Study On Longitudinal Vibration Of Drill String Based On FSI

In drilling engineering, the study on strong nonlinear solid-liquid mechanism of flexible multi-body system in turbulent environment, relationship between dynamic behavior and structural parameters, dynamic behavior of BHA and the impact on well trajectory by mud are always the key issues for prediction of well trajectory and simulation technology. They also constitute basic content for the development of trajectory design and the visualization system for monitoring. Unlike the past single longitudinal vibration analysis of drill string, according to the flexible multi-body system consists of derrick, TDS, traveling assembly and drill string, the longitudinal vibration analysis on round trip and in drilling was carried out based on FSI. It had great significance for avoiding resonance, extending drill string life and improving drilling rate by optimizing drilling parameters and BHA.On round trip process, because of the changes of velocity and environmental saltation when the drill string enter into or depart from the drilling fluid, the interaction among drill string, mud and well wall existed, through which the vibration appeared. And the vibration would bring interference for the transmission of mud pulse signal. The effect of mud inside and outside to drill string was converted to equivalent added mass, and the vibration and frequency equation of multi-body system were deduced on round trip. Then the exact solutions natural frequencies acquired by using Matlab software were compared with the numerical solution through Ansys software, which verified the rationality of theoretical analysis. In addition, according to the linear relationship between the dynamic load and velocity/time ratio, harmonic response analysis of drill string was performed; therefore, dynamic responses under different velocity and time ratios were obtained. This work provided reference for the optimizing of drilling parameter on round trip operation.Secondly, according to the characters of flexible multi-body drill string system and TDS, longitudinal vibration kinetic equations of solid-liquid coupling of the system were established based on elastic vibration theory and FEM. The longitudinal vibration model analysis on 1700m entire solid-liquid coupling drill string system was performed with the software ANSYS, along with the transient analysis under the axial load contained dead load and static WOB. The critical speed of resonance was obtained from the transformation of natural frequency. The influences to natural frequency by axial load and different BHA were summarized through the analysis of natural frequency and mode of vibration for different BHA. In transient analysis, the influence laws for drill string system by mud were got. The longitudinal vibration intensity on wellhead was most weak. The mud could reduce the longitudinal amplitude and weaken the intensity of longitudinal vibration, for which the drill string was reasonable protected and the working life was extending.