Environmentally assisted fracture in oil and gas production

摘要

This session had been organised jointly between EFC WP5 on environment sensitive fracture and EFC WP13 on corrosion in oil and gas production, which is chaired by P. Jackman. The nine papers covered various aspects of environmentally assisted cracking in alloys used in the oil and gas industry, including: mechanisms of stress corrosion cracking, hydrogen stress cracking, and sulphide stress corrosion cracking; test methods; fabricational aspects; predictive methodologies; and the performance of modern highly alloyed materials.The first paper, by D. Le Friant of the Ecole des Mines de St-Etienne et al., discussed mechanisms and test methods for stress corrosion cracking of pipeline steels in simulated ground water. This study had been stimulated by the failure of a French oil transmission pipeline in 1997 due to transgranular stress corrosion cracking. This had originated on the outside of the buried pipe in a region where disbanding of the asphalt coating had allowed contact between ground water and the surface of the steel and there was evidence of local overprotection by the cathodic protection (CP) system and the presence of hydrogen in the vicinity of the transgranular cracking with under-protection in other regions. Slow strain rate tests have been conducted in the laboratory on smooth and notched specimens of high sulphur, 0.025 percent, and low sulphur, 0.003 percent, pipeline steels in near neutral pH simulated groundwater (NS4 solution containing 122 mgL~(-1) KCl, 483 mg L~(-1) NaHCO_3, 181 mg L~(-1) CaCl_2.H_2O, 131 mg L~(-1) MgSO_4.7H_2O and acidified to pH 6.7 by bubbling with a CO_2/N_2 gas mixture). The results showed that in the higher sulphur X52 steel, transgranular cracking occurred at normal cathodic potentials of -1000 and -1200 mV(SCE), and with overprotection at -1700 mV(SCE) ladder-like hydrogen induced cracking occurred. In the lower sulphur steel (TUE360), transgranular cracking only occurred with overprotection at -1700 mV(SCE). The presence of shallow, 50-150 #mu#m deep, notches and cyclic loading exacerbated the cracking. It was proposed that a test in NS4 solution suing pre-notched specimens subjected to both cycling loading, 0.008 Hz, and cyclic polarization, 22 h at 1200 mV(SCE) and 2 h at E_(cor) -750 mV(SCE) per day, would be suitable for ranking the resistance of pipeline steels to transgranular cracking in groundwater.