Leakage of combustible gas at a certain minimum rate may be regarded as inevitable, or at least possible.

The questions which then arise are:-

• Will gas-air mixtures arise which are within the limits of flammability?

• How large a volume of such flammable mixture can accumulate in the module?

• Will it be located near a source of possible ignition?

EXPLOITS, faced with such a task, will take into account the following processes:

• the motion of the air, under the combined influences of:
  • ventilation induced by exposure to the external atmosphere;

  • buoyancy, ie density differences interacting with gravity, whether temperature- or concentration-driven;

  • the momentum of the gas jet, if this is appreciable;

  • any internal-ventilation fans;

  • the resistance exerted by the solid objects in the module;

• the turbulence generated by the flow past the solid objects;

• the resulting evening-out of composition differences.

PHOENICS, having received the appropriate information from EXPLOITS:

• creates a computational grid, ie a sufficiently-fine set of cells,

• sets up the corresponding set of balance equations for mass, momentum and energy;

• solves those equations; and

• prints the results on files which can be displayed and inspected in various ways.

3.4 Some typical findings

There will now be displayed some results generated by PHOENICS, in studies connected with the elucidation of what might have happened on Piper Alpha.

The first results concern the influence of a cross-wind (neglected in some earlier studies) on the distribution of combustible gas within a module.

Thereafter the influence of a floor grating will be illustrated.

The module is viewed from the top. The floor-level velocity vectors are shown. The wind entering the open end of the module, on the left, is aligned with the module axis.