Failure Analysis on Damage of Upset Tubing Threaded Connections Used for Coal-bed Gas Development

The threaded connections of 2-7/8″ (73.02 mm) upset tubing used for coal-bed gas development were seriously damaged after passing and washing well in gas production test. The oil field service condition and operation were surveyed and diagnosed. The extrinsic feature, chemical compositions, mechanical properties and microstructure of the failure connections were analyzed. The same batch upset tubing's thread parameters were inspected. The results show that the misoperation of make-up was the main reason for the damage failure of the threaded connections, and some suggestions were put forward for safe use of tubings.