Drill String Fatigue

Most drill string failures are due to fatigue, resulting from repeated cyclic bending loads and stresses in tensile or buckled drill pipes.
Fatigue prediction is usually based on S-N curves, a failure criterion and a damage accumulation rule, initially requiring a calculation of the drill pipe stress caused by bending when rotated in a dog leg. This bending stress calculation is the key point of the cumulative fatigue damage model.
The cumulative fatigue damage model as defined in API RP7G (ISO 10407) has been implemented in an advanced torque and drag model, which enables to track any given point of the drill pipe while drilling, such as the transition zone, the tool joints and the drill pipe body. This technique enables to fully track variation of stresses at a given point in the drill pipe.