Evaluation of emissions and spatial distribution of methane from offshore oil and gas platforms in the Liaodong Bay of China based on shipboard measurement

Offshore oil and gas platforms are a notable source of methane (CH₄) in the marine environment, yet there remains a lack of comprehensive understanding regarding their impact along the coastal areas of China. Besides, relevant emissions are generally derived from systematic inventory and rarely quantified by top-down methods. To this end, a ship-based measurement of CH₄ emissions from three offshore oil and gas platforms with different oil and gas production levels was conducted over the Liaodong Bay of China in August 2023. The results demonstrated that there were differences in CH₄ background concentrations around the three platforms (2291 ± 15 ppb, 2468 ± 32 ppb and 2100 ± 12 ppb). At moderate wind speeds, CH₄ levels showed a gradual increase as the ship approached the target upwind, with peak concentrations typically observed within a range of 2-3 km from the platform. Along with meteorological data and the Gaussian plume model, CH₄ emission rates were determined corresponding to the average values of 229 ± 48 kg h^-1, 59 ± 16 kg h^-1 and 13 ± 8 kg h^-1 for the three platforms. By analyzing the concurrent changes in carbon dioxide concentration, it was hypothesized that gas flaring was the primary source of CH₄ emissions from one of the platforms. Additionally, when considering the revised emission factors for the oil and gas industry from the IPCC and combining them with the production data from the three platforms, the results clearly demonstrated a substantial contrast between top-down emissions and inventory emissions. Diverse platforms and continuous monitoring are necessary to validate the aforementioned perspective and ultimately enhance the precision of CH₄ emission data provided to regulators and the general public.