A stress amplification model to evaluate the stress concentration of tool joint under complex loads

Abstract

The more and more severe drilling conditions make the load conditions of tool joint extremely complex. Under the complex load conditions, the stress concentration of tool joint is serious, so failure of tool joint often occur in the oilfield. However, few researchers have studied the stress concentration of tool joint under complex loads. In this paper, the stress distribution of an API tool joint, NC38, was analyzed with 3D elastic-plastic finite element method under complex loads. The stress concentration factors of the tool joint were calculated, and the influence of tensile and bending load on it was analyzed in detail. The results show that the existing model cannot effectively reflect the fact that the stress concentration of tool joint will increase with large tensile load in the crooked hole section. Based on the 3D finite element analysis, a stress amplification model is proposed and used to reflect the failure risk of tool joints under the combined action of axial tension and bending moment. The results show that this model is consistent with the fact that the failure risk of tool joint will increase with large tensile load in the crooked hole section.