Nonlinear dynamic characteristics of the drill-string for deep-water and ultra-deep water drilling

Riser will bend and deform in deep-water and ultra-deep water drilling, which may affect the dynamic characteristics of the drill-string. The analysis of drill-string dynamic characteristics is very difficult due to its super slenderness ratio. As the drill-string rotates in a curved narrow wellbore, the geometric nonlinearity and contact nonlinearity must be considered. In this study, a new method is proposed to analyze the drill-string dynamics considering the influence of riser deformation. The configuration of riser, based on different sea current velocities, is calculated to form a quasi-dynamic well trajectory combining with the fixed wellbore under mudline. In order to solve the dynamic problem of an extra-long drill-string model, the node iteration method is used to judge whether contact occurs at each node, and the Newmark method is used to calculate the spatial configuration of the drill-string at each time step. A program is successfully developed to analyze the dynamic characteristics of the drill-string in a deformed riser. The simulation results show that the deformation of the riser caused by ocean current has a great influence on the lateral vibration of the upper drill-string, and relatively little effect on the bottom-hole assembly. The contact and collision force near subsea wellhead is so large that may result in eccentric wear and leakage of the nearby riser.