Influencing Factors on Internal-Pressure Capacity, Collapse Resistance, and Fatigue Life of Coiled Tubing During Field Operations

Abstract

Coiled tubing is subjected to internal-to-external pressure differentials and repeated bending during field operations, while ovalization and cumulative fatigue damage progressively compromise structural integrity. To address inconsistencies among pressure-capacity and fatigue models and the simplified treatment of ovality, three complementary calculations are performed. The nominal first-yield pressure of a circular tube is obtained using the von Mises criterion. The ovality-corrected collapse estimate is determined from cross-sectional area conservation and principal-axis equilibrium, while complete tripping cycles are predicted using an empirical equivalent-stress formulation and Miner’s rule. The methodological contribution is a coordinated engineering assessment in which nominal first yield, ovality-corrected collapse, and complete tripping-cycle life are distinguished and evaluated using consistent geometric, loading, and output definitions. Within the investigated parameter range, increasing yield strength from 483 to 758 MPa raised the calculated internal-pressure yield limit from 116.88 to 183.42 MPa. Increasing ovality from 0 to 0.02 reduced the calculated ovality-corrected collapse estimate from 122.44 to 77.03 MPa, while increasing internal pressure from 0 to 20 MPa reduced the predicted number of complete tripping cycles from 190 to 41. The results clarify the effects of material strength, geometry, pressure, and bending conditions and support model-based tubing selection, pressure management during bending, and segment-specific life assessment. Field operating limits should nevertheless be established using applicable standards, measured material properties, and full-scale tests on coiled tubing of the same specification.