the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Aircraft-based mass balance estimate of methane emissions from offshore gas facilities in the Southern North Sea
Magdalena Pühl
Anke Roiger
Alina Fiehn
Alan M. Gorchov Negron
Eric A. Kort
Stefan Schwietzke
Ignacio Pisso
Amy Foulds
James Lee
James L. France
Anna E. Jones
Dave Lowry
Rebecca E. Fisher
Langwen Huang
Jacob Shaw
Prudence Bateson
Stephen Andrews
Stuart Young
Pamela Dominutti
Tom Lachlan-Cope
Alexandra Weiss
Grant Allen
Abstract. Atmospheric methane (CH4) concentrations have more than doubled since the beginning of the industrial age, making CH4 the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). The oil and gas sector represent one of the major anthropogenic CH4 emitters as it is estimated to account for 22 % of global anthropogenic CH4 emissions. An airborne field campaign was conducted in April–May 2019 to study CH4 emissions from offshore gas facilities in the Southern North Sea with the aim to derive emission estimates using a top-down (measurement-led) approach. We present CH4 fluxes for six UK and five Dutch offshore platforms/platform complexes using the well-established mass balance flux method. We identify specific gas production emissions and emission processes (venting/fugitive or flaring/combustion) using observations of co-emitted ethane (C2H6) and CO2. We compare our top-down estimated fluxes with a ship-based top-down study in the Dutch sector and with bottom-up estimates from a globally gridded annual inventory, UK national annual point-source inventories, and with operator-based reporting for individual Dutch facilities. In this study, we find that all inventories, except for the operator-based facility-level reporting, underestimate measured emissions, with the largest discrepancy observed with the globally gridded inventory. Individual facility reporting, as available for Dutch sites for the specific survey date, shows better agreement with our measurement-based estimates. For all sampled Dutch installations together, we find that our estimated flux of (122.7 ± 9.7) kg h-1 deviates by a factor 0.7 (0.35–12) from reported values (183.1 kg h-1). Comparisons with aircraft observations in two other offshore regions (Norwegian Sea and Gulf of Mexico) show that measured, absolute facility-level emission rates agree with the general distribution found in other offshore basins despite different production types (oil, gas) and gas production rates, which vary by two orders of magnitude. Therefore, mitigation is warranted equally across geographies.
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Magdalena Pühl et al.
Status: closed
-
RC1: 'Comment on acp-2022-826', Anonymous Referee #1, 26 Mar 2023
General comments:
This paper has estimated methane emissions from offshore gas facilities in the Southern North Sea based on airborne observations of CH4, CO2, C2H6. A mass balance approach was applied to the observed CH4 enhancements downwind of the targeted facilities to derive the emissions. Furthermore, the estimated emissions were compared to inventory studies, operator-based facility-level reporting, and other relevant and similar studies. Top-down quantification of CH4 emissions from offshore oil and gas facilities is much needed to independently evaluate various CH4 emissions, either expected or unexpected, and these efforts are very useful to help mitigate CH4 emissions. The manuscript is well structured and well written, and will be suitable for publication after addressing my comments below.
- Assuming a vertically well-mixed plume within the PBL and observing the enhancements that vary significantly across the transects at different heights are contradictory. It may be justifiable to assume the entrainment flux is small, and the top most transect can be extrapolated to the top of the PBL. Can the author justify the extrapolation to the sea surface? Note that the surface layer usually has distinct mixing scheme compared to the PBL. Also, LES simulations indicate that plumes downwind of point sources are not fully mixed kilometers from the source, and only time-averaged plumes show the Gaussian shape, e.g., Raznjevic et al., 2022.
- The comparison of the mass balance estimates with operator-based reporting is highly appreciated. From the main text, I understand that operator-based reporting is provided daily. As maintenance activities and planed venting may vary over hours, I wonder the detailed info was provided by the operator as well. This may potentially explain the still quite large discrepancies between the mass balance estimates and the operator-based reporting.
- The derivation of flux uncertainties is well described. However, some important info is lacking for a reader to understand why the uncertainties of the estimated fluxes are rather small compared to other studies, e.g., what are the wind speed and the wind speed and direction variabilities? Has the upwind transect been considered as backgrounds? How much differences are found in the calculated fluxes if a Kriging method is used to interpolate/extrapolate the plumes? What’s the flux uncertainty caused by the uncertainty in the PBL height? How is the uncertainty of the sum emissions of all facilities derived from the individual uncertainties?
