## 4 Discussion
### Factors Affecting Pressure Holding  
As can be seen from the application results in Table 3, the pressure-retaining effect of WPCS was closely related to the type of hydrate reservoir and operational feasibility of closing and sealing the ball valve in situ. In the case of drilling operations within muddy reservoirs, all four runs achieved a certain degree of pressure holding. On the other hand, the success rate of ball valve turn-over and sealing in the sandy reservoir was only 54.55%. This disparity can be attributed to two primary factors identified through analysis. Firstly, during the sealing process, the ball valve was blocked by falling sand (Figure 8c), which was a common issue contributing to ball valve sealing failure [33; 34]. Secondly, elevated concentrations of rock cuttings resulted in the jamming of the inner pipe assembly, rendering retrieval attempts futile. As shown in Figures 1, 2 and 8c, in the process of the ball valve turn-over sealing, the ball valve drive nipple, the upper ball valve seat (including the ball valve seal), the ball valve, and the lower ball valve seat (collectively known as the downward motion unit, DMU) made a downward movement as a whole, and the ball valve turned over due to the presence of the turn-over pin and the keyway on the ball valve. Therefore, the key to the ball valve turn-over was to achieve the overall downward movement and complete the 90\({}^{\circ}\) flip. In the process of the downward movement, the downward component forces included (1) the force driven by the thrust spring; (2) the gravity of DMU. The upward component forces included (1) the thrust of the bearing spring; (2) the buoyancy of the DMU; (3) the  
Figure 8: The physical picture of WPCS: (**a**) pressure-retaining core bit and BHA; (**b**) the water tunnels of core bit plugged by sand; (**c**) pressure-retaining ball valve turned over with sand. friction resistance at the side sealing ring of the upper ball valve seat (Figure 9). Notably, the buoyancy force and gravity of the moving unit were also constant. At the same time, the specifications of the driving spring and the bearing spring used in the 15 cycles were consistent, and the downward and upward forces generated in the initial state of the two remained constant. Therefore, the key to determining whether the downward motion unit could move downward was the resistance at the sealing ring on the upper ball valve seat. The friction resistance was significantly affected by the type and the lubrication method of the sealing ring. With the increase in sampling depth, the ambient pressure increased and the temperature decreased, resulting in varying degrees of increase in the viscosity of the lubricating medium, and this also led to changes in friction resistance. During runs 1 to 12, butter lubrication was employed for the side sealing ring of the upper ball valve seat, while runs 13 to 15 utilized a combination of oil and butter. It was observed that run 13 and run 14 successfully achieved in situ pressure-retaining ball valve turn-over sealing. (Figure 10). As can be seen from the application results in Table 3, the pressure-retaining effect of WPCS was closely related to the type of hydrate reservoir and whether the ball valve could be closed and sealed in situ. Figure 10: Several cases of ball valve turn-over sealing: (**a**) the unsuccessful case with no ball valve turn-over; (**b**) the successful turn-over sealing case of the ball valve. Figure 9: Force analysis near DMU. Upon raising the inner tube assembly above the ball valve, the DMU began to move downward, initiating the ball valve's interaction with the flip pin for support. Since the support reaction force of the flip pin does not pass through the ball valve's center of mass, so that the ball valve has a flipping trend, the flipping torque generated by the flip pin must be greater than the friction torque between the upper and lower ball valve seats and the ball valve.