Experimental Study On The Vortex-Induced Vibrations Of Marine Riser Transporting Fluid

The riser is the main conductor between the seabed and the platform. It suffers from wind, wave and current. When current flows across the riser at certain speed, there will be vortex shedding, and if the natural frequency of the riser is close to that of the vortex shedding, oscillation will make the vortex shedding frequency fixed nearby to the structural natural frequency and cause large amplitude vibration which is called'lock-in'state. When lock-in state appears, it will aggravate the fatigue damage, not only causing huge loss of project itself but also leading to secondary haphazard and environment pollution. As a project supported by the National Natural Science Foundation of China, the model experiment on the vortex-induced vibration of riser transporting fluid was conducted in the physical oceanography laboratory of Ocean University of China in 2005. The impact test and the test in the current were all conducted in the water flume. Various measures were obtained from the experiments, including frequency response, the time–domain tracing of in-line and cross-flow response. The experimental results analyzed by using these measures exhibit several valuable features. One of them is that effect of the internal fluid on the riser is not obvious, for the riser stiffness is high, but the effect will becomes obvious when the current velocity becomes high enough, and the exist of internal fluid will decrease the oscillation frequency of the riser. Another is that the first vibration frequency becomes higher owning to the current varied from lower speed to higher speed, while the second, even the third one will appear when the speed is high enough.