(70h) Numerical Modeling of Pool Fire inside an Offshore Methanol Storage Tank

Methanol is widely used in the oil and gas industry as a reactant for the production of methyl tertiary butyl ether (MTBE) and as down-hole fluid additive to reduce hydrate formation and prevent freezing of solutions during injection into gas wells. Methanol is classified as a flammable liquid with a flammable range from 6% to 36% bounding the lower and upper flammable limits, respectively.

There are many experimental and numerical studies carried out for the methanol liquid pool fires in an open air environment, or a wind tunnel, with multiple pool fire sizes. However, few numerical studies have been performed for methanol pool fires inside or emanating from a storage container (typical storage units containing man-way entrances). The inadequate oxygen inside the container will limit the duration of the fire, and extinction and re-ignition of sectional small fires propose a challenge for numerical simulation of such kind of fire. In this paper, the Fire Dynamics Simulator (FDS) tool is used in the analysis of methanol pool fires. FDS is a computational fluid dynamics (CFD) fire model using large eddy simulation (LES) turbulence model. The basis for FDS has been developed from the mathematical background common to many CFD models with an emphasis on the slow moving flow and heat transfer caused by fires.

Methanol tanks typically have a pressure vacuum safety valve (PVSV) installed to relieve static pressure inside the tank when the pressure difference between the vessel pressure and the ambient pressure is small. If the methanol pumps continue to run and the tank PVSV or man-way is opened, air (e.g. oxygen) may ingress and begins mixing with the denser methanol vapors to a flammable range. Assuming the vapors find an ignition source, a pool fire is possible with the flammable vapors burning above the methanol liquid surface. This paper considers two scenarios for air ingress that lead to pool fires, a PVSV opening during normal venting operations and a man-way opening during maintenance to the inside of the tank.  The relationship between the fire duration and tank fill levels are obtained and discussed.