Modeling and multi-objective optimization of a drill-string system in the tripping operation

The increase in deep and ultra-deep wells demands higher drilling efficiency and more drilling safety. Since the undesired oscillation associated with the tripping in/out of the drill-string increases the risk of downhole safety, it is important to master the axial dynamics of the drill-string system and choose appropriate tripping parameters. In this study, the drill-string system during the tripping operation is modeled as a continuous Euler–Bernoulli beam with the equivalent mass-spring-damper at the top and free at the bottom. The finite element method is employed to solve the axial dynamics of the drill-string system. Then, the dynamic characteristics of the underground drill-string for different system parameters are investigated in detail. Moreover, a multi-objective optimization framework is proposed, aiming to find the optimal tripping parameters and reach a balance between drilling safety and drilling efficiency. The results show that the axial vibration of the drill-string brings about the dynamic load fluctuation of the system, which becomes more serious in the tripping out process. The proposed optimization procedure can reach a desired balance between drilling efficiency and drilling safety and obtain appropriate tripping parameters for different conditions.