FPSO Vent Post Gas Dispersion Analysis - Alternatives to Minimize Hydrocarbons Return in Critical Conditions

Abstract FPSO Vent Post systems may have significant variations in terms of gas flow and concentrations along the offshore asset lifecycle. Therefore, this paper describes a series of CFD simulations in order to evaluate alternatives to improve the gas dispersion efficiency in terms of minimizing the risk of hydrocarbons plume return to the FPSO in unfavorable wind conditions combined with heavier-than-air vented gas compositions. The alternatives are mostly focused on simple modifications so that they may be considered feasible even for offshore platforms in operation. Considering the restrictions to implement modifications on operating offshore units, some alternatives were evaluated such as: redirecting the vent nozzles, diluting the tanks internal atmosphere with frequent use of inert gas and including air blowers next to the vent exit. The venturi type air blowers were tested in multiple configurations so that a database was formed in order to cover multiples FPSO designs. The configurations were tested in CFD simulations for the most critical environmental conditions, usually related to very low wind speeds. Among the considered options, the alternative of using venturi blowers was selected as one of the most promising and therefore, some configurations of the blowers next to the venting system for two FPSOs are presented. Simulations indicates that the system was able to reduce the risk of hydrocarbon plume return to the FPSO under critical concentrations.