Study on a Probabilistic Fire Risk Analysis and Structural Safety Assessment of FPSO Topside Module

Abstract

Many offshore platforms are often exposed to the flammable oil & gas circumstances

hence hydrocarbon fire is a big threat to the offshore platforms. Fire accident on the offshore platform occupies a major part of the total risks. Nevertheless, former procedures of fire risk analysis used in the industry are still not intact enough to evaluate the risks related to fire accident. For this reason a new procedure of fire risk analysis is established in this paper to overcome the deficiencies in the former FRAs. In the new FRA procedure, the key parameters used for constructing fire scenarios are carefully considered and discussed for fear of the too much conservative judgment. Some of the parameters are considered in a probabilistic way or by using historical database and even sometimes based on the information of P&ID and PFD. CFD fire simulation is carried out based on the identified representative fire scenarios. In the fire simulation, the dynamic effect of hydro carbon fire determined by the inventory condition is also properly reflected by applying an effective method called snapshot. Furthermore, for investigating the structural response under the fire, heat transfer analysis and thermal elasto-plastic structural analysis are both included in the new FRA procedure. In the former FRAs, a concept of exceedance curve with certain physical variable is generally used in the frequency analysis for combining the results of fire simulation and fire frequency calculation. The purpose of using exceedance curve is for deciding a DAL, which is used for structure design against the fire accident or determination of proper PFP application area. Whereas, in the new FRA procedure, a new concept of structural cumulative failure frequency is presented to take place of the conventional exceedance curve. The purpose of using cumulative failure frequency in the new FRA can be summarized as two aspects. One is to solve the connection problem between the determined design load and structure analysis existed in the former FRAs, and other one is to have the determination of PFP application area become more intuitively and precisely. Generally, in the offshore platform PFP (Passive Fire Protection) is the most common and useful measure to reduce the failure of offshore structure under the hydro carbon fires. However, wide use of the PFP leads to a considerable increase of total project cost and this indicates that minimizing or optimizing the application areas is quite important. In that sense, cumulative failure frequency presented in this paper can be a quite useful tool to determine the proper PFP application areas for the structures. Furthermore, in order to demonstrate more detail of the new FRA procedure, an example with FPSO separation module is presented at last.

Keywords : QRA(Quantitative Risk Assessment), FRA(Fire Risk Analysis), PFP(Passive Fire Protection), CFD, Snapshot