Fire and gas detection mapping study

Fire and Gas (F&G) Mapping Study

The step by step approach for F&G Mapping study is given below:

Document the process plant

This involves building up a model which gives the process data (vessel and pipe substances, pressures and temperatures and locations). This can be the same data that is used for the risk assessment. To this must be added vessel elevations and heights, and inter unit pipe routing, to allow for full 3D modelling. These are transferred to the GIS model for the analysis.

Document the fire and gas detection system

This involves defining the location, direction (for the fire detectors), path (for open path detectors), and detection thresholds (from the instrument specifications). These are transferred to the GIS model for the analysis.

Assessment of detectability

For gas cloud, a check is made to determine whether it is detected by one, two or more detectors. For fires, a check is made to determine whether each fire is within the detection zone of one or more detectors.

Sensitivity study

The impact of moving, adding and removing sensors is investigated for the more likely design improvements.

Plot the detector coverage zone

The range and aperture angle of the detectors is obtained from manufacturer specifications and is plotted on a 3D plot of the plant.

1. What is Fire and Gas (F&G) mapping study?

Prior to the release of a risk-based standard for the design of Fire and 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.

1. Why F&G mapping study?

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.

1. When F&G mapping study is needed?

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.

1. How to conduct a F&G mapping study

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.

What is fire and gas mapping study?

What is F&G mapping?

What is gas mapping?

What are the two types of gas detection system?