Ductile Instability Analysis of HSLA Coiled Tubing

Coiled tubing are thin walled steel tubes of 25 to 89 mm diameter and thousands meters long used in the oil industry for production and maintenance services. J Integral evaluation, based on elastoplastic fracture mechanics, has a central role in critical crack length evaluation for fracture instability of coiled tubing. The instability analysis requires the critical load determination at which the crack will grow in an unstable manner. Due to coiled tubing diameter and thickness, standard specimens could not be constructed. J-R curves were experimentally determined from four point bending tests of 1 m long coiled tubing specimens. Instability was evaluated through tearing modulus T, using J-T curves, where the instability point is found on the intersection of both curves, Tmaterial and Japplied, Tapplied. Two calculus schemes were employed for a through wall thickness cracked tube: J EPRI and the Reference Stress Method (RSM) of Ainsworth. For every coiled tubing specimen Jcritic, applied tearing modulus and critical crack extension were determined. Critical load was also determined.