Encyclopaedia Index

3. CFD applied to gas-dispersion


    3.1 A typical scenario
    3.2 The physical processes considered
    3.3 How PHOENICS takes these into account
    3.4 Some typical findings

3.1 A typical scenario

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

The questions which then arise are:-

Such questions are easy for EXPLOITS to resolve, if the location and rate of the leakage are specified, and if sufficient is known about the ventilation conditions.

3.2 The physical processes considered

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

3.3 How PHOENICS takes these into account

PHOENICS, having received the appropriate information from EXPLOITS:

The user is not required to intervene; but he may do so if he wishes.

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.

A downward-directed jet of combustible gas from a leaking flanged joint

This picture represents the corresponding concentration of combustible gas at floor level

This picture represents the corresponding concentration of combustible gas at the outlet cross-section

When the wind from the left also has a component ALONG the inlet face of the module, the velocity-vector pattern is quite different, as is shown here.

So also, of course, is the pattern of gas concentration on the floor.

and at the outlet section

Had such computer-simulation exercises been carried out at the time of the Cullen enquiry (as they could have been, albeit with less ease than now), the outcome of the enquiry might have been different.

Instead, much reliance was placed on the results of PHYSICAL-MODEL experiments; and these took no account of the side-wind effect, even though it was known that the wind direction was indeed oblique to th module end on the night of the disaster.

A remark: Computer models may be incompletely reliable; but physical models can be positively misleading, not least because they have to be conducted at much too small a Reynolds no. Now for the study of the influence of a grating on the spread of combustible vapour from the floor of a module.

This picture represents gas-concentration distributions on vertical planes across an oil-platform module. The wind-induced flow is from top left to bottom right. An appreciable amount of combustible mixture enters the upper part of the module.

This picture shows the consequence of taking account of the grating. Much less vapour enters the upper region.