Methodology for Recoil Analysis of Emergency Disconnection Drilling Riser in a Deepwater Environment

Abstract

In deepwater drilling, it is inevitable to disconnect the riser from the Low Marine Riser Package (LMRP) with the blowout preventer (BOP) under some extreme operating conditions. Since the riser is in tensile state under normal working conditions, elastic potential energy is stored in the riser. So, the riser will move upward like a spring under the combined action of the elastic potential energy and the frictional resistance generated by the discharge of drilling mud in riser, which is called recoil response. Recoil analysis of drilling riser is of great significance to the safety of deepwater drilling, from which the recoil response, main controlling factors and corresponding solutions can be defined. In this paper, the mechanical model and governing equations are established based on the mass-spring-damping system with two degrees of freedom. In this model, the frictional resistance on the riser inner wall when the drilling fluid is discharged, and the elastic potential energy stored in the riser are taken into account. In addition, the direct acting tensioner (DAT), which is simplified as sinusoidal motion and spring model, is selected as the top boundary condition of the disconnected riser. The tension force and spring stiffness is calculated based on the configuration and working principle of the DAT. Finally, the time-history displacement of the LMRP after disconnected from the BOP is obtained. On this basis, the parameter sensitivity analysis is presented. Analysis results show that with the increase of water depth and the amplitude of the DAT, the possibility of collision between them rises. For a determined riser system, there is an optimal heave motion frequency for the DAT to implement riser emergency disconnection. Corresponding measures should be taken to prevent the spring stiffness of the DAT from decreasing. Besides, the riser size has a comprehensive effect on the riser recoil response. This paper can provide reference for the research of riser emergency disconnection.