It is all-too-well known that:
Yet they MUST be estimated, if the satisfactoriness of safety
precautions is to be assessed, and approvals to build and operate
are to be given.
Computer simulations provide a means of making such estimates, by
way of predictions of:
Moreover, being an attachment to the general-purpose fluid-flow and
solid-stress-analysis code PHOENICS, it can call on all the further
capabilities of that code which it requires.
The following pictures illustrate the use of PHOENICS, with its
"fine-grid-embedding feature", to investigate one element of the
Piper Alpha scenario considered in the Cullen report (Ref.1).
Fine grids, ie large numbers of computational cells, are often
needed for such simulations, because small details, for example
the size of the gap between two flanges, often have very significant
effects.
3. Gas concentration, side view
4. Gas concentration and velocity vectors, top view
5. Velocity vectors, 3 dimensional view
6. Gas concentration contours, 3 dimensional view
Computer-based prediction methods are used for hazard assessment,
and for estimating the utility of counter-measures, because
experimental studies are far too expensive.
Experimental studies ARE carried out, on laboratory, intermediate or
full scales; but their main use is for testing of the validity
of the computer-simulation methods.
The recent full-scale-module experiments conducted by the Steel
Construction Institute at Spadeadam exemplify the difficulty:
The predictive techniques used for explosions have been
categorised (Ref 2) as being based on:
The programs CLICHE, SCOPE and EXTRAN fall into this category.
They can be regarded as rudimentary numerical models, falling far
short of them in realism and detail, but possessing the merits of
speed and cheapness.
Computer codes in this category are FLACS, EXSIM, REAGAS, COBRA and
EXPSIM, all of which have been specially designed for the simulation
of explosions.
EXPLOITS belongs to the "numerical-model" group; but it differs from
the others in being a special-purpose embodiment of a general-
purpose code, rather than a one-use-only computer creation.
All such codes seek to obtain SOLUTIONS OF THE FUNDAMENTAL EQUATIONS
of science applied to a sufficiently finely-divided set of imaginary
"cells" so configured as to represent, geometrically and in material
properties, the real-life object of study.
This attempt can be called: THE CFD APPROACH; for it employs the
techniques of Computational Fluid Dynamics.
With the increasing power of computers, and a lengthening record of
practical achievements, it is this approach which can be expected to
dominate hazaed analysis in the future.
However, this is so rarely the case that the method is used only by
those who have nothing better available. It may however become
useful again as the correlator of numerical predictions, when
sufficient of these have been carried out.
However the differences between a containable accident and an
intolerable disaster can easily depend on precisely those details
(for example the shapes of turbulence-promoting obstructions) for
which these models have no space.
Therefore, despite their greater sophistication and expense, the
"numerical models" can also not be trusted completely.
Nevertheless,
Provided that its predictions are treated as probabilities rather
than certainties, CFD renains the BATWEC, ie
"The Best Available Technique Without Excessive Cost".
1.1 Four types of simulation of relevance to oil-platform safety
How severe will be the damage, how lethal will be the missiles, and
what fires may subsequently spread, are all hard to estimate.
The software package EXPLOITS is designed to perform simulations of
all four kinds.
An example: the escape of gas from a leaky flanged joint
1.2 Why computer simulations are used for hazard assessment
Only computer simulations are cheap enough to permit exploration.
1.3 What kinds of computer predictions are available
and the same categorization principle can be applied to gas
dispersion, missile projection and fire spread.
1.4 Why computer predictions cannot be fully trusted
BUT the first two conditions can be fulfilled only at considerable
expense; and, until science advances further, the third is fulfilled
entirely only for non-turbulent flows and flames, ie those in small-
size apparatus.
1.5 Why numerical modelling is nevertheless still the BATWEC
the CFD approach does provide a valuable means of risk assessment;
and moreover it is one which must be used if UK HSE's "legal
requirement for operators to take new information into account"
(Ref.3) is to be complied with.