Fracture Cause Analysis on 5-1/2FH Double Shoulder Drill Pipe Box

The fracture causes of a 5-1/2FH double shoulder drill pipe box were analyzed by means of macroscopic examination, chemical composition analysis, mechanical property testing, metallographic examination, fracture analysis and so on. The results showed that fatigue fracture failure was happened to the first whole thread of the second shoulder on the double shoulder drill pipe box. The cracks initiated from the root of thread and developed to the outer surface to form the flowing outlets that led to the flushing of the fracture. Insufficient of make-up torque and bending stress on the box were the main reasons of the fatigue. In addition, the root radius of FH thread is small while stress concentration is large, which increased the risk of box fatigue.