Modelling of escalation of initial gas explosions on offshore installations

Gas explosion is one of the hazards to be considered in risk analysis. Scenarios initiated by gas explosions could lead to major catastrophe either by causing immediate collapse of the structure or by triggering a chain of events that finally culminate in total loss of installation. Scenarios of the latter category are denoted escalation scenarios of which the Piper Alpha accident was a tragic example. Very limited information has so far been published on prediction of escalation, and regarding theoretical models for predicting escalation paths initiated by offshore gas explosions, the authors are not aware of any publications. The objective of this paper is to describe the escalation of gas explosion phenomenon and to develop different procedures that could be used to investigate escalation of gas explosion in a risk analysis context. The underlying mechanisms of escalation of gas explosions are in the paper outlined through event trees and a flow diagram, and qualitative descriptions are given. Simplifications can be introduced at different levels of the basic flow diagram of escalation. The simplifications are concerned with the number of initial events chosen for analysis versus continuous descriptions of the initial events. The different types of simplifications are discussed and twomore » different algorithms for how escalation paths can be found are presented. These are denoted Forward Analysis of Escalation and Backward Analysis of Escalation, and the objectives of using procedures like these are to reveal the critical escalation paths leading to intolerable consequences and estimate the probability of these escalation paths occurring. Finally the models are illustrated by some simple examples.« less