With atmospheric methane concentrations rising and spurring increased social concern, there is a renewed focus in the oil and gas industry on methane emission monitoring and control. In 2019, a methane emission survey at a bp asset west of Shetland was conducted using a closed-cavity methane spectrometer mounted onboard a long-endurance fixed-wing uncrewed aerial vehicle (UAV). This flight represents the first methane emissions survey of an offshore facility with a miniature methane spectrometer onboard a UAV with subsequent flights performed. A small laser spectrometer was modified from an open-cavity system to a closed-cavity onboard the aircraft and yielded in-flight detection limits (3 seconds) of 1,065 ppb methane above background for the 2019/2020 sensor version and 150 ppb for the 2021 sensor versions. Through simulation, the minimum detection limits of sensors in mass flow rate were determined to be 50 kg/h for the 2019/2020 campaign and 2.5 kg/h for the 2021 campaigns, translating to an obtainable measurement for 23% and 82% of assets reporting higher than 1 kg/h according to the 2019 Environmental Emissions Monitoring System (EEMS) data set, respectively. To operationalize the approach, a simulation tool for flight planning was developed using a Gaussian plume model and a scaled coefficient of variation to invoke expected methane concentration fluctuations at short time intervals. Two methods were developed to calculate offshore facility-level emission rates from the geolocated methane concentration data acquired during the emission surveys. Furthermore, a Gaussian plume simulator was developed to predict plume behavior and aid in error analysis. These methods are under evaluation, but all allow for the rapid processing (<24 hours) of results upon landing the aircraft. Additional flights were conducted in 2020 and 2021 with bp and several UK North Sea operators through a Net Zero Technology Centre (NZTC)-funded project, resulting in a total of 18 methane emission survey flights to 11 offshore assets between 2019 and 2021. The 2019 flight, and subsequent 2020–2021 flights, demonstrated the potential of the technology to derive facility-level emission rates to verify industry emission performance and data.