STUDY ON TRANSIENT FUEL GASES DISPERSION AND FLAMMABILITY IN AIR FOR THE APPLICATION OF LEAKAGE IN OFFSHORE SAFETY MANAGEMENT

To meet world’s insatiable gas demand, offshore production of natural gas must be carried out efficiently, effectively and most importantly safely. The lack of fundamental understanding of how these offshore fuel gases behaves and interacts with air after leakage occurrence has prompted this study into action which is essential in offshore safety management. Gaseous fuels are being labeled the “silent killer” as the biggest hazard of a gas leak is fatal explosion. Undetected gas leak is due to poor understanding and prediction of fluid dynamics interaction between the fuel gases and air as well as the flammability envelope at different times. Using ANSYS Fluent 14.0, 2D simulation is carried out to study the transient mixing behavior of selected fuel gases which are natural gas (primarily methane), carbon monoxide, ethane, propane, butane and lastly acetylene. Shravan and Umit, 2009 studied on transient hydrogen mixing with air are used as a benchmark for our study. A simple geometry of 1m by 0.5m is used alongside different scenario settings. 3 scenarios analyzed are fuel gas release near the bottom enclosed geometry, comparison between a smaller leak volume and a bigger leak volume, and lastly fuel gas release at the top enclosed geometry. Navier Stokes equations describe the fluid dynamics of their hydrogen diffusion model alongside using laminar model. The major steps in the CFD simulation are Geometry Construction, Meshing, Model Setup, Solution and Display of results. The results have shown that the transient dispersion of fuel gases and flammability in air is affected by the mixing buoyancy effects, diffusivity differences and density differences between the fuel gas and air. Hydrogen and natural gas leak (less dense gases compared to air) has high risk; they form flammable vapor cloud at short times that could ultimately lead to fatal explosion in presence of ignition sources. Compared to heavier fuel gases such as propane and butane, vapor cloud formation is rather unlikely in cases when the leak is at bottom. Aside from potentially high risk gas explosion, carbon monoxide hazardous leak into open environment brings fatality to the platform workers even at ppm level. Closed geometry outlines the importance of proper ventilation whereas open geometry outlines the estimation of evacuation period after leakages occurrence. All in all, this thesis has
5
delivered a comprehensive understanding and comparison between different transient gaseous fuels mixing with air based on different geometry settings as well as how the formation and decay of flammable zones changes.