Definition
Drill Pipe Fatigue — Drill pipe fatigue is the progressive cracking of drill pipe caused by repeated cyclic bending stress as the pipe rotates through doglegs and curved hole sections, and is a leading cause of drill string washouts and twist-offs.
Also known as: drill pipe fatigue, fatigue failure, cyclic fatigue
What drill pipe fatigue is
Drill pipe fatigue is damage that accumulates when the pipe is bent and straightened over and over as it rotates. Every revolution in a curved section of hole reverses the bending stress on the pipe wall; enough of these cycles nucleate and grow a fatigue crack, which can lead to a washout (a hole in the wall that leaks) or a twist-off (the string parting downhole).
Fatigue concentrates where stress and bending are highest: at the tool-joint upset, the slip area, and anywhere the pipe passes through a severe dogleg. High rotary speeds and tension amplify the damage.
Managing fatigue
- Dogleg severity — limiting DLS in the drilled path is the single biggest control on fatigue.
- Transition design — HWDP between collars and drill pipe reduces the stiffness step that drives fatigue.
- Inspection — DS-1 and API RP 7G-2 inspection tracks cumulative wear and catches early cracks.
Renting fatigue-managed pipe
Rental pipe is inspected and rotated through service so fatigue is tracked and cracked joints removed. We supply inspected drill pipe and heavy-weight drill pipe for fatigue-critical directional work — contact us for the right transition string.