Controlled failure design of drillstring threaded connections

The threaded connection of the type used in hydrocarbon exploration is a critical component of the drillstring that is highly susceptible to fatigue damage. Fatigue crack growth can either lead to a catastrophic failure of the connection, known as a twist-off, or can be a non-critical failure, known as a wash-out. The design philosophy of Controlled Failure Design has been used in order to promote a 'friendly' non-critical failure in the connection, by the controlled application of compressive residual stresses. The influence of compressive residual stress and its effect on controlling fatigue crack shape has been experimentally investigated with the development of thread root cold rolling equipment. Through the control of the cold rolling process, it has been demonstrated that fatigue crack growth can be controlled to produce a localized, non-critical failure in the connection.