Optimization of Limit Drilling Parameters for High-Inclination Wells in the Bohai Sea Based on Drillstring Dynamics

With continued expansion of offshore oil and gas development, the number of high-inclination wells has increased rapidly. During drilling of such wells, vibration transmission from the bottom drillstring to the wellhead is significantly attenuated. Therefore, even when severe vibration occurs at the bit, surface monitoring may not accurately reflect downhole conditions. To analyze axial and lateral vibration behavior, this study considers drillstring–wellbore contact and bit–formation interaction. Based on the Lagrange equation and the S–N fatigue curve, a dynamic model of the drillstring in offshore high-inclination wells is developed using the beam element method. A dynamic safety evaluation model is then constructed using the calculated dynamic characteristics, forming a mechanical analysis and optimization approach for drillstrings in these wells. The technique was applied in a branch well in the Bohai Oilfield. Drillstring vibration under different wellbore trajectories and drilling parameters was examined, and limit drilling parameters were selected through fatigue life analysis. The recommended configuration includes 24 drill collars, a weight on bit of 100 kN, and a rotation speed of 60 r/min. These optimization guidelines support improved drilling efficiency and help ensure drillstring safety in offshore high-inclination well applications.