Methane Emissions from Offshore Oil and Gas Platforms in the Gulf of Mexico

Shipboard
measurements of offshore oil and gas facilities were
conducted in the Gulf of Mexico in February 2018. Species measured
at 1 s include methane, ethane, carbon-13 (13C) and deuterium
(D) isotopes of methane, and several combustion tracers. Significant
variability in the emission composition is observed between individual
sites, with typical ethane/methane ratios around 5.3% and 13C and D methane isotopic compositions around −40 and −240‰,
respectively. Offshore plumes were spatially narrower than expectations
of the plume width based on terrestrial atmospheric stability classes;
a modified Gaussian dispersion methodology using empirically measured
horizontal plume widths was used to estimate the emission rates. A
total of 103 sites were studied, including shallow and deepwater offshore
platforms and drillships. Methane emission rates range from 0 to 190
kg/h with 95% confidence limits estimated at a factor of 10. The observed
distribution is skewed with the top two emitters accounting for 20%
of the total methane emissions of all sampled sites. Despite the greater
throughput of the deepwater facilities, they had moderate emission
rates compared to shallow-water sites. Analysis of background ethane
enhancements also suggests a source region in shallow waters. A complete
1 s measurement database is published for use in future studies of
offshore dispersion.