Viscous Hydroelasticity of Riser for Pumping-Up Deep Ocean Water

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The aim of this research is to develop a dynamic analysis method for predicting deep-ocean-water pumping-up risers. Firstly, motion measurements of oscillated flexible pipe models were carried out to investigate conditions when vortex induced vibrations occur. The experimental results suggested that vortex induced vibrations occur at Keulegan-Carpenter number higher than 8, and the transverse motion amplitude becomes very large when the motions of the pipe bottom end form 8-shaped figures. Secondary, a symmetrical vortices model was developed to predict viscous forces acting on riser elements. This method is applicable to irregularly oscillating riser elements at Keulegan-Carpenter number lower than 8. Motion measurements of oscillated flexible pipe models were also carried out to validate a dynamic analysis method using the symmetrical vortices model. The results showed that the developed analysis method can estimate the riser motions in both regularly and irregularly oscillating conditions. Thirdly, effects of internal flow were experimentally and theoretically investigated. The results of motion measurements and the dynamic analyses considering the internal flow effects demonstrated that the horizontal displacement of the pipe with internal flow is larger than that without internal flow due to the reaction force of the intake flow. Finally, a dynamic analysis method considering transverse motions was developed to estimate motions with vortex induced vibrations. This method can simulate 8-shaped motions and motions with three antinodes.