The hydroelastic response is significant from the viewpoint of the structural design of a Mega-Float. Equipping a Mega-Float with some additional structures, such as vertical plates, is one of the ways to reduce the hydroelastic response easily. However, in general, wave drift force acting on the Mega-Float may be increased, when the Mega-Float is equipped with the additional structures to reduce the response. In the present study, we developed a novel additional structure that was effective in the reduction of both hydroelastic response and wave drift force. Furthermore, we estimated the response of Mega-Float equipped with the additional structures, and compared the result with the value measured in at-sea experiments. As a result, we confirmed both the effectiveness of the novel additional structure and the validity of the estimation method.
For subsea mining, the prediction of pressure loss due to the hydraulic transport of solid particles in the flexible pipe to connect the mining tool and the lifting system is important for the design of mining system. The configuration of the flexible pipe is expected to have an inclined part. In the present paper, the authors developed a mathematical model to predict the pressure loss in inclined pipes. The total pressure loss is expressed by the summation of the loss due to a liquid single-phase flow and the additional loss due to the existence of solid particles. The additional pressure loss can be divided into the variation in static pressure due to the existence of solid particles, the loss due to the particle-to-pipe wall friction and collisions, and the loss due to the particle-to-particle collisions. The empirical formula in horizontal pipes proposed by the other researchers was applied to the model of the last two losses. Furthermore, we carried out the experiment on hydraulic transport of solid particles in a pipe. In the experiment, alumina beads, glass beads, and gravel were used as the solid particles, and the inclination angles of the pipe were varied to investigate the effect of the pipe inclination on the pressure loss. The calculated pressure loss using the model was compared with the experimental data. As the results of the comparison, it was confirmed that the developed model could be applied to the prediction of the pressure loss in inclined pipes.
Subsea minerals exist in the deep water within Japan’s exclusive economic zone. There are many technical issues which should be addressed for subsea mining. The air-lift pumping systems are one of promising methods for subsea minerals transport. Flow assurance for three-phase flow is important to design the air-lift pumping system for subsea mining. It is important to establish methods for estimating void fractions and frictional pressure drops. To establish the methods for three-phase flow, we reviewed previous studies for two- or three-phase flow. There are some models to estimate the void fractions such as slip flow model and drift flux model. There are also some models to estimate the frictional pressure drops such as homogeneous model and separated flow model. We calculated void fractions and frictional pressure drops by existing correlation and compared calculated results with experimental data in two- or three-phase flow. In addition, we proposed the methods for estimating the void fractions and frictional pressure drops in three-phase flow. These had fewer number of experimental constants than existing correlations, these could calculate void fractions and frictional pressure drops in more various conditions than existing correlations.
The evaluation of pipe wear is necessary to design the mining system for development of seafloor massive sulfides. The authors conducted three types of the flow loop wear experiment to obtain the basic data of pipe wear during slurry transport. In these experiments, the characteristics of pipe wear for different inner materials and pipe inclinations were investigated. As a result of the experiments, the wear resistance of SUS304 was shown to be higher than those of plastic and rubber materials, and the wear volume of pipe inclined at 30 degrees was largest in the experiment. Then the influence of solid degradation on the wear rates was also investigated. The authors estimated the wall thickness reduction for the lifting pipe based on the results of the flow loop wear experiments. The estimation results showed that it took 89 days to reduce wall thickness by 1mm for a rigid riser made of SUS304.
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