Comparison of Risk Based Methodologies for Structural Response Analysis in Case of Fire in Offshore Process Facilities

In the context of offshore structures acting on production and storage of flammable liquids and gas, accidental fire analyses are an important asset to ensure the safe operation. These analyses allow the identification of possible hydrocarbon accidental release scenarios and critical structural elements as they combine a risk assessment with structural analysis. Based on the identification of the possible leak scenarios and their probability of occurrence and consequence (risk assessment), a detailed structural analysis is carried out for the relevant scenarios with the purpose of indicate Passive Fire Protection (PFP) to be applied on critical structural members that fail the survivability criterium.

Looking at a process plant on a FPSO (Floating, Production, Storage and Offloading unit), for instance, where one may find different liquid and gas process lines, the possibilities of accidental flammable liquid or gas releases leading to fire scenarios are significant. The risk assessment identifies the relevant scenarios, i.e., the scenarios associated to relevant frequency of occurrence and considerable inventory.

Currently, in the industry, different approaches are considered to determine which scenarios are relevant. In this paper, two approaches will be covered. The first will be called “Express Probabilistic Approach” as it is made through a probabilistic tool that summarizes the contribution of parameters that reinforce or mitigate the fire scenario. Among these important parameters are the actuation of process devices such as Shut Down Valves (SDVs), Blowdown Valves (BDVs)and flammable gas detectors. Some additional important variables are also accounted, such as the probability of different weather conditions, ventilation conditions, operating pressure and temperature and many other relevant ones. This approach considers in equal levels the frequency and the consequence of each scenario leading to a more detailed risk-based approach.

The second approach will be called “Scenario Based Approach”. Through this approach the mapped fire scenarios are ranked from the most to the less frequent and the relevant scenarios are selected applying screening steps that consider the temperature of the structure and necessity of PFP application. This approach considers the fire scenario consequence in higher level than the frequency of occurrence.

The main objective of this paper is to demonstrate how the process devices provide robustness to the Fire Risk Assessment (FRA). Besides, the final result of this analysis is the indication of the required PFP amount on a very specific way, indicating which elements need to be protected to ensure that the structure will withstand the dimensioning fire scenarios defined. Hence, a comparison between PFP indication based on each risk approach is also presented. This may be of interest since PFP application represents an additional cost and potentially also operational issues during the asset lifecycle for most operators and offshore units’ owners.