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ANALYSIS OF CONTACT STRESS AND STRESS CONCENTRATION FACTORS AT THE TOOL JOINTS OF OIL WELL DRILLSTRINGS

In this article using axisyrnetric elements in ANSYS standard code , threaded too l join ts of drill strings, which expose to failure in vertical drilling more than other members, have been modeled. Also, for modeling the contact between surfaces contact elements are used. Axial tensile and compressive forces , internal and external pressures arising from drill fluid on pipe , exerted torque by the mot or on the top of the well, and bending moment caused by drill collars bnckling are the main loads exerted to the model. According to the API standard, some preloads are also exerted to the too l joint to improve the fati gue behavior. Different types of loading composed of tension and compression, pnrepreload and combination of them are applied to the model. Some conclusive results have been drawn from the a nalysis including the location and value of maximum stress concent ration factor in Pin and Box, the effect. of preload 0 11 the stress concent ration factor and as a result on the fatigue life of the tool joint and t he contact stress distribution on the screw threads. Finally, convergence of the contact pressure on the contact surfaces has been investigated.