Fatigue Analysis Of The Oil Drill Pipe Connection

Oil drill pipe is one of the main equipments for oil and natural gas exploration, which is used to transfer rotational torque and transportation of drilling fluids. They are connected by drill pipe connections with the thread. Working under complex and cyclic loading, it’s inevitable to get stress concentration in the connection for its section shape changes a lot, and eventually leads to a fatigue failure. Accordingly, much emphasis is placed on the study of fatigue properties of the oil drill pipe connections both at home and abroad. The research and development of oil drill pipe products started late in China, and there is a large gap in the performance and quality compared with developed countries. So it’s significative to study deeply of the fatigue failure mechanism of the drill pipe for the optimization of its structure. In this paper, the fatigue properties of the drill pipe connection were deeply studied based on the research at home and abroad. The main content of the research are as follows.The basic fatigue theory of drill pipe connection and the correlation between the connection structure and its fatigue properties are introduced briefly in this article. The finite element models of the API connections under different loads were developed for the stress analysis, and studied the impact of the API connection structural defects on its fatigue strength. The structure optimization schemes were proposed in the thread, internal upset and connection, and draw the manufacturing drawings based the processing conditions. The FEA results showed that the stress distribution and concentration in the weak areas were improved by theses optimization schemes compared to the API connections, as well as the torsional properties and fatigue strength of the connections. The effectiveness of these schemes was also proved by the sample tests and field applications and has already created good economic benefits.According to the research results, a new drill pipe connection applied for the drilling of the offshore gas well is designed and developed based on the deep understanding of the fatigue mechanism of the oil drill pipe connections. This new connection can be used in the drilling of high-pressure nature gas well for its reasonable stress distribution, relaxative stress concentration, excellent fatigue resistance and good gas sealing performance. The determination of the angle, interference and other key parameters of the sealing surface were introduced. The make-up tests of the connection samples were conducted and the results meet the design requirements. In the further, the hydrostatic and pneumatic test will be conducted to verify its sealing performance.