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Dispersion and Ventilation

Background

This section covers:

Dispersion and ventilation are recognised as key topics in the control of fire and explosion hazards on offshore installations. They have been the subject of various research studies over the past 10 years and are the subject of increasingly detailed modelling by dutyholders as a means of demonstrating lower risk levels.

Strategy objectives

  1. identify areas of uncertainty in evaluation of ventilation and dispersion.
  2. promote the use of consistent methodologies in the evaluation of installations ventilation and dispersion analysis,
  3. Initiate research to increase knowledge and understanding of ventilation and dispersion analyses,

Knowledge of dispersion and ventilation

Standards, guidance and codes of practice on ventilation

The standards, guidance and codes of practice relevant to ventilation on offshore installations include BS 5925, ISO15138, BS EN60079-10, IP 15, API 500 and NORSOK H-001. A review of these documents has highlighted a number of areas of concern:

Research on gas dispersion

Notable research studies on dispersion on offshore installations include:

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Modelling and measurement capabilities

Dispersion models

Gas dispersion

There are a number of areas of uncertainty with regard to the modelling of offshore ventilation and dispersion:

Measurement capabilities

Measurement of ventilation rates in enclosed, mechanically-ventilated areas is relatively straightforward and well documented. Measurement of ventilation rates in relatively open conditions is much more difficult and depends on the flow regime. Under plug flow conditions air speed measurements or tracer gas techniques can be employed. The data would then have to be correlated with wind speed and direction. This approach can, however, be invalidated where 'short circuiting' of the air flow occurs. Smoke releases or point measurements of air velocity on a deck can highlight areas of recirculation or low wind speed, but do not provide any information on ventilation rate.

Industry practice

Wind tunnel modelling and a range of mathematical modelling techniques are often used to evaluate offshore air movement and dispersion. In a sample of recent Safety Cases only one provided validation of modelling against offshore experimental measurements, as required by ISO 15138.

Strategy development issues

Standards and guidance

Models

Dispersion calculations in exceedance modelling approaches

Updated 2012-12-13