Tensile and torsional loads stress distribution along the drill string for deep wells

Résumé

Common knowledge and rule of thumb tell us that the tensile failures will mostly occurs in the pipe body, while torsional failures will occurs 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 later can usually be operating up until 80% of the pipe body yield stress which is considered as 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 limit probably its tensional limit.By this I mean; exerting both a tension and torsion load on the drill string the yield stress of this later will weaken consequently applying a high tension effort combined with a significant torque lead to plastified the drill pipe body thus reducing fatigue life of the drill pipe.This problem is well observed in back-reaming operation.The results shows, fatigue damage from rotation in dogleg during back reaming operation is a significant problem if the severity is greater then the critical value.So the position of drill pipe in the string will influence the amount of fatigue damage it sustains .besidesthat Back reaming can reduce the fatigue life of the drill pipe significantly because of the reversal stress of the drill string under tension in a dog leg.

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