Study On Vibration Response Of Marine Riser Under Multi-field Coupling Of Wind And Wave

The riser in the coupling condition of wind-wave-current will present nonlinear fluid-structure interaction, as the condition is complex and changeable in medium and shallow marine environment. The numerical tanks for multi-field coupling of wind-wave-current have been created, which are based on the Fluent software, borderline method of wave generating, damping wave elimination technique, VOF method of free surface's treatment. The marine riser's vibration responses in coupling condition of wind, wave and current are investigated numerically by two-way coupling with a fluid-structure interaction computational method. The main works of this paper includes:(1) The numerical tanks for wave, the coupling condition of wave-current and the coupling condition of wind-wave-current have been created in present work. It is proved that the methods of creating numerical tanks is feasible through analyzing the pressure contours, velocity contours, and phase distribution by combining the numerical simulation results with the theory.(2) The vibration of the riser is investigated in the numerical tank of wave. The in-line vibration frequency of marine riser only in wave condition is equal to the wave frequency and the influences of the second order harmonic force to vibration in the cross-flow direction will decrease with the relative water depth increasing. And the vortices, which are big at top and small at bottom, will be shed from the riser.(3) When the riser is in the coupling condition of wave-current, the in-line vibration frequency is also equal to wave frequency and the cross-flow vibration frequency will appear multi-frequency phenomena with relative velocity increasing. The main frequency of the cross-flow vibration is gradually changed from 2 times of wave's frequency to the frequency of wave, the secondary frequency is 2 times, and the frequency of the three stage occurs. The balance position of the vibration decreases linearly with the increase of the relative velocity. The high relative velocity will lead to the vortex shed earlier from riser than low relative velocity.(4) The vibration responses are totally similar, as the influences of wind is little, between the coupling condition of wind-wave-current and the coupling condition of wave-current.(5) The in-line amplitude magnitude of riser is 10-3m and the cross-flo amplitude magnitude is 10-5m-10-4m in present work. The reasons for it are the large riser's stiffness, the small depth-diameter aspect ratio and reduced velocity. In the future, the vibration response simulation of large and long diameter ratio and flexible riser can be carried out in order to analyze the contribution of each field.