Research And Development Of In-situ Long-term Detection Technology For Deep-sea Dissolved Methane And Its Application

Ocean is the important source and sink of the atmospheric methane.Studies on dissolved methane in the ocean are helpful to understand the environment of the deep-sea,explore the natural gas hydrate reservoir,and prevent the submarine geological hazard.Marine in-situ sensor is an important part of undersea observatory network.Compared with the traditional methods of laboratory analyses,marine in-situ sensor has the advantages of small size,easy operation,high accuracy and can be placed in the designated position for continuous monitoring.At present,our country has started lately in such research field,which is in the stage of laboratory research or prototype test,only a few teaching and research units occupied in this direction.Based on the current domestic demand for the study of in situ methane sensor,and the sensor's advanced technologies and methods of international sensors,this study designs a real-time,long-term,high-precision instrument for detecting methane concentrations in a deep-sea.We have solved the key technologies mainly including seawater circulation,degasification and separation,and detection,and then discussed the integration project in detail.In addition,based on the system theory and method,the conversion formula and system sensitivity and detection limit have been analyzed.Combination with the analyses of simulation experiment in the laboratory and deep-sea,we have further discussed the overall system performance.This study on the in-situ detection system for deep-sea dissolved methane is very important to promote the development of marine survey technology and helpful for the utilization of ocean resources in our county.The following achievements have been withdrawn in this study.?1?Deep-sea dissolved methane in-situ detection instrument is mainly composed of pressure relief system,gas detection system and air-liquid treatment system.Its design principle is using high precision underwater gas sensor to test the dissolved gas out of in-situ seawater samples,to realize the in-situ high precision detection of light hydrocarbons such as methane dissolved in seawater.?2?Under the high pressure condition in a deep-sea,inner circulation system mainly relies on two-stage relief valve and flow control valve.Outside circulation system mainly relies on homemade waste water tank and pressurization drainage pumps for liquid storage and discharge.Liquid level sensor is installed in the waste water tank,which can effectively control the discharge time of waste liquid.?3?The modified polypropylene hollow fiber membrane has been used in the degassing detection system.Aiming at the closed working environment of the system,the optimization and modification of the mainstream degassing machine is carried out,and the degassing efficiency of the degassing device is proved to be more efficient.?4?The performance index of the optimal methane chromatographic column was determined by optimizing the selection of conventional chromatographic column.High-oxidized materials have been selected as the matrix material,such as SnO₂,and then mixed with some different dopant and drag reduction materials in order to make different formulations.According to the different sintering process,we have made a variety of gas sensitive element,and finally chose one with the highest sensitivity and least discreteness,which would be used to be the sensor probe for the detection system.?5?The power supply and control system of the detection system are integrated and the stability and coordination of the system are verified through the joint commissioning experiment.According to the detection principle of the detection instrument,the measured chromatographic data value is converted into the concentration of dissolved methane in seawater,and the sensitivity and detection limit of the detection instrument are deduced.?6?The performance of the system was verified with the help of self-made preparatory,the sea trial experiment and network connection test,which have obtained the accurate methane concentration of seawater,demonstrating the stability of the test system.