In recent years, a number of deepwater development projects are running worldwide. As the water depth increases, safety and reliability of the riser is a critical issue. One of the most important matters is vortex-induced vibration (VIV). In the deepwater area with strong current, for example in the Gulf of Mexico, a riser pipe’s VIV motion changes continually due to the change of hydrodynamic conditions. In order to assess safety and reliability of deepwater riser, investigation of the dynamic variation of riser VIV is important. This paper deals with top tensioned riser. To clarify the phenomena of dynamic variation of the riser’s vibration mode, a large-scale experiment using a 28.5 m long model was carried out at the Deep Sea Basin of the National Maritime Research Institute in Japan. In this experiment, the model’s bottom end was fixed, while top end was connected to a tensioner. The current generating system was installed to make flow from water surface to about 5m in depth. A measurement of riser VIV motion in current condition was carried out by changing the model’s top tension from self-weight balanced condition to four times of the self-weight. Vibration mode widely changed from high mode of up to 15th to low mode of 5th. In order to investigate an influence of the change of vibration mode on the riser VIV motion, a series of experiments were conducted with the periodic variation of top tension. It was found that the traveling wave occurred from top to bottom end in a moment of vibration mode changed.
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.
Seafloor Massive Sulfides (SMS), which were formed by deposition of precipitates from hydrothermal fluids vented from seafloor, is one of unconventional mineral resources beneath deep seafloors in the world. The authors have proposed the concept of seafloor mineral processing for development of SMS, where useful minerals included in SMS ores are separated on seafloor to be lifted while the remaining gangue is disposed on seafloor in appropriate ways. To apply column flotation, one of conventional methods in mineral processing, to seafloor mineral processing, the authors carried out simulating experiments of column flotation on deep seafloor using ores including copper, iron, lead and zinc as metallic elements. Prior to the experiments at high pressures, preparatory experiments at the atmospheric pressure were carried out to find out the optimum condition of the properties of pulp, a mixture of feed ore, water and chemical reagents. In flotation experiments at high pressures, formation and overflow of froth layer by bubbling were observed at 1MPa in both of pulps with pure water and artificial seawater. The analytical data showed that the concentration of metallic elements such as copper and zinc in the concentrates recovered from the experiments was higher than that in the feed ores while the concentration of silicon and calcium, which are assigned to gangue, in the concentrates was lower than that in the feed ores. These results suggest that column flotation can be applied to operation on seafloor.
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