Theoretical calculation was made for the flow field in the hydraulic transportation with deep-sea polymetallic nodule collector, and the relationship between particle size of polymetallic nodules and minimum conveying speed was obtained. The distribution of flow pattern in the conveying channel was simulated with the jet velocity of nozzle at 15 m/s, 20 m/s and 25 m/s respectively, and  rate of the flow at 30 mm away from the lower surface of the channel was measured. A laboratory test was conducted for nodules pick-up by using a collector with different jet velocities of nozzles, and it is found that the obtained results are consistent with the theoretical calculation and simulation results.

The lifting of polymetallic nodules was simulated by using a CFD-DEM coupling approach, and the influence of different swirling flow on hydraulic lifting of polymetallic nodules in vertical pipelines was explored. The results show that as swirl ratio increases, the intensity of swirl will be enhanced significantly, and both the maximum fluid velocity and the average axial velocity of particle group will increase. It is found that intensity of swirl can bring obvious impact to the distribution of particles with different velocity in the pipe, showing that particles with high velocity are gradually distributed around the pipe wall, while the particles with low velocity in the center of the pipe. Swirling flow can reduce the local concentration of particles, which is beneficial to reducing the risk of pipeline blockage due to higher concentration of particles caused by retention effect.

Based on the experimental study on fragmentation and pulverization characteristics of polymetallic nodules in the process of ore lifting, it is found that there is obvious fragmentation of nodules during lifting process. With the volume concentration of 6.5%, the mass percent of polymetallic nodules with particle size larger than 50 mm decreases from 75.58% to 23.78%; with the volume concentration of 15%, the mass percent decreases from 51.51% to 10.08%.Pulverization of nodules was observed in the experiment. With the volume concentration of 6.5%, the mass percent of the polymetallic nodules powder with particle size less than 0.1 mm is 1.79%, and the median particle size is 10.07 μm; with the volume concentration of 15%, the mass percent of polymetallic nodule powder with particle size less than 0.1 mm is 1.99%, and the median particle size of the powder is 9.47 μm. Based on the experimental data, it is estimated that as per mining of 10 million tons of polymetallic nodules, the powder discharge will cause tens of thousands of tons of Fe and Mn losses, as well as hundreds of tons of Co, Ni and Cu losses. Besides, the maximum redeposition thickness of the sediment plume induced by mining is more than 3 mm after diffusion and settlement, which will bring a significant impact to environment.

Based on the time-domain coupling analysis method, a three-dimensional nonlinear hydrodynamic model for mother vessel, deep-sea mining equipment and umbilical cable was established, and numerical calculation and analysis were also conducted for the hydrodynamic response characteristics of umbilical cable during launch and recovery. The results show that deep-sea mining equipment (weighing 10 t in water) can be safely launched and recovered at a speed of 0.8 m/s under condition of sea state 4. Due to the influence of both waves and currents, the deep-sea mining equipment will have strong oscillations and swings at the beginning of launch and at the end of recovery, which can be reduced by installing anti-swaying devices.

In order to improve the working performance of drum cutter in mining mechanism, the cutting process of drum cutter under different working conditions was numerically simulated by using discrete element method (DEM). The effects of cutting thickness, rotating speed and tractor speed on cutting resistance, productivity and specific energy consumption were investigated by single-factor analysis and orthogonal testing. With cutting resistance and specific energy consumption of the drum cutter as evaluation indexes, the best working parameters of the drum cutter were obtained by matrix analysis. The results show that as cutting thickness increases, both cutting resistance and productivity increase, while specific energy consumption decreases. With rotation speed increases, cutting resistance and productivity decrease, but specific energy consumption increases. And with tractor speed increases, both cutting resistance and productivity increase, while specific energy consumption decreases. It is concluded that the optimum working parameters of drum cutter are as follows: cutting thickness of 50 mm, rotation speed of 100 r/min, and tractor speed of 0.06 m/s.

In order to explore the reliability of type selection in the design of lift pipe for underwater transport system in deep sea mining, hydrodynamic parameters were checked with transport indices as input for design. Based on the settling velocity of polymetallic nodules, anti-fatigue threaded sleeve of P110 steel, with outer diameter of 244.48 mm and wall thickness of 20.24 mm, was selected for lift pipe. And a towing test performed in a pool shows that the resistance drag coefficient is 1.4 for the lift pipe at the upper end of the pump and 1.3 at the lower end of the pump. Hydrodynamic parameters were calculated and analyzed for the underwater transport system according to working conditions and launching conditions, showing that the safety factor of lift pipe can meet the requirements of API RP 2RD Standard. In the deep-sea polymetallic nodules mining test project, lift pipe in the at-sea acceptance test, with reliable joint connection, could meet the requirement for each transporting parameter, which has verified that the type selection of lift pipe in the design is acceptable.

