(176a) CFD Analysis of Gaseous Leaks with 80% CO2 on Offshore Facilities

Many recently discovered reservoirs have high levels of gaseous contaminants, particularly CO₂, and require corresponding evaluation plans. Correspondingly, HAZID/HAZOP studies need to be performed for related well-test jobs, as accidental leakage of gases around the offshore exploration and production facilities pose risks for the workers’ health and the environment. The primary focus of this work is to estimate the gas-dispersion concentration around the potential release sources on an offshore drillship.

The computational fluid-dynamics (CFD) simulations using ANSYS Fluent® were carried out to study the CO₂ gas-dispersion behavior from sources with 80% CO₂ content. In the Fluent® model developed for the simulations, the Realizable k-º model for turbulence was used for the multi-component gas flow, while the species-diffusion equation was solved for the gas-dispersion calculation. A series of scenarios was run for leaks in three SWT areas, relief line (overboard), and flare boom, and with different wind directions, speeds, and gas-leak rates. The “worst-case” scenarios were further studied. Parallel simulations were used to accelerate the computing time, as the simulation grid generally had 1‒2 million points. The results yielded 3-D gas concentration distribution, including critical safety-concentration (3,900 ppm of CO₂) level profiles. Meanwhile, real field measurements of CO₂ levels were carried out.

The analysis of the results indicates that: (1) high leak rate and low wind speed conditions have larger pollution area; (2) at low wind speed, the upstream of the flare is also polluted because the vertices bring gas to all directions; and (3) local layout of the ship affects the CO₂ distribution. Using these results makes it possible to (1) determine the risk of moderate (M) and non-tolerable (NT) CO₂ levels profiles; (2) optimize the location and quantity of CO₂ sensors in the surface well-test plant and drill floor accordingly; and (3) assess the risks of CO₂ disposal by the flare boom and the relief line overboard.

This work reveals the hazard of gas-leakage risks in the most recently discovered reservoirs in deepwater operations. The CFD simulation and field-tests results generated on this drillship can be widely used to estimate and monitor the gas leakage on the offshore vessels and rigs where the health, safety, and environment are critical for the deepwater operation.