsheet or software;
• Step 3: Evaluation of HC gas detector arrangement;
• Step 4: Optimization of HC gas detector quantity and arrangement (the coverage targets are achieved with the minimum number of detectors);
• Step 5: Modify the detector arrangement as per step 4.

A 3D simulation software (e.g Detect 3D) is usually used to perform this F&G mapping study take into account the obstruction of equipment, structure. A F&G mapping study is mainly based on the comparison of detector coverage versus the coverage target.

Detector Geographic Coverage – is generally defined as the fraction of the geometric area of a defined monitored process area that would be detected by the detector considering the defined voting arrangement (if a release were to occur) in a given geographic location.

Detector (Scenario) Coverage – For each defined release scenario, an area of gas dispersion is defined. Detector Scenario Coverage is defined as the fraction of the area that can be detected by the detector considering the defined voting arrangement in the area of gas dispersion.

This ISA technical report describes the analysis that should be undertaken and the effectiveness criteria that should be specified when an FGS is implemented.