Risk assessment of membrane type LNG storage tanks in Korea-based on fault tree analysis

For a liquefied natural gas (LNG) storage tank, the greatest concern is for the release of a large amount of LNG or its vapor due to the mechanical failures of main tank and its ancillary equipments or the malfunctions of various hardware components. Nowadays two types of LNG storage tank design, that is, 9%-Ni full containment and membrane concepts, are mostly applied to LNG industry. In Korea the membrane type has been nationally adopted from the beginning step of LNG project because of its higher flexibility in storage capacity comparing to the 9%-Ni type. All the while several huge membrane-type tanks have been built up and operating, the quantified results of risk associated with them has not been systematically delineated. Hence the method of fault tree analysis as a quantitative risk assessment has been here employed to identify and evaluate the risks related to the membrane-type LNG storage tank. Six top events leading greatly to the large release of natural gas are defined as internally induced major accidents and the failure frequencies of these events are calculated by using other sources of process equipment reliability data for the lack of membrane type-specific data.