Reliability-based Methodology For Sloshing Assessment of Membrane LNG Vessels Available to Purchase

Sloshing is a large scale phenomenon involving some very small scale effects. Indeed, overall volume and geometries, fluid densities etc. come into play as well as many local parameters such as local geometry, gas/liquid mixture or surface tension. Significantly, the local effects have a large influence on the local peak pressures, which explains their high variability. Moreover, these peak pressures influence greatly the design of the Cargo Containment Systems (CCS) of LNG vessels. For these reasons it is impossible for the time being and even in the near future, for numerical simulations to encapture both the global and highly stochastic local effects, and to give a useful estimation of sloshing peak pressures. Only model-scale tests can be envisaged to approach reality in its complex form. However, model sloshing testing is a representation that can diverge in regard to some aspects from full scale reality under cryogenic conditions. Over the years, GTT has developed a methodology to study sloshing and sloshing-induced loads. A large Research and Development program carried out over the past four years enables today the introduction of the most comprehensive evolution of GTT's methodology: the reliability-based methodology for sloshing assessment. This methodology is based on a long term approach which combines all conditions the vessel could experience during her lifetime using sophisticated statistical analysis. It compares finally the sloshing loads to the strength of the CCS for its different limit states via a reliabilitybased method. Aware that the model tests do not account for all the physics parameters, it is considered prudent to calibrate the estimations of the probability of failure for the different limit states of the CCS to values that are derived from actual experience at sea. The methodology therefore remains comparative. This paper describes today's major evolution. It encompasses hardware improvements, assumptions for sea-keeping calculations, loads and strength evaluations as well as final assessment.