Experimental investigation on the vibration characteristics induced by severe slugging in a pipeline S-shaped riser system

The investigation of the severe slugging characteristics and its induced vibration is an important basis for effectively avoiding fatigue damage or excessive stress in the offshore riser system. In this paper, a non-invasive optical method is used to study the vibration induced by severe slugging in a pipeline S-shaped riser system. Based on the image data of the gas-liquid interface structure and differential pressure fluctuation, severe slugging is classified into three sub-flow patterns (SS1, SS2, and SS3), and the flow characteristics and dynamic response of the riser are investigated at three sub-flow patterns. The two peaks in the spectral of the riser vibration response correspond to the fluctuation frequency of the differential pressure and the first natural frequency of the riser, respectively, and then a correlation for predicting the response frequency induced by severe slugging is established. The vibration amplitude of the riser is strongly related to the bubble length and the liquid slug velocity in the gas blowdown stage. The in-plane vibration responses of the riser are dominated by the first and second modes. The two modes coexist and switch over time due to the continuous change of slug characteristic parameters.