Some details:
- Both Line 115 and Line 302 refer to the Aerodyne instrument using the same technique, please be consistent with the name of the instrument; also, please be consistent in using American and British spelling, e.g., analyzer, analyser, tuneable or tunable
- L130: should be mole fractions? As analyzers report concentrations in mole fractions, not molar ratios. Also for other occurrences throughout the text.
- L131-132: not clear how backgrounds are calculated, please rephrase the sentence. It seems there is a typo here to interpolate between plume edge and side of the plume.
- L197-198: Can the authors clarify how the level of detection was derived? Not sure how the maximum uncertainty of all flux calculation parameters is defined?
- L291: As P9 is reported offline, no CO2 plume was detected, can the authors exclude flaring emissions from P9?
- L329-330, Figure 3, it would be nice to see the scatter plots of CH4 CO2 and CH4 vs. C2H6.
- L455: eq.(A3) is found nowehere.
Citation: https://doi.org/10.5194/acp-2022-826-RC1 -
CC1: 'Comment on acp-2022-826', Glen Thistlethwaite, 26 Apr 2023
Hi there, I'm the Technical Director for the NAEI and have worked on the EEMS and UK inventory data for about 20 years. This is really useful research but it would be useful if possible to be able to view some of the underlying data per installation, to understand the findings more clearly. As regards the article, you state in the paragraph on line 265 (and then come back with a comment ("Surprisingly..") on line 409) that you would expect consistency between EEMS and the NAEI; this is not always the case. You have slightly mis-represented the NAEI, as the EEMS data is ONE OF SEVERAL data inputs to the NAEI emission estimates. We also use data such as EUETS data and also operator-reported activity data which is gathered by the North Sea Transition Authority via a mechanism called the Petroleum Producers Reporting System (PPRS). In the compilation of the NAEI estimates, we also conduct rigorous time series consistency and completeness checks on the raw EEMS data, and revise or gap-fill the EEMS data in using them for the national inventory. Therefore it is not surprising to me that you are observing some inconsistencies between EEMS and NAEI. In general I would expect EEMS to be a de-minimis value, so I am interested in the data for site P3; that disparity (EEMS>NAEI) could arise through data aggregation within the point source reporting process, but without seeing the data I can't comment further. One other very minor point is that there is a typo in the title of Table E1, which should read "..Dutch (P7 to P11) installations..". Kind regards, Glen Thistlethwaite.
Citation: https://doi.org/10.5194/acp-2022-826-CC1 -
RC2: 'Comment on acp-2022-826', Tim Butler, 28 Apr 2023
I am posting this reviewer comment as the editor of this manuscript. During the open discussion it has been difficult to find a second anonymous referee to provide a review of the manusript. But based on the one available anonymous review and the additional open comment by Glen Thistlethwaite, I am satisfied that the paper could meet the standards for publication in ACP, being reconsidered after major revisions. I am posting this comment so that the open discussion phase can be closed, allowing the process to continue and the authors of the manuscript to post their responses to all reviews and to provide a revised manuscript for reconsideration.
Citation: https://doi.org/10.5194/acp-2022-826-RC2 - AC1: 'Comment on acp-2022-826', Magdalena Pühl, 01 Jul 2023
Status: closed
-
RC1: 'Comment on acp-2022-826', Anonymous Referee #1, 26 Mar 2023
General comments:
This paper has estimated methane emissions from offshore gas facilities in the Southern North Sea based on airborne observations of CH4, CO2, C2H6. A mass balance approach was applied to the observed CH4 enhancements downwind of the targeted facilities to derive the emissions. Furthermore, the estimated emissions were compared to inventory studies, operator-based facility-level reporting, and other relevant and similar studies. Top-down quantification of CH4 emissions from offshore oil and gas facilities is much needed to independently evaluate various CH4 emissions, either expected or unexpected, and these efforts are very useful to help mitigate CH4 emissions. The manuscript is well structured and well written, and will be suitable for publication after addressing my comments below.
- Assuming a vertically well-mixed plume within the PBL and observing the enhancements that vary significantly across the transects at different heights are contradictory. It may be justifiable to assume the entrainment flux is small, and the top most transect can be extrapolated to the top of the PBL. Can the author justify the extrapolation to the sea surface? Note that the surface layer usually has distinct mixing scheme compared to the PBL. Also, LES simulations indicate that plumes downwind of point sources are not fully mixed kilometers from the source, and only time-averaged plumes show the Gaussian shape, e.g., Raznjevic et al., 2022.
