Strength assessment of membrane LNG tank structure based on direct calculation of structural response

Expanding LNG market reinforces the demand for new concepts of LNG transportation. Membrane LNG vessel design widely applied until now, encounters new challenges due to requirement for larger vessel's capacities and more flexible operation in partially filled conditions. Present assessment procedures of LNG tank structure usually combine small scale sloshing loads measurement and containment system structural strength assessment, on a comparative base for the reference and target vessels. For the new LNG design, more rational methods become essential in the assessment procedure. Some improvements in the strength assessment procedure of membrane LNG tank structure is presented in this paper, combining small scale sloshing load measurements with direct FEM calculation of structural response. The complexity of problem is the consequence of: stochastic nature of impulsive sloshing loads, material used for the cargo containment system at cryogenic temperature and strong hydro-elastic interaction during impacts. Disadvantages of small scale testing and limits of today's numerical methods require that further in the future certain simplifications and assumptions should remain. In the paper, method for the design loads selection from the small scale sloshing measurements is described and discussed. The impulse, transferred over the corresponding impacted surface, is the base for the comparison of successive violent sloshing loads. The stochastic nature and statistics of measured loads are discussed. The structural analysis of LNG tank structure under selected design sloshing loads, using on-linear and time-dependant explicit FE calculations, is described. This paper presents Bureau Veritas recent developments and their applications in the field of sloshing assessment.