Page 558 - Marine Structural Design

534 Part V Risk Assessment

The explosion load may be categorized by its maximum overpressure. For instance,
if the overpressure is smaller than 0.2 bar, it is typically an ‘insignificant’ explosion;
if the overpressure is larger than 2 bar, it is considered to be a severe explosion.
In the Piper Alpha accident, an explosion due to ignited gas leakage set off an uncontrolled
fire, which in the end led to the total loss of the platform. In the last few years, large-scale tests
were conducted to study explosion modeling. The test results revealed that the blast loads due
to explosion had been significantly underestimated previously, and these loads cannot be pre-
designed in many cases. Therefore, the explosion risk picture may be even more severe than
previously thought.
30.3.1 Explosion Frequency
If the gas cloud formed by the gas leak is outside the flammable concentration range, or the
ignition source is lacking, no explosion will occur. Subsequently the gas cloud will dilute and
disappear. Thus, three factors may influence the explosion occurrence, i.e. gas leak sources,
ventilatioddispersion, and ignition sources. The overall explosion Erequency can be expressed
as:
(30.3)
where,
PEP = Frequency of explosion
P- = Probability of gas leakage
PCc = Probability of gas concentration
= Probability of ignition
Gas leak sources are important for the gas dispersion. Generally, the following aspects need to
be considered:
Location of the leak source, in a 3-dimensional space
Gas composition and characteristics, Le. temperature and specific weight
Leakrate
Direction of flow from the leak source
Unrestricted gas jet or diffuse gas leak

The ventilation conditions also have considerable influence on the dispersion of a gas leak and
the resulting gas cloud. Most platforms have natural ventilation, implying that the dispersion
of a gas leak will be strongly dependent on the wind speed and direction.
The actual location of the ignition point may vary considerably depending on the type of
ignition source. The ignition sources are generally identified as one of the following three
types:
Rotating equipment: Major equipment units, with a discrete distribution related to the
location of each unit.
Electrical equipment: A high number of possible sources, may be described as a
continuous distribution.