Research On A Model Of Nonlinear Coupled-vibration Of Drillstring

The vibration of drillstring is inevitable form of the movement during the drillingengineering. Due to the complexity of drillstring stress state in underground, the vibration ofthe drillstring is often the coupled vibration of longitudinal vibration, lateral vibration andtwisting vibration. The coupled vibration showed the characteristics of nonlinear during thedrilling engineering. So it is necessary to set up a actual model to research the nonlinearcoupled vibration. In this paper the boundary layer theory and the finite element were used tostudy the nonlinear coupled vibration during the drilling, the research results obtained below:First, it is established that the physical model of nonlinear coupled vibration of thedrillstring in the paper, the finite element was used to scatter the drillstring, considering theinfluence of the drilling fluid based on the boundary layer theory, the stiffness matrix andmass matrix were derived based on the nonlinear deformation. Then the nonliear coupledvibration rule of the drillstring is obtained.Second, the nonliear coupled vibration of the drillstring when the acceleration on liftingthe drillstring was anlyzed, then the nonliear coupled vibration rule of the acceleration onlifting the drillstring is obtained. It is concluded that with the accelerated time increases, thefrequency of the nonliear coupled vibration of the drillstring which the acceleration onincreases.Finally, the stability of the drillstring when the fluctuations in drilling pressure at the bitwas analyzed by, the finite element was used to obtain the critical instability frequency of thedrill string. It is concluded that with the increase of the drilling rotational speed, the stabilityof the drillstring will increase, with increase of the bit pressure, the stability of the drillstringwill reduce.The actual drilling data in Jilin Oilfield was used to analyze the change of theeccentricity of the drillstring, the drilling fluid in the drilling, the drilling fluid in annulus, thedensity of the drill fluid which impact the frequency of the drillstring vibration. It will providea reliable basis for regulating the drilling parameters.