Tensile and Torsional Loads Stress Distribution Along the Drill String for Deep Wells

Common knowledge and rule of thumb tell us that tensile failures will mostly occur in the pipe body, while torsion failures will occur in the tool joints. The total stresses on the drill string should be considered which are induced by combined bending, torsion and tensile stresses. The latter can usually be operating up until 80 % of the pipe body yield stress, a level which is considered as a safe working limit. Moreover, torque and drag are caused by the lateral forces and the friction between the borehole wall and the drill pipe. The lateral forces depend on the weight of the drill string. Torque and drag are sometimes overlooked when drilling simple wells. In deep well this is not acceptable. Proper decision made using correct torque and drag can make all the difference between TD drilling and suspended drilling. Therefore the over pull load plus the torque are the most important points to be considered in deep well drilling, since the resistance of the drill string body to the combined efforts will probably limit its tensional limit. By this I mean, exerting both a tension and torsion load on the drill string of this later will weaken its yield stress consequently applying a high tension effort combined with a significant torque lead to plastifying of the drill pipe body thus reducing its fatigue life. This problem is well observed in back-reaming operation. The results show that fatigue damage from rotation in dogleg during backreaming operation is a significant problem if the severity is greater than the critical value. So the position of drill pipe in the string will influence the amount of fatigue damage it sustains. Furthermore, back reaming can reduce the fatigue life of the drill pipe significantly because of the stress reversal of the drill string under tension in a dogleg.