Fluid-solid Coupling Analysis And Comprehensive Evaluation Of Working Characteristics Of Pipelines In Deep-ocean Mining System

Working characteristics of pipelines in a 1000m deep-ocean mining system was studied theoretically, numerically and experimentally, including space configuration, mechanical properties and transportation performance. Optimal layout of buoyancy blocks imposed on the flexible hose and walking path of the mining machine was proposed based on comprehensive evaluation of working characteristics of the lifting pipeline, which can provide a scientific basis for the design of the deep-ocean mining system.Main research results are as follows:(1) Based on simplified mechanical model of the lifting pipe in deep-ocean mining system, a new program of meshless local Petrov-Galerkin method in Matlab language was designed to calculate offset characteristic of the lifting pipe subjected to current resistance. Numerical results show that the maximum offset of the lifting pipe occurs near the end of buffer and the maximum tensile stress of the lifting pipe occurs near the end of the mining ship. The maximum offset of the lifting pipe increases with the increase of the towing speed in countercurrent flow. It decreases first and then increases as the towing speed increase in concurrent flow.(2) Three-dimensional nonlinear finite element model of the initial space configurations of the flexible hose was established to analyze effects of the layout of buoyancy blocks, current resistance, bearing form and elastic modulus on the characteristics of initial space configuration of the flexible hose in launching process. Numerical results show that the layout of buoyancy blocks greatly influence the height of space configuration and complete floating of the flexible hose in deep ocean. In order to ensure safety and transportation efficiency of the flexible hose, the buoyancy block should be imposed in the range of L/2-3L/4 for uniform distribution, L/3-2L/3 for concentrated distribution of single buoyancy block and L/4-3L/4 for concentrated distribution of double buoyancy blocks. The total buoyancy force of the buoyancy block must be restricted within into the range of 0.9G≦F≦1.5G (where G is net buoyancy force). Fixed or hinged constraints should be added at the two ends of the flexible hose. Current resistance and bearing form have little effect on the characteristics of initial space configuration of the flexible hose.(3) Based on fluid-solid coupling theory, three-dimensional nonlinear finite element model of fluid-solid coupling was established to investigate space configuration, mechanical properties and transportation performance of the flexible hose as well as influencing factors, including lifting velocity, yielding stress and viscosity coefficient of the mineral fluid, elastic modulus of the flexible hose and walking path and velocity of the mining machine. In order to improve stability and efficiency of the mining operation, the lifting velocity of the mineral fluid must be limited to a suitable ranges (e.g.2.5-4 m/s). Effective measures should be taken such as extension of the time of mineral crushing and desliming and decrease of the particle size, mineral concentration and sediment content). Concentration of the mineral fluid must be limited within 10%-25%. Suitable materials of moderate elastic modulus should be used for the flexible hose. Maximum stress of the flexible hose occurs when the mining machine turns in circle walking-path or changes the sign of walking displacement in square walking-path, which is key parameter for strength design of joints. Walking velocity of the mining machine must be controlled within 0.2-0.4m/s.(4) In view of difficult reflection of the similarity between the evaluated scheme and the optimal solution in traditional principal component analysis, an improved method of principal component analysis based on index optimization principle of data matrix standardization is proposed for comprehensive evaluation of working characteristics of the flexible hose, which was compared with the method of fuzzy hierarchy analysis. Similar evaluation results are obtained by using the two methods, i.e., space configurations of S-shape and saddle shape have higher evaluation indices and can be used for optimal design of the flexible hose. The improved method of principal component analysis independent of subjective factors can give real and reliable evaluation results, which is especially suitable for comprehensive evaluation of multiple-index working characteristics of the deep-ocean mining system.(5) Based on similarity theory, a test device for space configuration and transportation performance of the flexible hose is designed to experimentally study effects of the distribution of buoyancy blocks and buoyancy force on the initial space configuration of the flexible hose and effects of different initial space configurations on the transportation performance of the flexible hose for the first time. Test results of optimal space configurations are in good agreement with the numerical results of comprehensive evaluation.(6) By comprehensive consideration of the interaction of inner mineral fluid, external current and wave and the motion of mining machine and mining ship, three-dimensional nonlinear finite element model of fluid-solid coupling was established to analyze dynamical responses of the deep-ocean mining system influenced by towing velocity of the mining ship and the mining machine, wave period and nodule mass stored in the buffer. Numerical results show that for safety and efficiency of the mining operation, the velocity of the mining machine and the nodule mass should be limited within appropriate ranges (about 0.3-0.5m/s and 2000-4000kg) and the countercurrent mining operation must be avoided in the case of smaller wave period.