Knight, Matthew James; (2002) Reliable operation of drillstring threaded connections. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis presents an investigation into the two common failure modes of rotary shouldered connections of the type used in the drillstring in hydrocarbon exploration. The threaded connection is a critical component of the drillstring, which is highly susceptible to fatigue damage, due to the inherent stress concentrations around the thread form. 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 washout. The design philosophy of Controlled Failure Design, which accepts that not all failures can be avoided and is concerned with managing the manner in which the failure may occur, has been used in order to promote a "friendly" failure in the connection, i.e. a wash-out before twist-off. The significance of fatigue crack location, shape and size has been investigated analytically and experimentally, using structural integrity design codes, finite element stress analysis, and full scale experimentation. A custom-built fatigue test rig was designed and constructed for the full scale testing of NC-26 drill collar connections. Through a novel design in the test rig and the use of the ACFM inspection technique, crack growth data was collected during testing, which has to led a better understanding of the fatigue mechanism in drill collar threaded connections. The influence of compressive residual stresses and its effect on improved fatigue performance has been experimentally investigated through the development of thread root cold rolling equipment. The technique of cold rolling has been studied in order to maximise the fatigue life of the connection. Through the control of the cold rolling process, it has been demonstrated that fatigue crack growth can be controlled to produce a localised, non-critical wash-out failure, which leads to a more reliable operation of drillstring threaded connections.

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