(156d) Transient Analysis of Gaseous Contamination Dispersion on Offshore Facilities

Many recently discovered deepwater reservoirs contain high levels of gaseous contaminants, particularly CO₂, H₂S, and SO₂. As a result, specific plans are required because the accidental leakage of gases on offshore exploration and production facilities poses significant risks to personnel health and the environment. Traditional approaches to plume simulations focus on gas distribution in the steady state, which can be achieved only after a sufficiently long time from the beginning of leakage. This work describes a computational fluid dynamics (CFD) propagation analysis of the released gas around potential sources on an offshore rig under different scenarios. In particular, a scenario of gaseous blowout is considered, and the dynamics of hazardous gas spreading through the working area are analyzed. The numerical approach used in this study is similar to the approach applied to weather forecasting and air quality prediction.

CFD simulations were performed to study CO₂ dispersion from various sources containing 80% CO₂. CO₂ is used in this study as an example, but the same method can be applied to predict dispersion of other gases, including H₂S or SO₂. The Realizable k-º model for turbulence in a commercially available CFD tool was used for the multi-component gas flow, while the convective diffusion equation for species was solved for the gas dispersion calculation. A variety of dispersion scenarios were considered,   including leaks in three surface well test (SWT) equipment areas, relief lines (overboard), and flare booms, with different wind directions, speeds, and gas-leak rates. From these three areas, several “worst-case” scenarios and time-dependent gas contaminant distributions were studied in detail.

Analysis of the results indicates that (1) high leak rate and low wind speed conditions result in a larger pollution area; (2) at low wind speed, the contaminant gas propagates in all directions, including upstream of the leak point; and (3) the layout of the rig on the drill ship affects the local CO₂ distribution. Detailed transient analyses show that the gaseous contaminants cover the entire rig within approximately 30 seconds, even with a low leak rate. Meanwhile, the plume size increases, and the plume can change direction during expansion.

These results help (1) determine the risk of moderate (M) and non-tolerable (NT) CO₂ levels profiles; (2) optimize the location and quantity of CO₂ sensors on the SWT plant and drill floor accordingly; and (3) assess the risks of CO₂ disposal by the flare booms and the overboard relief lines.

This work can be very helpful in analyzing gas leakage-related risks in many recently discovered reservoirs in deepwater operations. The CFD approach developed in this study can be used to forecast and monitor gas leakage on offshore vessels and rigs, where accounting for health, safety, and environment factors is required.