Abstract
The main objective of this work is to study the dynamic behavior of gas expansion in marine risers. The term ‘riser equilibrium' is used to provide a more rational explanation of gas expansion and its relation to riser unloading. While handling a gas kick situation with the blowout preventer closed, the gas within the riser expands until its pressure equals the hydrostatic pressure of the mud column above, plus any applied back pressure. Buoyancy or slip will cause the gas to migrate, which in turn will cause further expansion, if allowed to, or an increase in pressure. But after a certain point, termed as ‘Riser Equilibrium Point', any small decrease in the hydrostatic pressure would trigger the gas to expand rapidly until enough back pressure is applied or the top of the gas reaches surface. This could cause an explosive unloading. Complete dissolution of gas in oil based muds and the relative delay in noticing any surface indications for the gas influx makes influx detection more complicated with oil based mud as opposed to water based mud. Estimation of various parameters to study these phenomena and their correlation to the gas expansion is studied through an analytical and iterative approach. All schemes are implemented in Matlab and give a basic understanding of the severity of the situation. Calculation of the riser equilibrium point is beneficial to understand the risks related to riser unloading and riser collapse through proper estimation of collapse load. Since the conventional ‘free gas' approach to calculate surface volume of gas during kill operations tend to overestimate the risks involved, this study is expected to provide a more comprehensive understanding of the situation providing a safer operational conditions to handle gas in risers.
