Vytyaz O. Development of scientific bases for assessing the efficiency of drill and flexible pipe strings

The thesis is devoted to comprehensive evaluation of serviceability of elements of long-operated drill strings and coiled tubing, by developing engineering methods of calculating dynamic operation mode, improving techniques and technology of well deepening and determining residual durability of pipes with operational defects. The mathematical model of dynamic mode of drilling experimental stand has been improved and mathematical models of coiled tubing string while drilling horizontal wells have been developed for the first time. New devices and bottomhole assembly were developed to improve rotary drilling of directional and horizontal sections of the well. A new design of shock absorber of longitudinal-torsional oscillations of drill string was created and manufactured, and its industrial tests were carried out. The experimental-calculating method for prediction and estimation of serviceability of long-operated elements of drill strings and coiled tubing has been suggested. It is based on the conducted researches of corrosive-active medium influence on the rate of corrosion-fatigue cracks development in long term operated steels of drill strings and coiled tubing elements, evaluation of operational factors influence on their performance, as well as determined by means of finite element method and analytical dependences of the stress intensity coefficients in the vicinity of transverse semi-elliptical cracks (schematized operational defects) on internal or external walls of long operated drill pipes of strength groups “K” and “L”.