DISCUSSION ON FAILURE OF DRILLING STRING

The forms and the study methods of drilling string failure are thoroughly analised.The traditional method has it limitation and is unable to describe properly the properties of material in terms of fatigue fracture.From the view point of mechanism,the failure of drilling string can be divided into the low stress fatigue failure,plastic fatigue fracture,fatigue and wear failure under the action of environmental medium and other forms of failure.Based on the categorization above,the concepts and methods of the local stress-stain method to study the extension speed of cracke,the nonproportional cycle concept to study the impact of combine load on the properties of material and the concept of spectral random load to study the randomness of the motion of drilling string are introduced.The study on the mechanism above is one necessary step to solve the issue on the failure of drilling string and is one of the key to the establishment of scientific criterion of break and to steer the practices.