Development of applicable ice valves for ice-valve-based pressure corer employed in offshore pressure coring of gas hydrate-bearing sediments

Because of sealing failures, less than 60% hydrate-bearing sediment pressure cores can be retrieved by pressure corers based on mechanical valves. The ice-valve-based pressure corer presents a promising solution to improve the recovery of pressurized hydrate-bearing sediment cores. This study aims to develop applicable ice valves for ice-valve-based pressure corer employed in offshore pressure coring of gas hydrate-bearing sediments. The pressure sustaining performance of the ice valves formed from both seawater and seawater mixed with bentonite were studied. Experimental results indicate that ice valves formed from pure seawater can sustain the pressure of only 3.2 MPa, which is far from meeting the requirement of pressure coring. By analyzing the freezing-equilibrium relationships in seawater, it can be concluded that there was a unfrozen zone containing calcium and magnesium ions kept in the axial center of the ice valve, which was considered to be the primary reason for the poor pressure sustaining capacity of sea ice valves. In addition, pressurizing tests of the ice valves made from seawater with different mass concentration of bentonite added were carried out and results showed that the sustained pressure can be greater than 38 MPa when the bentonite mass concentration was more than 5%, which was quite satisfactory for offshore pressure coring. Comprehensive analysis for the significant improvement of sustained pressure was also provided which reasonably and satisfactorily revealed the enhancement mechanism of bentonite on sea ice valves.