Stress Analysis and Fitness-For-Service Assessment of a Full Containment Steel-Steel LNG Tank for Major Leak Scenarios and Laboratory Testing of 9% Ni Steel Plates

Abstract Storage and containment systems are critical components of Liquefied Natural Gas (LNG) plants to ensure safe and reliable operations. Strict regulations are in place globally to ensure safety of operations of LNG tanks because of the amount of stored energy. In this study, structural integrity and safety of a FEED stage design of a 45-million-gallon capacity LNG tank with a secondary metal container was evaluated for various LNG release scenarios from the primary container. A secondary metal container is permitted in current versions of NFPA 59A and API 625; however, typical practice involves construction of concrete secondary containment systems. Nine percent Nickel steel (ASTM A553) secondary containment systems provide advantages in operation, maintenance, cost, and construction schedule compared to traditional reinforced concrete secondary containers for some projects and may provide modularization advantages. Laboratory testing of welded ASTM A553 steel plates at cryogenic temperatures (−196°C or −321°F) provided Charpy impact energy and fracture toughness data, which were compared with historical data and used for LNG tank assessments. Finite Element (FE) models of the tank were developed to evaluate the structural response of the secondary metal container to −168°C or −271°F temperature exposure due to sudden LNG release load cases. Seismic aftershock following a major leak from the primary container during the main seismic shock was also considered. The stresses calculated from the FE analyses were compared with the allowable stresses in API 620. API 579 fitness-for-service (FFS) assessments with postulated flaws in the secondary container body and welds in multiple orientations were performed using the stress demands from our detailed FE analyses. The study showed that the secondary container constructed of 9% Ni steel could safely withstand the loads resulting from the cryogenic release scenarios. The FFS assessment results using our test data provided insights on the suitability of the type of nondestructive testing (NDT) methods for identifying the maximum flaw sizes that meet the acceptable performance according to the Level 2 FFS assessment procedure (API 579). For most of the LNG release scenarios, visual inspection of the secondary container was sufficient to ensure that the tank would meet its design intent. The study provides fracture toughness values of welded 9% Ni steel plates’ base metal and weldment at cryogenic temperatures. The study also provides a methodology using detailed stress analysis and FFS assessments to evaluate the integrity of LNG storage containments for cryogenic spill scenarios.