Based on an introduction of Japan's operation mechanism for exploiting deep-sea mineral resources, including Japan's marine policies, management and implementation agencies related to ocean mining, as well as Japan's coordination mechanisms for pushing progress in ocean technologies, Japan's cross-ministerial Strategic Innovation Promotion Program is outlined in terms of ocean mining, and the research achievements made by Japan in this field are also summarized. Such research on Japan's management system and program for exploitation of deep-sea mineral resources can be of reference for China in developing strategies, formulating investment mechanism and coordination mechanism for deep-sea mining.

Based on the environmental characteristics of polymetallic nodules mining zone, different lifting modes for deep-sea polymetallic nodules, including continuous line bucket (CLB) system, autonomous submersible shuttle mining system and pipeline lifting system, were analyzed in terms of characteristics and applicability. With eight factors taken into consideration, including maneuverability, dry ore lifting capacity, launching and recovery efficiency, manufacturing and operation costs, suitability for water depth, adaptability to extreme sea condition and seabed disturbance, as well as environmental friendliness, a mathematical model covering multi-dimensional evaluation indices and an adaptability evaluation system were established. Analysis results show that CLB system and capsule pipeline lift system are not suitable for lifting polymetallic nodules, while autonomous submersible shuttle mining system, due to high cost in R & D, is not economically applicable. Due to the improvement in the reliability of underwater equipment and progress in the solution to sealing of heavy medium, pipeline lift system with heavy medium is expected to become a high-efficient lifting mode for deep-sea polymetallic nodules.

The concentration of plume caused by hydraulic collecting system during deep-sea polymetallic nodules mining under different working conditions were analyzed. Test results show that the concentration of discharged plume from conveying system is much higher than that of sediment plume caused by the disturbance of capture system, which is one of the main causes of turbidity current in the sea. In the test, the curve of slurry concentration presented a steep upward trend line followed by a steep downward line. Increasing oil pressure of the pump of capture system will lead to higher concentration of slurry. However, the oil pressure of the pump of conveying system won't result in obvious variation in slurry concentration.

The motion of spherical, tetrahedral and hexahedral particles flowing in a vertical pipe under different working conditions was simulated by using computational fluid dynamics-discrete element method (CFD-DEM), and the motion of irregular particles flowing in a vertical pipe was obtained. Based on comparison of local concentration and local flow rate, it is found that with lower flow rate at the inlet, the shape of particles can bring a larger influence to local concentration; while with a higher feed concentration and lower inlet flow rate, the shape of particles can bring obvious influence to local flow rate.

In order to explore disturbance of seabed sediment during mining of polymetallic nodules, a pool experiment was carried out to scour sediment by a waterjet from polymetallic nodules collector. Variation of gully depth, and migration and diffusion regulation of sediments were investigated in the experiment by setting the waterjet at different speed, with nozzle at different height from sediment, and collector at different traveling speed. The results show that the collecting process of polymetallic nodules brings scour to both particles and blocks of sediment, leading to uneven surface of the gully. Then, disturbed sediment migrates and diffuses through upper plume flow and bottom gravity flow.

As for pipes with different lengths, with heave compensation device replaced by elastic constraint, a vibration equation was established by using energy integration approach, and the dynamic response of lifting pump installed at different positions of pipes was analyzed by finite element method. Results show that the harmonic response of stress excited by the following single loads was in an ascending order: torque, wave in longitudinal direction, and a combination of wave and current in horizontal direction. Under excitation by all these three loads with the same phase, the harmonic response of the system increased significantly. When the installed lifting pump was changed from a high position to a low position, the maximum equivalent stress of pipeline increased after an initial falling down. The installation position of lifting pump on the pipe corresponding to the minimum of the maximum equivalent stress was the suboptimal installation position. Based on the suboptimal installation position for the lifting pump on the pipeline with specific length, an empirical formula was established for the suboptimal installation position, which can provide reference for the quantitative design of lifting system.

Aiming at complicated flow field of Y-shaped pipe in pump piping system and risk of ore leakage in conveying test for deep sea mining, numerical simulation was performed by using CFD-DEM method for ore particles with different velocities, different volume concentration and different particle sizes. Based on that, the movement features of particle groups and flow field distribution pattern in Y-shaped pipe were all analyzed. It is found that the flow field distribution in Y-shaped pipe is influenced by the curvature of pipe elbow and the flow distribution in the inlet of Y-shaped pipe. The particle groups flow against the inner wall of elbow, and deflect upward under the influence of upward flow. If upward flow with lower velocity, particles with large grain size and high volume concentration will hit the bottom of pipe, leading to some losses. As the upward velocity decreases and both particle volume concentration and grain size increase, more and more particles will be continuously lost on the bottom of pipe. Due to the existence of particle flow, both intensity and scale of the low-pressure vortex region in Y-shaped pipe become weaken.

