Analysis of Causes for Thread Gulling Failure of Φ139.7 mm×9.17 mm N80 LC Casing

In order to find out the causes for the thread gulling failure of the Φ139.7 mm×9.17 mm N80 LC casing connection as occurred during the casing in hole operation, relevant analysis are conducted involving chemical composition, mechanical properties, and metallographic structure of the failed casing.And the thread parameters of three completely-new casings as coupled are checked, and screw-down test is also made.Finally the thread gulling morphology is analyzed.The test results show that the material physiochemical properties and thread parameters of the casing are up to API Spec 5CT-2011, API Spec 5B-2008 and Q/SY 1572.1-2013.It is identified that the root causes for the thread gulling failure are improper thread cleaning and irregular operation as conducted at site.