Abstract
ABSTRACT A model of coiled tubing fatigue is presented. Failures at high internal pressure are predicted when the net cumulative axial plastic strain reaches a critical failure strain that can be approximated as the local fracture strain measured in a uniaxial tensile test. Failures at low internal pressure can be predicted using standard low-cycle fatigue approaches. Fatigue-life predictions using the new model are in close agreement with results of full-scale tests of 70 ksi yield, 1-¼-inch OD, 0.087-inch wall thickness steel coiled tubing manufactured by the conventional resistance weld and anneal process. Plots are developed that relate fatigue life to hoop stress and axial strain amplitude. Based on these curves a methodology is proposed that can be used together with full scale data as a guide for selecting drilling conditions to optimize tubing fatigue life.
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