Study On The Transient Contact/impact Theory And Modeling Analysis Of The DTH Drill String System

The dynamic system of DTH drilling process is related to the problem of transient contact/impact. In the numerical analysis and research, relationship between the contacts surfaces is set properly and reasonably which can reveal the dynamic process of impact and deformation. This paper starts from the feature of hertz contact/impact, analyzes the limitations of the theoretical formula of equivalent spring model, calculates the buckling boundary conditions and critical speed theory, verifies the results with numerical analysis method and points out the scope of its application, details are as following:First, this article studies the axial impact model of two flexible bars in different contact forms by theory of one-dimensional elastic wave, the impact of contact/impact buckling to the collision process. For the specificity of Hertz contact, buckling boundary conditions are established based on the closed state formed by the contact area and the buckling zone. The critical buckling radius is solved in given larger speed in MATLAB. The comparative analysis of the collision process of the impulse hammer and the drill rod at different radius of curvature in LS-DYNA, verify the correctness of buckling boundary conditions and buckling area closed condition as critical buckling condition having more project value than simple strength theory. The numerical analysis of collision process showed the limitations of buckling boundary conditions in solving the low-speed collisions.Second, the further analysis of differences between the numerical solution and theoretical solution of the equivalent spring model in low-speed collision is done, stress wave in low-speed collision has little effect on the contact/impact process, and impact force is mainly affected by local deformation. The corresponding critical speeds are obtained by different ideas at collision deformation with quasi-static Hertz collision theory method. Numerical analysis in Hertz low-speed contact collision revealed that critical velocity in quasi-static Hertz closed buckling zone is consistent with the soft collision. Third, the drill string is added to DTH drilling system model in LS-DYNA. The problems existed in the vibration model of flexible coupling shock of DTH and drill string drilling system were preliminarily discussed: how to accurately define the movement of the piston, how to apply gravity of drill string system, the contact algorithm among piston, bit, rock, and the drill string, flexible conversion settings and so on. All of the above studies have enriched and improved the DTH dynamic model.