The valve-controlled hydraulic cylinder system (VCHCS) is commonly used for actuators such as manipulators in deep-sea equipment, whose working performance is crucial to subsea tasks. Affected by the ambient pressure introduced by the pressure compensator, the viscosity of the hydraulic oil increases significantly. On this basis, the viscosity changes further when flowing in the slender pipeline, making the pipeline pressure loss substantially increase and subsequently affecting the working performance of the deep-sea VCHCS. Aiming at this issue, a detailed nonlinear mathematical model of the deep-sea VCHCS is established, in which the viscosity-pressure characteristics of the hydraulic oil is considered to take the viscosity changes in the pipeline into account. Besides, the hydrodynamic effects are also included in the model. Then the corresponding numerical simulation model of the deep-sea VCHCS is established, and its working performance at different depths is simulated and analyzed. When the depth is 11km, the extension and retraction movements are delayed by 52.50% and 43.12% respectively. The root cause of the delay is then analyzed and discussed. Finally, the parameters that affect the working performance are studied, and suggestions to reduce or eliminate the delay phenomenon are given. The results can provide theoretical support for the performance optimization of the deep-sea VCHCS.

The valve-controlled hydraulic cylinder system (VCHCS) is commonly utilized in the underwater manipulator, which is the most important tool for subsea tasks. Hydraulic oil viscosity is very sensitive to pressure. Therefore, when working at different depths under different ambient pressures in the sea, the hydraulic oil viscosity and the pipeline pressure loss in the deep-sea VCHCS vary greatly, which seriously affects the energy efficiency of the system. In addition, the control accuracy of the deep-sea VCHCS is also influenced by changes in the hydraulic oil viscosity and the pipeline pressure loss. In order to realize energy-saving control, this research introduces a proportional relief valve and develops a variable pump pressure control strategy. At the same time, a variable gain proportional-integral-derivative (PID) algorithm is designed to achieve precise control. A co-simulation model of the deep-sea VCHCS is then established, and many simulation analyses are carried out. Compared with traditional PID control with a constant pump pressure, the proposed method presents advantages such as lower energy consumption, better control accuracy, better resistance to load impact, and accuracy consistency under different working depths. Among them, when working at 11 km depth in the sea, the proposed method is capable of saving energy by 36.5% for the multi-step movement, by 30% for the harmonic movement, and by 47% for the complex movement. The present work in this research provides a solution that can realize energy saving and precise control of the deep-sea VCHCS at the same time in the wide span of depth in the sea.

The hydraulic collector is an important device for collecting seafloor polymetallic nodules. In this study, a hydraulic polymetallic nodule collector with two acquisition nozzles and one transmission nozzle is described. The numerical model of the hydraulic collector is established based on the solid–liquid two-phase flow method, and it is verified by experimental tests. On this basis, the collection mechanism of the hydraulic collector is analyzed, and the effects of structural parameters and working parameters on its collection performance are explored. The results show that the collection height and slant angle of the acquisition nozzle are key factors for collection efficiency, with optimal heights below 150 mm and angles between 45 and 49${}^{\circ }$. The recommended range for the center distance between the two acquisition confluence tubes is 650–730 mm. Excessive acquisition and transmission flow rates make a negligible contribution to improving the collection efficiency, but can also cause a significant increase in energy consumption. Therefore, the recommended ranges for acquisition and transmission flow rates are 140–160 m${}^3$/h and less than 80 m${}^3$/h, respectively. All of the results indicated that the parameters of the developed hydraulic collector were set reasonably, which thus ensured a balance between the collection efficiency and energy consumption.