Abstract
Abstract. Emissions of methane (CH4) from offshore oil and gas installations are poorly ground-truthed, and quantification relies heavily on the use of emission factors and activity data. As part of the United Nations Climate & Clean Air Coalition (UN CCAC) objective to study and reduce short-lived climate pollutants (SLCPs), a Twin Otter aircraft was used to survey CH4 emissions from UK and Dutch offshore oil and gas installations. The aims of the surveys were to (i) identify installations that are significant CH4 emitters, (ii) separate installation emissions from other emissions using carbon-isotopic fingerprinting and other chemical proxies, (iii) estimate CH4 emission rates, and (iv) improve flux estimation (and sampling) methodologies for rapid quantification of major gas leaks. In this paper, we detail the instrument and aircraft set-up for two campaigns flown in the springs of 2018 and 2019 over the southern North Sea and describe the developments made in both the planning and sampling methodology to maximise the quality and value of the data collected. We present example data collected from both campaigns to demonstrate the challenges encountered during offshore surveys, focussing on the complex meteorology of the marine boundary layer and sampling discrete plumes from an airborne platform. The uncertainties of CH4 flux calculations from measurements under varying boundary layer conditions are considered, as well as recommendations for attribution of sources through either spot sampling for volatile organic compounds (VOCs) ∕ δ13CCH4 or using in situ instrumental data to determine C2H6–CH4 ratios. A series of recommendations for both planning and measurement techniques for future offshore work within marine boundary layers is provided.
Highlights
Methane is a potent greenhouse gas in the atmosphere, with a global warming potential 84 times that of carbon dioxide when calculated over a 20-year period (Myhre et al, 2013)
Offshore oil and gas fields make up ∼ 28 % of total global oil and gas production and are expected to be significant sources of CH4 to the atmosphere, given that 22 % of global CH4 emissions are estimated to be from the oil and gas (O&G) sector (Saunois et al, 2016)
Son of Whole Air Sampler (SWAS) canister sampling was manually triggered during the flights according to in situ observations made by fastresponse instrumentation of CO2, C2H6 and CH4, with the aim of capturing specific oil and gas plumes
Summary
Methane is a potent greenhouse gas in the atmosphere, with a global warming potential 84 times that of carbon dioxide when calculated over a 20-year period (Myhre et al, 2013). Shipbased measurements of CH4 and associated source tracers have been made in both the Gulf of Mexico (Yacovitch et al, 2020) and in the North Sea (Riddick et al, 2019) The latter reported fluxes of CH4 from offshore O&G installations in UK waters that were derived from observations made from small boats at ∼ 2 m above sea level. We provide an overview of the measurement platform configuration and sampling strategy during these campaigns, including instrument comparisons for hydrocarbon plume detection, spot sampling strategies for VOCs and δ13CCH4, and flight planning to cope with complex boundary layer meteorology to allow for the estimation of emission fluxes. A sister publication will present the estimated facility level emissions in detail and discuss the results in a regional context
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