## 3 Application and Results  
From 11 May to 22 June 2023, the GMGS conducted a hydrate survey voyage in the South China Sea with China National Offshore Oil Corporation (CNOOC)'s \"Offshore Oil 708\", and carried out the first application of the WPCS designed in this paper in the deep ocean. The specific application conditions and test results are shown in Table 3.  
As indicated in Table 3, the investigation involved conducting a total of 15 pressure coring tests across six borelines, with operations conducted at a seawater depth ranging from 1751.2 m to 1769 m. And the drilling depth ranged from 76.5 m to 168.4 m during the submarine pressure core drilling. Furthermore, the penetration depth of individual core drilling ranged from 0.8 m to 2.5 m.  
Among the 15 core drilling tests conducted, 10 of them had a single-return penetration depth of 1.5 m-2.1 m, with a maximum penetration depth of 2.5 m and a minimum penetration depth of 0.8 m. The above single penetration depth was less than the maximum coring length that WPCS could carry out, that is, 3.5 m. In addition, the core recovery rate is a very important index for evaluating core quality. The core recovery rate refers to the ratio of core recovery length to penetration length. That is, the core recovery rate \(P=L_{c}/L_{p}\times 100\%\). Analysis of the fifteen core drilling tests revealed that only three achieved a core recovery rate exceeding 90%. Additionally, six tests yielded recovery rates ranging from 45% to 90%, while the remaining six tests resulted in recovery rates below 45% (Figure 5). During the pressure-holding core drilling operations across the above six specified boreholes, the in situ pressure at the bottom of the hole ranged from approximately 18 MPa to 20 MPa. Analysis presented in Table 3 illustrates notable disparities among the results from 15 pressure-holding seal tests and their impact on core recovery rates. Among these tests, 10 runs achieved different degrees of pressure-holding sealing, and the success rate of pressure-holding sealing was 66%. Notably, runs 13 and 14 successfully maintained in situ pressure-holding seals. Throughout the coring and retrieval operations, careful measures were taken to ensure the inner tube core barrel remained within the stable temperature and pressure regime of the gas hydrated stability zone. As depicted in Figure 6, the temperature  
Figure 4: The Pressure Core Analysis Transfer System: (**a**) the main part of the Pressure Core Analysis Transfer System; (**b**) pressure measurement of the hydrate core inside the autoclave before transfer; (**c**) transfer of the hydrate core inside the core tube with pressure holding. and pressure curve was recorded during the whole coring process of the WPCS in run 13, and the phase equilibrium curve of gas hydrate proves that the gas hydrate could be kept stable using WPCS. However, affected by other factors (such as hard sandy hydrate), the core recovery rate was 0. The other eight runs of tests only achieved partial pressure-holding sealing and failed to maintain the temperature and pressure stability region of hydrate. Additionally, there were five test runs that failed to achieve the pressure-holding sealing effect. Practical implications of drilling in sandy gas hydrate formations were evident, highlighting increased drilling complexities compared to clay-rich formations. Sandy cores exhibited heightened friction among grain particles, frequently resulting in blockages within the core liner. The temperature and pressure profiles depicted in Figure 7 exhibit consistent trends throughout the entire operational sequence. In the preparation stage before lowering the drill string, elevated temperatures on the deck caused a gradual increase in temperature, while pressure remained unchanged. (1) In the stage of lowering through the drill string to latch in the BHA, with the increase in the descending depth, the pressure gradually increased and the temperature gradually decreased. At this time, the changes in pressure and temperature were relatively linear.