Study On Dynamic Characteristics And Failure Mechanism Of Drill String In Air Drilling

Focused on the issue of dynamic characteristics and fatigue failure of drilling string in gas drilling, the paper makes investigation analysis on research results of the issue at home and broad. On this basis, the article carries out study on drill string movement law and its influencing factors by the simulation experiment of BHA motion state, and points out the several possible kinds of BHA motion states, the causes and conditions of tubular whirling and the influencing factors of whirl. This thesis also establishes the buckling differential equations with friction, presents its solution method, analyses the effect of friction and torque on tubular buckling and outlines the numerical relations between the maximum speed and critical axial force when drill strings are in sinusoidal buckling state. The research shows that there are three different BHA motion states at specified drilling pressure which respectively correspond with that: a drill string comes into stable positive whirling under critical rotate speed, irregular and unstable movement of BHA appears at critical speed, when rotate speed exceeds critical speed a drill string forms stable reverse motion. What's more, the analysis also indicates that these factors such as drilling pressure, the position and number of centralizer, friction of borehole wall and the location and number of sliding sleeve have a great impact on motion state of drilling string. The stability of drill string can be improved effectively after the installation of sliding stabilizer on it. Severe vibration is the main reason of drill str??failure in gas drilling. The author respectively introduces the inherent frequency of the drill string lateral vibration, tensional vibration and axial vibration, then manily analyses transverse resonance and presents the calculation method of critical speed.