The high ambient pressure in deep-sea conditions greatly increases the viscosity of hydraulic oil and then the pipeline pressure loss in deep-sea hydraulic systems. Large pipeline pressure loss can lead to a further change of viscosity on the basis of the viscosity increase caused by the ambient pressure when the hydraulic oil flows through the pipeline. Therefore, the classic Poiseuille’s law can no longer accurately calculate the pipeline pressure loss in deep-sea conditions since it treats the viscosity as a constant. In this paper, based on laminar flow theory and the viscosity-pressure characteristics of hydraulic oil, a novel equation for pipeline pressure loss is proposed, in which the viscosity change when flowing through the pipeline is taken into account. A CFD (Computational Fluid Dynamics) model of a pipeline in the deep-sea hydraulic system has been established, and CFD simulations have been conducted to verify the correctness of the proposed equation for pipeline pressure loss. Theoretical analysis shows that the proposed novel equation for pipeline pressure loss is equivalent to the classic Poiseuille’s law when the pipeline pressure loss or the viscosity change is low. The research results in this paper can provide theoretical support for work efficiency optimization, load capacity improvement, and precise control of deep-sea operation equipment or deep-sea hydraulic systems.

Hydraulic system is widely used as a power source in deep-sea operation equipment. The sealing performance and the relative movement of the parts in hydraulic components are significantly affected by the structural deformation at the clearance fit and the viscosity increase of the hydraulic fluid medium, which are both caused by high seawater pressure. In this paper, the deformation formula at the clearance fit in the deep-sea environment is deduced, which indicates that the deformed height of the fit clearance decreases linearly in the axial direction. A minimum clearance design criterion is proposed, and it is found that the smaller the difference between the bulk modulus of the matching parts and the fit radius are, the smaller the variation of the height of the fit clearance is under various working conditions and the smaller the required minimum initial height is. The leakage flow rate formula at the clearance fit in the deep-sea environment is deduced as well, which introduces modified factors to consider the effects of structural deformation, viscosity increase, and eccentricity. The calculation result shows that under the condition of 11 000 m deep sea, the leakage flow rate calculated by the classic formula is about five times larger than that of the modified formula. Multi-parameter fluid–solid-interaction simulations are carried out to verify the correctness of the deduced deformation and leakage flow rate formulas. The leakage flow rate of the situation with inclination involved is also analyzed through numerical simulations.

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