Experimental investigation on vortex-induced vibrations of a hang-off evacuated drilling riser

A model experiment has been conducted for a hang-off evacuation riser considering the lower marine riser package (LMRP) based on the strain gauge test technology. The finite element eigenvalue method is used to analyze the natural frequency and mode shapes of the hang-off riser considering the axial tension force and the weight of LMRP. The modal analysis method is used to restructure the motion and analyze the vortex-induced vibration characteristics of the hang-off riser. Then, the influence of evacuation velocity is discussed. Results indicate that the mode shapes amplitude of the experimental riser with LMRP is significantly larger than that of the ordinary cantilever. The dominant frequency of each measuring point is the same, and the end-cell-induced vibration small frequency is observed near the bottom end. The vortex shedding in the cross-flow (CF) direction produced a large lift force, which resulted in the vibration frequency of in-line direction dominated by the CF direction. When the reduced velocity (V-r) is less than 40, the frequency response of the hang-off riser model increased with the evacuation velocity, which was subjected to the Karman vortex shedding frequency. When the frequency response of the riser model reached, the second-order natural frequency with V-r exceeds 40. Consequently, the "lock-in" phenomenon was observed and the riser exhibited vibrations with the second-order model.