Page 562 - Marine Structural Design

538 Part V Risk Assessment

Install Fire and Blast Barriers
Escalation caused by explosions can be limited by fire and blast baxriers between modules and
areas. However, the barriers themselves may cause problems for keeping ventilation and
introduce more blocks. The constructionhepair of such barriers may involve extensive hot
work. This measure is therefore more effective in the early design stage.
Active Deluge on Gas Leakage
Leakage may be deluged out without causing any explosions or fires. Deluge may be
particularly effective in preventing so-called runaway flame accelerations. It may also lead to
a reduction of the peak overpressure.
The most critical aspect in the use of deluge is that it must be triggered prior to ignition, e.g.
on detection of a gas leak. Modeling of ignition has shown that the most likely interval
between release and ignition is two to three minutes. Thus deluge activation has to be within
the first half minute in order to be effective.
Improve Resistance of Equipment and Structures
The last possibility of reducing an explosion consequence is to improve the resistance of
equipment and structures to blast loads. However, it is not cost-effective to design structures
for the worst explosion case. Therefore this approach may be quite expensive.

30.4 FireRisk
In the offshore risk assessment, usually two types of fire risk are considered: the topside fire
and the fire on sea. The following sections mainly deal with the topside fire. Further, the
smoke effect analysis and the structural response under the fire are normally integrated into
the fire risk assessment.
The distinction between what is classified ‘fire’ and what is called ‘explosion’ is relatively
subjective. The total loss of the fixed platform ‘Piper Alpha’ was initiated by a small
explosion, but the damage was primarily due to fire.
30.4.1 Fire Frequency
Fire frequency analysis is very similar to explosion frequency analysis. The overall fire
frequency can be expressed as:
(30.4)
PFp = Phk * PCc ‘Ignition
where,
PFp = Frequency of fire
Pkak = Probability of gas leakage
PCc = Probability of gas concentration
P,gn,,jo,, =Probability of ignition
The flammable gaslair concentration range determines whether it is explosion or fire for a
given ignition. Further, fire scenarios are mainly caused from the following sources: blowout,
riser failure, pipeline failure, process equipment failure, and dropped object. The uncontrolled
hydrocarbon flow (blowout or riser failure) is considered as the main fire risk contributor to
the structures. Further, dropped objects may contribute to fire only when they lead to the