The only standard drillstring model in use today is the torque-drag model because of its simplicity and general availability. Field experience indicates that this model generally gives good results but sometimes performs poorly. For example, some friction loads predicted for casing running in horizontal wells have not been consistent with field data. In the standard torque-drag model, the drillstring shape is taken as the wellbore shape. However, given that the most common method for determining the wellbore shape is the minimum curvature method, this assumed wellbore shape forces the bending moment to be discontinuous at survey points. This defect is dealt with by neglecting the bending moment. A different approach assumes that the drillstring position corresponds with the minimum curvature wellbore only at discrete points. The obvious choice for these discrete points is at the positions of the tool joints in the drillstring. While these tool joints are fixed in position, they are allowed to rotate within the wellbore. These extra degrees of freedom allow solution of the bending moment problem, for continuity of bending moment can now be assured at each tool joint. Further, experimental studies of actual drillstrings have shown the potential to develop contact forces for lateral buckling that are significantly larger than predicted by smooth-pipe models. Thus, by the discrete-point assumption, we resolve two problems: bending moment continuity and underprediction of lock-up. This paper gives a complete description of the drillstring calculation. Typical torque-drag problems are studied to compare the two torque-drag formulations. These studies give comparisons in drag forces and torques for the two models, and for the new formulation, the magnitude of the bending moments.
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