| Successful deployment of mining system is the basic and premise of deep sea mining operation. According to the dynamics characteristics of mining system, the deploying process can be divided into two parts:the first part is from beginning of deployment to water entry of mining tool when a complicated air-body-water coupling process occurs, and the bottom of mining tool would bear a huge impact force, exerting big influence on deploying equipment, which even results in failure of deployment in hostile ocean conditions; the second part is from the time that mining tool fully immerses in sea water to mining tool reaching to seafloor, because of the effect of sea wave and motion of mining vessel, the mining system in deploying process is in unstable state, and the whole mass of immersed parts of mining system is more and more big, leading to considerable large load in connecting parts between mining system and mining vessel. The study of deployment and hydrodynamic characteristics of deep sea mining system is an important but seldom concerned subject. Based on the 1000m sea pilot system of deep-sea mining tool of China, exploratory researches are conducted which focus on deployment and hydrodynamic characteristics of mining system in key process of deployment.Efforts have been made and the results are presented as follows:1. The two-dimensional (2D) numerical wave tank is developed using CFD code Fluent. A piston-type wave-maker is incorporated in the computational domain to generate the desired waves, including linear waves of different ocean conditions,2-order Stokes wave and random wave based on Pierson-Moskowitz spectrum. Towards the end of the computational domain, the porous media technology is applied to form an artificial damping zone so that the wave energy is gradually dissipated in the direction of wave propagation. To verify their applicability in two dimensions, the computational results are compared with analytical solutions, showing good agreement; the effect of permeability of porous media is discussed, a suggestion for choosing permeability of porous media is made. It is shown that an efficient method for the simulation of waves is successfully established for research of wave-structure interaction by CFD codes.2. The hydrodynamic problems of a two-dimensional model of seafloor mining tool entering still water vertically are analyzed based on the velocity potential theory. Two conditions are mainly discussed. The first is that mining tool enters still water in constant speed. Pressure distribution, maximum impact pressure, impact duration time during the water entry of mining tool are presented at various deploying velocity, the two peak pressures in impact process are observed, the relationship between maximum impact pressure and deploying velocity is obtained. The results are compared and verified with that based on other prediction theories and methods. The second condition is that mining tool enters water in free fall motion. The time histories of velocity and pressure during the impact process are obtained. Residual air entrained between the bottom of mining tool and free water surface is observed, and influence which exerts on impact is discussed. The concept of impact added mass is presented to express the coupled function between air-rigid body-water during water entry process. Two dimensionless ratios are determined which the drop simulations should comply with to simulate an average slam:drop height ratio and mass ratio. The relationships between dimensionless ratios and several important parameters derived from simulation results are discussed.3. Based on the studies of numerical wave tank and mining tool entering still water, the mining tool impacting with sea water with the influence of ocean waves are studied. The condition that the mining tool is suspended still above the water surface is considered, the simulation results show the impact pressures of this condition are mainly determined by the instant acceleration of water particles on wave surface. The second condition is that mining tool enters waves, the time points and positions of which the pressures on bottom of mining tool reaches to their maximum values are obtained, and the time histories of impact pressure of bottom of mining tool, the pressure distributions and impact durations are presented, those results are compared with that of still water entry of mining tool, thus the influence that waves exert on water entry of mining tool is obtained. The similarity between mining tool entering wave and wedged-shape body entering still water is observed.4. Two numerical methods for assessing the added mass of components of deep sea mining system are mainly proposed in this work. The first method is referred to as the Hess-Smith method which is based on surface panel method and potential theory. The second method is referred to here as a Fluent method which is a constant acceleration method based on the CFD software Fluent. The added mass coefficient of components of mining system which are acquired from the two methods are approximately equal, both are applicable for infinite flow field, while for the condition of free surface, including water entry, mining tool touching down the seafloor, the Fluent method is more suitable.5. Deployment of seafloor mining tool is studied with the effects of wave and motion of vessel considered. Four ocean conditions which are proper for deployment are considered, and the effect of deploying velocity is also researched. The results indicate that the ocean condition and the deploying velocity play an important role to determine the kinematic and dynamic condition of the deployment process of deep sea mining system, and the research results can provide basic information for the design of the heave compensation system and reduction of dynamics effects of riser system on seafloor mining tool during the deploying process.6. Experiments of still water entry of body are conducted by six dimension-of-freedom (DOF) experiment platform and water tank. The experimental model is designed and experimental equipment is constructed. The impact pressures which mining tool model bear when it enters still water in different constant velocities are obtained by measuring devices, The comparisons between simulation results and experimental results indicate that the two results are identical, thus the accuracy and dependablity of simulations of water entry study based on CFD code Fluent can be verified. |
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