CAUSTIC FAILURE OF THERMAL INSULATED TUBING IN STEAM FLOODING

The coupling of the pre-stressed thermal insulated tubing string broke down inside the steam flooding welts, after the tubing were used in h igh temperature and high pressure steam for about two years. With furt her check, it was found that most of the weld joints between the inner and outer tubing also cracked. The failure analysis for the accidents has been carried out by the analysis of chemical composition for the steels, microstructure observation, mechanical property tests, and the investigation of the service environment etc.. The results show that the early damage of the pre-stressed thermal insulated tubing string i s mainly induced by the caustic stress corrosion, which can be strengt hened by an unstable microstructure of the coupling serviced at high t emperature for a long time. Moreover, the high pre-stress design on th e thermal insulated pipe for the cyclic steam injection may be not sui table to the situation of steam flooding, which would result in the we ld joint cracking.