Analysis of a noval stress relief structure of drill collar joint

It has been a hot issue of reducing the local stress concentration on the drill collar joint to improve the stress distribution. For the oil-drill collar, the failure often occurred at the root of the last engaged tooth in the pin near the shoulder, and the fatigue fracture was much related to the stress level. The larger the stress, the more easily fatigue fracture occurs. The new designed structure has accessed to the application, which was designed on the base of the stress-reducing structure of API. Detailed analysis was carried out on the new structure using finite element method. The stress distribution on the thread teeth and the contact normal force on the shoulder surface were compared among the API joint, the stress relief structure of API and the new structure. The results indicated that the new structure can effectively reduce the local stress peaks of the drill collar joints and improve the stress distribution, so it could provide reference of designing new drill collar joints and improving drill collar lifetime.