A Study On Structure Dynamics And Vortex Shedding Vibration Of Drilling Risers Under Hang Off Mode

Drilling platform is needed to be evacuated with the risers hung off in advance when encountering severe typhoons during drilling operation because of the consideration of safety. There exists a security risk due to vortex-induced vibration under the interaction of the ocean current and the navigation of platform.This paper applies the FEM software Orcaflex to simulate the dynamic response under different suspended length, current speed and driving directions of hard hang-off drilling riser system, and to analyzes the feasibility of hard hang-off risers when evacuated in hard hang-off mode to escape from typhoon. The study shows that it is needed to decrease the suspension length as the current speed increases and the dynamical load gets strengthen. When the course angle gets bigger, the platform sails downstream to upstream actually, then the surrounding load increases and the security reduces.SHEAR7 is used to study the VIV characteristics of the drilling risers, under hard hang-off mode when evacuated to avoid typhoon. Then the vortex-induced vibration responses of actual size riser system are analyzed in different sailing conditions and under different suspension lengths. It is found that it is not the shorter the better, we need to analyze the combined effects of ocean current, configuration of risers and the navigation condition of platform. The computation indicates that it is necessary to avoid the layout of buoyancy modules in the high current velocity region and avoid the platform sailing upstream.Finally, CFD software FLUENT with user defined function to simulate the vortex-induced vibration response of deepwater drilling riser and compute the fluid-structure interaction under 1DOF and 2DOF to obtain the fluid forces and VIV characteristics of actual size riser. The vortex-induced vibration amplitude, frequency and lift and drag coefficients have great relationship with the reduced velocity. In the lock-in area, the vortex-induced vibration amplitude, lift coefficient and drag coefficient are larger.