Multi-axial fatigue life prediction of drill collar thread in gas drilling

• Multi-axial fatigue life prediction suitable for drill collar thread is proposed.
• FEM is established and the fatigue life of API drill collar thread is calculated.
• A new design of double shoulder thread is proposed.
• Modified double shoulder thread can prolong the fatigue life to some thirty times.

With its advantages of lost circulation prevention, drilling speed improvement and reservoir protection, gas drilling technology has been widely applied in Sichuan and Xinjiang oilfields in China. However, drill collar failures have often occurred under high weight on bit (WOB) during gas drilling. These incidents have not only caused serious economic loss but hampered the development and application of gas drilling. Finite element analysis is used to determine the drill collar thread stress distribution and, using simulation of drill string dynamics, multi-axial fatigue life theory is used to calculate the life of a standard API drill collar connection. The computed results reveal the early drill collar fatigue failures in gas drilling. Then, the multi-axial fatigue life of drill collar with double shoulder thread is analyzed under the same loads. Analysis shows that the fatigue life of a double shoulder thread is some thirty times that of the API thread. So drill collar with double shoulder thread could be an efficient way to solve the fatigue failure problems of drill collar in gas drilling. The work presented in this paper can provide theoretical foundations for safe and efficient drilling with gas.