From boat-based observations we observed elevated CH 4 mole fractions, between 11 and 370 ppb above background, downwind of eight oil and gas production platforms in the North Sea when none of the platforms was engaged in either gas flaring or oil transfer and unloading. This suggests that all observed oil and gas platforms leak CH 4 during normal operations. Using the near-source CH 4 measurements in a simple Gaussian plume model (where the CH 4 emissions are calculated from the minute-averaged peak enhancement at the centre of the plume), we found the median of the calculated CH 4 emissions from offshore oil and gas installations to be 589 kg CH 4 d−1, with individual platforms' CH 4 emissions ranging from 98 to 1928 kg CH 4 d−1. Matching production data to our measurements we estimate (1) a median loss of CH 4 from the six platforms, unweighted by production, of 0.23 % (mean 0.54 %); and (2) the cumulative loss of CH 4 , weighted by total production, of 0.19 %. These results indicate that, of the platforms measured, those producing more gas leaked proportionally less of what they produced. Also, the two higher emitting platforms (nos. 5 and 6) include floating production storage and offloading vessels; we find these to have much larger loss rates than the three fixed platforms (nos. 4, 7 and. 8). However, we also acknowledge our sample size is small and the five platforms may not be indicative of the overall performance of platforms in the North Sea.

The 2015 emission-factor-based NAEI emissions are within the ranges calculated in this study, i.e. a median loss rate of 0.23 % and a production-weighted loss of 0.19 %, and also show larger losses come from lower-producing platforms. However, the NAEI provides the main source of emission for each installation, and their reported emissions from the six platforms are almost entirely due to flaring (83 %) and offshore oil loading (17 %), neither of which was taking place during our measurements. Typically, these activities are not continuous on North Sea platforms; consequently, emission rates are likely to be much higher at certain times than others. As flaring and oil loading did not coincide with our measurement campaign, the measured emissions presented here represent leakage only and do not account for intermittent emissions due to venting, flaring or oil-loading activities. This suggests a potentially large missing source of CH 4 emissions in the national UK CH 4 emission inventory.

The emission estimates presented here are from a pilot study and further work is needed to establish total CH 4 leakage rates from offshore oil and gas platforms. We have established, however, that CH 4 enhancements can be detected downwind of all production platforms during normal operations when neither venting, flaring or oil-loading activities are taking place. Our measurements used in a Gaussian plume model indicate leakage from offshore installations is likely larger than previously estimated. However, these emission estimates come with large uncertainties as they are based on relatively few measured platforms; assume values for the height of emission, lateral and vertical mixing ratio distributions; and may not meet all the Gaussian plume model assumptions.

When the CH 4 emissions are calculated for two different emission heights, the importance of identifying the source location and height above the sea becomes apparent. The median CH 4 emission from the five platforms is 6.8 g s−1 when the emissions are all assumed to come from the working deck, while the median emission is 2658 g s−1 (47 % of production) when all CH 4 is assumed to be emitted from the flare, i.e. the highest point of the platform. This analysis indicates that the median emission presented here, based on the assumption that the emissions occur from the working deck, is a conservative estimate. However, without further measurements the height of the emission source cannot be definitively determined, and this leaves the possibility that leakage is higher during normal operations than our results indicate. The other input variables that cannot be determined without further measurement are the lateral and vertical mixing ratio distributions, but we feel that following the study of Blackall et al. (2007) the estimates used in this study are sufficient to establish leakage from oil and gas platforms and to provide a rough estimate of their emissions. As with the emission height, mixing can be resolved with further measurement, including the use of aircraft to resolve the vertical and horizontal mixing of the plume.

It is clear that further studies are needed to provide additional data that will yield more definitive emission estimates. Using the near-source (< 1 km) observations of this paper (Fig. 2; Sect. S2, platform nos. 5 and 6) we can see that plumes from the leaks are compact (< 200 m wide) and in some cases difficult to detect from sea-level measurements (Sect. S2, platforms nos. 7 and 8). Making three-dimensional observations downwind of the platforms and using a sonic anemometer would help identify some of the unknowns presented here. Also, measuring more platforms over a longer time frame would improve the understanding of ambient leakage.

Any further measurements would be significantly easier with the cooperation of the oil and gas industry, which could benefit from the findings. If the emissions are as low as the industry currently estimates, further measurements confirming low leakage rates would improve consumer confidence in oil and gas extraction activities. Alternately, if emissions are higher than currently reported, additional measurements would give the industry an opportunity to identify common issues such as incomplete combustion at the flare (Fig. 2), reduce leakage, and improve the efficiency of platforms, thus potentially increasing profits from the extracted gas.

The continuous leakage of CH 4 from offshore production platforms observed here is consistent with observations of similar onshore operations (Omara et al., 2016; Riddick et al., 2019). Ambient leakage is not unexpected as these offshore production platforms are located in the inhospitable conditions of the North Sea, where wind speeds regularly exceed hurricane force and waves can reach the working deck. However, it is surprising that ambient leakage has not been explicitly factored into the UK national emissions inventory, which relies solely on operator self-reported emissions calculated using emission factors combined with specific processes like flaring. Without direct measurement, operators can remain unaware of small emissions that occur during normal operation.