Numerical analysis of the sediment erosion of the balance valve in a buoyancy regulation system was performed. A numerical model for the two-phase flow inside the balance valve was constructed based on the discrete phase model. The sediment erosion rate on the balance valve was discussed, and the effects of five parameters were considered. The effects of the sediment concentration and valve opening were found to be significant, while the effects of the pressure difference, sediment density, and size were found to be moderate. The erosion rate, according to the numerical results, increased linearly with the sediment concentration, so long-term operation of a buoyancy regulation system in high-concentration areas should be avoided. The erosion rate was the highest when the valve opening was 46.3%, so half-open operating conditions are not recommended. The erosion rate was proportional to the square root of the pressure difference. However, adjusting the pressure difference may not be an effective method for regulating the total erosion. The superposition of the secondary flow and the main stream caused particles to spiral along with the fluid, resulting in asymmetric erosion at the working edge. The erosion rate on the working edge decreased with the increase in the sediment size. Conversely, the erosion rate on the valve ball surface increased with the sixth power of the sediment size. Considering that large particles are more likely to cause a blockage, it is recommended to install a seawater pretreatment device at the inlet to prevent large sediments from entering the valve and to improve the working life of the buoyancy regulation system.

Under ultra-high-pressure full-ocean-depth conditions, the rolling bearings of seawater pumps are often subjected to coupled stress conditions, including high external pressure, oil–water emulsification, and sustained high loads. Early failure tends to occur, which severely compromises system stability and reliability. This study focuses on identifying the typical failure mechanisms of bearings and proposing key optimization measures. A high-pressure experimental system rated at 120 MPa was constructed. Long-term water injection and drainage cycling tests were performed, followed by teardown inspections of failed prototypes. The bearing degradation was found to involve multiple failure modes, including rolling element fracture, cage breakage, lubricant emulsification, and three-dimensional embedded abrasive wear. The combined effects of lubricant degradation and particulate contamination primarily caused these failures. Comparative tests were conducted on ceramic bearings, PEEK bearings, and tapered roller bearings. The results confirmed that the tapered roller bearing exhibited superior environmental adaptability under lubrication with No. 10 aviation hydraulic oil. To enhance system performance, two engineering measures were proposed: (1) the use of heavy-duty tapered roller bearings to increase load capacity and fatigue life; (2) the addition of molybdenum disulfide (MoS₂) anti-wear additives to the lubricant to improve lubrication stability and wear resistance. Validation results showed that, after optimization, the prototype achieved significantly higher mechanical efficiency under 120 MPa conditions, and bearing wear was substantially reduced. These findings provide theoretical support and engineering guidance for selecting bearings and developing lubrication strategies in high-pressure, deep-sea hydraulic systems.

Large-depth manned submersibles generally conduct diving and floating operations in an unpowered mode. On the premise of guaranteeing satisfactory diving and floating speed, this mode can significantly reduce energy consumption and prolong underwater operating time, which serves as one of the key foundations for the underwater operational performance of manned submersibles. Therefore, the study on prediction methods for unpowered diving and floating motion is of great engineering significance. At present, existing prediction methods are mainly established on the standard motion model, whose results are extremely sensitive to the handling of hydrodynamic terms in the model, thus limiting the prediction accuracy. In this paper, a prediction method for unpowered diving and floating motion of manned submersibles is proposed based on geometric similarity theory, by deriving the relationship between the submersible’s diving and floating speed and its net weight in water. Taking the “Jiao Long” manned submersible as a research object, the established model is employed to predict the 7000 m unpowered diving and floating motion using the 5000 m deep-sea trial data. Comparative analysis indicates that the proposed method achieves higher prediction accuracy than the conventional standard motion model, which preliminarily verifies its feasibility and effectiveness and its general applicability remains to be further examined in follow-up investigations. Meanwhile, many assumptions are adopted in the development of the proposed method. Therefore, this method is only applicable to the quasi-steady segments of the same manned submersible, and is not suitable for transient phases such as water entry, ballast release transitions, or motions involving significant attitude coupling. Furthermore, the method is only intended for operational application phases, rather than the demonstration and design phases of manned submersibles.

Reliable operation of adjustable ballast systems is essential for full-ocean-depth manned submersibles, but existing studies mostly focus on component-level performance optimization, leaving the multi-valve coupled instability mechanism under ultra-high pressure insufficiently understood. This paper investigates an uncontrolled water injection failure in the adjustable ballast system of the “Fendouzhe” submersible via fault tree analysis, high-pressure physical tests and AMESim dynamic simulations. Results show that the initial fit clearance of the downstream flow valve is the dominant prerequisite for instability, and a fluid–structure interaction positive feedback loop between the series valve assembly drives the flow surge fault. Removing the series flow valve fundamentally interrupts the coupling feedback loop. Full-range 0–115 MPa bench tests and follow-up sea trials confirm that the modified system eliminates the observed flow surge within the tested pressure range and operating conditions. This work provides a practical diagnostic framework and engineering reference for reliability design of high-pressure seawater hydraulic systems.

When a submersible is sitting on a seabed, it could lose buoyancy because of the bottom-sitting adsorption effect. In this article, a numerical calculation model and experimental scheme for eliminating the bottom-sitting adsorption effect of under-sea equipment were established. An analysis of the hydrostatic pressure variation on a submersible’s bottom was carried out, and a submerged water jet which was based on the method of soil liquefaction was proposed to solve the problem of reducing hydrostatic pressure. It was shown that a water jet could liquefy soil to restore hydrostatic pressure on the submersible’s bottom, and there was an optimal jet velocity to form the largest liquefied soil thickness. A rectangular pulsed jet was the best way to liquefy soil in terms of efficiency and the liquefaction degree, which can be seen from the calculation of the two-dimensional two-phase flow. Through the calculation of the three-dimensional two-phase flow, it was found that the soil liquefaction developed from the periphery to the center, and a variation in jet liquefaction with the top wall constraint was obtained. Finally, an experiment was carried out to prove that a submerged water jet could eliminate the bottom-sitting adsorption effect of a submersible. The results showed that the submerged jet was an efficient way to liquefy soil, and a submersible could quickly recover hydrostatic pressure on the bottom and refloat up independently.