Dispersion refers to the process by which particles, gases, or pollutants spread out from a source into the surrounding environment. This can occur in various mediums, such as air, water, or soil, and is influenced by factors like wind, temperature, and topography. In the context of air pollution, dispersion modelling is used to predict how pollutants will spread and dilute in the atmosphere, helping to assess their impact on air quality and public health

Air Dispersion Modelling study is a mathematical simulation governing the transport, dispersion and transformation of pollutants in the atmosphere.

Air Dispersion Modelling is a means of estimating downwind air pollution concentrations given information about the pollutant emissions and nature of the atmosphere.

Objective

The objectives of Air Dispersion Modelling study are as follows.

• Predict the Ground Level Concentration (GLC) of pollutants.
• Assess potential environmental and health effects from atmospheric pollutants

Methodology

The process of air dispersion modelling contains four stages (data input, dispersion calculations, deriving concentrations, and analysis).

The most used dispersion models are steady-state Gaussian-plume models, which are based on mathematical approximation of plume behaviour.

AERMOD is a straight-line, steady-state Gaussian plume model to predict the GLC’s for each study pollutant. The AERMOD Cloud modelling system is utilised to model the dispersion of pollutants over rural and urban areas, flat and complex terrain.

Surface and elevated releases, and multiple sources (including, point, area and volume sources) are also considered to determine ground level pollutant concentrations at specified receptor points.

The study inputs background concentrations of pollutant, meteorological conditions, source data such as emission rate at site and local topographical features to build the Atmospheric dispersion model. The results are predicted for the ground level concentrations of pollutants and in turn assess the potential environmental and health effects arising from the pollutants.

Gas Dispersion Study

Accidental leaks from pressurized tanks, pipelines, processing equipment and wellheads can result in the dispersion of hydrocarbon/toxic gases into the atmosphere and the formation of flammable/toxic gas clouds. Loss of containment and subsequent dispersion of fluids is a potential risk source for offshore and onshore infrastructure.

Objective

The objectives of Gas Dispersion Study are as follows:

• Identify potential gas dispersion release scenarios.
• Address the potential hazards related to flammable and toxic dispersion and its potential impairment of the installation/ facility.
• Provide recommendations to minimize the impairment due to flammable and toxic releases.

Methodology

The key steps anticipated in the Gas Dispersion Study are summarised below:

Failure Case Definition (Isolatable sections)

The initial key step is the identification of the release scenarios, which is based on Hazard Identification process usually carried out by internal review of the process design flow diagrams and instrumentation drawings, and layout configurations. Process conditions such as operating pressure, temperature, density, molecular weight, specific heat ratio and stream compositions shall be taken from Heat and Material Balance sheet. Once the scenarios are defined, then these are evaluated further for their potential consequence hazard zone.

Consequence Modelling: Flammable and Toxic Gas Dispersion

Gas dispersion modelling involves discharge/release modelling to determine the initial release rates and will be followed by flammable gas cloud and toxic gas cloud for each failure case/hole size.

The phenomenon of gas dispersion over distance (e.g. under the influence of the wind) is assumed to be represented simply by offsetting the various conditions at the release source by a dilution factor. The primary outputs here will be the gas concentration at the various targets of interest.

Impairment Criteria

The potential for impairment will be based on agreed criteria for each target, to be agreed prior to the start of the study. Typical criteria used for this kind of analysis will be put forward in the assumptions register prior to the start of the study.