Environmental Fate for Subsea Discharge of Hydrogen Sulphide Laden Seawater

Abstract There are numerous undeveloped sour gas fields worldwide where the content of hydrogen sulphide is so high that it is not economically viable to dispose the gas using conventional techniques. A new technique capable of removing high concentrations of hydrogen sulphide is being proposed, which requires the discharge of hydrogen sulphide laden seawater through a subsea outlet. Hydrogen sulphide can be harmful to the environment and to assess the risk of impacts, the dispersion, advection and breakdown of hydrogen sulphide is described using high resolution hydrodynamic and water quality models. The description of the hydrodynamic processes is based on the CORMIX near-field flow model coupled with a MIKE 3 far-field flow model while the chemical breakdown of hydrogen sulphide is described using a customised ECO Lab water quality model with the half-life being a function of available dissolved oxygen. The coupled models are used to derive the temporal and spatial variation of hydrogen sulphide and dissolved oxygen concentration within the model domain. The application of the modelling approach is demonstrated for a sour gas field development offshore Sarawak, Malaysia where it is proposed to discharge seawater with a hydrogen sulphide concentration in the excess of 800 mg/l. Initial modelling shows a risk of environmental impacts over a sizeable area and that it is important to not only consider hydrogen sulphide, but also the possible oxygen depletion in that area. With the coupled models it was possible to define and document a practicable option that with increased seawater dilution and discharge through a multi-port outlet results in a noticeable reduction in the risk of environmental impacts.