In the past 40 years, there have been over 60 major fire accidents reported on offshore platforms. The thermal effects imposed on process equipment and systems (such as pressure vessels, flare scrubbers, etc.) has the likely potential to result in catastrophic consequences such as explosions given that these process systems typically operate at very high pressures. In the event of an accidental fire, blowdown valves are engaged to relieve the excess pressure of the process system to acceptable values in a specific time period. During that time, however, the increased temperature of the blowdown system due to the impinging fire may result in the degradation of its structural capacity. The combination of a degraded vessel shell and a high internal pressure typically leads to sudden ruptures and explosions which could result in the escalation of the initial fire or injuries to personnel. Therefore, it is critical to accurately capture the pressure and temperature of the process system in a fire blowdown event.
A Blowdown analysis is typically carried out using simple thermodynamic software. While these software provide cost-effective approaches to studying process systems, they are unable to accurately compute the local effects of heat transfer and fluid behavior. Moreover, stress concentrations on the pressure vessel due to nozzle effects are not captured correctly using classical approaches and advanced analysis is required. Therefore, to accurately capture the rupture time of pressure vessels, scrubbers, etc., during a fire scenario, advanced approaches such as Computational Fluid Dynamics (CFD) are required.
In this paper, we will demonstrate a new approach in analyzing onshore and offshore process systems in an extreme fire scenario. Heat transfer through the vessel shell as well as the internal fluid will be accounted for by computing the rates of conduction, convection, and radiation. The time history of temperature and pressure as well as the change of phase of the fluid inside the pressure vessel will be captured in the analysis. A degradation study of the structural steel will be performed to predict the time of rupture of the process system to internal pressure using advanced analysis. Moreover, the effect of jet fire momentum on the vessels and their supports will be computed according to industry guidelines.
The methodology is validated by comparison with experimental data. The rupture time and rupture location will be computed and design recommendations will be provided to increase the strength of the process system at rupture locations. The current approach could provide accurate results to compute the response of process systems such as high-pressure vessels, scrubbers, etc., during offshore or onshore fire scenarios.