Dynamic Effect of a Flexible Riser in a Fully Connected Semisubmersible Drilling Rig Using the Absolute Nodal Coordinate Formulation

Abstract

The motion of a semisubmersible drilling rig must be checked in advance to satisfy the safety criteria of the rig. However, the complexity of the rig's connections makes it difficult to analyze the rig motion during the drilling operation because it is connected to the seabed by the blow-out preventer (BOP). The rig's connections consist of several pieces of risers, a telescopic joint, and a riser tensioner system. Also, from a macroscopic perspective, the risers should be regarded as flexible threads. Therefore, this study developed a rig motion analysis program considering the deformable effects of flexible risers and the full connectivity of the drilling rig. Flexible multibody dynamics (FMBD) based on the absolute nodal coordinate formulation (ANCF) is adapted for the mathematical modeling of the risers and joints. Acting as an external disturbance, a hydrodynamic force and current force are exerted on the drilling rig and the risers, respectively. The drilling rig is fully modeled including the riser tensioner system, telescopic joint, flexible risers, and upper and lower flex joints. The motion analysis with and without connections was fulfilled to verify the effect of connectivity. Moreover, we observed that the movement of the drilling rig increases as the current speed increases. Finally, the simulation is successfully applied to check the motions and tensions of the drilling rig in moderate and storm conditions.