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.
For subsea mining, the estimation of pressure loss in the pipe of lifting system and the flexible pipe to connect the mining tool and the lifting system is important to design the mining system. The configuration of flexible pipe is expected to have an inclined part. In the present paper, the authors carried out the experiment to measure the pressure loss in inclined pipes using alumina beads to investigate the effect of inclination angle of pipe on the pressure loss. Furthermore, a mathematical model to estimate the pressure loss in inclined pipes was proposed and validated through the experiments. As the result of the validation, it was confirmed that the proposed model could be applied to the pressure loss estimation in inclined pipes.
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