Study On The Fracture Properties Of The Surface Of The Drill Pipe Body And Joints

The drill pipe is the main drilling tool of transferring power and conveying drilling fluid. During the rotary drilling operation, the drill pipe is usually subjected to tension, internal and external pressure of drilling fluid, bending moment, torque, all various of impact loads and transverse impact between the drill pipe and bore wall. At the same time, the drill pipe is always in a seriously corrosive environment because of the drilling fluid and the corrosive medium in the well. So the drill pipe is the most weakest link in the whole drilling equipment. On the other hand, the crack always initiate on the surface of drill pipe in the process of manufacturing and using. Being in the corrosive environment and standing the complicated loads, the crack will propagate constantly until fracture failure happens. The failure of the drill pipe has become one of the main reasons for affecting the development and normal production of oil and gas field.The failure of the drill pipe is usually happens on the drill pipe body, the inner upset transitional section, the pin or box tool joint and the butt welding section between the drill pipe body and joint. At present, scholars at home and abroad have made many studies in exploring the failure reasons, analyzing the intensity, improving the structure of the drill pipe and the static fracture behaviors of the surface crack in the cylinder or the other simple geometry. However, there are less studies in the fracture behaviors of the cracked drill pipe body, the inner upset transitional section and the pin or box tool joint. Based on the actual operating loads of drill pipe in the drilling operation, using fracture mechanics combined with finite element software, a drill pipe defect is represented as a circumferential semi-elliptical surface crack in this paper, the fracture behaviors of the cracked drill pipe is quantitatively studied in different loads. The present paper provides a basis for revealing the failure mechanism of the drill pipe, predicting the fatigue life of the drill pipe, reducing the fracture accidents and drilling cost. The mainly work are as follows:1. The numerical model of the cracked drill pipe body is established, and then the dynamic response of the model is studied which separately stands tensile impact, eccentric compressive impact, torsion impact and transverse impact between the drill pipe and borehole wall. The dynamic fracture performance of the numerical model is also compared and evaluated quantitatively under all kinds of impact loads.2. The fracture behaviors of the inner upset transitional section of the drill pipe containing a crack is numerically studied under the combination of tension and torsion. For different loading ratios and crack depths, the transition aspect ratios are determined and variation laws of the dimensionless stress intensity factor are fitted along with the crack propagating. It provides a basis for study on the fatigue life of the drill pipe near the well head which is subjected to the combined loads of tension and torsion.3. The fracture behaviors of the cracked tool joint is numerically studied under the combined loads of tension, bending moment and torsion. For different loading ratios and crack depths, the transition aspect ratios are determined. And the variation laws of the dimensionless stress intensity factor are fitted along with the crack propagating under the combined loads of tension, bending moment and torsion. It provides a basis for study on the fatigue life of the tool joint which is near the well head or the angle buildup interval of the directional well.