Page 560 - Marine Structural Design

536 Part V Risk Assessment

The consequence of an explosion is also dependent on the space and environment in which the
gas cloud is contained. Therefore, it is natural to classify explosions into the following three
categories (Vinnem, 1999):
Confined Explosion (internal explosion), occurs within tanks, process equipment, pipes,
and closed rooms etc. For this kind of explosion, the combustion process does not need to
be fast in order to cause serious pressure build-up.
Partlj Confined Explosion, occurs inside partially opened buildings. Typical cases are
compressor rooms and offshore modules. The explosion pressure can only be relieved
through the vent areas, or if the surrounding enclosure fails.
Unconfined Explosion, occurs in open areas such as process plants. A truly unconfined,
unobstructed gas cloud ignited by a weak ignition source may produce low overpressures.
In a process plant, there are local areas, which are partly confined and obstructed. These
areas are causing high explosion pressures. However, if an unconfined gas cloud detonates,
the explosion pressure will be as high as 20 bar independent of confinement and
obstructions.
Depending on the amount of the explosion loads, the types of damage on structures include:
Direct catastrophic failure,
Considerable damage which may be firther extended by the subsequent fire,
Little or no damage to structures, but causing critical failure of safety systems and thereby
preventing control of the fire,
Damage to passive fire protection, thereby reducing the survivability of structural
members,
Damage to process equipment, thereby causing immediate escalation of the accident.
The damages to the structures may be predicted using simplified analysis (such as single
degree of freedom model for dynamics) and simulated using non-linear finite element analysis,
based on a methodology that is similar to collision analysis discussed in Part I1 Chapter 14 of
this book. The explosion consequence is also dependent on the overpressure loading duration
in relation to the natural period of the structure being subjected to the explosion loads, for
instance,
Impulsive loads with duration that is shorter than the natural period of the structure,
Dynamic loads with duration that is comparable with the natural period of the structure,
Quasi-static loads that are applied slowly.
The overpressure time history should be properly modeled in the explosion consequence
analysis as it may significantly affect the analysis results.
The acceptance criteria (performance requirements) include strength criteria for structural
failure and deformation criteria for operating critical equipment.
30.3.4 Explosion Risk Reduction
TO reduce the risk of explosion, the first priority is to reduce the frequency of its occurrence.
This may be achieved by the following three measures (Vinnem, 1999):