Heat treatment effects on collapse of JCO-E steel pipes under external pressure: Experiments and numerical predictions

Abstract

The paper examines the collapse resistance of thick-walled steel pipes, candidates for deep offshore applications, fabricated with the JCO-E process. It describes primarily a combined experimental, numerical and analytical work on the collapse of two JCO-E pipes, a 30-inch-diameter X60 pipe and a 24-inch-diameter X65 pipe, both with D/t ratio approximately equal to 20 and focuses on the effects of heat treatment. The JCO-E pipes are subjected to full-scale external pressure testing, which determines their collapse (ultimate) and propagation pressure. Prior to testing, both pipes have been subjected to a thermal heating, similar to the typical heating conditions for the application of a 3-layer coating system (i.e. 3-layer polypropylene system). Initial ovality and thickness measurements along the pipes have also been obtained. Furthermore, strip specimens, extracted from the ”as fabricated” and ”heat treated” pipes, have been subjected to uniaxial testing, so that the corresponding material properties are measured. In addition to experimental work, numerical calculations are performed using nonlinear finite element models that simulate the collapse testing procedure and employ the measured material properties. The experimental and numerical results on collapse and propagation pressure of the pipes are compared with the corresponding predictions of the DNV-ST-F101 standard, and the value of fabrication factor αfab is discussed. Finally, a numerical study is conducted to investigate the effect of yield strength recovery, due to heat treatment, on collapse of pipes with different values of D/t.