Fatigue of drillstring threaded connections
Analytical, experimental and numerical work is reported on failure and fatigue crack growth in rotary shouldered drillstring threaded connections, where fatigue failure occurs at critical (last engaged) threads. A comprehensive review is made of drillstring failure data in the period 1989 to 1994, where connection failure accounted for 58% of the total, confirming it as the principal cause of drillstring equipment failure over the period. Approximately equal proportions of pin and box failures occurred. An extensive programme of 2-D elastostatic finite element analyses was undertaken on drilling motor and drillcollar connections investigating peak stress response at the critical threads to preload (from connection make-up) and applied loads. A pronounced effect of preload on these peak notch stresses was found, particularly at the pin, revealing the stress concentration factor (based upon remote nominal pipe stress) to be inconstant and a strong function of applied load, its value reducing with increasing load. Fracture mechanics data in the form of finite element derived stress intensity factor solutions were produced. New K solutions in the form of Y functions were derived for tension loaded connections containing cracks: in a drilling motor box at its last engaged thread (without preload): and in an API NC-61 nonmagnetic drillcollar pin and box at their last engaged thread sites, both with preload. These solutions are all conveniently referenced to nominal stresses in the uniform pipe making their use in crack growth studies straightforward. A new solution is also given for a fully circumferential internal surface crack in a tube under axial tension. The growing fatigue crack is also shown to cause load to be shed from the last engaged thread onto adjacement threads. The dominance of the thread root notch singularity at short crack lengths (a/T<0.1) means that equations fitted to the majority of the Y function data using linear regression analysis are done so for two separate regions of a/ T.