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The sources and sinks of dissolved CH4 in offshore waters are becoming diversified with the rapid increase in human activities. The concentration and air–sea exchange flux of dissolved CH4 present new characteristics of more intense spatiotemporal evolution, and the contribution to atmospheric CH4 continues to increase. Herein, a new model based on navigable air–sea exchange flux observations was proposed, which replaced the traditional station-based sampling analysis and testing method, realizing the synchronous measurement of methane in the atmosphere and surface seawater carried by ships. Based on the Marine Geological Survey project of the China Geological Survey, comprehensive environmental surveys were conducted in April 2018, September 2018, and June 2019 in the Qiongdongnan area in the northern part of the South China Sea, and the dissolved methane content in the sea surface atmosphere and surface seawaters in 2019 were simultaneously obtained. The methane exchange flux ranges of the southeastern sea area were calculated as −0.001~−0.0023 μmol·m−2·d−1 and −0.00164~−0.00395 μmol·m−2·d−1 by using the Liss and Merlivat formula (LM86), the Wanninkhof formula (W92), and the field-measured wind speed. The feasibility of the navigational air–sea methane exchange flux observation system was proven in a sea trial, and the measurement accuracy and observation efficiency of air-sea flux were improved with the designed system, providing a new technical means for further research on multiscale air–sea interactions and global climate change.

Natural gas hydrate is easy to decompose and leak due to the changes in temperature and pressure during drilling, which causes safety accidents. Early monitoring of wellhead overflow is a practical and effective measure to prevent overflow blowouts and other accidents. Herein, a wellhead methane monitoring system for the open-circuit drilling of marine natural gas was designed. The system consisted of an overwater acoustic reception part and an underwater self-contained methane monitoring part, matching the construction environment of marine natural gas hydrate exploitation. Compared with the existing gas logging technology (measurement while drilling), the monitoring and early warning of wellhead methane content were realized at all stages of drilling, casing running, cementing, completion and fracturing in the process of natural gas hydrate exploitation. System communication and data acquisition tests were completed at different water depths through sea trials, which verified the effectiveness of the system design. The research results provide important theoretical and technical implications for promoting the development of early spill monitoring technology at the wellhead of open-circuit drilling for marine gas hydrates.