- The comparison of the mass balance estimates with operator-based reporting is highly appreciated. From the main text, I understand that operator-based reporting is provided daily. As maintenance activities and planed venting may vary over hours, I wonder the detailed info was provided by the operator as well. This may potentially explain the still quite large discrepancies between the mass balance estimates and the operator-based reporting.
- The derivation of flux uncertainties is well described. However, some important info is lacking for a reader to understand why the uncertainties of the estimated fluxes are rather small compared to other studies, e.g., what are the wind speed and the wind speed and direction variabilities? Has the upwind transect been considered as backgrounds? How much differences are found in the calculated fluxes if a Kriging method is used to interpolate/extrapolate the plumes? What’s the flux uncertainty caused by the uncertainty in the PBL height? How is the uncertainty of the sum emissions of all facilities derived from the individual uncertainties?
Some details:
- Both Line 115 and Line 302 refer to the Aerodyne instrument using the same technique, please be consistent with the name of the instrument; also, please be consistent in using American and British spelling, e.g., analyzer, analyser, tuneable or tunable
- L130: should be mole fractions? As analyzers report concentrations in mole fractions, not molar ratios. Also for other occurrences throughout the text.
- L131-132: not clear how backgrounds are calculated, please rephrase the sentence. It seems there is a typo here to interpolate between plume edge and side of the plume.
- L197-198: Can the authors clarify how the level of detection was derived? Not sure how the maximum uncertainty of all flux calculation parameters is defined?
- L291: As P9 is reported offline, no CO2 plume was detected, can the authors exclude flaring emissions from P9?
- L329-330, Figure 3, it would be nice to see the scatter plots of CH4 CO2 and CH4 vs. C2H6.
- L455: eq.(A3) is found nowehere.
Citation: https://doi.org/10.5194/acp-2022-826-RC1 -
CC1: 'Comment on acp-2022-826', Glen Thistlethwaite, 26 Apr 2023
Hi there, I'm the Technical Director for the NAEI and have worked on the EEMS and UK inventory data for about 20 years. This is really useful research but it would be useful if possible to be able to view some of the underlying data per installation, to understand the findings more clearly. As regards the article, you state in the paragraph on line 265 (and then come back with a comment ("Surprisingly..") on line 409) that you would expect consistency between EEMS and the NAEI; this is not always the case. You have slightly mis-represented the NAEI, as the EEMS data is ONE OF SEVERAL data inputs to the NAEI emission estimates. We also use data such as EUETS data and also operator-reported activity data which is gathered by the North Sea Transition Authority via a mechanism called the Petroleum Producers Reporting System (PPRS). In the compilation of the NAEI estimates, we also conduct rigorous time series consistency and completeness checks on the raw EEMS data, and revise or gap-fill the EEMS data in using them for the national inventory. Therefore it is not surprising to me that you are observing some inconsistencies between EEMS and NAEI. In general I would expect EEMS to be a de-minimis value, so I am interested in the data for site P3; that disparity (EEMS>NAEI) could arise through data aggregation within the point source reporting process, but without seeing the data I can't comment further. One other very minor point is that there is a typo in the title of Table E1, which should read "..Dutch (P7 to P11) installations..". Kind regards, Glen Thistlethwaite.
Citation: https://doi.org/10.5194/acp-2022-826-CC1 -
RC2: 'Comment on acp-2022-826', Tim Butler, 28 Apr 2023
I am posting this reviewer comment as the editor of this manuscript. During the open discussion it has been difficult to find a second anonymous referee to provide a review of the manusript. But based on the one available anonymous review and the additional open comment by Glen Thistlethwaite, I am satisfied that the paper could meet the standards for publication in ACP, being reconsidered after major revisions. I am posting this comment so that the open discussion phase can be closed, allowing the process to continue and the authors of the manuscript to post their responses to all reviews and to provide a revised manuscript for reconsideration.
Citation: https://doi.org/10.5194/acp-2022-826-RC2 - AC1: 'Comment on acp-2022-826', Magdalena Pühl, 01 Jul 2023
Magdalena Pühl et al.
Magdalena Pühl et al.
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