Flammable Gas Dispersion Calculation and Map Layers

Understanding how flammable gases disperse following a subsea release is critical for offshore risk assessment. Dropulyzer now includes advanced gas dispersion modelling that calculates hazard zones and visualises them directly on the interactive map, enabling users to identify areas where gas concentrations may reach dangerous levels under various weather conditions.

How Gas Dispersion Modelling Works

When hydrocarbons are released from a damaged subsea pipeline or equipment, the gas rises through the water column and surfaces as a pool. As this gas enters the atmosphere, it disperses downwind according to meteorological conditions. Dropulyzer uses the industry-standard Gaussian Plume Model with Briggs (1973) rural dispersion coefficients to calculate how far the gas cloud extends before falling below hazardous concentration levels.

The calculation determines two critical boundaries:

• Lower Flammability Limit (LFL) - The minimum gas concentration at which ignition is possible. Beyond this distance, the gas is too diluted to burn.
• 50% LFL - A more conservative threshold representing half the flammability limit, providing an additional safety buffer for personnel and operations.

Key Factors Influencing Dispersion

The extent of gas dispersion depends on several factors that users configure within their study:

Release Characteristics

• Mass flow rate from the damaged target
• Fluid properties including Lower Flammability Limit (LFL) and molar mass
• Leak size and severity category

Weather Scenarios

• Wind speed - higher speeds disperse the gas more quickly but carry it further
• Air temperature - affects gas density and buoyancy
• Atmospheric stability class (A through F) - determines vertical mixing intensity

Receptor Height

Users can specify the height above ground at which concentrations are evaluated, typically set to 1.5 metres to represent breathing zone height for personnel.

Multi-Scenario Analysis

Dropulyzer automatically calculates dispersion distances for every combination of release and weather scenario in your study. This comprehensive approach allows you to understand how hazard zones change under different meteorological conditions - from calm, stable atmospheres to windy, well-mixed conditions.

"By modelling dispersion across multiple weather scenarios, operators can identify worst-case conditions and plan appropriate safety measures for the full range of expected atmospheric conditions."

Visualising Hazard Zones on the Map

Once dispersion calculations are complete, Dropulyzer generates dedicated map layers for each release-weather combination. These layers display the hazard zones as concentric polygons around the affected target:

• Inner Zone (50% LFL) - Represents the higher concentration area closer to the release point, displayed with a darker fill.
• Outer Zone (LFL) - Shows the full extent where flammable concentrations exist, displayed with a lighter fill.

The visualisation adapts to different target types - creating corridor-style zones around linear targets such as pipelines, and expanded polygons around equipment installations. Users can toggle individual layers on and off to compare different scenarios or focus on specific releases.

Practical Applications

The gas dispersion feature supports several important risk assessment activities:

• Identifying safe zones for personnel during emergency response
• Planning ignition source exclusion zones around potential release points
• Evaluating the impact of weather variability on hazard extent
• Documenting compliance with safety standards and regulations
• Supporting emergency response planning and evacuation procedures

Scientific Foundation

The gas dispersion calculations in Dropulyzer are based on well-established scientific methods referenced in regulatory guidance documents:

• EPA/NOAA ALOHA Technical Documentation
• Turner's Workbook of Atmospheric Dispersion Estimates (EPA-454/R-92-002)
• Briggs (1973) Diffusion Estimation for Small Emissions

Conclusion

The flammable gas dispersion feature provides a powerful tool for understanding atmospheric hazards from subsea releases. By combining rigorous scientific modelling with intuitive map-based visualisation, Dropulyzer enables risk practitioners to communicate hazard zones effectively and make informed decisions about safety measures. The ability to analyse multiple weather scenarios ensures that assessments capture the full range of potential conditions, supporting robust and defensible risk management.

To use this feature, ensure your study includes active releases with defined leak rates and fluid properties, along with weather scenarios specifying wind speed, temperature, and stability class. The dispersion layers will automatically appear in the map view after running calculations.