Abstract: Along with the progress of drilling technology, the work condition of the borehole drill collar becomes harsher. Main reasons and vital area of causing drill collar screw failure are discovered by further analysis on these problems. This research analyzes the strength of API standard connection screw thread with large scale three-dimensional solid modeling software Pro/E and large scale finite element analysis software ANSYS, as well as the distribution of von Mises stress under different work condition and structure. Optimal research of stress-relief grooves of ZTNC56-80 drill collar thread is conducted and the optimal value found out. It is pointed out that the stress trend and the fillet of structure is not linear relation, when improving stress-relief groove structure of drill collar thread, we can not simply rely on increasing the fillet of structure, thus this research offers scientific basis for ameliorating stress-relief grooves of drill collar thread.

Key words: drill collar, stress concentration, stress-relief grooves, ANSYS, strength analysis