Dynamic Simulation of Gas Expansion in Marine Risers
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The main objective of the work is to study the dynamic behavior of gas expansion in marine risers. In case of well control situations in deep and ultra deep waters, gas influx shows no warnings if not little, making it difficult to handle the gas. It can be observed that the gas is completely soluble in the base oil of the drilling mud leaving no signs of the incident as observed from the surface. The dissolved gas starts to flash from the liquid when it experiences a reduction in pressure below the bubble point pressure of the system during circulation. Owing to the depth of the risers, gas may be trapped within the marine risers after successful shut-in of blow our preventers at the sea bed. Circulating the contaminated mud poses a major threat of explosive riser unloading when the gas expands rapidly when the hydrostatic pressure is not sufficient to contain the gas. This forms the basis of study of riser equilibrium point, above which a slight movement of gas can lead to riser unloading. Explicit and implicit models have been created to understand the dynamics of gas expansion during the circulation. Specific case of oil based mud when the gas is completely dissolved in the base oil is considered. To capture the time varying dynamics of the multiphase flow, drift flux models have been created for both water based and oil based mud. Effect of choke pressure and pressure losses due to friction and acceleration have been successfully investigated. Based on the study, a minimum threshold back pressure is recommended to be able to circulate out the contaminated mud by bypassing the riser equilibrium point and avoiding riser unloading.