This paper presents an investigation of the influence of raised invar edges on tank sloshing under partial-fill conditions. Through multiple scale wedge drop tests and 2-D sloshing model tests, the authors were able to show that raised invar edges tend to enhance the magnitude of sloshing pressures. The enhancement effects were found to be highly localized and vary with the size of loaded area. The findings from this work emphasize the importance of considering the physics of invar edge effects in defining the design pressure to be used in assessing the integrity of membrane LNG tanks.
Safe design of Membrane Insulation Systems (MISs) in LNG ships requires adequate assessment of sloshing loads and structural capacities. Dynamics of the MIS structural designs require that sloshing pressures be defined with fine spatial and temporal resolution on the order of 0.1 square meters and 10-4 seconds, respectively. The requirement of fine spatial and temporal resolution has posed significant challenges to analytically based methodologies for prediction of sloshing impact pressures. In the past, researchers have conducted model-scale sloshing tests in order to define prototype design pressures for structural integrity assessment (Bass et al. 1980), although using tanks with smooth walls. However, at prototype scale, the two widely-used MIS systems both have raised elements, corrugations in the case of MKIII and raised invar edges in the case of No.96. The MKIII primary membrane includes a square pattern of corrugation cells formed by the crossing rows of larger and smaller corrugations, both with spacings of roughly 340 mm. In the case of No.96 system, parallel rows of raised invar edges are present with spacings of roughly 500 mm and contain the weld used to join the sheets invar that make up the primary membrane. Recently ExxonMobil has developed sloshing assessment methodologies (Sandström et al. 2007, Kuo et al. 2009) using corrugated tanks for assessing integrity of the membrane LNG containment systems.
