(54bz) Investigating the Gas Dispersion from a Subsea Gas Release in Stratified Shallow Waters

Mary Kay O’Connor Process Safety Center – Qatar

Texas A&M University at Qatar, PO Box 23874, Education City, Doha, Qatar

Corresponding author: Moustafa Ali [moustafa.ali@qatar.tamu.edu]

Presenting author: Moustafa Ali

Objective:

Investigate the effects of thermal or saline stratification and high gas rates on the consequence modeling of subsea gas releases with numerical modelling. Phenomena which need better understanding like plume turbulence, bubble size distribution and hydrates formation are addressed. In addition, the potential hazards associated with the toxicity and flammability of surface reaching gas are analyzed.

Abstract

Although the repetitive subsea gas releases incidents occurring in the offshore oil and gas industry, the attention is limited on a case-by-case study; leading to quantitative risk assessment approaches that are limited to similar cases. A subsea gas release can result from a range of different causes including drilling operations; failures in flow lines, gas export lines, and subsea equipment. Such releases can have catastrophic impacts on the environment, offshore platforms and human lives (Olsen & Skjetne, 2016). Natural gas, in general and in middle east, covers a large portion of the world supply and, consequently, this type of incidents could pose significant risk for the related and nearby facilities, like Hydrogen Sulfide (H₂S). For example, 40% of the natural gas reserves in the world are sour gas fields in which middle east holds the highest reserves of sour gas. (Total E&P, 2007). While many approaches have been proposed for the description of underwater/subsea releases, these are not universal and still include deficiencies concerning plume turbulence, hydrates formation, high flowrates, stratified (thermally or owe to salinity) water bodies. According to (Rew, Gallagher, & Deaves, 1995), experimental data for underwater releases exist but are also limited to small and medium scale (compared to actual depth and flowrate).

In this study, we investigated the available computational methods for subsea gas releases applicable for “hot” (e.g. stratified) and saline water bodies like the ones of the offshore facilities at the State of Qatar. An Eulerian based computational fluid dynamics (CFD) model was configured in order to study representative gas release scenarios. Earlier, the CFD model was validated successfully against experimental data from SINTEF and Statoil (Engebresten, Northug, Sjoen, & Fannelop, 1997). It also demonstrated the required sensitivity for critical parameters such as the radial and centerline distribution of velocities and void-fraction. Then, the model was applied for conditions specific to Qatar’s offshore industry as a representation of shallow water and a sensitivity analysis was conducted against parameters of local interest such as high-flowrates, salinity, bubble distribution and stratified flows. These parameters are needed to understand the behavior of the gas in shallow depths and above the sea surface. Finally, a discussion on the potential risk level is given concerning fire & explosion and toxicity hazards like H₂S.

Keywords:

Offshore, subsea gas releases, shallow water, bubble plume, CFD, toxic gas, flammable, H₂S, QRA

References

Engebresten, T., Northug, T., Sjoen, K., & Fannelop, T. K. (1997). Surface Flow and Gas Dispersion from a Subsea Release of Natural Gas. In Proceedings of the Seventh (1997) International Offshore and Polar Engineering Conference (Vol. I). USA.

Olsen, J. E., & Skjetne, P. (2016). Current understanding of subsea gas release: A review. Canadian Journal of Chemical Engineering, 94(2), 209–219. https://doi.org/10.1002/cjce.22345

Rew, P. J., Gallagher, P., & Deaves, D. M. (1995). Dispersion of Subsea Releases. Review of prediction methodologies.

Total E&P. (2007). Sour Gas - A History of Expertise.