Based on the research data from ERA5 reanalysis data, the marine environment data of the central place of the testing area for deep-sea mining was taken to construct the input characteristics, and then the gray wolf optimization (GWO) algorithm was adopted to optimize the parameters of the support vector regression (SVR) algorithm. The sea wave height prediction effect obtained by GWO-SVR, traditional SVR and PSO-SVR based on particle swarm optimization (PSO) algorithm were compared and analyzed. Furthermore, three data models were constructed and 63 groups of tests were performed for exploring the influence of the surrounding environment and wave state on the prediction effect with GWO-SVR. The results show that GWO-SVR can effectively predict the sea wave height with higher prediction accuracy. When more marine environment data are used to construct input characteristics, the prediction accuracy of GWO-SVR algorithm can be effectively improved.

In order to improve the accuracy of deep-sea mineral estimation in the investigation of the basic morphological structure, the distribution of minerals and other occurrence of polymetallic nodules in the contract area using the modern image processing technology, a seabed mineral image segmentation system was proposed based on the combination of Sea-thru and Mask R-CNN. The experimental results show that Sea-thru can not only effectively enhance the seabed mineral images, but also restore the decayed mineral images, while Mask R-CNN can improve the segmentation accuracy of mineral particle images, thus providing more accurate mineral particle information, which can improve the accuracy of mineral resources estimation.

In order to isolate the relative motion of buffer station from hose and mining vehicle, a bottom-resting deep-sea mining system was proposed. With the influence of wave and current taken into consideration, a whole system coupling analysis model consisting of mining ship, tensioner and transportation pipe was established, by which the tension of the tensioner, equivalent stress of the transportation pipe and the angle of the flex joint under various designed working conditions were calculated. The results meet the relevant requirements of specifications, verifying the feasibility and safety of the bottom-resting deep-sea mining system.

A numerical model was established by adopting coupling computational fluid dynamics and discrete element method, and then was used to perform a numerical simulation for the solid-liquid two-phase flow in the delivery pump. In consideration of the interaction between the solid and liquid phases, the influence of solid particle size on the wear and failure of the impeller in the pump and the underlying mechanisms were explored by adopting the Euler-Lagrange model and Archard wear model. The results show that the blades undergo the most severe wear among the components of the impeller, with the wear concentrated at the outlet of the working surface of the long blade and the blade inlet. As the solid particle size increases, the wear in the blade inlet becomes increasingly severe while the blade working surface presents an opposite wear tendency. Meanwhile, there's less probability for the solid particles to impact against the working surface of the impeller, but they tend to impact the short blades.

Based on the idea of discontinuous polymetallic nodule mining system, a submerged floating cabin lifter was designed for the transportation of seabed polymetallic nodules. According to the functional requirements of the submerged floating cabin lifter, the overall structure and layout were designed and the hydraulic simulation analysis was carried out by using a fluid simulation software for studying the stress of the cabin lifter under different sea conditions. The calculation results show that when the cabin lifter is moved horizontally, the differential pressure resistance predominates, and the main body suffers the greatest resistance; when the cabin lifter is moved vertically with the acoustic bin in a position of facing the flow, it is beneficial to reduce the lateral water resistance and improve the operation and control of the cabin lifter. Also, the calculation of the force on the cabin lifter being moved vertically under different current velocities shows that the resistance variation in X direction increases obviously, but the forces in Y and Z directions show little variation with the increase of the movement speed.

In order to understand the movement rule of particles in the slurry pump, two-phase coupling of the unsteady flow of coarse particles and seawater in the lifting pump model was calculated by using the CFD-DEM coupling algorithm in consideration of volume effect of particles and collision effect in the phase. The spatial distribution and movement characteristics of the particles with different particle sizes in the pump were analyzed. The results show that the solution method with two-phase coupling can simulate the two-phase flow field in the slurry pump. The particle concentration in the flow domain is slightly higher than the average value along the pipeline. The motion of particles in the stratified fluid in the impeller and guide blade can be obviously observed. The energy obtained by small particles is higher than that of large particles, and the presence of particles increases the local vortices and phase losses. The calculation results are in good agreement with the test results, proving that the calculation method adopted in this paper is effective. The research results can provide theoretical support for the design and analysis of deep-sea slurry pump with high-efficiency and non-clogging characteristics.