Mechanical analysis on recoil response of deepwater drilling riser based on a complex mode method

Key words:  deepwater drilling  riser  emergency disconnection  recoil response  main control factors  control measures

Abstract:

In this study, a mechanical model and control equations for the recoil response of drilling riser were established by discrediting the riser as a mass-spring-damping system, in which the elastic potential energy stored in the riser due to axial tension in normal drilling operation and the viscous resistance acting on the riser during fluid discharge in recoil response were considered. The heave motion of the floating drilling vessel was taken as the boundary condition, and the riser axial deformation before disconnection was taken as the initial condition. The control equations were solved by a complex mode method considering the damping matrix in the governing equations cannot be diagonalized. Finally, the vibration displacement of the low marine riser package (LMRP) during recoil response was obtained. Then, the influence of water depth, wave height, spring stiffness of the top tensioner and disconnection phase angle on the vibration displacement of the LMRP was discussed in details. The analysis results show that, under the same operating conditions, the greater the water depth, the smaller the safe operating window of the riser emergency disconnection, and the higher the wave height, the more severe the riser recoil response. The stiffness of the top tensioner is an important factor determining the riser recoil characteristics. The possibility of collision between the LMRP and blow out preventer increases dramatically with the decrease of the stiffness of the top tensioner. While the disconnection phase angle has little effect on the riser recoil response.