A theoretical study has been performed on the lateral vibration of non-rotating drillpipe interacted with drilling fluid in vertical holes. Particular attention is paid to the interaction between drillpipe and drilling fluid near drillpipe neutral point zone. A new theoretical model has been established. The interaction between drillpipe and drilling fluid is characterized in this model. Dimensional analysis has been conducted to generalize this model. An innovative numerical method has been developed to solve the eigensystem. A FORTRAN program has been developed to model the behavior of drillpipe conveying fluid in vertical holes. Several numerical examples are provided to show the influences of drilling fluid on drillpipe dynamics. Fluid is considered to be flowing downward inside pipe and upward in annulus simultaneously. Results show that drilling fluid has significant influence on drillpipe dynamics.
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